Use of bentonite calcined clay as an adsorbent: equilibrium and thermodynamic study of Rhodamine B adsorption in aqueous solution.

The Rhodamine B adsorption was realized in batch using calcined bentonite clay. The effects of Rhodamine B initial concentration, pH, and temperature were evaluated and the conditions where the adsorption was favored were in 500 mg L-1, pH 3, and 35 °C. The equilibrium isotherms studied were from Langmuir and Freundlich. The coefficients of determination (R2 > 0.99) were found to confirm the best fitted to Langmuir isotherm, with a monolayer adsorption capacity (qmax) of 552.49 mg g-1. The kinetic data agreed well with the pseudo-second order model (R2 > 0.99). The in natura and calcined clay were characterized by the techniques of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 physisorption (BET), and scanning electron microscopy (SEM). Thermodynamic parameters including Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated to estimate the nature of Rhodamine B adsorption in clay. The results suggested that the adsorption was endothermic and spontaneous, with the enthalpy adsorption increasing with the increase of temperature. Therefore, calcined bentonite can be used as an efficient adsorbent for discoloration of large volume of residual water, presenting low-cost and high adsorptive capacity.