RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Goethite is a sink for antimonate Sb(OH)₆^– [Sb(V)] in soil and sediments. The pH- and ionic strength (I)-dependent mass distribution of Sb(V) between adsorbed and solution phases at equilibrium may be used to indirectly infer an adsorption mechanism (inner vs. outer sphere surface complexation) and to develop predictive surface complexation models. The objectives of this study were to characterize the adsorption of Sb(V) by goethite as a function of pH, I, and in the absence or presence of competing ligands, SO₄ or PO₄. Ligand adsorption was at a maximum in strongly acidic suspensions and decreased with increasing pH. Adsorption envelopes of Sb(V) where not influenced by I, but increasing I decreased SO₄ and increased PO₄ retention. In strongly acidic suspensions, SO₄ and PO₄ retention was not influenced by I. Ligand adsorption decreased the isoelectric point (IEP) of goethite from 8.5 to 6.3 [Sb(V)] and 4 (PO₄). The adsorption of SO₄ did not affect the IEP, although SO₄ reduced the zeta potential of goethite when pH < IEP. Antimonate and PO₄ adsorption was hysteretic; SO₄ adsorption was reversible. Sulfate did not impact Sb(V) retention; adsorbed Sb(V) generated greater negative surface charge, decreasing SO₄ adsorption. Both Sb(V) and PO₄ competed for adsorption sites, resulted in reduced retention of both in competitive systems. The triple layer formulation of the charge distribution multisite complexation model, coupled with inner sphere surface complexation, was used to describe the adsorption envelopes for Sb(V), SO₄, and PO₄, and to predict adsorption in competitive systems.