Adsorption of methylene blue dye using common walnut shell (Juglans regia) like biosorbent: implications for wastewater treatment

Methylene blue (industrial dye) presents adverse effects on the environment. Traditionally, this dye is removed by adsorption with commercial activated carbon. Researchers are exploring low-cost alternatives using agro-industrial waste. This study evaluates Chandler-type walnut shells for removing methylene blue from aqueous solution. Batch experiments with two nominal sizes reveal that pseudo-second-order kinetics best describe adsorption. Langmuir isotherms show adsorption capacities of 8.636 and 38.46 mg g−1 for analyzed sizes. The best removal capacity (97%) was observed for smaller particle sizes (2.36–1.18 to 1.18–0.6 mm) with an initial concentration of 200 mg L−1. High initial concentrations decrease removal due to porous saturation. The best removal capacity in continuous systems is 50.47 mg g−1, with a saturation time of 8 hours at 5 cm bed height, adsorbent size 1.18–0.6 mm, 60 mg L−1 initial concentration, and 15 mL min−1 flow rate. The Thomas model best fits the data, suggesting external mass transfer and intra-particle diffusion. This study provides crucial insights for scaling processes and shows a competitive removal efficiency of methylene blue using walnut shells compared to other biosorbents. Further investigation is needed to analyze channeling phenomena observed with increased bed heights in continuous systems.