Influence of pH and oxidation state on the interaction of arsenic with struvite during mineral formation

Struvite (MgNH₄PO₄•6H₂O) precipitated from animal and human wastes may be a sustainable source of fertilizer. However, arsenic, present in some wastes, may be removed with struvite. Here the sorption of As with struvite during mineral formation at pH 8-11 was assessed. The yield of struvite increased with pH, and was highest at pH 10. For recovered struvite, XRD indicated reduced crystallinity and particle size, and FT-IR suggested less distortion of phosphate tetrahedra with increased pH. The As impurity did not affect the crystallinity or particle size, but did contribute to phosphate distortion. Sorption of As(V) was observed at all pH values, and was highest at pH 10. As(III) sorption was consistently lower than that of As(V), but increased with pH. XAFS suggested coprecipitation of As(V), and adsorption of As(III) as the potential sorption mechanisms. Solids derived from As(III) solutions exhibited dual mechanisms due to the partial oxidation of As(III) to As(V) in solution prior to sorption. For struvite recovery in the presence of As, optimizing the pH to improve yields may increase the As content. Adsorbed As(III) could be removed prior to fertilizer application, however coprecipitated As(V) will release upon mineral decomposition, linking its cycling to that of phosphorus.