Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO-4 , CrO24 , and Cr2O27 ions in aqueous solutions. At low pH (~1-4), the availability of HCrO-4 ions attributes the electrostatic forces of attraction due to the available functional groups such as - NH+4 , - COOH, and - OH+2 , which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr (VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr (VI) removal and other heavy metals are elegantly discussed.

Automated summary

This paper provides a review on the sources, chemical speciation, and toxicity of Cr(VI) ions, as well as the applications of biochar materials for the remediation of Cr(VI) contaminated matrices. The mechanism of Cr(VI) adsorption onto biochar materials is discussed, and various models are used to understand the adsorption process. The practical applicability of biochar-based sorbents for Cr(VI) removal is also evaluated, and future directions for advanced biochar materials are proposed.