Adsorption characteristics of chitosan grafted copolymer on kaolin

Efficient destabilization of colloidal dispersions is the top challenge facing solid-liquid separation processes. In this study, an in-house synthesized environmentally friendly graft copolymer, chitosan-graft-polyacrylamide (chi-g-PAM), was investigated as a potential flocculant of fine kaolin dispersions. Chi-g-PAM was successfully prepared by combining the properties of synthetic monomer (acrylamide) and natural polymer (chitosan) using ceric ammonium nitrate as an initiator. The physical and chemical characteristics of the copolymer were analyzed using Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and Zetasizer Nano ZS in order to identify the active adsorption sites in the polymer structure.The performance of chi-g-PAM as a flocculant was evaluated by treating 5 wt% kaolin despersion (d50 and mean diameter of 6.63 and 9.24 μm, respectively) with different dosages of the polymer and calculating the initial settling rates (ISR). The flocculation mechanism and the adsorption capacity were investigated using zeta potential and total organic carbon (TOC) measurements. Results showed that ISR increased with increasing chi-g-PAM dosages before reaching maximum values at corresponding optimal ones; then, the settling rate slightly decreased. Chi-g-PAM showed a better flocculation and settling behavior (ISR 24.84 m/h) as compared to chitosan (ISR of 7.2 m/h) at optimun dosages and performed similar to commercial PAM (ISR of 25.92 m/h). Reliable correlation of zeta potential measurement and adsorption isotherms obtained from TOC mesaurements demonstrated that bridging and charge neutralization were the dominant adsorption mechanisms involved. The experimental adsorption data were analyzed using Langmuir and Freundlich models. The best fit was obtained using the Langmuir isotherm model with a correlation coefficient value of 0.991 as compared with 0.895 for the Freundlich model. The TOC method has proven to be suitable and feasible for explaining the adsorption mechanism and determination of the adsorbed amount of chi-g-PAM on kaolin.