<p>Removing harmful dyes from wastewater is crucial for reducing health hazards and safeguarding community health. In this research, a novel biocomposite was synthesized using sodium alginate (SA), bentonite (Bnt), and iron-modified walnut (<i>Juglans regia</i>) biochar (Fe@BC) to eliminate methylene blue (MB) from the water solution. The biocomposite (SA/Bnt/Fe@BC) was characterized by techniques including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) analysis. The prepared biocomposite exhibited a high removal efficiency and resulted in 95% adsorption with the preliminary concentration of MB dye (20 mg L⁻¹) using 1 g L⁻¹ of prepared adsorbent at pH 7 for 60 min. The adsorbent was efficient for greater levels of dye (upto 80 mg L⁻¹) and a wide pH range (2–10). This study also shows that the adsorbent’s maximum Langmuir adsorption capacity was 95.33 mg g⁻¹. The Freundlich isotherm models and pseudo-second-order (R² = 0.99) have shown superior suitability for precisely characterizing the adsorption process. Thermodynamic analysis revealed a spontaneous and endothermic adsorption process, with an enthalpy change (ΔH) measured at 7.53 kJ mol⁻¹ and an entropy change (ΔS) at 0.047 kJ mol⁻¹ K⁻¹. The reusability investigation further revealed that the adsorbent can retain its adsorption capacity after seven cycles. Further, the effect of co-existing ions, Fe²⁺, Ni²⁺, Cu²⁺, and, Zn²⁺, were also studied, which showed a strong selectivity of SA/Bnt/Fe@BC towards MB dye, making it an economical and environmentally sustainable adsorbent for the treatment of wastewater.</p>