TRACE REMOVAL OF HEXAVALENT CHROMIUM FROM DRINKING WATER USING HYBRID ANION EXCHANGE RESIN

Abstract

Out of the two forms of chromium naturally found in water, hexavalent chromium [Cr(VI)] is the most harmful and even at trace concentration level can cause serious health problems due to prolonged exposure via the drinking water route. In the wake of new findings, various regulatory agencies around the world have started consider lowering the maximum contaminant level (MCL) values for Cr(VI), which makes it important to develop effective Cr(VI) treatment technology. In this research, studies were conducted to investigate the applicability of anion exchange resins for the trace Cr(VI) removal from the matrix consisting of naturally occurring competing anions in concentrations typical of groundwater and to synthesize, characterize and validate hybrid sorbents prepared by dispersing iron sulphide nanoparticles within anion exchange resins for trace Cr(VI) removal. Batch and column studies with different strong and weak base anion exchange resins demonstrated that the selectivity of resins is in the sequence: IRA 400 ˃ IRA 900 ˃ IRA 910 ˃ LMP 64 ˃ IRA 96. During fixed-bed column study with solution resembling natural groundwater containing 200 μg/L of Cr(VI) at pH 7, IRA 400 removed 1.39 mg/g of Cr(VI) before exhaustion. Regeneration of IRA 400 using 10% NaCl solution at pH 4.0 recovered approximately 89% of the total Cr(VI) removed. Random effluent analysis during the batch and column studies showed no presence of Cr(III) species confirming no conversion of Cr(VI) due to any redox reaction; the interaction between anionic Cr(VI) species and the positively charged functional groups of anion exchange resins were solely responsible for the trace Cr(VI) removal. Next, novel hybrid sorbents with iron sulphide nanoparticles were synthesized via two-step in-situ synthesis process. Surface morphology analysis of the hybrid sorbent evidenced that nanoparticles were uniformly distributed within the parent resins. Transmission Electron Microscopy (TEM) analysis provided confirmations that the precipitated nanoparticles were spherical in shape and the majority of the particles were in the range 10 – 40 nm and amorphous in nature. X-Ray Photoelectron Spectroscopy (XPS) spectra of Fe 2p, S 2p and O1s of the interior hybrid sorbent confirmed that the precipitated nanoparticles comprised of FeS2. Validation studies demonstrated that hybrid sorbents had much higher Cr(VI) removal capacity than respective parent resins. A series of investigations involving manipulation of column runs, step-wise regeneration, digestions of exhausted hybrid sorbents and various characterization studies on virgin and exhausted hybrid sorbents were performed to elucidate the exact mechanism for enhanced Cr(VI) capacity. The Donnan membrane effect which stems from the electrostatic attraction caused by the positively charged functional groups fixed on the host anion exchange resin is primarily responsible for attracting Cr(VI) anions and populating them within the anion exchanger at concentrations higher than the bulk solution phase. Evidence showed that there was a redox reaction which took place between Cr(VI) anions and FeS2 nanoparticles within the hybrid sorbent. Due to redox reaction, some ion-exchange functional sites initially occupied by Cr(VI) got freed up so that further uptake of Cr(VI) anions can take place. The redox reaction produced Fe(III)-oxyhydroxides surfaces which provided further sites for Cr(VI) removal using ligand sorption which also played an important role in the enhanced Cr(VI) removal. Thus the synergetic effect of ion exchange followed by redox reaction and ligand sorption was responsible for the higher capacity for the selective removal of trace Cr(VI) concentration. Understanding that the primary reason behind such high capacity for the selective removal of trace concentration of Cr(VI) by the hybrid sorbent was the redox reaction between Cr(VI) with FeS2, the synthesis process was modified in order to load higher amount of FeS2 within the hybrid sorbent. The modified method involved the lowering of the Donnan exclusion potential on Fe2+ by using solutions with a lower dielectric constant than the aqueous solution of FeCl2 during the process of synthesis of the hybrid sorbent. Evaluation of modified hybrid sorbent thus prepared demonstrated higher loading of FeS2 nanoparticle phase and higher Cr(VI) removal capacity than the earlier version of the hybrid sorbent. The research resulted in the synthesis of a hybrid iron sulphide impregnated anion exchange resin which was simple to synthesize and was low cost with the distinction of easy field deployability in fixed-bed columns along with the possibility of regeneration and reuse. These attributes make the hybrid sorbent a good candidate for field applications for trace removal of hexavalent chromium from contaminated groundwater sources.