Chrome Tanning: An Indispensable Process for the Leather Industry

Chromium Tanning: Indispensable to the Leather Industry Raw hides, obtained largely from cattle and small ruminants, are a by-product or waste material of the meat and meat products industry. If not preserved under suitable conditions, they decompose and spoil easily. The leather processing industry, operating in facilities commonly known as tanneries, subjects this natural material to a succession of complex chemical and mechanical processes, transforming it into finished leather with properties suited to diverse applications—non-decomposing, non-spoiling, odor-free, shape-retaining, soft, flexible, strong and stable. In essence, this entire process involves converting the structure of collagen, the protein that forms the basic tissue structure of raw hide, into a more robust, durable and stable state. Collagen is the most abundant protein in animals. This fibrous, structural protein consists of molecules formed by three amino acid chains wound around each other. This helical structure forms microfibrils; microfibrils form fibrils; and fibrils form collagen fibers. Collagen exists in different types depending on the arrangement of chains formed by the combination of amino acids in its structure. Type I collagen consists of two α-1 and one α-2 chains. This is the strongest collagen, comprising 90 percent of the collagen in animals as well as in the human body. The animal's skin, which protects it from external agents and microorganisms while alive, can suffer structural breakdown after the animal's death either through microbial action or through autolysis by cell enzymes inherent in the skin's own structure. At the most advanced stage, raw hide can be broken down to the individual amino acids that constitute collagen protein. Raw hide, with a shrinkage temperature around 65°C, can easily shrink when the hydrogen bonds maintaining the skin as a whole are broken by the action of hot water. At further stages, the skin structure becomes gelatinous or even converts to glue. When water, which comprises 60-65 percent of raw hide, is removed, the skin becomes dry and acquires a hard, brittle structure; when placed in water, it absorbs moisture and swells. In this form, the service life and application range of raw hide is quite limited and can be considered unusable. Animal hides left in the environment can cause various diseases and foul odors. The leather sector both prevents these problems and performs an environmentally responsible action by converting raw hides into a high-value-added usable material. Furthermore, as a labor-intensive industry, it contributes to employment. Ultimately, as long as humanity continues its meat-eating habits to meet its animal protein requirements, animal husbandry will continue and leather will be obtained as a by-product. The transformation of raw hide into finished leather is a long-term process resulting from technological knowledge and dedication to the craft. Leather processing begins with pre-tanning operations aimed at removing soluble proteins, fats, hair or wool from the natural composition of raw hide, isolating the collagen fiber and bundle network, modifying and developing active groups on the collagen protein suitable for bonding. These process stages—known as soaking, hair removal, liming, deliming, bating and degreasing—have the basic purpose of cleaning raw hides and preparing them for the tanning process. The tanning stage is designed according to the properties of the tanning agent to be used. This is because the tanning agent used imparts its own characteristic to the leather. The choice of tanning agent is important in determining which human needs the produced leather will meet. Of course, the influence of fashion cannot be overlooked at this stage. Tanning agents are classified according to their sources. Mineral tanning agents such as chromium, aluminum, zirconium, titanium and iron; vegetable tanning agents obtained from different parts of plants such as quebracho, mimosa, chestnut, pine, oak gall, tara and gambir; and additionally aldehyde, polymer, oil and synthetic (syntans) based tanning agents are widely used in the leather industry either individually or in combination with each other. Post-tanning operations impart to the leather properties suited to the final product in which it will be used—such as tone, color, softness, flexibility, water resistance, strength, durability, surface feel and homogeneity—through neutralization followed by retanning, oiling, dyeing and finishing operations. During these processes, the leather is also made into a flat layer through mechanical operations. The leather processing process is schematized in the figure below. According to 2020 data, approximately 23 billion square feet of leather is produced worldwide today, and if the global market value of leather products reaches approximately USD 400 billion, the tanning industry's recognition in the second half of the 1800s of chromium's tanning power and the advantages it provided played a major role. This was an important and influential discovery for the leather industry. Until that time, tanning had primarily used tannins from plants obtained from the world's subtropical and tropical regions. Vegetable tanning involved a production process lasting months. But the advent of chromium greatly shortened the transformation of raw hides into finished leather. Moreover, leather tanned with this tanning agent was softer, more flexible, stronger, more easily dyed and possessed higher hydrothermal stability. This so increased interest in chromium that today chromium is the primary tanning agent in an average 90 percent of leather produced worldwide. Chromium (atomic number 24; atomic weight 51.996), which means color in Greek, is an important element for Turkey, which possesses the world's richest chromium ore deposits after the Republic of South Africa. Chromium is used to impart hardness to metals and in the manufacture of armored vehicles because of its extreme hardness and melting point of 1857°C. Its most important application is in the production of stainless steel when used together with nickel. Thus, the chromium oxide layer it forms covers the steel's surface like a film layer and provides resistance to corrosion. Chromium occurs in nature with a +3 charge and is therefore usable in powder form for tanning hides. Chromium (Cr) is one of the redox-active heavy metals. In aqueous environments it exists in trivalent and hexavalent forms. Cr(VI), depending on the pH of the aqueous solution, exists in various anionic forms such as chromate (CrO4 2−), hydrochromate (HCrO 4−) or dichromate (Cr2O7 2−). The permitted trivalent and hexavalent chromium concentrations in wastewater are 5 and 0.1 mg respectively. At micro levels, Cr(III) is a trace element essential in the human body for physiological functions. Furthermore, by enhancing insulin's effect, it plays a role in glucose, fat and protein metabolism. Nutritionists have established that a chromium (III) content in the range of 8.4 - 23.7 mg/1,000 kcal with an average of 13.4 mg/1,000 kcal is optimal for daily diets, but higher values are harmful to human health. Excess Cr(III) can even have mutagenic effects. Although Cr(III) complexes do not easily cross cell membranes, their accumulation around cells in the body causes morphological changes to the cell surface. Second, this causes cellular lipid injuries through disruption of cellular functions and integrity, ultimately leading to DNA damage. Chrome-tanned leather is known for its versatility, excellent hydrothermal stability, superior dyeability and softness. However, chromium tanning is considered the most polluting process globally because it releases trivalent chromium ions [Cr(III)] into water bodies. Nevertheless, compared with non-chromium tanning methods, chromium tanning is more suitable for the production of various leathers. Chrome-tanned leathers interact in better compatibility with retanning and oiling chemicals. However, Cr(III) in chromium tanning wastewater can easily be converted to Cr(VI) due to the presence of oxidizing agents such as dissolved oxygen and MnO2. Additionally, pH fluctuations at a wastewater treatment facility can accelerate the oxidation of Cr(III) to Cr(VI). Furthermore, stored leather and other leather products have been found to deteriorate over time due to the effects of moisture, temperature and UV radiation. During this process, lubricants, tanning agents and auxiliary substances replace molecules in the leather structure, and subsequently free radicals form. As a result, these radicals become a potential driving force for Cr(III) oxidation to Cr(VI). Leather products have been shown to contain Cr(VI) content over time, which makes it difficult to guarantee that Cr(III) used in the leather tanning process is harmless to consumers of leather products. Hexavalent chromium [Cr(VI)], due to its high toxicity, threatens terrestrial and marine life. Additionally, Cr(VI) is carcinogenic, mutagenic and allergenic to humans and causes organism death. During tanning, only 55-70 percent of all chromium salts entering the tanning liquor bind to the leather, with the remainder passing into wastewater. The main portion of chromium wastewater comes from the tanning bath, with a significant amount also being discharged from washing and retanning operations. While chromium concentrations in spent liquor range from 2,000 mg/l to 5,000 mg/l, environmental regulatory bodies worldwide permit no more than 20 mg/l of total chromium in wastewater discharged to public sewers. Practically, it is possible to reduce chromium concentrations in tannery wastewater streams to permitted levels, but this is capital-intensive, imposing operational cost burdens on tanneries. This difficulty requires suitable technologies that will minimize or eliminate chromium in wastewater for the sustainability of the leather industry. Chromium tanning technology continues to be an environmental concern on a global scale. To achieve sustainability and maintain a clean leather industry, chromium pollution must be reduced. Intensive work is underway on chromium-free tanning methods, that is, alternative tanning methods to chromium, but no method has yet been found that can provide this comprehensively. Other reduction methods include reducing chromium usage, reusing tanning baths and recovering chromium from spent baths. Beyond environmental pollution, chromium has significant negative effects on human health. Tannery workers can be exposed to chromium through inhalation or contact. Particularly hexavalent chromium can cause carcinogenic and even fatal effects. Therefore, serious occupational health and safety measures must be taken and workers must be encouraged to comply with these measures.     References 1) Zhang C, Lin J, Jia X, Peng B., (2016). A salt-free and chromium discharge minimizing tanning technology: The novel cleaner integrated chrome tanning process. Journal of Cleaner Production.; 112(1):1055-1063. Doi: doi.org/10.1016/j.jclepro.2015.07.155 2) Uddin,, M. M., Hasan, M. J., Mahmud,, Y., Tuj-Zohra, F., & Ahmed, S. (2020). Evaluating Suitability of Glutaraldehyde Tanning in Conformity with Physical Properties of Conventional Chrome- Tanned Leather. 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Total Environ. 17, 13e29. https://doi.org/10.1016/0048-9697(81)90104-2. 7) Oruka, R.O., Selvarajan, R., Ogola, H.J.O., Edokpayi, J.N., 2020, Contemporary and future direction of chromium tanning and management in sub Saharan Africa tanneries. Process Safety and Environmental Protection.369-386 Doi: https://doi.org/10.1016/j.psep.2019.11.013     Assistant Professor Fazlı Akyüz Department of Textiles, Clothing, Footwear and Leather, Leather Technology Program Istanbul University Technical Sciences Vocational School