Calcite Surface Adsorption of As(V), As(III), MMAs(V), and DMAs(V) and the Impact of Calcium and Phosphate

The objective of this study was to investigate the retention of arsenate (iAs(V)), arsenite (iAs(III)), monomethylarsenate (MMAsV), and dimethylarsenate (DMAsV) by calcite and the influences of dissolved Ca, Mg, and phosphate, by means of adsorption kinetics, adsorption isotherms, competitive adsorption, and chemical equilibrium relationships. Adsorption of iAs(V) by calcite was quite rapid during the first hour and remained relatively constant after the second hour at both low and high initial As concentrations. The relative adsorption of arsenicals by calcite decreased in the following order: iAs(V) > iAs(III) > DMAsV > MMAsV; however, in no case did As adsorption exceed 90% of added As, even at low initial As concentrations, and clear adsorption maxima were not observed, contrary to the adsorption behavior of phosphate on calcite, where quantitative adsorption at low P concentrations and clear adsorption maxima are commonly observed. Low concentrations of dissolved Ca (0.01 mol CaNO3 L-1) resulted in increased iAs(V) adsorption, compared with high concentrations (0.1 mol Ca L-1) that resulted in decreased As adsorption. Phosphate retarded iAs(V) adsorption; however, the reciprocal effect of iAs(V) on phosphate adsorption was negligible. The current results indicate the relatively minor influence of calcite on As adsorption compared with the substantial impact of Fe oxides and the very different adsorption behaviors of phosphate and iAs(V) by calcite, in spite of the similar ion sizes, pKa, and ionic symmetries.