Simultaneous removal of cadmium and tetracycline from aqueous solutions by oxalic acid and pyrite co-modified biochar: Performance and mechanism

Abstract

The remediation of combined contamination with heavy metals and antibiotics in soil and aqueous environments represents an ongoing challenge. In this study, a novel highly functionalized biochar-based composite (FeS₂@OA-BC) was synthesised by combining oxalic acid (OA) pre-treatment with ball-milling of FeS₂ for the simultaneous removal of cadmium (Cd²⁺) and tetracycline (TC) from aqueous solutions. FeS₂@OA-BC demonstrated exceptional performance in simultaneously removing 74.7 % of Cd²⁺ and 95.8 % of TC from the binary systems, meanwhile the degradation rate of TC reached up to 64.8 %. Moreover, no significant competitive or promoting effects between Cd²⁺ and TC removal were observed by FeS₂@OA-BC in binary systems. The adsorption of Cd²⁺ was primarily governed by three mechanisms: complexation with functional groups, Cd-π conjugation and cation exchange. Meanwhile, TC degradation relied on reactive oxygen species (ROS), where hydroxyl radicals (•OH) and hydrogen peroxide (H₂O₂) played dominant roles, with singlet oxygen (¹O₂) contributing minimally. The co-modification of OA and FeS₂ synergistically introduces abundant exogenous defect sulphur vacancies (SVs), enhancing molecular oxygen activation and stimulating more ROS for TC degradation, as well as promoting more functional groups as adsorption sites for Cd²⁺ complexation. This therefore ultimately led to the reinforcement of the concurrent removal of Cd²⁺and TC. Overall, FeS₂@OA-BC shows great promise for addressing combined pollution involving heavy metals and antibiotics in environmental systems.