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.
A new pyrolytic process was successfully used to prepare hollow-like spherical sludge chars (HSC). In the process, pre-formed cotton spheres with a low density were firstly coated by a layer of sludge paste containing ZnCl2 to form cotton fiber@sludge spheres and then pyrolyzed at 500 °C for 2 h. The resulting HSC possessed a diameter of about 3 mm, a low density of 0.1008 g cm−3 and a high surface area of 1008 m2 g−1. The column adsorption behavior showed that the adsorption capacity and rate constant of HSC for methylene blue reached 186.4 mg g−1 and 0.232 mL min−1 mg−1. The adsorption capacity is about 8.4 and 12 fold of the adsorption capacities of GAC (22.28 mg g−1) and SC (15.26 mg g−1), respectively, and the adsorption rate constant is about 1.3 fold of that of GAC and SC. The great adsorption capacity and rate constant can be considerably dependent on the special hollow-like spherical structure of HSC.
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