Co-precipitation of Cu and Zn in precipitation of struvite

Struvite recovered from wastewater can be used as a slow-release fertilizer. Nevertheless, hazardous metals easily precipitated with struvite would increase the ecological risk for its agricultural use. In this study, the influence of individual and coexistence of Cu and Zn on the precipitation of struvite was investigated. The loading of Cu and/or Zn in precipitates increased with the increase of initial metal concentrations (0.1-100 mg/L). Quantitative X-ray diffraction (QXRD) analysis revealed that the increase of Cu and/or Zn level in reaction solution disturbed crystal growth of struvite and promoted the formation of amorphous phase(s). Scanning electron microscopy (SEM) revealed the pit formation on struvite crystal surfaces, combined with X-ray photoelectron spectroscopy (XPS) data, the results indicated a surface interaction for the formation of Cu-OH and Cu-NH₃ on struvite surface at Cu of 0.1-10 mg/L. With the increase of Cu to 25-100 mg/L, the precipitation of amorphous Cu phosphate(s) was confirmed by XPS and QXRD. At Zn of 0.1-10 mg/L, the enrichment of Zn-PO₄ and Zn-OH on struvite surface was observed, whereas, the precipitation of amorphous Zn hydroxide(s) was confirmed at Zn of 25-100 mg/L. At Cu and Zn co-existed solution, the decrease of Cu-PO₄ and increase of Zn-PO₄ suggested the competitive binding of PO₄ between Cu and Zn. In addition, the formation of amorphous Mg hydroxide(s) and phosphate(s) was detected regardless of the addition of Cu in solutions. The overall results revealed that the existence of Cu and Zn during struvite formation can greatly affect its content by formation of different metal-containing products.