Eco-Friendly Valorization and Utilization of Plant Waste as a Source of Tannin for Leather Tanning

In the present study, bark, which is the waste part of a tree, was utilized as a source of tannin for leather tanning after its value addition. The barks of Acacia nilotica L. and Eucalyptus globulus, which are abundantly available in Pakistan, were selected in this study. Different extraction techniques including mechanical, soxhlet, reflux and ultrasonic extraction were used for the extraction of tannin from the selected barks. The medium of extraction included aqueous as well as different solvent mixtures in different ratios. The solvent mixtures used in this study were methanol-water and acetone-water. The methanol-water ratio was kept (50:50) and the acetone-water ratio was maintained at (50:50 and 70:30). In the first step, the process of extraction was optimized for solvent mixtures and extraction time by taking a fixed dose (5 g) of each bark and a fixed volume of each extracting solvent mixture (150 mL). The total phenolic contents (TPC) of the bark extracts were determined by colorimetric assay (as mg GAE/g bark), taking gallic acid as the reference standard. The tannin contents (TC) of the bark extracts were determined by using the standard hide powder method (SLC 117). It was concluded that among the different extraction techniques, and among the solvent mixtures acetone-water in ratio (70:30), ultrasonic extraction resulted in maximum extraction of tannin (196.1 and 125.2 mg/g), respectively for A. nilotica and E. globulus. FTIR analyses of the dried extracts obtained from both barks were compared with FTIR of the most commonly used commercial vegetable tanning agent in the leather industry, i.e., mimosa. Results were comparable, which confirmed the presence of condensed tannin in both barks. The extracted tannin was applied on a pickle and wet blue leather to evaluate its tanning and re-tanning capability. Mimosa, the most abundantly used vegetable tanning material in the leather industry, was applied as a reference standard on the same pickle and wet blue leather for comparison. Different physical properties, such as leather shrinkage temperature, leather softness, tensile strength, percent elongation and breaking strength, were measured to evaluate the tanning efficacy of the extracted tannins. The results showed that tannin obtained from A. nilotica showed comparable physical properties to those of mimosa. It was concluded from the results that tannin obtained from locally available bark, i.e., A. nilotica, can be utilized as a vegetable tanning agent for the leather industry.