Unraveling the interplay of common groundwater ions in arsenic removal by sulfide-modified nanoscale zerovalent iron

Sulphidation of nZVI (S-nZVI) has shown to significantly improve the arsenic removal capacity of nZVI, concurrently modifying the sequestration mechanism. However, to better apply S-nZVI for groundwater arsenic remediation, the impact of groundwater coexisting ions on the efficacy of arsenic uptake by S-nZVI needs to be investigated. This present study evaluates the potential of S-nZVI to remove arsenic in the presence of typical groundwater coexisting ions such as Cl⁻, HA, HCO₃⁻, PO₄³⁻ and SO₄²⁻ through batch adsorption experiments. Individually, PO₄³⁻ and HA had a dominant inhibition effect, while SO₄²⁻ promoted As(III) removal by S-nZVI. Conversely, for As(V) removal, HCO₃⁻ and SO₄²⁻ impeded the removal process. X-ray spectroscopic investigation suggests that the coexisting ions can either compete with arsenic for the adsorption sites, influence the S-nZVI corrosion rates and/or generate distinct corrosion products, thereby interfering with arsenic removal by S-nZVI. To investigate the cumulative effects of these ions, a 2^5–1 Fractional Factorial Design of experiments was employed, wherein the concentration of all the ions were varied simultaneously in an optimized manner, and their impact on arsenic removal by S-nZVI was observed. Our results shows that when these ions are present concurrently, PO₄³⁻, SO₄²⁻ and HA still exerted a dominant influence on As(III) removal, whereas HCO₃⁻ was the main ions affecting As(V) removal, although the combined influence of the ions was not merely a summation of their individual effects. Overall, the finding of our study might provide valuable insight for predicting the actual performance of S-nZVI in field-scale applications for the remediation of arsenic-contaminated groundwater.