TRACE ELEMENT ASSOCIATION WITH STRUVITE DURING RECOVERY OF PHOSPHORUS FROM CONTAMINATED WATER

Struvite (MgNH₄PO₄.6H₂O) recovered from wastes may be a viable source of phosphate for use as fertilizer. However, the potential for interaction of trace elements (As, Cr, Zn, Cu) with struvite during and after mineralization requires assessment. This research determines the reactivity and sorptive properties of struvite in simulated contaminated waters. The impact of initial recovery conditions is assessed, and several standard and advanced techniques are used to evaluate generated products. Results reveal changing the initial solution pH can affect the crystallization kinetics, morphology, particle size and sorptive properties of recovered struvite; optimal conditions for struvite crystallization can also enhance the interaction of contaminant sorbates; and low concentrations of sorbed contaminants impact the surface, structure and stability of the mineral. In uncontaminated systems, raising the pH from 8-10 increased recovered P from 40 to 100%, with 96 to 99% of the precipitate identified as struvite. Under conditions optimal for struvite recovery, As and Cr can sorb at concentrations in excess of recommended fertilizer limits by incorporation into the mineral structure or the formation of surface-bound precipitates as determined using XAFS. The micronutrient Zn reacts with struvite, distorting phosphate groups even at low concentrations based on vibrations in FTIR spectra. Structural changes induced by impurity association affect struvite thermal stability and decomposition by STA, which can be a proxy for susceptibility to nutrient and volatile release. These results confirm the association of trace elements with struvite both during and after mineralization. Under specific recovery conditions concentrations may exceed recommended contaminant levels. However, even at low concentrations structural changes induced by contaminant associations can impact functionality and utility of the mineral for targeted repurposing.