Removal of arsenic(III) from groundwater by nanoscale zero-valent iron

Nanoscale zero-valent iron (NZVI) was synthesized and tested for the removal of As(Ill), which is a highly toxic, mobile, and predominant arsenic species in anoxic groundwater. We used SEM-EDX, AFM, and XRD to characterize particle size, surface morphology, and corrosion layers formed on pristine NZVI and As(Ill)-treated NZVI. AFM results showed that particle size ranged from 1 to 120 nm. XRD and SEM results revealed that NZVI gradually converted to magnetite/maghemite corrosion products mixed with lepidocrocite over 60 d. Arsenic(III) adsorption kinetics were rapid and occurred on a scale of minutes following a pseudo-first-order rate expression with observed reaction rate constants (k(obs) of 0.07-1.3 min(-1) (at varied NZVI concentration). These values are about 1000x higher than k(obs), literature values for As(III) adsorption on micron size ZVI. Batch experiments were performed to determine the feasibility of NZVI as an adsorbent for As(III) treatment in groundwater as affected by initial As(III) concentration and pH (pH 3-12). The maximum As(III) adsorption capacity in batch experiments calculated by Freundlich adsorption isotherm was 3.5 mg of As(III)/g of NZVI. Laser light scattering (electrophoretic mobility measurement) confirmed NZVI-As(III) inner-sphere surface complexation. The effects of competing anions showed HCO3, H4SiO40, and H2PO42- are potential interferences in the As(III) adsorption reaction. Our results suggest that NZVI is a suitable candidate for both in-situ and ex-situ groundwater treatment due to its high reactivity.