Introduction

Leather manufacturing technology has been in existence since Stone Age when cavemen used animal skins for apparels, tents and tarpaulins. The royalty of texture and durability of transformed animal skin has made leather manufacturing as one of the largest industries of the world. Indian leather industry accounts for 12.9% of the world's total leather production by providing a broad range of consumer goods such as apparels, footwear, bags etc. (Dixit, Yadav, Dwivedi, & Das, 2015). The transformation of animal skin into leather is achieved by a variety of processing steps which results in different forms of leather. Structurally, the raw animal hide is a natural polymer of collagen. In order to prevent natural decomposition of hide, water molecules are removed from collagen and flexibility is restored by using tanning chemicals (Harlan & Feairheller, 1977). Several tanning processes such as chrome tanning, vegetable tanning, aldehyde tanning, synthetic tanning and alum tanning determine the form and mechanical characteristics of finished leather material (Maina, Ollengo, & Nthiga, 2019). Leather tanning is a wet process that requires an enormous amount of water and mostly 90% of water is wasted as effluents. Tanning effluents contribute to the deposition of toxic tannins which damages land and water ecosystems.

Tannery effluents have been ranked as highly polluting agents among all other industrial wastes. The presence of metals in tannery waste is chronically toxic for organic life. The by-products of tanning industries cause significant environmental pollution unless pre-treated before discharge. Even treated effluents from leather industries are high in biochemical oxygen demand (BOD), chemical oxygen demand (COD) and heavy metal toxicity beyond permissible levels (Gupta, Gaumat, & Mishra, 2012). Due to this, the tanning industry has affected the lives of workers as well as nearby residents in the area.

There have been reports of occupational cancer and severe DNA damage due to chromium toxicity among tannery workers in Kanpur, India (Kvitko et al., 2012; Rastogi, Pandey, & Tripathi, 2008). The leather industries are also responsible for heavy metal contamination in agricultural soil in nearby areas which affects the irrigation process. Soil pollution at phytotoxic concentration affects plants and causes the risk to human health (Liknaw & Tekalign, 2017). Similarly, tannery effluents in water have deleterious effects on aquatic ecosystems. Due to tannery effluents dispersal in soil, water and air, toxic chemicals have been incorporated in the food chain with rapid interactions with the environment. This review elaborately discusses specific toxicity sources in the leather manufacturing process, toxicity analysis bioassays and provides alternatives for sustainable leather industries.

Transformation process

Animal skin to Leather

In 2019-2020, there has been an export of 4.6 billion US dollars from leather industries in India (IBEF, 2020). The industry has transformed drastically from being a mere raw material supplier to exporter of value added leather products. Leather manufacturing process is technically a channeled process involving three sub processes – beam house operations, tanning and crusting (Figure 1).

Beam house operations are carried out on animal hide as pre-treatment required before tanning. Firstly, the hides undergo a curing process with salts and chemicals to prevent bacterial putrefaction and ensure preservation. Then, they are trimmed and soaked to restore moisture and get rid of salts in wastewater (Il, Fs, Environment, & IL. & FS. Environment, 2010). The excess tissue, fats are removed to obtain uniform thickness and dehairing is performed by incubation in drums containing calcium hydroxide and sharpening agents. The flexibility is restored by hide bating and incubation in ammonium salts and proteolytic enzymes (U. N. I. D. O., 2000). Next step involves picking, which is required to adjust low pH to enhance the effects of tannins in the next process step.

The tanning process restores flexibility to make the material suitable for various applications. The two most common types of tanning methods include vegetable tanning and chrome tanning. Vegetable tanning is a three weeks long process which ensures complete penetration of dyes into the animal hide. Further, the hides are treated with sodium bicarbonate and sulphuric acid for bleaching and tannins removal. For flexibility, lingo-sulphate, oils or corn sugar are added as finishing steps. Chrome tanning is achieved when hide reacts with trivalent chromium salt. Subsequently, fat-liquoring is done by applying oil on the leather surface to provide the necessary moisture for flexibility (Heidemann, 1993). Chrome tanning is preferred over vegetable tanning due to better properties of finished leather in terms of softness, thermal ad water stability and less processing time.

Crusting is performed after tanning and lubrication of hiding and sometimes dye is incorporated at this step. Crusting ensures fixation of chemicals added in the entire process for leather softening. The final leather finish is provided by a surface coating that includes oiling, brushing, buffing, spray coating, embossing and glazing (Bienkiewicz, 1983). The environmental impact of leather tanning process involving toxic chemicals, tannins, resins, biocides along with negligent waste disposal and emissions are responsible for the fatalistic shape of the leather industry.

Impact of leather industry effluents on the environment

Wastewater effluents

The leather industry holds significant economic responsibilities, but its impact on the environment is undeniable. Almost 90% of leather tannery pollutants have been reported to be discharged during the pre-tanning and tanning process. According to Thanikaivelan et al., tanning process contributes to disturbed pH, increase in chemical oxygen demand (COD), sulphates and total dissolved solids (TDS) in wastewater (Thanikaivlaen, Rao, & Nair, 2001). The dehairing process of animal hides accounts for 84% BOD, 75% COD and 92% suspended solids (SS) in tannery effluents (Marsal, Cot, Boza, Celma, & Manich, 1999). Using sodium sulphide hinders the efficacy of waste treatment plants along with unusual consequences on the environment (Bailey, Tunick, & Kuzma, 1982). The post tanning process has been reported to result in heavy metal leaching in water, causing ecological imbalance and increased river salinity. This has led to agricultural production loss and increased toxicity in drinking water.

Solid waste sludge and volatile effluents

Tanneries generate huge amounts of sludge which mostly comprise non-biodegradable tanned residues creating a clearance burden in terms of waste management. The tanned residues are generally resistant to chemical, microbial and thermal degradation due to perceived tanning chemicals (Han, Jaurequi, Tang, & Nimni, 2003). It is reported that tannery residues in water bodies affect aquatic life and agricultural activities. Different tannery chemicals, including hydrogen sulphide, hydrocarbons, amines, ammonia, and aldehydes are major effluents emitted into the atmosphere. During pH adjustments in the tanning process, hydrogen sulphide is generally leached in wastewater (Dixit et al., 2015). Emissions of chromium can occur during chromate reduction reaction, leaching of chromium particulates from powder or during the buffing step. These volatile organic compounds released as effluents from tanning industries have been reported to be a major threat to the atmosphere.

Chemicals toxicity

Due to severe environmental threats posed by tanning chemicals, regulatory bodies have developed standards and control measures for chemicals used in leather industries. According to EU (2003 a), authorities have posed strict instructions on the labeling of products that contain more than 0.5% phthalates as di-butyl phthalate (DBP) and di-ethyl hexyl phthalate (DEHP) is used in the leather coating. Phthalates have been reported to induce reproductive toxicity and its low degradability result in their environmental persistence. Chemicals used in leather finishing such as o-phenyl phenol (OPP), formaldehyde and azo dye derivatives have been reported as carcinogens (Mohan, Saravanabhavan, Thanikaivelan, Rao, & Nair, 2008). Inorganic chemicals chromate, cadmium sulphate, are used for good leather color, but these impart heavy metals toxicity to the environment (Table 1). According to Srivastava and Nair, chromium (III) can lead to apoptosis and proteins structural changes (Shrivastava & Nair, 2001). A study conducted by Afaq et al. reported that leather dyes Bismarck brown and acid leather brown decrease RBC counts in marine life forms (Afaq & Rana, 2009). Thus for creating a balance of economic development and environmental repairment, leather processes need to be intricately studied to develop greener and sustainable solutions.

Evaluation of leather toxicity

Leather is a multipurpose product with a standard of texture, mechanical strength and durability properties. To achieve these characteristics, tannins' toxicity burden is increasing along with technology development. High concentration traces of tannins in effluents affect metabolic processes due to their ability to interact with organic compounds. Tanning chemicals have been responsible for leaching of heavy metals leading to lethal consequences of carcinogens, and mutagens exposure, causing genetic toxicity to flora and fauna (Siyanbola, Ajanaku, James, Olugbuyiro, & Adekoya, 2011). Hence it is essential to identify the toxicity potential of leather industry effluents.

In a study conducted by Roy et al., effluent samples were collected from a tannery discharge site in Tamil Nadu, India and several bioassays were performed to evaluate toxicity on different life forms (Table 2). According to physicochemical analysis, the effluent contained very high levels of Nickel, Chromium, Zinc and Iron with other heavy metals like Cobalt, Cadmium, and Manganese etc. at a relatively lower concentration. It was found that tannery effluent severely affected protein and chlorophyll synthesis in plant Lemna minor. .

Also, the effluent negatively affected human blood cells by inducing chromosome fragmentation, cellular damage and hemolytic activity (Afaq et al., 2009; Roy, Nagarchi, Das, Achuthananthan, & Krishnamurthy, 2015). Overall the study suggested that tannery effluent was highly toxic for plants, microbes and human beings. The toxicity was majorly attributed to the presence of heavy metals. Hence it is extremely necessary to study the toxicity of leather industry effluents before discharging them to the natural environment.

Sustainable future of Leather

Environmental sustainability has become a burning issue all around the world. With the growing industrialization, it is necessary to reduce the negative impact on the ecosystem. There could be two concepts that can be implemented – using sustainable, eco-friendly technologies and vigilant treatment of waste (Dixit et al., 2015). For cleaner processes, less toxic alternatives should be employed for animal hides treatments. The technology designing should support reuse and recycling processes specifically in tanning procedures. Vegetable tanning can be used in place of chromium tanning (Mahdi, Palmina, Gurshi, & Covington, 2009).

Similarly, toxic effects of leather dyes can be reduced by incorporating natural dyes with non-toxic mordants in tanning procedures (Inayat, Khan, Waheed, & Deeba, 2010). Also, Sargassum seaweeds can be used to bio-remediate chromium from tannery effluents, or chromium salt can be manufactured as reductant (Aravindhan, Madhan, Rao, Nair, & Ramasami, 2004). It has been reported that using ammonia free de-liming and bating methods with acids and carboxylic acid esters, 97% of ammonia load can be reduced in effluents (Frendrup, 1995).

Enzymatic dehairing is an alternative to avoid chemicals involved in beam house operations (Valeika, Beleska, & Sirvaityte, 2012). Additionally, the conventional methods of waste disposal have been ineffective in the case of leather industries.

The problem of chromium leaching from effluents to groundwater exposes several chemicals towards the ecological niche. Proper treatment of solid waste such as anaerobic digestion to produce biogas should be introduced in leather industry practices (Dhayalan et al., 2007; Gnanamani & Bai, 1992).

Keratin hydrolysates can be employed in chrome tanning along with leather strength (Chakraborty & Sarkar, 1998). Flesh waste can be utilized for byproducts such as fat liquor, soap, biodiesel etc. Filtration technologies such as activated carbon filters, reverse osmosis, cross flow microfilters can be employed for tertiary wastewater treatment (Gallego-Molina, Mendoza-Roca, Aguado, & Galiana-Aleixandre, 2013; Labanda, Khaidar, & Llorens, 2009).

Membrane bioreactors can be used for treating tannery sludge waste and crystallization saline streams can be employed for salt bearing wastewater from tanneries (Keerthi, Suganthi, Mahalakshmi, & Balasubramanian, 2013). Alternatively, sustainable leathers through biotechnology innovations have transformed the way of making leather material. Startups like Mycotech (Indonesia), Pinatex (London) and Malai (India) have developed leather alternatives maintaining similar properties and finish (Ladha, 2019). The upcoming new class of mushroom and plants based leathers has capabilities to reduce adverse environmental impact in terms of animal abuse, toxicity exposure and supply-demand paradigm of animal leather. Although the commercial acceptance of these leather similars remain unexplored, these innovations can reduce toxicity load on the environment and improve the image of the leather industry.

Conclusion

Leather industry has been famous for producing economically beneficial value added products along with undeniable ecological damage by toxicity exposure. The leather manufacturing process involves a series of technically essential steps which are responsible for producing toxic chemicals, heavy metals and carcinogens affecting environmental and occupational sustainability. The leather toxicity exposure affects agricultural practices, plants and aquatic life forms. The exposure to leather industry chemicals in the form of effluents has created toxicity load on the overall food chain, thus affecting human health. Hence there is a need to vigilantly evaluate the toxicity of leather industry effluents on different life forms. This study elaborately discuss specific toxicity source in the leather manufacturing process and toxicity evaluation assays concerned with biological life forms. The study describes alternatives to leather processes that can help in developing a sustainable future of leathers.

Funding Support

The authors declare that they have no funding support for this study.

Conflict of Interest

The authors declare that they have no conflict of interest for this study.