Removal performance and mechanisms of toxic hexavalent chromium (Cr(VI)) with ZnCl2 enhanced acidic vinegar residue biochar

Highlights: • Vinegar residue biochar (VB) is nitrogen and silicon-rich product. • ZnCl₂ can inhibit the production of toxic nitriles in biochar. • Modified VB exhibits excellent ability to remove Cr (VI) (236.81 mg g⁻¹). • Si-O of VB can be converted into Si-OH to provide indirect adsorption sites. • Mechanisms of Cr(VI) removal included physical and chemical adsorption. Acidic vinegar residue (VR) and toxic hexavalent chromium (Cr(VI)) are unfavorable substances due to their toxicity against the environment. In this study, modified biochar was prepared to investigate the removal mechanisms of Cr(VI). The results showed that ZnCl₂ could yield highly aromatic products with improved pore structures. The adsorption capacity of modified biochar reached the highest efficiency (236.81 mg g⁻¹) when the mass ratio of ZnCl₂/VR was 1, which is higher than the control (9.96 mg g⁻¹). In addition, Cr(VI) adsorption coexisted with physical and chemical adsorption. The mechanisms of modified biochar to Cr(VI) removal included electrostatic attraction, pore filing, reduction and surface complexation. Notably, as a fermented product, VR biochar was a nitrogen-rich product; the formation of the amino group could provide a direct solid site for Cr(VI) adsorption. Subsequently, amorphous silica could be converted into silanol to provide additional adsorption sites. This work establishes the theoretical basis for efficient Cr(VI) removal and VR reuse.