Uptake of Methyl Red dye from aqueous solution using activated carbons prepared from Moringa Oleifera shells

The present work was successfully designed to prepare effective adsorbents from Moringa oleifera shells (PMOS) and chemically activated by zinc chloride (ZnCl₂) and calcined at different temperatures as 200, 300, 400 and 500 °C, for methyl red (MR) dye removal. Thereafter, the prepared materials were characterized using diverse analytical techniques as SEM, FTIR, XRD and BET. The results showed that the activated carbon prepared at 500 °C had a larger specific surface area (610.031 m²/g) compared to its original precursor (only 3.16 m²/g) or even the rest of prepared adsorbents. The maximum MR sorption capacity of the PMOS of 500 °C was the highest as much as 28.67 mg/g at ambient temperature.

The sorption capacity of raw M. oleifera shells and the tests of calcination effect of this material on the improvement of their capacity were studied in batch system by varying operating conditions such as: contact time, dye concentration, adsorbent dose, pH and temperature. The results confirmed that PMOS calcined at 500 °C with 1.0 g/L of dose and in acidic to neutral media at 25 °C, gives the most significant elimination rate (25.46 mg/g). Thermodynamic study shows that the retention of MR is an exothermic physisorption using PMOS calcined at 200 and 300 °C. Unlike materials calcined at 400 and 500 °C, the process was exothermic chemisorption.

In addition, adsorption isotherms and kinetics were studied using experimental data fitting to further understand and describe the dynamic equilibrium, dynamic kinetics, and mechanism of MR adsorption onto the calcined materials. As compared to Freundlich isotherm model, the Langmuir isotherm model provided a better fit with the experimental data for the different calcined PMOS (at 200 °C, 300 °C, 400 °C and 500 °C) exhibiting a maximum monolayer adsorption capacity of 25.45, 27.10, 28.13 and 28.91 mg/g, respectively. The linear pseudo-first-order kinetic model was found to be suitable for describing the adsorptive kinetics of all prepared activated carbons.