Study On The Mechanism Of Vegetable Tannin-aldehyde Combination Tannage & Adsorptive Behaviors Of Collagen Immobilized Tannin For Cu(Ⅱ)

Using mimosa as vegetable tannin agent and oxazolidine as aldehydic tanning agent, the approaches of calorimetry, hydrogen bond and hydrophobia bond breaking and chemical modifications of collagen were employed to investigate the effects of hydrogen bond, hydrophobic bond and side amino group of collagen on vegetable tanning, aldehyde tanning and vegetable-aldehyde combination tanning. The results indicated that the aldehyde predominantly reacted with the side amino group of collagen even it was used after vegetable tanning. But the aldehyde could further react with vegetable tannin and form cross-linkage between collagen and vegetable tannin if there are highly nucleophilic sites in tannin molecules. As a result, the synergistic effect of the combination tanning in increasing hydrothermal stability of collagen fibers was achieved and the association of vegetable tannin and collagen was remarkably stabilized. This mechanism is appropriate for description of the combination tanning with condensed tannins that contain nucleophilic sites. But for hydrolysable tannins, no aldehydic cross-linkage between collagen and tannin could be observed. In general, this research revealed the fact that the actual features of vegetable-aldehyde combination tanning should be different from the ones previously assumed. With the mechanism suggested in this paper, all the phenomenain vegetable tannin -aldehyde-collagen interaction can be well understood.The high hydrothermal stability of tannin-aldehyde combination tannage can also be explained by the cooperating units theory as follows: covalent cross-link bonds formed by aldehyde between the collagen and vegetable tannin may produce large cooperating units in the melting processes of collagen. So more melting energy is required in the shrinking process of collagen and then high shrinking temperature is achieved.Using (+)-catechin and (+)-gallocatechin as models, the chemical properties of condensed tannin were elucidated based on the approach of computational chemistry. Both Hartree-Fork and MP2 methods were used in the calculations. The results showed that, in consideration of the atomic charges and the distribution of HOMO, A-ring of the flavonoid structure is the nucleophilic center. This implies that A-ring is more reactive to aldehydes. As to (+)-catechin, C-6 and C-8 positions are of nearly equal reactivity to aldehydes, according to their energies transformation of the reaction. C-8 of (+)-gallocatechin is more reactive than C-6 position in the same molecule. Difference of B-rings of (+)-catechin and (+)-gallocatechin influences on the distribution of HOMO in molecules and consequently leads to difference of relative reactivity of C-6 and C-8-positions in flavonoid structures.Using collagen fibre as matrices, the immobilized tannins adsorbents based on black wattle, larch and myrica rubra tannins were prepared through crosslinking reaction. As novel adsorbents, their ability of recovering Cu(II) from waste water was studied. Experiment showed that the adsorption isotherms of the immobilized tannins for Cu(II) can be described by Freundlich model and their adsorption capacities are inthe sequence of immobilized myrica rubra tannin > immobilized black wattle tannins> immobilized larch tannin. Furthermore, the adsorption characteristics of the immobilized black wattle tannin that has higher thermal stability were investigated in detail. It was found that the influence of temperature on adsorption isotherms is inconsiderable, but the effect of pH is significant. Higher pH of solution leads to higher equilibrium adsorption capacity. The data of adsorption ratio could be well fitted by pseudo-second-order rate model, and the adsorption capacity that was calculated by the model was consistent with actual measurement, particularly when the adsorption temperature higher than 308K was employed. Studies also indicated the immobilized black wattle tannin had excellent properties of column adsorption kinetics, as well as highly effective availability of bed. Me...