Elucidating the dominant role of π–π interactions in methylene blue removal via porous biochar: A synergistic approach of experimental and theoretical mechanistic insights

Abstract

Despite the extensive application of porous carbon materials for dye-contaminated wastewater treatment, the molecular-level mechanisms governing pore design and surface functionality remain inadequately resolved. This study employs a synergistic experimental and density functional theory (DFT) approach to systematically investigate methylene blue (MB) adsorption mechanisms on porous carbons. Wheat straw was selected as the precursor due to its abundant availability, cost-effectiveness, and renewable nature, offering an environmentally friendly alternative to conventional adsorbents. High-performance porous carbon materials can be effectively fabricated by combining the KOH activation with pre-carbonized wheat straw as a carbon source. Kinetic analyses demonstrate chemisorption-dominated monolayer adsorption governed by pseudo-second-order kinetics, while equilibrium data align with the Langmuir model, revealing chemisorption contributions. KPC-3 exhibits exceptional performance with a maximum adsorption capacity of 657.56 mg/g, correlated to its hierarchical porosity (BET surface area: 1554 m²/g) and optimized pore distribution. Intraparticle diffusion of MB within KPC-3 was found to be the rate-controlling step, exhibiting pH-independent characteristics. DFT simulations confirmed chemisorption dominance at oxygen-functionalized sites, with π-π interactions substantially augmenting adsorption energy. Maximum electron transfer occurred at the zig_lactone group. MB⁺ adsorption is primarily controlled by micropore and mesopore structures via spatial confinement effects that amplify π-π interactions. Rational material engineering techniques are advanced by this study, which unequivocally identifies pore topology as the pivotal design parameter for optimizing dye removal efficiency in carbon-based adsorbents.