Talc-graphite schist as a natural organo-mineral complex for methylene blue remediation: kinetic and isotherm study

Talc-graphite schist (TGSH) is a natural organo-mineral complex that was derived from the regional metamorphism of organic matter-bearing calcareous clays under the lowest temperature and pressure of the green schist facies (573–632 K and 1–3 Kbars, respectively). The application of TGSH that is widely available in the Meatiq area (Eastern Desert, Egypt) was investigated for the first time ever in the remediation of methylene blue dye (MB) from aqueous solutions. The characterization of the TGSH was carried out using various techniques (XRF, XRD, SEM, FT-IR and BET surface area), and its MB removal capacity was estimated at different experimental conditions. The MB adsorption by TGSH was time- and pH-dependent process, where 120 min was sufficient enough to attain equilibrium. The MB adsorption capacity by the TGSH was directly proportional to the applied temperature, confirming that the MB adsorption process was endothermic in behavior. Based on the determination coefficients, the pseudo-second-order equation (R² = 0.939) explained MB adsorption data better than the first-order one (R² = 0.653). Meanwhile, intra-particle diffusion was not the sole controlling rate in MB removal by the TGSH. Moreover, the nonlinear regressions of Langmuir model (R² = 0.876) explained the equilibrium data better than the other applied models (Temkin, R² = 0.793, and Freundlich, R² = 0.752). On the other hand, the estimated q_max (maximum removal capacity) of Langmuir was 9.41 mg/g at ambient temperature. Hydrogen bonding and electrostatic interaction were the governing mechanisms of MB removal by TGSH with variable impact in accordance with the prevailing pH condition.