Interaction of divalent metals with struvite: sorption, reversibility, and implications for mineral recovery from wastes.

Phosphorus (P) recovered from wastewater as struvite (MgNHPO·6HO) can meet high P demands in the agricultural sector by reuse as a P fertiliser. Heavy metals are prevalent in wastewaters and are common fertiliser contaminants, therefore struvite as a sorbent for metals requires evaluation. Struvite sorption experiments were conducted in model solutions with cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) at 1-5 μM concentrations from pH 7-10. The struvite metal loading increased with dissolved metal concentration and pH, ranging from 2 to 493 mg kg. Highest loadings were observed for 5 μM Pb, which exceeded the 120 mg kg European Union (EU) struvite fertiliser limit at all pH values. At 5 μM concentrations, Ni and Cd loadings exceeded EU limits of 100 mg kg at pH 10, and 60 mg kg at pH 8-10, respectively. In desorption experiments, 10-85% metal was released after resuspension in metal-free solutions, with a positive correlation between initial loading and amount desorbed. Distortions of the struvite phosphate band, by Fourier transformation infrared (FTIR) spectroscopy, indicated lowered symmetry of phosphate vibrations with metal sorption. X-ray absorption fine structure spectroscopy (XAFS) analysis of pH 9 solids indicated tetrahedral coordination for Cu and Zn, octahedral coordination for Co and Ni, and Pb in 9-fold coordination. Precipitation of Pb-phosphate minerals was a primary mechanism for Pb sorption. The results provide insight into metal contaminant sorption with struvite in wastewaters, and the potential for metal desorption after recovery.