Exploring and Studying the Adsorption Mechanisms of the Herbicides 2,4,5-T and 2,4-D on Activated Carbon from Walnut Shells, Using Theoretical DFT Analyses and a Central Composite Design

Water depollution via adsorption is an energy-efficient technique that uses activated carbon produced from walnut shells to remove different pollutants. Our research integrates theoretical and experimental approaches to comprehensively understand the subject. By analyzing organic functional groups in the walnut shell-derived activated carbon (AC-Ws), evaluating adsorption performance, employing DFT analysis, and optimizing conditions, we provide valuable insights into the application of AC-Ws for removing pesticide pollutants from wastewater. The properties of this activated carbon (AC-Ws) are identified by techniques such as XRD, FTIR, SEM/EDX, pHzpc, and Boehm titration. A response surface methodology (RSM) has been utilized to develop an adsorption experiment for aqueous solutions of two herbicides, 2,4,5-trichlorophenoxyacetic acid (2.4.5-T), and 2,4-Dichlorophenoxyacetic acid (2,4-D). Abbreviations for technical terms will be explained when first used, and objective language will be used throughout. Grammar and spelling will conform to British standards. The experiment examined the removal of the herbicides based on three independent variables; pH, AC-Ws dosage, and temperature, within specific ranges (pH: 4–10, AC-Ws dosage: 0.1–0.3 g/L, temperature: 20–40 °C). The research conducted 17 experiments using Design-Expert software. Optimal conditions included pH 4 and AC-Ws dosage of 0.3 g/L for 2.4.5-T and pH 5 and AC-Ws dosage of 0.1 g/L for 2,4-D. These conditions demonstrated removal efficiency of 98% and desirability function of 1.000 for 2.4.5-T and 92% and desirability function of 1.000 for 2,4-D. The findings suggest that the adsorption process was thermodynamically favorable and exothermic. The experimental data was described accurately by the pseudo-second order kinetic model, and the Freundlich isotherm model showed good alignment with the equilibrium data. Using the Boehm method, organic functions were identified on the adsorbent's surface, and the density functional theory (DFT) was applied to evaluate the adsorption mechanism on AC-Ws surface sites.