Insight into the adsorption properties of methoxy bentonite (MXBE) as an environmental adsorbent for malachite green dye: statistical physics modelling

Natural bentonite was functionalised by methanol to obtain a novel methoxy bentonite structure (MXBE) of enhanced adsorption capacity for malachite green dye (MG.D). The adsorption properties of MXBE adsorbent were illustrated based on the classic equilibrium studies and the advanced equilibrium modelling according to the statistical physics theory. The uptake of MG.D by MXBE follows the kinetic properties of the Pseudo-First order model (R-2 > 0.9) and exhibits the equilibrium behaviour of a classic Langmuir isotherm (R-2 > 0.9). This demonstrates the physical adsorption of MG.D in homogeneous and monolayer form considering the Gaussian energy value (4.24-6.53 kJ/mol). The description of the adsorption system according to the statistical physics theory and the assumption of the Monolayer model with one energy site was accomplished considering the steric and energetic parameters. The saturation adsorption capacity of MXBE during the uptake of MG.D is 435.12 mg/g (313.13 K) and this value depends on the saturation of the structure with numerous active adsorption sites (Nm = 121 to 240.6 mg/g) and the efficiency of each active site to adsorb up to three MG.D molecules (n = 1.92 to 2.68). Considering the adsorption energy (Delta E = 16.02 to 17.94 kJ/mol), the uptake of MG.D occurred by physical mechanisms involving mainly hydrogen bonding and dipole bonding forces. The recognised thermodynamic functions (entropy, internal energy, free enthalpy) validate the spontaneous and endothermic behaviours of the MG.D adsorption reactions by MXBE.