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 (HPO42−), cations (Ca2+, Mg2+) and neutral H4SiO4 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 Ca2+, Mg2+, HPO42−, or by their removal by cation – and 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 Ca2+ or Mg2+, 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 Ca2+ or Mg2+ for charge neutralisation. When expressing the charge balance of the different groundwaters as the molar ratio (Ca2+ + Mg2+)/P, a linear relationship was found with the cumulative Fe floc volume, with a plateau at molar ratios >500. At environmentally relevant concentrations, H4SiO4 was found more likely to compete with As(III) for adsorption capacity than HPO42−. As(III) removal was strongly related to Fe removal - independent of Ca2+ or Mg2+ presence - indicating that As(III) is primarily adsorbed at an early stage in the flocculation process.