Mechanistic modeling of arsenic retention on natural red earth in simulated environmental systems.

Arsenic retention on natural red earth (hereafter NRE) was examined as a function of pH, ionic strength, and initial arsenic loading using both macroscopic and spectroscopic methods. Proton binding sites on NRE were characterized by potentiometric titrations yielding an average pH(zpc) around 8.5. Both As(III)- and As(V)-NRE surface configurations were postulated by vibration spectroscopy. Spectroscopically, it is shown that arsenite forms monodentate complexes whereas arsenate forms bidendate complexes with NRE. When 4<pH<8 and [total arsenic as As(III) or As(V)]=0.385 micromol/L both arsenite and arsenate exhibit near 100% adsorption for a 10-fold variation of ionic strength that is ascribed to inner-sphere complexation of surface bonding. Arsenite exhibits an apparent bond-switching mechanism from inner-sphere to outer-sphere at excess As(III) loading (total arsenic as As(III) or As(V)]=38.5 micromol/L. Competitive effect of arsenate for arsenite adsorption sites was observed when [initial As]=0.385 micromol/L. In dual adsorbate systems the Gamma(As(III)) was reduced over 20%, showing a competition of arsenite for arsenate binding sites (or vice versa). All experimental data were quantified with a 2pK generalized diffused layer model considering two site types for both protons and anions binding using reaction stoichiometries, as follows: [table, see text].