Mechanistic insights into Cr(VI) removal by acid modified biochar via coupled redox and complexation processes

Abstract

Highly toxic hexavalent chromium (Cr(VI)) poses severe threats to ecosystems and human health. Biochar from waste materials have shown promising advances for Cr(VI) remediation, but combining agricultural and industrial wastes for biochar fabrication remains largely unexplored. This study developed an innovative phosphoric acid modified biochar adsorbent through co-pyrolysis of agricultural straw and industrial tire wear particles (TWPs), demonstrating the Cr(VI) removal efficiency from aqueous solutions. The phosphoric acid modified biochar (PBC-3) exhibited outstanding performance with a 90.43% removal rate and an adsorption capacity of 10.851 mg·g-1, and the adsorption process was mainly chemical adsorption. The removal of Cr(VI) was achieved by electrostatic attraction, surface complexation and redox reaction. Some Cr(VI) was converted into Cr(III) and metallic Cr, and was fixed on biochar. The other part was directly combined with oxygen (O) in biochar to CrO3. Further investigations confirmed the stability of PBC-3 under diverse environmental conditions, and its practical applicability was demonstrated through cyclic experiments and real-water tests. This work presents a sustainable strategy for transforming agricultural and industrial waste into high-performance adsorbents, offering new insights into the design of functional biochar materials for heavy metal remediation.