Adsorption of Reactive Red 120 dye from aqueous solutions using sepiolite clay with a channel-structured morphology: Mechanism, kinetics, isotherm, and thermodynamic studies

The effective treatment of dye-containing wastewater remains a major environmental challenge due to the high stability and solubility of reactive dyes. In this study, the adsorption behavior of Reactive Red 120 (RR120), an anionic reactive azo dye, from aqueous solutions was investigated using natural sepiolite clay possessing a channel-structured morphology. Batch adsorption experiments were performed to evaluate the effects of contact time (1–200 min), initial dye concentration (50–500 mg L−1), adsorbent dosage (0.1–1.5 g), solution pH (2–10), and temperature (25–50°C). The adsorption equilibrium was reached within approximately 75 min. The Langmuir isotherm provided the best fit to the equilibrium data, yielding a maximum monolayer adsorption capacity of 168.4 mg g−1 at 298 K. Thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic, with an activation energy of 32.41 kJ mol−1. Adsorption kinetics were analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, while equilibrium data were evaluated using Langmuir, Freundlich, and Temkin isotherm models. The kinetic data were best described by the pseudo-second-order model, while thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic. FTIR and XRD analyses further supported the important role of the channel-structured framework of sepiolite in RR120 adsorption. Unlike many clay-based adsorbents that require chemical modification, natural sepiolite exhibited high adsorption performance due to its intrinsic fibrous morphology, nano-channel structure, and high surface area. These findings demonstrate that natural sepiolite is a promising, low-cost, and sustainable adsorbent for the treatment of dye-contaminated wastewater.