New insights into the interaction between heavy metals and struvite: Struvite as platform for heterogeneous nucleation of heavy metal hydroxide

Abstract

During the struvite (MgNH4PO4·6H2O) recovery from agricultural or industrial wastewater, the extensive existence of heavy metals would pose great threats to the planting and environment. This work revealed the association of kinds of heavy metals, as possible substances in the wastewater, with the struvite. The struvite has been synthesized in situ and employed to contact with both high and low concentration heavy metal (including Cu, Ni, Pb, Zn, Mn, Cr(III)) wastewater. The heavy metal precipitation rates under different pH values from 6.0 to 10.0, and under a series of struvite addition amount have been investigated. The precipitation of heavy metals without addition of struvite is functioned as control experiments. The struvite’s presence can enhance the heavy metal precipitation rate under all pH values. The Ni and Mn have relatively lower precipitation rate compared with other metals. For extremely low heavy metal concentration the precipitation rate is 99.1%, 97.9%, 99.9%, 98.9%, 96.9%, and 98.3%, which is much higher than that of control group (10.5%, 8.7%, 13.2%, 14.1%, 6.7%, and 10.5%). Through XPS and TEM & EDS analysis, the heavy metal hydroxides is found to be precipitated on the struvite surface. Based on TEM observations, it has been found the copper hydroxides had nucleation and growth on the struvite’s surface. The heterogeneous nucleation mechanism has been proposed, which provides relationship model of sorption, nucleation, and precipitation of heavy metals in the form of hydroxides on struvite surfaces. It has been calculated that the △Gi value that thermodynamic activation energy barrier constraints on the metal hydroxide formation can be intensively abated through heterogeneous nucleation on struvite.