This study optimized the adsorption parameters for removing Bemacid Red dye using biochar derived from coagulation-flocculation sludge. The biochar was comprehensively characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, thermogravimetric analysis, X-ray fluorescence, and point of zero charge determination. Adsorption experiments were conducted by varying the adsorbent dose, temperature, contact time, and initial dye concentration. A Box-Behnken design with response surface methodology was used to optimize the conditions. The experimental results closely matched the model predictions. The analysis of variance validated the model. The optimum conditions were 1.2 g. L−1, 45 °C, 40 min, and 80 mg. L−1 initial concentration, achieving 97.27 % removal efficiency. B3LYP/6-31G(d) quantum calculations supported the results. Global reactivity descriptors and local Fukui indices were calculated to elucidate adsorption mechanisms. This study highlights the effectiveness of low-cost coagulation sludge biochar for removing dyes from textile wastewater and provides a comprehensive understanding of the adsorption process through experimental and theoretical approaches.