Low concentrations of Cu2+ in synthetic nutrient containing wastewater inhibit MgCO3-to-struvite transformation

Abstract

Simultaneous major nutrient nitrogen (N) and phosphorus (P) recovery from wastewater is key to achieving food–energy–water sustainable development. In this work, we elucidate the reaction kinetics, crystalline structure and chemical composition of the resulting solid precipitate obtained from simulated N and P containing wastewater solution using widely abundant low solubility magnesite (MgCO₃) particles in the presence of common transition metal ions, such as zinc (Zn²⁺) or copper (Cu²⁺). We show that up to 100 ppm Zn²⁺ from the simulated wastewater can be incorporated into the struvite lattice as isolated distorted Zn²⁺ while even at very low concentrations of ∼5 ppm Cu²⁺ ions almost completely inhibit struvite crystal formation. The resulting solid precipitate distinctly affects soil microbial biomass carbon and soil dehydrogenase enzyme activity. These results show a cautionary case where abundant natural mineral MgCO₃ exhibits very different chemistry in Cu²⁺ containing simulated wastewater and does not readily adsorb or retain NH₄⁺ and PO₄³⁻ ions, unlike less sustainable but more water-soluble magnesium sources, such as MgCl₂, at the equivalent [Mg²⁺] : [NH₄⁺] : [PO₄³⁻] molar ratio of 1.4 : 1 : 1.