Phosphate adsorption and precipitation on calcite under calco-carbonic equilibrium condition

Phosphate (PO₄^3-) removal on calcite often entails two processes: adsorption and precipitation. Separating these two processes is of great importance for assessment of PO₄^3- stability after removal. Thus, this study was aimed at finding a critical range of conditions for separating these two processes in calco-carbonic equilibrium, by adjusting PO₄^3- concentration, reaction time and pH. PO₄^3- removal kinetic results showed that: (I) At pH7.7, PO₄^3- removal was mainly by adsorption at initial PO₄^3- concentration ≤2.2 mg L^-1 and reaction time ≤24 h, with dominant precipitation occurring at initial PO₄^3- concentration ≥3 mg L^-1 after 24 h reaction; (II) At pH8.3, adsorption was the key removal process at initial PO₄^3- concentration ≤7.5 mg L^-1 and reaction time ≤24 h, whereas precipitation was observed at initial PO₄^3- concentration of 10 mg L^-1 after 24 h reaction, (III) At pH 9.1 and 10.1, PO₄^3- removal mechanism was mainly by adsorption at initial PO₄^3- concentration ≤10 mg L^-1 within 24 h reaction. Based on the kinetic results, it is suggested that PO₄^3- precipitation will occur after 24 h reaction when saturation index of amorphous calcium phosphate is between 1.97 and 2.19. Besides, increasing PO₄^3- concentration does not cause a continuous decline of PO₄^3- removal percentage. Moreover, experimental removal data deviated largely from the theoretical adsorption value by CD-MUSIC model. These indicate occurrence of precipitation which is in agreement with the kinetic result. Therefore our study will provide fundamental reference information for better understanding of phosphorous stabilization after removal by calcite.