Adsorption of 2,4-D Using Zinc Chloride–Activated Pyrolyzed Coal: Efficiency, Reusability, Statistical Physics and Thermodynamic Insights
摘要
The production of coal-derived adsorbents stood out as an alternative for coal waste management. This study investigates the production of activated mineral coal (AMC) using ZnCl2, a well-known chemical activation agent. The prepared material was characterized and used for adsorbing 2,4-D herbicide (2,4-dichlorophenoxyacetic acid) in an aqueous solution. The characterization results indicated a substantial enhancement of the material's textural properties after activation, resulting in the surface area increasing from 2.87 to 347 m2 g⁻1, as well as an increase in the presence of surface hydroxyl groups. 2,4-D adsorption onto AMC was highly pH-dependent, and the most suitable conditions were found to be pH 2 and an adsorbent dosage of 0.88 g L-1. The adsorption process was best described by a pseudo-second-order kinetic model, with faster adsorption (h0 = 0.722–6.924 mg g-1 min-1) for higher initial concentrations (C0 = 10–50 mg L-1). The equilibrium data were well described by the Sips isotherm model, with a maximum theoretical adsorption capacity of approximately 60 mg g⁻1 at 328 K. Thermodynamic analysis suggested that the adsorption of 2,4-D was spontaneous and endothermic. Advanced isotherm models indicate that the process occurs via double-layer formation on the adsorbent surface, involving a single energy source. The adsorption energy suggests that the process occurs predominantly by physical forces. Pore filling, hydrophobic interactions, electrostatic attraction, hydrogen bonding, and π–π interactions were proposed as the main mechanisms. The regeneration and reuse experiments showed a small reduction in the adsorption capacity (only about 10% after six cycles), indicating the potential for prolonged use of the adsorbent.