Synthesis and characterization of avocado pit activated carbon-incorporated chitosan composite beads for harnessing methylene blue adsorption: DFT insights and box-behnken design optimization

Background: Methylene blue (MB), a toxic dye in industrial wastewater, requires efficient removal methods due to its environmental and health risks. This study uses activated carbon derived from avocado pits, modified with chitosan to form the CS@ACAP composite, to enhance adsorption. The research focuses on optimizing the synthesis, evaluating adsorption efficiency, and exploring the adsorption mechanisms through experimental and theoretical analyses. Methods: Activated carbon was synthesized and optimized using the Box-Behnken design, considering key parameters such as sulfuric acid concentration, activation time, and temperature. The material was then modified with chitosan to create the CS@ACAP composite. Both materials were characterized using FTIR, SEM, XRD, BET, pHpzc, and EDS analyses. Adsorption experiments were performed under varying conditions pH (2-10), contact time (10-180 min), adsorbent mass (0.02-0.14 g) to determine the maximum adsorption capacity (qmax). Isotherm and thermodynamic models, along with DFT simulations, were used to analyze the adsorption behavior and provide molecular-level insights into methylene blue interactions with the materials. Significant Findings: The CS@ACAP composite exhibited a higher adsorption capacity (81.0 mg/g) compared to activated carbon (40.2 mg/g). Activated carbon achieved optimal adsorption at pH 10, with 160 min and 0.12 g of adsorbent, while CS@ACAP required only 110 min and 0.08 g. Both materials followed the Langmuir model in isotherm studies. Thermodynamic analysis showed exothermic adsorption on activated carbon and endothermic adsorption on CS@ACAP. DFT simulations confirmed stronger MB interactions with CS@ACAP, underscoring its potential as an efficient adsorbent. These findings position CS@ACAP as a promising, eco-friendly material for wastewater treatment.