Study on the mechanisms and performance of Cr(VI) reduction and transformation in soil by formic acid and Fe(III)/Fe(II) co-modified straw biochar

The remediation of soil contaminated with the heavy metal chromium (Cr) is a critical environmental challenge requiring urgent mitigation. Biochar, owing to its excellent adsorption capacity and environmental compatibility, has been widely applied to heavy metal remediation; however, its intrinsic properties require further improvement. This study used straw as feedstock to produce biochar, subsequently modified with formic acid, FeCl3·6H2O, and FeSO4·7H2O. Iron-loaded modified biochar (FFBC) with varying Fe(III)/Fe(II) ratios was synthesised. Orthogonal experiments and multiple characterisation techniques were employed to evaluate its adsorption performance and the underlying mechanism for Cr(VI) removal. All experiments were conducted in artificially contaminated soil. Results show that FFBC1, prepared with FeCl3 and FeSO4 at a 1:1 ratio, achieved 90.44% adsorption efficiency at a Cr(VI) concentration of 150 mg/kg and a dosage of 3%, identifying this ratio as the key factor governing adsorption performance. FTIR and XPS analyses revealed that introducing functional groups such as C—O and —COOH, together with Fe–O structure formation, substantially improved Cr(VI) adsorption and reduction capacities. Partial reduction of Cr(VI) to Cr(III) effectively reduced heavy metal toxicity in soil. This method is environmentally sound and highly efficient, indicating strong potential for engineering-scale applications.