Journal of Hazardous Materials 279 (2014) 597–604
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Removal of CdTe in acidic media by magnetic ion-exchange resin: A potential recycling methodology for cadmium telluride photovoltaic waste Teng Zhang ∗ , Zebin Dong, Fei Qu, Fazhu Ding, Xingyu Peng, Hongyan Wang, Hongwei Gu Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s • • • •
Sulfonated magnetic microsphere was prepared as one strong acid cation-exchange resin. Cd and Te can be removed directly from the highly acidic leaching solution of CdTe. Good chemical stability, fast adsorbing rate and quick magnetic separation in strong acidic media. A potential path for recycling CdTe photovoltaic waste.
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Article history: Received 24 February 2014 Received in revised form 30 June 2014 Accepted 2 July 2014 Available online 21 July 2014 Keywords: Cadmium telluride Magnetic microsphere Magnetic separation Ion-exchange resins
a b s t r a c t Sulfonated magnetic microspheres (PSt-DVB-SNa MPs) have been successfully prepared as adsorbents via an aqueous suspension polymerization of styrene-divinylbenzene and a sulfonation reaction successively. The resulting adsorbents were confirmed by means of Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS) and vibrating sample magnetometer (VSM). The leaching process of CdTe was optimized, and the removal efficiency of Cd and Te from the leaching solution was investigated. The adsorbents could directly remove all cations of Cd and Te from a highly acidic leaching solution of CdTe. The adsorption process for Cd and Te reached equilibrium in a few minutes and this process highly depended on the dosage of adsorbents and the affinity of sulfonate groups with cations. Because of its good adsorption capacity in strong acidic media, high adsorbing rate, and efficient magnetic separation from the solution, PSt-DVB-SNa MPs is expected to be an ideal material for the recycling of CdTe photovoltaic waste. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cadmium telluride (CdTe) is an important semiconductor material with high commercial value. It is commonly applied as light absorber due to its unique advantages including superior light absorption properties, outstanding module performance at high temperature, short energy payback time and the smallest carbon footprint among current photovoltaic (PV) technologies on a life cycle basis [1–3]. Recently, the efficiency record of CdTe solar cell and its module have been refreshed to 19.6% [4] and 16.1% (0.72 m2 module area) [5], respectively. CdTe PV is expected to have a continuous market growth [6,7].
∗ Corresponding author. Tel.: +86 1082547146. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.jhazmat.2014.07.024 0304-3894/© 2014 Elsevier B.V. All rights reserved.
However, the growth of CdTe PV production and installation will face to two major concerns: (1), potential environmental impacts of hazardous cadmium contamination; and (2), possible shortage of tellurium as a scarce element in future. Meanwhile, Te is normally a by-product of Cu production in industry. Overexploitation of Cu to meet the supply of Te will result in a major disposal problem of refinery wastes, and will seriously affect the market of Cu production. In order to resolve the above problems, it is always in need to develop proper methodology for the recycling of CdTe PV waste [8]. A variety of technologies and methodologies for the recycling of CdTe PV modules have been developed, and some of them are even in commercial operation [9]. Compared with the mechanically scraping and vacuum blasting, diluted acids leaching is a widely applied process in industry for completely removing all metals from glass substrates, which can be used to treat almost
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all types of waste streams including intact or crushed modules, and production residues. In addition, some hydrometallurgical processes (e.g. precipitation, liquid–liquid extraction, electro-winning, ion-exchange and oxidation/reduction) have also attracted much attention because these technologies can be commercialized in short time. In the process of First Solar (U.S.), CdTe film leached in sulfuric acid/hydrogen peroxide solution is precipitated in three stages by increasing the solution’s pH value. The precipitated metals are filtered for dewatering and then the resulting filter cake is packaged for metals recovery [10]. However, the grading sedimentation and filtering process are tedious, and the complicated solubility of Te with different valence state (For example, Te(IV) is sparingly soluble in acid media, insoluble in neutral media, and soluble in alkaline media; and Te(VI) is soluble in acid media, and insoluble in alkaline media [11].) will unavoidably cause the loss of Te in the precipitation solution. Fthenakis et al. used cation-exchange resins to separate Cd from Te in sulfuric acid media. In this process, Cd could be retained in the cation-exchange resin, while Te remained in the solution. Subsequently, Cd was eluted from the ion-exchange resin by acid, and then was recovered by electrowinning; while the Te remained in the solution was precipitated by sodium carbonate and sodium sulfide [11]. During this process, the ion adsorbents need to be packed into the ion-exchange columns. Hence, the resulting column pressure will limit the throughput of CdTe acid leaching solution. Additionally, the ion-exchange process commonly suffers from the problems of resin fouling and the unavoidably complicated regeneration process. Electrochemical approach has been carried out for removal, separation and retrieval of CdTe and CIGS films from PV module waste [12–14]. However, although the recycling technology can be easily integrated in a production line to manage the waste disposal, the concentration of metals ions in the electrolyte should usually reach several thousands ppm (mg/L) in an electrolytic process so that to keep this process efficient and economy. Thus, it is necessary to develop an enrichment technology of Te and Cd element from an acid leaching solution with high efficiency, high throughput and convenient operation process. Recently, there is a growing interest to use micro/nanoscale superparamagnetic adsorbents for the removal and preconcentration of heavy metals (Cd(II) [15–17], Cr(VI) [16,18], Pb(II) [15,16], As(V) [18], Hg(II) [16,19], La(III) [20], etc.) from water. The magnetic adsorbents can be easily separated from the solution via an external magnetic field, and the micro/nano size allows them to have high adsorption capacity and high adsorbing rate for metal ions in spite of low amount of consumption. Most studies were focused on removing the metal ions from nearly neutral aqueous solution with low concentration, but it is seldom reported that the Cd or Te ions are separated from acid solution directly by superparamagnetic adsorbents because the high affinity of adsorbents with protons will limit their ion-exchange capability for cations in acid solutions. In the present work, a sulfonated magnetic poly(styrenedivinylbenzene) microspheres (PSt-DVB-SNa MPs) is prepared upon this purpose, and its preparation followed the illustrated process. Fe3 O4 particles (Fe3 O4 MPs) were synthesized via a solvothermal method, which were then coated with a thin layer of silica through a sol–gel process to obtain silica–Fe3 O4 composites (Fe3 O4 @SiO2 MPs). After that, the hydroxyl groups in Fe3 O4 @SiO2 MPs were hydrolyzed with siloxane groups in 3-(trimethoxysilyl)propyl methacrylate (3-MPS) to prepare 3-MPS-coated MPs. Subsequently, the unsaturated bonds in 3MPS-coated MPs were co-polymerized with styrene/divinybenzen to prepare magnetic poly(styrene-divinylbenzene) microspheres (PSt-DVB MPs) in an aqueous suspension. After the final sulfonation process, the sulfonated magnetic microspheres (PSt-DVB-SNa MPs) were prepared as strong acid cation-exchange resin. Its removal
ability for Cd and Te in the acid leaching solution was evaluated subsequently, and the adsorption properties of removal process were also clarified. 2. Materials and methods 2.1. Materials Ferric chloride hexahydrate (FeCl3 ·6H2 O), sodium acetate (CH3 COONa), ethylene glycol (HOCH2 CH2 OH), tetraethyl orthosilicate (TEOS), 3-(trimethoxysilyl)propyl methacrylate (3-MPS, Aldrich), divinylbenzene (DVB), benzoyl peroxide (BPO) and gelatin (CAS: 9000-70-8) were of analytical grade and used as received without further purification. Commercial styrene (St) was washed with 10% NaOH aqueous solution and deionized water to remove the inhibitor, and then distilled under reduced pressure prior to use. 2.2. Synthesis of sulfonated magnetic microspheres (PSt-DVB-SNa MPs) Typically, FeCl3 (13.5 g, 49.99 mmol) and sodium acetate (CH3 COONa) (36 g, 43.90 mmol) were dissolved in ethylene glycol (500 mL) under stirring. The obtained yellow solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 200 ◦ C for 8 h. The resulting black Fe3 O4 MPs were washed with ethanol and deionized water, and then dried in vacuum oven. After that, Fe3 O4 MPs (4 g) were further washed by ultrasonic in a 0.1 M hydrochloric acid solution and deionized water, and then were well dispersed in a mixture of ethanol (1600 mL), deionized water (400 mL) and concentrated ammonia solution (20 mL, 28 wt%). TEOS (2.4 g) was dropped into the above solution. The reaction was continued for 6 h under stirring and the resulting product Fe3 O4 @SiO2 MPs were separated by using a magnet and washed with ethanol and deionized water. Dried Fe3 O4 @SiO2 MPs (6.4 g) were added in 800 mL of ethanol/water (volume ratio = 1) solution, and then the mixture was treated by ultrasonic for 30 min to allow a uniform dispension of magnetic particles in the solution. 3-MPS (12 mL) was dropped into the solution at 60 ◦ C. The reaction was continued under vigorous stirring for 8 h. The prepared 3-MPScoated MPs were washed with ethanol and deionized water, and dried at 60 ◦ C in vacuum oven. The dried 3-MPS-coated MPs (8 g) were then dispersed in a mixture containing 40 g of St, 10 g of DVB and 1 g of BPO under ultrasonic and vigorous stirring. Then the prepared ferrofluid was dropped in 250 mL of aqueous solution containing gelatin (2.5 g) under stirring to prepare an emulsion. The prepared emulsion was gradually heated from ambient temperature to 90 ◦ C for 8 h. After the reaction, PSt-DVB MPs were magnetically separated and washed with warm deionized water (60 ◦ C). The dried PSt-DVB MPs (10 g) were sulfonated with concentrated sulfuric acid for 8 h under mild stirring at 90 ◦ C. The PSt-DVB-SNa MPs were washed with deionized water until pH value was neutral. 2.3. Batch studies and evaluation of adsorption ability The waste CdTe sputtering target was grinded and then was dissolved by the solutions of sulfuric acid and hydrogen peroxide (H2 O2 ). After filtered through a porous membrane with a pore size of 0.45 m, the resulting tellurium–cadmium containing solution was further used for the adsorption experiment. The PSt-DVB-SNa MPs were soaked in deionized water for 12 h and filtered before the adsorption process. Then the adsorption experiments were carried out in the same manner by using batch technique to determine the effect of contacting time, adsorbent dosage, adsorbent cycle times and initial concentration of Cd and
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Te ion. In each type of study, a certain volume of acid solution containing Cd and Te was transferred into wide-mouth bottles using a FINNPIPETTE pipette, and a certain amount of PSt-DVBSNa MPs were then added into each bottle. Afterwards, the bottles were continuously shaken on a shaker platform. At certain intervals of contacting time, the solution was withdrawn and diluted by 5% HNO3 . The concentration of Cd and Te were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES). For the desorption and regeneration experiment, 1.0 g of PStDVB-SNa MPs was added into the acid solution containing tellurium and cadmium (10 mL) to reach the adsorbent equilibrium. Then the PSt-DVB-SNa MPs loaded with Cd and Te were eluted by 1 M H2 SO4 and activated by 1 M NaOH alkaline solution. To test the reusability of the resin, this adsorption–desorption cycle was repeated three times. The tellurium and cadmium concentrations of the filtered liquid samples were diluted by 5% HNO3 and determined by ICPOES after each step. Above experiments were repeated for three times and the average values were used for interpretation of results.
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Fig. 1. Wide-angle XRD patterns of pure Fe3 O4 MPs (A) and Fe3 O4 @SiO2 MPs (B).
2.4. Characterization and analysis methods Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 focus X-ray diffraction with Cu KR radiation. Fourier transform infrared spectra (FTIR) were recorded on a TENSOR27 (Bruker Optic GMBH) IR spectrophotometer in the region of 4000–400 cm−1 as a KBr disc. The surface morphology and elements of each sample were inspected using a scanning electron microscope (SEM; S-4800, Hitachi) with an energy dispersive spectrometer (EDS). Transmission electron microscopy (TEM) images were obtained from a JEM-2000FX transmission electron microscope. Quantitative determination of the metal-ion contents was performed by ICP-OES (Optima 8000, PerkinElmer). The detection limit of ICP for Cd element is 0.2 ppb (g/L) at 26.502 nm. Magnetic characterization was carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7410). The dark field imaging of the samples were observed on an Olympus BX51 optical microscope. The PH value of the solution was measured by a pHB-8A pH meter. 3. Results and discussion 3.1. Preparation and characterization of PSt-DVB-SNa MPs For a magnetic ion-exchange resin used in acid solution, it is necessary to have a magnetic core resisting the strong acid. In the present work, Fe3 O4 MPs synthesized through solvothermal method [21] was further coated with a dense silica shell through the sol–gel reaction of TEOS (ca. Fe3 O4 @SiO2 MPs). Wide-angle XRD patterns of pure Fe3 O4 MPs and Fe3 O4 @SiO2 MPs are shown in Fig. 1. The diffraction peaks of Fe3 O4 MPs (curve A) can be readily indexed to a face-centered cubic structure of magnetite according to JCPDS card No. 19-0629. Fe3 O4 @SiO2 MPs has diffraction peaks similar to that of the parent Fe3 O4 MPs, which indicates that Fe3 O4 MPs is well set in the silica matrix. The broad band centered at 2 = 22◦ (curve B) is assigned to the characteristic reflection from amorphous SiO2 shell (JCPDS 29-0085). According to the SEM image in Fig. 2(A), the spherical Fe3 O4 @SiO2 MPs with size of 300–500 nm can be observed and the core–shell structure can be clearly confirmed by the TEM image shown in Fig. 2(B). The FT-IR spectra of Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B), 3-MPScoated MPs (C), PSt-DVB MPs (D) and PSt-DVB-SNa MPs (E) are shown in Fig. 3. The absorption bands related with Fe–O (595 and 631 cm−1 ) are observed in spectrum A, and the absorption bands
Fig. 2. SEM (A) and TEM (B) images of Fe3 O4 @SiO2 MPs.
assigned to Si–O–Si (1086 and 807 cm−1 ), Si–OH (936 cm−1 ) and Si–O (468 cm−1 ) in spectrum B can confirm the formation of SiO2 shell in Fe3 O4 @SiO2 MPs accompanying with a slight decrease in Fe–O intensity. In order to be well mixed with St and DVB for the aqueous suspension polymerization, Fe3 O4 @SiO2 MPs was firstly modified with 3-MPS molecules, and then was used for preparing
Fig. 3. FT-IR spectra of Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B), 3-MPS-coated MPs (C), PSt-DVB MPs (D) and PSt-DVB-SNa MPs (E).
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PSt-DVB MPs. As shown in spectrum C, the enhanced Si–O–Si signals at about 1300–1093 cm−1 originate from the hydrolytic polycondensation reaction of 3-MPS with SiO2 shell and aliphatic ester bonds of 3-MPS. Moreover, the new peaks at around 2920 and 2852 cm−1 are attributed to the stretching vibration of C–H bonds in 3-MPS. In spectrums D and E, the weak peaks at around 3033 cm−1 and three peaks at 1596, 1500, and 1447 cm−1 are assigned to the stretching vibration of C–H in the phenyl ring and vibration of the phenyl framework. Compared with spectrum D of PSt-DVB MP, the remarkable peaks at 1195 and 1045 cm−1 in spectrum E can be ascribed to the sulfonic groups on PSt-DVB-SNa MPs, which confirms the successful sulfonation of PSt-DVB MPs. Magnetic properties of the prepared samples are investigated by VSM at room temperature in an applied magnetic field and their hysteresis loops are shown in Fig. 4. Pure Fe3 O4 MPs, Fe3 O4 @SiO2 MPs and PSt-DVB-SNa MPs have magnetization saturation values of 75.8, 58.1 and 8.2 emu/g, respectively, at room temperature. It should be noted that the resulted PSt-DVB-SNa MPs still shows strong magnetization and superparamagnetism, which suggests that it can quickly respond to the external magnetic field and be re-dispersed once the external magnetic field is removed. This is advantageous for magnetic separation. In addition, the magnetization values of resulted PSt-DVB-SNa MPs can easily be adjusted by controlling the content of 3-MPS-coated MPs during the aqueous suspension polymerization for preparing the PSt-DVB MPs. As shown in Fig. 5(A), the size of PSt-DVB-SNa MPs is about 500 m, which is observed in dark field by an optical microscope. Upon placement of a NdFeB magnet beside the bottle, the PSt-DVBSNa MPs are quickly attracted to the side of the bottle within five seconds, which illustrates its strong magnetization (Fig. 5(B)). The surface morphology of PSt-DVB-SNa MPs is further examined by SEM (Fig. 5(C)), and the elemental composition on the surface of PSt-DVB-SNa MPs is analyzed by EDS (Fig. 5(D)). The signals of Fe, Si, O, C, S and Na confirm the incorporation of magnetic Fe3 O4 @SiO2 and poly (styrene-divinylbenzene) with sodium sulfonate group (–SO3 Na). When the 3-MPS-coated MPs are enwrapped in the
Fig. 4. Magnetization curves of pure Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B) and 3MPS-coated MPs (C).
PSt-DVB MPs, the PSt-DVB provides a strong protection of 3-MPScoated MPs to maintain their magnetism. The sulfonation reaction generally happened with the PSt-DVB shell, and will not much influence the internal 3-MPS-coated MPs. 3.2. Batch studies and evaluation of adsorption ability 3.2.1. Leaching process of CdTe It has been reported that an optimal leaching condition with sulfuric acid/hydrogen peroxide is 478 mL of 1.0 M H2 SO4 and 4.8 mL H2 O2 per kilogram of CdTe PV modules in consideration of leaching efficiency and solid-to-liquid ratio of leaching system [11]. In our experiment, 1.17 g of CdTe powder was leached in a solution containing 478 mL of 1.0 M of H2 SO4 and 4.8 mL H2 O2 based on the approximate composition of 0.05 wt% Cd and 0.06 wt% Te in PV
Fig. 5. Photo images in dark field photograph (A), separation process by a magnet (B), SEM images (C) and EDS spectroscopy (D) of PSt-DVB-SNa MPs.
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Fig. 6. Concentration of Cd and Te in acid leaching solution after each leaching process (1st time: 1.17 g of CdTe powder is leached with 478 mL of 1.0 M of H2 SO4 and 4.8 mL H2 O2 , 2nd time: 1.17 g of CdTe powder is leached by the 1st time leaching solution, 3rd time: 1.17 g of CdTe powder is leached by the 2nd time leaching solution).
module. Actually if the leaching solution containing Cd and Te can be used again to leach CdTe waste after the first leaching, there will be at least two advantages: (1), the dosage of acid in recycling process will be reduced greatly; and (2), the decrease of H+ concentration after the former leaching can promote the complexion of metal cations in acid media with the sulfonate groups, which will increase the adsorption capacity of the PSt-DVB-SNa MPs. Therefore, the concentration of Cd and Te was firstly investigated after the leaching solution was reused twice or thrice. As shown in Fig. 6, the first leaching process of CdTe was completed within about 90 min, and the equilibrium concentrations of Cd and Te reach about 1112 and 1340 mg/L, respectively. After that, the second 1.17 g of CdTe powder was added into the first leaching solution again. All CdTe powder was dissolved, and the equilibrium concentrations of Cd and Te were about 2274 and 2720 mg/L, respectively. However, the second leaching process achieved the equilibrium at about 200 min, which indicates that the leaching rate of CdTe slightly slows down. When the third 1.17 g of CdTe powder was leached again in the twice-used solution for 300 min, there was still a trace of insoluble substance in the leaching container. Compared with the concentration of Cd (about 3476 mg/L), the concentration of Te (about 3760 mg/L) is less triple that in the first leaching solution. It can be attributed to the poor solubility of tetravalent Te(IV) (Te4+ ) in acidic media. After the initial leaching solution was reused two times, H2 O2 was depleted constantly and not enough to oxidize Te(IV) (Te4+ ) into Te(VI) (TeO6 6− ). Te(VI) is soluble in acid media but Te(IV) is sparingly soluble in acid media, which leads to some insoluble Te(IV) substance. Hence, in our experiment, the acid solution of Cd and Te after twice leaching (twice leaching solution) was used to investigate the adsorption capacity of PSt-DVB-SNa MPs in the following section.
3.2.2. Effect of contacting time on adsorption 10 g of PSt-DVB-SNa MPs was added into 50 mL of twice leaching solution, and the effect of contacting time on the removal of Cd and Te is presented in Fig. 7. As shown in Fig. 7(A), the adsorption equilibrium was reached within 5 min, and about 60% of Cd and 25% of Te were removed. This process is much faster than that using conventional ion-exchange resins, which usually requires at least 20 min or even more time [11]. The efficient adsorption benefits from the small size of PSt-DVB-SNa MPs.
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Fig. 7. Removal efficiency (E, %) of Cd and Te vs. contacting time (A) and adsorption capacity (Qt , mg/g) of PSt-DVB-SNa MPs vs. contacting time (B) (initial concentrations of Cd and Te in twice leaching solution: [Cd], 2274 mg/L; [Te], 2720 mg/L; ratio of PSt-DVB-SNa MPs dosage to twice leaching solution volume: 1 g/5 mL; pH ≈0.4).
In the twice leaching solution of CdTe, the dissolved Cd existed predominantly as uncomplexed Cd2+ , whereas Te existed in the form of Te4+ , TeO(OH)+ and TeO6 6− . Thereinto, the positively charged Cd2+ , Te4+ and TeO(OH)+ can complex with the –SO3 − group on PSt-DVB-SNa MPs. Fig. 7(B) shows that the adsorption capacity of Cd (7.5 mg/g) is much higher than that of Te (2.5 mg/g) in a strong acid solution (pH ≈0.4), which indicates that PSt-DVBSNa MPs has much higher affinity with Cd2+ than with Te4+ and TeO(OH)+ in an acidic media. 3.2.3. Effect of PSt-DVB-SNa MPs dosage on the adsorption Based on the above results, about 60% of Cd and 25% of Te were removed when the dosage of PSt-DVB-SNa MPs in twice leaching solution volume was 1 g/5 mL, the dependence of Cd and Te removal on different adsorbent dosages (0.5 g/5 mL, 1.0 g/5 mL, 2.5 g/5 mL, 5.0 g/5 mL and 10.0 g/5 mL) was investigated to determine the best adsorbent dosage. As shown in Fig. 8(A), the removal efficiency of Cd increased sharply from 44.8% to 93.4% with the increase of
Fig. 8. Removal efficiency (E, %) of Cd and Te (A) and selectivity coefficient (B) vs. dosages of PSt-DVB-SNa MPs (initial concentrations of Cd and Te in twice leaching solution: [Cd], 2274 mg/L; [Te], 2720 mg/L) (Note: The selectivity coefficient (k) for Cd and Te was calculated by E[Cd]/E[Te], where E[Cd] and E[Te] are the removal efficiency of Cd and Te at equilibrium state.).
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PSt-DVB-SNa MPs dosage from 0.5 g/5 mL to 2.5 g/5 mL. After this period, the removal of Cd did not show much improvement by increasing the amount of magnetic adsorbents. When the adsorbent dosage increased to 10.0 g/5 mL, the concentration of Cd in the leaching solution was beyond the detection limit of ICP and all Cd was removed. Similar with Cd, the removal efficiency of Te also increased from 8.3% to 71.5% with the increase of PSt-DVB-SNa MPs dosage from 0.5 g/5 mL to 5.0 g/5 mL, and then showed no significant improvement from 71.5% to 76.8% even if a large amount of adsorbents (10.0 g/5 mL) were used. It is obvious that enough PStDVB-SNa MPs can only remove all Cd2+ , Te4+ and TeO(OH)+ , but leave 23.2% of TeO6 6− in the leaching solution. Fig. 8(B) shows the competitive adsorption of Cd and Te on the PSt-DVB-SNa MPs. The adsorption difference is described as the selectivity coefficient (k) for Cd and Te. When the adsorbent dosage was 0.5 g/5 mL, the removed Cd was much more than removed Te, and the selectivity coefficient reaches 5.4. With the increase of PStDVB-SNa MPs dosage from 0.5 g/5 mL to 5.0 g/5 mL, the selectivity coefficient for Cd and Te decreases sharply from 5.4 to 1.4. After that, it showed no significant change. Obviously the adsorption difference for Cd and Te is also related to the adsorbent dosage besides the affinity. When the adsorbent dosage is less than 5.0 g/5 mL, the sulfonate groups on PSt-DVB-SNa MPs are insufficient relative to cations in twice leaching solution. The removal of Cd predominated because the affinity of sulfonate groups with Cd is stronger than that with Te. In this case, the removal efficiency is decided by the affinity. With the adsorbent dosage increasing over 5.0 g/5 mL, a large number of sulfonate groups are available enough to adsorb cations. The removal efficiency for Cd and Te is mainly decided by the number of sulfonate sites rather than the affinity. Hence all Cd2+ , Te4+ and TeO(OH)+ are adsorbed on PSt-DVB-SNa MPs and the selectivity coefficient for Cd and Te is close to 1.
3.2.4. Effect of multi-steps extraction As the results showed above, 2.5 g of PSt-DVB-SNa MPs can remove 93.4% of Cd and leave about 60% of Te in the 5 mL of leaching solution. However, excessive adsorbents in the leaching solution will cause high viscosity and poor mobility, which is not suitable for the practical mixing process and magnetic separation. It will be helpful to dilute the twice leaching solution before it is used for the adsorption process. Based on the dosage ratio of PSt-DVB-SNa MPs in twice leaching solution (2.5 g/5 mL), 5 mL of twice leaching solution ([Cd], 2274 mg/L; [Te], 2720 mg/L) was firstly diluted five times by 20 mL of deionized water, and then the dilute solution (25 mL; [Cd], 454.8 mg/L; [Te], 544 mg/L) was adsorbed by 2.5 g of PSt-DVB-SNa MPs. After magnetic separation to remove used 2.5 g of PSt-DVBSNa MPs, the resulting solution was treated again by another 2.5 g of PSt-DVB-SNa MPs in order to remove residual Cd and Te in the solution. The above process (adsorbing-separating) was repeated five times. Removal efficiency of Cd and Te for whole process (multiple adsorbing) and for each time is presented in Fig. 9(A) and (B). As shown in Fig. 9(A), about 94.0% of Cd and 52.7% of Te were removed from the acid solution during the first adsorption. After that, removal efficiency of Cd in the leaching solution showed no significant improvement even if a large amount of fresh adsorbents were used. Different from Cd, the removal efficiency of Te cations becomes constant after multi-steps extraction and about 20% of TeO6 6− is still left in the leaching solution. Fig. 9(B) shows that the removal efficiency of Cd and Te for each time decreases sharply after the first adsorption. After four times of adsorption, the concentration of Cd2+ was beyond the detection limit of ICP and removal efficiency of Cd was almost zero, which indicated the removal of all Cd2+ . Similarly, removal efficiency of Te cations for each time
Fig. 9. Removal efficiency (E, %) of Cd and Te for whole process (A) and for each step (B) vs. the number of replication adsorption (initial concentrations of Cd and Te: [Cd], 454.8 mg/L; [Te], 544 mg/L).
decreased from 52.7% to 1.5%. All these can be attributed to gradual decrease of initial concentration for each time. 3.2.5. Adsorption isotherms and adsorption kinetics 25 mL of leaching solution with different initial concentration was treated by 2.5 g of PSt-DVB-SNa MPs for 60 min, and the relationship between adsorption capacity and initial concentration is presented in Fig. 10. The adsorption capacity (Qe ) almost increased linearly with the increase of initial concentration in the low concentration range. However, PSt-DVB-SNa MPs can remove more Cd than Te at the high concentration, which is due to the strong affinity between the Cd2+ and sulfonate groups. It is further proved that the adsorption capacity is controlled by the affinity of cations with functional groups when the adsorbent dosage is relatively insufficient. Adsorption isotherm is used to describe the interactive behavior between solution and adsorbent. In this study, Langmuir model Ce /Qe = 1/(bQmax ) + Ce /Qmax ) and Freundlich model (ln Qe = ln KF + (1/n)ln Ce ) [19] were used to determine the adsorption equilibrium between the adsorbent (PSt-DVB-SNa MPs) and cations (Cd2+ , Te4+ and TeO(OH)+ ). The model parameters obtained by applying Langmuir and Freundlich models to the experimental
Fig. 10. Effect of initial concentration on the adsorption capacity (Qe ) of Cd and Te.
T. Zhang et al. / Journal of Hazardous Materials 279 (2014) 597–604
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Table 1 Adsorption isotherm parameters of Cd and Te catoins. Metal cations
Langmuir isotherm
Cd Te
Freundlich isotherm
Qmax (mg/g)
b (L/mg)
R2
KF (mg/g)
1/n
R2
16.667 9.804
0.010 0.001
0.994 0.992
2.324 3.110
0.445 0.602
0.836 0.992
Table 2 Adsorption kinetic parameters of Cd and Te cations. Metal cations
Cd Te
C0 (mg/L)
2274 2720
Pseudo-first-order
Pseudo-second-order
k1 (min−1 )
Qe.cal (mg/g)
R2
k2 (min−1 )
Qe,cal (mg/g)
R2
0.02 0.016
0.549 0.617
0.812 0.916
0.111 0.080
8.333 2.857
0.999 0.995
data are given in Table 1. Based on the correlation coefficient (R2 ) of each model, the Langmuir model can obviously describe the experiment data more precisely than Freundlich model. The maximal absorption capacity for Cd and Te cations are 16.667 and 9.804 mg/g calculated by Langmuir model, respectively. From the above facts, it can be concluded that the monolayer Langmuir adsorption isotherm is more suitable to explain the adsorption of Cd and Te cations by PSt-DVB-SNa MPs. In order to evaluate the kinetic mechanism which controls the adsorption process, the pseudo-first-order model ln(Qe − Qt ) = ln Qe − k1 t and pseudo-second-order model t/Qt = 1/k2 Qe2 + t/Qt [20] were employed based on the experimental data (in Fig. 7). The kinetic parameters for adsorption of Cd and Te by PSt-DVB-SNa MPs are given in Table 2. Based on the correlation coefficient (R2 ) of each kinetic model, the pseudo-second-order model can obviously describe the experiment data more precisely than pseudo-first-order kinetic model. As we well known, the pseudo-second-order model assumed that the determining rate step may be chemical adsorption, which takes place through a chemical process involving valence forces through sharing or exchange of electron between sorbent and sorbate. It is consistent with the chemical mechanism of ion-exchange process between cations and –SO3 − group on PSt-DVB-SNa MPs (In this case, the adsorption process comes from the high affinity of –SO3 − groups for metal cations, and promoted by ionic bond forces through the electrons sharing between –SO3 − groups for metal cations, which is mainly the chemical reactive adsorption.).
3.2.6. Desorption and regeneration cycles of adsorbents In order to examine the reusability of the prepared PSt-DVBSNa MPs, 2.5 g of PSt-DVB-SNa MPs were added into 25 mL of leaching solution (the initial concentrations of Cd and Te: [Cd], 454.8 mg/L; [Te], 544 mg/L). About 94.0% Cd and 52.7% Te were removed after the first adsorption, and then the PSt-DVB-SNa MPs loaded Cd and Te was regenerated in 1 M H2 SO4 and activated by 1 M NaOH alkaline solution. The adsorption–desorption experiments were repeated three times and the results were shown in Fig. 11. After four adsorption/desorption cycles, the removal efficiency of Te showed no obvious change and the removal efficiency of Cd decreased slightly from 95.0% to 90.7%. These results show the potential reusability of PSt-DVB-SNa MPs for the CdTe recovery from acid media. 4. Conclusions In summary, an effective methodology to synthesize sulfonated magnetic microspheres has been developed and the as-prepared magnetic adsorbents show a number of important features for concentrating Cd and Te. All Cd and 80% of Te are removed from the twice leaching solution of CdTe within 5 min even if its concentration is less than 5.3 mg/L, which is attributed to the strong affinity of Cd ion with sulfonate groups. The adsorption process of Te is different from Cd especially when the adsorbent dosage is relatively insufficient. The difference of adsorbing rates between Cd and Te mainly comes from their different affinity with sulfonate groups. With its unique advantages, the magnetic separation by using PStDVB-SNa MPs can directly concentrate Cd and Te cations from the leaching solution of CdTe, which can be further separated and recycled by the electrochemical or chemical approach. Te anions (TeO6 6− ) left in the leaching solution can be used to prepare some compounds of Te by chemical methods. As an efficient methodology, the magnetic separation is expected to be used in the recycling of CdTe photovoltaic waste. Acknowledgements This work was financially supported by the Foundation of Director in Institute of Electrical Engineering, Chinese Academy of Sciences, China (Grant No. Y140151CSC). We gratefully thank Dr. Dongliang Wang for the dark field imaging of samples. References
Fig. 11. The adsorption–desorption cycles of PSt-DVB-SNa MPs (residual concentration (A) and removal efficiency (B) of Cd and Te in leaching solution).
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of CdTe in acidic media by magnetic ion-exchange resin: A potential recycling methodology for cadmium telluride photovoltaic waste Teng Zhang ∗ , Zebin Dong, Fei Qu, Fazhu Ding, Xingyu Peng, Hongyan Wang, Hongwei Gu Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
h i g h l i g h t s • • • •
Sulfonated magnetic microsphere was prepared as one strong acid cation-exchange resin. Cd and Te can be removed directly from the highly acidic leaching solution of CdTe. Good chemical stability, fast adsorbing rate and quick magnetic separation in strong acidic media. A potential path for recycling CdTe photovoltaic waste.
a r t i c l e
i n f o
Article history: Received 24 February 2014 Received in revised form 30 June 2014 Accepted 2 July 2014 Available online 21 July 2014 Keywords: Cadmium telluride Magnetic microsphere Magnetic separation Ion-exchange resins
a b s t r a c t Sulfonated magnetic microspheres (PSt-DVB-SNa MPs) have been successfully prepared as adsorbents via an aqueous suspension polymerization of styrene-divinylbenzene and a sulfonation reaction successively. The resulting adsorbents were confirmed by means of Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS) and vibrating sample magnetometer (VSM). The leaching process of CdTe was optimized, and the removal efficiency of Cd and Te from the leaching solution was investigated. The adsorbents could directly remove all cations of Cd and Te from a highly acidic leaching solution of CdTe. The adsorption process for Cd and Te reached equilibrium in a few minutes and this process highly depended on the dosage of adsorbents and the affinity of sulfonate groups with cations. Because of its good adsorption capacity in strong acidic media, high adsorbing rate, and efficient magnetic separation from the solution, PSt-DVB-SNa MPs is expected to be an ideal material for the recycling of CdTe photovoltaic waste. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cadmium telluride (CdTe) is an important semiconductor material with high commercial value. It is commonly applied as light absorber due to its unique advantages including superior light absorption properties, outstanding module performance at high temperature, short energy payback time and the smallest carbon footprint among current photovoltaic (PV) technologies on a life cycle basis [1–3]. Recently, the efficiency record of CdTe solar cell and its module have been refreshed to 19.6% [4] and 16.1% (0.72 m2 module area) [5], respectively. CdTe PV is expected to have a continuous market growth [6,7].
∗ Corresponding author. Tel.: +86 1082547146. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.jhazmat.2014.07.024 0304-3894/© 2014 Elsevier B.V. All rights reserved.
However, the growth of CdTe PV production and installation will face to two major concerns: (1), potential environmental impacts of hazardous cadmium contamination; and (2), possible shortage of tellurium as a scarce element in future. Meanwhile, Te is normally a by-product of Cu production in industry. Overexploitation of Cu to meet the supply of Te will result in a major disposal problem of refinery wastes, and will seriously affect the market of Cu production. In order to resolve the above problems, it is always in need to develop proper methodology for the recycling of CdTe PV waste [8]. A variety of technologies and methodologies for the recycling of CdTe PV modules have been developed, and some of them are even in commercial operation [9]. Compared with the mechanically scraping and vacuum blasting, diluted acids leaching is a widely applied process in industry for completely removing all metals from glass substrates, which can be used to treat almost
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all types of waste streams including intact or crushed modules, and production residues. In addition, some hydrometallurgical processes (e.g. precipitation, liquid–liquid extraction, electro-winning, ion-exchange and oxidation/reduction) have also attracted much attention because these technologies can be commercialized in short time. In the process of First Solar (U.S.), CdTe film leached in sulfuric acid/hydrogen peroxide solution is precipitated in three stages by increasing the solution’s pH value. The precipitated metals are filtered for dewatering and then the resulting filter cake is packaged for metals recovery [10]. However, the grading sedimentation and filtering process are tedious, and the complicated solubility of Te with different valence state (For example, Te(IV) is sparingly soluble in acid media, insoluble in neutral media, and soluble in alkaline media; and Te(VI) is soluble in acid media, and insoluble in alkaline media [11].) will unavoidably cause the loss of Te in the precipitation solution. Fthenakis et al. used cation-exchange resins to separate Cd from Te in sulfuric acid media. In this process, Cd could be retained in the cation-exchange resin, while Te remained in the solution. Subsequently, Cd was eluted from the ion-exchange resin by acid, and then was recovered by electrowinning; while the Te remained in the solution was precipitated by sodium carbonate and sodium sulfide [11]. During this process, the ion adsorbents need to be packed into the ion-exchange columns. Hence, the resulting column pressure will limit the throughput of CdTe acid leaching solution. Additionally, the ion-exchange process commonly suffers from the problems of resin fouling and the unavoidably complicated regeneration process. Electrochemical approach has been carried out for removal, separation and retrieval of CdTe and CIGS films from PV module waste [12–14]. However, although the recycling technology can be easily integrated in a production line to manage the waste disposal, the concentration of metals ions in the electrolyte should usually reach several thousands ppm (mg/L) in an electrolytic process so that to keep this process efficient and economy. Thus, it is necessary to develop an enrichment technology of Te and Cd element from an acid leaching solution with high efficiency, high throughput and convenient operation process. Recently, there is a growing interest to use micro/nanoscale superparamagnetic adsorbents for the removal and preconcentration of heavy metals (Cd(II) [15–17], Cr(VI) [16,18], Pb(II) [15,16], As(V) [18], Hg(II) [16,19], La(III) [20], etc.) from water. The magnetic adsorbents can be easily separated from the solution via an external magnetic field, and the micro/nano size allows them to have high adsorption capacity and high adsorbing rate for metal ions in spite of low amount of consumption. Most studies were focused on removing the metal ions from nearly neutral aqueous solution with low concentration, but it is seldom reported that the Cd or Te ions are separated from acid solution directly by superparamagnetic adsorbents because the high affinity of adsorbents with protons will limit their ion-exchange capability for cations in acid solutions. In the present work, a sulfonated magnetic poly(styrenedivinylbenzene) microspheres (PSt-DVB-SNa MPs) is prepared upon this purpose, and its preparation followed the illustrated process. Fe3 O4 particles (Fe3 O4 MPs) were synthesized via a solvothermal method, which were then coated with a thin layer of silica through a sol–gel process to obtain silica–Fe3 O4 composites (Fe3 O4 @SiO2 MPs). After that, the hydroxyl groups in Fe3 O4 @SiO2 MPs were hydrolyzed with siloxane groups in 3-(trimethoxysilyl)propyl methacrylate (3-MPS) to prepare 3-MPS-coated MPs. Subsequently, the unsaturated bonds in 3MPS-coated MPs were co-polymerized with styrene/divinybenzen to prepare magnetic poly(styrene-divinylbenzene) microspheres (PSt-DVB MPs) in an aqueous suspension. After the final sulfonation process, the sulfonated magnetic microspheres (PSt-DVB-SNa MPs) were prepared as strong acid cation-exchange resin. Its removal
ability for Cd and Te in the acid leaching solution was evaluated subsequently, and the adsorption properties of removal process were also clarified. 2. Materials and methods 2.1. Materials Ferric chloride hexahydrate (FeCl3 ·6H2 O), sodium acetate (CH3 COONa), ethylene glycol (HOCH2 CH2 OH), tetraethyl orthosilicate (TEOS), 3-(trimethoxysilyl)propyl methacrylate (3-MPS, Aldrich), divinylbenzene (DVB), benzoyl peroxide (BPO) and gelatin (CAS: 9000-70-8) were of analytical grade and used as received without further purification. Commercial styrene (St) was washed with 10% NaOH aqueous solution and deionized water to remove the inhibitor, and then distilled under reduced pressure prior to use. 2.2. Synthesis of sulfonated magnetic microspheres (PSt-DVB-SNa MPs) Typically, FeCl3 (13.5 g, 49.99 mmol) and sodium acetate (CH3 COONa) (36 g, 43.90 mmol) were dissolved in ethylene glycol (500 mL) under stirring. The obtained yellow solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 200 ◦ C for 8 h. The resulting black Fe3 O4 MPs were washed with ethanol and deionized water, and then dried in vacuum oven. After that, Fe3 O4 MPs (4 g) were further washed by ultrasonic in a 0.1 M hydrochloric acid solution and deionized water, and then were well dispersed in a mixture of ethanol (1600 mL), deionized water (400 mL) and concentrated ammonia solution (20 mL, 28 wt%). TEOS (2.4 g) was dropped into the above solution. The reaction was continued for 6 h under stirring and the resulting product Fe3 O4 @SiO2 MPs were separated by using a magnet and washed with ethanol and deionized water. Dried Fe3 O4 @SiO2 MPs (6.4 g) were added in 800 mL of ethanol/water (volume ratio = 1) solution, and then the mixture was treated by ultrasonic for 30 min to allow a uniform dispension of magnetic particles in the solution. 3-MPS (12 mL) was dropped into the solution at 60 ◦ C. The reaction was continued under vigorous stirring for 8 h. The prepared 3-MPScoated MPs were washed with ethanol and deionized water, and dried at 60 ◦ C in vacuum oven. The dried 3-MPS-coated MPs (8 g) were then dispersed in a mixture containing 40 g of St, 10 g of DVB and 1 g of BPO under ultrasonic and vigorous stirring. Then the prepared ferrofluid was dropped in 250 mL of aqueous solution containing gelatin (2.5 g) under stirring to prepare an emulsion. The prepared emulsion was gradually heated from ambient temperature to 90 ◦ C for 8 h. After the reaction, PSt-DVB MPs were magnetically separated and washed with warm deionized water (60 ◦ C). The dried PSt-DVB MPs (10 g) were sulfonated with concentrated sulfuric acid for 8 h under mild stirring at 90 ◦ C. The PSt-DVB-SNa MPs were washed with deionized water until pH value was neutral. 2.3. Batch studies and evaluation of adsorption ability The waste CdTe sputtering target was grinded and then was dissolved by the solutions of sulfuric acid and hydrogen peroxide (H2 O2 ). After filtered through a porous membrane with a pore size of 0.45 m, the resulting tellurium–cadmium containing solution was further used for the adsorption experiment. The PSt-DVB-SNa MPs were soaked in deionized water for 12 h and filtered before the adsorption process. Then the adsorption experiments were carried out in the same manner by using batch technique to determine the effect of contacting time, adsorbent dosage, adsorbent cycle times and initial concentration of Cd and
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Te ion. In each type of study, a certain volume of acid solution containing Cd and Te was transferred into wide-mouth bottles using a FINNPIPETTE pipette, and a certain amount of PSt-DVBSNa MPs were then added into each bottle. Afterwards, the bottles were continuously shaken on a shaker platform. At certain intervals of contacting time, the solution was withdrawn and diluted by 5% HNO3 . The concentration of Cd and Te were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES). For the desorption and regeneration experiment, 1.0 g of PStDVB-SNa MPs was added into the acid solution containing tellurium and cadmium (10 mL) to reach the adsorbent equilibrium. Then the PSt-DVB-SNa MPs loaded with Cd and Te were eluted by 1 M H2 SO4 and activated by 1 M NaOH alkaline solution. To test the reusability of the resin, this adsorption–desorption cycle was repeated three times. The tellurium and cadmium concentrations of the filtered liquid samples were diluted by 5% HNO3 and determined by ICPOES after each step. Above experiments were repeated for three times and the average values were used for interpretation of results.
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Fig. 1. Wide-angle XRD patterns of pure Fe3 O4 MPs (A) and Fe3 O4 @SiO2 MPs (B).
2.4. Characterization and analysis methods Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 focus X-ray diffraction with Cu KR radiation. Fourier transform infrared spectra (FTIR) were recorded on a TENSOR27 (Bruker Optic GMBH) IR spectrophotometer in the region of 4000–400 cm−1 as a KBr disc. The surface morphology and elements of each sample were inspected using a scanning electron microscope (SEM; S-4800, Hitachi) with an energy dispersive spectrometer (EDS). Transmission electron microscopy (TEM) images were obtained from a JEM-2000FX transmission electron microscope. Quantitative determination of the metal-ion contents was performed by ICP-OES (Optima 8000, PerkinElmer). The detection limit of ICP for Cd element is 0.2 ppb (g/L) at 26.502 nm. Magnetic characterization was carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7410). The dark field imaging of the samples were observed on an Olympus BX51 optical microscope. The PH value of the solution was measured by a pHB-8A pH meter. 3. Results and discussion 3.1. Preparation and characterization of PSt-DVB-SNa MPs For a magnetic ion-exchange resin used in acid solution, it is necessary to have a magnetic core resisting the strong acid. In the present work, Fe3 O4 MPs synthesized through solvothermal method [21] was further coated with a dense silica shell through the sol–gel reaction of TEOS (ca. Fe3 O4 @SiO2 MPs). Wide-angle XRD patterns of pure Fe3 O4 MPs and Fe3 O4 @SiO2 MPs are shown in Fig. 1. The diffraction peaks of Fe3 O4 MPs (curve A) can be readily indexed to a face-centered cubic structure of magnetite according to JCPDS card No. 19-0629. Fe3 O4 @SiO2 MPs has diffraction peaks similar to that of the parent Fe3 O4 MPs, which indicates that Fe3 O4 MPs is well set in the silica matrix. The broad band centered at 2 = 22◦ (curve B) is assigned to the characteristic reflection from amorphous SiO2 shell (JCPDS 29-0085). According to the SEM image in Fig. 2(A), the spherical Fe3 O4 @SiO2 MPs with size of 300–500 nm can be observed and the core–shell structure can be clearly confirmed by the TEM image shown in Fig. 2(B). The FT-IR spectra of Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B), 3-MPScoated MPs (C), PSt-DVB MPs (D) and PSt-DVB-SNa MPs (E) are shown in Fig. 3. The absorption bands related with Fe–O (595 and 631 cm−1 ) are observed in spectrum A, and the absorption bands
Fig. 2. SEM (A) and TEM (B) images of Fe3 O4 @SiO2 MPs.
assigned to Si–O–Si (1086 and 807 cm−1 ), Si–OH (936 cm−1 ) and Si–O (468 cm−1 ) in spectrum B can confirm the formation of SiO2 shell in Fe3 O4 @SiO2 MPs accompanying with a slight decrease in Fe–O intensity. In order to be well mixed with St and DVB for the aqueous suspension polymerization, Fe3 O4 @SiO2 MPs was firstly modified with 3-MPS molecules, and then was used for preparing
Fig. 3. FT-IR spectra of Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B), 3-MPS-coated MPs (C), PSt-DVB MPs (D) and PSt-DVB-SNa MPs (E).
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PSt-DVB MPs. As shown in spectrum C, the enhanced Si–O–Si signals at about 1300–1093 cm−1 originate from the hydrolytic polycondensation reaction of 3-MPS with SiO2 shell and aliphatic ester bonds of 3-MPS. Moreover, the new peaks at around 2920 and 2852 cm−1 are attributed to the stretching vibration of C–H bonds in 3-MPS. In spectrums D and E, the weak peaks at around 3033 cm−1 and three peaks at 1596, 1500, and 1447 cm−1 are assigned to the stretching vibration of C–H in the phenyl ring and vibration of the phenyl framework. Compared with spectrum D of PSt-DVB MP, the remarkable peaks at 1195 and 1045 cm−1 in spectrum E can be ascribed to the sulfonic groups on PSt-DVB-SNa MPs, which confirms the successful sulfonation of PSt-DVB MPs. Magnetic properties of the prepared samples are investigated by VSM at room temperature in an applied magnetic field and their hysteresis loops are shown in Fig. 4. Pure Fe3 O4 MPs, Fe3 O4 @SiO2 MPs and PSt-DVB-SNa MPs have magnetization saturation values of 75.8, 58.1 and 8.2 emu/g, respectively, at room temperature. It should be noted that the resulted PSt-DVB-SNa MPs still shows strong magnetization and superparamagnetism, which suggests that it can quickly respond to the external magnetic field and be re-dispersed once the external magnetic field is removed. This is advantageous for magnetic separation. In addition, the magnetization values of resulted PSt-DVB-SNa MPs can easily be adjusted by controlling the content of 3-MPS-coated MPs during the aqueous suspension polymerization for preparing the PSt-DVB MPs. As shown in Fig. 5(A), the size of PSt-DVB-SNa MPs is about 500 m, which is observed in dark field by an optical microscope. Upon placement of a NdFeB magnet beside the bottle, the PSt-DVBSNa MPs are quickly attracted to the side of the bottle within five seconds, which illustrates its strong magnetization (Fig. 5(B)). The surface morphology of PSt-DVB-SNa MPs is further examined by SEM (Fig. 5(C)), and the elemental composition on the surface of PSt-DVB-SNa MPs is analyzed by EDS (Fig. 5(D)). The signals of Fe, Si, O, C, S and Na confirm the incorporation of magnetic Fe3 O4 @SiO2 and poly (styrene-divinylbenzene) with sodium sulfonate group (–SO3 Na). When the 3-MPS-coated MPs are enwrapped in the
Fig. 4. Magnetization curves of pure Fe3 O4 MPs (A), Fe3 O4 @SiO2 MPs (B) and 3MPS-coated MPs (C).
PSt-DVB MPs, the PSt-DVB provides a strong protection of 3-MPScoated MPs to maintain their magnetism. The sulfonation reaction generally happened with the PSt-DVB shell, and will not much influence the internal 3-MPS-coated MPs. 3.2. Batch studies and evaluation of adsorption ability 3.2.1. Leaching process of CdTe It has been reported that an optimal leaching condition with sulfuric acid/hydrogen peroxide is 478 mL of 1.0 M H2 SO4 and 4.8 mL H2 O2 per kilogram of CdTe PV modules in consideration of leaching efficiency and solid-to-liquid ratio of leaching system [11]. In our experiment, 1.17 g of CdTe powder was leached in a solution containing 478 mL of 1.0 M of H2 SO4 and 4.8 mL H2 O2 based on the approximate composition of 0.05 wt% Cd and 0.06 wt% Te in PV
Fig. 5. Photo images in dark field photograph (A), separation process by a magnet (B), SEM images (C) and EDS spectroscopy (D) of PSt-DVB-SNa MPs.
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Fig. 6. Concentration of Cd and Te in acid leaching solution after each leaching process (1st time: 1.17 g of CdTe powder is leached with 478 mL of 1.0 M of H2 SO4 and 4.8 mL H2 O2 , 2nd time: 1.17 g of CdTe powder is leached by the 1st time leaching solution, 3rd time: 1.17 g of CdTe powder is leached by the 2nd time leaching solution).
module. Actually if the leaching solution containing Cd and Te can be used again to leach CdTe waste after the first leaching, there will be at least two advantages: (1), the dosage of acid in recycling process will be reduced greatly; and (2), the decrease of H+ concentration after the former leaching can promote the complexion of metal cations in acid media with the sulfonate groups, which will increase the adsorption capacity of the PSt-DVB-SNa MPs. Therefore, the concentration of Cd and Te was firstly investigated after the leaching solution was reused twice or thrice. As shown in Fig. 6, the first leaching process of CdTe was completed within about 90 min, and the equilibrium concentrations of Cd and Te reach about 1112 and 1340 mg/L, respectively. After that, the second 1.17 g of CdTe powder was added into the first leaching solution again. All CdTe powder was dissolved, and the equilibrium concentrations of Cd and Te were about 2274 and 2720 mg/L, respectively. However, the second leaching process achieved the equilibrium at about 200 min, which indicates that the leaching rate of CdTe slightly slows down. When the third 1.17 g of CdTe powder was leached again in the twice-used solution for 300 min, there was still a trace of insoluble substance in the leaching container. Compared with the concentration of Cd (about 3476 mg/L), the concentration of Te (about 3760 mg/L) is less triple that in the first leaching solution. It can be attributed to the poor solubility of tetravalent Te(IV) (Te4+ ) in acidic media. After the initial leaching solution was reused two times, H2 O2 was depleted constantly and not enough to oxidize Te(IV) (Te4+ ) into Te(VI) (TeO6 6− ). Te(VI) is soluble in acid media but Te(IV) is sparingly soluble in acid media, which leads to some insoluble Te(IV) substance. Hence, in our experiment, the acid solution of Cd and Te after twice leaching (twice leaching solution) was used to investigate the adsorption capacity of PSt-DVB-SNa MPs in the following section.
3.2.2. Effect of contacting time on adsorption 10 g of PSt-DVB-SNa MPs was added into 50 mL of twice leaching solution, and the effect of contacting time on the removal of Cd and Te is presented in Fig. 7. As shown in Fig. 7(A), the adsorption equilibrium was reached within 5 min, and about 60% of Cd and 25% of Te were removed. This process is much faster than that using conventional ion-exchange resins, which usually requires at least 20 min or even more time [11]. The efficient adsorption benefits from the small size of PSt-DVB-SNa MPs.
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Fig. 7. Removal efficiency (E, %) of Cd and Te vs. contacting time (A) and adsorption capacity (Qt , mg/g) of PSt-DVB-SNa MPs vs. contacting time (B) (initial concentrations of Cd and Te in twice leaching solution: [Cd], 2274 mg/L; [Te], 2720 mg/L; ratio of PSt-DVB-SNa MPs dosage to twice leaching solution volume: 1 g/5 mL; pH ≈0.4).
In the twice leaching solution of CdTe, the dissolved Cd existed predominantly as uncomplexed Cd2+ , whereas Te existed in the form of Te4+ , TeO(OH)+ and TeO6 6− . Thereinto, the positively charged Cd2+ , Te4+ and TeO(OH)+ can complex with the –SO3 − group on PSt-DVB-SNa MPs. Fig. 7(B) shows that the adsorption capacity of Cd (7.5 mg/g) is much higher than that of Te (2.5 mg/g) in a strong acid solution (pH ≈0.4), which indicates that PSt-DVBSNa MPs has much higher affinity with Cd2+ than with Te4+ and TeO(OH)+ in an acidic media. 3.2.3. Effect of PSt-DVB-SNa MPs dosage on the adsorption Based on the above results, about 60% of Cd and 25% of Te were removed when the dosage of PSt-DVB-SNa MPs in twice leaching solution volume was 1 g/5 mL, the dependence of Cd and Te removal on different adsorbent dosages (0.5 g/5 mL, 1.0 g/5 mL, 2.5 g/5 mL, 5.0 g/5 mL and 10.0 g/5 mL) was investigated to determine the best adsorbent dosage. As shown in Fig. 8(A), the removal efficiency of Cd increased sharply from 44.8% to 93.4% with the increase of
Fig. 8. Removal efficiency (E, %) of Cd and Te (A) and selectivity coefficient (B) vs. dosages of PSt-DVB-SNa MPs (initial concentrations of Cd and Te in twice leaching solution: [Cd], 2274 mg/L; [Te], 2720 mg/L) (Note: The selectivity coefficient (k) for Cd and Te was calculated by E[Cd]/E[Te], where E[Cd] and E[Te] are the removal efficiency of Cd and Te at equilibrium state.).
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PSt-DVB-SNa MPs dosage from 0.5 g/5 mL to 2.5 g/5 mL. After this period, the removal of Cd did not show much improvement by increasing the amount of magnetic adsorbents. When the adsorbent dosage increased to 10.0 g/5 mL, the concentration of Cd in the leaching solution was beyond the detection limit of ICP and all Cd was removed. Similar with Cd, the removal efficiency of Te also increased from 8.3% to 71.5% with the increase of PSt-DVB-SNa MPs dosage from 0.5 g/5 mL to 5.0 g/5 mL, and then showed no significant improvement from 71.5% to 76.8% even if a large amount of adsorbents (10.0 g/5 mL) were used. It is obvious that enough PStDVB-SNa MPs can only remove all Cd2+ , Te4+ and TeO(OH)+ , but leave 23.2% of TeO6 6− in the leaching solution. Fig. 8(B) shows the competitive adsorption of Cd and Te on the PSt-DVB-SNa MPs. The adsorption difference is described as the selectivity coefficient (k) for Cd and Te. When the adsorbent dosage was 0.5 g/5 mL, the removed Cd was much more than removed Te, and the selectivity coefficient reaches 5.4. With the increase of PStDVB-SNa MPs dosage from 0.5 g/5 mL to 5.0 g/5 mL, the selectivity coefficient for Cd and Te decreases sharply from 5.4 to 1.4. After that, it showed no significant change. Obviously the adsorption difference for Cd and Te is also related to the adsorbent dosage besides the affinity. When the adsorbent dosage is less than 5.0 g/5 mL, the sulfonate groups on PSt-DVB-SNa MPs are insufficient relative to cations in twice leaching solution. The removal of Cd predominated because the affinity of sulfonate groups with Cd is stronger than that with Te. In this case, the removal efficiency is decided by the affinity. With the adsorbent dosage increasing over 5.0 g/5 mL, a large number of sulfonate groups are available enough to adsorb cations. The removal efficiency for Cd and Te is mainly decided by the number of sulfonate sites rather than the affinity. Hence all Cd2+ , Te4+ and TeO(OH)+ are adsorbed on PSt-DVB-SNa MPs and the selectivity coefficient for Cd and Te is close to 1.
3.2.4. Effect of multi-steps extraction As the results showed above, 2.5 g of PSt-DVB-SNa MPs can remove 93.4% of Cd and leave about 60% of Te in the 5 mL of leaching solution. However, excessive adsorbents in the leaching solution will cause high viscosity and poor mobility, which is not suitable for the practical mixing process and magnetic separation. It will be helpful to dilute the twice leaching solution before it is used for the adsorption process. Based on the dosage ratio of PSt-DVB-SNa MPs in twice leaching solution (2.5 g/5 mL), 5 mL of twice leaching solution ([Cd], 2274 mg/L; [Te], 2720 mg/L) was firstly diluted five times by 20 mL of deionized water, and then the dilute solution (25 mL; [Cd], 454.8 mg/L; [Te], 544 mg/L) was adsorbed by 2.5 g of PSt-DVB-SNa MPs. After magnetic separation to remove used 2.5 g of PSt-DVBSNa MPs, the resulting solution was treated again by another 2.5 g of PSt-DVB-SNa MPs in order to remove residual Cd and Te in the solution. The above process (adsorbing-separating) was repeated five times. Removal efficiency of Cd and Te for whole process (multiple adsorbing) and for each time is presented in Fig. 9(A) and (B). As shown in Fig. 9(A), about 94.0% of Cd and 52.7% of Te were removed from the acid solution during the first adsorption. After that, removal efficiency of Cd in the leaching solution showed no significant improvement even if a large amount of fresh adsorbents were used. Different from Cd, the removal efficiency of Te cations becomes constant after multi-steps extraction and about 20% of TeO6 6− is still left in the leaching solution. Fig. 9(B) shows that the removal efficiency of Cd and Te for each time decreases sharply after the first adsorption. After four times of adsorption, the concentration of Cd2+ was beyond the detection limit of ICP and removal efficiency of Cd was almost zero, which indicated the removal of all Cd2+ . Similarly, removal efficiency of Te cations for each time
Fig. 9. Removal efficiency (E, %) of Cd and Te for whole process (A) and for each step (B) vs. the number of replication adsorption (initial concentrations of Cd and Te: [Cd], 454.8 mg/L; [Te], 544 mg/L).
decreased from 52.7% to 1.5%. All these can be attributed to gradual decrease of initial concentration for each time. 3.2.5. Adsorption isotherms and adsorption kinetics 25 mL of leaching solution with different initial concentration was treated by 2.5 g of PSt-DVB-SNa MPs for 60 min, and the relationship between adsorption capacity and initial concentration is presented in Fig. 10. The adsorption capacity (Qe ) almost increased linearly with the increase of initial concentration in the low concentration range. However, PSt-DVB-SNa MPs can remove more Cd than Te at the high concentration, which is due to the strong affinity between the Cd2+ and sulfonate groups. It is further proved that the adsorption capacity is controlled by the affinity of cations with functional groups when the adsorbent dosage is relatively insufficient. Adsorption isotherm is used to describe the interactive behavior between solution and adsorbent. In this study, Langmuir model Ce /Qe = 1/(bQmax ) + Ce /Qmax ) and Freundlich model (ln Qe = ln KF + (1/n)ln Ce ) [19] were used to determine the adsorption equilibrium between the adsorbent (PSt-DVB-SNa MPs) and cations (Cd2+ , Te4+ and TeO(OH)+ ). The model parameters obtained by applying Langmuir and Freundlich models to the experimental
Fig. 10. Effect of initial concentration on the adsorption capacity (Qe ) of Cd and Te.
T. Zhang et al. / Journal of Hazardous Materials 279 (2014) 597–604
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Table 1 Adsorption isotherm parameters of Cd and Te catoins. Metal cations
Langmuir isotherm
Cd Te
Freundlich isotherm
Qmax (mg/g)
b (L/mg)
R2
KF (mg/g)
1/n
R2
16.667 9.804
0.010 0.001
0.994 0.992
2.324 3.110
0.445 0.602
0.836 0.992
Table 2 Adsorption kinetic parameters of Cd and Te cations. Metal cations
Cd Te
C0 (mg/L)
2274 2720
Pseudo-first-order
Pseudo-second-order
k1 (min−1 )
Qe.cal (mg/g)
R2
k2 (min−1 )
Qe,cal (mg/g)
R2
0.02 0.016
0.549 0.617
0.812 0.916
0.111 0.080
8.333 2.857
0.999 0.995
data are given in Table 1. Based on the correlation coefficient (R2 ) of each model, the Langmuir model can obviously describe the experiment data more precisely than Freundlich model. The maximal absorption capacity for Cd and Te cations are 16.667 and 9.804 mg/g calculated by Langmuir model, respectively. From the above facts, it can be concluded that the monolayer Langmuir adsorption isotherm is more suitable to explain the adsorption of Cd and Te cations by PSt-DVB-SNa MPs. In order to evaluate the kinetic mechanism which controls the adsorption process, the pseudo-first-order model ln(Qe − Qt ) = ln Qe − k1 t and pseudo-second-order model t/Qt = 1/k2 Qe2 + t/Qt [20] were employed based on the experimental data (in Fig. 7). The kinetic parameters for adsorption of Cd and Te by PSt-DVB-SNa MPs are given in Table 2. Based on the correlation coefficient (R2 ) of each kinetic model, the pseudo-second-order model can obviously describe the experiment data more precisely than pseudo-first-order kinetic model. As we well known, the pseudo-second-order model assumed that the determining rate step may be chemical adsorption, which takes place through a chemical process involving valence forces through sharing or exchange of electron between sorbent and sorbate. It is consistent with the chemical mechanism of ion-exchange process between cations and –SO3 − group on PSt-DVB-SNa MPs (In this case, the adsorption process comes from the high affinity of –SO3 − groups for metal cations, and promoted by ionic bond forces through the electrons sharing between –SO3 − groups for metal cations, which is mainly the chemical reactive adsorption.).
3.2.6. Desorption and regeneration cycles of adsorbents In order to examine the reusability of the prepared PSt-DVBSNa MPs, 2.5 g of PSt-DVB-SNa MPs were added into 25 mL of leaching solution (the initial concentrations of Cd and Te: [Cd], 454.8 mg/L; [Te], 544 mg/L). About 94.0% Cd and 52.7% Te were removed after the first adsorption, and then the PSt-DVB-SNa MPs loaded Cd and Te was regenerated in 1 M H2 SO4 and activated by 1 M NaOH alkaline solution. The adsorption–desorption experiments were repeated three times and the results were shown in Fig. 11. After four adsorption/desorption cycles, the removal efficiency of Te showed no obvious change and the removal efficiency of Cd decreased slightly from 95.0% to 90.7%. These results show the potential reusability of PSt-DVB-SNa MPs for the CdTe recovery from acid media. 4. Conclusions In summary, an effective methodology to synthesize sulfonated magnetic microspheres has been developed and the as-prepared magnetic adsorbents show a number of important features for concentrating Cd and Te. All Cd and 80% of Te are removed from the twice leaching solution of CdTe within 5 min even if its concentration is less than 5.3 mg/L, which is attributed to the strong affinity of Cd ion with sulfonate groups. The adsorption process of Te is different from Cd especially when the adsorbent dosage is relatively insufficient. The difference of adsorbing rates between Cd and Te mainly comes from their different affinity with sulfonate groups. With its unique advantages, the magnetic separation by using PStDVB-SNa MPs can directly concentrate Cd and Te cations from the leaching solution of CdTe, which can be further separated and recycled by the electrochemical or chemical approach. Te anions (TeO6 6− ) left in the leaching solution can be used to prepare some compounds of Te by chemical methods. As an efficient methodology, the magnetic separation is expected to be used in the recycling of CdTe photovoltaic waste. Acknowledgements This work was financially supported by the Foundation of Director in Institute of Electrical Engineering, Chinese Academy of Sciences, China (Grant No. Y140151CSC). We gratefully thank Dr. Dongliang Wang for the dark field imaging of samples. References
Fig. 11. The adsorption–desorption cycles of PSt-DVB-SNa MPs (residual concentration (A) and removal efficiency (B) of Cd and Te in leaching solution).
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