Synthesis of a Polyaniline-Modified Magnetic Zn/Fe Bimetallic Sludge Biochar for Aqueous Cr(VI) Adsorption

Heavy metals are recognized as a growing environmental concern due to their toxicity and long-term stability. In this study, a magnetic composite biochar material (PDFMB) was synthesized using iron-based sludge and Chinese medicine residue, with zinc chloride and polyaniline serving as modifiers, to adsorb and reduce Cr(VI) in water. The optimal preparation conditions were determined through preliminary experiments, and the adsorption performance was evaluated through material characterization and Cr(VI) adsorption tests. The removal of Cr(VI) by PDFMB was highly pH-dependent, with an optimal pH of 2, indicating that acidic conditions favored adsorption. The maximum adsorption capacity for Cr(VI) at 308 K and pH 5 was 111.11 mg/g. The adsorption kinetics followed a pseudo-second-order model, while the Langmuir isotherm provided a good fit to the adsorption isotherm data. Thermodynamic analysis revealed that the removal process was spontaneous and endothermic. The anions Cl‒, SO42‒, and PO43‒ inhibited the removal efficiency of Cr(VI), with inhibition strength following the order Cl‒ < SO42‒ < PO43‒. The key mechanisms involved in Cr(VI) removal included physical and chemical adsorption, chelation, and the reduction of Cr(VI) to Cr(III). The PDFMB composite exhibited high stability and reusability, achieving an 85.25% removal efficiency after six cycles. Owing to its high efficiency, magnetic separability, and excellent recyclability, PDFMB represents a highly promising candidate for addressing Cr(VI) pollution.