Metal induced degradation of leather

Large quantities of leather waste are generated from manufacturing processes and disposal of leather goods. Environmental directives encourage diversion of solid waste from landfills through viable recycling methods. Since leather is resistant to microbial degradation, direct recycling of leather waste through biodegradation methods are impractical. In this research, the mechanism and potential application of metal induced oxidative degradation of leather were studied. Experimental work on the effects of retanning vegetable tanned samples with salts of the first row transition metals indicated the presence of unstable tannin-metal interactions in samples containing V(IV), Fe(II) and Cu(II) salts. Denaturation of collagen at ambient conditions was observed in leather samples treated with V(IV) and Fe(II) salts. Autodegradation of leather samples containing Fe(II) and V(IV) ions occurred during at ambient conditions of storage. The process of degradation in the leathers was characterised in terms of the decline in hydrothermal stability, progressive lowering of tensile strength, loss of metal-tannin binding interactions, changes in the fibre structure and the occurrence of gelatinisation. Direct evidence on the reduction of V(V) by tannins was obtained using electron paramagnetic resonance spectroscopy. Results of spectrophotometric studies of polyphenol-metal ion interaction in acidic media indicated that Fe(II) and V(IV) ions act as catalysts in the oxidation of tannins. To explain the phenomena of metal induced degradation, a mechanism involving redox cycling of the iron ions (Fe2+/Fe3+) and vanadium ions (V02+/V02+) was proposed. In the semi-metal tanned leathers, Fe(II) and V(IV) species are oxidised by air, producing superoxide anion. Subsequently Fe(III) and V(V) are reduced back to the lower oxidation states by the o- diphenol moieties of tannins. In this way, the metal ions act as catalysts for the oxidation of tannins. Autoxidation of Fe(II) and V(IV) species in aqueous media results in the formation of superoxide anion (02“) and hydrogen peroxide (F1202). The subsequent formation of hydroxyl radicals (OFT) in the leathers, through metal catalysed decomposition of hydrogen peroxide (Fenton reaction), is considered to be the major cause for the observed oxidative denaturation of the fibrous collagen. An investigation on the oxidative effect of a dilute solution of Fenton reagent (0.2M H202, 0.03M Fe2+) showed that hydroxyl radical mediated oxidation causes a rapid decomposition of the tanning structures and denaturation of the collagen in less than a day. Autodegraded semi-metal tanned leathers and Fenton-treated leathers showed increased degree of susceptibility to digestion by bacterial enzymes. Based on the results of this research, it is recognised that Fenton reaction may be used as an efficient method of oxidative pre-treatment of leather waste for enhancing its biodegradability particularly in the context of recycling through composting and anaerobic digestion.