Simultaneous removal of Cr, Cu, Zn, and Cd by nano zero-valent iron modified sludge biochar in high salinity wastewater

Cr, Cd, Zn, and Cu commonly coexisted in the electroplating wastewater with high salinity and organics, while their removal was often investigated individually. In this study, the simultaneous removal of Cr, Cd, Zn, and Cu was investigated by nano zero-valent iron modified sludge biochar (nZVI@SBC). The results show that nZVI@SBC could significantly improve the removal efficiency of the four heavy metals (HMs) in high salinity wastewater. NO3– (N-NO3–) and humic acid (HA) could promote the removal of Cr, Cd, Zn, and Cu, and PO43- (P-PO43-) could enhance the removal of Cd. EPR analysis indicated the generation of 1O2 in different water matrices. The 1O2 intensity was increased in the presence of NO3– and HA, which accelerated the corrosion of nZVI and promoted the electron transfer in the system. To further understand the reaction mechanism, the variation of nZVI@SBC structures and compositions during the treatment process were characterized by SEM-EDS, XRD, BET, and XPS. Cr and Cu were removed through adsorption and co-precipitation after reduction by nZVI, while Cd and Zn were removed by alkali precipitation and complexation. This work improves the understanding of simultaneous removal of HMs by nZVI@SBC and provides a cost-effective approach for the electroplating wastewater treatment.