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Oxalic acid, one of the common organic acids secreted by microorganisms, was used to modify the biochar of sludge (SB) and rice husk (RB), and the adsorption function of biochar was combined with the reductive property of oxalic acid to repair Cr(VI). The biochar materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform attenuated total reflection infrared spectroscopy (ATR-IR), and the mechanism of chromium hexavalent removal was studied. Adsorption experiments showed that the removal rate of Cr(VI) with the modified SB and RB was 1.68 and 2.19 mg/g within 4h, which was 7.4 and 33 times of that before modification. Oxalic acid modification can enhance the repair of Cr(VI) by biochar in three aspects: 1. increases the BET surface area (109.34-174.13%) and decrease the average pore width (86.3-93.7%) of biochar, which increases the adsorption sites. 2. the addition of reducing oxygen-containing functional groups improves biocahr reduction performance. 3. improve reduced and unreduced Cr ion forms stable minerals on the surface of biochar. Therefore, oxalic acid can not only improve the adsorption and reduction of Cr(VI) by biochar, but also further improve the fixation/passivation of Cr ion after reduction and reduce the migration of Cr ion