Co-Aging Process of Sludge Char and Soil Minerals Affects Electron-Donating and Mediating Capacities for the Reduction of Cr(Vi)

Sludge hydrochar used for chromium-contaminated soil remediation is susceptible to natural aging with soil minerals, which may alter its electrochemical properties and affect its remediation performance. In this study, the effect of co-aging with soil minerals (soluble mineral salts [FeCl3, MnCl2, AlCl3, and CaCl2] and clay minerals [kaolinite]) on the redox activity of sludge hydrochar for Cr(VI) reduction was investigated. The subsequent aging process revealed that Fe3+ and Al3+ ions oxidized the stabilized alkanes to functional groups with high electron-donating capacities, such as -OH and C-O, because of their oxidizing properties or electron transfer facilitated by the formation of the crystal structure, while enhancing the electron-donating and electron-transferring capacities of the co-aging hydrochar. However, Mn2+ was easily oxidized to Mn(III) and Mn(IV), which protected alkanes from oxidation. Simultaneously, Ca2+ and kaolinite were encapsulated on the hydrochar surface, hindering electron transfer during oxidation. None of the three substantially enhanced the electron-supplying ability of the hydrochar during the co-aging process. Because of the richness of soil minerals, it is vital to consider redox reactions as a critical factor when applying hydrochar for soil remediation.