Removal of high-concentration phosphate by calcite: Effect of sulfate and pH

Sulfate is a common anion in wastewater, and the effect of sulfate on high-concentration phosphate removal by calcite has not been fully investigated. We studied high-concentration phosphate removal as a function of pH in both the presence and absence of sulfate. Solution pH greatly affected phosphate-removal mechanism via calcite and phosphate-calcite reaction products. Phosphate removal was favorable in both acidic and strong basic solutions; solutions with an initial pH between 8.0 and 11.0 were significantly less effective. The effect of sulfate on phosphate removal depended on solution pH. In the acidic solution, phosphate removal was enhanced by a low concentration of sulfate but inhibited by a high concentration of sulfate (i.e., a sulfate-to-phosphate ratio greater than 6.0). Although sulfate can promote calcite to dissolve, high concentrations of sulfate compete with phosphate-forming calcium sulfate first as precursor, and calcium sulfate eventually converts into Hydroxyapatite (HAp). In the strong base media, phosphate removal increased with increasing sulfate concentration. This increase in phosphate removal was attributed to the increase of solubility of calcite in the presence of sulfate. The process of phosphate removal fit a four-parameter logistic function model regardless of the presence of sulfate, but it did not fit traditional kinetic models.

► Sulfate is important factor that affect phosphate removal by calcite.
► pH affected phosphate removal mechanism and phosphate–calcite reaction product.
► The effect of sulfate on phosphate removal was dependent on solution pH.
► The process of phosphate removal fit a four-parameters logistic function model.