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 (qmax) 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.