Competitive sorption of tungstate, molybdate and phosphate mixtures onto goethite

The interactions of MoO₄^2- + WO₄ ^2-, MoO₄^2- + PO₄^3-, WO₄^2- + PO₄^3- systems on goethite were studied to better understand the competitive adsorption/desorption of these anions in the natural environment. Tungstate has a strong adsorption affinity for goethite; molybdate adsorption onto goethite is reversible (as shown by the overlapping of adsorption and desorption isotherms), while tungstate and phosphate adsorption are irreversible. The activation energy needed by P, W and Mo to adsorb/desorb to/from goethite follows the order: P > W > Mo. The goethite surface contains adsorption sites that are specific to one anion, i.e. phosphate, molybdate and tungstate. For the binary system containing MoO ₄^2- + PO₄^3-, phosphate may react with molybdate to form phosphomolybdate, which strongly attaches to the goethite surface and becomes very difficult to remove. No such phenomenon was observed in the binary system containing MoO₄^2- + WO ₄^2- The Sheindorf-Rebuhn-Sheintuch (SRS) equation was applied to obtain coefficients accounting for the competitive adsorption interactions of tungstate, molybdate, and phosphate. The SRS equation shows that tungstate and phosphate have a comparable competitive effect; furthermore, both significantly compete with molybdate adsorption.