Iron Sulfide-Coated Sand for Remediation of Arsenic(III)-Contaminated Anoxic Groundwater.

Abstract

This study investigated the feasibility of using reduced iron sulfide (FeS) as a permeable reactive barrier (PRB) material for remediating As(III) contaminated groundwater under anoxic conditions. FeS is more advantageous than more commonly used materials such as zero valent iron (ZVI) or Fe(III)-based materials in treating As(III)-contaminated groundwater, under anoxic conditions, as other Fe(III)-based adsorbents display a lower removal capacity and may release immobilized arsenic back into solution by reductive dissolution. To use nano-sized FeS as a PRB material, a method was developed to coat a natural sand giving a coating 1.2 to 4.0 mg FeS/g. The removal capacity of the FeS-coated sand was 30%, 70% and 300% at pH 5, 7 and 9, respectively, of the maximum capacity of nanoscale FeS. Although some reduction of uptake capacity was observed at pH 5 and 7, these capacities are greater to iron or aluminum based adsorbents under anoxic conditions. Spectroscopic methods such as x-ray absorption spectroscopy and x-ray photoelectron spectroscopy showed that the As(III) uptake mechanisms of FeS-coated sand as the precipitation of orpiment (As2S3) at pH 5 and the adsorption of arsenite at pH 9. The prominent differences in As(III) removal mechanisms as a function of pH also affects the column capacity of FeS-coated sand. At pHs where precipitation dominates, the column capacity is closely related to the amount of sulfide and column capacity was greater than that of batch system due to the continuous supply of sulfide from the dissolution of FeS. In contrast, at pH 9 where adsorption dominates, the As(III) removal capacity of the column system is comparable as long as the sorption non-linearity is considered and the retention time is adequately long. Overall, this dissertation supports FeS as a promising material for use in a PRB to remove As(III) under anoxic conditions.