Evidence for surface precipitation of phosphate on goethite

Recent studies have suggested that the interaction between phosphate and goethite includes ternary adsorption/surface precipitation as well as surface complex formation. The ternary adsorption/surface precipitation process envisioned involves the dissolution of the goethite crystal and subsequent adsorption of iron on the surface-bound phosphate. Further evidence to support the suggested process is needed. The process was investigated using two approaches. First, the sorption of iron spiked into a slurry of phosphated goethite and the effect of the iron sorption on phosphate uptake kinetics were investigated to determine whether iron would be adsorbed on the phosphated surface and whether it would enhance phosphate adsorption. Lead was also spiked into solution for comparison. Second, changes in the zeta-potential of phosphated goethite were monitored with time. Adsorption of iron on the surface of phosphated goethite should increase the zeta-potential of the goethite. Iron spiked into a phosphated goethite slurry was adsorbed on the solid with a concurrent adsorption of phosphate. The iron adsorption did not change the slow phosphate adsorption kinetics. Lead spiked into the solution was also sorbed but to a lesser extent than iron and with a lower apparent P:Pb mole ratio. Lead addition also changed the phosphate adsorption kinetics. With time, the zeta-potential of phosphated goethite became more positive, returning almost to the potential of unphosphated goethite at low surface coverages. The slow increase in zeta-potential over time indicates that long-term reactions are occurring on the goethite surface, most likely involving the dissolution of goethite to release iron and the subsequent reaction between the iron and surface-bound phosphate. These results provide strong support for the surface precipitation model, and are inconsistent with models envisioning the reaction between phosphate and the goethite surface as involving only monolayer surface complex formation.