A novel hybrid ferrous sulfide impregnated anion exchanger for trace removal of hexavalent chromium from contaminated water

Abstract

To effectively remove trace concentration of potential cancer-causing Cr(VI) from impaired drinking water, a novel hybrid material was synthesized through an in-situ synthesis process by dispersing ferrous sulfide nanoparticles within an anion exchanger. Characterization studies revealed that the hybrid material, named hybrid ferrous sulfide impregnated anion exchanger (HISIIX), contained uniformly distributed ferrous sulfide nanoparticles of size 10-40 nm within the anion exchanger host. Apart from FeS₂ nanoparticles, it also included nanoparticles of FeO and FeOOH. The incorporation of ferrous sulfide nanoparticles within the anion exchanger contributed to the significant differences in the Cr(VI) uptake capacity of HISIIX. Validation studies using fixed-bed column proved that HISIIX had significantly high Cr(VI) uptake capacity and was able to run for 4200 bed volumes (BVs) before a breakthrough of 50 μg L^-1 when subjected to a synthetic aqueous solution containing 200 μg L^-1 Cr(VI). Cr(VI) uptake capacity of the parent anion exchanger and HISIIX were determined to be 1.39 mg g^-1 and 3.44 mg g^-1, respectively, when the columns were allowed to run until exhaustion. Ferrous sulfide nanoparticles acted as a reducing agent transforming Cr(VI) anions into Cr(III) precipitates. It also produced sites for further removal of Cr(VI) anions through ligand sorption upon oxidation. The anion exchanger substrate attracted anions selectively via the Donnan membrane principle, resulting in a synergy of three different processes - ion exchange, redox reaction, and ligand sorption that gave the HISIIX a high capacity for the selective Cr(VI) removal from contaminated water.

Keywords: Anion exchanger; Cr(VI) removal; Ferrous sulfide; Hybrid material; In-situ synthesis; Reaction mechanism.