Journal of Applied Electrochemistry

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Can dissolved Pu-complexes react on the anode? .... L-1 HNO3 + 2.0 mol L-1 NaCl; their experiments show that no severe microstructural .... acid solution containing silver nitrate have been developed to treat transuranic wastes [1-5] and for the .... or 0.050 mol L. -1 ... measured at 1.45 V/SCE on Ti/Pt, and 2.5 V on Nb/BDD.
Journal of Applied Electrochemistry Performance of Ti/Pt and Nb/BDD anodes for dechlorination of nitric acid and regeneration of silver (II) in a tubular reactor for the treatment of solid wastes in nuclear industry --Manuscript Draft-Manuscript Number:

JACH-D-14-01022R1

Full Title:

Performance of Ti/Pt and Nb/BDD anodes for dechlorination of nitric acid and regeneration of silver (II) in a tubular reactor for the treatment of solid wastes in nuclear industry

Article Type:

S.I. : ESEE 10

Keywords:

silver(II); boron doped diamond; electrolysis; tubular reactor; nuclear wastes; electrode service life

Corresponding Author:

Andre Jean Savall, Ph.D. University Toulouse, FRANCE

Corresponding Author Secondary Information: Corresponding Author's Institution:

University

Corresponding Author's Secondary Institution: First Author:

Karine Groenen Serrano, PhD

First Author Secondary Information: Order of Authors:

Karine Groenen Serrano, PhD Andre Jean Savall, Ph.D. Laure Latapie, Engineer Charlotte Racaud, PhD Philippe Rondet, Ingineer Nathalie Bertrand, PhD

Order of Authors Secondary Information: Funding Information: Abstract:

One of the problems frequently encountered in the processing of nuclear fuels is the recovery of plutonium contained in various solid wastes. The difficulty is to make soluble the plutonium present as the refractory oxide PuO2. The dissolution of this oxide in nitric acid solutions is easily performed by means of silver(II) a strong oxidizing agent which is usually electrochemically generated on a platinum anode. However, certain solid residues that must be treated to separate actinides contain important quantities of chloride ions that require after dissolution in nitric acid a preliminary electrochemical step to be removed before introducing Ag(I) for Ag(II) electrogeneration. Research is conducted to find electrocatalytic materials being able to replace massive platinum in view to limit capital costs. In the present work a set-up including a two-compartment tubular reactor with recirculation of electrolytes was tested with anodes made of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for the removal of chlorides (up to 0.1 M) and for silver(II) regeneration. The study showed that these two anodes are effective for the removal of chlorides contained in 6M HNO3 solution as gaseous chlorine, without producing the unwanted oxyanions of chlorine. Furthermore, the regeneration rate of silver(II) on Nb/BDD anode is approximately equal to that obtained on Ti/Pt anode for the same hydrodynamic conditions in the tubular reactor. Accordingly, dechlorination as well as silver(II) regeneration can be performed in the same reactor equipped either with a Nb/BDD or a Ti/Pt anode. Besides, the service life of Nb/BDD anodes estimated Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

by accelerated life tests conducted in 6M HNO3 can be considered as very satisfactory compared to that observed with Ti/Pt anodes. Response to Reviewers:

Answers to the reviewers Reviewer #1: This paper contains results of good experimental work of high importance and is recommended for publication after some smaller changes (most related to language). 1.The language is a little strange here and there and another language check is recommended, see for example: a.Abstract line 8: "Researches are developed" could be "Research is conducted" b.p.4 line 18: "Two of anode constituted with" could be "The two anodes were" c.p.5 line 22: "The time to return on initial conditions was during 4 min." could be "The time to return to initial conditions was 4 min." d.p.6 line 19 "submitted" could be "subjected" All these corrections were introduced. But for item b the sentence was changed in reason of other comments: “Two anodes of quasi-cylindrical shape formed by 8 rectangular plates made of expanded titanium or expanded niobium, covered by a layer of Pt (5 m) or BDD ( 1 m) respectively, were tested.” 2.p.3 reaction (1). Eo=1.396 V vs SHE seems too high, should be 1.37 V vs SHE We have introduced the value for Cl2 released as a gas (from Pourbaix) : E° = 1.359 V vs SHE). 3.p. 5 What was the cathode reaction? Hydrogen evolution I guess, but why was then air needed to trap nitrogen oxides? At a concentration of 13.6 mol L-1 the cathodic reduction of HNO3 forms mainly NO2 by a complex mechanism (cf. Ref. 16 and references therein); there is no hydrogen evolution. We have changed the sentence to clarify this point: “The atmosphere above the cathodic compartment was swept by a flow of air to trap nitrogen dioxide formed by the complex mechanism of reduction of the concentrated nitric acid (13.6 mol L-1) used as catholyte [16]. At this high concentration there is no hydrogen evolution during HNO3 reduction that makes safe the process.” 4.p.6 line 11: why was Ag(II) generation only determined during the first five minutes of electrolysis? By extrapolation of the variation of the experimental Ag(II) concentration at t = 0 one can obtain the initial rate of Ag(I) oxidation, that is the maximum value of the regeneration rate of Ag(II). This remark was introduced in the text as follows: “The rate obtained by extrapolation at t = 0 corresponds to the maximal rate of regeneration of Ag(II) for a set of given operating conditions.” 5.p.10 last two lines and heading of Table 1: Not clear if Table 1 shows theoretical values from eq 13, or if it contains experimental values. Please clarify. Could you give the limiting current also in the heading of Table 1? The heading of Table 1 was clarified; the limiting values of the current intensity were introduced and the text was also modified in introduction of Table 1 (page 10). 6.p.11 line 8: do I understand right that equation 17 may be the reason for this increase in Ag(II) formation at I>I lim? We have chosen to begin with experimental results presented in Table 1 and Fig. 4 and then to give an explanation based on the set of Equations 14-17. Thus, we have introduced a short remark in P. 11, Line 11, linked with this equation set: “…competitive reactions may happen for I > Ilim (see below Eqs. 14-17)”. 7.p.11 line 13: "the first term of the second member" - what does this mean? Please rewrite. This was replaced by “the production rate Rgen in Eqs. 9 and 13 is…” 8.Conclusions: line 3-4: "Therefore, sequence of both steps in the same reactor is possible because conductive diamond shows a kinetic performance comparable to that of platinized titanium" sounds odd. Something like: "It is possible to run the two steps sequentially in the same reactor using Nb/BDD anodes, as has been shown for Pt/Ti anodes." may be better. This suggestion was taken into account. 9.Can dissolved Pu-complexes react on the anode? The highest degree of oxidation known for the plutonium is 6; the cation (PuO2)2+ formed by oxidation of PuO2 is not oxidizible at the anode. We did not modify the text on this point because the properties of the plutonium are not the heart of this manuscript.

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  Reviewer #2 The paper shows an interesting study of the elimination of chloride ions and the generation of silver(II) for applications in the nuclear industry. Further applications of electrochemical engineering and calculations under different parameters are important for industrial applications. The paper should be accepted with minor corrections after the authors address the comments below. Page 2 Line 15; please consider replacing: "In the aim of using the same anode material to…" for "In order to use the same anode material to…" This improvement was introduced. Page 4 Materials and methods Line 2; what are subcritical geometry requirements? To clarify this expression we added the following simple definition: “such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide”. Figure 1 should show the water temperature controlled equipment or connections. Fig. 1 was modified in this sense. Figure 1 should indicate how the ceramic membrane was supported and detailed description of the ceramic membrane. A short explanation was introduced in the text (in page 4): “Every plate was welded on its width to a clamping collar made of the same metal, and was linked to the electrical connector across the lid (Teflon) of the reactor. At the bottom of the reactor, at the anolyte inlet, these plates were mechanically fixed to the base of a cone (Teflon). In the upper part of the reactor the clamping collar was also used to maintain the cylindrical ceramic separator.” The dimensions of the rectangular plates of expanded titanium or niobium covered with Pt should be stated. The design of Fig. 1 was modified to introduce further equipment. In contrast some requested information on the equipment (details on ceramic, supports, plate sizes), considered as sensitive, cannot be entirely described. How was the electrolyte flow rate measured? In Fig. 1 we have introduced the scheme of the classic flowmeter used to measure the electrolyte flow. Page 6 Line 6; is the expression: "Silver(II) was continuously titrated by UV-visible spectroscopy thanks to…" correct? If yes how can Ag(II) be titrated by UV-Vis? Please revise. To clarify this point we added at this place the following explanation: “Silver(II) forms a brown complex in 6 mol L-1 nitric acid (Eq. 7) which was continuously titrated by UV-visible spectroscopy thanks to an immersed probe (Hellman; = 580 nm)”. Page 7 Line 1; at the end of the line the word should be "mol" not "mole" Corrected Line 4 after equation (2); can the authors calculate the mean linear flow rate over the electrode surface? If it is consistent with industrial conditions there should be a reference. The mean linear flow rate value v (m s-1) over the electrode was introduced after Eq. 4. Line 7 after the equation (2); it could be: "in order to…" rather than "in view to…" Corrected Line 13 after equation (2); the residual method should have a reference In the book of F. Walsh this experimental technique was explained; so we have added this reference [18]. How the authors justify the calculation of the mass transfer coefficient with only two points? How do the mass transfer coefficients compare with the literature? The potential values measured under galvanostatic condition were very stable and these two points were considered as significant to evaluate (for example) the operating time for a complete electrolysis in the real industrial equipment operating under the same hydrodynamic conditions. Our aim was to have at one’s disposal (specially for Areva Group) a good estimation of the mass transfer coefficient. Thus, we added the Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

following comment in P. 7 after line 13: “By operating under these conditions the values of the anode potential are very stable and the I-E curve is reproducible.” The authors should report the value kS to compare with other values in the literature that report kA where A is the electrode area. The values of the mass transfer coefficient are strictly valuable for the two expanded metals used. The real surface of the material is difficult to estimate. Thus the mass transfer coefficients k are given to be used for these materials. Equations 3 and 4 could be expressed as logarithmic correlation and compare with other electrochemical reactors that report k =a vb where "b" is an exponent, "a" is a constant and "v" is the mean linear flow rate. To be able to find a logarithmic correlation between k and v we have introduced just after Eqs. 3 and 4 the value of the mean linear flow rate v for a given value of the anolyte flow rate . Thus it will be easy to find the values of the coefficients contained in the relation linking k and v Did the authors observed limiting current plateau for the oxidation of silver(I)? We have observed the same current plateau that was obtained in the case of the oxidation of Ag(I) in a filter-presse reactor (ref. 11). The following explanation was added: “Steady-state polarisation curves (not shown) were obtained under galvanostatic conditions at different flow rates for the tubular reactor. Silver(I) oxidation appears distinctly before water discharge; the oxidation wave of silver(I) presents the classic shape of the progressive change of the kinetic limitation from charge transfer to mass transfer with increasing potential. The mass transfer coefficient was determined using the limiting current measured at 2.04 and 2.36 V (vs. SHE) for Ti/Pt and Nb/BDD respectively.” Figure 3 and equation 5; is the dechlorination process first order kinetics? At the concentration of chloride anion (< 0.1 mol L-1) the process is mass transfer limited; consequently, the dechlorination process is expressed by a rate law of the first order (Eq. 2). Page 9 Line 7; it would be better to state"…chromatography in order to detect…" rather than "…chromatography in the objective to detect…" This suggestion was taken into consideration Please provide an interpretation of the value given by the term S/Vsol. As shown by Eqs. 5 and 6 the S/V ratio is important to calculate the electrolysis time to reach a given conversion for a process limited by mass transfer.To understand well its importance we have introduced the following comment after Eq. 6: “Equations 5 and 6 show that the higher the value of the S/Vsol ratio is, the shorter is the duration of electrolysis.” As the BDD electrode is used to generate OH radicals, would be the main reaction? Why the generation of Ag(II) is not faster in BDD if these electrodes are very efficient to generate the radicals. The efficiency of the BDD electrode has a tendency to decrease when the current density increases as shown in Table 1. Eqs. 16 and 17 were introduced to give an explanation; the rate of H2O2 formation from OH radical is very fast and H2O2 reacts with Ag(II). The production flux of OH radical (current density) should be adjusted to the flux of silver(I) towards the BDD anode. Our explanation follows Eq. 17. What is the effect of the radicals on chlorine ions? As regards to the effect of the OH radical on chlorine ions we do not know for the moment the details of the mechanism (transfer of electron or transfer of oxygen atom), but it is the subject of reflection that we wish to develop. Page 12 Line third before the end of the page; state the value of the high anodic current density. We have introduced the value of the current density: 50 kA m-2. Page 13 Line 4; are there references for the duration of the BDD electrodes? Since the submission of the manuscript we have found a paper by Chen et al. (1997); this reference was introduced in the revised paper (ref [26]). Thus we added a short introduction of this paper in P. 13 (Section 3.4): “Chen et al. [26] tested boron-doped diamond films at 5 kA m-2 in a solution of 1.0 mol L-1 HNO3 + 2.0 mol L-1 NaCl; their experiments show that no severe microstructural or morphological damage after periods of time up to 20h. Thus,”… We also added a recent paper of Chaplin et al. [25] presenting some considerations on Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

the BDD stability. Table 1; would the use of current density be more meaningful? We think that current density can be easily calculated with data given in the heading of the Table 1 Why conversion at 30 A cannot be calculated? Due to the important release of heat under this current intensity the temperature of the electrolyte raised too much (> 5°C) in spite of the use of a thermoregulation system of the reservoir. Consequently, we didn’t take into consideration the value of the stationary concentration of Ag(II) for these conditions. We have added a short remark in Table 1: (a)“Due to excessive heat release the temperature of the electrolyte increased too much to measure accurately the conversion.”   Reviewer #3 The proposed paper is a detailed study, with a typical chemical engineer's approach and perspective, on the performance and lifetime of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for (a) the electrochemical elimination of chlorides from concentrated HNO3 solutions used in transuranic waste treatments, and (b) for the subsequent electrochemical generation from Ag(I) salts (implying prior abatement of chlorides)of the Ag(II) oxidizing agent to be used for dissolution of PuO2. The study appears well contextualized, clearly described, with sound and accurate experimental protocol and detailed modelization. The reported results are interesting (chloride abatement without detected generation of oxyanions, good electrode lifetimes). The applicative issue (treatment of transuranic waste) is surely important. Therefore I think that the paper complies with the Journal's scope and can be accepted for publication.   Reviewer #4 This paper investigates and compares the performances of two anodes for the subsequent removal of chlorides and the generation of Ag(II) in view of the dissolution of PuO2 in HNO3 for the recovery of Pu. I think the manuscript is suitable for the Journal of Applied Electrochemistry after the following minor issue will be addressed: 1-In the introduction, the author should write the equation of the expected reaction of PuO2 oxidation. This equation was introduced (Without number because it is not any more used after). 2-What does "sub-critical geometry "(page 4) mean? To clarify this expression we added the following simple definition “such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide”. 3-The properties of the porous ceramic diaphragm (e.g. material, porosity) should be specified. The nature of this kind of ceramic was given in a previous paper (ref. 8). The characteristics of this ceramic are considered as a property of the Areva Group. 4- The sentence (page 11)" In fact, if the electrolyses started under conditions for which (alpha) was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state (alpha) reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD competitive reactions between hydroxyl radicals and silver species can be expected according to Eqs. (14)-(17)" is not clear. Please arrange it better. The sentence was slightly shortened in reason, indeed, of the interpretation which follows equations 14-17: “In fact, if the electrolyses started under conditions for which was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD a more complex mechanism may be expected according to Eqs. (14)-(17) [21-23].” END

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Manuscript Click here to download Manuscript: JACH-D-14-01022_RevisedManuscript_YellowMarks.docx

Performance of Ti/Pt and Nb/BDD anodes for dechlorination of nitric acid and regeneration of silver (II) in a tubular reactor for the treatment of solid wastes in nuclear industry K. Groenen Serranoa • A. Savall*a • L. Latapiea Ch. Racaudb • Ph. Rondetb • N. Bertrandc a

Université de Toulouse, CNRS, Laboratoire de Génie Chimique, F-31062 Toulouse, France AREVA NP, 25 avenue de Tourville F-50120 Equeurdreville, France c AREVA NC, 1 place Jean Millier F-92400 Courbevoie, France Corresponding author: savall@ chimie.ups-tlse.fr b

Abstract One of the problems frequently encountered in the processing of nuclear fuels is the recovery of plutonium contained in various solid wastes. The difficulty is to make soluble the plutonium present as the refractory oxide PuO2. The dissolution of this oxide in nitric acid solutions is easily performed by means of silver(II) a strong oxidizing agent which is usually electrochemically generated on a platinum anode. However, certain solid residues that must be treated to separate actinides contain important quantities of chloride ions that require after dissolution in nitric acid a preliminary electrochemical step to be removed before introducing Ag(I) for Ag(II) electrogeneration. Research is conducted to find electrocatalytic materials being able to replace massive platinum in view to limit capital costs. In the present work a setup including a two-compartment tubular reactor with recirculation of electrolytes was tested with anodes made of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for the removal of chlorides (up to 0.1 M) and for silver(II) regeneration. The study showed that these two anodes are effective for the removal of chlorides contained in 6M HNO3 solution as gaseous chlorine, without producing the 1

unwanted oxyanions of chlorine. Furthermore, the regeneration rate of silver(II) on Nb/BDD anode is approximately equal to that obtained on Ti/Pt anode for the same hydrodynamic conditions in the tubular reactor. Accordingly, dechlorination as well as silver(II) regeneration can be performed in the same reactor equipped either with a Nb/BDD or a Ti/Pt anode. Besides, the service life of Nb/BDD anodes estimated by accelerated life tests conducted in 6M HNO3 can be considered as very satisfactory compared to that observed with Ti/Pt anodes.

Keywords Silver (II) • Boron doped diamond • Electrolysis • Tubular reactor • Nuclear wastes •

Electrode service life

1. Introduction Silver(II) is a strong oxidising agent (E° = 1.98 V/SHE) capable of attacking many organic and inorganic substances. Electrochemical processes involving Ag(II) regeneration in nitric acid solution containing silver nitrate have been developed to treat transuranic wastes [1-5] and for the safe low-temperature destruction of a wide variety of contaminated organic waste materials [6,7]. In particular, silver(II) generated by anodic oxidation is a method of choice for the dissolution of plutonium oxide PuO2 [1, 3, 8]: PuO2 (solid) + 2 Ag2+ → (PuO2)2+(solution) + 2 Ag+ Dissolution of PuO2 is of considerable interest as this process is used in (i) aqueous reprocessing of uranium-plutonium fuel, (ii) fuel fabrication by reconversion of weaponsgrade Pu, and (iii) regeneration of Pu from several types of residues produced during conversion processes [3, 8]. But during some of these stages, pyrochemical processes used to recover and purify Pu metal generate ashes rich in chloride salts. Consequently, in the

2

silver(II) process operated by the nuclear industry, chlorides contained in solid wastes must be first eliminated before dissolving PuO2 by chemical attack of Ag(II) in nitric acid solution [8]. Indeed, the presence of chloride ions leads to the precipitation of Ag(I) cations to form AgCl, thus making impossible the regeneration of Ag(II) by electrolysis. Although a chemical process of chloride removal from pyrochemical residues by sparging nitrogen dioxide in the solution was proposed by Pierce et al. [9], the implementation of an electrochemical process involving direct chloride oxidation can be considered as a simpler technique [8, 10]: 2 Cl- → Cl2(gas) + 2 e

(E° = 1.359 V/SHE)

(1)

Thus, in the process of PuO2 dissolution it would be convenient to use the same electrochemical reactor to first eliminate chloride as gaseous chlorine by electrolysis before adding silver nitrate for the dissolution of PuO2 by regenerating Ag(II). In a previous study, we showed that the Nb/BDD anode can efficiently generate silver(II) in 6 mol L-1 nitric acid solution: compared with platinised electrodes (Ti/Pt 2 and 5 m and Nb/Pt 5 m), the generation speed of Ag(II) was very similar and the conversion rate was of the same order of magnitude [11]. In order to use the same anode material to perform the sequential electrochemical steps, the study of anodic chlorine evolution on BDD in nitric acid at 6 mol L-1 was undertaken in a tubular reactor. This typical design is suitable for an electrochemical cell with separated cathodic and anodic compartments requiring a diaphragm with good mechanical and chemical resistance; it benefits indeed from a good experience feedback in nuclear industry [3, 10]. This attempt is based on the recognized stability and the high oxygen overpotential of diamond electrodes making them excellent candidates for chlorine evolution from dilute chloride solution [12]. In fact, Ferro et al. [12] have shown by comparison of current-potential curves for chlorine and oxygen evolution that under the same acidic condition (pH = 3.5) the

3

faradaic yield for chlorine evolution was very high on BDD anode. However, oxidation of chloride ions on diamond can form the undesirable chlorate and perchlorate anions [13-15]. Although these last observations were made in neutral or alkaline solutions, it was necessary to test if these unwanted products were capable of being formed under electrochemical dechlorination conditions in concentrated nitric acid. This paper presents kinetic studies conducted in a tubular reactor to perform (i) the electrochemical dechlorination of 6 mol L-1 nitric acid solutions containing chloride at concentration up to 0.1 mol L-1 and (ii) the silver(I) oxidation at initial concentration of 0.1 mol L-1. In addition, the service life of Nb/BDD anodes was estimated by accelerated life tests in nitric acid at 6 mol L-1. The main objective was to test the performance of the promising Nb/BDD anode and to compare it to that of Ti/Pt.

2. Materials and method 2.1. Electrochemical reactor Dechlorination tests were performed under galvanostatic condition in a proprietary electrolysis set-up designed according to sub-critical geometry requirement (Fig. 1); such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide. Electrolyses were achieved in a tubular reactor consisting of two coaxial compartments separated by a porous ceramic diaphragm [8]. The anodic compartment was a temperature-controlled cylinder made of glass. The cathode, in the central compartment was made of stainless steel (47 cm2). Two anodes of quasi-cylindrical shape formed by 8 rectangular plates made of expanded titanium or expanded niobium, covered by a layer of Pt (5 m) or BDD (1 m) respectively, were tested. Plates made of Ti/Pt were supplied by MAGNETO special anodes B.V. (NL) while the ones made of Nb/BDD were supplied by Condias GmbH (DE). Assuming that for an expanded metal both faces are equipotential, the active surface of these two electrodes was 521 cm2. 4

Every plate was welded on its width to a clamping collar made of the same metal, and was linked to the electrical connector across the lid (Teflon) of the reactor. At the bottom of the reactor, at the anolyte inlet, these plates were mechanically fixed to the base of a cone (Teflon). In the upper part of the reactor the clamping collar was also used to maintain the cylindrical ceramic separator. The cell was connected to a direct current supply (Delta Elektronika BV SM 52V-30A; NL). The cell (Fig. 1) was connected in its upper part by a glass pipe with a temperature-regulated glass tank containing 1.5 L of 6 mol L-1 HNO3 solution. The anolyte was recirculated at a flow rate up to 25 L min-1 to ensure a satisfactory renewal of the boundary layer over the full length of the anode. Chlorine evolved in the anodic compartment was trapped in three scrubbers placed in series and containing 5 mol L-1 NaOH and 0.05 mol L-1 Na2SO3. The atmosphere above the cathodic compartment was swept by a flow of air to trap nitrogen dioxide formed by the complex mechanism of reduction of the concentrated nitric acid (13.6 mol L-1) used as catholyte [16]. At this high concentration there is no hydrogen evolution during HNO3 reduction that makes safe the process.

2.2 Chemicals and analysis procedure Nitric acid 68 % (VWR, AnalaR Normapur®) was used to prepare electrolytic solutions, whereas sodium chloride and silver nitrate (Acros Organics, ACS reagent 99%) were used as model of chloride salt for the dechlorination tests and for the electrogeneration of Ag(II) respectively. Before each dechlorination experiment the anolyte (1.5 L of 6 mol L-1 HNO3 + 0.1 mol L-1 NaCl) was recirculated between the anodic compartment and the reservoir during one hour to reach a stationary temperature of 60 °C. Samples of the anolyte taken before starting and regularly during dechlorination experiments were immediately diluted with pure water

5

(1v/100v) in reason of the high concentration of nitric acid. Chloride, chlorate and perchlorate were analysed by ionic chromatography using a Dionex column (ICS3000, IonPac® AS19, 4 x 250 mm) and a conductometric detection. The flow rate of the pump was set to 1.0 mL min1

. The mobile phase composition was constant (80% of NaOH 5 mmol L-1 + 20 % of NaOH

100 mmol L-1) for 1 min and then the gradient was from 20 % to 90 % of NaOH 100 mmol L1

during 27 min to provide good separation of all the peaks in a single chromatogram. The

time to return to initial conditions was 4 min. The end-point of electrochemical dechlorination was detected by a potentiometric titration with a constant imposed current intensity between two indicator platinum electrodes (0.28 cm2 each). In the region of the end-point, when the concentration of chloride reaches a value lower than 10-3 mol L-1, the potential difference E between the electrodes varies considerably for an applied current intensity. This sharp change of E (750 mV) which marks exactly the end of the electrolysis arises when the oxidation limiting current of Cl- becomes lower than the applied intensity equal to 50 µA. Silver(II) forms a brown complex (Eq. 7) in 6 mol L-1 nitric acid which was continuously titrated by UV-visible spectroscopy thanks to an immersed probe (Hellman;  = 580 nm) placed in the glass tube connecting the exit of the anode compartment with the reservoir of anolyte. The probe was standardized by back potentiometry with cerium(III) nitrate hexahydrate and Mohr’s salt (Acros Organics, 99.5 and 99 % respectively) as described in [11]. The rate of generation of Ag(II) was measured by considering the variation of its concentration during the first five minutes of electrolysis. The rate obtained by extrapolation at t = 0 corresponds to the maximal rate of regeneration of Ag(II) for a set of given operating conditions.

2.3 Stability of electrodes 6

Ageing tests were performed in a test bench composed of six electrochemical cells of 100 mL capacity, without separator, thermo-regulated at 30°C, under agitation and with zirconium cathode (7 cm2). The samples were submitted to galvanostatic runs in 6 mol L-1 HNO3 at high current density (j = 50 kA m-2). The generated vapours were evacuated by pumping and trapped in a sodium hydroxide solution. The concentration of nitric acid was adjusted every day after concentration control. Anodes (1.8 cm2 each; 3 of the Ti/Pt-5m type from Magneto, and 3 of the Nb/BDD type from Condias) were subjected to tests up to their deactivation. It was considered that anodes were deactivated when the cell potential exceeded 10 V [17]. The variation of the cell voltage as a function of time for the 6 tested anodes are reported and discussed in this paper.

3. Experimental results and discussion 3.1 Mass transfer rate Under industrial conditions, concentration of chloride ions in nitric acid can reach 1 mol L-1 [9, 10] whereas for the dechlorination process the final concentration should be lower than 103

mol L-1 to avoid AgCl precipitation when AgNO3 is introduced. The rate of reaction (1) may

be controlled by charge transfer or mass transport depending on the applied current density, chloride concentration, and hydrodynamic conditions which may depend on the rate of gas evolution at the anode [11]. If a sufficient potential, or current intensity, is applied to operate reaction (1) by mass transport control, the reaction rate can then be identified to the limiting current Ilim corresponding to the maximum reaction rate: (2) where n is the number of electrons exchanged (n = 1), F the Faraday constant (96498 C mol1

), S the electrode surface (m²) and

the chloride concentration (mol m-3).

7

The mass transfer coefficient, k, was measured at 60°C for dechlorination in the tubular reactor under anolyte recirculation at a flow rate

= 20 L min-1 consistent with industrial

conditions. Measurements were carried out in 6 mol L-1 nitric acid with sodium chloride at initial concentration of 0.040 mol L-1 or 0.050 mol L-1, lower than that used typically in the industrial process (

= 0.1 mol L-1) in order to obtain current-potential curves presenting

a net diffusion plateau. Fig. 2 presents plots obtained point by point under galvanostatic conditions for electro-oxidation of chloride. For successive controlled current intensity values the electrode potential was registered after 3 seconds. Chloride oxidation appears distinctly before water discharge as a short plateau of 150 mV length. By operating under these conditions the values of the anode potential are very stable and the I-E curve is reproducible. The mass transfer coefficient k was then determined using the limiting current intensity measured at 1.45 V/SCE on Ti/Pt, and 2.5 V on Nb/BDD. Current intensities were corrected by the residual current [18] measured in nitric acid at 6 mol L-1. The mass transfer coefficient k, measured at 60 °C, was equal to 6.01 x 10-5 m s-1 at the Ti/Pt 5µm anode, whereas it was equal to 4.84 x 10-5 m s-1 at the Nb/BDD anode. The discrepancy between the two values of k results probably from different induced hydrodynamic conditions established by the different mesh size of the expanded metals. Values of k were used to calculate the

= f(t) curves

(Eq. 5-6). In the case of silver(I) anodic oxidation the mass transfer coefficient k was measured in the tubular reactor at 30 °C for different flow rates  (from 3.5 to 20 L min-1) for two anodes of the same geometric surface S = 521 cm2. Steady-state polarisation curves (not shown) were obtained under galvanostatic conditions at different flow rates for the tubular reactor. Silver(I) oxidation appears distinctly before water discharge; the oxidation wave of silver(I) presents the classic shape of the progressive change of the kinetic limitation from charge transfer to mass transfer with increasing potential [11]. The mass transfer coefficient was determined 8

using the limiting current measured at 2.04 and 2.36 V (vs. SHE) for Ti/Pt and Nb/BDD respectively. Linear regression of k values (m s-1) as function of  (L min-1) conducts to the following correlations: k x 106 (Ti/Pt) = 1.78  + 14.72

(3)

k x 106 (Nb/BDD) = 1.962  + 0.1596

(4)

For a flow rate of 20 L min-1 (mean linear flow rate in the anodic compartment v = 0.3 m s-1) the values of k are: 5 x 10-5 m s-1 and 4 x 10-5 m s-1 for the Ti/Pt and Nb/BDD anodes respectively.

3.2 Dechlorination Dechlorination experiments were conducted under galvanostatic conditions at applied current intensity equal or higher than the initial limiting current intensity ( values of the ratio of the anode surface on the anolyte volume (

) for two

). When  = 1 (at t = 0),

the process is mass transfer limited and the chloride concentration varies as [18]: ( )

(

)

(5)

where: (

)

(6)

Equations 5 and 6 show that the higher the value of the S/Vsol ratio is, the shorter is the duration of electrolysis. Fig. 3a shows that 6 mol L-1 nitric acid solutions, initially containing 0.1 mol L-1 of chloride, can be dechlorinated until 10-3 mol L-1 on the Ti/Pt anode in around 2500 s at a current intensity (I = 30 A) equal to the initial limiting current

(

) at t = 0. For the same

operating conditions: configuration of the tubular reactor (S/Vsol = 35 m-1) and galvanostatic 9

conditions (=1) Fig. 3b (curve I) shows that with a Nb/BDD anode chlorides can be removed in 3000 s. According to the value of the mass transfer coefficient of these two electrodes, electrolysis durations to reach the final chloride concentration of 10-3 mol L-1 were slightly higher for the Nb/BDD anode than for Ti/Pt. As expected, with a surface electrode reduced by half (S/Vsol = 17.4 m-1) and an increasing of the ratio  to 2.47 have no accelerating effect (Fig. 3b, curve II). The dotted lines presented in Fig. 3 calculated with Eq. (5) fit correctly with experimental results in accordance with a limitation by mass transfer (

).

All electrolyses were followed by ionic chromatography in order to detect the eventual formation of chlorate and perchlorate. As the detection limit of the technique was evaluated around 1 ppm for these two oxyanions because of the high concentration of nitric acid (0.06 M, after dilution of the samples by a factor 100) it can be assumed that the concentration of these undesirable anions were (if they are present) at concentration lower than 100 ppm (i.e. 1 mM). In summary, in this device characterized by a ratio S/Vsol = 35 m-1, electrochemical dechlorination can be performed satisfactorily in less than 50 minutes under standard and safe industrial conditions; therefore, the process carried out with the Nb/BDD could be adapted for industrial application.

3.3 Silver(I) oxidation In the anodic compartment, Ag(II) generated in aqueous nitric acid solution at 6 mol L-1 is stabilized by nitrate ion; thus in the absence of a reducing agent, silver(II) is present as a dark brown nitrate complex [6]: Ag+ + NO3- →

Ag(NO3)+ + e-

(7)

However, the increase of the Ag(II) concentration gives a rise to the rate of oxidation of the water according to reaction: 10

2 Ag(NO3)+ + H2O →

2 Ag+ + 2 HNO3 + ½ O2

(8)

Thus, the concentration of Ag(II) reaches a quasi-stationary value [11]. Referring to a batch recirculation reactor system, the model is based on the mass balance in which the actual rate of Ag(II) generation is expressed as the difference between the production rate Rgen by electrolysis and the destruction rate by reaction with water ( )

:

(9)

The rate of Ag(II) production (mol s-1 L-1) is given by: (10) where jAg is the part of the current density used to generate Ag(II); in a first approximation the volume of the electrochemical reactor is considered as small (around 60 mL) with regard to the total volume of the anolyte (Vsol = 1.5 L). The kinetic law of the chemical reaction (8) in nitric acid [11, 19, 20] involves Ag(II) and Ag(I) concentrations as follows: ( )

(11)

( )

Some values of

are compiled in [11].

depends on the temperature and has been

determined experimentally by following the temporal decrease of [Ag(II)] at different temperatures. In the range of 20-35 °C, one obtains: (12)

( )

For a constant current intensity I higher than the initial limiting current, the partial current jAgS is calculated by Eq. (2) applied to the anodic oxidation of Ag(I). In this case, the generation rate of Ag(II) is expressed by Eq. (13): ( )

( )

( ) ( )

(13)

Simulations have been performed by numerically solving Eq. (13) using a finite difference method. 11

Table 1 shows experimental values of the initial speed of generation of Ag(II) for three current intensities as well as the corresponding conversion of Ag(I) at the stationary state. The Nb/BDD electrode presents performance slightly lower than that of the Ti/Pt electrode for the two highest currents (15A and 30A). For a current intensity lower than 15 A, the generation rates of Ag(II) obtained with Nb/BDD are similar than those obtained with Ti/Pt. Results presented in Table 1 show that an increase in the current intensity for the Ti/Pt anode at a value upper than the limiting current increases frankly the rate of silver(II) generation, whereas this rate decreases for the same change in current intensity (15 to 30 A) in the case of the Nb/BDD anode. This discrepancy suggests that, in the case of the BDD electrode, competitive reactions may happen for

(see below Eqs. 14-17). Accordingly, using a

BDD anode needs a perfect control of the current to optimise Ag(II) generation. Fig. 4 (a and b) shows experimental results (points) for galvanostatic electrolyses performed in the tubular reactor. The experimental stationary Ag(II) concentration is slightly higher for Ti/Pt than for Nb/BDD (19 and 17 mol m-3 respectively); in accordance with numerical values of the mass transfer coefficient evaluated for the two materials (Eqs. (3) and (4)), the production rate Rgen in Eqs. 9 and 13 is higher for the Ti/Pt anode. Fig. 4 shows also the simulation of Ag(II) concentration variation (curves) during the transient and stationary periods of electrolysis. It is seen that the simple model described by Eq. (13) fits better for Ti/Pt (difference less than 2%) than for Nb/BDD (difference around 10%). In fact, if the electrolyses started under conditions for which  was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state  reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD a more complex mechanism may be expected, according to Eqs. (14)-(17) [21-23]: 

H2O  Ag+ +



OH + H+ + e

OH →

Ag2+ + OH-

(14) (15) 12



OH +



OH →

H2O2

2 Ag(NO3)+ + H2O2 →

(16) 2 Ag+ + O2 + 2 H+

+ 2 NO3-

(17)

Reaction (15) suggested by Barker and Fowles [22] and which corresponds to a fast reaction in water (k15 = 6.3 x 109 L mol-1 s-1) could be favored in concentrated nitric acid because of neutralization of OH- and complexation of Ag(II) by the high concentration of nitrate. On the opposite, Ag(II) is reduced by another pathway involving hydrogen peroxide easily formed close to the BDD anode (k16 = 5.5 x 109 L mol-1 s-1) [21]. Reaction (17) considered to be pseudo-first order is very fast (k17 = 300 s-1) and the steady-state concentration of H2O2 should be very low [23]. Let us note furthermore that some traces of silver oxide (AgO) formed on the electrode could activate oxygen evolution [24]. In summary, electrochemical generation of silver(II) proceeds on Nb/BDD anode at rate similar to that on the Ti/Pt anode; however, the process is complicated by side reactions implying the chemical reduction by water (Eq. 8) and by hydrogen peroxide generation in the case where BDD anode is submitted to a current higher than the mass transfer limiting current.

3.4 Electrode stability The service life of an electrode material is a mattering parameter to estimate its time of use and validate its industrialization. It has been shown that diamond electrodes are etched electrochemically under certain experimental conditions [25 and ref. therein]. The mechanisms that can cause diamond film corrosion or detachment are complex [17]. Chen et al. [26] tested boron-doped diamond films at 5 kA m-2 in a solution of 1.0 mol L-1 HNO3 + 2.0 mol L-1 NaCl; their experiments show that no severe microstructural or morphological damage after periods of time up to 20h. Thus, it was important to test a series of samples of Ti/Pt 5mm and Nb/BDD under the real conditions of use. For a first evaluation accelerated life tests were conducted. 13

Fig. 5 shows the variation of the cell potential during electrolysis under very high anodic current density (50 kA m-2) for six electrodes. The relative jump of potential indicates the total deactivation of an electrode. The lifetime of three Ti/Pt anodes were respectively of 880, 1280 and 1550 hours whereas those of the diamond anodes were 1620, 1720 and >1810 hours. A theoretical extrapolation based on a practical current density of 1500 A m-2 leads to a minimal life expectancy of more than 3 years for Ti/Pt (value in agreement with the industrial experience on the Ti/Pt) and approximately 6 years for Nb/BDD.

Conclusions This study performed with a tubular electrochemical cell has shown that the Nb/BDD anode represents an interesting alternative to the Ti/Pt anode for the dechlorination as well as for the regeneration of Ag(II) in nitric acid solution. It is possible to run the two steps sequentially in the same reactor using Nb/BDD anodes, as has been shown for Pt/Ti anodes. Results obtained with Ti/Pt 5µm and Nb/BDD anodes were satisfactory since dechlorination reached 99 %, value in accordance with the practical objective. Furthermore, the formation of oxyanions of chlorine was not detected during electrochemical dechlorination. The specific particularity of the Nb/BDD electrode lies in the service life practically double than that of Ti/Pt. Monitoring the rate of Ag(I) oxidation on the BDD anode imposes however the current density to be controlled to avoid the generation of hydroxyl radicals susceptible to slow down the rate of the Ag(II) electrochemical regeneration via hydrogen peroxide formation.

14

References

[1] Ryan JL, Bray LA, Wheelwright EJ, Bryan GH (1990) Catalyzed electrolytic plutonium oxide dissolution (CEPOD-The past seventeen years and future potential). Proceed. International Symposium to Comm. 50th Anni. of Transuranium Elements, Washington, DC, August 26-31 [2] Ryan JL, Bray LA, Boldt AL (1987) Dissolution of PuO2 or NpO2 using electrolytically regenerated reagents. US Pat. 4,686,019 [3] Koehly G, Bourges J, Madic C, Lecomte M (1988) Process for the recovery of plutonium contained in solid waste. US Patent 4,749,519 [4] Bourges J, Madic C, Koehly G, Lecomte M (1986) Dissolution du bioxyde de plutonium en milieu nitrique par l’argent(II) électrogénéré. J Less-Common Met 122:303-311 [5] Zundelevich Y (1992) The mediated electrochemical dissolution of plutonium oxide: kinetics and mechanism. J Alloys Comp 182:115-130 [6] Steele DF (1990) Electrochemical destruction of toxic organic industrial waste. Platinum Metals Rev 34:10-14 [7] Steele DF (1990) Regeneration of nitrous acid by oxidation with direct application of heated nitric acid. US Pat 4,925,643 [8] Brossard M-P, Belmont J-M, Baron P, Blanc P, J-C. Broudic J-C (2003) Dechlorination, dissolution, and purification of weapon grade plutonium oxide contaminated with chlorides: tests performed in the CEA Atalante facility for the aqueous polishing part of MOX fuel fabrication facility. Proceedings of Global 2003:485-491 [9] Pierce RA, Campbell-Kelly RP, Visser AE, Laurinat JE (2007) Removal of chloride from acidic solutions using NO2. Ind Eng Chem Res 46:2372- 2376

15

[10] Koehly G, Madic C, Saulze J-L (1989) Process for eliminating the chloride ions present in contaminated solid wastes, such as incineration ashes contaminated by actinides. US Patent 4,869,794 [11] Racaud C, Savall A, Rondet Ph, Bertrand N, Groenen Serrano K (2012) New electrodes for silver(II) electrogeneration: Comparison between Ti/Pt, Nb/Pt, and Nb/BDD. Chem Eng J 211-212:53-59 [12] Ferro CS, de Battisti A, Duo I, Comninellis Ch, Haenni W, Perret A (2000) Chlorine evolution at highly boron-doped diamond thin film electrodes. J Electrochem Soc 147:2614-2619 [13] Polcaro AM, Vacca A, Mascia M, Palmas S, Rodiguez Ruiz J (2009) Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides. J Appl Electrochem 39:2083-2092 [14] Bergmann MEH, Rollin J, Iourtchouk T (2003) The occurrence of perchlorate during drinking water electrolysis using BDD anodes. Electrochim Acta 54:2102-2107 [15] Sanchez-Carretero A, Saez C, Canizares P, Rodrigo MA (2011) Electrochemical production of perchlorates using conductive diamond electrolyses. Chem Eng J 166:710714 [16] Lange R, Maisonhaute E, Robin R, Vivier V (2013) On the kinetics of the nitrate reduction in concentrated nitric acid. Electrochem Comm 29:25-28 [17] Kraft A (2007) Doped diamond: a compact review on a new, versatile electrode material. Int J Electrochem Sci 2:355-385 [18] Walsh F (1993) A first course in electrochemical engineering (Ch. 6). The Electrochemical Consultancy (Romsey) Ltd, UK [19] Po HN, Swinehart JH, Allen TL (1968) The kinetics and mechanism of the oxidation of water by Ag(II) in concentrated nitric acid solution. Inorg Chem 7:244-249

16

[20] Lehmani A, Turq P, Simonin JP (1996) Oxidation of water and organic compounds by silver(II) using a potentiometric method. J Electrochem Soc 143:1860-1865 [21] Kapałka A, Fóti G, Comninellis Ch (2009) The importance of electrode material in environmental electrochemistry. Formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes. Electrochim Acta 54:2018–2023 [22] Barker GC, Fowles P (1970) Pulse radiolytic induced transient electrical conductance in liquid solutions. Part 3 Radiolysis of aqueous solutions of some inorganic systems. Trans Faraday Soc 66:16611669 [23] Rance PJW, Nikitina GP, Korolev VA, Kirshin MYu, Listopadov AA, and Egorova VP (2003) Features of electrolysis of nitric acid solutions of silver: I. Behavior of Ag(II) in HNO3 solutions. Radiochemistry 45:346-352 [24] Panizza M, Duo I, Michaud P-A, Cerisola G, Comninellis Ch (2000) Electrochemical generation of Silver(II) at boron-doped diamond electrodes. Electrochem Solid State Lett 3:550-551 [25] Chaplin BP, Hubler DK, Farrell J (2011) Understanding anodic wear at boron doped diamond film electrodes. Electrochim Acta 89:122–131 [26] Chen Q, Granger MC, Lister TE, Swain GM (1997) Morphological and microstructural stability of boron‐doped diamond thin film electrodes in an acidic chloride medium at high anodic current densities. J Electrochem Soc 144:3806-3812

17

Table Click here to download Table: JACH-D-14-01022_Table 1_Revised_YellowMarks.docx

Table 1: Experimental values of Ag(II) generation rate and conversion at stationary state as functions of current intensity for Nb/BBD and Ti/Pt anodes in the tubular reactor. [Ag(I)]0 = 0.05 mol L-1, [HNO3] = 6 mol L-1, T=30°C,

= 20 L min-1, S = 521 cm2. Limiting current intensity: 9.9 A

(Nb/BDD); 12.6 A (Ti/Pt).

Current /A

Nb/BDD Generation rate -2

mol m min -2

-1

Ti/Pt 5µm

Conversion

Generation rate

Conversion

%

-2

%

mol m min

-1

34.4

6.19 x 10

-2

36.83

7.3

5.99 x10

15

7.31 x 10-2

33.04

9.36 x 10-2

41.82

30

6.59 x 10-2

(a)

1.29 x 10-1

(a)

(a)

Due to excessive heat release the temperature of the electrolyte increased too much to measure accurately the conversion.

Figure Click here to download Figure: JACH-D-14-01022_FiguresRevised_YellowMarks.docx

Captions of figures

Fig. 1: Dechlorination set-up. (1) Power supply (2) Thermoregulated tubular reactor (3) Anode (Nb/BDD or Ti/Pt) (4) Cathode (SS) (5) Diaphragm (6) Tank (anolyte) (7) Tank (catholyte) (8) Chlorine absorber (9) NOx absorber (10) flowmeter. Fig. 2. Current-potential curves for Cl- oxidation on Ti/Pt and Nb/BDD anodes in 6 mol L-1 HNO3; S = 521 cm2. (1) [Cl-] = 0.050 mol L-1 ; (2) [Cl-] = 0.040 mol L-1 ; (3) ground current in [HNO3] = 6 mol L-1; T = 60°C; flow rate  = 20 L min-1.

Fig. 3. Calculated (dotted lines) and experimental (symbols) [Cl-] = f(t) curves on (a) Ti/Pt, and (b) Nb/BDD anodes. Vsol (anolyte) = 1.5 L; [NaCl]0 = 0.1 mol L-1 in 6 mol L-1 HNO3; T = 60 °C; flow rate = 20 L min-1; I = 30 A. Modelling using (a) k = 6.01 x 10-5 m s-1 and  = 1 for Ti/Pt (S = 521 cm2;

= 30 A), and (b) k = 4.84 x 10-5 m s-1 for Nb/BDD. Exp. 1-3

(curve I): anode surface S = 521 cm2;  = 1.23;

= 24.3 A. Exp. 4 (curve II): anode surface

S = 260.5 cm2 (anode made with 4 rectangular plates) ;  = 2.47;

= 12.2 A.

Fig. 4: Theoretical (line) and experimental (symbol) variation of Ag(II) concentration during electrolysis. Conditions: [Ag(I)]0 = 0.05 mol L-1; T = 30°C; [HNO3] = 6 mol L-1; I = 15 A, k = 4.24 x 10-5 m s-1, kH2O = 3.54 10-3 s-1. (a) Anode Ti/Pt: k = 5 x 10-5 m s-1 (b) Anode Nb/BDD: k = 4.24 x 10-5 m s-1.

Fig. 5: Accelerated life tests of (a) Ti/Pt anodes and (b) BDD anodes in 6 mol L-1 HNO3 solution for I = 50 kA m−2 at 30°C.

1

Figures

Fig. 1

2

25

(1)

(a) Ti/Pt

(2)

20

15

15

I /A

I /A

20

10

10

5

5

0 0.5

1

1.5 E vs. SCE/ V

2

(1)

25

(3)

2.5

(2) (3)

(b) Nb/BDD

0 1

1.5

2

2.5 E vs. SCE/ V

3

3.5

Fig. 2

3

100

(a) Ti/Pt theoretical Exp. 1 Exp. 2 Exp. 3

75

50 25 0

(b) Nb/BDD

Exp. 1 Exp. 2 Exp. 3 Exp. 4 theoretical

75

[Cl-] / mol m-3

[Cl-] mol m-3

100

II

50 25

I

0

0

1000

2000 Time / s

3000

4000

0

1000

2000 Time /s

3000

4000

Fig. 3

4

20

(a) Ti/Pt

[Ag(II)] / mol m-3

[Ag(II)] / mol m-3

20

15 10

5

theoretical

experimental

0

(b) Nb/BDD

15 10 5

theoretical

experimental

0 0

500

1000

Time /s

1500

2000

0

500

1000

1500

2000

Time /s

Fig. 4

5

40

(a) Ti/Pt (1)

Ti/Pt (2)

Ti/Pt (3)

30 20 10 0

Cell potential / V

Cell potential / V

40

(b) BDD (1)

BDD (2)

BDD (3)

30 20 10 0

0

500

1000 Time /h

1500

2000

0

500

1000 Time /h

1500

2000

Fig. 5

6

Attachment to manuscript Click here to download Attachment to manuscript: JACH-D-14--01022_Answers to the reviewers.docx

Answers to the reviewers

Reviewer #1: This paper contains results of good experimental work of high importance and is recommended for publication after some smaller changes (most related to language). 1. The language is a little strange here and there and another language check is recommended, see for example: a.

Abstract line 8: "Researches are developed" could be "Research is conducted"

b.

p.4 line 18: "Two of anode constituted with" could be "The two anodes were"

c. p.5 line 22: "The time to return on initial conditions was during 4 min." could be "The time to return to initial conditions was 4 min." d.

p.6 line 19 "submitted" could be "subjected"

All these corrections were introduced. But for item b the sentence was changed in reason of other comments: “Two anodes of quasi-cylindrical shape formed by 8 rectangular plates made of expanded titanium or expanded niobium, covered by a layer of Pt (5 m) or BDD ( 1 m) respectively, were tested.” 2.

p.3 reaction (1). Eo=1.396 V vs SHE seems too high, should be 1.37 V vs SHE

We have introduced the value for Cl2 released as a gas (from Pourbaix) : E° = 1.359 V vs SHE). 3. p. 5 What was the cathode reaction? Hydrogen evolution I guess, but why was then air needed to trap nitrogen oxides? At a concentration of 13.6 mol L-1 the cathodic reduction of HNO3 forms mainly NO2 by a complex mechanism (cf. Ref. 16 and references therein); there is no hydrogen evolution. We have changed the sentence to clarify this point: “The atmosphere above the cathodic compartment was swept by a flow of air to trap nitrogen dioxide formed by the complex mechanism of reduction of the concentrated nitric acid (13.6 mol L-1) used as catholyte [16]. At this high concentration there is no hydrogen evolution during HNO3 reduction that makes safe the process.” 4. p.6 line 11: why was Ag(II) generation only determined during the first five minutes of electrolysis? By extrapolation of the variation of the experimental Ag(II) concentration at t = 0 one can obtain the initial rate of Ag(I) oxidation, that is the maximum value of the regeneration rate of Ag(II). This remark was introduced in the text as follows: “The rate obtained by extrapolation at t = 0 corresponds to the maximal rate of regeneration of Ag(II) for a set of given operating conditions.” 5. p.10 last two lines and heading of Table 1: Not clear if Table 1 shows theoretical values from eq 13, or if it contains experimental values. Please clarify. Could you give the limiting current also in the heading of Table 1?

The heading of Table 1 was clarified; the limiting values of the current intensity were introduced and the text was also modified in introduction of Table 1 (page 10). 6. p.11 line 8: do I understand right that equation 17 may be the reason for this increase in Ag(II) formation at I>I lim? We have chosen to begin with experimental results presented in Table 1 and Fig. 4 and then to give an explanation based on the set of Equations 14-17. Thus, we have introduced a short remark in P. 11, Line 11, linked with this equation set: “…competitive reactions may happen for I > Ilim (see below Eqs. 14-17)”. 7.

p.11 line 13: "the first term of the second member" - what does this mean? Please rewrite.

This was replaced by “the production rate Rgen in Eqs. 9 and 13 is…” 8. Conclusions: line 3-4: "Therefore, sequence of both steps in the same reactor is possible because conductive diamond shows a kinetic performance comparable to that of platinized titanium" sounds odd. Something like: "It is possible to run the two steps sequentially in the same reactor using Nb/BDD anodes, as has been shown for Pt/Ti anodes." may be better. This suggestion was taken into account. 9.

Can dissolved Pu-complexes react on the anode?

The highest degree of oxidation known for the plutonium is 6; the cation (PuO2)2+ formed by oxidation of PuO2 is not oxidizible at the anode. We did not modify the text on this point because the properties of the plutonium are not the heart of this manuscript.

Reviewer #2 The paper shows an interesting study of the elimination of chloride ions and the generation of silver(II) for applications in the nuclear industry. Further applications of electrochemical engineering and calculations under different parameters are important for industrial applications. The paper should be accepted with minor corrections after the authors address the comments below. Page 2 Line 15; please consider replacing: "In the aim of using the same anode material to…" for "In order to use the same anode material to…" This improvement was introduced. Page 4 Materials and methods Line 2; what are subcritical geometry requirements? To clarify this expression we added the following simple definition: “such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide”. Figure 1 should show the water temperature controlled equipment or connections. Fig. 1 was modified in this sense. Figure 1 should indicate how the ceramic membrane was supported and detailed description of the ceramic membrane. A short explanation was introduced in the text (in page 4): “Every plate was welded on its width to a clamping collar made of the same metal, and was linked to the electrical connector across the lid (Teflon) of the reactor. At the bottom of the reactor, at the anolyte inlet, these plates were mechanically fixed to the base of a cone (Teflon). In the upper part of the reactor the clamping collar was also used to maintain the cylindrical ceramic separator.” The dimensions of the rectangular plates of expanded titanium or niobium covered with Pt should be stated. The design of Fig. 1 was modified to introduce further equipment. In contrast some requested information on the equipment (details on ceramic, supports, plate sizes), considered as sensitive, cannot be entirely described. How was the electrolyte flow rate measured? In Fig. 1 we have introduced the scheme of the classic flowmeter used to measure the electrolyte flow. Page 6 Line 6; is the expression: "Silver(II) was continuously titrated by UV-visible spectroscopy thanks to…" correct? If yes how can Ag(II) be titrated by UV-Vis? Please revise. To clarify this point we added at this place the following explanation:

“Silver(II) forms a brown complex in 6 mol L-1 nitric acid (Eq. 7) which was continuously titrated by UVvisible spectroscopy thanks to an immersed probe (Hellman;  = 580 nm)”. Page 7 Line 1; at the end of the line the word should be "mol" not "mole" Corrected Line 4 after equation (2); can the authors calculate the mean linear flow rate over the electrode surface? If it is consistent with industrial conditions there should be a reference. The mean linear flow rate value v (m s-1) over the electrode was introduced after Eq. 4. Line 7 after the equation (2); it could be: "in order to…" rather than "in view to…" Corrected Line 13 after equation (2); the residual method should have a reference In the book of F. Walsh this experimental technique was explained; so we have added this reference [18]. How the authors justify the calculation of the mass transfer coefficient with only two points? How do the mass transfer coefficients compare with the literature? The potential values measured under galvanostatic condition were very stable and these two points were considered as significant to evaluate (for example) the operating time for a complete electrolysis in the real industrial equipment operating under the same hydrodynamic conditions. Our aim was to have at one’s disposal (specially for Areva Group) a good estimation of the mass transfer coefficient. Thus, we added the following comment in P. 7 after line 13: “By operating under these conditions the values of the anode potential are very stable and the I-E curve is reproducible.” The authors should report the value kS to compare with other values in the literature that report kA where A is the electrode area. The values of the mass transfer coefficient are strictly valuable for the two expanded metals used. The real surface of the material is difficult to estimate. Thus the mass transfer coefficients k are given to be used for these materials. Equations 3 and 4 could be expressed as logarithmic correlation and compare with other electrochemical reactors that report k =a vb where "b" is an exponent, "a" is a constant and "v" is the mean linear flow rate. To be able to find a logarithmic correlation between k and v we have introduced just after Eqs. 3 and 4 the value of the mean linear flow rate v for a given value of the anolyte flow rate . Thus it will be easy to find the values of the coefficients contained in the relation linking k and v Did the authors observed limiting current plateau for the oxidation of silver(I)? We have observed the same current plateau that was obtained in the case of the oxidation of Ag(I) in a filter-presse reactor (ref. 11). The following explanation was added:

“Steady-state polarisation curves (not shown) were obtained under galvanostatic conditions at different flow rates for the tubular reactor. Silver(I) oxidation appears distinctly before water discharge; the oxidation wave of silver(I) presents the classic shape of the progressive change of the kinetic limitation from charge transfer to mass transfer with increasing potential. The mass transfer coefficient was determined using the limiting current measured at 2.04 and 2.36 V (vs. SHE) for Ti/Pt and Nb/BDD respectively.” Figure 3 and equation 5; is the dechlorination process first order kinetics? At the concentration of chloride anion (< 0.1 mol L-1) the process is mass transfer limited; consequently, the dechlorination process is expressed by a rate law of the first order (Eq. 2). Page 9 Line 7; it would be better to state"…chromatography in order to detect…" rather than "…chromatography in the objective to detect…" This suggestion was taken into consideration Please provide an interpretation of the value given by the term S/Vsol. As shown by Eqs. 5 and 6 the S/V ratio is important to calculate the electrolysis time to reach a given conversion for a process limited by mass transfer.To understand well its importance we have introduced the following comment after Eq. 6: “Equations 5 and 6 show that the higher the value of the S/Vsol ratio is, the shorter is the duration of electrolysis.” As the BDD electrode is used to generate OH radicals, would be the main reaction? Why the generation of Ag(II) is not faster in BDD if these electrodes are very efficient to generate the radicals. The efficiency of the BDD electrode has a tendency to decrease when the current density increases as shown in Table 1. Eqs. 16 and 17 were introduced to give an explanation; the rate of H2O2 formation from OH radical is very fast and H2O2 reacts with Ag(II). The production flux of OH radical (current density) should be adjusted to the flux of silver(I) towards the BDD anode. Our explanation follows Eq. 17. What is the effect of the radicals on chlorine ions? As regards to the effect of the OH radical on chlorine ions we do not know for the moment the details of the mechanism (transfer of electron or transfer of oxygen atom), but it is the subject of reflection that we wish to develop. Page 12 Line third before the end of the page; state the value of the high anodic current density. We have introduced the value of the current density: 50 kA m-2. Page 13 Line 4; are there references for the duration of the BDD electrodes? Since the submission of the manuscript we have found a paper by Chen et al. (1997); this reference was introduced in the revised paper (ref [26]). Thus we added a short introduction of this paper in P. 13 (Section 3.4):

“Chen et al. [26] tested boron-doped diamond films at 5 kA m-2 in a solution of 1.0 mol L-1 HNO3 + 2.0 mol L-1 NaCl; their experiments show that no severe microstructural or morphological damage after periods of time up to 20h. Thus,”… We also added a recent paper of Chaplin et al. [25] presenting some considerations on the BDD stability. Table 1; would the use of current density be more meaningful? We think that current density can be easily calculated with data given in the heading of the Table 1 Why conversion at 30 A cannot be calculated? Due to the important release of heat under this current intensity the temperature of the electrolyte raised too much (> 5°C) in spite of the use of a thermoregulation system of the reservoir. Consequently, we didn’t take into consideration the value of the stationary concentration of Ag(II) for these conditions. We have added a short remark in Table 1: (a) “Due to excessive heat release the temperature of the electrolyte increased too much to measure accurately the conversion.”

Reviewer #3 The proposed paper is a detailed study, with a typical chemical engineer's approach and perspective, on the performance and lifetime of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for (a) the electrochemical elimination of chlorides from concentrated HNO3 solutions used in transuranic waste treatments, and (b) for the subsequent electrochemical generation from Ag(I) salts (implying prior abatement of chlorides)of the Ag(II) oxidizing agent to be used for dissolution of PuO2. The study appears well contextualized, clearly described, with sound and accurate experimental protocol and detailed modelization. The reported results are interesting (chloride abatement without detected generation of oxyanions, good electrode lifetimes). The applicative issue (treatment of transuranic waste) is surely important. Therefore I think that the paper complies with the Journal's scope and can be accepted for publication.

Reviewer #4 This paper investigates and compares the performances of two anodes for the subsequent removal of chlorides and the generation of Ag(II) in view of the dissolution of PuO2 in HNO3 for the recovery of Pu. I think the manuscript is suitable for the Journal of Applied Electrochemistry after the following minor issue will be addressed: 1-In the introduction, the author should write the equation of the expected reaction of PuO2 oxidation. This equation was introduced (Without number because it is not any more used after). 2-What does "sub-critical geometry "(page 4) mean? To clarify this expression we added the following simple definition “such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide”. 3-The properties of the porous ceramic diaphragm (e.g. material, porosity) should be specified. The nature of this kind of ceramic was given in a previous paper (ref. 8). The characteristics of this ceramic are considered as a property of the Areva Group. 4- The sentence (page 11)" In fact, if the electrolyses started under conditions for which (alpha) was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state (alpha) reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD competitive reactions between hydroxyl radicals and silver species can be expected according to Eqs. (14)-(17)" is not clear. Please arrange it better. The sentence was slightly shortened in reason, indeed, of the interpretation which follows equations 14-17: “In fact, if the electrolyses started under conditions for which  was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state  reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD a more complex mechanism may be expected according to Eqs. (14)-(17) [21-23].”

END

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Performance of Ti/Pt and Nb/BDD anodes for dechlorination of nitric acid and regeneration of silver (II) in a tubular reactor for the treatment of solid wastes in nuclear industry K. Groenen Serranoa • A. Savall*a • L. Latapiea Ch. Racaudb • Ph. Rondetb • N. Bertrandc a

Université de Toulouse, CNRS, Laboratoire de Génie Chimique, F-31062 Toulouse, France AREVA NP, 25 avenue de Tourville F-50120 Equeurdreville, France c AREVA NC, 1 place Jean Millier F-92400 Courbevoie, France Corresponding author: savall@ chimie.ups-tlse.fr b

Abstract One of the problems frequently encountered in the processing of nuclear fuels is the recovery of plutonium contained in various solid wastes. The difficulty is to make soluble the plutonium present as the refractory oxide PuO2. The dissolution of this oxide in nitric acid solutions is easily performed by means of silver(II) a strong oxidizing agent which is usually electrochemically generated on a platinum anode. However, certain solid residues that must be treated to separate actinides contain important quantities of chloride ions that require after dissolution in nitric acid a preliminary electrochemical step to be removed before introducing Ag(I) for Ag(II) electrogeneration. Research is conducted to find electrocatalytic materials being able to replace massive platinum in view to limit capital costs. In the present work a setup including a two-compartment tubular reactor with recirculation of electrolytes was tested with anodes made of boron doped diamond coated niobium (Nb/BDD) and platinum coated titanium (Ti/Pt) grids for the removal of chlorides (up to 0.1 M) and for silver(II) regeneration. The study showed that these two anodes are effective for the removal of chlorides contained in 6M HNO3 solution as gaseous chlorine, without producing the 1

unwanted oxyanions of chlorine. Furthermore, the regeneration rate of silver(II) on Nb/BDD anode is approximately equal to that obtained on Ti/Pt anode for the same hydrodynamic conditions in the tubular reactor. Accordingly, dechlorination as well as silver(II) regeneration can be performed in the same reactor equipped either with a Nb/BDD or a Ti/Pt anode. Besides, the service life of Nb/BDD anodes estimated by accelerated life tests conducted in 6M HNO3 can be considered as very satisfactory compared to that observed with Ti/Pt anodes.

Keywords Silver (II) • Boron doped diamond • Electrolysis • Tubular reactor • Nuclear wastes •

Electrode service life

1. Introduction Silver(II) is a strong oxidising agent (E° = 1.98 V/SHE) capable of attacking many organic and inorganic substances. Electrochemical processes involving Ag(II) regeneration in nitric acid solution containing silver nitrate have been developed to treat transuranic wastes [1-5] and for the safe low-temperature destruction of a wide variety of contaminated organic waste materials [6,7]. In particular, silver(II) generated by anodic oxidation is a method of choice for the dissolution of plutonium oxide PuO2 [1, 3, 8]: PuO2 (solid) + 2 Ag2+ → (PuO2)2+(solution) + 2 Ag+ Dissolution of PuO2 is of considerable interest as this process is used in (i) aqueous reprocessing of uranium-plutonium fuel, (ii) fuel fabrication by reconversion of weaponsgrade Pu, and (iii) regeneration of Pu from several types of residues produced during conversion processes [3, 8]. But during some of these stages, pyrochemical processes used to recover and purify Pu metal generate ashes rich in chloride salts. Consequently, in the

2

silver(II) process operated by the nuclear industry, chlorides contained in solid wastes must be first eliminated before dissolving PuO2 by chemical attack of Ag(II) in nitric acid solution [8]. Indeed, the presence of chloride ions leads to the precipitation of Ag(I) cations to form AgCl, thus making impossible the regeneration of Ag(II) by electrolysis. Although a chemical process of chloride removal from pyrochemical residues by sparging nitrogen dioxide in the solution was proposed by Pierce et al. [9], the implementation of an electrochemical process involving direct chloride oxidation can be considered as a simpler technique [8, 10]: 2 Cl- → Cl2(gas) + 2 e

(E° = 1.359 V/SHE)

(1)

Thus, in the process of PuO2 dissolution it would be convenient to use the same electrochemical reactor to first eliminate chloride as gaseous chlorine by electrolysis before adding silver nitrate for the dissolution of PuO2 by regenerating Ag(II). In a previous study, we showed that the Nb/BDD anode can efficiently generate silver(II) in 6 mol L-1 nitric acid solution: compared with platinised electrodes (Ti/Pt 2 and 5 m and Nb/Pt 5 m), the generation speed of Ag(II) was very similar and the conversion rate was of the same order of magnitude [11]. In order to use the same anode material to perform the sequential electrochemical steps, the study of anodic chlorine evolution on BDD in nitric acid at 6 mol L-1 was undertaken in a tubular reactor. This typical design is suitable for an electrochemical cell with separated cathodic and anodic compartments requiring a diaphragm with good mechanical and chemical resistance; it benefits indeed from a good experience feedback in nuclear industry [3, 10]. This attempt is based on the recognized stability and the high oxygen overpotential of diamond electrodes making them excellent candidates for chlorine evolution from dilute chloride solution [12]. In fact, Ferro et al. [12] have shown by comparison of current-potential curves for chlorine and oxygen evolution that under the same acidic condition (pH = 3.5) the

3

faradaic yield for chlorine evolution was very high on BDD anode. However, oxidation of chloride ions on diamond can form the undesirable chlorate and perchlorate anions [13-15]. Although these last observations were made in neutral or alkaline solutions, it was necessary to test if these unwanted products were capable of being formed under electrochemical dechlorination conditions in concentrated nitric acid. This paper presents kinetic studies conducted in a tubular reactor to perform (i) the electrochemical dechlorination of 6 mol L-1 nitric acid solutions containing chloride at concentration up to 0.1 mol L-1 and (ii) the silver(I) oxidation at initial concentration of 0.1 mol L-1. In addition, the service life of Nb/BDD anodes was estimated by accelerated life tests in nitric acid at 6 mol L-1. The main objective was to test the performance of the promising Nb/BDD anode and to compare it to that of Ti/Pt.

2. Materials and method 2.1. Electrochemical reactor Dechlorination tests were performed under galvanostatic condition in a proprietary electrolysis set-up designed according to sub-critical geometry requirement (Fig. 1); such a geometry does not have the ability to sustain a fission chain reaction what gives security for the operators during the treatment of plutonium dioxide. Electrolyses were achieved in a tubular reactor consisting of two coaxial compartments separated by a porous ceramic diaphragm [8]. The anodic compartment was a temperature-controlled cylinder made of glass. The cathode, in the central compartment was made of stainless steel (47 cm2). Two anodes of quasi-cylindrical shape formed by 8 rectangular plates made of expanded titanium or expanded niobium, covered by a layer of Pt (5 m) or BDD (1 m) respectively, were tested. Plates made of Ti/Pt were supplied by MAGNETO special anodes B.V. (NL) while the ones made of Nb/BDD were supplied by Condias GmbH (DE). Assuming that for an expanded metal both faces are equipotential, the active surface of these two electrodes was 521 cm2. 4

Every plate was welded on its width to a clamping collar made of the same metal, and was linked to the electrical connector across the lid (Teflon) of the reactor. At the bottom of the reactor, at the anolyte inlet, these plates were mechanically fixed to the base of a cone (Teflon). In the upper part of the reactor the clamping collar was also used to maintain the cylindrical ceramic separator. The cell was connected to a direct current supply (Delta Elektronika BV SM 52V-30A; NL). The cell (Fig. 1) was connected in its upper part by a glass pipe with a temperature-regulated glass tank containing 1.5 L of 6 mol L-1 HNO3 solution. The anolyte was recirculated at a flow rate up to 25 L min-1 to ensure a satisfactory renewal of the boundary layer over the full length of the anode. Chlorine evolved in the anodic compartment was trapped in three scrubbers placed in series and containing 5 mol L-1 NaOH and 0.05 mol L-1 Na2SO3. The atmosphere above the cathodic compartment was swept by a flow of air to trap nitrogen dioxide formed by the complex mechanism of reduction of the concentrated nitric acid (13.6 mol L-1) used as catholyte [16]. At this high concentration there is no hydrogen evolution during HNO3 reduction that makes safe the process.

2.2 Chemicals and analysis procedure Nitric acid 68 % (VWR, AnalaR Normapur®) was used to prepare electrolytic solutions, whereas sodium chloride and silver nitrate (Acros Organics, ACS reagent 99%) were used as model of chloride salt for the dechlorination tests and for the electrogeneration of Ag(II) respectively. Before each dechlorination experiment the anolyte (1.5 L of 6 mol L-1 HNO3 + 0.1 mol L-1 NaCl) was recirculated between the anodic compartment and the reservoir during one hour to reach a stationary temperature of 60 °C. Samples of the anolyte taken before starting and regularly during dechlorination experiments were immediately diluted with pure water

5

(1v/100v) in reason of the high concentration of nitric acid. Chloride, chlorate and perchlorate were analysed by ionic chromatography using a Dionex column (ICS3000, IonPac® AS19, 4 x 250 mm) and a conductometric detection. The flow rate of the pump was set to 1.0 mL min1

. The mobile phase composition was constant (80% of NaOH 5 mmol L-1 + 20 % of NaOH

100 mmol L-1) for 1 min and then the gradient was from 20 % to 90 % of NaOH 100 mmol L1

during 27 min to provide good separation of all the peaks in a single chromatogram. The

time to return to initial conditions was 4 min. The end-point of electrochemical dechlorination was detected by a potentiometric titration with a constant imposed current intensity between two indicator platinum electrodes (0.28 cm2 each). In the region of the end-point, when the concentration of chloride reaches a value lower than 10-3 mol L-1, the potential difference E between the electrodes varies considerably for an applied current intensity. This sharp change of E (750 mV) which marks exactly the end of the electrolysis arises when the oxidation limiting current of Cl- becomes lower than the applied intensity equal to 50 µA. Silver(II) forms a brown complex (Eq. 7) in 6 mol L-1 nitric acid which was continuously titrated by UV-visible spectroscopy thanks to an immersed probe (Hellman;  = 580 nm) placed in the glass tube connecting the exit of the anode compartment with the reservoir of anolyte. The probe was standardized by back potentiometry with cerium(III) nitrate hexahydrate and Mohr’s salt (Acros Organics, 99.5 and 99 % respectively) as described in [11]. The rate of generation of Ag(II) was measured by considering the variation of its concentration during the first five minutes of electrolysis. The rate obtained by extrapolation at t = 0 corresponds to the maximal rate of regeneration of Ag(II) for a set of given operating conditions.

2.3 Stability of electrodes 6

Ageing tests were performed in a test bench composed of six electrochemical cells of 100 mL capacity, without separator, thermo-regulated at 30°C, under agitation and with zirconium cathode (7 cm2). The samples were submitted to galvanostatic runs in 6 mol L-1 HNO3 at high current density (j = 50 kA m-2). The generated vapours were evacuated by pumping and trapped in a sodium hydroxide solution. The concentration of nitric acid was adjusted every day after concentration control. Anodes (1.8 cm2 each; 3 of the Ti/Pt-5m type from Magneto, and 3 of the Nb/BDD type from Condias) were subjected to tests up to their deactivation. It was considered that anodes were deactivated when the cell potential exceeded 10 V [17]. The variation of the cell voltage as a function of time for the 6 tested anodes are reported and discussed in this paper.

3. Experimental results and discussion 3.1 Mass transfer rate Under industrial conditions, concentration of chloride ions in nitric acid can reach 1 mol L-1 [9, 10] whereas for the dechlorination process the final concentration should be lower than 103

mol L-1 to avoid AgCl precipitation when AgNO3 is introduced. The rate of reaction (1) may

be controlled by charge transfer or mass transport depending on the applied current density, chloride concentration, and hydrodynamic conditions which may depend on the rate of gas evolution at the anode [11]. If a sufficient potential, or current intensity, is applied to operate reaction (1) by mass transport control, the reaction rate can then be identified to the limiting current Ilim corresponding to the maximum reaction rate: (2) where n is the number of electrons exchanged (n = 1), F the Faraday constant (96498 C mol1

), S the electrode surface (m²) and

the chloride concentration (mol m-3).

7

The mass transfer coefficient, k, was measured at 60°C for dechlorination in the tubular reactor under anolyte recirculation at a flow rate

= 20 L min-1 consistent with industrial

conditions. Measurements were carried out in 6 mol L-1 nitric acid with sodium chloride at initial concentration of 0.040 mol L-1 or 0.050 mol L-1, lower than that used typically in the industrial process (

= 0.1 mol L-1) in order to obtain current-potential curves presenting

a net diffusion plateau. Fig. 2 presents plots obtained point by point under galvanostatic conditions for electro-oxidation of chloride. For successive controlled current intensity values the electrode potential was registered after 3 seconds. Chloride oxidation appears distinctly before water discharge as a short plateau of 150 mV length. By operating under these conditions the values of the anode potential are very stable and the I-E curve is reproducible. The mass transfer coefficient k was then determined using the limiting current intensity measured at 1.45 V/SCE on Ti/Pt, and 2.5 V on Nb/BDD. Current intensities were corrected by the residual current [18] measured in nitric acid at 6 mol L-1. The mass transfer coefficient k, measured at 60 °C, was equal to 6.01 x 10-5 m s-1 at the Ti/Pt 5µm anode, whereas it was equal to 4.84 x 10-5 m s-1 at the Nb/BDD anode. The discrepancy between the two values of k results probably from different induced hydrodynamic conditions established by the different mesh size of the expanded metals. Values of k were used to calculate the

= f(t) curves

(Eq. 5-6). In the case of silver(I) anodic oxidation the mass transfer coefficient k was measured in the tubular reactor at 30 °C for different flow rates  (from 3.5 to 20 L min-1) for two anodes of the same geometric surface S = 521 cm2. Steady-state polarisation curves (not shown) were obtained under galvanostatic conditions at different flow rates for the tubular reactor. Silver(I) oxidation appears distinctly before water discharge; the oxidation wave of silver(I) presents the classic shape of the progressive change of the kinetic limitation from charge transfer to mass transfer with increasing potential [11]. The mass transfer coefficient was determined 8

using the limiting current measured at 2.04 and 2.36 V (vs. SHE) for Ti/Pt and Nb/BDD respectively. Linear regression of k values (m s-1) as function of  (L min-1) conducts to the following correlations: k x 106 (Ti/Pt) = 1.78  + 14.72

(3)

k x 106 (Nb/BDD) = 1.962  + 0.1596

(4)

For a flow rate of 20 L min-1 (mean linear flow rate in the anodic compartment v = 0.3 m s-1) the values of k are: 5 x 10-5 m s-1 and 4 x 10-5 m s-1 for the Ti/Pt and Nb/BDD anodes respectively.

3.2 Dechlorination Dechlorination experiments were conducted under galvanostatic conditions at applied current intensity equal or higher than the initial limiting current intensity ( values of the ratio of the anode surface on the anolyte volume (

) for two

). When  = 1 (at t = 0),

the process is mass transfer limited and the chloride concentration varies as [18]: ( )

(

)

(5)

where: (

)

(6)

Equations 5 and 6 show that the higher the value of the S/Vsol ratio is, the shorter is the duration of electrolysis. Fig. 3a shows that 6 mol L-1 nitric acid solutions, initially containing 0.1 mol L-1 of chloride, can be dechlorinated until 10-3 mol L-1 on the Ti/Pt anode in around 2500 s at a current intensity (I = 30 A) equal to the initial limiting current

(

) at t = 0. For the same

operating conditions: configuration of the tubular reactor (S/Vsol = 35 m-1) and galvanostatic 9

conditions (=1) Fig. 3b (curve I) shows that with a Nb/BDD anode chlorides can be removed in 3000 s. According to the value of the mass transfer coefficient of these two electrodes, electrolysis durations to reach the final chloride concentration of 10-3 mol L-1 were slightly higher for the Nb/BDD anode than for Ti/Pt. As expected, with a surface electrode reduced by half (S/Vsol = 17.4 m-1) and an increasing of the ratio  to 2.47 have no accelerating effect (Fig. 3b, curve II). The dotted lines presented in Fig. 3 calculated with Eq. (5) fit correctly with experimental results in accordance with a limitation by mass transfer (

).

All electrolyses were followed by ionic chromatography in order to detect the eventual formation of chlorate and perchlorate. As the detection limit of the technique was evaluated around 1 ppm for these two oxyanions because of the high concentration of nitric acid (0.06 M, after dilution of the samples by a factor 100) it can be assumed that the concentration of these undesirable anions were (if they are present) at concentration lower than 100 ppm (i.e. 1 mM). In summary, in this device characterized by a ratio S/Vsol = 35 m-1, electrochemical dechlorination can be performed satisfactorily in less than 50 minutes under standard and safe industrial conditions; therefore, the process carried out with the Nb/BDD could be adapted for industrial application.

3.3 Silver(I) oxidation In the anodic compartment, Ag(II) generated in aqueous nitric acid solution at 6 mol L-1 is stabilized by nitrate ion; thus in the absence of a reducing agent, silver(II) is present as a dark brown nitrate complex [6]: Ag+ + NO3- →

Ag(NO3)+ + e-

(7)

However, the increase of the Ag(II) concentration gives a rise to the rate of oxidation of the water according to reaction: 10

2 Ag(NO3)+ + H2O →

2 Ag+ + 2 HNO3 + ½ O2

(8)

Thus, the concentration of Ag(II) reaches a quasi-stationary value [11]. Referring to a batch recirculation reactor system, the model is based on the mass balance in which the actual rate of Ag(II) generation is expressed as the difference between the production rate Rgen by electrolysis and the destruction rate by reaction with water ( )

:

(9)

The rate of Ag(II) production (mol s-1 L-1) is given by: (10) where jAg is the part of the current density used to generate Ag(II); in a first approximation the volume of the electrochemical reactor is considered as small (around 60 mL) with regard to the total volume of the anolyte (Vsol = 1.5 L). The kinetic law of the chemical reaction (8) in nitric acid [11, 19, 20] involves Ag(II) and Ag(I) concentrations as follows: ( )

(11)

( )

Some values of

are compiled in [11].

depends on the temperature and has been

determined experimentally by following the temporal decrease of [Ag(II)] at different temperatures. In the range of 20-35 °C, one obtains: (12)

( )

For a constant current intensity I higher than the initial limiting current, the partial current jAgS is calculated by Eq. (2) applied to the anodic oxidation of Ag(I). In this case, the generation rate of Ag(II) is expressed by Eq. (13): ( )

( )

( ) ( )

(13)

Simulations have been performed by numerically solving Eq. (13) using a finite difference method. 11

Table 1 shows experimental values of the initial speed of generation of Ag(II) for three current intensities as well as the corresponding conversion of Ag(I) at the stationary state. The Nb/BDD electrode presents performance slightly lower than that of the Ti/Pt electrode for the two highest currents (15A and 30A). For a current intensity lower than 15 A, the generation rates of Ag(II) obtained with Nb/BDD are similar than those obtained with Ti/Pt. Results presented in Table 1 show that an increase in the current intensity for the Ti/Pt anode at a value upper than the limiting current increases frankly the rate of silver(II) generation, whereas this rate decreases for the same change in current intensity (15 to 30 A) in the case of the Nb/BDD anode. This discrepancy suggests that, in the case of the BDD electrode, competitive reactions may happen for

(see below Eqs. 14-17). Accordingly, using a

BDD anode needs a perfect control of the current to optimise Ag(II) generation. Fig. 4 (a and b) shows experimental results (points) for galvanostatic electrolyses performed in the tubular reactor. The experimental stationary Ag(II) concentration is slightly higher for Ti/Pt than for Nb/BDD (19 and 17 mol m-3 respectively); in accordance with numerical values of the mass transfer coefficient evaluated for the two materials (Eqs. (3) and (4)), the production rate Rgen in Eqs. 9 and 13 is higher for the Ti/Pt anode. Fig. 4 shows also the simulation of Ag(II) concentration variation (curves) during the transient and stationary periods of electrolysis. It is seen that the simple model described by Eq. (13) fits better for Ti/Pt (difference less than 2%) than for Nb/BDD (difference around 10%). In fact, if the electrolyses started under conditions for which  was 1.2 and 1.5 for Ti/Pt and Nb/BDD respectively, at the stationary state  reached values around 1.9 and 2.3 respectively, what implies oxygen evolution on Pt whereas on BDD a more complex mechanism may be expected, according to Eqs. (14)-(17) [21-23]: 

H2O  Ag+ +



OH + H+ + e

OH →

Ag2+ + OH-

(14) (15) 12



OH +



OH →

H2O2

2 Ag(NO3)+ + H2O2 →

(16) 2 Ag+ + O2 + 2 H+

+ 2 NO3-

(17)

Reaction (15) suggested by Barker and Fowles [22] and which corresponds to a fast reaction in water (k15 = 6.3 x 109 L mol-1 s-1) could be favored in concentrated nitric acid because of neutralization of OH- and complexation of Ag(II) by the high concentration of nitrate. On the opposite, Ag(II) is reduced by another pathway involving hydrogen peroxide easily formed close to the BDD anode (k16 = 5.5 x 109 L mol-1 s-1) [21]. Reaction (17) considered to be pseudo-first order is very fast (k17 = 300 s-1) and the steady-state concentration of H2O2 should be very low [23]. Let us note furthermore that some traces of silver oxide (AgO) formed on the electrode could activate oxygen evolution [24]. In summary, electrochemical generation of silver(II) proceeds on Nb/BDD anode at rate similar to that on the Ti/Pt anode; however, the process is complicated by side reactions implying the chemical reduction by water (Eq. 8) and by hydrogen peroxide generation in the case where BDD anode is submitted to a current higher than the mass transfer limiting current.

3.4 Electrode stability The service life of an electrode material is a mattering parameter to estimate its time of use and validate its industrialization. It has been shown that diamond electrodes are etched electrochemically under certain experimental conditions [25 and ref. therein]. The mechanisms that can cause diamond film corrosion or detachment are complex [17]. Chen et al. [26] tested boron-doped diamond films at 5 kA m-2 in a solution of 1.0 mol L-1 HNO3 + 2.0 mol L-1 NaCl; their experiments show that no severe microstructural or morphological damage after periods of time up to 20h. Thus, it was important to test a series of samples of Ti/Pt 5mm and Nb/BDD under the real conditions of use. For a first evaluation accelerated life tests were conducted. 13

Fig. 5 shows the variation of the cell potential during electrolysis under very high anodic current density (50 kA m-2) for six electrodes. The relative jump of potential indicates the total deactivation of an electrode. The lifetime of three Ti/Pt anodes were respectively of 880, 1280 and 1550 hours whereas those of the diamond anodes were 1620, 1720 and >1810 hours. A theoretical extrapolation based on a practical current density of 1500 A m-2 leads to a minimal life expectancy of more than 3 years for Ti/Pt (value in agreement with the industrial experience on the Ti/Pt) and approximately 6 years for Nb/BDD.

Conclusions This study performed with a tubular electrochemical cell has shown that the Nb/BDD anode represents an interesting alternative to the Ti/Pt anode for the dechlorination as well as for the regeneration of Ag(II) in nitric acid solution. It is possible to run the two steps sequentially in the same reactor using Nb/BDD anodes, as has been shown for Pt/Ti anodes. Results obtained with Ti/Pt 5µm and Nb/BDD anodes were satisfactory since dechlorination reached 99 %, value in accordance with the practical objective. Furthermore, the formation of oxyanions of chlorine was not detected during electrochemical dechlorination. The specific particularity of the Nb/BDD electrode lies in the service life practically double than that of Ti/Pt. Monitoring the rate of Ag(I) oxidation on the BDD anode imposes however the current density to be controlled to avoid the generation of hydroxyl radicals susceptible to slow down the rate of the Ag(II) electrochemical regeneration via hydrogen peroxide formation.

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