Adsorption characteristics of polyvinyl alcohols in solution on expanded graphite

Abstract

Expanded graphite (EG) adsorbent was prepared with 50 mesh graphite as raw materials, potassium permanganate as oxidant, and vitriol as intercalation compound. Three kinds of polyvinyl alcohol (PVA) with different degree of polymerization (DP) in aqueous solution were used as adsorbates. We have studied the influence of initial PVA concentration, temperature and ionic strength on adsorption capacity. Langmuir constants and Gibbs free energy change ( Δ G°) were calculated according to experimental data respectively. Thermodynamic analysis indicates the equilibrium adsorbance of PVA on EG increase with the rise of SO42- concentration. Adsorption isotherms of PVA with different degree of polymerization are all types and we deduce PVA molecules lie flat on EG surface. Adsorption processes are all spontaneous. Kinetic studies show that the kinetic data can be described by pseudo second-order kinetic model. Second-order rate constants and the initial adsorption rate rise with the increasing of temperature and half-adsorption time decreases with the increasing of temperature. The adsorption activation energy of each PVA is less than 20 kJ·mol-1, physical adsorption is the major mode of the overall adsorption process.