Advanced simultaneous adsorption of heavy metals and organic dyes from aqueous solutions using CTAB-functionalized broccoli waste biochar: A sustainable approach for water remediation

Addressing the simultaneous contamination of wastewater with dyes and heavy metals is a major environmental challenge requiring cost-effective and sustainable treatment strategies. In this study, broccoli waste was converted into biochar and subsequently modified using sulfuric acid (H 2 SO 4 ) and cetyltrimethylammonium bromide (CTAB) to enhance its physicochemical properties for the concurrent adsorption of methylene blue (MB), malachite green (MG), Congo red (CR), cadmium (Cd 2+ ), and lead (Pb 2+ ) from aqueous solutions. SEM analysis revealed that acid and surfactant treatments transformed the initially smooth biomass surface into a highly porous and rough structure with increased surface heterogeneity, while FTIR and BET results confirmed the successful incorporation of sulfate and quaternary ammonium groups, leading to improved surface charge distribution and pore architecture. Batch adsorption experiments demonstrated that the modified biochar exhibited significantly enhanced removal efficiencies—up to 99.9% for MB, 95.9% for MG, 99.4% for CR, 98.6% for Pb 2+ , and 62.9% for Cd 2+ —under optimized conditions. Adsorption was strongly pH-dependent, with cationic dyes and metal ions favoring near-neutral to slightly alkaline media, while the anionic CR showed maximum uptake near the point of zero charge. Kinetic studies fitted well to the pseudo-second-order model (R 2 > 0.99), indicating chemisorption-dominated mechanisms involving electron sharing or exchange. Isotherm analysis revealed that MB followed the Freundlich model, while MG and CR were best described by the Langmuir model, suggesting a combination of monolayer and multilayer adsorption on heterogeneous surfaces. Thermodynamic parameters confirmed that MB adsorption was spontaneous and strongly exothermic, whereas CR adsorption changed from endothermic on raw biochar to spontaneous and exothermic after CTAB modification. Finally, CTAB-modified broccoli biochar demonstrates high adsorption capacity, rapid kinetics, and excellent reusability, highlighting its potential as an efficient, low-cost, and sustainable biosorbent for the simultaneous removal of multi-class pollutants from industrial wastewater. • Broccoli waste-derived biochar developed as an eco-friendly and low-cost adsorbent • Surface modification with sulfuric acid and CTAB enhanced porosity and roughness • High removal efficiencies achieved: 99.9% for MB, 95.9% for MG, 98.6% for CR, 62.9% for Cd, and 98.6% for Pb • Adsorption strongly pH-dependent, with distinct optimal conditions for dyes and heavy metals • Kinetic data fitted well to the pseudo-second-order model, implying surface-controlled adsorption. • Reusability confirmed with considerable efficiency maintained over five cycles • Promising sustainable solution for simultaneous removal of dyes and heavy metals from complex wastewater