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 γ-Al2O3 to better understand the competitive adsorption of these anions in the natural environment. The Freundlich isotherms of anionic adsorption onto γ-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 soluteʹs 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 γ-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 aij and relative affinity coefficients αij 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.