A bauxite waste of alumina manifacture, i.e., red mud (RM), is an oxide-like adsorbent capable of removing radiocesium and strontium. The adsorption behavior of these radionuclides is dominated by the surface charge of the adsorbent and the number of available adsorption sites. In this study, the surface charge densities (sigma), microscopic acidity constants (pK(s)), and site distributions of the RM in 10(-3)-1 M concentrations of NaCl, CsCl, and SrCl(2) solutions were evaluated from potentiometric titration data. The reciprocal slopes of the pH-sigma curves are higher than those predicted by double-layer theory. This suggests that surface charge and the counter charge are located in a region inside the surface because the porous and/or gel surface layer is permeable to these ions. Ionic strength dependency of sigma in CsCl solutions is similar to those found for other oxides. In SrCl(2) and NaCl solutions, at any pH the surface charge decreases as the electrolyte concentration increases. This behavior of the RM may be attributed to the existence of differently charged oxide surface sites of variable affinity for electrolyte ions. Uptake of protons on these sites could be interpreted in terms of H(+) adsorption and well described by the Freundlich equation. The empirical Freundlich parameters were used to characterize a site distribution function which provides information about the affinity ratio of the adsorption sites to H(+) and supporting electrolyte cations. Copyright 2000 Academic Press.