Effects of Dissolved Calcium on Arsenate Sorption at the Kaolinite-Water Interface

The effects of dissolved calcium (Ca) on arsenate (As(V)) reactivity and surface speciation at the kaolinite-water interface were studied as a function of pH using pseudo-equilibrium adsorption/desorption experiments and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses. The EXAFS analyses indicated that As(V) tetrahedral molecules were coordinated on aluminum octahedral via bidentate binuclear (similar to 3.11 angstrom) bonding at pH 4.5 to 6.75 and total As loading levels of 978 to 1,783 mu mol/L kg(-1). There was no evidence for As(V)-Ca ternary species when both As(V) and Ca were present (0.68-1.3 mmol/L). In the adsorption experiments, As(V) adsorption gradually increased with increasing pH and decreased at pH above 6. Interestingly, As(V) sorption was suppressed when 1 mmol/L of Ca was added to the systems at pH 4 to 8. The As(V) desorption was also pronounced when the systems contained Ca. These macroscopic results indirectly suggest that the weakly sorbed As(V) surface species are outcompeted by the ion-exchange process of Ca at the broken edge site of kaolinite. Results of macroscopic data and EXAFS analysis suggest that both outer-sphere and inner-sphere surface As(V) species might coexist on kaolinite surfaces. In predicting the dissolved As(V) in subsurface environments, weakly sorbed As(V) species might be influenced by not only anionic species, but also cationic species in soil solutions and in pore water. Quantifying the effects of various sorbed cations may be important in predicting arsenic transport processes in subsurface environments.