Use of Water-Compatible Polystyrene−Polyglycidol Resins for the

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Use of Water-Compatible Polystyrene-Polyglycidol Resins for the Separation and Recovery of Dissolved Precious Metal Salts Yu-Lung Lam, Die Yang, Chi-Yuet Chan, Kwong-Yu Chan,* and Patrick H. Toy* Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong

The use of polystyrene-polyglycidol (PS/PG) water-compatible resins for the extraction of the precious metal salts gold(I) cyanide and silver(I) cyanide from aqueous phase was studied. The resins studied include polyglycidol grafted onto a polystyrene core, a thiol version of this base PS/PG resin in which the terminal hydroxyl groups are replaced with thiol groups, and a magnetic PS/PG resin. More than 99% of the gold and silver ions of the original solutions could be extracted using the resins, and reverse extraction of the loaded metal ions from the resins was performed. Thus, recycling of the resins was possible and no deterioration in extraction performance of the reused materials was observed. The attractiveness of these resins in water purification is augmented by the fact that the PS/PG resins examined can be easily synthesized from inexpensive commodity chemical starting materials. Introduction In recent years there has been increasing environmental concern regarding metal salt pollution in wastewater. The conventional technologies used for metal extraction for water purification include ion-exchange resins, polymer ultrafiltration, and microemulsion techniques. Such extraction processes are generally composed of four separate operations: (1) binding of dissolved metal salts to an extracting material, (2) separation of the metal loaded extracting material from the aqueous phase, (3) elution of loaded metal ions, and (4) recycling of extracting material. Each of the above-mentioned conventional technologies suffers from drawbacks in various steps of the extraction process, such as slow extraction rates, long separation times, difficult reverse extraction, and no recycling of extracting material. In order to overcome some of these deficiencies, metal salt extraction using an aqueous biphasic extraction system (ABES) has recently been reported.1,2 In this strategy, water-soluble polymer poly(ethylene glycol) (PEG) is used as the extracting material. The use of PEG in an ABES has the advantage that it is virtually nontoxic, nonflammable, and relatively inexpensive.3 However, such an ABES suffers from the fact that a large quantity of a salt, such as sodium sulfate, is required to achieve phase separation of the aqueous and PEG phases once the extraction is complete. Though most of the salts used for such applications are inexpensive, they do potentially introduce corrosion issues. This method is particularly uneconomical for treating effluents with parts per million concentrations of dissolved metals. With respect to the problems mentioned above, a solid phase polymer of PEG with a polystyrene (PS) core was first used for metal extraction in this work. A PS core provides a hydrophobic backbone while PEG has hydrophilic properties. This solid phase polymer was commonly applied in biological research for biomolecules such as proteins. Polyglycidol (PG), a polymer with a structure similar to that of PEG, was synthesized and immobilized on polystyrene (PS) surface. This resin is called PS/PG resin, and it is water compatible and partially soluble in aqueous solution. As opposed * To whom correspondence should be addressed. E-mail: hrsccky@ hku.hk (K.-Y.C.); [email protected] (P.H.T.).

to common ion-exchange resins in which binding sites (charges) are hidden inside a hydrophobic substrate (polystyrene or divinylbenzene), the PS/PG structure has exposed swellable external hydrophilic groups. Because of the PS core, the resin can be readily separated by sedimentation or centrifugation without the need of salt addition. The synthesized PS/PG resin was tested for precious metal extraction such as gold(I) and silver(I) ions. Reverse extraction of metal ions from the resin was also investigated. Furthermore, since thiols have strong affinity to gold, the extraction capability of PS/PG resin was strengthened and possibly made more selective by coupling to thiol groups,4 and the resin became thiol-terminated PG (PS/PG-SH). A magnetic aqueous biphasic system was reported for biomolecules, and an increased rate of separation was achieved.5 Using methodologies well-known in the art, a magnetite core coated with the PS/PG resin was also synthesized for extraction in order to promote separation efficiency, easy separation, and repeated extractions. Methods Preparation of Materials for Extraction. I. PS/PEG Resins. PS/PEG resin was purchased directly from Rapp Polymere GmbH. The resin was used for extraction without any modification. The resin is called TentaGel S 30 900, with a diameter of 90 µm. II. Synthesis of Polystyrene/Polyglycidol (PS/PG) Resins. In a 100 mL three-necked flask, hydroxymethylated PS beads (1 g, 7.1 mmol of OH groups/g of polymer) were suspended in dry diglyme (25 mL). After swelling for 1 h, KOtBu (0.8 g, 7.1 mmol) in THF solution was added to deprotonate the hydroxyl groups at 40 °C over 12 h. The mixture was heated to 120 °C, and 50 mL of dry DMF was added. A solution of freshly distilled glycidol (24 mL, 26.4 g, 356 mmol) in dry DMF (40 mL) was added dropwise to the reaction mixture. The mixture was stirred for 12 h at 120 °C and then terminated with 1 N HCl. The mixture was filtered and washed with water and MeOH and finally dried in a vacuum to give the product, with a yield of 21.6 g. The final white resins are composed of core polystyrene, c.a. 5% by weight, covered with polyglycidol, c.a. 95% by weight. The resins swelled in water upon stirring. The resins can be filtered or settled as a bottom layer by gravity. This synthesis is a modification of that of Haag et al.6 A

10.1021/ie801904h CCC: $40.75  2009 American Chemical Society Published on Web 04/10/2009

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Figure 1. Scanning electron microscopic image of polystyrene/polyglycidol (PS/PG).

Figure 2. Scanning electron microscope image of magnetite/polystyrene/ polyglycidol (MAG/PS/PG).

scanning electron microscopic (SEM) image of a PS/PG/ magnetite particle is shown in Figure 1. III. Synthesis of Polystyrene/Thiolated-Polyglycidol (PS/ PG-SH) Resins. Mesyl chloride (1.0 g, 9 mmol) was added dropwise to a mixture of PS/PG (0.5 g, 6.0 mmol of OH groups/g of polymer) and pyridine (1.4 g, 18 mmol) in anhydrous CH2Cl2. The reaction mixture was stirred for 12 h at room temperature under N2 atmosphere. The resulting resin was filtered, washed with water and MeOH, and then dried in a vacuum to give 0.73 g of the product PS/PG-OMs. Thiourea (1.6 g, 21 mmol) was added to the PS/PG-OMs (0.73 g) in ethanol (20 mL). The reaction mixture was refluxed overnight. After workup, 0.48 g of the newly formed polymer (PS/PG-SH) was obtained with the loading of 5.4 mmol/g S. The resulting yellow resins swelled in water upon stirring. IV. Synthesis of Polystyrene/Polyglycidol Resins with a Magnetite Core (MAG/PS/PG). Magnetite nanoparticles were first prepared7-9 by mixing 11.1 g of FeCl3 · 6H2O and 5.6 g of FeSO4 · 7H2O in 50 mL of deionized water followed by addition of 20 mL of concentrated ammonia at 70 °C. Then 1.5 g of oleic acid was added to the mixture and stirred for 30 min; sonification was applied throughout. The mixture was heated at 110 °C for 30 min to remove water and excess ammonia. The synthesized magnetite nanoparticles are about 10 nm in size. The oleic acid grafted magnetite was mixed together with 4-vinylbenzyl alcohol, divinylbenzene (DVB), and azobis(isobutyronitrile) (AIBN) in a three-necked flask with the mass concentrations of 0.48%, 4.26%, 0.54%, and 0.09%, respectively. The reaction mixture was stirred and degassed at room temperature under N2 over 1 h. Deionized (DI) water (94.54 wt %), with 0.09% dissolved poly(vinyl alcohol) (PVA) was also added to the mixture. Polymerization was performed for 16 h at 70 °C. The polymer was washed with excess DI water and dried in a vacuum to give the product of PS coated magnetite. In a 100 mL three-necked flask, hydroxymethylated magnetic beads (1 g) were suspended in dry diglyme (25 mL). After swelling for 1h, KOtBu (0.8 g, 7.1 mmol) in THF solution was added to deprotonate the hydroxyl groups at 40 °C over 12 h. The mixture was heated to 120 °C and 50 mL of dry DMF was added. A solution of freshly distilled glycidol (24 mL, 26.4 g, 356 mmol) in dry DMF (40 mL) was added dropwise to the reaction mixture. The mixture was stirred for 12 h at 120 °C and then terminated with addition of 1 M HCl. The mixture was filtered and washed with water and methanol and finally

dried in a vacuum to give 3.5 g of the product with a weight increase of 250%. In our experiment, magnetite core coated with the PS/PG resin was synthesized, and Figure 2 is a SEM image of it. With a magnetite core, the resin can be controlled by a magnetic field to increase the separation efficiency. Extraction Experiments. To investigate the extraction at low gold concentration, a stock solution of 15 mg/L Au was prepared from a corresponding quantity of potassium gold(I) cyanide (obtained from Kenlap PCG Manufacturer Company Limited). A 21.96 mg sample of potassium gold(I) cyanide was dissolved and diluted to 1 L of deionized water. Another stock solution of 15 mg/L Ag was prepared from a corresponding quantity of potassium silver(I) cyanide (obtained from Kenlap PCG Manufacturer Company Limited). A 27.68 mg sample of potassium silver(I) cyanide was dissolved and diluted to 1 L of deionized water. The solutions were then used for metal extraction without modification unless otherwise stated. For each metal extraction, 10 mL of solution was mixed with 0.5 g of polymer. After stirring, the polymer was settled by sedimentation or magnetic separation and the aqueous phase was decanted for metal concentration determination. To evaluate the uptake capacity of resin and also investigate reverse extraction, another set of experiments were performed with high inlet Au or Ag concentration of 100 mg/L and larger quantities of solution and resin. The extraction procedure was similar with but with 50 mL of solution and 10 g of resin. Repeated extractions were performed on metal loaded resin with 100 mg/L metal concentration. Recovery of metal can be achieved by several alternatives. A common method is by means of electrodeposition. After metal extraction, the clean aqueous phase could be discarded and the resin phase subject to electrowinning using a platinum wire as the counter electrode; the gold was plated onto a clean nickel or copper wire. Results and Discussion Extraction of Gold and Silver by Different Resins. Results of the gold and silver extraction experiments using different resins are summarized in Table 1. The results indicate that gold can be fully extracted from the aqueous phase to the polymer phase. The determination of gold in aqueous phase after extraction showed that the percentage of gold extracted to the

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Table 1. Extraction Results of Gold and Silver by Different Resins type of solid phase

aqueous phase

polystyrene/poly(ethylene glycol) polystyrene/poly(ethylene glycol) polystyrene/polyglycidol polystyrene/polyglycidol polystyrene/thiolated polyglycidol polystyrene/thiolated polyglycidol magnetite/polystyrene/poly(ethylene glycol) magnetite/polystyrene/poly(ethylene glycol)

potassium potassium potassium potassium potassium potassium potassium potassium

metal concn before extraction (mg/L)

metal concn after extraction (mg/L)

% removal of Au from aqueous phase

15 15 15 15 15 15 15 15

99 94.4

gold cyanide solution silver cyanide solution gold cyanide solution silver cyanide solution gold cyanide solution silver cyanide solution gold cyanide solution silver cyanide solution

pH was 7 as excess protons formed HCN and volatile HCN was easily removed. The percentage extraction of silver using such control pH treated silver solution was higher than 99%. The mechanism of extraction is based on the special binding between the metal cyanide complex and the polymer resin. The lone pair electrons of oxygen on the PEG or PG group can attract the positively charged gold atoms in gold cyanide complexes, as illustrated in Figure 3. Recovery of Gold and Silver. After extraction, gold can be reverse extracted and recovered via chemical means. An alternative convenient means of electrodeposition is illustrated in Figure 4. First, gold solution was mixed with PS/PG resin for extraction as shown in step 1 (Figure 4). After extraction, the gold content in the polymer phase increased significantly. In step 3, the aqueous phase could then be discarded and only a wet polymer phase was retained for metal recovery. In step 4, using a platinum wire as the anode, the gold was plated onto a clean nickel or copper wire. The electrical resistance in the polymer required a high plating voltage. In this experiment, the current was 0.1 A and the voltage was 15 V. The voltage could be reduced to about 1 V by adding a small amount of salt such as sodium chloride or sodium sulfate. During electrodeposition (Figure 5), the gold ions loaded on the resin are reduced by the applied current. The change in oxidation state and charge neutralization disrupt the attractive interaction with the C-O-C group. The desorbed gold atoms are immediately deposited onto the nickel or copper surface. The nickel wire was weighed before and after electrodeposition so that the mass of gold plated could be calculated. See Table 2. Similarly, silver recovery was also performed (see Table 3). In the experiment, only about 40-50% of gold or silver was

Figure 3. Partial charges on polymer chain, which can attract gold cyanide complex.

Figure 4. Schematic illustration of gold recovery in electrodeposition. Goldbearing phase is indicated in yellow.

polymer phase was greater than 99%. For silver extraction, the percentage of silver extracted to the polymer phase was 99%. The silver solution had to be treated with diluted hydrochloric acid to remove excess cyanide anions. While [Au(CN)2]- is the only stable complex between gold and cyanide, [Ag(CN)2]-, [Ag(CN)3]2-, and [Ag(CN)4]3- can coexist.10 The additional ligands in [Ag(CN)3]2- and [Ag(CN)4]3- affect the binding of the complexes with the polymer. Therefore, the percentage extraction of silver with excess cyanide ions was lower than that of gold. [Ag(CN)3]2- and [Ag(CN)4]3- can be removed by adding HCl to shift equilibrium toward [Ag(CN)2]- and remove free cyanide (KCN). Free cyanide, if present, reacts with HCl to form HCN, which can escape from the solution. If excess HCl was added, it reacted with [Ag(CN)2]- and the white precipitates AgCl formed could be removed by filtration. The

Figure 5. Mechanism of metal desorption from resin followed by electrodeposition. Table 2. Results of Gold Recovery After Reverse Extraction and Recycling of Resins sample description

Au concn in 100 mL of lean soln (mg/L)

% removal

Au in 10 g of polymer phase (mg)

Au recovered by electrodeposition (mg)

% recovered by electrodeposition

after first extraction after second extraction

99

5 (5 - 2.1) + 5 ) 7.9

2.1 3.8

42 48

Table 3. Results of Silver Recovery After Extraction and Recycle of Resins sample description

Ag concn in 100 mL of lean soln (mg/L)

% removal

Ag in 10 g of polymer phase (mg)

Ag recovered by electrodeposition (mg)

% recovered by electrodeposition

after first extraction after second extraction

19.26 14.43

80.74 85.57

4.04 (4.04 - 2.1) + 4.28 ) 6.22

2.1 2.8

52 45

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Figure 6. Infrared spectrum of PS/PG resin.

Figure 8. Comparsion of thiolated PS/PG and unthiolated PS/PG on gold capacity for repeated extractions of dissolved gold.

Figure 7. Infrared spectra of PS/PG resin before extraction (upper curve) and after loading with Au(CN)2- (lower curve).

stripped from the resins. The metal loaded resins could be reused for repeated extraction until saturation. Infrared Analysis of Resins. To investigate the structure and functional groups on the resin, infrared spectra were obtained from the resins. From the infrared spectrum of PS/PG resin (Figure 6), O-H (3391 cm-1), C-H (2901 cm-1), aromatic CdC (1643, 1450, and 1350 cm-1), and C-O (1077 cm-1) functional groups could be observed.11 The presence of these groups is consistent with the structure of the resin. The infrared spectrum obtained for PS/PG resin after extraction is presented in Figure 7 (lower spectrum). Compared with the PS/PG resin before extraction, several additional absorption peaks were observed between 2000 and 2800 cm-1. They correspond to the presence of adsorbed gold cyanide ions. This observation is in agreement with the extraction mechanism proposed previously.12 Enhancement of the Extraction Capability. In order to enhance the extraction performance of the polymer, a functional group was introduced into the PS/PG copolymer.

Figure 9. Comparsion of thiolated PS/PG and nonthiolated PS/PG on capacity for repeated extractions of dissolved silver.

The functional group incorporated is a thiol group, and some of oxygen atoms in the polymer are replaced by sulfur atoms. The sulfur content in thiolated PS/PG resin was about 20% (m/m) from X-ray fluorescence (XRF) analysis. The lone pair

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PS/PG can be a good candidate to replace poly(ethylene glycol) in poly(ethylene glycol)-based aqueous biphasic extraction systems (PEG ABES) without needing the addition of salt. The system is simple and convenient and should be considered for larger scale operations. Acknowledgment This research is a joint project between the University of Hong Kong and Kenlap PGC Manufacturer Company Limited and has been supported by an award from the Innovation and Technology Fund (UIM/166). Literature Cited

Figure 10. Effect of concentration of sodium cyanide on gold eluted from thiolated PS/PG resins.

electrons in the sulfur atoms have a strong affinity toward gold and silver ions. The thiolated PS/PG was used for silver and gold extractions. It was found that the thiolated resin showed higher extraction capability as shown in Figures 8 and 9. With five consecutive extractions, the thiolated resin still showed good extraction performance while the unthiolated one was saturated after the third or fourth extraction. Gold could be eluted from the thiolated polymer by adding sodium cyanide. A range of sodium cyanide with different concentrations was used to elute the gold from the thiolated resin saturated with gold. From Figure 10, an adequate sodium cyanide concentration for elution is about 0.5 M. Conclusions In conclusion, a polystyrene-graft-polyglycidol (PS/PG) resin was synthesized and used for gold and silver extractions. In a two-cycle extraction-elution experiment, the resin can be reused without deterioration in extraction capability. In addition, a significant increase in loading could be accomplished by introducing thiol groups to the polymer. Metal recovery from the polymer phase is possible by electrodeposition or sodium cyanide leaching. In the case of elution of Au loaded thiolated PS/PG using NaCN solution, pure electroplating salt can be obtained in the elute after drying without the need for electrowinning. Further extraction studies on Au and Ag recovery in multimetal solution are needed before application to electroplating wastewater can be confirmed.

(1) Zhang, T. X.; Li, W. J.; Zhou, W. J.; Gao, H. C.; Wu, J. G.; Xu, G. X.; Chen, J.; Liu, H. Z.; Chen, J. Y. Extraction and separation of gold (I) cyanide in polyethylene glycol-based aqueous biphasic systems. Hydrometallurgy 2001, 62, 41–46. (2) Fontana, D.; Ricci, D. Poly(ethylene glycol)-based aqueous biphasic systems: effect of temperature on phase equilibria and on partitioning of 1,10-phenanthroline-copper(II) sulphate complex. J. Chromatogr., B 2000, 743, 231–234. (3) Chen, J.; Spear, S. K.; Huddleston, J. G.; Rogers, R. D. Polyethylene glycol and solutions of polyethylene glycol as green reaction media. Green Chem. 2005, 7, 64–82. (4) Du, Y. J.; Brash, J. L. Synthesis and Characterization of ThiolTerminated Poly(ethylene oxide) for Chemisorption to Gold Surface. J. Appl. Polym. Sci. 2003, 90, 594–607. (5) Wikstro¨m, P.; Flygare, S.; Gro¨ndalen, A.; Larsson, P.-O. Magnetic Aqueous 2-Phase SeparationsA New Technique to Increase Rate of PhaseSeparation, Using Dextran Ferrofliud or Larger Iron-Oxide Particles. Anal. Biochem. 1987, 167, 331–339. (6) Roller, S.; Turk, H.; Stumbe, J.-F.; Rapp, W.; Haag, R. Polystyrenegraft-Polyglycerol Resins: A New Type of High-Loading Hybrid Support for Organic Synthesis. J. Comb. Chem. 2006, 8, 350–354. (7) Ramirez, L. P.; Landfester, K. Magnetic Polystyrene Nanoparticles with a High Magnetite Content Obtained by Miniemulsion Processes. Macromol. Chem. Phys. 2003, 204, 22–31. (8) Lopez-Perez, J. A.; Lopez-Quintela, M. A.; Mira, J.; Rivas, J. Preparation of Magnetic Fluids with Particles Obtained in Microemulsions. IEEE Trans. Magn. 1997, 33, 4359–4362. (9) Kang, Y. S.; Risbud, S.; Rabolt, J. H.; Stroeve, R. Synthesis and Characterization of Nanometer-Size Fe3O4 and γ-Fe2O3 Particles. Chem. Mater. 1996, 8, 2209–2211. (10) Jones, L. H.; Penneman, R. A. Infrared Absorption Studies of Aqueous Complex Ions: I. Cyanide Complexes of Ag(I) and Au(I) in Aqueous Solution and Adsorbed on Anion Resin. J. Chem. Phys. 1954, 22, 965–970. (11) Stuart, B. Modern Infrared Spectroscopy; Wiley: New York, 1996. (12) Lukey, G. C.; van Deventer, J. S. J.; Chowdhury, D. C. S.; Huntington, S. T.; Morton, C. J. The Speciation of Gold and Copper Complexes on Ion-exchange Resins containing Functional Groups. React. Funct. Polym. 2000, 44, 121–143.

ReceiVed for reView December 10, 2008 ReVised manuscript receiVed March 13, 2009 Accepted March 14, 2009 IE801904H