The rates of formation of carbon and gases from high energy ball milling of organic compounds
Soo, Quai Siang Kelvin
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Mechanochemical reactions induced by high energy ball milling have been known as a method of compound destruction but the mechanics behind the destruction has not been well understood. This study focuses on reactions induced by air, resulting in end product formation of principally carbon dioxide, methane and elemental carbon. The use of ball milling to destroy pollutants in soils has several advantages. Including the use of a sealed container reducing direct human exposure to compounds and intermediates, low maintenance coupled with low level of technological sophistication and the ability to break compounds -down into harmless carbon, water and gases, principally methane, carbon dioxide and hydrogen. This makes ball milling a very attractive mitigation technology for organic pollutants. This research has found that at 0.1% concentration (typical for seriously polluted soils), most of the organic compounds were destroyed under a standard set of conditions and proved very effective against a wide variety of chemical structures ranging from polynuclear aromatics (PAHs) to linear hydrocarbons. Initial work to quantitatively isolate the elemental carbon from reactions was only partially successful. Pure samples of carbon were obtained, analysed by Raman spectroscopy and shown to be partially graphitic but it was not possible to get quantitative results. Methane formation from a variety of organic compounds was investigated in detail. It was found that gases such as nitrogen and oxygen can significantly alter the rates methane production, suggesting that a number of mechanisms are competing. By changing the atmospheres in the jar it possible to change the rates and types of gas formation, from this it was postulated that organic compounds degrade into intermediates which further degrade to the observed gases. It was also found that carbon dioxide was able to form methane from available hydrogen sources, either organic or water. When milling organic compounds in air or oxygen, the formation of carbon dioxide initially predominates over methane but as the reaction progresses this is reversed. This research has shown that ball milling is a more complex series of equilibria than previously thought and has also found an unexpected pathway where carbon dioxide is converted to methane.