CTAB-functionalized clinoptilolite alginate microbeads for enhanced Congo Red adsorption: performance, kinetics and equilibrium analysis

Congo Red (CR) is an anionic dye that persists in industrial effluents due to its stability against biodegradation, water solubility, and resistance to the conditions found in industrial wastewater. Natural clinoptilolite is a low-cost adsorbent for water treatment. However, its negatively charged surface limits its ability to remove anionic dyes effectively. In this study, clinoptilolite microbeads functionalized with cetyltrimethylammonium bromide (CTAB) were developed using a calcium-crosslinked alginate matrix to enhance Congo Red (CR) adsorption. The functionalization and characterization of the microbeads were performed using Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) as well as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Batch adsorption experiments were performed to evaluate the effects of the initial dye concentration, microbeads dosage, and the contact time on Congo Red removal. Under the investigated batch conditions, the CTAB-functionalized microbeads achieved a maximum experimental adsorption capacity of 51.69 mg g⁻ 1 and a Congo Red removal efficiency of 76.93% using 0.075 g adsorbent after 15 h. Adsorption kinetics were best described by the pseudo-second-order model, while equilibrium data showed better agreement with the Langmuir isotherm models, indicating enhanced adsorption affinity following CTAB modification. In addition to enhanced performance, the microbeads configuration offers practical benefits over powder including less material loss during the recovery process and easier to handle in the wastewater treatment process. In summary, microbeads CTAB-functionalized clinoptilolite are a promising adsorbent for laboratory-scale Congo Red removal.