Acid-Activated Biochar for Efficient Elimination of Amoxicillin From Wastewater

In exploring the potential of agricultural waste as an efficient adsorbent, acid-activated coconut husk biochar was prepared, characterized and applied to remove amoxicillin from wastewater. The mechanism of adsorption was also investigated through isotherm and kinetic studies. The biochar was activated with HNO₃ for 2 h at 25°C and investigated under varying experimental conditions of pH (1–11), initial amoxicillin concentration (0.5–2.5 mg/L), adsorbent dosage (2–20 mg) and contact time (0–24 h). The adsorption process was characterized by rapid attainment of adsorption equilibrium and high values of adsorption capacity. Acid activation had a significant impact on the adsorptive capacity of biochar as removal efficiency improved from 90% (over an hour) to 98.9% (within 40 min). The FTIR results confirmed the characteristic adsorption peaks of C-H, =C-H, C=C, C-O, C=O and O-H, suggesting the dominant presence of aromatic and carbonyl functional groups on the surface of the biochar. Adjustments in peak shapes and transmittance were observed for these functional groups after the adsorption; highlighting the likely interaction between the biochar and the amoxicillin during the adsorption which has been succinctly proposed. The SEM micrographs showed the growth of leaf-like flakes of irregular shapes and pores essential for the adsorption. The rate and mechanism of adsorption were driven by the variable surface charge of the adsorbent and the degree of ionization of the adsorbates, which were largely controlled by the solution pH. Adsorption rates were highest in the acidic medium, peaking around a pH of 3, then remained relatively stable between pHs of 5 and 7 before reducing dramatically with increasing pH in the alkaline region. The adsorption best followed the Langmuir model of isotherm while the kinetics mimicked the pseudo–second order (PSO). The findings suggest that HNO₃-activated coconut biochar is a promising, low-cost and porous adsorbent effective for amoxicillin removal.