Selective adsorption of tannin from flavonoids by organically modified attapulgite clay

In this study, selective adsorption of tannin on organically modified attapulgite clay was investigated. Tannin was selectively removed from model tannin/flavonoids mixtures in the solution. In adsorption isotherm experiments of tannin, obtained data fitted well to the Freundlich model. The surface sites of the octodecyl trimethyl ammonium chloride (OTMAC)-attapulgite were heterogeneous. The effect of pH on tannin adsorption was also investigated and the results showed that the adsorbed amount of tannin was highest at pH 6.0. The enthalpy values of adsorption of tannin on OTMAC-attapulgite at pH 4.0, 6.0 and 8.0 were −49.80, −74.31 and −38.62 kJ/mol, respectively. At 2-5.3 and 7.4-10, the driving forces for the adsorption may be the hydrophobic force and hydrogen bonding. And at pH 5.3-7.4 the electrostatic force may be the most important one. These facts implied that the selective adsorption of tannin on OTMAC-attapulgite was driven by the collaboration of hydrogen bonding, electrostatic force and hydrophobic interactions of tannin molecular with adsorbent. The low adsorption of isoflavones of soybean and the puerarin was ascribed to less phenolic hydroxyls of them than those of tannin, which made the low electrostatic interactions and hydrogen bonding interactions between the tannin and OTMAC-attapulgite adsorbent.