Temperature impact assessment on struvite solubility product: a thermodynamic modeling approach
Résumé
This work addresses the problem of phosphorus recovery from wastewater by struvite precipitation, which is chemically known as magnesium ammonium phosphate hexahydrate MgNH4PO4·6H2O. The struvite solubility product values that are reported in the literature were found to vary significantly, from one solution to another and over the range of the experimental conditions as well. The various factors affecting the struvite solubility include pH, ionic strength and temperature. The struvite solubility product is yet a very important parameter to determine the supersaturation ratio. A thermodynamic model for phosphate precipitation is proposed to determine the phosphate conversion rate and the value of struvite solubility product for a temperature range between 15 and 35 °C. This model is based on numerical equilibrium prediction of the study system Mg–NH4–PO4–6H2O. The mathematical problem is represented by a set of nonlinear equations that turns, to an ill-conditioned system mainly due to the various orders of magnitude of the involved variables. These equations have first been solved by an optimization strategy with a genetic algorithm to perform a preliminary search in the solution space. The procedure helps to identify a good initialization point for the subsequent Newton–Raphson method. A series of experiments were conducted to study the influence of pH and temperature on struvite precipitation and to validate the proposed model.