There are many factors that need to be taken into account to determine how long it takes for a 100 per cent cotton t-shirt to decompose in a landfill site.
First of all, depending on the environment it is found in, a swatch of cotton fibre can decompose in a matter of two weeks if given the proper conditions. On the other hand, some cotton has been found in caves that was over 10 000 years old!
This is because for optimum microbial activity to occur, warm and moist conditions are required (about 26.7 to 35° C is the optimum for microbial activity, with an approximate moisture of nine per cent) as well as an abundance of oxygen. However, if the cotton is in a landfill or another site that is very hot, very cold, very dry, and/or with little oxygen, microbial activity will be significantly reduced, and therefore the decomposition rate will be much slower. As well, the temperature of the soil and the air above-ground, as well as the depth at which the shirt is buried in the landfill site will also have an impact on the decomposition rate of the cloth.
Cotton fibre after ginning is composed of approximately 95 per cent cellulose, with the remaining five per cent being mainly composed of wax, pectins and proteins.
The biodegradation of cellulose is caused by enzymes known as "cellulases". These are produced by various microorganisms, such as bacteria or fungi. Since the decomposition process is considered aerobic (or "occurring with oxygen"), the greater the amount of oxygen the t-shirt is exposed to in the landfill site, (with other factors such as temperature and moisture level taken into account), the more rapid the decomposition rate of the cotton fibre will be.
Cellulosic material is considered biodegradable. However, dyes and other treatments to the t-shirt must be taken into account when estimating the decomposition rate of the garment, as they may contribute to a slowing of the process.
Of course, the most favourable option when faced with a t-shirt that you have outgrown or don't wear anymore would be to find a way to re-use it, by either passing the t-shirt on to someone else, donating it to a charitable organization, or by finding a creative way to re-use the material!
Source:
National Cotton Council and Cotton Inc. (United States)
Related sites
Waste Management Industry Survey: Business and Government Sectors
National Cotton Council of America Educational Materials
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