Competitive sorption of tungstate, molybdate and phosphate mixtures onto goethite

摘要

The interactions of MoO_4~(2-) + WO_4~(2-), MoO_4~(2-) + PO_4~(3-), WO_4~(2-) + PO_4~(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-) + PO_4~(3-), phosphate may react with molybdate to form phospho-molybdate, which strongly attaches to the goethite surface and becomes very difficult to remove. No such phenomenon was observed in the binary system containing MoO_4~(2-) + WO_4~(2-). The Shein-dorf-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.