Recovery of Pt (IV) from aqueous solutions using

Separation Science and Technology

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Recovery of Pt (IV) from aqueous solutions using magnetic functionalized cellulose with quaternary amine Ahmed M. Yousif, Sherif A. Labib, I.A. Ibrahim & Asem A. Atia To cite this article: Ahmed M. Yousif, Sherif A. Labib, I.A. Ibrahim & Asem A. Atia (2018): Recovery of Pt (IV) from aqueous solutions using magnetic functionalized cellulose with quaternary amine, Separation Science and Technology To link to this article: https://doi.org/10.1080/01496395.2018.1534866

Published online: 22 Oct 2018.

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SEPARATION SCIENCE AND TECHNOLOGY https://doi.org/10.1080/01496395.2018.1534866

Recovery of Pt (IV) from aqueous solutions using magnetic functionalized cellulose with quaternary amine Ahmed M. Yousifa, Sherif A. Labiba, I.A. Ibrahimb, and Asem A. Atiaa a Chemistry Department, Menoufia University, Shebin El-Kom, Egypt; bChemistry Department, Central Metallurgical Research and Development Institute, Helwan, Cairo, Egypt

ABSTRACT

ARTICLE HISTORY

A new magnetic cellulose hybrids anchored with quaternary amine moieties were fabricated and used in recovering Pt(IV) ions from acidic solutions. Kinetic and thermodynamic parameters of adsorption process were reported through using batch experiments. An equilibrium uptake capacity of 178 mg g−1 was achieved within 55 min. The adsorption process was found to follow the pseudo-second-order kinetics and fitted well Langmuir and D-R adsorption isotherms revealing monolayer surface coverage and physisorption mechanism, respectively. At low pH values and high chloride concentration ranges, ion exchange was proposed as the predominant mechanism for the adsorption of Pt(IV) ions on the sorbent. The obtained sorbent showed good durability, easy separation, and regeneration from the adsorption medium.

Received 11 December 2017 Accepted 8 October 2018

Introduction Platinum is one of the precious metals used in many applications, including in electronic devices, catalysis, biomedical devices, space materials, and jewelry. However, its limited resources are becoming depleted. To meet the future demand and conserve resources, it is necessary to process spent platinum-containing materials, such as electronic scraps, catalysts, and used equipment.[1] The natural resources of platinum and platinum group metals (PGMs) are limited and mainly found in South Africa, North America, Canada, and Russia. South Africa is considered as the world’s largest producer of platinum. The PGMs concentration is low, in the range of 2–10 ppm, and generally associated with base metal sulfide minerals.[2] There are many methods used in the recovery of PGMs and the most commonly used preconcentration methods are solvent extraction,[3] adsorption/ion exchange, and coprecipitation.[4–6] Adsorption processes using a solid chelating resin represent an important alternative for the recovery of low concentrations of PGMs because they have advantages of ease in phase separation and high enrichment efficiency; moreover, they require no organic solvent.[7–9] The complicated chemistry behavior of the PGMs makes their separation difficult. So, it is an urgent matter to develop more efficient, simple, and selective preconcentration methods through the exploitation of their fine chemistry differences.[10]

Platinum; recovery; kinetic; thermodynamic

Biosorbents have considerable interest over other sorbents in the recovery of precious metal ions from aqueous solutions because they are inexpensive and eco-friendly. Cellulose-based adsorbents have advantages for possessing high adsorption rates because of their hydrophilic nature as well as fine fibrous morphology. In addition, the adsorbed metal may be recovered simply by burning the cellulose matrix.[11] Organic-inorganic composites show better characteristics if compared to individual organic and inorganic components. Presence of an inorganic component in the polymer matrix leads to improved properties such as enhanced magnetic properties, increased heat resistance, and unique surface and textural properties.[12] Many efforts were conducted toward recovery of platinum ions from different sources using modified sorbents with different functional groups or active sites as tri-noctylamine hydrochloride,[13] acrylic copolymers,[14] polyethyleneimine,[15] thiolated mesoporous silica,[16] polyamine functionalized polystyrene,[17] polyacrylonitrile,[18] l-lysine,[19] and glycine.[20] In this study, a cellulose-magnetite hybrid was fabricated and anchored with quaternary amine moieties. The characteristics of the obtained sorbents were investigated using various analytical and spectral tools. The adsorption properties for the sorbent toward Pt(IV) ions from aqueous solutions at various experimental conditions were studied. The obtained adsorption data

CONTACT Sherif A. Labib [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst. © 2018 Taylor & Francis

KEYWORDS

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were analyzed to get the adsorption parameters such as equilibrium adsorption capacity, kinetics, and thermodynamics of the adsorption process. Applicability of the sorbents to flow streams was examined using batch method.

Materials and methods Chemicals Cellulose powder, sodium hydroxide (NaOH), urea, hydrochloric acid, dimethyl formamide (DMF), thionyl chloride (SOCl2), and ammonium hydroxide were Sigma-Aldrich products (Germany). Ferric chloride hexahydrate (FeCl3.6H2O) with 98% purity and ferrous chloride (FeCl2) with 98% purity were purchased from Oxford. Tetraethylenepentamine (TEP) and glycidyl trimethylammonium chloride (GTA) were Aldrich products. A Platinum wire was used as a source of Pt(IV) ions. All other chemicals were of analytical reagent grade and were used as received. Double distilled water was used throughout the experimental work.

Preparation of sorbent Synthesis of chlorinated cellulose In 200 mL of N,N-dimethyl formamide (DMF) was suspended a sample of 10 g cellulose (Cell) previously activated at 80°C for 12 h, followed by the slow addition of 35 mL of thionyl chloride (SOCl2) at 80°C, under mechanical stirring. After the addition was complete, stirring was continued at the same temperature for another 4 h. The obtained cellulose chloride (CellCl) was washed with several portions of dilute ammonium hydroxide solution and the supernatant after each treatment was removed to bring the pH to neutral. The suspension was exhaustively treated with distilled water to complete the washing process. The solid was then separated by filtration and dried under vacuum at room temperature. [21] Synthesis of chlorinated magnetic cellulose A sample of 4 gm of Cell-Cl was dispersed in 500 mL solution of 1.5 mol L−1 sodium hydroxide which referred to as solution 1. The magnetic particles were prepared by using the coprecipitation method. [22–24] 0.04 mol of FeCl3.6H2O and 0.02 mol of FeCl2 were dissolved in 50 mL of 0.5 M HCl solution. This mixture was added to solution 1 dropwise at 80 °C under vigorous stirring for 30 min. An external magnetic field was used to separate obtained particles which then repeatedly washed with distilled water followed

by drying at 50 °C until complete dryness and referred to as M-Cell-Cl. Immobilization of magnetic cellulose with amine moiety 10 mL of tetraethylenepentamine (TEP) was reacted with 4.0 g M-Cell-Cl under reflux and with mechanical stirring for 3 h. The solution was filtered through a sintered glass filter and the solid was dried under vacuum at room temperature for 24 h and referred to as RPA. [25] Immobilization of magnetic cellulose with quaternary amine moiety In 50 mL of DMF/water mixture (1:1, v/v) was suspended 4.0 g of RPA. 6 mL of glycidyl trimethylammonium chloride (GTA) was added to this mixture and then heated at 60 oC for 24 h. The product was washed with distilled water, methanol, acetone, and then dried in air and referred to as RQA. [26] Preparation of solutions A platinum standard stock solution (1000 ppm) was prepared by dissolving platinum wire in aqua regia (HCl+ HNO3, 3 + 1). The obtained solution was evaporated almost to dryness. Conc. HCl (5 mL) was added and the solution was evaporated once again. The residue was dissolved in few drops of 1 M HCl. The obtained solution was diluted to 50 mL with 0.1 mol L–1 HCl. Working standards were prepared by serial dilution of the stock solution with 0.10 M HCl. Hydrochloric acid solution (0.1 M) was used as a blank solution. [27] A solution of 2 M HCl + 0.5 M thiourea was used to elute Pt(IV) ions from loaded RQA. Equipment and methods of characterization FT-IR spectra were recorded on a Pye-Unicam Sp-88 spectrophotometer between 4000 cm−1 and 400 cm−1 using KBr pellets technique. The surface area and the pore distribution were calculated by Brunauer-EmmettTeller (BET) and Barret-Joyner-Halenda (BJH) equations at 77 ± 1 K by a Quanta chrome NOVA automated gas sorption system using N2 gas as the adsorbate. Thermogravimetric analysis (TGA) was carried out in a nitrogen atmosphere using Shimadzu DT/ TG-50 with a heating rate of 10°C/min and flow rate of N2 of 20 mL/min. The morphology of the sorbent particles was examined using an environmental Scanning Electron Microscope (SEM). The use of environmental SEM allows the direct observation of materials without

SEPARATION SCIENCE AND TECHNOLOGY

previous metallization of the samples. The topography of the samples was observed using secondary electron flux while the backscattered electrons were used for the identification and localization of metals at the surface of the materials by phase contrast. Detection of elements at the surface of sorbents was carried out using Energy Dispersive X-ray microanalysis ESEM-EDX system (Quanta FEG 200, equipped with an Oxford Inca 350 (EDX) system). The residual concentration of Pt(IV) ions was determined in the form of the PtCl62– complex in the hydrochloric acid solution whose concentration varies from 0.01 to 2 M by measuring the absorbance at 260 nm using spectrophotometric method.[27] A 25 mL of Pt solution was heated on a water bath to remove nitrogen oxides, acidified with conc. HCl, and then left to evaporate to near dryness. After quantitative transfer to a measuring flask with 0.1 mol L–1 HCl, an aliquot of the solution is pipetted in the cell and the platinum is determined. The absorbance of the solution was measured at 260 nm against the reagent blank. Hydrochloric acid (0.1 M) was used as a blank solution. The calibration curve was constructed by measuring the absorbance against concentration at 260 nm. The calibration curve was obtained by measuring the absorbance of standard solutions of Pt(IV) ions in the range 0.4–1.8 ppm. The correction coefficient was 0.994. The measurements were carried out using Cicell/7400 UV-VIS spectrophotometer.

Uptake experiment using batch method Effect of pH The adsorption measurements were conducted by placing 0.1 g of RQA in several flasks each contains 100 mL of Pt(IV) ions solution (150 ppm) at different initial pH values in the range of 1–4 using HCl. The contents of flasks were equilibrated at 25ºC and 250 rpm for 1 h. The uptake of Pt(IV) ions was obtained by measuring the residual concentration of Pt(IV) ions spectrophotometrically. Each data point of batch experiments was taken as the average of three measurements. Effect of contact time The uptake of Pt(IV) ions as a function of contact time was carried out by shaking 0.1 g of RQA with 100 mL (150 ppm) of Pt(IV) ions solution at 25ºC and initial pH = 2. Two milliliters of the supernatant were taken at different time intervals where the residual concentration of the metal ions was determined as mentioned above.

3

Adsorption isotherms Portions of 0.1 g of RQA were placed in several flasks containing 100 mL of Pt(IV) metal ions solution at different concentrations (100–150 ppm). The acidity of all solutions was adjusted to pH = 2 and the flasks were conditioned at 25, 35, 45, and 55ºC for 1 h. Two milliliters of the solution were taken at the end of the experiment where the residual concentration of the metal ion was determined.

Regeneration experiments To a 100 mL (100 ppm) of Pt(IV) solution was equilibrated 0.1 g of RQA for 30 min at pH = 2. After the determination of the metal ion uptake, the resin was recovered and washed thoroughly with water. The adsorbed metal ions were released from the resin by shaking with 25 mL mixture of (2 M HCl + 0.5 M thiourea) and then washing with distilled water for reuse in the second uptake run. The efficiency of regeneration for each resin was calculated according to Eq.1 Efficiency ¼

Uptake in the 2nd run 100 Uptake in the 1st run

(1)

Results and discussion Sorbents characterization The synthesis route of RQA is given in Scheme 1. The structure of the prepared sorbent and its intermediates were elucidated using various analytical and spectral tools. FT-IR spectra of cellulose, MCell-Cl, and RQA were presented in Fig. 1. The spectrum of cellulose (Fig. 1A) showed absorption bands at 3362, 2903, and 1056 cm−1 referring to υOH, υC-H and antisymmetric stretching vibration of C-O-C bridge. The weak stretching peak (Fig. 1B) at 878 cm−1 assigned to C-Cl, the strong band at 582 cm−1 assigned to Fe-O vibration, and the bands at 3420 and 1630 cm−1 assigned to the stretching and bending vibrations of hydroxyl groups on the surface of Fe3O4 revealed that the cellulose was coated on the surface of magnetite.[28] The characteristic peak at 2924 cm−1 (Fig. 1C) corresponding to aliphatic C–H and the peak at 1635 cm−1 corresponding to the N– H stretching bond elucidate the formation of RQA.[22] The outlined results of the FT-IR analysis refer to the functionalization of cellulose layer shell (stretched on the magnetic Fe3O4 particles) with amine moiety. Thermogravimetric analyses (TGA) of cellulose and RQA showed diverse behavior as shown in Fig. 2. The TGA thermogram of cellulose displayed a dehydration weight loss of 6% in the temperature range 22–91°C. The main thermal decomposition step of cellulose was

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Scheme 1. Simplified representation for synthesis of RQA.

observed in the temperature range 276–360°C with a sharp weight loss of 76% corresponding to the combustion of cellulose giving finally a 6% of ash residue. The weight loss in the case RQA showed a gradual weight loss 5% in the temperature range 42–254°C according to dehydration of the resin then showed a gradual weight loss of 24% in the range 253–380°C and 62% of the weight of RQA is still stable enough in the range of 380–600°C. This indicated that RQA has higher thermal stability than pure cellulose.

BET-surface area, BJH pore volume and the average pore diameter for RQA was calculated from the N2 adsorption isotherm method. Values of the surface area, the pore volume, and the average pore diameter were found as117.1 m2 g−1, 0.20 cm3 g−1, and 17.4 Å for RQA. These results indicated enhancements in textural properties of RQA (which is considered as a mesoporous material) if compared with cellulose itself. The high surface area of RQA (117.1 m2 g−1) if compared with cellulose (21 m2 g−1) may be explained by the


5

corresponding to the adsorbed Pt(IV) ions on RQA/ Pt if compared with the free RQA resin (Fig. 3C)

Adsorption studies

Figure 1. Infrared spectra of cellulose (A), Cell-Cl (B) and RQA(C).

Effect of pH The results of pH experiments showed in Fig. 4A. demonstrated that the maximum adsorption capacity has occurred at pH 2.0 that metal ions were adsorbed on protonated amine groups of RQA.[30] The speciation of Pt(IV) depends on both pH and chloride concentrations. At acidic solutions, the adsorption mechanism of Pt(IV) on RQA is assumed to be electrostatic attraction and ion exchange as shown in Scheme 2. At low pH, platinum is usually present in solution in its most stable form (Pt(IV)) that can forms stable complexes especially with amino group chelation sites of RQA, due to their characteristics of soft acid. Moreover, the amount of chloride in the solution is high enough to favor the formation of chloro-anionic species (PtCln (n−4)−) that will be adsorbed on protonated amine groups of RQA.[7] The protonation of amine groups on the RQA induced an electrostatic attraction of anionic metal complexes and increased the number of available binding sites for Pt (IV) metal ions uptake. In the presence of chloride ions, the interaction between metal anions and active sites of the RQA are shown below: 2RNH3 þ Cl þPtCl6 2 ! ðRNH3 þ Þ2 PtCl6 2 þ2Cl (2)

Figure 2. Thermogravimetric analysis (TAG) of cellulose (A) and RQA (B).

formation of proposed stretched cellulose thin layer on the surface of magnetite particles.[29] The surface micrograph of RQA before and after adsorption of Pt(IV) ions was carried out using SEM analysis as shown in Fig. 3A,B. The sample of RQA before adsorption of Pt(IV) ions showed irregular uniform monoliths (Fig. 3A). The surface image of the sorbent loaded with Pt(IV) ions (RQA/Pt) showed agglomeration of small particles into large lumps with a shiny appearance (Fig. 3B). Formation of such lumps may be explained by particle-particle bridging through Pt(IV) ions. The bright appearance of RQA/Pt may indicate the adsorption of Pt(IV) ions on the surface of RQA and was confirmed by EDX measurements shown in Fig. 3C, D. In Fig. 3D, the sorbent showed signals at 2.1 ,9.5 ke.V,

At lower pH, the excess of Cl− anions from HCl (used for adjusting pH) involves competition between the Pt(IV) chlorocomplexes and the counter anions for the sorption on protonated sites. Increasing the pH results in a decrease of this competition and the predominance of species more favorable for sorption around pH 2.0. At higher pH values decrease the sorption percentage may be explained by the presence of less-adsorbable Pt(IV) species such as hydroxyl complexes. Also, the decreased adsorption observed may be due to the electrostatic repulsion between surface sites of adsorbent and Pt(IV) metal ions. Several previous studies in the literature also reported that the higher adsorption of Pt(IV) was obtained at pH 2.0–3.0. [31–33] Effect of contact time The adsorption of Pt(IV) ions on RQA as a function of contact time at initial pH = 2 and 25°C is shown in Fig. 4B. The equilibrium uptake capacity of 178 mm g−1 was achieved within 55 min. The observed fast kinetics and high adsorption capacity for the resin RQA may be attributed to the hydrophilic and swelling characters of cellulose

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Figure 3. SEM photomicrographs of free (A) and Pt-loaded RQA(B). (C) and (D) graphs represent EDX measurements for free and Ptloaded RQA, respectively.

Figure 4. (A) Effect of pH on the uptake of Pt(IV) ions from an initial concentration 150 ppm at 25°C using RQA, (B) effect of contact time on the uptake of Pt(IV) ions from an initial concentration of 200 ppm at 25°C using RQA.

constituting the hybrid and may be due to its higher concentration of active sites capable to bind Pt(IV) ions. The obtained adsorption/time data were applied to two simplified kinetic models, including pseudo-firstorder model and pseudo-second-order model. The pseudo-first order model was expressed in Eq. 3.[34] k1 t (3) logðqe qt Þ ¼ log qe 2:303

where k1 is the pseudo-first-order overall rate constant (min−1), qe, and qt (mg g−1) refer to the amount of metal ion adsorbed at equilibrium and at time t, respectively. The pseudo-second-order model was expressed in Eq. 4[35] t 1 1 ¼ þ t 2 qt k2 qe qe

(4)


7

Scheme 2. The adsorption of hexa-chelated platinum chloride anion, [PtCl6]2- on the sorbent active sites in high chloride solutions.

where k2 is the pseudo-second-order rate constant of adsorption (g mmol−1 min1), qe, and qt (mmg g−1) refer to the amount of metal ion adsorbed at equilibrium and at time t, respectively. The kinetic parameters were determined either from the linear plot of (t/qt) versus t (Fig. 5A) for pseudo-second order (the related data were mentioned in Table 1) or from that of log (qe - qt) versus t for pseudo-first order (related Figure and data were not involved). The validity of each model was checked by the fitness of the straight line (R2 ) as well as the consistency between calculated and experimental values of qe which were reported in Table 1. The data were found to fit well the pseudo-second-order model. The interaparticle diffusion model may be listed as in Eq. 5.[35]

qt ¼Xi þkdif t 0:5

(5)

where qt is the amounts of metal ion adsorbed at time t, kdif is intraparticle diffusion rate constant (mmol g min−0.5), and Xi represents the boundary layer diffusion effects. The plot of qt versus t0:5 was shown in Fig. 5B. The plot gave straight lines with intercepts values of 62.7. The large value of intercept indicates that the intraparticle diffusion is not the rate determining step and there is a considerable effect of the boundary layer. On the other hand, the chemical reaction between active sites and Pt(IV) ions may be the rate-determining step in this case.[36] The steeper slope indicated the faster uptake of Pt(IV) ions due to the higher concentration of the active sites in the surface of sorbent.

Figure 5. (A) Pseudo-second-order plot of the uptake of Pt(IV) ions from an initial concentration of 200 ppm at 25°C using RQA, (B) the intraparticle diffusion plots for the adsorption of Pt(IV) ions on RQA.

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Table 1. Pseudo-first-order and pseudo-second-order parameters for the adsorption of Pt(IV) ions on RQA from an initial concentration of 200 ppm, pH = 2 and 25°C. Pseudo-first order qe (exp) K1 min−1 qe (calc) (mg mi (mg g−1))(mg −1 −1 i2 (min ) g−1) Resin g ) RQA 177 0.09 229 0.92

Pseudo-second order K2 (g mg−1 min−1) 0.96

qe (calc) (mg R2 g−1) 188 0.99

Equilibrium adsorption The adsorption isotherms of Pt(IV) ions on the investigated resin at different temperatures were shown in Fig. 6A. At 25°C and pH = 2, maximum uptake values were achieved at 177 mg g−1 (0.97 mmol g−1). The adsorption curves indicated that the uptake of platinum decreases as the temperature increases. This might be due to the fact that the interaction between Pt(IV) ions and the active groups of the studied resin was weaker at higher temperatures. At a fixed temperatures, the uptake increases as the equilibrium concentration increase until reaching equilibrium. The adsorption data obtained were treated according to Langmuir equation as it is the most widely used isotherm equation for modeling equilibrium data. The linear form of Langmuir equation is expressed in Eq. 6.[36] Ce Ce 1 ¼ þ (6) qe Qmax KL Qmax where Ce is the equilibrium concentration of Pt(IV) ions solution (mmol L−1), qe is the amount adsorbed of Pt(IV) ions at equilibrium concentration (mmol g−1), Qmax is the maximum adsorption capacity of Pt(IV) ions (mmol g−1) and KL is the Langmuir binding constant which is related to the energy of adsorption (L mmol−1). Plotting Ce/qe against Ce gave a straight line with slope and intercept equal to 1/Qmax and 1/KLQmax, as shown in Fig. 6B. From the values of slopes and

intercepts, the values of Qmax and KL for adsorption of Pt(IV) ions at different temperatures were calculated and reported in Table 2. Data of Table 2 showed that the values of Qmax obtained from Langmuir plots were close to that experimentally obtained. These findings reveal that the adsorption process was mainly proceeded through monolayer coverage according to Langmuir model. It was also seen that both Qmax and KL values decrease as the temperature increases (Table 2), this may be attributed to the bond weakening due to the increased vibrational energy with increasing temperature.[37] The essential features of Langmuir adsorption isotherm can be expressed in terms of separation factor (RL) which is expressed by Eq. 7.[38] RL ¼

1 1 þ KL Co

(7)

where KL is the Langmuir equilibrium constant and Co is the initial concentration of metal ion solution. The values of RL indicates the adsorption process to be irreversible (RL = 0), favorable (0 < RL < 1) and unfavorable (RL = 1). At 25°C, The values of RL lie between 0.2 and 0.27 for all concentration ranges indicating the suitability of adsorption process. Thermodynamic calculations. To obtain the thermodynamic parameters of the adsorption reaction of Pt (IV) ions on RQA, the values of KL at different temperatures were processed according to Van’t Hoff equation. [36] ln KL ¼

ΔH o ΔSo þ RT R

(8)

where ΔHº (kJ mol−1) and ΔSº (kJ mol K−1) are enthalpy and entropy changes, respectively, R is the gas constant. Plotting ln KL against 1/T as shown in

Figure 6. (A) Adsorption isotherms of Pt(IV) ions on RQA at different temperatures and pH = 2, (B) Langmuir plots for the adsorption of Pt(IV) ions on RQA.


Table 2. Values of Langmuir parameters for adsorption for Pt (IV) ions on RQA at different temperatures and at pH = 2. RQA Temp (°C) 25 35 45 55

qe (mg g−1) 175 171 166 161

Qmax (mg g−1) 181 167 156 153

KL (L mg−1) 0.019 0.017 0.015 0.013

R2 0.99 0.98 0.99 0.99

Fig. 7 gave straight lines of slopes and intercepts equal to ΔHº/R and ΔSº/R, respectively from where values of ΔHº and ΔSº were calculated and reported in Table 3. The negative values of ΔH° indicate the exothermic nature of the adsorption process and the negative values of ΔS° suggest high orderness of adsorption system at equilibrium due to the interaction between active sites and metal ions. The observed decrease in both values of Qmax and KL with elevated temperatures may be another indicator for the exothermic nature of the adsorption process. The Gibbs free energy change (ΔG°) is the fundamental criterion of spontaneity. (ΔG°) values were calculated from Eq. 9.[36] ΔG ¼ ΔH TΔS

(9)

The values of ΔGº were listed in Table 3. The obtained negative values of ΔG° indicate the spontaneity of the adsorption process.[39] Langmuir isotherm is insufficient

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to explain the chemical or physical properties of the adsorption process. However, the mean adsorption energy (E) calculated from the D-R isotherm can provide important information about these properties. The D-R isotherm apart from being an analog of the Langmuir isotherm is more general because it does not assume a homogenous surface or constant adsorption potential, and may be expressed as follows.[35] ln qe ¼ ln Qmax βε2

(10)

where qe is the adsorption capacity at equilibrium concentration (mg g−1), Qmax is the maximum adsorption capacity (mg g−1), ε is Polanyi potential, and β is the activity coefficient related to adsorption mean free energy (mol2 kJ−2) which may be calculated from plotting (ln qe) against ε2 as shown in Fig. 8. Polanyi potential (ε) is calculated using the following equation[35] 1 (11) ε ¼ RT ln 1 þ Ce where R is the gas constant (8.314 × 10−3 kJ mol−1 K−1), T is the temperature (K), and Ce is the equilibrium concentration in (mg L−1). The mean adsorption energy (E, kJ mol−1) can be obtained from the β value of the D-R isotherms using the following equation 1 E ¼ pffiffiffiffiffi 2β

(12)

For E < 8 kJ/mol, the adsorption process follows physisorption mechanism. If E is in the range 8–16 kJ/mol, the adsorption process follows chemisorption mechanism. KDR , ß, and E values listed in Table. 4 indicate that adsorption of Pt(IV) ions on the resin probably proceed via physisorption mechanism. The strength of bonding

Figure 7. Van’t Hoff plots for the uptake of Pt(IV) ions on RQA. Table 3. Thermodynamic parameters for the adsorption of Pt (IV) ions on RQA. Resin RQA

ΔH° (kJ mol−1) −18.7

ΔS° (kJ mol−1 K−1) −0.04

Temp (K) 298 308 318 328

ΔG° (kJ mol−1) −6.78 −6.38 −5.98 −5.58

TΔS° (kJ mol−1) −11.92 −12.32 −12.72 −13.12

Figure 8. DR-isotherm for the adsorption of Pt(IV) ions on RQA.

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Table 4. Values of D-R isotherm parameters for the adsorption of Pt(IV) ions. RQA Temp (°C) 25 35 45 55

Qmax (mg g−1) 228 216 212 182

β¶(J2 mol−2) 0.0144 0.0142 0.014 0.013

E (kJ mol−1) 5.90 5.93 5.97 6.20

R2 0.996 0.977 0.996 0.990

Table 5. Comparison of the adsorption capacities of Pt(IV) ions on different sorbents. Adsorbent Lysine modified cross-linked chitosan resin Thiourea-modified chitosan microspheres Glycine-modified chitosan Bayberry tannin immobilized collagen fiber membrane Fe3O4 nanoparticles PA-Lignin Amberlite IRC 718 Poly(vinylbenzylchloride–acrylonitryle– divinylbenzene) modified with tris(2aminoethyl)amine RQA

Adsorption capacity (mg g−1)

Ref.

129.26 129.87 122.47 45.8

(7 9)

13.27 42.93 66.334 245

[40]

178

This work

[20] [31]

[41] [42] [43]

between the metal ion and resin clearly decreases as the temperature increases. RQA showed higher adsorption capacity if compared with reported values achieved by other sorbents as shown in Table 5. Regeneration experiments Regeneration efficiency of RQA reached 65% ± 0.4% using 25 mL of 2 M HCl and up to 90 ± 0.5% by using 25 mL of a mixture of 2 M HCl and 0.5 M thiourea after four cycles. The regenerated resin showed uptake capacities comparable to the fresh ones.

Conclusions A new magnetic cellulose hybrids anchored by quaternary amine moieties were fabricated and used to recover Pt(IV) ions from acidic solutions. The new sorbents showed higher uptake capacity of 178 mg g−1 at faster kinetics if compared to other resins reported in the literature. The observed fast kinetics and high adsorption capacity for the resin RQA may be attributed to the hydrophilic and swelling characters of cellulose constituting the hybrid and may be due to its higher concentration of active sites capable to bind Pt(IV) ions. The adsorption reaction was found to follow the pseudo-second order kinetics and fitted well with Langmuir and D-R adsorption isotherms. At low pH

values and high chloride concentration ranges, ion exchange was proposed as the operating mechanism for the adsorption of Pt(IV) ions by the sorbent. The obtained sorbents showed good durability, easy separation, and regeneration from the adsorption medium. These characteristics obtained from batch experiments make the presented new sorbent promising in the field of platinum recovery from chloride solutions.

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