<p>This work aimed to develop and optimize a novel magnetic biochar (MB) adsorbent from sorghum residue biomass (SRB) modified with Fe₃O₄ to remove cadmium (Cd(II)) from aqueous solution effectively and examined its applicability for paint industry effluent. The MB was developed through pyrolysis of the preprocessed SRB at 450&#xa0;°C with subsequent modification with Fe₃O₄. Proximate, FTIR, SEM, and XRD analyses were implemented for its characterization. The Cd(II) level in the paint effluent and aqueous solution was determined using MP-AES. The MB adsorption process was comprehensively optimized for factors affecting Cd(II) adsorption including pH, adsorbent dosage, contact time, and Cd(II) initial concentration. The optimization of the adsorption process and binary interaction effects of operational variables were examined using batch adsorption experimentation with the central composite design under the response surface method (RSM) using a design-expert. Maximum Cd(II) removal of 97.5% was achieved from aqueous solution at pH 6, adsorbent dose of 1.25&#xa0;g/100&#xa0;mL, initial Cd concentration of 5.2&#xa0;mg/L, and contact time of 45&#xa0;min. This result was in agreement with the RSM predicted value of 97.3%. However, for the actual paint effluent, the Cd(II) removal efficiency was slightly reduced to 95.14% due to other ions in the effluent competing for adsorbent sites. Moreover, the experimental data best fit the Langmuir isotherm and pseudo-second-order kinetic model with <i>R</i>² values of 0.986 and 0.999, respectively. The thermodynamic study of the adsorption of Cd(II) ions on the MB revealed that spontaneous, endothermic, and with increased randomness at the adsorbent-solution interface, more effective at high temperatures. The adsorbent demonstrated good regeneration capacity, maintaining 52.3% removal efficiency after five adsorption–desorption cycles. The study reveals that MB-derived SRB/Fe3O4 can effectively remove Cd(II) from wastewater.</p> Graphical Abstract <p></p>