Modification of tannin extracted from the bark of Acacia auriculiformis for the antibacterial activity and application of metal adsorption

Tannins offer capability as a bio-derived chemical feedstock. Their current application is limited primarily to leather browning and wood board adhesives. However, proper derivatization may change both chemical and physical properties and thereby permit further utilization of polyphenols. This study focused on modification of tannin to obtain green polymeric materials. Tannin extracted from Acacia auriculiformis (Kaththi Karuvel) bark was modified by reacting with different aliphatic alkyl carboxylic acid (C4 and C18) to produce esterified tannin. Various weight percentage of alkyl chain lengths were introduced into the tannin by esterification. The resulted modified tannins were characterized by FT-IR and ATR spectroscopic techniques. Bivalent cations (Mg2+, Cu2+ and Cd2+) were used to evaluate the adsorption properties of both short and long chain fatty acid tannin esters. The esterified tannins were used to determine the inhibition effect against clinical isolates of Staphylococcus aureus and Escherichia coli aerobically using nutrient agar medium. Vulnerabilities were determined using standard agar well-diffusion method. Esterified tannins showed effectual ion exchange and antibacterial ability compared to that of virgin tannin. Partially esterified products showed higher exchange capacity compared to fully esterified tannin where C18-carboxylic acid-ester product has shown the highest value of ion adsorption compared to C4 carboxylic acid-ester product. The highest ion adsorption capacity was shown by the tannin stearates (1:1) for Cd2+ (0.020 mg/l), Mg2+ (13.852 mg/l) and Cu2+ (15.650 mg/l). In case of antibacterial activity, tannin stearates (1:6) possessed the highest inhibition against Staphylococcus aureus at 2.5 mg/ml concentration (LD50:1.2mg/ml), and S. aureus was more susceptible to this extract compared to E. coli.