Insight into adsorption equilibrium, kinetics and thermodynamics of Malachite Green onto clayey soil of Indian origin

This study investigates the adsorption potential of clayey soil of Indian origin to remove Malachite Green, a cationic dye from its aqueous solution. To this end, the effect of operational parameters including initial pH, adsorbent dose, contact time, initial dye concentration and temperature were studied in batch adsorption experiments. scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were used to characterize the adsorbent material. The experimental equilibrium data were tested by three widely used isotherm models namely, Langmuir, Freundlich and Dubinin–Radushkevich (D–R). It was found that adsorption of Malachite Green on clayey soil correlated well with the Langmuir isotherm model, implying monolayer coverage of dye molecules onto the surface of the adsorbent. The maximum adsorption capacity was found to be 78.57 mg g−1 at 303 K. Kinetics of the adsorption process was tested by pseudo-first-order and pseudo-second-order kinetics, and intraparticle diffusion mechanism. Pseudo-second-order kinetic model provided a better correlation for the experimental data studied in comparison to the pseudo-first-order model. Intraparticle diffusion was not the sole rate-controlling factor. The activation energy of the adsorption process (Ea) was found to be 45.82 kJ mol−1 by using the Arrhenius equation, indicating chemisorption nature of Malachite Green adsorption onto clayey soil. A thermodynamic study showed spontaneous nature and feasibility of the adsorption process. A negative enthalpy (ΔH°) value indicated that the adsorption process was exothermic. The results indicate that clayey soil can be used as an effective and low-cost adsorbent to remove Malachite Green from aqueous solution.