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 [FeCl 3, MnCl 2, AlCl 3, and CaCl 2 ] and clay minerals [kaolinite]) on the redox activity of sludge hydrochar for Cr(VI) reduction was investigated. The subsequent aging process revealed that Fe 3+ and Al 3+ 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-mediating facilitated by the formation of the crystal structure, while enhancing the electron-donating and electron-mediating capacities of the co-aging hydrochar. However, Mn 2+ was easily oxidized to Mn(III) and Mn(IV), which protected alkanes from oxidation. Simultaneously, Ca 2+ and kaolinite were encapsulated on the hydrochar surface, hindering electron-mediating during oxidation. None of the three substantially enhanced the electron-donating capacity 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.