Natural formation of vinyl chloride in the terrestrial environment

Vinyl chloride is a highly reactive and toxic substance which is widely used in industry. It is the parent compound of poly(vinyl chloride) (PVC), one of the most important industrial polymers. Until now, it was thought that vinyl chloride found in the environment is exclusively man-made or results from the degradation of other anthropogenic substances, such as trichloroethylene and tetrachloroethylene. Here, we demonstrate that vinyl chloride also has natural sources. Soil air and ambient air from a rural area in Northern Germany were investigated for volatile chlorinated halocarbons. The concentrations of vinyl chloride in the soil air were significantly enhanced as compared to ambient air, indicating a natural formation of this compound in the soil. A series of laboratory experiments using different soils and model compounds was conducted, which clearly proved that vinyl chloride could be produced during soil processes. We propose that this highly reactive compound can be formed during the oxidative degradation of organic matter in soil, for example, in a reaction between humic substances, chloride ions and an oxidant (ferric ions or hydroxyl radicals). The redox-sensitive aromatic compounds in soil such as catechols and o-quinones can be degraded to CO2, accompanied by the release of vinyl chloride and other volatile chlorinated compounds. This process could have started in the Late Silurian to Early Devonian, 400 million years ago, when the first soils on earth evolved.