Synthesis of Bifunctional Poly(vinyl Phosphonic Acid-co-glycidyl Metacrylate-co-divinyl Benzene) Cation-exchange Resin and Its Indium Adsorption Properties from Indium Tin Oxide Solution

Poly(vinyl phosphonic acid-co-glycidyl methacrylate-co-divinyl benzene) (PVGD) and PVGD containing an iminodi-acetic acid group (IPVGD), which has indium ion selectivity, were synthesized by suspension polymerization, and their indium adsorption properties were investigated. The synthesized PVGD and IPVGD resins were characterized using Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spec-troscopy (EDS) and mercury porosimetry. The cation-exchange capacity, the water uptake and the indium adsorption properties were investigated. The cation-exchange capacities of PVGD and IPVGD were 1.2-4.5 meq/g and 2.5-6.4 meq/g, respectively. The water uptakes were decreased with increasing contents of divinyl benzene (DVB). The water uptake values were 25%-40% and 20%-35%, respectively. The optimum adsorption of indium from a pure indium solution and an artificial indium tin oxide (ITO) solution by the PVGD and IPVGD ion-exchange resins were 2.3 and 3.5 meq/g, respectively. The indium adsorption capacities of IPVGD were higher than those of PVGD. The indium ion adsorption selectivity in the artificial ITO solution by PVGD and IPVGD was excellent, and other ions were adsorbed only slightly.


Introduction
Rare metals with conductivity and transparency, such as indium, are essential for the production of display panels, such as LCD, OLED and PDP.China possesses most of the rare metals and recently weaponized these resources.Therefore, a secure supply of rare metals, which are necessary to the semiconductor industry and the munitions industry, is urgently needed.Intensive research on indium recovery technologies and material development have been conducted in developed countries (USA, Japan) to secure these resources from seawater and ITO etching waste fluid [1][2][3][4].
However, the amount of indium in seawater is very low.It is important to develop an adsorption and separa-tion material with high selectivity.To separate indium from ITO waste fluid which is produced during the display etching process called "urban mining," the development of a selective adsorption and separation material for the recovery of indium from the etching waste fluid in highly acidic conditions is required [5][6][7].
Ion exchange resins are widely used for the recovery of indium from seawater due to the simplicity of the exchange process and the regeneration ability of the resin.However, the development of a highly selective ion exchanger is needed because the concentration of indium in seawater is low [8][9][10][11][12].
The chemical extraction method, which uses nitric acid or a combination of nitric acid and hydrochloric acid, the electro-dialysis method, and the ion exchange method are used for the recovery of indium from ITO etching fluid [2,4].Chemical extraction using acids has high efficiency, but it is difficult to separate indium and tin because both of these elements are simultaneously extracted from seawater due to the difficulty involved in controlling the pH.The electrodialysis method has high selectivity, but it suffers from contamination of the membrane, which is a fundamental part of the electrodealysis method, due to the high acidity of the ITO etching waste fluid, which causes a decrease in selectivity.The ion exchange method can improve the weaknesses of other methods; however, it is necessary to develop a highly selective adsorption material for the recovery of traces of indium from dilute solutions.
The most widely used ion exchange resins have phosphate groups for the selective adsorption of indium.However, the development of a novel resin is necessary because these resins have low mechanical stability and low adsorption capability.
We synthesized a bifunctional poly(VPA-co-GMAco-DVB) cation exchanger with a high ion exchange capacity, good mechanical strength, and easy functionalization capability of selective indium adsorption groups for the selective absorption and separation of indium.We investigated its structure, ion exchange capacity, water uptake, and the optimal conditions for the synthesis of the resin and the adsorption of indium via adsorption tests using artificial ITO solutions.In this paper, it was experimental with "February.4. 2013.Chungnam National University, 79 Daehangno, Yuseong-gu, Daejeon 305-764, Republic of Korea".

Cation-Exchangers
The highly selective bifunctional poly(VPA-co-GMAco-DVB) cation exchangers were synthesized by suspension polymerization [10,[11][12][13].Table 1 and Figure 1 show the synthesis conditions, the polymerization scheme and the functionalization of the poly(VPA-co-GMAco-DVB) cation exchangers with iminodiacetic acid, respectively.The polymerization was conducted in a 1000 mL four-neck round-bottom flask equipped with a mechanical stirrer (IKA ® RW20 digital, IKA company, Osaka, Japan), a condenser, a nitrogen inlet, a thermometer and a dropping funnel.VPA, GMA and DVB were dissolved in acetonitrile (ACN) and PVA at 70˚C.The solution was placed under continuous, strong agitation until all of the monomers and 2,2'-Azobis isobutyro nitile (AIBN), which was used as an initiator, were completely dissolved.The polymerization was performed in airtight equipment at 70˚C and maintained with stirring (400 rpm) for 24 hours.The bifunctional poly(VPA-co-GMA-co-DVB) resins were separated by vacuum suction and washed using distilled water until a pH of 7 was achieved.The poly(VPA-co-GMA-co-DVB) resins were dried in a vacuum oven for 24 hours at 50˚C.The yield of the copolymer resins was determined using Equation (1): where W d is the weight of clean and dry polymer beads (g) and W m is the initial weight of the monomer (g).
Table 2 shows the functionalization conditions.The reaction for functionalizing the poly(VPA-co-GMA-co-DVB) cation exchangers was conducted in a 500 mL four-neck round-bottom flask equipped with a mechanical stirrer (IKA ® RW20 digital, IKA company, Osaka, Japan), a condenser, a nitrogen inlet, a thermometer and a dropping funnel.The poly(VPA-co-GMA-co-DVB) copolymer was placed in the reactor with 300 mL of DMF.The functionalization was performed at 120˚C for 24 hours with stirring (400 rpm).After introducing the IDA groups in the GMA of the copolymer, the remaining unreacted epoxide groups were hydrolyzed with a dilute HCl solution for 2 hr at 80˚C.Subsequently, the poly (VPA-co-GMA-co-DVB) cation exchangers with IDA groups were washed with methanol and then dried at 50˚C in a vacuum oven.

Characterization of Poly(Vinyl Phosphonic Acid-co-metacrylic Acid)
The structures of the poly(VPA-co-GMA-co-DVB) resins and the bifunctional poly(VPA-co-GMA-co-DVB) cation exchangers were characterized using a FT-IR spectrometer (IR Prestige-21, Shimadzu, Kyoto, Japan).The KBr pellets, which contained 1 mg of the sample and 150 mg of KBr, were prepared on a press using a 60 -70 kN of compression force for 10 minutes under vacuum.
The FT-IR spectra were obtained over a wavenumber range of 4000 -600 cm −1 , the resolution was 4 cm −1 , and 20 scans were recorded.
The morphologies of the poly(VPA-co-GMA-co-DVB) cation exchangers were analyzed using a scanning electron microscope (SEM), and the elemental analyses of these exchangers were conducted using energy dispersive X-ray spectroscopy (EDS, JSM-7000F, JEOL, Akishima, Japan).Incident electron-beam energies from 0.5 to 30 keV were used.In all cases, the beam was at a normal incidence to the sample surface, and the measurement time was 100 s.All of the surfaces of the samples were covered with osmium using the ion sputtering method.
The acidic resistance of the PVGD and IPVGD cation exchange resins was investigated using the weight measurement method.One gram of resin and a 5% HCl standard solution (30 ml) were placed in a 50 ml Erlenmeyer flask.The flask was placed in a shaking water bath at 50˚C.The resin was removed from the flask after 60 minutes, washed with distilled water and dried in a heating oven at 50˚C.The durability was calculated using Equation (2): where W w and W d are the weights of the sample before and after treatment with acid, respectively.The crush strength of the PVGD and IPVGD cation exchange resins before and after acid treatment were sted with a universal testing machine (UTM).The

Water Uptake and Cation Exchange
A 1 g, ple was immersed in DI water for 24 hr.te specimens were prepared according to the ASTM standard.All specimens were measured 5 times.

Capacity dried sam
The sample was removed from the DI water and wiped with absorbent paper to remove excess water adhered to the surface.The sample was then weighed on a balance.The water uptake was calculated using Equation (3) [14,15]:

   
Water uptake % 100 where W w and W d are the weights of the samp e the ion-exchange capa le in wet and dry conditions, respectively.Titration was used to determin city (IEC) of the cation exchange resins.The sample was equilibrated in 100 mL of a 0.1 N NaOH solution at room temperature for 24 hr before it was removed, and then 20 mL of the NaOH solution was titrated with a 0.1 mol/L HCl solution, which contained a drop of phenolphthalein solution (0.1% in ethanol) as a pH indicator.The experimental IEC was calculated according to Equation (4): where V and V are the volume of the NaOH so ere adjusted with a n ligible mount f a 0. or 0.1 M grams of the bifunctional poly(VPA-co-GMA-co-DVB) cation exchange resins were immersed in a 50-mL indium solution, and the cation exchange resins were mixed with an indium solution in a shaker, which was operated at 200 rpm for 24 hr to reach equilibrium.

Indium Adsorption Property of indium from
The indium adsorption property was determined using the EDTA titration method.A certain amount of sa as immersed in a 100 mg/L indium solution, and then 40 mL was taken from the indium solution.1-(2-Pyridylazo)-2-naphtol (PAN) was used as an indicator.The amount of adsorbed indium was calculated using Equation (5): Amount of adsorbed indium mmol g

2.5
In in ETDA EDTA resin where C in and V in are the molar concentratio volume of the initial indium solution, respectively.C VPA-co-GMA-co-DVB) microindium were dipped into a solu-n and the EDTA and V ETDA are the molar concentration and the consumed volume of the EDTA solution, respectively.

Durability Test
The bifunctional poly( beads that had adsorbed tion of 0.1 mol/L HCl with stirring for 4 hr at 25˚C to Copyright © 2013 SciRes.OJPChem desorb indium (III) ions.The solution was then filtered and washed with water.The obtained bifunctional poly (VPA-co-GMA-co-DVB) microbeads were used in the adsorption experiment.This entire process was repeated for 10 cycles to ascertain the reusability of the bifunctional poly(VPA-co-GMA-co-DVB) microbeads.

Preparation of PVGD an Cation-Exchange Resins d IPVGD
VPA-co-GMA ifunctional poly(VPAspectra for confirming the ion of poly(vinyl phosphonic Figure 2 shows the conversions of the poly( -co-DVB) resins and the b co-GMA-co-DVB) cation exchanger.The conversions were calculated from Equation (1), where the initial weights of the monomers revealed the weights before the synthesis of the poly(VPA-co-GMA-co-DVB) resins in the cases of both synthesis and functionalization.The conversion was increased slightly to increase the GMA molar ratio, and the maximum conversion was 93%.The use of divinyl benzene (DVB) as a crosslinking agent did not affect the conversion.However, the conversions of the bifunctional poly(VPA-co-GMA-co-DVB) cation exchanger ranged between 85% and 93%, and the values increased with increasing GMA molar ratio.

Structure Analysis
Figure 3 shows the FT-IR structure and functionalizat acid-co-glycidyl methacrylate-co-divinyl benzene) (PVGD) and poly(vinyl phosphonic acid-co-glycidyl methacrylate-co-divinyl benzene) containing iminodiacetic acid group (IPVGD) cation exchange resins.As shown in Figure 3(a) (PVGD spectrum), the strong broad band at approximately 3500 cm −1 was attributed to OH vibrations.The bands resulting from methylene C-H stretching vibrations were observed at approximately 2941 cm −1 .The strong band at 1194 cm −1 was ascribed to the stretching vibrations of P=O, and the absorption band at 965 cm −1 was ascribed to the P-OH stretching band.In addition, a strong band at 1720 cm −1 resulted from C=O stretching vibrations in GMA, and the other band at 450 -1650 cm −1 was due to stretching vibrations in the vinyl groups of DVB.These results confirmed which PVGD cation exchange resins were synthesized.Figure 3(b) shows the FT IR spectrum of poly(vinyl phosphonic acid-co-glycidyl methacrylate-co-divinyl benzene) containing iminodiacetic acid group (IPVGD) cation exchange resins.The strong band at 1660 cm −1 was attributed to COOH vibrations, and the intensity of the broad band at approximately 3450 cm −1 , which was attributed to OH, was increased.This result confirmed which IPVGD cation exchange resin was synthesized [16].

SEM-EDS of PVGD and IPVG
Figure 4 and Table 4 show the morpholo and the elemental compositi bifunctional IPVGD cation exchange resins determined using SEM-EDS analyses.The surfaces of the resins were smoothly spheres, and their average particle sizes were approximately 50 μm.Figure 4 shows SEM photographs of the resins.The morphologies of the resins before and after functionalization were not observed to be different in PVGD and IPVGD.
Table 4 shows the proportions of carbon, oxygen and phosphorous in the PVGD an osphorus oxygen contents were determined to be 1.00%, 1.96%, 2.68% and 3.76%, respectively, in the PVGD resins.Their contents were increased by increasing the VPA monomer ratio.Meanwhile, the oxygen compositions for IPVGD resins were higher than that of PVGD.Their values were 1.00%, 1.96%, 2.68% and 3.76%, respectively.These results confirmed which Open Access OJPChem

Figu
ter u the PVGD and IPVGD resins.The water increasing DVB contents.The water uptakes of the IPVGD resins were higher than PV re performed at pH 4. The cation-exchange resin was placed in uptake was decreased with those of PVGD because the hydrophilicity of the resins increased due to the carboxyl group.The crosslinking ratios were also determined to affect the water uptake of the hydrophilic polymers.The water uptake was also dependent on the amount of crosslinking agent.The water uptake values decreased to a minimum of 75% as a result of the increased crosslinking density of the resins.
The IEC provides an indication of the acid group content in the PVGD and IPVGD resins.The experimental IEC values are given in Figure 6.This result confirmed that the IEC values decreased.The IEC values of the GD and IPVGD resins were 2.5 -5.6 meq/g and 2.7 -6.4 meq/g, respectively.In addition, the values for the IPVGD resins were higher than those of the PVGD resins because of the increase of the hydrophilicity due to the introduction of the COOH groups.This result led to the conclusion that the IPVG resins were acceptable adsorbents for indium from ITO and the dilute solutions.

Indium Adsorption Properties
Adsorption isotherm experiments to examine the adsorption of indium on the cation-exchange resin we  C q K q C q  (5) where C e is the equilibrium concentration (mmol/L), q e is the adsorption capacity at equilibrium (m q m is the maximum amount of solute exchanged per gram of micro-beads (mmol/g resin).K L mol/g resin) and and K F are the Langmuir constant and the Freundlich constant related to the adsorption capacity, respectively, and n is a constant to be determined.
Figure 7 shows the experimental adsorption isotherms of In 3+ on the cation-exchange resin.Figures 8(a 3. Finally, the Langmuir and Freundlich models calculated from the data in Table 5 are also illustrated in Figure 7.The Langmuir constants, such as K L and q m , were obtained using the linear plot of e e C q vs. C e from Equa-  a m nolayer of ad orptio whe as th reundlich model onside s equi brium n a h teroge eous surf ce, wh re the sorpti n ener y is no homo eneous for all adsorptio sites Thus, he Fr ndlic odel can be app ied to ulti-la er adsorption There fo cation-exchange resin is considered to be the adsorption of a monolayer. The maximum value of q m , the maximum amount of solute exchanged per gram of poly(GMA-co-PEGDA) microbeads, was attained at a 90% molar ratio of GMA, and the value was 0.614 mmol/g resin.

Acidic Res
The indium adsorbents endured in the strong acid solution because the pH of the ITO etching solution was low.Thus, this experiment measured the acid resistance of the PVGD and IPVGD resins.Figure 9 sh the acidic resistance for t The weight losses of the P minimal.According to this result, the PVGD and IPVGD resins were very excellent in the low pH solution.In general, the acidic resistance was influenced by the degree of crosslinking of the resins.The degree of crosslinking of the PVGD and IPVGD resins increased with increasing DVB content in the resins.When the degree of crosslinking was increased, the skeletal structure of the resins increased the rigidity.Thus, the mechanical strength was higher than that before crosslinking.

Durability
The durability was also investigated.The maximum sorption capacity changes of indium versus the number of reuses are provided in Figure 10 for up to 10 reuses.Each the number of reuse tim (6).It is observed that t indium onto PVGD and IPVGD decreased only slightly with an increasing number of reuse times, which indicates that the prepared microbeads have a good reusability.

Conclusion
PVGD and IPVGD, which have indium ion selectivities, MA-coanger ranged between 85% and 93%, creased with increasing GMA molar were synthesized by suspension polymerization and the indium adsorption properties were investigated.The conversions of the bifunctional poly(VPA-co-G ir DVB) cation exch and the values in ratio.The synthesized PVGD and IPVGD resins were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS).According to the FTIR result, it was confirmed that the IPVGD cation exchange resin was synthesized.The surfaces of the resins were wrinkled spheres, and their average particle sizes were approximately 50 μm.However, the morphology of resins before and after functionalization did not show a difference between PVGD from IPVGD.The contents were increased by increasing the VPA monomer ratio.The water uptakes of IPVGD resins were higher than those of PVGD because the hydrophilicity of the resins increased due to the carboxyl group.The IEC val-ant number (2013008092)) 05 ues increased as the DVB concentration decreased, which was accompanied by a consequential increase in the GMA concentration.When the degree of crosslinking was increased, the skeletal structure of the resins increased in rigidity.

Figure 2 .Figure 3 .
Figure 2. The effect of the GMA content on the conversion.

5 Figure 5 .
Figure 5.The effect of DVB mole ratio on the water uptake.

Figure 6 .
Figure 6.The effect of DVB mole ratio on the ion-exchange capacity.contact with the solution in a shaker operating at 200 r ) and (b) are the linear plots of the Freundlich and Langmuir models obtained from the experimental data in Figure 7.All constants derived from Figure 7 are listed in Table

Table 3 . Chemical composition of artificial ITO waste wa-
pure and artificial ITO solutions (see Table3) were conducted under ambient conditions using the batch technique.The experiments examining the adsorption onto the bifunctional poly(VPA-co-GMA-co-DVB) cation exchange resins were performed at pH 4. The pH values ter.