Abstract
Core Ideas Sb(V) adsorption by goethite decreases with increasing pH and is unaffected by ionic strength. Sb(V) adsorption is hysteretic. Adsorbed Sb(V) generates a downward shift in the goethite isoelectric point. The adsorption of Sb(V) decreases in the presence of phosphate, but is unaffected by sulfate. Inner sphere mono‐ and bidentate Sb(V) complexation by goethite is predicted. 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.
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