Influence of process parameters on the heavy metal (Zn2+, Cu2+ and Cr3+) content of struvite obtained from synthetic swine wastewater.

Struvite recovered from swine wastewater can be used as a good slow release fertilizer. Nevertheless, the presence of heavy metals would be easily precipitated with struvite and increase the ecological risk for its agricultural use. This paper investigated the possibility of using process variables for heavy metal (Cu2+, Zn2+ and Cr3+) minimization during struvite crystallization in swine wastewater. The heavy metal content, effect ratios (ER) of the citric acid concentration under varying conditions were tested and their SEM, EDS and XRD patterns were compared for morphology analysis. The results show that an increase in pH decreased the content of Cu, Zn and Cr in recovered precipitates. Heavy metal content in the precipitates increased markedly with their initial concentrations in the solution. The effect ratio calculation indicates that Cr has the strongest co-precipitation potential, followed by Zn and Cu. An increase in citric acid concentration reduced the heavy metal removal efficiency (14.3, 27.7 and 28.1% for Cu, Zn and Cr, respectively) but did not decrease their content in struvite precipitates. What is more, increase of total ammonia nitrogen (TAN) to soluble phosphate molar ratio significantly decreased Cu, Zn removal efficiency (52.2 and 50% respectively), while Mg:PO4P molar ratio had much less effect.