Freshly formed elemental sulfur-sulfidated zero-valent manganese for enhanced chromium(VI) removal from wastewater

Both zero-valent manganese (ZVMn) and sulfide possess high reducibility and strong affinity towards heavy metals in high oxidation states. The feasibility and behavior of combining these two components to create sulfidated zero-valent manganese (SZVMn) for hexavalent chromium (Cr(VI)) removal, however, remain largely uninvestigated. This study aims to explore the behaviors and mechanisms of Cr(VI) removal utilizing SZVMn, which was synthesized by borohydrides reduction of divalent manganese salt, followed by sulfidation using freshly generated elemental sulfur. SZVMn comprised a mixture of low-crystallinity Mn0, MnOx, and MnSx, with the fraction of MnSx increasing with higher S/Mn molar ([S/Mn]) ratio. The optimal [S/Mn] ratio was determined to be 0.1, achieving the highest Cr(VI) removal capacity of 190.8 mg/g. At [S/Mn] ratio exceeding 0.1, Cr(VI) removal declined due to higher consumption of Mn0 as a result of increased S dosing during material formation. SZVMn outperformed bare ZVMn in terms of preservation and resistance to coexisting ions. The removal of Cr(VI) occurred primarily through the rapid reduction of Cr(VI) to Cr(III) via Mn0 and MnSx initially, followed by slow reduction via dissolved/surface-bound Mn2+, along with precipitation of Cr(III) as Cr(OH)3. This study provides a deep understanding of the interactions between Cr(VI) and SZVMn derived from freshly-generated elemental sulfur, offering new insights into Cr(VI) removal processes.