Adsorption Equilibrium, Kinetics and Thermodynamic Studies of Aqueous Phase Abatement of Basic dyes Using Clay Interpolated with Cationic Surfactant

Abstract

The adsorptive uptake of two synthetic dyes – Saffranin-O (Basic Red (BR2)) and Methylene Green (Basic Green (BG5)) from aqueous solution at  different adsorbent dose, pH, adsorbent-adsorbate contact time and temperature by hexadecyltrimethyl ammonium bromide (CTAB) intercalated  kaolinite (CTIK) clay has been studied and compared with native raw kaolinite (NRK) clay. NRK and CTIK were characterized using different analytical  techniques including pH at point of zero charge (pzc), bulk density, methylene blue adsorption (MBA) surface area, scanning electron microscopy  (SEM) coupled with Energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray Diffraction (XRD). The FT-IR and XRD  analyses results for CTIK compared to NRK revealed, respectively, that Amide I and II, hydroxyl, and carbonyl functional groups on CTIK surface are  involved in characteristic active sites for adsorption and the reflections at 2θ degree = 5.79° calculated using Braggs formula showed interlayer  space expansion (Δd) equivalent to 8.028 Å. The experimental data were fitted with the Freundlich, Langmuir, and Sip’s isotherm models for  adsorption equilibrium determination whereas pseudo-first order, pseudo-second order and Elovich equation were employed for adsorption kinetic  studies. To validate the accuracy of models' prediction, several error statistical goodness-of-fit criteria were applied. Using nonlinear regression  curve, the adsorption kinetics is best interpreted with the Elovich model (R² > 0.98) for the basic dyes adsorption onto both adsorbents followed by  the pseudo-second-order model (R² > 0.97). The best fit isotherm model followed the order; Sips (R² > 0.99) > Langmuir (R² > 0.96) > Freundlich (R²  > 0.89). Model applicability tests revealed an irreversible adsorptive reaction and chemosorption based on kinetic and isotherm models, respectively.  The thermodynamic parameters revealed that the adsorption process was spontaneous and endothermic in nature. CTIK was more  effective for basic dye uptake than NRK and had a higher affinity for BG5 than BR2 dye.