Pollutant adsorption-degradation mechanisms of Zn-Fe/biochar catalyst via peroxodisulfate activation

• Novel Zn-Fe/biochar catalyst enables 96% tetracycline and 100% rhodamine B degradation. • Surface charge modulation achieves pollutant-specific adsorption-degradation synergy. • 99% antibiotic removal in real wastewater without pH adjustment. • Radical and non-radical pathway synergy lowers toxicity of degradation intermediates. A bimetallic Zn-Fe/biochar composite catalyst was developed using walnut shell biochar as a support for Zn-Fe-containing surface species synthesized via reverse microemulsion. Its performance and mechanism in degrading tetracycline (TC) and rhodamine B (RhB) via peroxodisulfate (PDS) activation were systematically evaluated and compared to the traditional Fenton method. Characterization revealed positively charged Zn-Fe-containing surface species, contributing to electrostatic adsorption of anionic TC while biochar maintained affinity for cationic RhB. The composite exhibited high surface area and pore volume, facilitating mass transfer. Zn-Fe/biochar can efficiently activate PDS, generating hydroxyl radicals as important active species, and synergistically interacting with singlet oxygen generated through non-radical pathways. This achieved 96% TC and 100% RhB removal within 60 min, with degradation intermediates showing 40–70% reduced toxicity. Compared to Fenton, the system demonstrated wider pH adaptability (effective at pH 6.46–7.87 without adjustment), higher contaminant removal (>99% TC in pharmaceutical wastewater), lower metal leaching (Fe <20.8 μg·L -1 , Zn <3.7 μg·L -1 after five cycles), minimal secondary pollution (no iron sludge), and excellent COD reduction (residual COD of 56 mg·L -1 in pharmaceutical and 59 mg·L -1 in aquaculture wastewater). This work establishes Zn-Fe/biochar as a dual-functional and sustainable catalyst derived from biowaste. It offers a strategy not only for efficient recalcitrant pollutant removal via charge-directed synergy but also for concurrent phosphate adsorption, outperforming Fenton in efficiency, stability, and environmental safety while contributing to biomass valorization.