Competitive Adsorption of Arsenate and Phosphate on Calcite

Calcite, the most stable calcium carbonate in soil, is a major part of soil solid phase in arid and semiarid regions. This mineral significantly affects the chemical behavior of ions including oxyanions and transition metal ions in the soil. Arsenate and phosphate are two important oxyanions in natural systems like soil and because of similar ionic properties strongly compete for the colloids surface charge via adsorption and desorption reactions. Because of the importance of this reaction in controlling the equilibrium concentrations of arsenate and phosphate in soil, in this research, arsenate adsorption on calcite was measured and modeled as a function of arsenate concentration and ionic strength and also in competition with phosphate. In addition, charging behavior of calcite was measured by acid-base titration at various ionic strength. Based on the titration data, calcite point of zero charge (PZC) was obtained at pH=8.2. Adsorption isotherms showed that arsenate adsorption is affected by the ionic strength and the initial concentration of arsenate. Adsorption of arsenate was high at low ionic strength and was decreased with increasing the ionic strength. Arsenate adsorption was also decreased with increasing phosphate concentration, but arsenate had no effect on phosphate adsorption indicating phosphate is adsorbed stronger than arsenate on calcite. The experimental data were successfully (R2=0.998) described with a single set of parameters by CD-MUSIC model, considering two inner sphere surface complexes ≡Ca2O2AsO2 and ≡Ca2O2PO2. Overall, the experimental data and model parameters implied that the stronger interaction of phosphate with calcite increases bioavailability and mobility of arsenate in calcareous soils.