Predominance Diagrams for NH4+-Mg2+-PO43--H+-H2O System

The thermodynamics of NH4+-Mg2+-PO43--H+-H2O system was investigated based on the construction of predominance diagrams. The lgC(T,Mg)-lgC(T,P) and lgC(TP)-pH diagrams were constructed at an arbitrary Mg/P molar ratio with consideration of the ion strength influence(C-T,C-Mg: total concentration of magnesium; C-T,C-P: total concentration of phosphorus; C-T,C-N: total concentration of nitrogen). The thermodynamic stable zones of struvite (MgNH4PO4 center dot 6H(2)O), bobierrite (Mg-3(PO4)(2)center dot 8H(2)O), newberyite (MgHPO4 center dot 3H(2)O), and magnesium hydroxide (Mg(OH)(2)) were determined. The results show that struvite and bobierrite are the dominating phases in a wide range of pH value. Struvite and newberyite coexist with solute phase at low pH value and high total concentration of phosphorus while struvite and magnesium hydroxide are more stable at the alkaline condition. The minimum total concentration of nitrogen appears at pH value of 9.08 similar to 9.52 while struvite and bobierrite coexist with the solute phase. The predominance diagrams could be used to predict the precipitation-dissolution equilibrium of struvite for ammonia nitrogen removal and recovery from wastewater.