Effect of extracellular polymers in the wastewater on the incorporation of heavy metals with struvite and their adsorption mechanism

Abstract

Struvite crystallization has been regarded as an effective method to recover phosphorus (P) from wastewater, but the complicated substances might pose unknown effect on struvite formation. At present, the response of struvite incorporation with heavy metals to extracellular polymers (EPS) in the wastewater and its adsorption mechanism remains elusive. Herein, we analyzed the residues of different heavy metals in struvite at different concentrations of sodium alginate (SA, chosen as model EPS) and explored the effect of EPS on the adsorption of metals. The results demonstrated that higher concentrations of SA (> 50 mg/L) were favorable for obtaining struvite with lower heavy metal contents, but had little effect on struvite crystallization at higher pH values of 9.0. The inhibition of SA on heavy metals was mainly caused by its strong complexing ability for metals, mitigating their migration from solution to struvite crystals. Based on the instrument analysis of crystals and the previous study results, we innovatively proposed that transient adsorption and post adsorption occurred intertwined during the crystallization process. The transient adsorption might be dominated in the real wastewater due to the affinity of high levels of organics matter on metals, and thus inhibits their co-precipitation with struvite. The mechanisms of metal adsorption on crystals are closely associated with heavy metal forms which depends on the presence of ligands and their type in the wastewater. Whereas, the effect of SA on adsorption of heavy metals will be attributed to the adsorption and complexation of SA (EPS) with heavy metals in the wastewater