Insights into the simultaneous removal of Cr6+ and Pb2+ by a novel sewage sludge-derived biochar immobilized nanoscale zero valent iron: Coexistence effect and mechanism

Cr6+ and Pb2+ are both highly toxic pollutants and commonly co-exist in some industrial effluents and contaminated waters. In this study, simultaneous removal of Cr6+ and Pb2+ by a novel sewage sludge-derived biochar immobilized nanoscale zero-valent iron (SSB-nZVI) was systematically investigated. It was well demonstrated that a porous structure was successfully formed on the SSB-nZVI when the starch was used as an additive. A synergistic effect on the adsorption and reduction over the SSB-nZVI was achieved, resulting in nearly 90 and 82% of Cr6+ and Pb2+ removal within 30 min, respectively. Cr6+ was reduced prior to Pb2+. A low pH could accelerate the corrosion of nZVI as well as phosphate leaching. When Malachite green was added as a coexisting organic pollutant, its effective removal was found due to the formation of a Fenton-like system. The SSB-nZVI could be run consecutively three times with a relatively satisfactory performance. Most of Cr6+ was converted into Cr2O3 and Cr(OH)3 on the SSB-nZVI surface, whereas most of Pb2+ species existed as Pb(OH)2 (or PbO). A possible reaction mechanism on the SSB-nZVI involved the adsorption, reduction and precipitation of both Cr6+ and Pb2+ over the particles. Present study sheds light on the insight of the fate and transport of Cr6+ and Pb2+ in aquatic environment, as well provides helpful guide for the remediation of coexistence of pollutants in real applications.