Removal of phosphate from aqueous solution by phosphate-modified calcite

Previous literatures have shown that calcite can be used to effectively remove phosphate from aqueous solution. In this study, the calcite-phosphate reaction product, i.e., phosphate-modified calcite was reused to remove phosphate from aqueous solution.The phosphate removal kinetics of natural calcite and phosphate-modified calcite were compared. The effect of various experimental conditions such as solution pH, initial phosphate concentration, temperature, Ca2+and HCO3-on phosphate removal by phosphate-modified calcite was investigated. The phosphate removal efficiency of phosphate-modified calcite was much higher than that of natural calcite. When the reaction time was 2 h, the phosphate removal efficiency of phosphate-modified calcite was 72%under the experimental conditions, which was much higher than that of natural calcite(35%). The phosphate removal efficiency of phosphate-modified calcite was relatively high at pH 5-7, slightly decreased with increasing pH from 7 to 10, and significantly decreased with increasing pH from 10 to 12. The amount of phosphate removed by phosphate-modified calcite increased with increasing initial phosphate concentration. The phosphate removal efficiency of phosphate-modified calcite decreased with increasing initial phosphate concentration at a relatively high initial phosphate-phosphorus concentration(60-160 mg·L-1). The phosphate removal efficiency of phosphate-modified calcite increased with increasing temperature. The phosphate removal kinetics of phosphate-modified calcite followed a pseudo-second-order kinetic model. The removal of phosphate by phosphate-modified calcite occurred predominantly via the precipitation of calcium phosphate according to two consecutive phases: first, the dissolution of calcite produced an increase in the Ca2+concentration; then the Ca2+ions reacted with the phosphate ions to form the precipitate of calcium phosphate. The precipitation of calcium phosphate took place at the surface of phosphate-modified calcite, which acted as substrate for the heterogeneous nucleation of calcium phosphate. The presence of Ca2+ions in aqueous solution represented a further source of Ca2+ions that were available for the precipitation of calcium phosphate, thus resulting in an increase in phosphate removal efficiency of phosphate-modified calcite. Coexisting HCO3-in aqueous solution was unfavorable for the removal of phosphate by phosphate-modified calcite. As a general conclusion, the calcite-phosphate reaction product can be reused to remove phosphate from aqueous solution, and it is a more effective media for the removal of phosphate than the original calcite.