Optimized paraquat removal using Bi₄O₅Br₂: synthesis, performance evaluation, and mechanistic insights.

Paraquat (PQ), a widely used bipyridinium herbicide, poses severe environmental and public health risks due to its high toxicity and persistence in aquatic systems. In this study, a Bi₄O₅Br₂ composite was synthesized via a double-solvent-derived route using CAU-17 as a precursor, and subsequently employed as an efficient adsorbent for PQ removal from water. Comprehensive characterization using XRD, FTIR, BET, FE-SEM, and EDX revealed a well-defined lamellar mesoporous structure (specific surface area: 63.12 m²/g, average pore size: ~7 nm) and a homogeneous elemental distribution, offering abundant and accessible active sites for adsorption. The adsorption performance was systematically optimized through response surface methodology (RSM) based on a Box–Behnken design (BBD), considering adsorbent dosage, pH, contact time, and initial PQ concentration as key parameters. The model showed excellent fitting and predictive accuracy, and the results highlighted that adsorbent dosage and solution pH were the most influential factors governing removal efficiency, whereas contact time exhibited a moderate effect and PQ concentration played a comparatively minor role. Under the optimal conditions (adsorbent dosage of 0.0187 g, pH 6.47, contact time 38.26 min, and initial PQ concentration 3.89 mg L⁻¹), a maximum removal efficiency of 97.47% was achieved. Mechanistic investigation revealed that PQ removal occurred predominantly through electrostatic interactions between the negatively charged Bi₄O₅Br₂ surface (under near-neutral to alkaline conditions) and cationic PQ²⁺ molecules, supplemented by physical adsorption and hydrogen bonding. FTIR analysis before and after adsorption confirmed the participation of surface hydroxyl groups through slight shifts and intensity reductions in the O–H stretching band, suggesting their involvement in electrostatic binding and hydrogen bonding. The mesoporous architecture of Bi₄O₅Br₂ facilitated rapid intraparticle diffusion, enabling fast attainment of equilibrium. Compared with previously reported adsorbents, Bi₄O₅Br₂ demonstrated superior performance at an ultralow dosage, highlighting its cost-effectiveness and scalability for practical water treatment. Overall, these findings establish Bi₄O₅Br₂ as a promising candidate for the remediation of herbicide-contaminated water. Future work should focus on regeneration performance, stability under repeated use, and validation using real wastewater matrices.