The incorporation of Pb2+ during struvite precipitation: Quantitative, morphological and structural analysis

Struvite precipitated from wastewaters is an important fertilizer. However, struvite derived from wastewater usually contains toxic Pb, which can bring contamination to soil and even plants. Thus, understanding the incorporation mechanisms of Pb2+ during struvite precipitation is critical to its safe application. Here the influence of Pb concentration on struvite precipitation was assessed. When the initial Pb concentrations were at the range of 0.1-1 mg/L, the formation of pitting and roughening on struvite crystal surfaces was observed by scanning electron microscopy (SEM), indicating a surface interaction between Pb and struvite. Combined with Xray photoelectron spectra (XPS), the results confirmed that the formed Pb-enriched layer with Pb-OH and Pb-PO4 bonds was absorbed on struvite surface during precipitation. When Pb concentrations were increased to 10-100 mg/L, the precipitation of dominating Pb phase, crystalline Pb10(PO4)(6)(OH)(2), was confirmed by X-ray diffraction (XRD). Combined with XPS, the amorphous Pb hydroxide/phosphate and Mg phosphate were also detected in struvite solids. Our findings revealed that at low concentrations (0.1-1 mg/L), Pb can affect the mineral surface by surface absorption, whereas Pb precipitated as separated phase(s) (e.g. Pb-10(PO4)(6)(OH)(2), Pb hydroxide and/ or phosphate) at high Pb concentrations (10-100 mg/L). Thus, the initial Pb2+ concentrations in wastewater will dictate final struvite contents and Pb-bearing phases in recovered solids.