Sequestration, recovery, and reuse of cationic, anionic, and non-ionic dyes using bentonite

This study quantitatively evaluated the adsorption performance of natural bentonite for removing three dye classes—cationic (Basic dye: BEZACRYL RED GRL), anionic (Reactive dye: AVITERA LIGHT RED SE), and non-ionic (Disperse dye: BEMACRON BLUE HP3R) from synthetic textile wastewater. Batch adsorption experiments were conducted under varying conditions of contact time (15–90 min), adsorbent dosage (20–60 g L⁻¹), pH (4 and 12), and temperature (25–100 °C), with dye concentrations quantified by UV–Vis spectroscopy. At a contact time of 30 min and room temperature (25 °C), maximum removal efficiencies reached 99.98% and 99.93% for cationic dye, 65.26% and 77.13% for anionic dye, and 94.04% and 79.40% for non-ionic dye at pH 4 and pH 12, respectively. For non-ionic dyes, the removal efficiency improved slightly at elevated temperature (nearly at 60 °C). Kinetic analysis showed that cationic dye adsorption followed the pseudo-second-order (PSO) model (R² = 0.9999), indicating a chemisorption mechanism driven by electrostatic interactions with negatively charged bentonite basal planes. Non-ionic dye adsorption fitted better to the pseudo-first-order (PFO) model (R² = 0.9821), consistent with physisorption via van der Waals forces and hydrogen bonding. Anionic dye adsorption showed poor fits to both models (R² < 0.60), reflecting weak uptake due to electrostatic repulsion from the negatively charged bentonite surface. Equilibrium data were best described by the Langmuir isotherm, suggesting monolayer adsorption with maximum capacities of 0.486, 0.373, and 0.483 mg g⁻¹ for cationic, anionic, and non-ionic dyes, respectively. Recovered dyes were reused in textile printing, producing stable and vibrant prints for cationic and non-ionic dyes, with slightly reduced intensity for anionic dyes. This work demonstrates bentonite’s dual role as an efficient, low-cost adsorbent and a means for resource recovery, providing novel insights into simultaneous wastewater treatment and dye reuse across multiple dye classes.

Graphical abstract