Experimental results and theoretical statistical modeling of malachite green adsorption onto MCM–41 silica/rice husk composite modified by beta radiation

Abstract

The effect of beta particles (β−) on the structure and adsorption properties of the synthetic MCM–41 silica with rice husk MCM–41/RH composite was investigated for the first time. Based on XRD, SEM, and zeta potential results, the energy of β–radiation was sufficient to produce a morphs composite (β–MCM–41/RH) with negative surface charge and irregular internal porous structure. The adsorption of malachite green (MG) by the β–MCM–41/RH composite was performed under different conditions of solution pH, shaking time, initial MG concentration, adsorbent dose, and temperature. Modification of the MCM–41/RH composite by radiation process enhanced its maximum uptake efficiency for MG from 29.68 to 97.11%. The pseudo-second-order model with a correlation coefficient (R2 = 0.999) fitted well the MG adsorption kinetic. Linear and non-linear forms of the traditional isotherm models showed that Freundlich equation described well the MG adsorption data. Different statistical physics models on the basis of the grand canonical ensemble were also applied for a deeper interpretation of the MG adsorption mechanism. Based on the fitting experimental results, monolayer with two energy sites was found to be the best model in describing the MG adsorption phenomena. Through this model, the number of adsorbed MG molecules per site, the density of receptor sites, the removed amount of MG at saturation, the concentration at half saturation, and the adsorption energy were determined. The measured adsorption energies (E1 = −18.345 and E2 = −15.43 kJ/mol) suggested that electrostatic attraction and hydrogen bond interactions were involved in adsorption process. The parameters of thermodynamic functions including the configurational entropy, free enthalpy, and internal energy indicated that the MG adsorption onto the β–MCM–41/RH composite was exothermic and spontaneous in nature.