A porous bentonite–coconut husk composite for the enhanced adsorption of selected emerging contaminants from aqueous solution

Abstract

The preparation of bentonite clay–coconut husk composite (BECH) via microwave-assisted carbonization, the activation of BECH with HCl (BECH-H) and KOH (BECH-K) and the adsorption of ciprofloxacin, tetracycline, and bisphenol A from aqueous solutions by the resultant carbon materials are highlighted in this study. The adsorbents (BECH, BECH-H and BECH-K) were characterised by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and N₂ adsorption–desorption analysis using the BET model. The results from these analyses showed sufficient interactions between bentonite and coconut husk. The adsorbents exhibited high specific surface areas (284–456 m² g⁻¹) showing potential to be used in adsorption studies. The Sips model provided the best fit to the adsorption data when data from the equilibrium experiment were fitted to the Freundlich and Sips models. BECH-H and BECH-K demonstrated the highest affinity for bisphenol A with BECH-H having the highest adsorption capacity (472.9 mg g⁻¹). BECH also had the greatest preference for the adsorbents, having adsorption capacities of 199.7 mg g⁻¹ for ciprofloxacin, 183.5 mg g⁻¹ for bisphenol A and 281.7 mg g⁻¹ for tetracycline. Results showed high removal efficiencies for the ECs studied (>90%) at a low dosage of the adsorbents (50 mg). The kinetic data of all the adsorbents are best described by the pseudo-second-order kinetic model. Adsorption reactions were heterogeneous in nature with chemisorption being the rate-limiting step. These findings suggest that these adsorbents could be excellent materials for effectively treating EC-contaminated water.