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 mu mol/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)l = 38.5 mu mol/L. Competitive effect of arsenate for arsenite adsorption sites was observed when [initial As] = 0.385 mu mol/L. In dual adsorbate systems the I-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+ double left right arrow FeOH2+, 4.744, FeOH double left right arrow FeO- + H+, -9.03, AlOH + H+ double left right arrow AlOH2+, 7.229, AlOH double left right arrow AlO- + H+, -9.316, FeOH(s) + H3AsO3(aq) double left right arrow FeHasO(3(s))(2-) + H2O, 6.798, AlOH(s) + H3AsO3(aq) double left right arrow AlHAsO3(s)2- + H2O, 5.319, FeOH(s) + H2AsO4(aq)- double left right arrow FeHAsO4(s)2- + H2O, 11.88, AlOH(s) + H2AsO4(aq)- double left right arrow AlHAsO4(s)2- + H2O, 9.061. (c) 2005 Published by Elsevier Inc.