Thermodynamics Analysis and Removal of P in a P-(M)-H(2)O System

In order to efficiently remove phosphorus, thermodynamic equilibrium diagrams of the P-H(2)O system and P-M-H(2)O system (M stands for Fe, Al, Ca, Mg) were analyzed by software from Visual MINTEQ to identify the existence of phosphorus ions and metal ions as pH ranged from 1 to 14. The results showed that the phosphorus ions existed in the form of H(3)PO(4), H(2)PO(4)(−), HPO(4)(2−), and PO(4)(3−). Among them, H(2)PO(4)(−) and HPO(4)(2−) were the main species in the acidic medium (99% at pH = 5) and alkaline medium (97.9% at pH = 10). In the P-Fe-H(2)O system ((P) = 0.01 mol/L, (Fe(3+)) = 0.01 mol/L), H(2)PO(4)(−) was transformed to FeHPO(4)(+) at pH = 0–7 due to the existence of Fe(3+) and then transformed to HPO(4)(2−) at pH > 6 as the Fe(3+) was mostly precipitated. In the P-Ca-H(2)O system ((P) = 0.01 mol/L, (Ca(2+)) = 0.015 mol/L), the main species in the acidic medium was CaH(2)PO(4)(+) and HPO(4)(2−), and then transformed to CaPO(4)(−) at pH > 7. In the P-Mg-H(2)O system ((P) = 0.01 mol/L, (Mg(2+)) = 0.015 mol/L), the main species in the acidic medium was H(2)PO(4)(−) and then transformed to MgHPO(4) at pH = 5–10, and finally transformed to MgPO(4)(−) as pH increased. The verification experiments (precipitation experiments) with single metal ions confirmed that the theoretical analysis could be used to guide the actual experiments.