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

Phosphate (PO43-) removal on calcite often entails two processes: adsorption and precipitation. Separating these two processes is of great importance for assessment of PO43- 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 PO43- concentration, reaction time and pH. PO43- removal kinetic results showed that: (I) At pH7.7, PO43- removal was mainly by adsorption at initial PO43- concentration ≤2.2 mg L-1 and reaction time ≤24 h, with dominant precipitation occurring at initial PO43- concentration ≥3 mg L-1 after 24 h reaction; (II) At pH8.3, adsorption was the key removal process at initial PO43- concentration ≤7.5 mg L-1 and reaction time ≤24 h, whereas precipitation was observed at initial PO43- concentration of 10 mg L-1 after 24 h reaction, (III) At pH 9.1 and 10.1, PO43- removal mechanism was mainly by adsorption at initial PO43- concentration ≤10 mg L-1 within 24 h reaction. Based on the kinetic results, it is suggested that PO43- precipitation will occur after 24 h reaction when saturation index of amorphous calcium phosphate is between 1.97 and 2.19. Besides, increasing PO43- concentration does not cause a continuous decline of PO43- 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.