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 μmol/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 μmol/L. Competitive effect of arsenate for arsenite adsorption sites was observed when [initial As] = 0.385 μmol/L. In dual adsorbate systems the Γ_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:FeOH + H⁺ ↔ FeOH⁺₂,4.744,FeOH ↔ FeO^- + H⁺,-9.03,AlOH + H⁺ ↔ AlOH⁺₂,7.229,AlOH ↔ AlO^- + H ⁺,-9.316,FeOH_(s) + H₃AsO_3(aq) ↔ FeHAsO^2-_3(s) + H₂O,6.798,AlOH_(s) + H₃AsO_3(aq) ↔ AlHAsO^2-_3(s) + H₂O,5.319,FeOH_(s) + H₂AsO^- _4(aq) ↔ FeHAsO^2-_4(s) + H ₂O,11.88,AlOH_(s) + H₂AsO^- _4(aq) ↔ AlHAsO^2-_4(s) + H ₂O,9.061.