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Chromium (Cr) contamination in soil and water poses high toxicity risks to organisms and threatens food and water security worldwide. Biochar has emerged as a promising material for cleaning up Cr contamination owing to biochar's strong capacity to immobilize Cr. This paper synthesizes information on biochar modification for the efficient remediation of Cr contamination in soil and water, and critically reviews mechanisms of Cr adsorption on pristine and modified biochars. Biochar modification methods include physical activation via ball milling or ultraviolet irradiation, chemical activation via magnetization, alkali/acid treatment, nano-fabrication or loading of reductive agents, and biological activation via integrating biochars with microorganisms and their metabolites. Modified biochars often have multi-fold enhancement in Cr adsorption/reduction capacity than pristine biochars. Iron (Fe)-supported magnetic biochars have the most promising Cr removal abilities with high reusability of the biochars. Pre-pyrolysis modification with Fe could load Fe 3 O 4 micro-/nanoparticles on biochars, and increase the surface area and electrostatic attraction between chromate anions and biochar surfaces, and reduce Cr(VI) to Cr(III). Post-pyrolysis modification could enrich oxygen-containing functional groups such as C O and –OH on biochar surfaces and promote Cr reduction and adsorption. Future research directions for Cr mitigation using advanced biochar products are discussed in this review. [Display omitted] • Chromium (Cr) biogeochemistry and its remediation using pristine/modified biochars. • Negatively charged O-containing groups on biochar surfaces act as electron donors. • Iron-modified biochars exhibit high Cr removal ability and reusability. • Post-pyrolysis modification enriches O-containing functional groups on biochar. • Biochar-based nanocomposites are effective reductants and adsorbents for Cr(VI). [ABSTRACT FROM AUTHOR]