Modeling transport of arsenic through modified granular natural siderite filters for arsenic removal

Groundwater arsenic (As) contamination is a hot issue, which is severe health concern worldwide. Recently, many Fe-based adsorbents have been used for As removal from solutions. Modified granular natural siderite (MGNS), a special hybrid Fe(II)/Fe(III) system, had higher adsorption capacity for As(III) than As(V), but the feasibility of its application in treating high-As groundwater is still unclear. In combination with transport modeling, laboratory column studies and field pilot tests were performed to reveal both mechanisms and factors controlling As removal by MGNS-filled filters. Results show that weakly acid pH and discontinuous treatment enhanced As(III) removal, with a throughput of 8700 bed volumes (BV) of 1.0 mg/L As(III) water at breakthrough of 10 μg/L As at pH 6. Influent HCO₃^- inhibited As removal by the filters. Iron mineral species, SEM and XRD patterns of As-loading MGNS show that the important process contributing to high As(III) removal was the mineral transformation from siderite to goethite in the filter. The homogeneous surface diffusion modeling (HSDM) shows that competition between As(III) and HCO₃^- with adsorption sites on MGNS was negligible. The inhibition of HCO₃^- on As(III) removal was connected to inhibition of siderite dissolution and mineral transformation. Arsenic loadings were lower in field pilot tests than those in the laboratory experiments, showing that high concentrations of coexisting anions (especially HCO₃^- and SiO₄^4-), high pH, low EBCT, and low groundwater temperature decreased As removal. It was suggested that acidification and aeration of high-As groundwater and discontinuous treatment would improve the MGNS filter performance of As removal from real high-As groundwater.