Adsorption and reduction of Cr(VI) by C-P-O groups in biochar: Performance and mechanisms

Abstract

Applying agricultural and industrial wastes into solid functional adsorbents for environmental remediation of heavy metal pollution remains a challenge. Herein, an efficient alkaline-pretreated H₃PO₄-modified cotton straw biochar (PCS) was developed to remove Cr(VI) by adsorption and reduction. Detailed characterization demonstrated that PCS has a substantial specific surface area (301.6 m² g⁻¹) and C-P-O groups (not in pristine biochar). Adsorption experiments were better described by pseudo-second-order and Langmuir model, which indicated that the adsorption followed the physicochemical diffusive monolayer adsorption mechanism. According to the alkaline extraction method and X-ray photoelectron spectroscope (XPS) analysis, the C-P-O groups on PCS participated in the adsorption of Cr(VI), converting 20.2 % of it to Cr(III). The remarkable adsorption capacity of PCS was 138.9 mg g⁻¹, mainly attributed to pore filling, electrostatic attraction, complexation, and reduction. The PCS exhibited strong tolerance in common anionic and different water matrices and had the potential for practical application. The above findings can provide decision-making for the remediation of chromium-containing wastewater, as well as for the production of solid functional adsorbents in industry and agriculture.