Development and optimization of sustainable activated biochar from waste pigeon pea stalks for efficient adsorptive removal of methylene blue from water

This study explores the valorization of pigeon pea stalk waste (PPSW) through the synthesis of activated biochar (AB) with enhanced adsorption properties for the efficient removal of methylene blue (MB) from aqueous solutions. Addressing both agricultural waste disposal and water pollution, the research converts PPSW into high value adsorbents. Two types of AB were synthesized using sodium hydroxide (NaOH) and zinc chloride (ZnCl₂) as activating agents, designated as NaOH/AB and ZnCl₂/AB, respectively. The synthesis process was optimized using an L9 orthogonal array design, evaluating four key parameters: activation temperature, activation time, reagent to carbon ratio, and stirring speed. The influence of these parameters on the adsorption capacity (AC) and removal efficiency (RE) of MB was further examined through parametric studies. Statistical optimization and analysis were performed using ANOVA to validate the significance of each factor. The synthesized ABs were characterized using advanced analytical techniques. For NaOH/AB, optimal conditions yielding 100 % RE were found to be an activation temperature of 40 °C, activation time of 12 h, reagent-to-carbon ratio of 2:1, and stirring speed of 400 rpm. In the case of ZnCl₂/AB, maximum AC of 7.51 mg/g and 100 % RE were achieved at 80 °C, 12 h, a 6:1 reagent-to-carbon ratio, and 600 rpm stirring speed. The results confirm that AB derived from PPSW using NaOH and ZnCl₂ exhibits excellent adsorption performance and is highly effective in the complete removal of MB from contaminated water.