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Journal of Molecular Liquids 249 (2018) 361–370

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Adsorption behavior of zirconium and molybdenum from nitric acid medium using low-cost adsorbent Mostafa M. Hamed, H.E. Rizk ⁎, I.M. Ahmed Hot Laboratories and Waste Management Center, Atomic Energy Authority, 13759 Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 8 October 2017 Received in revised form 6 November 2017 Accepted 7 November 2017 Available online 10 November 2017 Keywords: Zirconium Molybdenum Adsorption Nitric acid medium Charcoal

a b s t r a c t The highly radioactive waste liquors from the processing of nuclear reactor fuels contain zirconium and molybdenum, which occur among the fission products. Treatment of wastewater by economical and effective methods is very crucial in the area of development and technological advancements. In the present work, the adsorption behavior of Zr(IV) and Mo(VI) on charcoal from nitric acid medium was investigated using batch technique. Variations of the distribution coefficients as a function of HNO3 concentration in the range 0.05–3.0 M were presented. Some of the separation possibilities were pointed out. The obtained results indicate that the selectivity of Zr(IV) is higher than Mo(VI) at high acidities. The adsorption kinetics data following the pseudo-second order model indicated that the rate-controlling step is chemical adsorption. The data also followed the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms. The values of enthalpy and entropy changes show that the overall adsorption process was endothermic (ΔH N 0) and increasing entropy (ΔS N 0), and it was spontaneous (ΔG b 0). This study shows that the low-cost adsorbent, charcoal, is an effective adsorbent for the retention of Zr(IV) and Mo(VI) species from nitric acid medium. The feasibility of the removal of 95Zr and 99Mo from simulated intermediate level waste solutions was tested. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Zirconium is used in nuclear reactor technology in the form of zircaloys as cladding material for the uranium fuel and for other reactor internals. Due to ageing effect of reactor vessels, erosion starts and leads to contamination of the coolant water with radioactive species of zirconium, thus posing hazards to the systems and environment [1]. Zirconium and its compounds are also extensively used in chemical industries, steel and cast manufacturing, surface agent on satellites, lamp filaments, welding fluxes, in vacuum tubes, preparation of water repellent textiles, dyes pigments, ceramics, glass and abrasives. It is, therefore, important to decontaminate the reactor coolant water from radioactive zirconium and from other industrial effluents, before their safe disposal into water bodies [1,2]. Molybdenum has strategic and industrial importance due to its various applications, e.g., as an alloying agent in steels and cast iron, solid lubricants, reactor vessels, and in special batteries. High concentrations of molybdate (N 5 ppm) cause environmental problem, which makes the removal of molybdenum ion from wastewater and ground water a task of great significance from the environmental point of view [2]. Molybdenum-99 is the most important radionuclide used in nuclear medicine practice [3]. Molybdenum is one of the fission products and ⁎ Corresponding author. E-mail address: [email protected] (H.E. Rizk).

https://doi.org/10.1016/j.molliq.2017.11.049 0167-7322/© 2017 Elsevier B.V. All rights reserved.

is present in significant concentration in the High Level Waste of Purex reprocessing process. Due to the complex chemical properties of molybdenum, it forms soluble and insoluble complexes which give rise to various problems, mainly in concentration, storage and solidification of high level waste [3]. High-level radioactive liquid waste generated from the extraction cycle in fuel reprocessing by PUREX process, contains fission products, actinides, lanthanides, corrosion products, various chemical additives and etc. [4]. In order to minimize an adverse impact of high-level radioactive liquid waste on the environment, a variety of extraction processes have been developed in recent years to separate harmful radionuclides from high level radioactive liquid waste [5]. Solvent extraction with noctyl(pheny1)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) was found to be effective extractant for the actinides and fission products from a concentrated HNO3 solution. Since CMPO shows excellent chemical-stability in nitric acid medium and is a commercially available compound for actinide partitioning from high level radioactive liquid waste [4,5]. Alkyl phosphoric acids were also used to separate zirconium from molybdenum in presence of long-lived fission products 90 Sr, 99Tc, 106Ru, 137Cs, 144Ce and 147Pm [6]. In the partitioning process, the short lived elements (Zr, Mo, etc.), transuranium elements, platinum group metals, and Cs/Sr can be separated by solvent extraction methods [4]. However, these partitioning processes essentially utilize liquid–liquid extraction technology by a mixture of an organic extractant and a hydrocarbon diluent. A large

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M.M. Hamed et al. / Journal of Molecular Liquids 249 (2018) 361–370

quantity of so-called secondary-waste liquid containing the hydrolytic and radiolytic degradation products of organic compounds can thus be generated [4]. Furthermore, a great number and large scale of equipments are often required in the extraction processes. Therefore, a separation technique which utilizes minimal organic solvents and compact equipment is very desirable for the high level radioactive liquid waste partitioning [4]. Thus, it is important to explore some new separation systems or methods superior to the current separation process which has been widely applied to uptake zirconium and molybdenum. Adsorption has been found to be superior to other techniques for wastewater treatment due to low-cost, flexibility and simplicity of design, ease of operation, insensitivity to toxic pollutants and for avoiding the formation of secondary-waste [7]. A number of laboratory-scale studies were performed to investigate the potential application of commercial carbonaceous material as adsorbent for the removal of uranium and heavy metal ions and organic materials from different wastewaters [7–13]. The results indicated that this material is a very suitable adsorbent in wastewater management. Furthermore, high chemical, radiation, and thermal stability, rigid porous structure, and mechanical strength impart present considerable advantages over polymeric materials [13]. However, few studies have been done on the use of charcoal for the adsorption of heavy metals and radionuclides from nitric acid medium. Since transactinide elements (Z N 103) are characterized by short half-lives, studies of their chemical properties require the use of fast and efficient radiochemical methods and techniques involving sorption offer these advantages [14–17]. Moreover, analogies in the behavior of Zr and Rf (Z = 104), as well as of Mo and Sg (Z = 106) have unambiguously been demonstrated [14–17]. In order to reach a better understanding of the chemical properties of the first transactinide elements i.e. Rf and Sg, experiments involving sorption process was conducted with their homologues [14–17]. To the best of our knowledge, the application of charcoal in sorption of Zr(IV) and Mo(VI) ions from nitric acid media has not been reported in the literature. To achieve a better understanding of the adsorption mechanism of Zr(IV) and Mo(VI) ions from HNO3 medium onto the charcoal and to find an effective separation method of these ions from High-level radioactive liquid waste in the MAREC (Minor Actinides Recovery from HLLW by Extraction Chromatography) process, the adsorption behavior of Zr(IV) and Mo(VI) ions with charcoal in a broad range of HNO3 concentration was examined. 2. Experimental 2.1. Materials and chemicals ZrOCl2·8H2O and (NH4)6Mo7O24·4H2O were used as sources for Zr(IV) and Mo(VI), respectively. Various concentrations of Zr(IV) and Mo(VI) with given concentration of HNO3 were prepared as needed. All the chemicals used were of analytical reagent (AR) grade and supplied by Merck or Aldrich, except charcoal activated which was supplied by Loba Chemical (India), and all the chemicals were used as received. 2.2. Radioactive isotopes The radioactive isotopes 95Zr (t1/2 = 64.4 d) was artificially produced through the irradiation of spec-pure ZrOCl2.8H2O powder while 99 Mo (t1/2 = 66.0 h) was produced from 235U nuclear fission as a fission product at the 2nd Egyptian Research Reactor at the Inshas site. 2.3. Instrumentation Shimadzu UV–Visible Recording Spectrophotometer (UV-160A) manufactured and supplied by Shimadzu Kyoto, Japan was used for estimation of Zr(IV) and Mo(VI) concentration.

The adsorbent was characterized with respect to its pore structure and surface area using nitrogen adsorption/desorption at 77 K which was conducted using a gas sorption analyzer (Quantachrome, NOVA 1000e series, USA), FTIR spectrum of the charcoal sample was recorded using a Nicolet spectrometer from Meslo, USA to identify the functional groups. Simultaneous differential thermal and thermogravimetric analyses (DTA/TGA) of activated carbon were carried out using DTA and TGA with sample holder made of platinum using a Shimadzu DTG-60/60H thermal analyzer, Japan. It was used for the measurements of the phase changes and weight losses of the sample at heating rate of 10 °C/min in presence of nitrogen gas to avoid thermal oxidation of the adsorbent sample. The sample was heated in a platinum crucible from room temperature to 1000 °C. 2.4. Distribution studies and capacity Accurate and reliable knowledge of the distribution coefficients of the metal ions between the solution and charcoal is needed in adsorption study to estimate their transport rates. Both distribution coefficient determinations and capacity measurements were performed by batch technique at constant volume to mass ratio of 100 mL/g. A mixture of charcoal (0.1 g) and the solution (10 mL) of either separate Zr(IV) or Mo(VI) was allowed to stand overnight in a shaker thermostat adjusted at room temperature with shaking to ensure establishment of equilibrium. The solid adsorbent was then separated from its solution by filtration, and the concentrations of Zr(IV) and Mo(VI) in filtrate were assayed by spectrophotometric methods using Alizarin red S and alcoholic ammonium thiocyanate as chromogenic reagents [18]. The distribution coefficient (Kd mL/g) values for the individual Zr(IV) and Mo(VI) were determined at different nitric acid concentrations using the equation [19]:

Kd ¼

C o −C e V ðmL=gÞ m Ce

ð1Þ

2.5. Separation factor (α) The separation factor is a guiding measure of the ability of the adsorption system to separate two metal ions (such as Zr and Mo) of equal concentration. The separation factor is sometimes also called the ‘selectivity’. It is used as a measure of possibility of chromatographic separation and is also expressed as the ratio of the distribution coefficients of the ions to be separated. If the Zr(IV) is preferred for the adsorZr bent the value of αZr Mo is N1. The opposite is true, i.e., αMo b 1 means that Mo(VI) is preferred [20]. K d ðZrÞ ; Separation factor αZr Mo ¼ K d ðMoÞ

ð2Þ

where Kd(Zr) and Kd(Mo) are the distribution coefficients of the two competing Zr(IV) and Mo(VI) species in the adsorption system. 2.6. Equilibrium studies Sorption onto the charcoal was studied using 0.1 g, 10 mL of solution at 0.1 M nitric acid concentration, and different initial metal ion concentrations (20–400 mg/L for Mo and 50–400 mg/L for Zr). The solutions were shaken until equilibrium was reached. The experimental data obtained from this experiment were evaluated using Langmuir and Freundlich isotherm models. The uptake efficiency (% R) and amount of Zr(IV) and Mo(VI), qe (mg/g) retained on the charcoal were calculated using the following


363

equations respectively [19,20]: % R¼

C o −C e 100 Co

qe ¼ ðC o −C e Þ

V ðmg=gÞ m

ð3Þ

ð4Þ

where Co and Ce are the initial and equilibrium concentrations of the metal ions in the aqueous solution (mg/L); V is the volume of the solution (L); and m is the mass of charcoal (g). 2.7. Kinetic and thermodynamic studies A fixed amount (0.1 g) of charcoal was added to a 10 mL sample with an initial metal ion concentration of 100 mg/L for both metal ions at 0.1 M nitric acid concentration, and then aliquots were taken at several time intervals until the adsorption equilibrium was reached to determine the kinetics and thermodynamics of adsorption. These experiments were conducted at 25, 45, and 65 °C. 3. Results and discussion Understanding of adsorption technique is possible with knowledge of the optimal conditions, which would show a better design and modeling process. Thus, the effect of some parameters like HNO3 concentration, contact time and initial concentration of metal ions on the uptake process was investigated. Adsorption studies were performed by batch technique to obtain the equilibrium data. 3.1. Adsorbent characteristics 3.1.1. Pore structure parameters The qualitative information of the adsorption process and surface area can be obtained from the shape of adsorption isotherm. There are five basics types (Types I\\V) of adsorption isotherms. The N2 adsorption/desorption isotherm of charcoal is shown in Fig. 1a. The adsorption isotherm is classified as Type I isotherm indicating that the charcoal is mainly microporous [21] in nature. The pore structure is also a major factor affecting the adsorption process. According to the IUPAC classification of pore dimensions, there are three broad classifications grouped as micropore (diameter, d ≤ 2 nm), mesopore (2 b d b 50 nm), and macropore (d ≥ 50 nm) [21]. The hysteresis indicates the presence of meso-porosity on the surface of the charcoal. The amount of nitrogen adsorbed on the charcoal gradually increases in the whole relative pressure range, implying that this adsorbent contains a small amount of mesopores. Characteristic features of the mesopores isotherm are its hysteresis loop, which is associated with capillary condensation taking place in mesopores [21]. The present of micropores and mesopores in the charcoal also indicated by the pore size distribution as depicted in Fig. 1b. The pore size distribution seems located in the micropore region. The pore characteristic of charcoal adsorbent is summarized in Table 1. As shown in Table 1, the BET surface area is 693 m2/g. Charcoal has 0.393 cm3/g of total pore volume and 0.085 cm3/g of micropore and 0.308 cm3/g of mesopore volume, which show charcoal is reasonably good for adsorption. 3.2. Surface chemistry Fourier transform infrared (FTIR) spectrum revealed the chemical functionalization of adsorbent surface. The surface functional group is an significant characteristics of the carbonaceous materials since it determines the surface properties of the adsorbents and has significant role on their behaviors. In the FTIR spectrum (Fig. 2), a number of absorption bands observed at about 3655, 3450, 1712, 1650, 1580, 1300–1000 and 773 cm−1 prove the presence of hydroxyl and carboxyl

Fig. 1. Nitrogen adsorption/desorption isotherm (a) and pore size distribution (b) of charcoal.

groups on the surface of charcoal [22]. The broad band around 3440 cm−1 is due to an O\\H stretching vibration mode of hydroxyl groups and adsorbed water, while \\CH2 vibration appears at about 2920 cm− 1. The presence of \\NH groups are also indicated by the band at 3437 cm−1 [9,10]. A weak band at 1720 cm−1 is attributed to C_O stretching vibration in the carboxylic groups. The bands at about 1600 cm−1 were ascribed to C_O stretching from ketones or carboxylic acid groups. Presences of C`C or C`N are indicated by the band at 2356 cm−1 [10]. The enhanced aromatic C_C double bonds can be observed at 1580 cm−1. The peak occurring at about 1450 cm−1 is attributed to C\\H bending vibrations. The band observed at around 1420 cm−1 is ascribed to the C\\O\\H stretching vibration in the carboxylic group [9]. The several bands in the region 1000–1300 cm− 1 can be ascribed to C\\O or C\\O\\C stretching vibrations in lactonic, ether, phenol groups etc. [10,22]. It can be concluded from the vibrational assignments of the spectrum that the chemical structure of charcoal surface contains different C\\O, C_O, C\\O\\C, and C_C bonds.

Table 1 N2 adsorption/desorption analysis of charcoal. Parameter

Value 2

BET surface area (m /g) Langmuir surface area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Mesopore volume (cm3/g) Average pore diameter, APD (nm)

693.0 1065 0.393 0.085 0.308 0.60

364


100

%Transmittance

99 98 97 96 95 94

4000

3500

3000

2500

2000

1500

1000

500

Wavenumbers , cm-1 Fig. 2. FT-IR spectrum of charcoal.

3.3. Thermal analysis The thermal properties of charcoal were studied to investigate its thermal stability at high temperature operating condition. Thermogravimetric analysis, TGA, and differential thermal analysis, DTA, of charcoal sample up to 1000 °C are shown in Fig. 3. The sample has lost water content up to 100 °C, coupling with the volatilization of hydrocarbons and exhibits no mass loss until around 500 °C. After about 500 °C mass loss rate has increased depending on the removal of volatiles organic components of sample and at 850 °C total mass loss has reached to about 95%. At a temperature above 850 °C, no further weight loss was observed. In the DTA, two peaks were observed at 86 and 536 °C, associated to the different mass losses observed in TGA. The endotherm at 86 °C indicating loss of water molecules which may be present as free and/or adsorbed on the porous of sorbent matrix [8,10].

3.4. Effect of nitric acid concentration on adsorption and selectivity Interaction of chemical compounds with charcoal may alter the surface properties; therefore, adsorption behavior of Zr(IV) and Mo(VI) ions was investigated in acid solutions of nitric acid, having a concentration range from 0.05 to 3.0 M, using 0.1 g of charcoal. Adsorption behavior of Zr(IV) and Mo(VI) ions from aqueous solution as a function of HNO3 concentration was studied and the data were illustrated in Fig. 4(a, b). The results clarify that the adsorption of both ions decreases with increasing HNO3 concentration from 0.05 to 3.0 M. Similar trends have been observed for the adsorption of Zr, Mo, Ag, Pb and Cd ions on different adsorbents [1,3–6]. The decrease in adsorption of Zr(IV)

Fig. 3. TGA and DTA for the charcoal.

and Mo(VI) with an increase in acid concentration may be attributed to the competition between the excess of H+ and NO−3 ions in the medium and negatively or positively charged species present in the solution. Also, a higher acid concentration appears to suppress hydrolysis of both ions in hydrolyzed species, resulting in lower uptake [1,5,6]. The % adsorbed amount was decreased from about 83 to 40% for Zr(IV) and decreased from 93% to 1% for Mo(VI) as HNO3 concentration increased from 0.05 up to 3.0 M. Using Visual MINTEQ version 3.1 speciation program we have calculated the speciation of Zr(IV) and Mo(VI). Fig. 4(c, d) shows the speciation of Zr(IV) and Mo(VI) species as a function of HNO3 concentrations as estimated by MINTEQ program. It was shown that Zr+4 and ZrOH+3 were the dominant ionic species at all HNO3 concentrations and the other species in the form of Zr(OH)4 and Zr(OH)− 5 can be neglected. A speciation profile of Mo(VI) and its species at different HNO3 concentrations showed that the predominant species is neutral MoO3(H2O)3 species. In the present study, maximum adsorption was observed at 0.05 M HNO3 i.e., positively charged Zr+4, ZrOH+3 and neutral MoO3(H2O)3 species predominate. As observed that N80% adsorption of Zr(IV) and N90% adsorption of Mo(VI) occurred in 0.1 M HNO3. Therefore this concentration of HNO3 was used for all the subsequent experiments regarding the optimization of conditions for the adsorption of Zr(IV) and Mo(VI) on charcoal. Interactions of Zr(IV) and Mo(VI) ions in different nitric acid solutions were investigated by the batch equilibration method under comparable experimental conditions to make possible evaluation of the obtained Kd values for the retention and separation of these ions. Fig. 4b displays the variation of the Kd values of Zr(IV) and Mo(VI) ions onto charcoal as a function of HNO3 concentration in the range from 0.05 to 3.0 M HNO3. The obtained results may be attributed to the greater competition between the metal ions and H+, NO− 3 and/or HNO3 in solution to penetrate or exchange with function groups on the charcoal surface [1–6]. Table 2 shows that Kd value of Zr(IV) and Mo(VI) decreases with increasing HNO3 concentration and vice versa. The Kd values of Mo(IV) at low concentration of HNO3 (0.05–0.2 M) are higher than that for Zr(IV) ions, then, the Kd values of Zr(IV) ions become higher than Mo(IV) ions at high concentration of HNO3 (0.5–3.0 M). This trend indicates that the number of sites selective for Zr(IV) is significantly higher than that for Mo(VI) at high HNO3 concentration. 3.5. Separation factor (α) Solid phase adsorbents often are used to separate individual metal ion species from each other. Typically this is regarding to the selective ability of these adsorbents [8,23]. On the other hand, it is very attractive to illustrate the selectivity behavior of Zr(IV) and Mo(VI) ions in terms of distribution coefficient values [8,23]. A study on the distribution behavior of ions in different media gives an idea about the possible ion separations as well as the eluants that could be used for separation. The separation factor (α) values are presented in Table 2 (complete separation is achieved if α N 1). However, Kd values that are derived from partition can be used to estimate the separation factors α (Eq. (2)) for Zr(IV) and Mo(VI) ions [20,24]. From the data listed in Table 2, it can be seen that: The αMo Zr values, related to separation between Mo(VI) and Zr(IV) ions, decrease with increase of HNO3 concentration. For the separation of Zr(IV) from Mo(VI) ions, the αZr Mo values increase with the HNO3 concentration, contrary to the αMo Zr . The maximum separation factor (αZr Mo = 54) was obtained at a concentration of 3.0 M HNO3. In this work high separation factor (αZr Mo = 54) was achieved at 3.0 M HNO3 and V/m ratio 100 mL/g, this is large enough for practical purposes [20,24]. The values of separation factor between Zr(IV) and Mo(VI) were found to be 2.5–54 at 0.5–3.0 M HNO3. This indicates


1400

Mo(VI) 100 mg/L Zr(IV) 100 mg/L

(a)

100

1000

80 60 40

800 600 400

20 0 0.0

Mo(VI) 100 mg/L Zr(IV) 100 mg/L

(b)

1200

Kd, mL/g

Sorption efficiency, %

120

200

0.5

1.0

1.5

2.0

2.5

0 0.0

3.0

0.5

1.0

[HNO3], M 100

2.0

2.5

3.0

100

(d)

80

Aqueous species, %

Aqueous species, %

1.5

[HNO3], M

(c)

90

365

70 60 50 40 30 20 10

80 60 40 20 0

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

[HNO3], M

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

[HNO3], M

Fig. 4. Variation of (a) sorption efficiency (b) Kd and (c, d) theoretical distribution diagrams of Zr(IV) and Mo(VI) species as a function of HNO3 concentration (with Zr and Mo total concentration of 100 mg/L).

that the charcoal has a preferential selectivity toward Zr(IV) than Mo(VI) at high concentration of nitric acid as shown in Table 2. Therefore, the studied adsorbent, charcoal, may be promising for the effective separation of Zr(IV) from Mo(VI). 3.6. Effect of contact time and temperature In order to establish the equilibrium time for maximum adsorption and to know the kinetics of adsorption process, Zr(IV) and Mo(VI) adsorption on charcoal was investigated as a function of contact time and the results are shown in Fig. 5(a, b). It is seen that the sorption efficiency increases with the increase in shaking time for both metal ions. Fig. 5 also shows that the adsorption of Zr(IV) and Mo(VI) by charcoal is very rapid and the equilibrium is reached within 60 and 30 min for Zr(IV) and Mo(VI), respectively. Therefore, 60 and 30 min shaking time was chosen to be sufficient for the adsorption of Zr(IV) and Mo(VI) on charcoal and was used for all further tests. Table 2 Separation factor (α) of Zr(IV) and Mo(VI) adsorbed onto charcoal. [HNO3], mol/L

0.05 0.1 0.2 0.5 1.0 2.0 2.5 3.0

Kd, mL/g

Separation factor (α)

Mo(IV)

Zr(IV)

αZr Mo

1329 1150 348.4 104 64.0 19.4 5.3 1.0

488 456 300 257 154 106 52.0. 53.85

0.37 0.40 0.86 2.47 2.40 5.46 9.81 53.85

It can be seen from Fig. 5 that the sorption efficiency of both studied ions increases with the increase in temperature, which meaning the adsorption on charcoal is endothermic. For both studied ions, the surface coverage increased at higher temperatures, this may be attributed to increased penetration of Zr(IV) and Mo(VI) species inside charcoal pores at higher temperatures or the induction of new active sites or formation of more than one adsorbate layer on the surface of charcoal. The increase in the adsorption efficiency may be also a result of increase in the mobility of metal ion species with increasing temperature. An increasing number of Zr(IV) and Mo(VI) species may also acquire sufficient energy to undergo an interaction with active sites at the surface of charcoal [8,10,25]. Furthermore, increasing temperature may produce a swelling effect within the internal structure of the charcoal which enabling large amount of Zr(IV) and Mo(VI) species to penetrate further [8,10]. 3.7. Sorption kinetic modeling In order to examine the controlling mechanism of the adsorption process, the pseudo-first-order and the pseudo-second-order rate equation were utilized to analyze the experimental sorption data at various temperatures. The linearized-integral form of the pseudo first-order model is represented by [19]: k1 logðqe −qt Þ ¼ logqe − t 2:303

ð5Þ

where k1 is the rate constant of ions adsorption, qe and qt are the amounts of Zr(IV) and Mo(VI) adsorbed (mg/g) at equilibrium and at time t. Values of k1 was calculated from the slope of the plot of log(qe

366


(a)

80

[Zr] = 100 mg/L [HNO3] = 0.1 M

60

298 K 318 K 338 K

40

[Mo] = 100 mg/L

80

[HNO3] = 0.1 M

298 K 318 K 338 K 60

0 0.8

(b)

100



100

50

100

150

Time, min

200

0

250

50

250

0.4 0.2

log(qe- qt)

log(qe-qt)

-0.4 -0.8

298 K 318 K 338 K

-1.2

0.0 -0.2 -0.4

298K 318K 338K

-0.6 -0.8 -1.0

0

10

20

Time, min

30

40

-1.2

50

0

5

10

Time, min

15

20

3.5

6

(e) Zr(IV)

(f) Mo(VI)

3.0 2.5

4

2

t/qt, min.g/mg

t/qt, min.g/mg

200

(d) Mo(VI)

0.6

0.0

0

150

Time, min

0.8

(c) Zr(IV)

0.4

-1.6

100

298 K 318 K 338 K

2.0 1.5

0.5 0.0

0

10

20

30

298 K 318 K 338 K

1.0

40

0

5

10

Time, min

15

20

25

30

35

Time, min

Fig. 5. Effect of contact time (a, b), pseudo first-order (c, d) and pseudo second-order (e, f) plots for the sorption of Zr(IV) and Mo(VI).

− qt) against t (Fig. 5c,d) and the values of kinetic parameters for two ions were presented in Table 3. The results show that the experimental qe values are not in agreement with calculated qe values and the correlation coefficients (R2) is low. Accordingly, the adsorption of Zr(IV) and Mo(VI) onto charcoal does not follow this kinetic model.

The linearized-integral form of the pseudo-second-order model is expressed as follows [19]: t 1 1 ¼ þ t qt k2 q2e qe

ð6Þ

Table 3 The calculated parameters of the pseudo first-order and pseudo second-order kinetic models of Zr(IV) and Mo(VI) at different temperatures. Ion

Zr(IV)

Mo(VI)

Temp., K

298 318 338 298 318 338

Pseudo-first-order parameter

qexp (mg/g)

Pseudo-second-order parameter

k1, (min−1)

qcalc (mg/g)

R2

k2 (g/mg·min)

qcalc (mg/g)

R2

0.060 0.080 0.050 0.144 0.138 0.153

2.330 2.355 1.640 3.380 3.080 2.920

0.935 0.959 0.968 0.984 0.987 0.967

0.109 0.124 0.110 0.058 0.062 0.128

8.19 8.40 9.00 9.80 9.90 10.0

0.9999 0.9998 0.9997 0.9998 0.9998 0.9997

8.20 8.40 9.45 9.20 9.35 9.50


367

models are applied to describe the sorption characteristics of Zr(IV) and Mo(VI) onto charcoal.

where k2 is the pseudo-second-order constant. This rate was determined from the intercept of the plot of t/qt versus t (Fig. 5e, f). The values of kinetic parameters were listed in Table 3. Values of R2 are N0.99 and the experimental qe are in accordance with calculated qe illustrate that the adsorption of Zr(IV) and Mo(VI) onto charcoal fits the pseudosecond-order model.

3.9. Langmuir isotherm model Langmuir isotherm is an empirical model assuming that adsorption occurs at a limited number of positions on the adsorbent surface, and the adsorbed layer is one molecule or monolayer adsorption in thickness. The fundamental postulate of this model, the amount of ions adsorbed can be expressed by [31]:

3.8. Adsorption isotherms The distribution of Zr(IV) and Mo(VI) between charcoal and metal ions in solution, when the adsorption system is at equilibrium, is of valuable importance in clarifying the maximum adsorption capacity of the charcoal toward the Zr(IV) and Mo(VI). The analysis of equilibrium sorption data of Zr(IV) and Mo(VI) is essential to promote equations that represent the results and would be useful for design purposes. Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) [23,26–33]

Ce 1 1 ¼ þ Ce qe bQ max Q max

ð7Þ

where Ce is the equilibrium concentration (mg/L) in the solution, qe is the amount of ions sorbed on charcoal (mg/g) at equilibrium, Qmax is 2.0

14

(a) Zr(IV)

(b) Mo(VI)

1.8

12 1.6

8 6

298K 318K 338K

4

1.4

Ce/qe, g/L

Ce/qe, g/L

10

1.2

298 K 313 K 338 K

1.0 0.8

2 0.6

0 0

50

100

1.3 1.2

150

200

Ce, μg/mL

0

250

20

30

40

50

Ce μg/mL

60

70

1.6

(c) Zr(IV)

1.4

(d) Mo(VI)

1.2

1.1

1.0

1.0

log qe

log qe

10

0.9

298 K 318 K 338 K

0.8

0.8 0.6

298 K 318 K 338 K

0.4

0.7

0.2

0.6 0.5

1.0

-8.2

1.5

2.0

log Ce

2.5

-0.2 0.0

0.2

0.4

0.6

-7.5

-8.4

(e) Zr(IV)

0.8

1.0

log Ce

1.2

1.4

1.6

1.8

(f) Mo(VI)

-8.0

-8.6

-8.5

-8.8

-9.0

ln qe

ln qe

-9.0 -9.2 -9.4

298 K 318 K 338 K

-9.6 -9.8

-9.5

298 K 318 K 338 K

-10.0 -10.5

-10.0 200

300

400

500

600 2

ε

700

800

900

-11.0 300

400

500

600

700 2

800

900

ε

Fig. 6. Langmuir (a, b), Freundlich (c, d) and D–R (e, f) isotherm plots for the sorption of Zr(IV) and Mo(VI).

1000

1100

368


the maximum theoretical monolayer capacity (mg/g), and b is the equilibrium constant of sorption related to the sorption energy. The equilibrium sorption data for Zr(IV) and Mo(VI) at different temperatures has been correlated with this model, Fig. 6. The graphic presentations of (Ce/qe) versus Ce give straight lines for both Zr(IV) and Mo(VI) sorbed onto charcoal, as presented in Fig. 6a, b, confirming that this expression is indeed a reasonable representation of chemisorption isotherm. The values of Qmax and b were calculated from the intercept and slope of the linear plots of Ce/qe vs. Ce for different temperatures and are listed in Tables 4, 5 along with coefficients of correlation (R2). The Langmuir constants Qmax and b increased with temperature showing that adsorption capacity and equilibrium constant are enhanced at higher temperatures and indicating the endothermic nature of adsorption. The maximum monolayer capacity (Qmax) obtained from the Langmuir model was 17.54 and 51.40 mg/g for Zr(IV) and Mo(VI) at 25 °C, respectively. One of the essential characteristics of the Langmuir model could be expressed by dimensionless constant called separation factor or equilibrium parameters, RL [26]: RL ¼

1 1 þ bC o

ð8Þ

where Co is the highest initial metal ion concentration (mg/L). The value of RL indicates the type of isotherm to be irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1), or unfavorable (RL N 1). The value of RL lies in the range 0.043–0.0292 for Zr(IV) and 0.085–0.071 for Mo(VI) ions, Tables 4, 5. All the RL values were found to be b1 and N0 indicating a highly favorable sorption of Zr(IV) and Mo(VI) on charcoal for all studied concentrations [26,31–33]. 3.10. Freundlich isotherm model Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. The Freundlich equation agrees well with the Langmuir equation over moderate concentration ranges. The logarithmic form of Freundlich equation may be written as [31–33]: 1 logqe ¼ logK f þ logC e n

ð9Þ

where Kf is constant indicative of the relative sorption capacity of charcoal ((mg/g)(L/mg)1/n) and 1/n is the constant indicative of the intensity of the sorption process. The value of 1/n determines the magnitude of adsorption in Freundlich isotherm. If the 1/n value is N 1, then the adsorption is not favorable, whereas if the value becomes 1/n is b1, then it represents a favorable adsorption. The graphic illustration of log qe versus log Ce is shown in Fig. 6c, d and the numerical values of the constants 1/n and Kf are computed from the slope and the intercepts, by means of a linear least square fitting method, and also given in

Table 4 Isotherm parameters for adsorption of Zr(IV) at different temperatures. Model

Isothermal model parameters

298 K

318 K

338 K

Langmuir

Qmax (mg/g) b (L/g) RL R2 1/n Kf (mg/g)(L/mg)1/n R2 qm (mmol/g) β (mol2/KJ2) EDR (KJ/mol) R2

17.54 0.056 0.043 0.999 0.339 2.85 0.960 0.521 0.0040 11.18 0.990

18.18 0.070 0.034 0.999 0.328 3.260 0.950 0.529 0.0033 12.31 0.988

18.64 0.082 0.0295 0.999 0.310 3.670 0.957 0.535 0.0028 13.36 0.996

Freundlich

D-R

Table 5 Isotherm parameters for adsorption of Mo(VI) at different temperatures. Model

Isothermal model parameters

298 K

318 K

338 K

Langmuir

Qmax (mg/g) b (L/g) RL R2 1/n Kf (mg/g)(L/mg)1/n R2 qm (mmol/g) β (mol2/KJ2) EDR (KJ/mol) R2

51.40 0.027 0.085 0.993 0.799 1.62 0.974 3.23 0.0066 8.70 0.993

53.50 0.031 0.075 0.991 0.798 1.90 0.990 4.28 0.0061 9.10 0.990

57.10 0.033 0.071 0.996 0.757 2.04 0.984 4.23 0.0052 9.81 0.994

Freundlich

D-R

Tables 4, 5. The values of R2 suggest that the data is not good represented by Freundlich isotherm. 3.11. Dubinin–Radushkviech isotherm (D–R isotherm) Most of the isotherm equations, including Langmuir and Freundlich isotherm models, do not explain the sorption mechanism well. These isotherms are insufficient to explain the chemical and physical characteristics of adsorption process. However, Dubinin–Radushkevich (D– R) isotherm can be used to find the free energy of sorption (EDR) and hence the sorption mechanism. D-R model can be used to describe sorption on both homogeneous and heterogeneous surfaces. The D–R isotherm model was applied to the equilibrium data to assess the nature of the sorption process, i.e. whether it is physical or chemical sorption, and it is used to calculate the mean free energy of sorption. D-R-model does not assume an energetically homogenous surface and proposes a non-homogenous distribution of sorption sites. In particular, it assumes that the ionic species bind first with the most energetically favorable sites and that multilayer sorption then occurs. The linear form of D-R isotherm is given as the following [23,29,30]: ln qe ¼ ln qm −βε2

ð10Þ

where β is a constant related to sorption energy and qm the sorption affinity or sorption capacity and ε is the Polanyi potential which can be calculated as the following: 1 ε ¼ RT ln 1 þ Ce

ð11Þ

where Ce is the equilibrium concentration of adsorbate species in solution (mmol/L), R (J/mol·K) is the gas constant, and T (K) is the absolute temperature. The D-R constants qm and β values were calculated from the linear plots of ln qe vs. ε2 for the experimental data (Fig. 6e, f) for sorption of Zr(IV) and Mo(VI) by charcoal and are given in Tables 4, 5. The mean free energy of sorption (EDR) is the free energy change when 1 mol of ion transferred to the charcoal surface from infinite solution. EDR is calculated from the β value using following equation: 1 EDR ¼ pffiffiffi 2β

ð12Þ

The mean free energy of sorption, EDR, gives information about the sorption mechanism as physical or chemical sorption. If EDR is in the range of 8–16 kJ/mol, sorption is governed by chemical ion exchange mechanism. In the case of EDR b 8.0 kJ/mol, physical forces may affect the sorption mechanism; if EDR N 16 kJ/mol, it could be supposed that particle diffusion may affect the sorption [23,29,30]. The D-R isotherm constants found from fitting the adsorption data to these isotherms for three temperatures are presented in Tables 4, 5 together with the correlation coefficients, R2.


7 6 5

lnKc

From Tables 4, 5, the sorption affinity (qm) of the charcoal lies in the range 0.521–0.535 mmol/g for Zr(IV) and 3.23–4.23 mmol/g for Mo(VI) ions, indicating that the charcoal had a greater affinity for Mo(IV) in this acidity condition (0.1 M HNO3). The mean free energies from the D-R model for the sorption process are in the range 11.20–13.40 kJ/mol and 8.70–9.80 kJ/mol for Zr(IV) and Mo(VI), respectively. It's therefore deduced that Zr(IV) and Mo(VI) species adsorption onto charcoal most likely involves a chemisorption process (the mean adsorption energy values fall into the energy range of chemisorption process, (8 to 16 kJ/mol)). It could be concluded from the isotherms constants (Tables 4, 5) that the Langmuir and D–R models fitted the experimental data better. In all cases, the Freundlich isotherm stated the least fit of experimental data as indicated by R2 values. The fact that the Langmuir isotherm fits the experimental data very well indicate almost complete monolayer coverage of the charcoal particles. The sorption capacities qm (from D-R model) values were extremely different from the sorption capacity qmax (from Langmuir model) values. This difference, which has been reported in other works [26,32]. Probably because of the different hypotheses considered in the formulation of the isotherm models [32].

369

4 3

Mo(VI) Zr(IV)

2 1 2.9

3.0

3.1

3.2

3.3

3.4

3 1/Tx 10 , K-1 Fig. 7. Van't Hoff plots for the sorption of Zr(IV) and Mo(VI).

3.12. Thermodynamic studies The Gibbs free energy change, ΔGo, is the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if ΔGo is a negative quantity. The ΔGo of the reaction is given by [25,30]: ΔGo ¼ −RT ln K C

ð13Þ

Fe 1− F e

ð14Þ

KC ¼

3.13. Application

where, KC is the sorption equilibrium constant and Fe is the fraction attainment of Zr(IV) or Mo(VI) ions sorbed at equilibrium. These parameters are leading to understand the mechanism of ion adsorption. The change of adsorption equilibrium constant with temperature, as outlined in Table 6, showed that equilibrium constant values increase with increases in temperature, thus implying a push of ions/charcoal interactions at higher temperature [33]. It is clear to see that the ΔGo values at all sorption temperatures were negative values. This notarize that the sorptive removal process is spontaneous and thermodynamically acceptable. Moreover, the value of ΔGo becomes more negative with the increase of sorption temperature, indicating more efficient sorption of Zr(IV) and Mo(VI) at high temperature. The thermodynamic parameters (ΔHo and ΔSo) for Zr(IV) or Mo(VI) sorption on charcoal can be calculated from the following relationship [33]: ln K C ¼

ΔSo −ΔH o 1 þ R R T

Table 6 Thermodynamic parameters for the adsorption of Zr(IV) and Mo(VI) ions. Ion

Temp., (K)

KC

Zr(IV)

298 318 338 298 318 338

18.20 73.70 124.0 13.50 48.18 90.02

Sorptive removal of radionuclides onto adsorbents such as charcoal is important from purification; nuclear industry, environmental and radioactive waste disposal point of view [35–39] Charcoal contains oxygenated functional groupings on its surface more over different pores, which are capable of sorbing radionuclide ions from aqueous wastes. Furthermore, high chemical radiation and thermal stability, rigid porous structure and mechanical strength impart considerable advantages over polymeric materials [8–10]. The removal of 95Zr and 99Mo by use of charcoal has been carried out from nitric acid solutions of a simulated intermediate level waste (ILW), prepared according to the composition of the waste in the Purex Process [34], and given in Table 7. In this context, an aliquot (10 mL) of the simulated ILW [33] was equilibrated with 0.5 g charcoal. After centrifuging a 5 ml sample was taken from the supernate solution, and the γ-measurement was compared to that before the charcoal treatment of the solution. The results of the removal are shown in Table 8.

ð15Þ

where R is the gas constant, and T is the absolute temperature. The thermodynamic parameters can be derived from the slope and intercept of the plots of ln KC versus 1/T (Fig. 7). The values of ΔHo and ΔSo are tabulated in Table 6. The values of the standard enthalpy changes (ΔHo) are positive and larger than 40 kJ/mol, implying the removal of Zr(IV) and Mo(VI) is

Mo(VI)

endothermic and via chemisorption (Table 6). Furthermore, the positive values of ΔSo show the freedom of Zr(IV) and Mo(VI) species during the sorptive removal and the high degree of randomness at solid-liquid interface during the removal of Zr(IV) and Mo(VI) species by charcoal from waste solutions [33].

ΔHo (kJ/mol)

ΔSo (J/mol·K)

40.54

161.10

40.10

156.57

ΔGo (kJ/mol) −7.18 −11.37 −13.54 −6.44 −10.25 −12.64

Table 7 Intermediate level waste simulate. Element

Amount, g/l

Al Ca Cr Cs Cu Fe U Mg Mn Mo Na Ni Sr Zn Zr HNO3: 1 M

0.230 1.500 0.080 0.0036 0.150 0.380 0.080 0.750 0.080 0.380 81.14 0.080 0.001 0.150 0.080

370


Table 8 Removal efficiency of 95Zr and 99Mo from intermediate level waste (ILW). Sample ILW-simulateb

Radionuclide spiked

Removal efficiency, %a

95

74.0 85.2

99 a b

Zr Mo

Just one treatment. As in Table 7.

The data in Table 8 indicate that the concentration of 95Zr and 99Mo decreased by about 74.0 and 85.2%, respectively with just one treatment. From the above data it can concluded that the charcoal can be used as a promising sorbent for the removal of 95Zr and 99Mo from different nitric acid solutions as well as in the treatment of radioactive liquid wastes. 4. Conclusion This study shows that the charcoal is an effective adsorbent for the removal of Zr(IV) and Mo(VI) species from HNO3 medium. The values of Kd, indicated show that Zr(IV) species distribute better than Mo(VI) species in charcoal, confirming that it has a higher affinity to Zr(IV) compared to Mo(VI) at high acidity (0.5–3.0 M HNO3). The adsorption kinetics was best described by a pseudo-second-order model. Adsorption isotherms were better described by the Langmuir and D-R models in comparison to the Freundlich model. The thermodynamic parameters indicated that the adsorption of these ions onto charcoal is an endothermic and spontaneous process. The removal of 95Zr and 99Mo by use of the charcoal from nitric acid solutions of simulated ILW from the PUREX PROCESS indicated the efficiency of the process. References [1] N. Khalid, M. Daud, S. Ahmad, Zirconium decontamination from aqueous media using lateritic minerals, Radiochim. Acta 93 (2005) 427–433. [2] A.A. Atia, A.M. Donia, H.A. Awaad, Synthesis of magnetic chelating resins functionalized with tetraethylenepentamine for adsorption of molybdate anions from aqueous solutions, J. Hazard. Mater. 155 (2008) 100–108. [3] S.K. Pathak, S.K. Singh, A. Mahtele, S.C. Tripathi, Studies on extraction behaviour of molybdenum (VI) from acidic radioactive waste using 2(ethylhexyl) phosphonic acids, mono 2(ethylhexyl) ester (PC-88A)/n-dodecane, J. Radioanal. Nucl. Chem. 284 (2010) 597–603. [4] E.H. Lee, D.S. Hwang, K.W. Kim, Y.J. Shin, J.H. Yoo, Removal of Zr and Mo from the simulated radwast solution by denitration with formic acid (I), J. Korean Ind. Eng. Chem. 6 (1995) 404–411. [5] A. Zhang, Y. Wei, M. Kumagai, Properties and mechanism of molybdenum and zirconium adsorption by a macroporous silica-based extraction resin in the MAREC process, Solvent Extr. Ion Exch. 21 (2003) 591–611. [6] I.L. Jenkins, A.G. Wain, The extraction of molybdenum from radioactive wastes, J. Appl. Chem. 14 (1964) 449–454. [7] L.P. Lingamdinne, Y. Chang, J. Yang, J. Singh, E. Choi, M. Shiratani, J.R. Koduru, P. Attr, Biogenic reductive preparation of magnetic inverse spinel iron oxide 2 nanoparticles for the adsorption removal of heavy metals, Chem. Eng. J. 307 (2017) 74–84. [8] M.M. Hamed, M.M.S. Ali, M. Holiel, Preparation of activated carbon from doum stone and its application on adsorption of 60Co and 152 + 154Eu: equilibrium, kinetic and thermodynamic studies, J. Environ. Radioact. 164 (2016) 113–124. [9] A.A. El-Sayed, M.M. Hamed, H.A. Hmmad, S. El-Reefy, Collection/concentration of trace uranium for spectrophotometric detection using activated carbon and firstderivative spectrophotometry, Radiochim. Acta 95 (2007) 43–48. [10] M.M. Hamed, Sorbent extraction behavior of a nonionic surfactant, Triton X-100, onto commercial charcoal from low level radioactive waste, J. Radioanal. Nucl. Chem. 302 (2014) 303–313. [11] S.E. Rizk, M.M. Hamed, Batch sorption of iron complex dye, naphthol green B, from wastewater on charcoal, kaolinite, and tafla, Desalin. Water Treat. 56 (2015) 1536–1546. [12] D. Kołodyńska, J. Krukowska, P. Thomas, Comparison of sorption and desorption studies of heavy metal ions from biochar and commercial active carbon, Chem. Eng. J. 307 (2017) 353–363.

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