Effective Removal of Azithromycin from Aqueous Solution Using Coconut Husk Derived Boric Acid-Modified Biochar

The occurrence of azithromycin in aquatic environments highlights the need for effective remediation approaches. In this study, three boric acid-modified coconut husk biochars prepared using nominal H₃BO₃ precursor concentrations of 0.1%, 2%, and 5% were evaluated for azithromycin adsorption from aqueous solution. The materials exhibited different adsorption performances under the investigated conditions, with 5% B-CB showing the highest Langmuir-predicted maximum adsorption capacity (q_max) of 170.45 mg g^-1, followed by 2% B-CB (158.84 mg g^-1) and 0.1% B-CB (136.92 mg g^-1). The equilibrium data were best statistically represented by the Langmuir model among the isotherm models evaluated. Kinetic analysis showed that the pseudo-second-order (PSO) model provided a better statistical description of the adsorption kinetics, with 5% B-CB showing the highest goodness-of-fit (R² = 0.99), compared with 0.96 and 0.95 for 0.1% and 2% B-CB, respectively. The positive ΔH° values of 11.08, 6.74, and 6.19 kJ mol^-1 for 0.1%, 2%, and 5% B-CB, respectively, indicated an endothermic adsorption process under the investigated conditions. FTIR analysis indicated qualitative changes in surface functionalities consistent with boric acid-modified coconut husk biochar following H₃BO₃-assisted modification; however, the actual boron content of the final biochars was not quantitatively determined. Furthermore, 5% B-CB retained measurable adsorption capacity over five adsorption-desorption cycles, and its fixed-bed performance was evaluated under the investigated operating conditions. Overall, the results indicate that azithromycin adsorption performance varied with the nominal H₃BO₃ precursor concentration, with 5% B-CB exhibiting the highest adsorption capacity among the materials investigated under the experimental conditions.