Experimental and Theoretical Studies of Methyl Orange Uptake by Mn-Rich Synthetic Mica: Insights into Manganese Role in Adsorption and Selectivity.

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 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 () of Mn-mica and the total removed MO layers () slightly changed with temperature. The decrease in the MO adsorption capacities ( = ·) from 190.44 to 140.33 mg·g in the temperature range of 25-55 °C was mainly controlled by the parameter. The results of adsorption energies revealed that MO uptake was an exothermic (i.e., negative Δ values) and a physisorption process (Δ < 40 kJ mol ). 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.