Influence of HPO42-, H4SiO4, Ca2+, Mg2+ on Fe floc growth and As(III) removal in aerated, natural groundwater

Abstract

Our aim was to systematically investigate the influence of anions (HPO 4 2-), cations (Ca 2+ , Mg 2+ ) and neutral H 4 SiO 4 on Fe flocculation and As(III) removal in the complex natural water matrix. For this purpose, three different anaerobic groundwaters were selected and manipulated by dosing of Ca 2+ , Mg 2+ , HPO 4 2-, or by their removal by cationand anion exchange. The change in Fe floc volume and of dissolved Fe and As were followed in aerated jar experiments. Fe floc growth was improved by addition of Ca 2+ or Mg 2+ , and hindered by their removal. This hindered floc growth was more severe for groundwaters with higher P:Fe ratios, where Fe flocs carry a larger net negative surface charge, and rely stronger on Ca 2+ or Mg 2+ for charge neutralisation. When expressing the charge balance of the different groundwaters as the molar ratio (Ca 2+ + Mg 2+ )/P, a linear relationship was found with the cumulative Fe floc volume, with a plateau at molar ratios > 500. At environmentally relevant concentrations, H 4 SiO 4 was found more likely to compete with As(III) for adsorption capacity than HPO 4 2-. As(III) removal was strongly related to Fe removal -independent of Ca 2+ or Mg 2+