Abstract

Manganese – containing mica (Mn – mica) was synthesized at 200 °C/96 h using Mn-carbonate, Al – nitrate, silicic acid, and high KOH concentration under hydrothermal conditions. Mn – mica was characterized and tested as a new adsorbent for the removal of methyl orange (MO) dye from aqueous solutions. Compared to naturally occurring mica, the Mn – mica with manganese in the octahedral sheet resulted in enhanced MO uptake by four times at pH 3.0 and 25 °C. The pseudo – second order equation for kinetics and Freundlich equation for adsorption isotherm fitted well to the experimental data at all adsorption temperatures (i.e., 25, 40 and 55 °C). The decrease of Langmuir uptake capacity from 107.3 to 92.76 mg·g⁻¹ within the temperature range of 25 – 55 °C suggested that MO adsorption is an exothermic process. The role of manganese in MO selectivity and the adsorption mechanism was analyzed via the physicochemical parameters of a multilayer adsorption model. The aggregated number of MO ions per Mn – mica active site (n) was superior to unity at all temperatures signifying a vertical geometry and a mechanism of multi – interactions. The active sites number (D_M) of Mn – mica and the total removed MO layers (N_t) slightly changed with temperature. The decrease in the MO adsorption capacities (Q_sat = n·D_M·N_t) from 190.44 to 140.33 mg·g⁻¹ in the temperature range of 25 – 55 °C was mainly controlled by the n parameter. The results of adsorption energies revealed that MO uptake was an exothermic (i.e., negative ΔE values) and a physisorption process (ΔE < 40 kJ mol ⁻¹). Accordingly, the adsorption of MO onto Mn – mica was governed by the number of active sites and the adsorption energy. This study offers insights into the manganese control of the interactions between MO ions and Mn – mica active sites.