Enhanced adsorption of ciprofloxacin from aqueous solutions using functionalized banana stalk

Abstract

Activated carbon from banana stalk, an eco-friendly agricultural waste, was prepared and investigated for its absorptive potentials in removing ciprofloxacin (CIP), an antibiotic from aqueous media. Orthophosphoric acid was used to modify the agricultural waste. The textural characteristics and adsorptive properties of the activated banana stalk (BSAC) were investigated using SEM, FTIR, Boehm titration (BT), and pHpzc analytical techniques respectively. Image from the SEM of BSAC showed well-developed pores, supporting the trapping of CIP molecules to the surface, while the FTIR revealed notable bands associated with specific functional groups responsible for enhanced and efficient uptake of CIP. The Boehm titration revealed the total acidic group to be 0.699 mmol/g and the basic group to be 0.1582 mmol/g, suggesting the predominance of acidic groups, and this was supported by the pHpzc value of 4.5. The most favorable interaction between the BSAC surface and the CIP molecules were in the in zwitterionic form of the CIP. The adsorptive uptake in this study was optimum at pH 8. Experimental data were studied using five different models of adsorption isotherm, namely, the Langmuir, Freundlich, Temkin, D-R, and Sips. The Langmuir isotherm (R2 = 0.9824) best described the experimental data with an optimum monolayer capacity for adsorption of 49.7 mg/g at 323 K. The D-R isotherm showed that the mean free energy ranged from 1.29 to 3.54 kJ/mol, suggesting that the mechanism of adsorption for the uptake was physisorption in nature. The adsorption process was best explained by the pseudo-second-order kinetic model with R2 values between 0.9103 and 0.9995. Thermodynamic results obtained proved that the sorption of CIP antibiotics onto BSAC was endothermic, spontaneous, and thermodynamically favored (ΔH = + 106.561 kJ/mol, ΔS = + 0.44797 kJ/mol, and ΔG = − 36.265 kJ/mol). BSAC prepared in this study is about six times cheaper than the commercially available activated carbon indicating the cost-effectiveness of this work. This study, therefore, establishes that the modification of banana stalk waste into activated carbon is efficient for the adsorptive uptake of ciprofloxacin antibiotics from aqueous media.