Tannins ellagic acid based

In the adsorption of some metal ion by tannin adsorbents [16], tannins are widely distributed in nature and have multiple adjacent phenolic hydroxyl groups and exhibit specific chelation ability toward metal ions [17]. According to the chemical stractures of tannins, they can usually be classified into hydrolyzable tannins, condensed tannins and complex tannins. Hydrolyzed tannins yield galhc acid or ellagic acid when hydrolyzed by acid, base or some enzymes [18]. Turkish sumac tannin (hydrolyzable tannin) is illustrated in Fig. 28.1 whose basic stracture is of flavan-3-ols. 

Most hydrolyzable tannins contain a central core of glucose or other polyhydric alcohol esterified with gallic acid (gallotannins) or hexahydroxydiphenic acid (ellagitannins) (Fig. 2). The latter is isolated as its stable dilactone, ellagic acid. These Types of tannins are readily hydrolyzed by acids, bases or certain enzymes. 

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