Thermodynamic Principles of Dittmarite Precipitation

Dittmarite is a naturally occurring mineral which is also produced by municipal wastewater treatment processes. It contains ammonium:magnesium:phosphate in molar ratios of 1:1.25:1, respectively. Dittmarite (MgNH₄PO₄·H₂O) has a very stable crystalline structure, its solubility constant (K_so) is a product of the activity coefficient (ɣ) and the activity of the individual species that make up the compound. The conditional solubility product, P_S = C_T, Mg C_T,NH₄ C_T,PO₄ = K_so/αMg αNH₄ αPO₄ ɣMg ɣNH₄ ɣPO₄. A theoretical plot of -log Ps vs. pH gives a pH of 10.7 for maximum dittmarite precipitation, albeit precipitation may begin at lower pH values. The pK_so for dittmarite at 25 °C is determined to be 13.359 from the final equilibrium pH values taking all relevant magnesium, phosphate and ammonium species into account. The geochemical speciation model, MINTEQA2 was used to verify the optimum conditions for the precipitation of dittmarite. Such theoretical predictions allow the control of phosphorus and nitrogen discharges from animal and municipal wastewaters and serve as key factors in preventing surface water eutrophication and groundwater pollution.