Chalky limestone as sustainable water purification from eriochrome black T: kinetics, isotherm, and equilibrium

A sustainable, eco-friendly, and effective adsorbent has been employed for the removal of eriochrome black T (EBT) toxic dye from the effluent. This adsorbent, derived from natural chalky limestone, underwent thorough characterization using various investigative tools, including X-ray diffraction, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller, field emission scanning electron microscopy, transmission electron microscopy, dynamic light scattering, zeta potential, thermogravimetric analysis (TGA), and ultraviolet–visible technique. The chalky limestone exhibited distinctive properties, achieving an efficient removal percentage and high capacity within a short timeframe. Kinetic studies were conducted to assess the reaction speed, employing both pseudo-first-order and pseudo-second-order models, yielding R² values of 0.67 and 0.62, respectively. Furthermore, thermal studies were carried out using Langmuir and Freundlich models to elucidate the interaction nature between the active sites of chalky limestone and EBT dye molecules, resulting in R² values of 0.75 and 0.91, respectively. The research findings indicated that the adsorption process achieved equilibrium in just five minutes with adsorption capacity (q_t) about 1.99 mg g⁻¹, and the chalky limestone showed the ability to be reused for up to five cycles without any decrease in removal efficiency or requiring a desorption step.