Genuine leather processing is an art been practiced since ancient times [4, 10], aiming to remove unwanted components such as elastin, keratin, hemoglobin, proteoglycans, lipids, convert raw leather and hide defects, including the basic stages: preprocessing, tanning, and scaling or surface coating as the summary in Fig. 1 [4, 7, 10-15].
- The preprocessing stage consisted of many steps such as soaking, peeling, softening, degreasing, bleaching, and waxing. Essentially, the raw leathers are cut, soaked to remove salt or other solids, and restore moisture lost during the previous storage. They are then further peeled to remove excess tissue, muscle, or fat until achieving a desired uniform thickness. Degreasing, bleaching, and waxing are usually performed mainly by applying the lime solution (calctum hydroxide) and abrasives such as ammonium salt solution with protease at 27-32 °C or applying thermal, oxidizing methods. The last step is the pickling step using a salt solution and sulfuric acid [7, 14].
- Tanning is the process of converting raw leathers into a stable, pliable, residue-free material suitable for a wide range of finish applications.
Different tanning processes produce different leather types such as vegetable-tanned leather, chrome leather, aldehyde leather, and alum- dyed leather [7, 14, 16]. Typically, raw leathers are immersed in the corresponding treatment solutions to re-bleaching and remove tannins clinging to their surface. The oils are then applied to the skin several times before they are rubbed and dried.
- The scaling stage involves several steps including impregnation, re- wetting, delamination, re-coloring, re-tanning, oiling, polishing,
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whitening, plating, embossing, ironing, brushing, glazing, and finally is
product shaping [7, 14].
\Raw hides7
tt tạm tạm tài
Leather
———— —ơ
Gaseous emissions
Figure 1 The genuine leather processing and associated material streams [15].
The genuine leather segment is still leading the leather market [6, 7, 10].
However, the genuine leather processing caused a multitude of wastes from animal meat, feathers, debris, exhaust gases, and volatile solvents, to numerous wastewater containing harmful chemicals such as chromium, tannins compounds, oils, resins, biocides, detergents [4, 5, 7, 11, 13, 14, 17]. Before
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the tanning stage, lime, salt solution, and sulfuric acid accounted for 84% of biochemical oxygen demand (BOD), 75% of chemical oxygen demand (COD), and 92% of suspended solids (SS) from tanneries [6, 7, 11]. Currently, almost 85% of raw leather is treated with chrome tanning due to its inexpensive method. However, poor chromium absorption (only 50-70%) in this method leads to significant changes in COD which caused severe heavy metal contamination of more than 55,000 hectares of land [5-7, 14, 18]. For solid waste, sludge from tanneries inactivates waste treatment systems because they are difficult to biodegrade. In addition, the treatment of various chemicals during the tanning process strengthens the leather's resistance to microbial degradation. These wastes are a serious threat to ecosystems and aquatic systems [7, 11, 17]. Approximately 4 million tons of solid waste is generated by the leather industry annually. One ton of raw leather is treated, an estimated 60 m? of clean water is used and nearly 650 kg of solid waste is generated [13, 19]. For the exhaust gas, volatile organic compounds such as ammonia, hydrogen sulfide, volatile hydrocarbons, amines, and aldehydes generated during delamination, liming, or drying are entirely possible hazardous to the atmosphere if not properly controlled [7, 10, 18]. Moreover, Table 1 describes the uses, lethal dose (LD50), and toxicity of chemicals still used in the genuine leather industry. Among all, benzyl butyl phthalate (BBP), diethyl hexyl phthalate (DEHP), and dibutyl phthalate (DBP) are toxic plasticizers in microporous artificial leather coatings. Formaldehyde, azo dyes, and dibutyltin (DBT) are carcinogenic chemicals in the leather finishing stage. N-methyl pyrrolidone is a reproductive toxicant, and has been used as a binder, plasticizer, flattener, wetting agent, or expander. Furthermore, inorganic pigments such as toxic heavy metals, lead chromate, and cadmium sulfate are usually used due to their durability and vibrant color [7, 14].
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Table 1. Uses, LD 50, and toxicity of some chemicals used in the leather industry [7].
Chemicals Uses LD50 Target organs
(mg/kg)
Benzyl butyl phthalate The production of a microporous artificial 2330 Eyes, lungs, liver, reproductive
(BBP) coating/water vapor-permeable materials. system
Bis(2-ethylhexyl) Used as plasticizers in a processing of shoes 30,000 Liver and testes phthalate (DEHP) soles, and artificial leather manufacturing
Di butyl phthalate Used as a Phthalate plasticizer in the artificial 7499 Eyes, lungs, gastrointestinal tract,
(DBP) leather industry testes
Anthracene Used as a tanning agent 16,000 Kidney, liver, fat and carcinogen Short chain Additive for the leather renders smoothness, or 3090 Liver, kidney, thyroid and carcinogen chlorinated paraffin's a leather oiling agent
Cobalt dichloride Used in leather dyeing and finishing as well 80 Lungs, liver, kidney, heart, skin found in tanned leather
Nonyl phenol Leather finishing 1475 Blood, lungs, eyes, skin
Methyl Biocide, microbiological protection 1800 Skin, eyes and carcinogen isothiazolinone
N-Methyl pyrrolidone Coalescence, plasticizers, wetting agent 3914 Eyes, kidney, lymphatic system, liver, Formaldehyde
Heavy metalsArsenic Chromium Organotin compounds
(Dibutyl tin) Azo dyes (Orange IT)
Leather finishing Leather finishing Used as a tanning agent
As a catalyst Used for Dyeing
lung, testes
Eyes, lungs and carcinogen 763 Liver, kidneys, skin, lungs, lymphatic
system and carcinogen 3250 Kidney, haematopoietic system
175 Gastrointestinal tract, liver and carcinogen
3418 Blood, liver, testes and carcinogen
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Strict regulations have been issued and many authorities worldwide current required the leather industry to have reformed its manufacturing methods to the lowest risk to the environment [20]. In several countries, wastewater from tanneries has been subject to general industrial discharge laws or an obligatory standard [7, 13]. However, illegal dumping and many pollution problems caused by tanneries remain unresolved due to the high waste treatment cost. Numerous eco-friendly and sustainable technologies have been applied to solve the pollution of the leather industry [4, 10, 12-14, 17, 18, 21, 22]. In general, the feasibility of these solutions depends heavily on quantity and quality reduction of pollution sources, waste treatment cost, reproducibility of the process, and market opportunity [7, 22]. Instead, the reuse of agroforestry by-products or bio-waste is being one of the most possible approaches that both scientists and producers priority selected to perfect the alternative leather materials notably vegan leathers, including fruit leathers, waxed cotton leathers, button leather, wood leathers, ocean plastic leathers, mushroom-derived leathers, bacterial cellulose-derived leathers or Kombucha leathers [2, 23-26].
1.3 Kombucha leather
Kombucha leather is a bacterial cellulose-derived leather, a type of vegan leather that resemble leather but is not made from animal skin. Kombucha leathers are high social responsibility, novelty, environmentally friendly, non- deforestation, less energy consumption, less harmful emissions, require very little space, water, or chemicals to produce [2, 20, 24, 25, 27, 28]. Essentially, Kombucha leathers have low production costs and are possibly applied in footwear, handbags, automotive interiors, interior design and decoration [23, 25, 28, 29]. However, in the case of prolonged exposure to sunlight, heat, friction,
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and moisture, Kombucha leathers or generally vegan leather degrade faster than genuine leather, leading to vegan leathers are in a shorter lifespan and require a higher level of care and maintenance. Fig. 2 illustrated a design of a naturally dyed and waterproof biotechnological leather from reconstituted bacterial cellulose, without depending on season and weather and possesses both the value of preserving the ecosystem and protecting animals [30].
Bacterial cellulose reconstitution and Bacterial cellulose dyeing
production and purification
Obtention of dyed and waterproofed
BC leather
Figure 2 A design of a naturally dyed and waterproof biotechnological leather from reconstituted bacterial cellulose [30].
Recently, the price of genuine leather is continuously increasing plus the constant pressure of animal rights groups (PETA - People for the Ethical Treatment of Animals), meat-free eaters, strict regulations on animal rights, and eco-protecting led to enhancing in the awareness of both consumers, designers, producers about ethical consumption that encouraged they select, use, design, research the alternative leather products suitable the evolving requirements of
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cleaner production and sustainable development [25]. Vendors and producers are moving towards quality control processes or developing Kombucha leather and vegan leather with superior properties such as high elasticity, good water resistance, and more ecofriendly. However, the use of non-biodegradable composite materials as a plasticizer for vegan leather is creating debates related to ecological environment protection [25]. In addition, the R&D costs of these leather products are still high. Therefore, it is necessary to continue to improve the environmental friendliness, production optimization, or new useful properties adding aiming the vegan leather products completely become good for the planet, good for animals, and good for the soul of the user.