Preferential, synergistic sorption and reduction of Cr(VI) from chromium–rhodamine B mixed wastewater by magnetic porous biochar derived from wasted Myriophyllum aquaticum biomass.

Eradication of heavy metals and dyes simultaneously from wastewater is urgently needed to safeguard public and environmental health. In this study, magnetic porous biochar derived from wasted Myriophyllum aquaticum (MPMaB) was synthesized by KOH-activation and co–precipitation method to treat chromate and rhodamine B (RhB)-bearing wastewater. The KOH activation significantly improved the pore structure of biochar with a high specific surface area of 937.1 m² g⁻¹. The sorption performance of MPMaB for Cr(VI) and RhB in single and co-solutes conditions was evaluated. In single system, a pH-dependent sorption pattern for Cr(VI) by MPMaB was revealed and the estimated sorption capability reached 175.4 mg g⁻¹, whereas the Langmuir-based sorption capacity of RhB was 175.4 mg g⁻¹ pH-independently. MPMaB partially transformed Cr(VI) to less toxic Cr(III) (approximately 59.3%). Synergistic sorption of Cr(VI) with the coexistence of RhB was observed, where synergistic effect ranged from 119% to 527% depending on pH. For example, the sorption capacity of Cr(VI) on MPMaB, at pH 2, augmented from 175.4 mg g⁻¹ (single system) to 208.3 mg g⁻¹ (binary system). Preferential sorption of Cr(VI) was found and was further confirmed by the post-sorption of Cr(VI) (or RhB) by MPMaB pre-sorbed with RhB (or chromate). Chromate sorption mechanisms mainly include electrostatic interactions and complexation, while the sorption of RhB is ascribed to π–π interactions, pore filling and hydrogen bonding. Additionally, MPMaB showed excellent reusability and maintained high removal efficiency after 5 cycles. In short, MPMaB can efficiently treat chromium and dyes–containing wastewater as sustainable and environmentally friendly adsorbent. [Display omitted] • Wasted macroalgae was used to synthesize magnetic porous biochar with high SSA. • MPMaB exhibited excellent sorption ability for Cr(VI) and RhB with good reusability. • SynErgistic sorption (119%–527%) of Cr(VI) was revealed with coexistence of RhB. • Preferential sorption of Cr(VI) than RhB was observed at pH 2. • MPMaB can partially transformed Cr(VI) to Cr(III), concurrent with Cr(VI) sorption. [ABSTRACT FROM AUTHOR]