Green Chemistry and Green Chemistry Applications in Tanning

Since the earliest humans, only one living species has done harm to the world in the name of development. Yet we humans, as this damaging intelligent species, despite our tremendous ego, all our advancement, our superior (!) technologies, and our bottomless accumulation of "knowledge that we know," still have only one planet where we can live. Our one and only world. From the moment we transitioned from hunter-gatherer societies to agricultural societies, as we found comfort we began reproducing more, rapidly increasing the homo sapiens population. Population growth meant the need for more food. To obtain this sustenance, we spread across the world, burned forests in new lands, cleared fields, hunted animals, and domesticated those we could; we benefited from their meat, milk, and other resources they provided. We wondered how we could produce more and began seeking answers to this curiosity, along with questions about other matters. We thought, and as we developed our systems of thought, we created sciences, as we did with many things we created. This gave us the ability to displace other inhabitants of this planet we call Earth from their places and homes, including members of our own species. For these purposes, we consumed many species.

While destroying, we learned new things. As we learned, we developed, and as we developed, we began to pollute more. We turned our world into a dump, and as if that were not enough, we even filled the orbit of our world with technological waste we created.

Today, human use of natural resources, development policies, and the resulting unlimited and unconscious consumption have caused environmental problems and led to major threats. Rapidly advancing technology began to cause ozone layer depletion, global warming, stack gases, acidification, eutrophication, carcinogens threatening human health, eco-toxicity, depletion of fossil fuels, excessive soil and water use, and similar problems. The universal ecological, energy, and health problems encountered prompted scientists to recall the Native American proverb: "When the last river has dried up, when the last tree has disappeared, when the last fish has died; the white man will understand that money cannot be eaten." The industrialization that began with the Industrial Revolution, particularly accelerating after World War II, caused excessive environmental pollution. Especially with industrialization and modernization, population growth, urbanization, improved living standards, and the effects of competition and wars, efforts to create environmental awareness and consciousness began to increase from the second half of the 20th century onward, alongside environmental pollution and degradation.  

Figure 1: The first book that raised awareness about environmental problems

Environmental concerns were first voiced at the 1949 United Nations Conference on Conservation and Use of Resources. However, the book "Silent Spring," published in the 1960s, played an important role in raising awareness about environmental issues. This historic book increased awareness about ecological perception and raised concerns about the dangers of excessive use of natural resources, leading to important government initiatives. Chemistry, which at its simplest can be defined as an important science that examines matter and the changes it undergoes, has penetrated every detail of our daily lives and is a discipline that greatly affects our lives and comfort. Initially considered the source of inventions necessary for modern life, chemistry today is present in every moment of our daily lives—from what we eat and drink, to our clothing and all types of cleaning, medicines, cosmetics, and even lethal weapons—yet many people view it as the fundamental source of universal pollution that threatens our world. While chemistry is the source of many problems humanity faces—energy, transportation, heating, technology, and lighting—the solutions will also come through the contributions of chemistry science. Eliminating the factors that create these problems is a much more effective method than solving the problem itself. The method to be used for eliminating the source of the negative effects created by traditional chemistry methods is "Green Chemistry." Green Chemistry emerged in the United States in 1990 with the "Pollution Prevention Act." With the passage of this law, preventing the formation of waste that causes pollution was targeted for the first time. Looking at chemistry's successful history of over 150 years, Green Chemistry is a relatively young concept. This movement began in 1990 with a pollution prevention initiative calling for industry to reduce or eliminate pollution at its source rather than cleaning it up afterward. In 1991, the EPA (U.S. Environmental Protection Agency) Office of Pollution and Hazardous Waste Prevention was established, and it was first articulated by Paul T. Anastas, who worked in this office.

In the 2000s, Green Chemistry began to appear in university classrooms. Despite being a relatively new discipline, Green Chemistry provides opportunities for research and production in a safe, inexpensive, and environmentally responsible way, leading to its adoption in industry as well.

Today's conscious consumers demand that the production processes of products they purchase be greener and more sustainable. The application of Green Chemistry concepts enables the safer and more effective processes that industry demands and needs. The fundamental concept of Green Chemistry is the use of chemical knowledge and skills to reduce or eliminate the formation or use of hazardous substances during the planning, production, and application of chemicals in order to minimize threats to the health of users and the environment. For this reason, concern about eliminating or minimizing toxic waste generation has become greater than dealing with previously generated waste. In other words, Green Chemistry is a branch of chemistry that prioritizes the development of chemical methods and substances aimed at protecting the environment. Additionally, Green Chemistry enables the regulation of chemical products and processes to eliminate or reduce the formation and use of substances that could be harmful to human health and the environment. In sustainable development, Green Chemistry or clean processes are the design or redesign of chemical products and operations that reduce or eliminate the use and production of hazardous materials and substances while making chemical transformation processes more reliable in terms of human health and environmental friendliness. Therefore, Green Chemistry is a universally accepted term for defining the movement toward more environmentally acceptable processes and products. In the Green Chemistry approach, 12 basic principles are accepted by academic units and industries to reduce waste generation, energy, and natural resource use (given in Figure 2).

Figure 2: The 12 Principles of Green Chemistry

1) Prevention: Instead of treating and cleaning generated waste, waste generation should be prevented. 2) Atom economy: Production processes should be provided that can increase the efficiency of materials spent during the production phase, the amount in the final product, and energy. This is achieved with very little byproduct and waste generation. 3) Reduction of hazardous chemicals: Processes should be designed to use and produce harmless substances that do not harm the natural environment and human health. 4) Design of safe chemicals: Designs should be made to minimize the negative effects of chemical substances and other toxic substances that harm the environment and human health. 5) Use of safe solvents and auxiliary substances: If auxiliary substances (solvents, separation agents, etc.) are not necessary during production, they should not be used; if they must be used, the least hazardous ones should be selected. 6) Energy efficiency: Considering environmental and economic conditions, the energy requirements of chemical processes should be determined and minimized. Production should be carried out as much as possible at atmospheric pressure and room temperature. This way, less energy is consumed. 7) Use of renewable raw materials: Renewable and food-based raw materials should be consumed due to technical and economic reasons. 8) Reduction of byproducts: Operations such as the use of unnecessary groups, and the modification of chemical and physical processes for a certain period should be reduced or not used if possible. This is because when unnecessary substances are used in these operations, waste can be generated. 9) Catalysts: By using catalysts in production, efficiency increases. Catalysts are always superior to stoichiometric chemicals. 10) Design for degradation: Chemicals should be designed so that after completing their lifespan, they break down into degradation products that do not create waste—that is, products that do not harm the environment. 11) Monitoring and solving pollution prevention: Analytical methods should be developed for continuous monitoring and control of production in the formation of hazardous substances. 12) Safer chemistry to prevent accidents: Chemical substances to be used and their physical states should be selected to minimize accidents such as fire, explosion, or leakage.

It is also important to emphasize here that Green Chemistry is not something taken seriously only in developed countries. Some pioneering research in the 1980s and various research and some industrial application examples were carried out in many countries, including the United States, India, and China in the 1990s.

Due to limited testing regulations, a relatively small portion of chemicals formally defined as harmful has been effectively used in daily products with little information about their toxicities and environmental effects. The European Union's REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) policy will change this situation seriously. As a result of this policy, some chemical substances are expected to be restricted, become extremely expensive, or not be used. Increasing information about chemicals and an increasing number of chemical substances being classified as "hazardous" in some way will have health and safety effects that make the use of these substances more costly and difficult. Additionally, there is no doubt that it will cause local authorities and governments to limit and increase the costs of disposal of waste containing these substances. In terms of sustainable development, Green Chemistry or clean processes are the design or redesign of chemical products and operations that reduce or eliminate the use and production of hazardous materials and substances while making chemical transformation processes more reliable in terms of human health and environmental friendliness. The use of Green Chemistry in the leather industry is still very limited. The leather industry can actually be considered as an industrial branch that processes raw hides, which are waste materials or byproducts of the meat and meat products industry, and prevents them from deteriorating and having negative environmental effects.

Leather processing is the process of equipping raw hides obtained from animals with properties suitable for their intended use through a series of mechanical and chemical operations.

However, both the raw materials it uses and the chemical substances, chemical reactions, and waste it releases during the conversion of these raw materials into finished leather have caused the leather processing industry to be recognized as one of the polluting industrial branches. The leather industry is primarily identified by the odor it releases into the environment, which corresponds to the gaseous form of various pollutants emitted into the air. Water used is released into receiving environments containing many organic and inorganic substances. This results in high, and sometimes very high, values of various parameters known as wastewater pollution parameters. Such a result means that the existence of living organisms found in the receiving environments where the wastewater is discharged is in danger. It indicates that the water can no longer be used. In a world where water resources are dwindling, this is obviously a threatening problem. Solid waste is mostly leather scraps that remain from mandatory mechanical operations performed at different stages of leather processing, containing or not containing chemicals used. For these reasons, the leather industry is among the industrial branches that are under continuous observation and examination by environmental organizations and authorities. With the development of environmental awareness, changes in consumption concepts and production models have made the leather industry, which is trying to change its negative image, willing to show greener or cleaner approaches. The leather industry is primarily investigating the possibilities of benefiting from Green Chemistry through chemical substances and production methods based on them that will help create clean, cleaner production methods, and in particular through close relationships with the chemical industry. For this reason, researchers have been forced to seek environment-friendly products and production processes, particularly as alternatives to chromium sulfate-based tanning methods using +3 valence basic chrome sulfate. Tanning is the basic operational stage of leather production.

At this stage, the degradable leather protein is transformed into a non-degradable form by establishing various chemical bonds with the tanning agent introduced into the structure.

Furthermore, tanning, which improves the durability and usability of leather products, has the advantage of preventing raw hide putrefaction and giving it resistance to chemical substances, heat, and microbiological degradation. For leather tanning, primarily inorganic tanning agents such as chromium, aluminum, iron, and zirconium; organic tanning agents such as vegetable tannins; aldehydes; synthetic tanning agents; and tanning methods in which these are used in various proportions together are mostly considered as traditional tanning methods. Chromium is the main tanning agent used in the production of approximately 90% of finished leather produced worldwide. However, the resources used in obtaining chromium tanning agents are quite limited, their consumption during leather production—that is, their uptake by leather—is poor (55-65%), and the recovery and reusability of chromium from leather waste is very low. Additionally, in chromium tanning, chromium and other inorganic substances cause a significant increase in biological oxygen demand (BOD), chemical oxygen demand (COD), total solids (TS), total dissolved solids (TDS), suspended solids (SS), and salt values, causing problems in wastewater treatment as well. Solid waste such as chrome chips, skiving waste, trimming scraps, and sanding dust is generally discarded, which increases costs on one hand and causes additional problems on the other. Chromium (III) found in solid waste causes pollution problems, is not sufficiently removed in treatment systems, and converts to carcinogenic and mutagenic chromium (VI), which is one of the factors encouraging the search for environment-friendly green alternatives. Tanning methods created by combining different tanning agents provide an opportunity to perform tanning using low amounts of chromium or none at all. The basic problem here is which tanning agents should be used together, in what proportions, and consequently whether the superior properties that chromium provides to leather can be achieved with the resulting method. Examples include the combined use of chromium with silica or chromium with iron in low-chromium methods.

In chromium-free tanning methods, the combined use of organic-based tanning agents is in the foreground.

Examples include combinations of synthetic tannins with vegetable tanning agents or systems composed of a combination of metal salts other than chromium with a biocatalyst, urea, melamine, phenol, formaldehyde, and their condensation products. However, while these methods reduce pollution caused by tanning, they fall short in terms of the desired product properties of leather. Within the scope of Green Chemistry, research on chromium-free tanning methods that can replace chromium tanning in the leather industry is noteworthy for the leather industry. For this purpose, studies conducted with different mineral tanning agents, vegetable tanning agents, synthetic tanning agents, aldehydes, and other tanning agents alone or in their combinations, treated within the scope of clean technology, are relevant. In experiments with chromium-free tanning methods such as combinations of unnatural amino acids with aldehydes, octa (aminopropylsilsesquioxane) with tetrakis phosphonium sulfate combination, D-Lysine with aldehyde combination, very positive results have been obtained both in the physical and organoleptic properties of leather and in terms of wastewater and solid waste. However, it is seen that there are still deficiencies when compared with quality properties obtained from chromium-tanned leathers.

After chromium, the most widely used tanning method is tanning with vegetable materials. This method takes longer than chromium tanning, is a labor-intensive system, and has higher costs.

However, product quality is lower. Nevertheless, the tanning agents used in vegetable tanning and called tannins are plant extracts accepted as a good alternative for Green Chemistry. These can be obtained through simple methods from different parts of different plants with high tannin content. The main role here is played by biophenolic structured compounds. Additionally, their antioxidant properties are also important. The use of biologically-based alternatives instead of harmful synthetic substances such as synthetic phenols, diisocyanates, chromium, and the like is considered healthier by researchers from an environmental perspective. Strategies for the transition to Green Chemistry in the context of tanning operations in the leather industry: (1) paradigm shift from traditional chemical processes to Green Chemistry; (2) improving traditional tanning operations to help reduce total solids content and other harmful chemical substance outputs; (3) utilizing Green Chemistry to increase tanning agent uptake and improve structural water resistance; (4) strengthening the stability of collagen's triple helix structure with intramolecular and intermolecular bonds; (5) modifying potential groups in terms of biological, physical, and chemical degradation; and (6) significant reduction of pollution loads and toxic chemicals in leather processing waste and bringing environmental problems to controllable levels. Assoc. Prof. Fazlı Akyüz / Istanbul University Cerrahpaşa - Vocational School of Technical Sciences - Department of Textiles, Clothing, Footwear and Leather Leather Technology Program

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