Synthesis of Hydrophilic Sulfur-Containing Adsorbents for Noble Metals Having Thiocarbonyl Group Based on a Methacrylate Bearing Dithiocarbonate Moieties

Novel hydrophilic sulfur-containing adsorbents for noble metals were prepared by the radical terpolymerization of a methacrylate bearing dithiocarbonate moieties (DTCMMA), hydrophilic monomers, and a cross-linker. -e resulting adsorbents efficiently and selectively adsorbed noble metals (Au, Ag, and Pd) from various multielement aqueous solutions at room temperature owing to the thiocarbonyl group having high affinity toward noble metals. -e metal adsorption by the adsorbents was proceeded by simple mixing followed by filtration. -e noble metal selectivity of the adsorbent obtained from DTCMMA and N-isopropylacrylamide was higher than that of the adsorbent obtained from DTCMMA and N,N-dimethylacrylamide due to the lower nonspecific adsorption.


Introduction
Noble metals such as Au, Pd, and Ag are essential in the modern life and applied in various fields such as jewelry [1], synthetic catalysts [2][3][4][5], and materials for electronic industry [6]. e amounts of noble metals in mines are inherently very low, and the recent increase in their use is shortening their reserves to production ratios.erefore, recovery and reuse of these metals from various used materials is very essential.For example, the recycling from industrial wastewater and urban mines is promising owing to the high amounts and contents.
In this study, we designed a hydrophilic adsorbent bearing thiocarbonyl moieties for selective adsorption of noble metals in an organic solvent free manner.As the source for thiocarbonyl groups, we focused on a methacrylate bearing dithiocarbonate moieties (5-(methacryloyloxy) methyl-1,3-oxathiolane-2-thione, DTCMMA) [61], which can be prepared via a simple reaction of glycidyl methacrylate with carbon disulfide.In order to adsorb noble metals from water in a facile manner, DTCMMA was copolymerized with hydrophilic monomers and a cross-linker to make the adsorbents swellable and insoluble.Selectivity of the adsorption was evaluated using three solutions containing multiple metal ions.

2.2.
Instruments.IR spectra were recorded on a HORIBA FT-720 spectrometer.Inductive-coupled plasma mass spectrometry (ICP-MS) measurements were performed on a PerkinElmer ELAN DRC II spectrometer.Operating conditions were as follows: nebulizer gas flow, 0.91-1.01mL/min; ICP RF power, 1.1 kW; lens voltage, 7.4 V; pulse stage voltage, 900 V; dwell time, 60 ns; sweeps, 3 times; readings per replicate, 3 times; and flow rate, 0.96 mL/min.e dissolution media for ICP-MS measurements of the samples after metal adsorption experiments were coincided with the multielement solutions, namely, 10% HCl aq. for A, 5% HNO 3 aq.for B, and 5% HNO 3 aq.for C.

Metal Adsorption from Multielement Solution with Hydrophilic Sulfur-Containing Adsorbents (Typical Procedure).
A multielement solution (2.0 mL, 10 mg/L of metal ions) and a hydrophilic sulfur-containing adsorbent ([C�S]/[total metal] � 4.5/1) were stirred in a glass vial at room temperature for 1.5 h.en, the adsorbent was removed by filtration and washed 2 times with 100 mL amounts of water.e amounts of adsorbed metals were calculated from the concentrations of metals in the filtrate measured by ICP-MS.

Competitive Metal Adsorption from Pd/Cu Mixed
Solution with 2 (Typical Procedure).CuCl 2 (54.9 mg/mL) in 1 N HCl solution (2.0 mL), PdCl 2 (0.36 mg/mL) in 1 N HCl solution (2.0 mL), and adsorbent 2 (50.0 mg) were added to a glass vial and stirred at room temperature for 1.5 h.en, the adsorbent was removed by filtration and washed 2 times with 100 mL of water.e amounts of adsorbed metals were calculated from the concentrations of metals in the filtrate measured by ICP-MS.

2
Advances in Materials Science and Engineering respectively.e products could be swelled by water to yield soft gels.e structures were con rmed by FT-IR spectroscopic analysis (Figure 1).Characteristic peaks assignable to C S moieties in the DTCMMA unit were observed at 1192 cm −1 [60].Peaks assignable to the amide moieties and carbonyl group in the units originating from the hydrophilic monomers and MBAA were also observed (1: 3438 cm −1 for NH in NIPAM, 3303 cm −1 for NH in MBAA, Advances in Materials Science and Engineering with the feed ratio (theoretical values for 1 and 2: 73.3 and 81.6 μmol).

Metal Adsorption Experiments to Find the Metals Adsorbed by the Hydrophilic Metal Adsorbents Using Multielement Solutions.
e hydrophilic metal adsorbents were used at the molar ratio of [C S]/[total metal] 4.5/1.e metal adsorption experiments were conducted by dispersing the adsorbents in multielement solutions, and the adsorbents adsorbing metals were separated by ltration.e amounts of the adsorbed metals were calculated by measuring the amounts of the metals in the ltrates by ICP-MS.
e metal adsorption ratios of some metals were calculated to be negative values, possibly because the apparent metal concentrations after the adsorption increased by uptake of water by the adsorbents resulting in increase in the concentration of metals.
First, a multielement solution A was employed for metal adsorption experiments at room temperature for 1.5 h and 24 h (Figure 2).As a result, Au was adsorbed almost quantitatively by both of the metal adsorbents in 1.5 h.Pd was also adsorbed quantitatively in 24 h, but the adsorption ratios at 1.5 h were di ered to be 52.3 and 98.7% for 1 and 2, respectively.e faster adsorption by 2 can be ascribed to the hydrophilicity of the DMAA unit higher than that of the NIPAM unit.Other metals were negligibly adsorbed.e selectivity of the metal adsorption by the sulfur-containing metal adsorbents can be ascribed to the very soft Lewis basicity of the thiocarbonyl group in the dithiocarbonate moieties having high a nity with the very soft Lewis acids Pd and Au.
Second, a multielement solution B was employed for the adsorption at room temperature for 24 h (Figure 3).As a result, all the adsorption ratios were below 10%.e low adsorption originated from the low a nity between the very soft thiocarbonyl group and the hard rare earth elements.
ird, a multielement solution C was employed for the adsorption at room temperature for 1.5 and 24 h (Figure 4).e exact metal concentrations of Na, Mg, Al, K, Ca, Fe, Zn, and Ba could not be obtained using the standard curves based on this multielement solution, probably due to the leakage of these metals from the vials used in this experiment.As a result, Ag was adsorbed almost quantitatively by both of the metal adsorbents within 1.5 h.Se, originating from SeO 2 and existing as selenous acid, was also adsorbed at 36.2 and 31.4% by 1 and 2, respectively, and the adsorption ratios were increased to ca. 60% at 24 h.We presume that selenous acid coordinated to the Lewis basic carbonyl groups in 1 and 2 as a hard Brønsted acid.e selectivity of 1 was excellent by the negligible adsorption of other metals, but that of 2 was lower due to the slight adsorption of most of the elements, suggesting nonspeci c adsorption described later.e experiments using these three multielement solutions proved that the hydrophilic sulfur-containing adsorbents, especially 1, are e ective adsorbents for noble metals with high selectivity and e ciency even in the presence of various elements.

Control Experiment Using
Cross-Linked Polymers without DTCMMA Unit.We con rmed the e ect of the DTCMMA unit using analogous cross-linked polymers without DTCMMA unit.e cross-linked polymers were synthesized by the radical copolymerization of the hydrophilic monomers NIPAM (1′) and DMAA (2′) with MBAA in DMSO at 60 °C under a nitrogen atmosphere.Control experiments with 1′ and 2′ were conducted using the multielement solutions A and C, which contain the metals adsorbed by 1 and 2. e metal adsorption experiments were conducted at room temperature for 24 h in a similar manner with the adsorption experiments using the sulfur-containing adsorbents.Most of the elements in A were hardly adsorbed as the cases of 1 and 2, but the adsorption ratios of Au by 1′ and 2′ were slightly higher (19.0% and 16.7%, resp.)(Figure 5).A plausible reason for the adsorption of Au is the adsorption with the amide moieties as reported for poly (NIPAM), poly(DMAA), and poly(N,N-diethylacrylamide) gels [63], which should be occurred for the amide moieties in 1′ and 2′ leading to the nonspeci c adsorption.In the adsorption from the multielement solution C, 1′ negligibly adsorbed metals, indicating that nonspeci c adsorption is ignorable (Figure 6).Contrary to 1′, 2′adsorbed most of the metals though to lesser expects.ese control experiments supported the excellent selectivity of 1 toward noble metals than 2. A possible reason is the harder Lewis basicity of the carbonyl group in DMAA units that is likely to be responsible for nonspeci c adsorption. e di erence in the Lewis basicity was supported by FT-IR absorption of the carbonyl groups in 1′ and 2′ appeared at 1635 cm −1 and 1610 cm −1 , respectively.
is result convinced us that the DTCMMA unit in the sulfur-containing adsorbents served as the important group for adsorption of noble metals.used with other metals such as catalysts for Sonogashira coupling reaction combined with copper halides [64].For this reason, extraction of Pd from mixtures containing excess amounts of other metals such as Cu is an important subject.Accordingly, we conducted competitive adsorption experiments for Pd from mixtures containing excess amounts of Cu with 2. e feed molar ratios of the C S moieties in 2 and the metals examined were [C S] : [Pd] 0 :[ Cu] 0 8.9 : 1 : 100 and 8.9 : 1 : 200.e experiment was conducted for 1.5 h in a similar manner with the case of the multielement solutions.As a result, 2 adsorbed Pd in an e ective and selective manner at both of the feed ratios in spite of the excess amounts of Cu (Figure 7). is result suggested the excellent selectivity of 2 as a Pd adsorption agent from solutions containing various base metals such as wastewater from Cu mines and waste solutions after Pd-catalyzed reactions [65].

Conclusions
We developed hydrophilic sulfur-containing adsorbents having thiocarbonyl groups based on a methacrylate bearing dithiocarbonate moieties and investigated the e ect of the structure adjacent to the thiocarbonyl group.
e hydrophilic sulfur-containing adsorbents 1 and 2 could selectively adsorb noble metals, Au, Ag, and Pd, from multielement aqueous solutions A and B. Au and Ag were adsorbed within 1.5 h by 1 and 2. e adsorption of Pd from multielement solution A with 2 proceeded faster than 1 owing to the higher hydrophilicity of DMAA than NIPAM but 1 also quantitatively adsorbed Pd for 24 h.Selective adsorption of Pd could be achieved with 2 in the presence of excess amounts of Cu by the high noble metal selectivity.e selectivity of 1 to noble metals was higher than 2 due to the lower nonspeci c adsorption.
e high selectivity of the hydrophilic sulfur-containing adsorbents is advantageous for practical recycling of noble metals from industrial and mining wastewater.

Figure 7 :
Figure 7: Competitive metal adsorption from mixed solutions of Pd and excess amount of Cu by 2.
Adsorption from Mixed Solution of Pd and Excess Amount of Cu by 2. Pd is a noble metal, and the concentration in nature is very low.In addition to the content in Pd mines, Pd is slightly contained in ores of other metals such as Cu.In industrial and catalytic uses of Pd, Pd are often