Effects of zeta-potential on enhanced removal of hexavalent chromium by polypyrrole-graft-chitosan biodegradable copolymer

Abstract

In the present research, chitosan grafted polypyrrole (PPy-g-CS) biodegradable copolymer particles were used as novel adsorbent to remove hexavalent chromium ions, Cr (VI), from water solutions and compared with the PPy. The effects of zeta (ζ)-potentials of these nanoparticles on the uptake properties of Cr(VI) ions were investigated as a function of pH. It was observed that the ζ-potential of PPy was shifted significantly from + 14 mV (pHinitial=4.8 for PPy) to + 47 mV (pHinitial=4.8 for PPy-g-CS) owing to the covalently grafting of polycationic CS chains with PPy chains. Thus, effective sites (−NH3+)for adsorption of Cr(VI) ions were created at the surfaces of the substrate. Adsorption efficiency of the PPy-g-CS copolymer observed to change by changing the pH of the medium which resulted in the variation of ζ-potential. The uptake capability of the PPy-g-CS copolymer increased compared to the pristine PPy. The maximum removal of Cr(VI) at the optimum pH of 4.8 and at 25 °C was 98 % and 73 % for PPy-g-CS and PPy, respectively. The adsorption kinetics was pseudo-second-order and inconsistent with the Freundlich isotherm. According to the thermodynamic parameters, the adsorption was endothermic (ΔH°PPy-g-CS = 7.12 kJ/mol), spontaneous (ΔG°PPy-g-CS = −546.43 kJ/mol) and driven by the increased entropy (ΔS°PPy-g-CS = 247.55 J/mol.K) of the system. In conclusion, the biodegradable PPy-g-CS graft copolymer particles with enhanced positive ζ-potential (+47 mV at pH=4.8) and adsorption capability (128 mg/g) would be a good candidate as an environmentally friendly adsorbent for the removal of negatively charged pollutants from water resources.