Utilization of Pinus kesiya and Schima wallichii Biomass-Derived Activated Carbon for Methylene Blue Removal: Adsorption Performance and Mechanistic Insights

Biomass material offers several advantages to synthesize activated carbon because of its economic viability, renewability, availability, and unique physicochemical properties. This piece of work reports activated carbon prepared from biomass of Pinus kesiya and Schima wallichii via ZnCl2 activation for potential application to remove methylene blue from water. The optimum adsorption parameters—pH, adsorbent dose, and agitation speed—were obtained using Taguchi design of experiment. Experimental equilibrium data of adsorption of methylene blue onto the adsorbents fitted well with Langmuir isotherm with maximum adsorption capacity of 116.28 mg g−1 and 95.24 mg g−1 for Pinus kesiya activated carbon and Schima wallichii activated carbon respectively. The kinetic experimental data followed a pseudo-second-order equation for both the adsorbents. The potential rate-controlling step in the adsorption of methylene blue onto the adsorbents was predominantly intraparticle diffusion with two stages of adsorption for Pinus kesiya activated carbon and three stages of adsorption for Schima wallichii activated carbon. Density functional study investigation suggested that methylene blue adsorption onto activated carbon is predominantly chemisorption, and the presence of a carboxylic acid functional group on the activated carbon surface has a higher methylene blue adsorption affinity with an adsorption energy of −171.85 kJ mol−1 compared to —CHO and —OH functionalized carbon, pristine activated carbon models. Our work indicates that activated carbon derived from Pinus kesiya and Schima wallichii biomass could be an efficient adsorbent to remove methylene blue from water.