The use of TG/DSC-FT-IR to assess the effect of Cr sorption on struvite stability and composition

Struvite (MgNH₄PO₄·6H₂O; MAP) can be recovered from animal and human wastes for use as fertilizer. This encourages the sustainable use of phosphorus (P), closing the human P cycle. The toxic metalloid chromium (Cr) is a common component of wastes, and can substitute for P in geochemical and biological systems. Thus, its sorption to, and effect on the stability and composition of recovered MAP requires assessment. MAP precipitated from solutions with 1-100 μM Cr(III) had higher Cr loadings compared to those reacted in the presence of Cr(VI), indicative of higher sorption affinity of the lower oxidation state. Simultaneous thermal analysis of unreacted MAP revealed an endothermic peak at 126 ± 0.5 °C by DSC with a mass loss of 52.9% by TG. Sorption of Cr produced minimal effects on the transition temperature and overall mass loss. The inflection in the TG curve indicated that Cr increased the temperature of maximum decomposition, but also the mass loss at this point. Combining TG results with FT-IR spectra revealed that for initial concentrations of 10-50 μM Cr(III) and 1-5 μM Cr(VI), NH₄⁺ was added, and H₂O(s) lost from the MAP structure. The change in composition was consistent with substitution of Cr(III) or Cr(VI) into the MAP structure. The TG/DSC-FT-IR technique confirmed that Cr contamination affects the MAP composition and may accelerate the release of nutrients upon mineral decomposition. This has implications for the use of MAP fertilizers and subsequent cycling of P and contaminants in agricultural systems.