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Degradation of Vegetable-tanned Leathers Part 2: - The Marine Environment:

Updated: Mar 4

By A. Clare

Degradation of Vegetable-tanned Leathers Part 2: - The Marine Environment:

The application of vegtans in leather making is increasing and, therefore, in turn, one can expect to see more of this leather entering the waste stream. All litter has the potential to enter our oceans (through illegal dumping or transport from terrestrial sites via wind/water) and, the subsequent impact these waste materials have on the marine environment is a growing global concern. In a previous blog, the impact of ‘vegan’ leather in the marine environment was discussed, but what about leathers tanned with ‘natural’ materials such as vegtans? How will vegetable-tanned leather break down and, is it a better option for protecting our oceans?



Degradation in the marine environment:


Photooxidation of vegetable-tanned leather may occur if the leather is retained in the euphotic zone for a prolonged period. The euphotic zone is the uppermost layer of the ocean where light penetrates (0-200 m). If vegetable-tanned leather becomes retained within this upper bathymetry, it may experience prolonged irradiation. The absorption of UV light is likely to cause the leather to become brittle and fragment into smaller pieces (Figure 1) – particularly if combined with wave action. Over time, fouling by microorganisms will cause vegetable-tanned leather to become negatively buoyant and sink to ocean depths. If vegetable tanned leather is exposed to a temperature high enough to cause thermal degradation, then further degradation is likely to occur.


Figure 1 - Photooxidation of vegetable-tanned leather in the euphotic zone.


Hydrothermal vents are areas of high temperature which occur at spreading centres on the seafloor. Hydrothermal vents form when seawater seeps into fractures in the seafloor and becomes geothermally heated by magma (Figure 2). This heating builds up pressure and causes seawater containing dissolved minerals to rise towards the surface and exit the crust. Hydrothermal vents have a global distribution and have been found on almost all seafloors studied to date. Vegetable-tanned leathers may potentially reach these habitats after sinking.


Figure 2 - Hydrothermal vent formation.


Black smokers are the most common type of hydrothermal vent and are located 1–3 km beneath the surface. The temperature of active black smokers ranges between 330–380 °C. These vents release a low pH fluid (pH 2-3) containing high concentrations of dissolved metal sulfides. If vegetable-tanned leather reaches black smoker sites, both thermal- and chemical degradation, in terms of acid hydrolysis, may occur.


Hydrothermal vents also occur as white smokers, which are found between 750-900 m. White smokers are cooler than black smokers, with temperatures ranging between 200-300 °C. Furthermore, these vents are highly alkaline (pH 9-11) and release precipitates of silica, barium- and calcium-sulfate. Vegetable-tanned leather at white smokers will, likely experience alkali hydrolysis as well as thermal degradation.


Biodegradation in the marine environment:


Although water is essential to both bacteria and fungi life cycles, the amount of water present has a big impact on survival. In aqueous environments, bacteria are highly mobile and can extract nutrients from their surroundings by secreting enzymes. However, for fungi, too much water can also pose a problem, because secreted enzymes can be lost by rapid diffusion into the water column. This is reflected by the low abundance and diversity of fungi in the marine environment.


Although bacteria are diverse in our oceans, the antibacterial properties of vegetable-tanned leathers prevent bacterial breakdown. Furthermore, the low abundance of fungi in marine environments further reduces the potential for these ‘naturally’ tanned leathers to biodegrade.


Approximately 0.6% of studied fungi are of marine origin and these are predominantly found in nutritionally rich environments, such as marine sediments. Fungi are reported as the dominant microbes within marine sediments, which are rich in organic matter descending from surface waters. The presence of fungi in marine sediments suggest that the sinking (through fouling by microorganisms), and eventual burial of vegetable tanned leathers in sediments, presents an opportunity for biodegradation in the marine environment.


However, in 1982, leather artefacts were recovered from the Mary Rose Tudor warship, which was buried under marine sediment for 437 years. This demonstrates the limited ability for marine fungi to degrade this ‘naturally’ tanned material, even in habitats where these microbes are abundant.



Conclusion:


The increased application of vegetable tannins, to lower the leather industry’s environmental impact, will increase the amount of vegetable-tanned leather entering our oceans.


Oxidative degradation has the potential to degrade leather in both surface-waters, through photooxidation, and the deep sea, through thermal degradation at hydrothermal vent sites. Depending on the type of vent system, vegetable-tanned leather may simultaneously be chemically degraded by acid- (black smokers) or alkali-hydrolysis (white smokers).


Biodegradation of vegetable-tanned leathers in the marine environment is unlikely to occur, due to the low diversity of fungi. Although fungi can be found in abundance in some marine ecosystems, such as marine sediments, historical leather artefacts have been recovered from these sites intact.


Although vegetable-tanned leathers have the potential to degrade in the ocean, biologically useful nutrients will not be produced during their breakdown. The lack of biodegradation in the ocean implies vegetable-tanned leather is likely only to degrade and fragment into smaller pieces. Therefore, ‘natural’ vegetable-tanned leather is likely to behave similarly to plastic after entering the marine environment. An unanswered question is can marine organisms higher than microbes ingest these fragments and what is the implication of that ingestion? Like micro-plastics, would it result in negative ecological impact, or like other organic material that enters the hydrosphere could it be a source of nutrition?



References:


Richards, T.A., Jones, M.D., Leonard, G. and Bass, D., 2012. Marine fungi: their ecology and molecular diversity. Annual review of marine science, 4, pp.495-522.


Vyskočilová, G., Ebersbach, M., Kopecká, R., Prokeš, L. and Příhoda, J., 2019. Model study of the leather degradation by oxidation and hydrolysis. Heritage Science, 7(1), p.26.

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