Insights into the simultaneous removal of Cr 6+ and Pb 2+ by a novel sewage sludge-derived biochar immobilized nanoscale zero valent iron: Coexistence effect and mechanism.

Cr ⁶⁺ and Pb ²⁺ are both highly toxic pollutants and commonly co-exist in some industrial effluents and contaminated waters. In this study, simultaneous removal of Cr ⁶⁺ and Pb ²⁺ 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 Cr ⁶⁺ and Pb ²⁺ removal within 30 min, respectively. Cr ⁶⁺ was reduced prior to Pb ²⁺ . 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 Cr ⁶⁺ was converted into Cr ₂ O ₃ and Cr(OH) ₃ on the SSB-nZVI surface, whereas most of Pb ²⁺ species existed as Pb(OH) ₂ (or PbO). A possible reaction mechanism on the SSB-nZVI involved the adsorption, reduction and precipitation of both Cr ⁶⁺ and Pb ²⁺ over the particles. Present study sheds light on the insight of the fate and transport of Cr ⁶⁺ and Pb ²⁺ in aquatic environment, as well provides helpful guide for the remediation of coexistence of pollutants in real applications.
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