Physicochemical and thermodynamic study of malachite green adsorption on raw and modified corn straw

Abstract

AbstractPhysicochemical and thermodynamic interpretations of malachite green (MG) adsorption on raw (RCS) and modified corn straw (MCS) are detailed in this work. Experimental isotherms were constructed at different temperatures (from 298 to 328 K). To interpret these curves, three models named multilayer model with saturation, double layer model with two energies, and monolayer model with one energy were developed based on the principle of grand canonical ensemble in statistical physics. The double layer model with two energies was the more adequate to represent the adsorption process. Thermodynamic functions such as configurational entropy (Sa), free adsorption enthalpy (G), and internal energy (Eint) were studied. The number of MG molecules per site (n) ranged from 0.33 to 2.33 and the adsorbed quantity at saturation (Qasat) ranged from 164.15 to 528.19 mg g−1 and both increased with the temperature. The density of the receptor sites (NM) ranged from 248.71 to 104.88 and decreased with the temperature. For RCS and MCS, the first and second energies, ϵ1 and ϵ2, increased with the temperature, with ϵ1 being higher than ϵ2. The adsorption energies of MCS (7.77–11.19 kJ mol−1) were higher than the adsorption energies of RCS (6.17–10.88 kJ mol−1). The adsorption process occurred by physisorption and the disorder increased at low equilibrium concentrations, but decreased at high concentrations. The free enthalpy and internal energy described with care the spontaneity of the studied system.