Adsorption of tannic acid from aqueous solution onto chitosan/NaOH/fly ash composites: Equilibrium, kinetics, thermodynamics and modeling

In this work the tannic acid adsorption efficiencies for fly ash (FA), NaOH treated fly ash (NaFA) and Chitosan modified NaOH treated fly ash (Chitosan/NaOH/fly ash composite) were compared. Chitosan/NaOH/fly ash composite presented higher tannic acid adsorption efficiency than FA and NaFA. This may be attributed to the H-bonding imparted by Chitosan molecules on the surface of NaFA, consequently leading to organic partioning. The adsorption of tannic acid onto Chitosan/NaOH/fly ash composite as a function of initial tannic acid concentration, contact time, adsorbent dosage, temperature, ionic strength and solution pH were investigated for their optimization. The adsorbents were characterized by XRD, FE-SEM, surface area - porosity measurement and FT-IR spectroscopy. The adsorption kinetics of tannic acid followed a pseudo-second order model. The equilibrium adsorption data were best fitted by Redlich-Peterson isotherm models. The tannic acid adsorption slightly decreased with increasing ionic strength adjusted by NaCl. The adsorption was relatively high at pH 5.5-7.0 and decreased above pH 7.0. The value of Delta G degrees, Delta S degrees and Delta H degrees indicated spontaneous and exothermic process.