Equilibrium, kinetic and thermodynamic studies for the purification of wastewater from Congo red using simple and low cost adsorbents

This manuscript reports the purification of wastewater from Congo red (CR) by adsorbing it on sawdust of Pinus walichiana (PW) and Populus ciliata (PC) used as adsorbents. The adsorbents were characterized by Field emission scanning electron microscopy (FESEM), Energy dispersive X-rays (EDX) spectroscopy, X-rays diffraction (XRD) and BET surface area and pore size analysis. Different adsorption models like Langmuir, Temkin, Freundlich and Dubinin-Radushkevich (R-D) were applied to the system. The adsorption of CR on sawdust of both plants followed Freundlich adsorption isotherm to greater extent than Langmuir and Temkin isotherms which means that the surface of adsorbents was mostly heterogeneous and there occurred adsorbate-adsorbate interaction which influenced the adsorption process. The process also obeyed D-R model and mean free energy of adsorption was calculated to be 7.11 kJ.mol^-1 showing that the process of adsorption was physical. Adsorption of CR on both adsorbents followed film diffusion, particle diffusion, pseudo first order and Elovich models. It means that the process of adsorption was mixed diffusional in nature, occurred through physical process, the sites for adsorption increased exponentially with adsorption and multimolecular layer was formed on the surface of adsorbents. ∆H was 3.77 kJ.mol^-1 and 15.92 kJ.mol^-1, ∆S was 19.49 kJ.mol^-1 and 85.91 kJ.mol^-1 while ∆G was (-1.75 to -2.33) kJ.mol^-1 and (-8.39 to 10.97) kJ.mol^-1 for PW and PC, respectively. It means that the adsorption process was endodermic, resulted in increase in entropy and spontaneous for both the adsorbents. Isoelectric point (IEP) was determined by salt addition method, and it was found to be 6.9 for sawdust of PW and 5.1 for PC. The process of adsorption was found to be higher in the low pH range for both the adsorbents.