Monday, November 13, 2006
87-7

Adsorption of As(V), As(III) and Methyl Arsenic by Calcite and the Impact of Counterion and Coion Species.

Robert Jones and Richard Loeppert. Texas A&M University, Soil & Crop Sciences Dept., 2474-TAMU, College Station, TX 77843-2474

Calcite has been implicated as playing a possible role in the retention and solubility of arsenic in calcareous soils and various other environments which contain an abundance of carbonates. Calcite surface chemistry is influenced by environmental factors such as pH, alkalinity and the partial pressure of CO2 and may be influenced by dissolved aqueous species such as Ca2+, Mg2+, phosphate and sulfate. Sorption processes at the calcite-water interface might also be impacted by these environmental parameters. The objective of this research is to investigate the retention of arsenate (AsV), arsenite (AsIII), monomethyl arsenate (MMAsV) and dimethyl arsenate (DMAsV) by calcite and access the impact of dissolved Ca2+, Mg2+, phosphate and sulfate on arsenic solubility, adsorption and precipitation phenomena. Adsorption isotherms, reaction kinetics and equilibrium relations were evaluated for arsenic in the presence of these common groundwater ions. Arsenic, phosphorus and sulfur concentrations were determined using ICP-MS, calcium and magnesium by FAAS and alkalinity by titration. Aqueous speciation and solubility relations of arsenic with respect to calcite and the possible formation of calcium arsenates were modeled using MINTEQA2 by imputing data for batch pH, alkalinity, calcium and arsenic concentrations. Adsorption isotherms were modeled using the Freundlich equation. The relative adsorption of arsenicals was shown to decrease in the following order: AsV > AsIII > DMAV > MMAV. Increased dissolved calcium concentrations resulted in an increase in arsenic adsorption while phosphate was shown to be the primary sorbed species when at equimolar concentrations with arsenic. The surface interactions of calcite with arsenic in these aqueous systems are quantified and modeled with the results having implications to calcite sorption behavior in soil and groundwater environments.