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Gold recovery onto poly(acrylamide-allylthiourea) hydrogels synthesized by treating with gamma radiation
Analytica Chimica Acta 547 (2005) 18–25
Gold recovery onto poly(acrylamide-allylthiourea) hydrogels synthesized by treating with gamma radiation ¨ ur C¸elikbıc¸ak, Nurettin S¸ahiner, Bekir Salih ∗ A. G¨ulden Kılıc¸, Savas¸ Malcı, Om¨ Hacettepe University, Department of Chemistry, 06532 Ankara, Turkey Received 28 October 2004; received in revised form 17 March 2005; accepted 21 March 2005 Available online 25 April 2005
Abstract Poly(acrylamide-1-allyl-2-thiourea) hydrogels, Poly(AAm-ATU), were synthesized by gamma irradiation using a 60 Co ␥ source at different irradiation dose rates and in a monomer mixture with different 1-allyl-2-thiourea contents. These hydrogels were used for the specific gold recovery from single and competitive media. It was observed that the gold adsorption capacity onto the hydrogels was high at low pHs and reached a maximum value at pH 0.5. It was found that the adsorption capacity of the hydrogels for gold ions in acidic media around pH 0.5 was high and about 940 mg g−1 dry hydrogel. Adsorption of these hydrogels for gold ions was found to be very fast and also these hydrogels were showed extremely high selectivity to the gold ions in acidic media even when the concentrations of the other metal ions were extremely higher than that of the gold. Because of the high specificity of these hydrogels to gold ions compared with the other metal ions at low pHs, all matrix effects could be easily eliminated by adsorbing gold ions onto the hydrogels at around pH 0.5 and desorbing into 0.8 M thiourea in 3.0 M HCl. The swellability of the synthesized hydrogels varied with irradiation dose rates and increased at high irradiation dose rates. The minimum swellability of the hydrogels was found to be at least 1000% which made it attractive for gold to penetrate into the hydrogels and react with all the functional groups in the interior surface of the hydrogels. © 2005 Elsevier B.V. All rights reserved. Keywords: Acryl amide; 1-Allyl-2-thiourea; Hydrogels; ␥-Irradiation; Swelling; Characterization; Gold recovery; Matrix elimination
1. Introduction In the recent years, hydrogels have gained a great importance for various applications instead of the other polymers in bulk, microbeads and membrane forms. Consequently, the synthesis and characteristic studies of new hydrogels have become more important due to their environmental sensitivity [1]. Hydrogels have three-dimensional networks containing hydrophilic functional groups are not dissolved in water but swell absorbing more than 95% water [2]. Hydrogels have also some different properties such as their capacity to allow controlled diffusion [3], changing their swellability by ionic strength [4], pH [5], temperature [6] chemical activity to interact with adsorbates [7] and selectivity to some species (i.e., metal ions, proteins, enzymes and some organic pollu∗
Corresponding author. Tel.: +90 3122977975; fax: +90 3122992163. E-mail address: [email protected] (B. Salih).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.03.042
tants) [8–11]. Hydrogels have been used widely in such different fields as medical applications [12], water purification [13], controlled release [14], contact lenses [15], drug delivery system [16], and selective binding metal ion application [2]. Hydrogels are special a class of cross-linked polymers and they can be prepared by different polymerization techniques. Using radiation technology, the polymerization procedure, the cross-linking structure and also the cross-linked density of the hydrogels could be controlled easily [17]. The advantages of hydrogels is that they are easy to obtain with some different polymerization techniques and interacting species with the active functional group on/in to the polymeric sorbent can penetrate inside the cavities of the hydrogel by the swellability of the hydrogels. In recent years, a great deal of interest has been observed in relation to the applicability of chelating agents for the removal and separation of metal ions from heavy metal con-
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taminated water, and the recovery and pre-concentration of precious metal ions from different media [18–20]. Among many sorptive materials, various forms of synthetic polymers containing complexing molecules, which are abundant at low cost, have emerged as one of the most important matrices for the synthesis of new sorbents [21,22]. In the case of some different hydrogels having an ionizable and hydrogen bonding capacity, different hydrogels show variable swelling behavior in the solution at different pH, temperature and ionic strength. Recently, there has been a substantial growth interest in the use of radiation-induced graft copolymerization of functionalized monomer onto polymeric hydrogels for chelating sorbent materials [23–25]. In the proposed study, poly(acrylamide-1-allyl-2thiourea) hydrogels were synthesized by a 60 Co ␥-radiation source at various radiation dose rates and different compositions of the two monomer mixture namely, acryl amide and 1-allyl-2-thiourea. The characteristics of the hydrogels were examined by scanning electron microscope (SEM), elemental analyzer and swelling experiments in this study. Hydrogels produced at different irradiation dose rates were used to examine the pH effect, adsorption kinetics, adsorption capacity, selectivity, recovery and matrix elimination during the gold recovery in real samples. 2. Experimental 2.1. Materials Acryl amide and 1-ally-2-thiourea were purchased from Aldrich (Milwaukee, USA), and both of them were used as received. Hydrochloric acid and thiourea were obtained from Aldrich (Stinheim, Germany). NaAuCl4 was obtained from Merck (Germany). All other chemicals were of reagent grade and were purchased from BDH (England). 2.2. Preparation of Poly(AAm-ATU) hydrogels Acryl amide-1-ally-2-thiourea-water ternary mixture containing different amount of monomer and water was placed in polyvinyl chloride straws of 3 mm diameter and irradiated in a 60 Co ␥ source (Russian made PX-␥-30 Isslodovatelj, 2.97 kGy h−1 ) at variable dose rates. Hydrogels were taken out from the straws and washed several times with distilled water to remove all unreacted monomers and the low molecular weight polymeric matters from the hydrogel. Then the hydrogels were dried first in the air then in a vacuum oven at 60 ◦ C for 5 days, and then stored in desiccators until use. 2.3. Characterization of Poly(AAm-ATU) hydrogels The swelling ratio of the hydrogels was obtained as follows to study the swellability of the hydrogels: 0.2 g of dry hydrogel was placed in a 100 mL cylindrical glass tube. Fifty milliliters of deionized water was added onto the hydrogels, and the hydrogels were allowed to swell at room temperature
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with occasional shaking. The hydrogels were periodically removed and weighed using an electronic balance (Mettler Toledo, Switzerland) to calculate the swellibility of the hydrogels. Swelling ratio was calculated by the following equation. Wswollen − Wdry × 100 Swelling ratio (%) = Wdry The elemental analysis of the Poly(AAm-ATU) hydrogels was performed using an elemental analyzer (Leco, CHNS932, USA) to examine the 1-allyl-2-thiourea content of the hydrogel structure produced during the hydrogel synthesis using mixture of water and different amounts of monomer. The 1-ally-2-thiourea content of the hydrogels was calculated from the sulfur stoichiometry originating from the 1-allyl-2thiourea only. For the SEM images of the hydrogels obtained at different irradiation dose rates, a CAMECA SU-30 SEMProbe (France) scanning electron microscope was used after the samples had been coated with carbon using a Polaron TB500 sputter coater. 2.4. Gold adsorption onto Poly(AAm-ATU) Gold ion adsorption from single metal ion aqueous solutions was investigated in batch-wise adsorption experiments. The effects of the pH, adsorption kinetics and the initial concentration of the gold ions in the adsorption media on the gold ion adsorption were studied. Twenty milliliters of 300 mg L−1 gold ion solution was treated with 0.01 g of dried hydrogel for 90 h at different pH values in the 0.5–7.0 pH range adjusted with HNO3 and NaOH at room temperature. For the kinetics of the gold ion adsorption onto the Poly(AAm-ATU) hydrogels, different hydrogels were synthesized at different irradiation dose rates and treated with 20 mL of 1980 mg L−1 gold ion solution at pH 0.5 in the course of 170 h. At different periods 100 L supernatant was discharged and measured with a graphite furnaceatomic absorption spectrometer (GF-AAS) after the dilution. For the gold ion uptake from a single solution, 50 mL of aqueous gold ion solutions with different concentrations (10–1980 mg L−1 ) were treated for 90 h at room temperature with 0.01 g of dried hydrogels synthesized at different irradiation dose rates and at pH 0.5. 2.5. Gold desorption from Poly(AAm-ATU) Twenty milliliters of 0.8 M thiourea in 3 M HCl solution was used to desorbe all gold ions adsorbed onto the Poly(AAm-ATU) hydrogels. Gold ion adsorbed Poly(AAmATU) hydrogels were placed into a beaker containing 20 mL of desorption agent solution. Desorption medium was stirred magnetically at 500 rpm for 48 h. At the end of desorption period, a part of the supernatant solution was removed and measured by GF-AAS.
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2.6. Matrix elimination for gold analysis in anode slime and method validation In order to examine the matrix effects of other transition metal ions and the validation of the discovered method, an anode slime sample obtained from an electrolytic copper production plant (HES Cable, Kayseri, TURKEY) was prepared in solution form as follows. Homogenized anode slime was dried at 110 ◦ C in an oven overnight and then 5.0 g of dried sample was weighted. The dried sample was added in to the 50 mL of aqua regia solution containing 1.0 mL Br2 solution and heated until dryness at 150 ◦ C. The process was repeated three times adding 1.0 mL Br2 solution each time. Finally, the dried solubilized sample was dissolved using 50 mL of 1.0 M of HCl solution and filtered using a filter paper (Whatmann no. 4). Twenty milliliters of the anode slime sample solution was treated with 0.1 g of dried Poly(AAm-ATU) hydrogels for 72 h at pH 0.5. Finally, the hydrogels were removed from the adsorption medium and desorbed into 20 mL of 0.8 M thiourea in a 3 M HCl solution for 48 h. For the validation of the method, 50 mL solubilized anode slime samples (containing 0, 100 and 200 g L−1 standard gold ions) were treated with 0.1 g of dried Poly(AAm-ATU) hydrogels and after the desorption using 0.8 M thiourea in 3 M HCl solution, the gold ions were measured by GF-AAS. Finally, the gold ion recovery was calculated to examine the validity of the discovered method, in this study, for leading to gold ion uptake.
3. Results and discussion 3.1. Characteristics of Poly(AAm-ATU) hydrogels In this study, the synthesized Poly(AAm-ATU) hydrogels showed a highly open pore structure with a large internal surface area leading to high adsorption capacity with low
diffusional resistance in the hydrogel matrix. In Fig. 1, the scanning electron microscope images of Poly(AAm-ATU) hydrogels produced at 30 and 70 kGy irradiation dose rates are given. When the images were compared to one another, it could be seen clearly that hydrogel surface was very smooth and more or less the cross-linking density seem to be high for hydrogel produced at 70 kGy irradiation dose. This shows that the cross-linking density of the hydrogels synthesized at high irradiation dose rates was higher than the hydrogels produced at low irradiation dose rates. Also it has been concluded that the amount of allylthiourea present in the hydrogels was increased with increasing irradiation dose rates. This evaluation is also verified by the elemental analysis results used to obtain the amount of allylthiourea in the hydrogels at different irradiation dose rates. In order to examine the effect of the irradiation dose rate on the Poly(AAm-ATU) hydrogel synthesis, the equilibrium swelling behavior of the hydrogels produced at 30, 40, 50, 60 and 70 kGy irradiation dose rates and an irradiated solution mixture containing 10% 1-allyl-2-thiourea were examined with the aqueous solution at pH 7.0. The swelling behavior of the hydrogels is given in Fig. 2. It may be noted that the equilibrium swelling ratio of the hydrogels decreased with the increasing irradiation dose rates. The decrease in the equilibrium swelling ratio of the hydrogels synthesized at high dose rates is because of the high degrees of cross-linking occurring inside the hydrogels. Elemental analysis experiments of hydrogels synthesized at 70 kGy irradiation dose rate and different 1-allyl2-thiourea contents in the monomer mixture before the irradiation were carried out using an elemental analyzer. Using especially the sulfur content, existing only in the 1-allyl-2thiourea monomer, amount of 1-allyl-2-thiourea was calculated and this is reported in Fig. 3. In Fig. 3, percent of 1allyl-2-thiourea in the hydrogels is plotted against the 1-allyl2-thiourea percentage in the irradiated mixture at 70 kGy irradiation dose rate. When the 1-allyl-2-thiourea percent in the
Fig. 1. Scanning electron microscope images of Poly(AAm-ATU) hydrogels. (A) Irradiated at 30 kGy dose rates and (B) irradiated at 70 kGy dose rates. 1-Allyl-2-thiourea content: 10%; acryl amide content: 90%. Both irradiation mixture compositions are the same.
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3.2. Gold adsorption onto Poly(AAm-ATU)
Fig. 2. Swelling behavior of Poly(AAm-ATU) hydrogels produced with a 10% 1-allyl-2-thiourea content in the irradiation mixture at different irradiation dose rates.
irradiated mixture was increased, the percentage in the synthesized hydrogels also increased and reached 8.4% in the hydrogels produced using 10% of 1-allyl-2-thiourea in the irradiated mixture. For up to a 5.0% 1-allyl-2-thiourea content in the irradiation mixture, the 1-allyl-2-thiourea content in the synthesized hydrogels increased rapidly up to 6.9%, and then increased very slowly with increasing irradiation dose rates and finally reached 8.4% in the hydrogels. During the investigation of the production of Poly(AAmATU) hydrogels, FT-IR spectra of 1-allyl-2-thiourea, acryl amide and several types of hydrogel were recorded. For this part of the work, FT-IR spectra of the hydrogels produced at different irradiation dose rates and containing different amounts of 1-allyl-2-thiourea from 0 to 10.0% have been discussed in detail in elsewhere [26].
Fig. 3. 1-Allyl-2-thiourea content in the hydrogels produced vs. the 1-allyl2-thiourea content in the irradiated mixture.
3.2.1. pH effect In the adsorption experiment, the pH is a very important parameter which is involved in the removal of the target metal ion specifically and in obtaining an increase in the adsorption capacity. For selective adsorption, besides the use of a specific ligand modified sorbent, selectivity could be achieved by adjusting a medium pH to different values. In this study, pH selective adsorption of gold ions was found to be possible at low pHs, which means in a very acidic media. In order to examine changes in the adsorption capacity of the gold ions on/in to the hydrogels with the pH, hydrogels synthesized at a 70 kGy irradiation dose rate and in a 10% allylthiourea irradiation mixture was treated with 20 mL of 250 mg L−1 gold ion solutions at between 0.5 and 7.0 pH for 72 h, that being the pre-determined adsorption time. Also, Poly(acryl amide) hydrogels produced at a 70 kGy irradiation dose rate were treated with same gold ion solutions using the same experimental parameters. All the results for the pH experiments are given in Fig. 4 for Poly(AAm-ATU) and Poly(acryl amide). It will be noted that at low pH values up to pH 0.5, the adsorption capacity of gold ions onto the Poly(AAm-ATU) increased and reached about 400 mg g−1 dried hydrogels. Also, the adsorption capacity of gold ions was found to be 280 mg g−1 dried hydrogel at pH 7.0. This shows that the adsorption capacity of the gold ion was increasing with decreasing pH values, but this change was not dramatic. High adsorption capacity at low pH around pH 0.5 is really important to eliminate the adsorption of the other metal ions which are showing a high matrix effect in the gold ion recovery and pre-concentration. Gold ion adsorption onto the homo polymer of Poly(acryl amide) also changed with the changing of the pH. In this case, the change in the adsorption capacity for gold ions was in the opposite direction. Gold adsorption capacity of the Poly(acryl
Fig. 4. pH effect on gold ion adsorption onto the Poly(AAm-ATU) and Poly(AAm) hydrogels produced at 70 kGy irradiation dose rates.
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Fig. 5. Gold ion uptake kinetics onto the hydrogels containing different amounts of 1-allyl-2-thiourea and produced at different irradiation dose rates.
amide) was increased when the pH was increased and also the gold ion adsorption capacity of the Poly(acryl amide) was really low at about 78 mg g−1 dried hydrogel compared to the Poly(AAm-ATU) hydrogels. Therefore, Poly(AAmATU) hydrogels were found to be more favorable for use in selective gold ion adsorption compared to the Poly(acryl amide) hydrogels, using low pHs for the elimination of the other metal ions. 3.2.2. Adsorption kinetics To study the equilibrium adsorption time, five different hydrogels (about 0.1 g on a dried basis) produced at different irradiation dose rates such as 30, 40, 50, 60 and 70 kGy were treated with 20 mL of 1980 mg L−1 gold ion solutions at pH 0.5 for 168 h. About 100 L aliquot from the supernatant was removed at different times and measured by GF-AAS. The results are given in Fig. 5. From Fig. 5, it is observed that gold ion adsorption capacity increased rapidly up to 28 h adsorption time then adsorption capacity increased slowly and then reached the equilibrium adsorption time at around 90 h. In the first 48 h, adsorption can reach at least 80% of the maximum adsorption capacity. All these results show that the adsorption time compares favorably with values in the literature for applications with a high level of gold ion recovery. 3.2.3. Initial gold ion concentration effect Five different hydrogels synthesized at different irradiation dose rates were treated with gold ion solutions at pH 0.5 for 90 h at room temperature. Twenty milliliters of 1980 mg L−1 gold ion solutions were treated with 0.1 g of Poly(AAm-ATU) and the supernatant was removed from the solutions after 90 h adsorption time. The gold ion concentration was then measured by GF-AAS. It was found that maximum gold adsorption was about 940 mg gold g−1 dried hydrogel when the hydrogels produced at 70 kGy was used (Fig. 6). This adsorbed gold ion value is seen to be really high, and the process seems to be very feasible for industrial
Fig. 6. Effect of the initial concentration on gold ion uptake onto the Poly(AAm-ATU) hydrogels containing different amount of 1-allyl-2thiourea and produced at different irradiation dose rates.
applications. Also, by increasing the irradiation dose rate for the synthesis of the hydrogels from 30 to 70 kGy, the gold ion adsorption capacity was increased by more than 50%. This is due to the penetration of more 1-allyl-2-thiourea into the structure of the hydrogel as a cross-linking agent when the irradiation dose rate is increased during the hydrogel synthesis. 3.3. Gold desorption from Poly(AAm-ATU) To examine the reusability of the Poly(AAm-ATU) hydrogels in a continuous gold ion adsorption process, desorption of the gold ions from the hydrogels was examined. Some different types of desorption agent were tested for this purpose, but a sodium cyanide solution and 20 mL of 0.8 M thiourea in 3 M HCl solution were found suitable to desorbe gold ions from the Poly(AAm-ATU) hydrogels with a higher than 96% desorption ratio. Because of the high toxicity of the sodium cyanide solution, an acidic thiourea solution was chosen as the best desorption agent in this study. Gold ion adsorbed Poly(AAm-ATU) hydrogels were placed into a beaker containing 20 mL of desorption agent solution and the desorbed gold ion concentration was measured by GF-AAS. In all experiments, the minimum desorption ratio was found to be 96%. This desorption ratio is suitable for at least 10 repeated process for the same hydrogels without loosing more than 5% adsorption capacity of the hydrogels (data not shown). 3.4. Matrix elimination for gold analysis in anode slime and method validation In natural samples, there are many different types of matrix affecting the measurements for the desired metal ions. These are either of an organic or inorganic nature. To elimi-
A.G. Kılı¸c et al. / Analytica Chimica Acta 547 (2005) 18–25 Table 1 Metal ion concentrations of the anode slime sample solution before and after the method was applied Element
Concentration of the metal ions (mg L−1 ) In the initial solution
Au Pb Cd Cu Ni Cr Zn Co Fe
0.792 1679 2.56 34.1 47.2 36.2 0.573 5.96 9408
± ± ± ± ± ± ± ± ±
0.007 12 0.02 0.11 0.13 0.22 0.009 0.05 19
In the desorption medium after the method applied 0.009 1687 2.54 34.2 46.9 36.4 0.571 5.95 9394
± ± ± ± ± ± ± ± ±
0.002 18 0.03 0.15 0.14 0.28 0.011 0.07 25
All results were given for five parallel experiments with a 95% confidence interval.
nate the organic matrices, different ashing experimental conditions might be set up to eliminate the high amount of the matrices in GF-AAS. But the elimination of the inorganic matrices is more difficult than the organic matrices. In the elimination of high amount of inorganic matrices, different application methods could be used. In our case, selective adsorption of the gold ion onto the Poly(AAm-ATU) hydrogels and the desorption of only the adsorbed gold ions into the desorption medium give an advantage for the determination of the gold in the medium separated from the matrices. Some main elemental matrix concentrations are given in Table 1
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before and after the method was applied to the anode slime sample solution. Only gold ions were adsorbed onto the hydrogels and other metal ions remained in the solution at pH 0.5. This shows that the hydrogels have a specific affinity to gold ions and no affinity to the other metal ions at pH 0.5. To examine the ability of matrix elimination on/in to the synthesized Poly(AAm-ATU) hydrogels, solubilized anode slime sample solutions were measured directly and after the adsorption–desorption process by GF-AAS. The atomic absorption and background signal curves of both samples are given in Fig. 7A and B. As seen in Fig. 7A, the background signal level was really high compared to the atomic absorption signal, which is characteristic of a large number of matrices in the solubilized anode slime sample. When the adsorption–desorption procedure was applied to the anode slime sample using Poly(AAm-ATU) hydrogels, the atomic absorption signal was increased and the background signal level was completely dead although there was a sharp peak at low intensity showing a background signal. This sharp peak was because of the thiourea. It was concluded that the background from the thiourea did not affect our results critically because of the low intensity and different location of background and atomic absorption signals in the spectrum. As a result, it can be concluded that high amount of matrices can be eliminated, and that precise measurements are possible in a GF-AAS system for gold ion determination using the Poly(AAm-ATU) hydrogels after the adsorption–desorption cycle. For the validation of the method, the same solubi-
Fig. 7. Atomization signal curves of gold. (A) Directly measured from anode slime sample solution after the solubilization, (B) after the adsorption–desorption cycle applied to the anode slime sample solution, (C) after adsorption–desorption cycle applied to the same anode slime sample solution after adding 100 g L−1 standard gold ion solution, and (D) after adsorption–desorption cycle applied to the same anode slime sample solution after adding 200 g L−1 standard gold ion solution.
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Table 2 Recovery of gold ions from anode slime sample solution Gold ion concentration (g L−1 )
Sample
Added Anode slime Anode slime Anode slime
Recovery (%)
Found 39.6 ± 2.4 138.0 ± 5.2 240.9 ± 9.7
0 100 200
– 98.2 ± 2.1 100.0 ± 3.7
All results were given for five parallel experiments with a 95% confidence interval.
lized anode slime sample solution was mixed with 200 and 400 g L−1 standard gold ion solutions in 1:1 ratio to get 100 and 200 g L−1 gold concentrations and also anode slime solution was diluted with an equal volume of water and measured by GF-AAS without adding any standard gold solution. The method was applied to recover and measure the concentration of gold ions in the anode slime sample and the other anode slime samples containing 100 and 200 g L−1 standard gold ions. Anode slime sample solutions containing 100 and 200 g L−1 standard gold ion were measured by GF-AAS after the adsorption–desorption cycle had been applied and the atomic and background signals are given in Fig. 7C and D. As seen from these Figures, the atomic absorption signal of the gold was increased and the background signal was decreased completely. Recovery of gold ions from the anode slime sample solutions without the addition of gold ions, and after standard gold ions had been added were calculated and are given in Table 2. From Table 2, it may be concluded that the recovery of gold was not less than 98% and that the validation of the method discovered in this study seems to be satisfactory for the gold recovery processes.
interference is in the presence of high amounts of chloride and sulphate as anions and also reasonably high amounts of main cations such as toxic and transition metal ions might be present in different natural samples. In each case, recovery of gold was found to be higher than 95%.
4. Conclusion It was shown that highly swellable hydrogels with an open structure can be easily synthesized by gamma irradiation using a ternary mixture of water–acryl amide-1-allyl2-allylthiourea. Increasing the 1-allyl-2-allylthiourea content in the irradiated mixture and the irradiation dose rate, also increased the 1-allyl-2-allylthiourea content of the hydrogels. Gold uptake was increased by the presence of high amount of 1-allyl-2-allylthiourea in the hydrogels at pH 0.5. At pH 0.5, the other metal ion matrix effects were eliminated, and the recovery of gold ions and also pre-concentration of gold ions were performed perfectly. Validation of method discovered, in this study, was tested adding known amounts of standard gold ions to the solubilized anode slime sample. It was found that the method worked efficiently at pH 0.5 despite extremely high concentration of the other metal ions (in some cases, more than 9000-fold) compared to the gold ions. The gold ion adsorption capacity of the synthesized hydrogels was found to be very high for the recovery of pure gold and the adsorption time of the gold ions on/in to the hydrogels was reasonably fast for feasible gold recovery applications in the industry.
3.5. Effect of matrix ions
Acknowledgements
The effect of matrix ions on the recovery of gold in the aqueous gold ion solution was investigated using 20 mL of 50 g L−1 gold ion solution and the recovery of gold in each solution containing different matrices is given in Table 3. No
Authors gratefully acknowledge Prof. Renato Zenobi from ETH, Z¨urich-Switzerland for his help in obtaining the Elemental Analyzer results. This work was supported in part by the Hacettepe University Scientific Research Foundation under the contract no. HU 0302601007.
Table 3 Effect of matrix ions on the recovery of gold Ions
Added as
Concentration (mg L−1 )
Gold recovery (%)
Mg2+
MgCl2 CaCl2 Cu(NO3 )2 Co(NO3 )2 Ni(NO3 )2 Zn(NO3 )2 Cd(NO3 )2 Pb(NO3 )2 Fe(NO3 )3 Cr(NO3 )3 Al(NO3 )3 NaCl Na2 SO4
5000 5000 2500 2500 2500 2500 5000 5000 2500 2500 5000 30000 30000
98 97 97 98 99 98 96 95 99 96 101 98 97
Ca2+ Cu2+ Co2+ Ni2+ Zn2+ Cd2+ Pb2+ Fe3+ Cr3+ Al3+ Cl− SO4 2−
Gold ion concentration: 50 g L−1 ; volume: 20 mL; amount of hydrogel: 0.05 g and N = 4).
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Gold recovery onto poly(acrylamide-allylthiourea) hydrogels synthesized by treating with gamma radiation ¨ ur C¸elikbıc¸ak, Nurettin S¸ahiner, Bekir Salih ∗ A. G¨ulden Kılıc¸, Savas¸ Malcı, Om¨ Hacettepe University, Department of Chemistry, 06532 Ankara, Turkey Received 28 October 2004; received in revised form 17 March 2005; accepted 21 March 2005 Available online 25 April 2005
Abstract Poly(acrylamide-1-allyl-2-thiourea) hydrogels, Poly(AAm-ATU), were synthesized by gamma irradiation using a 60 Co ␥ source at different irradiation dose rates and in a monomer mixture with different 1-allyl-2-thiourea contents. These hydrogels were used for the specific gold recovery from single and competitive media. It was observed that the gold adsorption capacity onto the hydrogels was high at low pHs and reached a maximum value at pH 0.5. It was found that the adsorption capacity of the hydrogels for gold ions in acidic media around pH 0.5 was high and about 940 mg g−1 dry hydrogel. Adsorption of these hydrogels for gold ions was found to be very fast and also these hydrogels were showed extremely high selectivity to the gold ions in acidic media even when the concentrations of the other metal ions were extremely higher than that of the gold. Because of the high specificity of these hydrogels to gold ions compared with the other metal ions at low pHs, all matrix effects could be easily eliminated by adsorbing gold ions onto the hydrogels at around pH 0.5 and desorbing into 0.8 M thiourea in 3.0 M HCl. The swellability of the synthesized hydrogels varied with irradiation dose rates and increased at high irradiation dose rates. The minimum swellability of the hydrogels was found to be at least 1000% which made it attractive for gold to penetrate into the hydrogels and react with all the functional groups in the interior surface of the hydrogels. © 2005 Elsevier B.V. All rights reserved. Keywords: Acryl amide; 1-Allyl-2-thiourea; Hydrogels; ␥-Irradiation; Swelling; Characterization; Gold recovery; Matrix elimination
1. Introduction In the recent years, hydrogels have gained a great importance for various applications instead of the other polymers in bulk, microbeads and membrane forms. Consequently, the synthesis and characteristic studies of new hydrogels have become more important due to their environmental sensitivity [1]. Hydrogels have three-dimensional networks containing hydrophilic functional groups are not dissolved in water but swell absorbing more than 95% water [2]. Hydrogels have also some different properties such as their capacity to allow controlled diffusion [3], changing their swellability by ionic strength [4], pH [5], temperature [6] chemical activity to interact with adsorbates [7] and selectivity to some species (i.e., metal ions, proteins, enzymes and some organic pollu∗
Corresponding author. Tel.: +90 3122977975; fax: +90 3122992163. E-mail address: [email protected] (B. Salih).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.03.042
tants) [8–11]. Hydrogels have been used widely in such different fields as medical applications [12], water purification [13], controlled release [14], contact lenses [15], drug delivery system [16], and selective binding metal ion application [2]. Hydrogels are special a class of cross-linked polymers and they can be prepared by different polymerization techniques. Using radiation technology, the polymerization procedure, the cross-linking structure and also the cross-linked density of the hydrogels could be controlled easily [17]. The advantages of hydrogels is that they are easy to obtain with some different polymerization techniques and interacting species with the active functional group on/in to the polymeric sorbent can penetrate inside the cavities of the hydrogel by the swellability of the hydrogels. In recent years, a great deal of interest has been observed in relation to the applicability of chelating agents for the removal and separation of metal ions from heavy metal con-
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taminated water, and the recovery and pre-concentration of precious metal ions from different media [18–20]. Among many sorptive materials, various forms of synthetic polymers containing complexing molecules, which are abundant at low cost, have emerged as one of the most important matrices for the synthesis of new sorbents [21,22]. In the case of some different hydrogels having an ionizable and hydrogen bonding capacity, different hydrogels show variable swelling behavior in the solution at different pH, temperature and ionic strength. Recently, there has been a substantial growth interest in the use of radiation-induced graft copolymerization of functionalized monomer onto polymeric hydrogels for chelating sorbent materials [23–25]. In the proposed study, poly(acrylamide-1-allyl-2thiourea) hydrogels were synthesized by a 60 Co ␥-radiation source at various radiation dose rates and different compositions of the two monomer mixture namely, acryl amide and 1-allyl-2-thiourea. The characteristics of the hydrogels were examined by scanning electron microscope (SEM), elemental analyzer and swelling experiments in this study. Hydrogels produced at different irradiation dose rates were used to examine the pH effect, adsorption kinetics, adsorption capacity, selectivity, recovery and matrix elimination during the gold recovery in real samples. 2. Experimental 2.1. Materials Acryl amide and 1-ally-2-thiourea were purchased from Aldrich (Milwaukee, USA), and both of them were used as received. Hydrochloric acid and thiourea were obtained from Aldrich (Stinheim, Germany). NaAuCl4 was obtained from Merck (Germany). All other chemicals were of reagent grade and were purchased from BDH (England). 2.2. Preparation of Poly(AAm-ATU) hydrogels Acryl amide-1-ally-2-thiourea-water ternary mixture containing different amount of monomer and water was placed in polyvinyl chloride straws of 3 mm diameter and irradiated in a 60 Co ␥ source (Russian made PX-␥-30 Isslodovatelj, 2.97 kGy h−1 ) at variable dose rates. Hydrogels were taken out from the straws and washed several times with distilled water to remove all unreacted monomers and the low molecular weight polymeric matters from the hydrogel. Then the hydrogels were dried first in the air then in a vacuum oven at 60 ◦ C for 5 days, and then stored in desiccators until use. 2.3. Characterization of Poly(AAm-ATU) hydrogels The swelling ratio of the hydrogels was obtained as follows to study the swellability of the hydrogels: 0.2 g of dry hydrogel was placed in a 100 mL cylindrical glass tube. Fifty milliliters of deionized water was added onto the hydrogels, and the hydrogels were allowed to swell at room temperature
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with occasional shaking. The hydrogels were periodically removed and weighed using an electronic balance (Mettler Toledo, Switzerland) to calculate the swellibility of the hydrogels. Swelling ratio was calculated by the following equation. Wswollen − Wdry × 100 Swelling ratio (%) = Wdry The elemental analysis of the Poly(AAm-ATU) hydrogels was performed using an elemental analyzer (Leco, CHNS932, USA) to examine the 1-allyl-2-thiourea content of the hydrogel structure produced during the hydrogel synthesis using mixture of water and different amounts of monomer. The 1-ally-2-thiourea content of the hydrogels was calculated from the sulfur stoichiometry originating from the 1-allyl-2thiourea only. For the SEM images of the hydrogels obtained at different irradiation dose rates, a CAMECA SU-30 SEMProbe (France) scanning electron microscope was used after the samples had been coated with carbon using a Polaron TB500 sputter coater. 2.4. Gold adsorption onto Poly(AAm-ATU) Gold ion adsorption from single metal ion aqueous solutions was investigated in batch-wise adsorption experiments. The effects of the pH, adsorption kinetics and the initial concentration of the gold ions in the adsorption media on the gold ion adsorption were studied. Twenty milliliters of 300 mg L−1 gold ion solution was treated with 0.01 g of dried hydrogel for 90 h at different pH values in the 0.5–7.0 pH range adjusted with HNO3 and NaOH at room temperature. For the kinetics of the gold ion adsorption onto the Poly(AAm-ATU) hydrogels, different hydrogels were synthesized at different irradiation dose rates and treated with 20 mL of 1980 mg L−1 gold ion solution at pH 0.5 in the course of 170 h. At different periods 100 L supernatant was discharged and measured with a graphite furnaceatomic absorption spectrometer (GF-AAS) after the dilution. For the gold ion uptake from a single solution, 50 mL of aqueous gold ion solutions with different concentrations (10–1980 mg L−1 ) were treated for 90 h at room temperature with 0.01 g of dried hydrogels synthesized at different irradiation dose rates and at pH 0.5. 2.5. Gold desorption from Poly(AAm-ATU) Twenty milliliters of 0.8 M thiourea in 3 M HCl solution was used to desorbe all gold ions adsorbed onto the Poly(AAm-ATU) hydrogels. Gold ion adsorbed Poly(AAmATU) hydrogels were placed into a beaker containing 20 mL of desorption agent solution. Desorption medium was stirred magnetically at 500 rpm for 48 h. At the end of desorption period, a part of the supernatant solution was removed and measured by GF-AAS.
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2.6. Matrix elimination for gold analysis in anode slime and method validation In order to examine the matrix effects of other transition metal ions and the validation of the discovered method, an anode slime sample obtained from an electrolytic copper production plant (HES Cable, Kayseri, TURKEY) was prepared in solution form as follows. Homogenized anode slime was dried at 110 ◦ C in an oven overnight and then 5.0 g of dried sample was weighted. The dried sample was added in to the 50 mL of aqua regia solution containing 1.0 mL Br2 solution and heated until dryness at 150 ◦ C. The process was repeated three times adding 1.0 mL Br2 solution each time. Finally, the dried solubilized sample was dissolved using 50 mL of 1.0 M of HCl solution and filtered using a filter paper (Whatmann no. 4). Twenty milliliters of the anode slime sample solution was treated with 0.1 g of dried Poly(AAm-ATU) hydrogels for 72 h at pH 0.5. Finally, the hydrogels were removed from the adsorption medium and desorbed into 20 mL of 0.8 M thiourea in a 3 M HCl solution for 48 h. For the validation of the method, 50 mL solubilized anode slime samples (containing 0, 100 and 200 g L−1 standard gold ions) were treated with 0.1 g of dried Poly(AAm-ATU) hydrogels and after the desorption using 0.8 M thiourea in 3 M HCl solution, the gold ions were measured by GF-AAS. Finally, the gold ion recovery was calculated to examine the validity of the discovered method, in this study, for leading to gold ion uptake.
3. Results and discussion 3.1. Characteristics of Poly(AAm-ATU) hydrogels In this study, the synthesized Poly(AAm-ATU) hydrogels showed a highly open pore structure with a large internal surface area leading to high adsorption capacity with low
diffusional resistance in the hydrogel matrix. In Fig. 1, the scanning electron microscope images of Poly(AAm-ATU) hydrogels produced at 30 and 70 kGy irradiation dose rates are given. When the images were compared to one another, it could be seen clearly that hydrogel surface was very smooth and more or less the cross-linking density seem to be high for hydrogel produced at 70 kGy irradiation dose. This shows that the cross-linking density of the hydrogels synthesized at high irradiation dose rates was higher than the hydrogels produced at low irradiation dose rates. Also it has been concluded that the amount of allylthiourea present in the hydrogels was increased with increasing irradiation dose rates. This evaluation is also verified by the elemental analysis results used to obtain the amount of allylthiourea in the hydrogels at different irradiation dose rates. In order to examine the effect of the irradiation dose rate on the Poly(AAm-ATU) hydrogel synthesis, the equilibrium swelling behavior of the hydrogels produced at 30, 40, 50, 60 and 70 kGy irradiation dose rates and an irradiated solution mixture containing 10% 1-allyl-2-thiourea were examined with the aqueous solution at pH 7.0. The swelling behavior of the hydrogels is given in Fig. 2. It may be noted that the equilibrium swelling ratio of the hydrogels decreased with the increasing irradiation dose rates. The decrease in the equilibrium swelling ratio of the hydrogels synthesized at high dose rates is because of the high degrees of cross-linking occurring inside the hydrogels. Elemental analysis experiments of hydrogels synthesized at 70 kGy irradiation dose rate and different 1-allyl2-thiourea contents in the monomer mixture before the irradiation were carried out using an elemental analyzer. Using especially the sulfur content, existing only in the 1-allyl-2thiourea monomer, amount of 1-allyl-2-thiourea was calculated and this is reported in Fig. 3. In Fig. 3, percent of 1allyl-2-thiourea in the hydrogels is plotted against the 1-allyl2-thiourea percentage in the irradiated mixture at 70 kGy irradiation dose rate. When the 1-allyl-2-thiourea percent in the
Fig. 1. Scanning electron microscope images of Poly(AAm-ATU) hydrogels. (A) Irradiated at 30 kGy dose rates and (B) irradiated at 70 kGy dose rates. 1-Allyl-2-thiourea content: 10%; acryl amide content: 90%. Both irradiation mixture compositions are the same.
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3.2. Gold adsorption onto Poly(AAm-ATU)
Fig. 2. Swelling behavior of Poly(AAm-ATU) hydrogels produced with a 10% 1-allyl-2-thiourea content in the irradiation mixture at different irradiation dose rates.
irradiated mixture was increased, the percentage in the synthesized hydrogels also increased and reached 8.4% in the hydrogels produced using 10% of 1-allyl-2-thiourea in the irradiated mixture. For up to a 5.0% 1-allyl-2-thiourea content in the irradiation mixture, the 1-allyl-2-thiourea content in the synthesized hydrogels increased rapidly up to 6.9%, and then increased very slowly with increasing irradiation dose rates and finally reached 8.4% in the hydrogels. During the investigation of the production of Poly(AAmATU) hydrogels, FT-IR spectra of 1-allyl-2-thiourea, acryl amide and several types of hydrogel were recorded. For this part of the work, FT-IR spectra of the hydrogels produced at different irradiation dose rates and containing different amounts of 1-allyl-2-thiourea from 0 to 10.0% have been discussed in detail in elsewhere [26].
Fig. 3. 1-Allyl-2-thiourea content in the hydrogels produced vs. the 1-allyl2-thiourea content in the irradiated mixture.
3.2.1. pH effect In the adsorption experiment, the pH is a very important parameter which is involved in the removal of the target metal ion specifically and in obtaining an increase in the adsorption capacity. For selective adsorption, besides the use of a specific ligand modified sorbent, selectivity could be achieved by adjusting a medium pH to different values. In this study, pH selective adsorption of gold ions was found to be possible at low pHs, which means in a very acidic media. In order to examine changes in the adsorption capacity of the gold ions on/in to the hydrogels with the pH, hydrogels synthesized at a 70 kGy irradiation dose rate and in a 10% allylthiourea irradiation mixture was treated with 20 mL of 250 mg L−1 gold ion solutions at between 0.5 and 7.0 pH for 72 h, that being the pre-determined adsorption time. Also, Poly(acryl amide) hydrogels produced at a 70 kGy irradiation dose rate were treated with same gold ion solutions using the same experimental parameters. All the results for the pH experiments are given in Fig. 4 for Poly(AAm-ATU) and Poly(acryl amide). It will be noted that at low pH values up to pH 0.5, the adsorption capacity of gold ions onto the Poly(AAm-ATU) increased and reached about 400 mg g−1 dried hydrogels. Also, the adsorption capacity of gold ions was found to be 280 mg g−1 dried hydrogel at pH 7.0. This shows that the adsorption capacity of the gold ion was increasing with decreasing pH values, but this change was not dramatic. High adsorption capacity at low pH around pH 0.5 is really important to eliminate the adsorption of the other metal ions which are showing a high matrix effect in the gold ion recovery and pre-concentration. Gold ion adsorption onto the homo polymer of Poly(acryl amide) also changed with the changing of the pH. In this case, the change in the adsorption capacity for gold ions was in the opposite direction. Gold adsorption capacity of the Poly(acryl
Fig. 4. pH effect on gold ion adsorption onto the Poly(AAm-ATU) and Poly(AAm) hydrogels produced at 70 kGy irradiation dose rates.
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Fig. 5. Gold ion uptake kinetics onto the hydrogels containing different amounts of 1-allyl-2-thiourea and produced at different irradiation dose rates.
amide) was increased when the pH was increased and also the gold ion adsorption capacity of the Poly(acryl amide) was really low at about 78 mg g−1 dried hydrogel compared to the Poly(AAm-ATU) hydrogels. Therefore, Poly(AAmATU) hydrogels were found to be more favorable for use in selective gold ion adsorption compared to the Poly(acryl amide) hydrogels, using low pHs for the elimination of the other metal ions. 3.2.2. Adsorption kinetics To study the equilibrium adsorption time, five different hydrogels (about 0.1 g on a dried basis) produced at different irradiation dose rates such as 30, 40, 50, 60 and 70 kGy were treated with 20 mL of 1980 mg L−1 gold ion solutions at pH 0.5 for 168 h. About 100 L aliquot from the supernatant was removed at different times and measured by GF-AAS. The results are given in Fig. 5. From Fig. 5, it is observed that gold ion adsorption capacity increased rapidly up to 28 h adsorption time then adsorption capacity increased slowly and then reached the equilibrium adsorption time at around 90 h. In the first 48 h, adsorption can reach at least 80% of the maximum adsorption capacity. All these results show that the adsorption time compares favorably with values in the literature for applications with a high level of gold ion recovery. 3.2.3. Initial gold ion concentration effect Five different hydrogels synthesized at different irradiation dose rates were treated with gold ion solutions at pH 0.5 for 90 h at room temperature. Twenty milliliters of 1980 mg L−1 gold ion solutions were treated with 0.1 g of Poly(AAm-ATU) and the supernatant was removed from the solutions after 90 h adsorption time. The gold ion concentration was then measured by GF-AAS. It was found that maximum gold adsorption was about 940 mg gold g−1 dried hydrogel when the hydrogels produced at 70 kGy was used (Fig. 6). This adsorbed gold ion value is seen to be really high, and the process seems to be very feasible for industrial
Fig. 6. Effect of the initial concentration on gold ion uptake onto the Poly(AAm-ATU) hydrogels containing different amount of 1-allyl-2thiourea and produced at different irradiation dose rates.
applications. Also, by increasing the irradiation dose rate for the synthesis of the hydrogels from 30 to 70 kGy, the gold ion adsorption capacity was increased by more than 50%. This is due to the penetration of more 1-allyl-2-thiourea into the structure of the hydrogel as a cross-linking agent when the irradiation dose rate is increased during the hydrogel synthesis. 3.3. Gold desorption from Poly(AAm-ATU) To examine the reusability of the Poly(AAm-ATU) hydrogels in a continuous gold ion adsorption process, desorption of the gold ions from the hydrogels was examined. Some different types of desorption agent were tested for this purpose, but a sodium cyanide solution and 20 mL of 0.8 M thiourea in 3 M HCl solution were found suitable to desorbe gold ions from the Poly(AAm-ATU) hydrogels with a higher than 96% desorption ratio. Because of the high toxicity of the sodium cyanide solution, an acidic thiourea solution was chosen as the best desorption agent in this study. Gold ion adsorbed Poly(AAm-ATU) hydrogels were placed into a beaker containing 20 mL of desorption agent solution and the desorbed gold ion concentration was measured by GF-AAS. In all experiments, the minimum desorption ratio was found to be 96%. This desorption ratio is suitable for at least 10 repeated process for the same hydrogels without loosing more than 5% adsorption capacity of the hydrogels (data not shown). 3.4. Matrix elimination for gold analysis in anode slime and method validation In natural samples, there are many different types of matrix affecting the measurements for the desired metal ions. These are either of an organic or inorganic nature. To elimi-
A.G. Kılı¸c et al. / Analytica Chimica Acta 547 (2005) 18–25 Table 1 Metal ion concentrations of the anode slime sample solution before and after the method was applied Element
Concentration of the metal ions (mg L−1 ) In the initial solution
Au Pb Cd Cu Ni Cr Zn Co Fe
0.792 1679 2.56 34.1 47.2 36.2 0.573 5.96 9408
± ± ± ± ± ± ± ± ±
0.007 12 0.02 0.11 0.13 0.22 0.009 0.05 19
In the desorption medium after the method applied 0.009 1687 2.54 34.2 46.9 36.4 0.571 5.95 9394
± ± ± ± ± ± ± ± ±
0.002 18 0.03 0.15 0.14 0.28 0.011 0.07 25
All results were given for five parallel experiments with a 95% confidence interval.
nate the organic matrices, different ashing experimental conditions might be set up to eliminate the high amount of the matrices in GF-AAS. But the elimination of the inorganic matrices is more difficult than the organic matrices. In the elimination of high amount of inorganic matrices, different application methods could be used. In our case, selective adsorption of the gold ion onto the Poly(AAm-ATU) hydrogels and the desorption of only the adsorbed gold ions into the desorption medium give an advantage for the determination of the gold in the medium separated from the matrices. Some main elemental matrix concentrations are given in Table 1
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before and after the method was applied to the anode slime sample solution. Only gold ions were adsorbed onto the hydrogels and other metal ions remained in the solution at pH 0.5. This shows that the hydrogels have a specific affinity to gold ions and no affinity to the other metal ions at pH 0.5. To examine the ability of matrix elimination on/in to the synthesized Poly(AAm-ATU) hydrogels, solubilized anode slime sample solutions were measured directly and after the adsorption–desorption process by GF-AAS. The atomic absorption and background signal curves of both samples are given in Fig. 7A and B. As seen in Fig. 7A, the background signal level was really high compared to the atomic absorption signal, which is characteristic of a large number of matrices in the solubilized anode slime sample. When the adsorption–desorption procedure was applied to the anode slime sample using Poly(AAm-ATU) hydrogels, the atomic absorption signal was increased and the background signal level was completely dead although there was a sharp peak at low intensity showing a background signal. This sharp peak was because of the thiourea. It was concluded that the background from the thiourea did not affect our results critically because of the low intensity and different location of background and atomic absorption signals in the spectrum. As a result, it can be concluded that high amount of matrices can be eliminated, and that precise measurements are possible in a GF-AAS system for gold ion determination using the Poly(AAm-ATU) hydrogels after the adsorption–desorption cycle. For the validation of the method, the same solubi-
Fig. 7. Atomization signal curves of gold. (A) Directly measured from anode slime sample solution after the solubilization, (B) after the adsorption–desorption cycle applied to the anode slime sample solution, (C) after adsorption–desorption cycle applied to the same anode slime sample solution after adding 100 g L−1 standard gold ion solution, and (D) after adsorption–desorption cycle applied to the same anode slime sample solution after adding 200 g L−1 standard gold ion solution.
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Table 2 Recovery of gold ions from anode slime sample solution Gold ion concentration (g L−1 )
Sample
Added Anode slime Anode slime Anode slime
Recovery (%)
Found 39.6 ± 2.4 138.0 ± 5.2 240.9 ± 9.7
0 100 200
– 98.2 ± 2.1 100.0 ± 3.7
All results were given for five parallel experiments with a 95% confidence interval.
lized anode slime sample solution was mixed with 200 and 400 g L−1 standard gold ion solutions in 1:1 ratio to get 100 and 200 g L−1 gold concentrations and also anode slime solution was diluted with an equal volume of water and measured by GF-AAS without adding any standard gold solution. The method was applied to recover and measure the concentration of gold ions in the anode slime sample and the other anode slime samples containing 100 and 200 g L−1 standard gold ions. Anode slime sample solutions containing 100 and 200 g L−1 standard gold ion were measured by GF-AAS after the adsorption–desorption cycle had been applied and the atomic and background signals are given in Fig. 7C and D. As seen from these Figures, the atomic absorption signal of the gold was increased and the background signal was decreased completely. Recovery of gold ions from the anode slime sample solutions without the addition of gold ions, and after standard gold ions had been added were calculated and are given in Table 2. From Table 2, it may be concluded that the recovery of gold was not less than 98% and that the validation of the method discovered in this study seems to be satisfactory for the gold recovery processes.
interference is in the presence of high amounts of chloride and sulphate as anions and also reasonably high amounts of main cations such as toxic and transition metal ions might be present in different natural samples. In each case, recovery of gold was found to be higher than 95%.
4. Conclusion It was shown that highly swellable hydrogels with an open structure can be easily synthesized by gamma irradiation using a ternary mixture of water–acryl amide-1-allyl2-allylthiourea. Increasing the 1-allyl-2-allylthiourea content in the irradiated mixture and the irradiation dose rate, also increased the 1-allyl-2-allylthiourea content of the hydrogels. Gold uptake was increased by the presence of high amount of 1-allyl-2-allylthiourea in the hydrogels at pH 0.5. At pH 0.5, the other metal ion matrix effects were eliminated, and the recovery of gold ions and also pre-concentration of gold ions were performed perfectly. Validation of method discovered, in this study, was tested adding known amounts of standard gold ions to the solubilized anode slime sample. It was found that the method worked efficiently at pH 0.5 despite extremely high concentration of the other metal ions (in some cases, more than 9000-fold) compared to the gold ions. The gold ion adsorption capacity of the synthesized hydrogels was found to be very high for the recovery of pure gold and the adsorption time of the gold ions on/in to the hydrogels was reasonably fast for feasible gold recovery applications in the industry.
3.5. Effect of matrix ions
Acknowledgements
The effect of matrix ions on the recovery of gold in the aqueous gold ion solution was investigated using 20 mL of 50 g L−1 gold ion solution and the recovery of gold in each solution containing different matrices is given in Table 3. No
Authors gratefully acknowledge Prof. Renato Zenobi from ETH, Z¨urich-Switzerland for his help in obtaining the Elemental Analyzer results. This work was supported in part by the Hacettepe University Scientific Research Foundation under the contract no. HU 0302601007.
Table 3 Effect of matrix ions on the recovery of gold Ions
Added as
Concentration (mg L−1 )
Gold recovery (%)
Mg2+
MgCl2 CaCl2 Cu(NO3 )2 Co(NO3 )2 Ni(NO3 )2 Zn(NO3 )2 Cd(NO3 )2 Pb(NO3 )2 Fe(NO3 )3 Cr(NO3 )3 Al(NO3 )3 NaCl Na2 SO4
5000 5000 2500 2500 2500 2500 5000 5000 2500 2500 5000 30000 30000
98 97 97 98 99 98 96 95 99 96 101 98 97
Ca2+ Cu2+ Co2+ Ni2+ Zn2+ Cd2+ Pb2+ Fe3+ Cr3+ Al3+ Cl− SO4 2−
Gold ion concentration: 50 g L−1 ; volume: 20 mL; amount of hydrogel: 0.05 g and N = 4).
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