Temperature-dependent phosphorus precipitation and chromium removal from struvite-saturated solutions.

The effect of temperature from 25 to 300°C on the precipitation of phosphorus (P) from struvite-saturated (MgNH(4)PO(4)·6H(2)O) solutions was explored. Scanning electron microscopy (SEM) revealed reduced particle size and a change in morphology from elongated to rhombohedral crystals with temperature. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) indicated that at 25°C, the precipitate was struvite, while newberyite (MgHPO(4)·3H(2)O) formed at 100°C, and magnesium pyrophosphate (Mg(2)P(2)O(7)) at 300°C. Increased temperature reduced the association of ammonium and water of crystallization with the solid and increased P polymerization. The behavior of dissolved chromium (Cr) under these conditions was also assessed. Removal of Cr with the solid phase from Cr(III) solutions was observed at all temperatures, whereas removal from Cr(VI) solutions was significant only at 300°C. X-ray absorption fine structure spectroscopy (XAFS) revealed that regardless of initial oxidation state in solution, Cr(III) was associated with the solid, interacting by the adsorption of short-range Cr polymers. Therefore, for struvite-saturated solutions, increasing the temperature changed both the mineralogy of the P phase recovered and enhanced the interaction of otherwise unreactive Cr(VI) with the substrate. These results have implications for the temperature-enhanced recovery of P from wastewater.