| Using synthetic swine wastewater, bench-scale experiments were carried out to investigate the effects of pH, varied initial n(Ca)/n(P) molar ratio, carbonate (CO2-3), magnesium (Mg2+) as well as coexistence of CO2-3 and Mg2+ on calcium phosphate (CP) precipitation for phosphorus removal. The composition of the obtained precipitate was analyzed and characterized by X-ray diffraction (XRD). The results showed that with pH value of 8.0 and initial n(Ca)/n(P) of 3.33, through extending the reaction time to 360 min, the removal rate of phosphate could reach 92.2%. Higher pH value and initial n(Ca)/n(P) were beneficial to calcium phosphate precipitation. However, with pH ≥10.0 and the initial n(Ca)/n(P) ≥3.33, these two factors had negligible effects on the improvement of phosphate removal rate. At pH=9.0, the effects of the separately existing CO2-3 and Mg2+ on the precipitation of phosphate was very small, with the phosphate removal rate keeping stable at 94.1%, averagely. The reaction of CO2-3 and Ca2+ caused the formation of prior production of calcite (CaCO3) and reduced the purity of HAP. The coexisting of Mg2+ made the precipitate be transformed into amorphous calcium phosphate (ACP); through XRD analysis of the heat treated precipitate, it was found that Mg2+ incorporated into the calcium phosphate, forming Mg-substituted whitlockite (Ca,Mg)3(PO4)2, which changed the structure of the precipitate. When CO2-3 and Mg2+ coexisted, the interaction of CO2-3 and Mg2+ would be beneficial to calcium phosphate precipitation with controlled CO2-3 and Mg2+ concentration and elevated solution pH value (≥9.0). |
