Abstract
To reveal the effects of solution conditions on the crystallization of magnesium phosphates during phosphorus recovery from wastewater, the geochemical aquatic modeling program PHREEQC (version 2.11) was applied to the calculations of ion speciation and saturation index (SI) of different magnesium phosphates in a chemically defined crystallization system. The results showed that the relationship between SI of magnesium ammonium phosphate (MgNH4PO4·6H2O, MAP), Mg3(PO4)2·8H2O (MP-8W) and Mg3(PO4)2·22H2O (MP-22W), and the respective composition ion concentration was of logarithmic function. There was polynomial function relationship between SI values of magnesium phosphates and solution pH value. The optimum pH for MAP crystallization was around 9.0, while it was 9.0–10.0 for MP-8W and MP-22W. The SI values decreased as the carbonate concentration increased. Meanwhile, the SI values, decreasing with the increase in solution ionic strength, were of logarithmic function to the solution ionic strength. Besides, the SI values were also of polynomial function to the solution temperature. Under different pH values, the SI values presented different variation trends as the temperature rose. For the case study of P removal from synthetic swine wastewater, the P removal trend at different pH, N:P and Mg:N was closer to the predictions of thermodynamic modeling.
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