A review of the green chemistry approaches to leather tanning in imparting sustainable leather manufacturing

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Abstract

Green chemistry approaches studied for leather tanning are critically reviewed. The tanning performance and mechanical properties are discussed. The mechanisms of tanning for various tanning agents and future directions are outlined.

Abstract

Leather is made of collagen protein polymer, which is used in the manufacture of a variety of products including footwear, automotive upholstery, garments, and sports equipment. Animal skins/hides are converted into leather using a series of chemical processes. Of them, the tanning process is the most important chemical process that converts animal skins into leather by stabilising collagen fibre so that they do not putrefy. However, it is a hazardous process because of the use of various toxic chemicals in tanning, re-tanning and fatliquoring treatments producing toxic effluent. Over the years many tanning treatments based on chromium sulphate, and vegetable and synthetic tannins in combination with heavy metals, have been developed but tanning with chromium sulphate (known as Cr-tanning) is still the most effective, cheap, and widely used tanning process in the leather industry. Although the development of various improved Cr-tanning methods highly reduced the chemical and water consumption in leather tanning, it is still under scrutiny because of the production of effluent containing a harmful level of Cr and there is strong evidence that when the treated leather is disposed into the environment, part of the released trivalent chromium is converted into carcinogenic hexavalent chromium. Many sustainable alternatives to Cr-tanning based on chemical and enzymatic crosslinking, various bio-derived polymers, enzymes, modified zeolites, and nanostructured materials have been developed over the years with limited success. The alternative methods are either not as effective as Cr-tanning, affect the dyeability and other functional and organoleptic properties of leather, and or are cost-prohibitive. In this comprehensive review article, various tanning methods used in industry or studied in the laboratory are critically reviewed, and their advantages and disadvantages are outlined. The consumption of tanning agents, total chemicals including various auxiliaries and fatliquoring agents, and water in tanning, and the tanning performance and mechanical properties of the processed leather are compiled and compared. The reaction mechanisms of novel tanning agents with leather collagens and the future directions to make leather tanning more sustainable are outlined. This review article will be a guide for academicians/researchers/manufacturers involved in leather processing to develop more sustainable leather materials.

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Chromium in Drinking Water: Sources, Metabolism, and Cancer Risks

Anatoly Zhitkovich (2011)

Drinking water supplies in many geographic areas contain chromium in the +3 and +6 oxidation states. Public health concerns are centered on the presence of hexavalent Cr that is classified as a known human carcinogen via inhalation. Cr(VI) has high environmental mobility and can originate from anthropogenic and natural sources. Acidic environments with high organic content promote the reduction of Cr(VI) to nontoxic Cr(III). The opposite process of Cr(VI) formation from Cr(III) also occurs, particularly in the presence of common minerals containing Mn(IV) oxides. Limited epidemiological evidence for Cr(VI) ingestion is suggestive of elevated risks for stomach cancers. Exposure of animals to Cr(VI) in drinking water induced tumors in the alimentary tract, with linear and supralinear responses in the mouse small intestine. Chromate, the predominant form of Cr(VI) at neutral pH, is taken up by all cells through sulfate channels and is activated nonenzymatically by ubiquitously present ascorbate and small thiols. The most abundant form of DNA damage induced by Cr(VI) is Cr-DNA adducts, which cause mutations and chromosomal breaks. Emerging evidence points to two-way interactions between DNA damage and epigenetic changes that collectively determine the spectrum of genomic rearrangements and profiles of gene expression in tumors. Extensive formation of DNA adducts, clear positivity in genotoxicity assays with high predictive values for carcinogenicity, the shape of tumor–dose responses in mice, and a biological signature of mutagenic carcinogens (multispecies, multisite, and trans-sex tumorigenic potency) strongly support the importance of the DNA-reactive mutagenic mechanisms in carcinogenic effects of Cr(VI). Bioavailability results and kinetic considerations suggest that 10–20% of ingested low-dose Cr(VI) escapes human gastric inactivation. The directly mutagenic mode of action and the incompleteness of gastric detoxification argue against a threshold in low-dose extrapolation of cancer risk for ingested Cr(VI).

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P. Ramnani, Rani Gupta (2006)

Microbial keratinases have become biotechnologically important since they target the hydrolysis of highly rigid, strongly cross-linked structural polypeptide "keratin" recalcitrant to the commonly known proteolytic enzymes trypsin, pepsin and papain. These enzymes are largely produced in the presence of keratinous substrates in the form of hair, feather, wool, nail, horn etc. during their degradation. The complex mechanism of keratinolysis involves cooperative action of sulfitolytic and proteolytic systems. Keratinases are robust enzymes with a wide temperature and pH activity range and are largely serine or metallo proteases. Sequence homologies of keratinases indicate their relatedness to subtilisin family of serine proteases. They stand out among proteases since they attack the keratin residues and hence find application in developing cost-effective feather by-products for feed and fertilizers. Their application can also be extended to detergent and leather industries where they serve as specialty enzymes. Besides, they also find application in wool and silk cleaning; in the leather industry, better dehairing potential of these enzymes has led to the development of greener hair-saving dehairing technology and personal care products. Further, their prospective application in the challenging field of prion degradation would revolutionize the protease world in the near future.

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A timeline of hydrogen sulfide (H 2 S) research: From environmental toxin to biological mediator

Csaba Szabo (2018)

The history of H 2 S - as an environmental toxin - dates back to 1700, to the observations of the Italian physician Bernardino Ramazzini, whose book “De Morbis Artificum Diatriba” described the painful eye irritation and inflammation of “sewer gas” in sewer workers. The gas has subsequently been identified as hydrogen sulfide (H 2 S), and opened three centuries of research into the biological roles of H 2 S. The current article highlights the key discoveries in the field of H 2 S research, including (a) the toxicological studies, which characterized H 2 S as an environmental toxin, and identified some of its modes of action, including the inhibition of mitochondrial respiration; (b) work in the field of bacteriology, which, starting in the early 1900’s, identified H 2 S as a bacterial product - with subsequently defined roles in the regulation of periodontal disease (oral bacterial flora), intestinal epithelial cell function (enteral bacterial flora) as well as in the regulation of bacterial resistance to antibiotics; and (c), work in diverse fields of mammalian biology, which, starting in the 1940’s, identified H 2 S as an endogenous mammalian enzymatic product, the functions of which - among others, in the cardiovascular and nervous system - have become subjects of intensive investigation for the last decade. The current review not only enumerates the key discoveries related to H 2 S made over the last three centuries, but also compiles the most frequently cited papers in the field which have been published over the last decade and highlights some of the current ‘hot topics’ in the field of H 2 S biology.