Rice Husk-Derived Biochar for the Adsorptive Removal of the Herbicide 2,4-Dichlorophenoxyacetic Acid from Water

2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most widely used herbicides in agriculture, and its occurrence in aquatic environments has prompted the search for efficient removal strategies. In this context, the use of agricultural residues as precursors for adsorbent materials represents a promising approach for the treatment of contaminated water. Therefore, this study evaluated the use of rice husk-derived biochar for the removal of 2,4-D from aqueous solutions, considering adsorption conditions, kinetics, equilibrium, thermodynamics, and desorption. Batch adsorption experiments were performed to investigate the effects of solution pH (2.0, 3.0 and 4.0), adsorbent dosage (0.1 and 0.2 g), contact time (0–360 min), initial 2,4-D concentration (20–240 mg L−1), and temperature (15.0–45.0 °C), and the residual 2,4-D concentration was quantified by HPLC-DAD. Adsorption kinetics were better described by the pseudo-second-order model (R2 = 0.8727), while the Langmuir, Sips, and Redlich–Peterson models satisfactorily described the equilibrium data (R2 ≈ 0.9). Langmuir maximum adsorption capacities ranged from 20.40 to 25.37 mg g−1, while FTIR analysis indicated the involvement of surface functional groups in 2,4-D adsorption. Thermodynamic analysis revealed negative ∆G° values (−34.0 to −37.7 kJ mol−1) and a mildly positive enthalpy change (ΔH° = 12.45 kJ mol−1), indicating a spontaneous and mildly endothermic adsorption process. Desorption close to 100% was achieved using 0.1 M NaOH:methanol (50:50, v/v), demonstrating high desorption efficiency and reversible interactions under the tested conditions. These results demonstrate the potential of rice husk biochar as a sustainable adsorbent for 2,4-D removal from aqueous systems and highlight the valorization of agro-industrial waste.