Modeling competitive adsorption of molybdate, sulfate, selenate, and selenite using a Freundlich-type multi-component isotherm

This study examined the interactions of MoO42- + SO42-, MoO42- + SeO42-, and MoO42- + SeO32- systems on gamma-Al2O3 to better understand the competitive adsorption of these anions in the natural environment. The Freundlich isotherms of anionic adsorption onto gamma-Al2O3 in single and binary solutes were also investigated to estimate the competition between these anions. Experimental results indicate that a higher concentration of competitive solute yields a higher efficiency of the competitive solute's prevention of MoO42- adsorption. The most significant result was found in the MoO42- + SeO32- system. The Freundlich isotherm constant (n) increases with the competitive solute concentration. The suitability of a Freundlich-type isotherm, the Sheindorf-Rebuhn-Sheintuch (SRS) equation, and the modified SRS equation in representing the competitive adsorption of MoO42-, SO42-, SeO42- and SeO32- on gamma-Al2O3 surface, was also examined. Each set of isotherm data was found to conform to linear SRS expressions, allowing competition coefficients to be derived on a concentration basis for each binary-solute system. The competition coefficient a(ij) and relative affinity coefficients ay can be seen as a way to quantify competitive interactions. The proposed SRS and modified SRS equations are simple mathematical expressions accounting for competitive interactions of anions present in a mixture for the range of concentrations over which each individual component exhibits Freundlich behavior. (C) 2002 Elsevier Science Ltd. All rights reserved.