Plastic is not evil

We have all seen the pictures of coastlines littered with plastics bottles and bags and we also cringe at the thought that these materials will persist for many years after they are disposed.  However, it is also not reasonable to do away with plastics completely as their inexpensive production and ease of disposal are a staple to sterile environments such as hospitals.  Yet, these fleeting applications for polymers are not appropriate for their persistence in our environment. In the last decade the chemical community has been working on the development of biodegradable polymers that can degrade through chemical hydrolysis or enzymatic processes. The applications for such polymers are when they are soiled by food or other biological substances and cannot be recycled. The benefits of these polymers are that they are generated from renewable resources and then degrade into CO2, CH4, water, biomass, and other natural substances in a short amount of time.

This bag will self desctruct in 18 months ..

Last week, a study, published in Environmental Science and Technology, carefully modeled the methane release of rapidly degrading polymers in landfills to gauge the benefits of biodegradable polymers. The researchers conclude that the polymer that they looked at (poly(3-hydroxybutyrate-co-3-hydroxyoctanoate (PHBO)) degraded and released a significant amount of methane too fast to be sufficiently captured by landfills with gas collection systems. Methane is a concern because like CO2 it is a greenhouse gas but is over 20 times more effective in trapping heat in the atmosphere. Their suggestion therefore were to develop biodegradable polymers that degraded slower to account for the delay in the gas collection systems to reach a critical concentration of GHG emissions to operate.

There are a couple of questions concerning this conclusion, as the authors only analyzed a single biodegradable polymer. There are other more common biodegradable polymers like poly(-e-caprolactone) PCL or poly(lactic acid) (PLA). PCL, in landfill reactors, degrades at a much slower rate than PHBO. Also there have been studies that have shown that biodegradable characteristics depend on the type of polymer and degradation conditions.

Although the authors of the paper do have a good point to have manufacturers of these types of polymers to consider the environmental impact of their products even after disposal. They put it best at the conclusion of their paper, I am paraphrasing: If the emissions for producing a biodegradable material is comparable to that of producing a material from petroleum-based feedstocks, and yet the disposal emissions is higher for the biodegradable materials, is it then a viable alternative?

But also this paper is a reminder for the need to create an infrastructure for biodegradable polymer disposal. Now that worldwide consumption of these types of polymers perhaps exceeds 68 million kg, we should worry about its disposal emissions. A possibility that was not considered in the paper was to analyze composting conditions for biodegradable polymers. A recent study in Korea compared the aerobic and anaerobic environments of poly(caprolactone) and poly(butylene succinate). They concluded that pretreatment technologies coupled with composting would be a means to decrease the waste of biodegradable polymers. This is important for them as the Korean government recently encouraged substituting biodegradable polymers for non-biodegradable polymers.

In addition to considering the disposal emissions of our materials, we should consider an infrastructure for disposal of biodegradable materials that would reduce or mitigate the emissions from these materials as they are an advantage because that do not persist in our environment and are made from non-petroleum-based (renewable) feedstocks.

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  1. Oil from Plastic Waste | STEM_Wonk - October 5, 2011

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