Thermodynamic Prediction of Non Additive Effects in Struvite Crystallization for Phosphorus Recovery from Wastewater

Phosphorus recovery from wastewater by struvite crystallization is a promising green process, but its efficiency is strongly affected by the simultaneous presence of competing ions. In practical wastewater matrices, calcium, carbonate, ammonium, magnesium, and phosphate species interact over a wide pH range, so precipitation cannot always be predicted from single component or binary equilibria. This work examines how non additive thermodynamic effects modify struvite stability and define operational windows for phosphorus recovery. A thermodynamic speciation and precipitation model was developed for multicomponent aqueous systems containing magnesium, ammonium, phosphate, calcium, and carbonate species. Struvite formation was evaluated as a function of pH, ionic strength, and competing ion concentrations. A reference additive state was constructed under identical physicochemical conditions by suppressing multicomponent coupling effects. The difference between the full system and the additive reference was used to identify cooperative or antagonistic contributions to precipitation behavior. The simulations show that coexisting ions shift the precipitation boundary of struvite and change the pH interval in which phosphorus recovery is thermodynamically favored. Calcium and carbonate species can reduce struvite selectivity by promoting competing solid phases or by redistributing phosphate among aqueous complexes. Near critical pH values, small compositional changes produce large variations in saturation state, indicating that additive models may overestimate or underestimate recovery efficiency. Similar boundary shifts were also observed in metal removal scenarios, confirming that non additive behavior is a general feature of multicomponent water treatment systems.

Conclusions: The proposed thermodynamic approach provides a practical tool for predicting robust operating windows in phosphorus recovery by struvite crystallization. By accounting for non additive interactions, it supports process optimization, improves selectivity control, and contributes to more resilient environmental and green water treatment processes.