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I came across this article on wikipedia on ethylene glycol stating that it decomposes fairly quickly. I was under the impression that ethylene glycol was dangerous to let leach into soil, but I think I'm mistaken as wikipedia states

it [ethylene glycol] breaks down in air in about 10 days and in water or soil in a few weeks. It enters the environment through the dispersal of ethylene glycol-containing products, especially at airports, where it is used in deicing agents for runways and aeroplanes.

  1. What does it decompose into ($\ce{CO2 + H2O}$?) and what is the mechanism?
  2. Does this mean that a tub of ethylene glycol exposed to air will eventually contain only water?
  3. Is the reaction the same when ethylene glycol penetrates soil around runways?
  4. What is the activation energy for the decomposition and at what temperature will it no longer decompose?
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  1. This image highlights the oxidation process of ethylene glycol fairly well

enter image description here

( Taken from Ethylene glycol: properties, synthesis, and applications )

  1. Since it has little tendency to evaporate, unless it is kept in direct sunlight, it would probably oxidize to a mixture of the above liquid compounds (glyoxal, glyoxalic acid) and not directly to water. If it is kept in sunlight, the reaction would be photochemically catalysed and a it may degrade to carbon dioxide faster, but according to theoretical principles you would not get much water.

  2. Ethylene glycol is relatively hydrophilic and also has a low tendency to separate from water, so the only major degradation process available to it in soil would be biodegradation by either aerobic or anaerobic process, which is NOT the same as atmospheric oxidation1

  3. Not sure about reaction energetics regarding the same

Most of the danger regarding glycol spills (and other chemicals in the environment) comes from absorption by other organisms; since glycol is comparatively less volatile and more hydrophilic than, say ethanol, it has a greater tendency to be absorbed by organisms.

1 Taken from William R. Roy, in Handbook of Antistatics (Second Edition), 2016

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