Adsorptive Characteristics of Tannin Removal from Aqueous Solutions and Coir Industry Effluents Using Calcined and Uncalcined Hydrotalcites

The adsorptive properties of hydrotalcite, Mg 6 Al 2 (OH) 16 CO 3 · x H 2 O (HT) and the calcined product, Mg 3 AlO 4 (OH) (HTC) for tannin were investigated. The adsorbents were characterized by means of chemical analyses, X-ray diffraction (XRD), potentiometric titration, and surface area analyzer (N 2 adsorption−desorption isotherms). All adsorption experiments were carried out in batch techniques with laboratory-prepared samples and wastewater obtained from a coir industry. The maximum adsorption capacity was observed at the pH range 3.0−5.0 and at an S/L ratio of 2 g/L, for both the adsorbents. Sorption capacity was found to be very high for HTC. The kinetic data were analyzed using a first-order kinetic model. An increase of ionic strength of the medium caused an increase in the tannin adsorption, indicating the occurrence of inner-sphere complex mechanism as well as hydrogen bonding between phenolic-OH groups of tannin molecule and the hydrogen-bonding sites on the adsorbents. The isotherm data were correlated with the Langmuir isotherm model and the tannin adsorption capacity was found to be 312.31 and 397.65 μmol/g for HT and HTC, respectively, at 30 °C. The coir industry effluent samples were treated by HT and HTC to demonstrate their efficiencies in removing tannin from wastewaters. Thermal regeneration studies were carried out by calcinating the exhausted adsorbents.