De novo formation of chloroethyne in soil.

To date, chloroethyne in the environment has been proposed to occur as a reactive intermediate during the reductive dechlorination of tri- and tetrachloroethene with zerovalent metals. Such artificial conditions might possibly be found at organohalide-contaminated sites that are surrounded by remediation barriers made of metallic iron. In this paper, it is shown that the highly reactive chloroethyne is also a product of natural processes in soil. Soil air samples from three differentterrestrial ecosystems of Northern Germany showed significant chloroethyne concentrations, besides other naturally produced monochlorinated compounds, such as chloromethane, chloroethane and chloroethene. Measured amounts range from 5 to 540 pg chloroethyne in air purged from 1 L of soil. A possible route of chloroethyne formation in soil is discussed, where chloroethyne is probably produced as a byproduct of the oxidative halogenation of aromatic compounds in soil. A series of laboratory studies, using the redox-sensitive catechol as a discrete organic model compound, showed the formation of chloroethyne when Fe3+ and hydrogen peroxide were added to the system. We therefore propose that the natural formation of chloroethyne in soil proceeds via oxidative cleavage of a quinonic system in the presence of the ubiquitous soil component chloride.

Formation of volatile iodinated alkanes in soil: results from laboratory studies.

Volatile iodinated organic compounds play an important role in the tropospheric photochemical system, but the current knowledge of the known sources and sinks of these alkyl iodides is still incomplete. This paper describes a new source of alkyl iodides from the pedosphere. Different organic-rich soils and humic acid were investigated for their release of volatile organoiodides. Six volatile organoiodides, iodomethane, iodoethane, 1-iodopropane, 2-iodopropane, 1-iodobutane and 2-iodobutane were identified and their release rates were determined. We assume an abiotic reaction mechanism induced by the oxidation of organic matter by iron(III). The influence of iron(III), iodide and pH on the formation of alkyl iodides was investigated. Additionally, different organic substances regarded as monomeric constituents of humus were examined for the production of alkyl iodides. Two possible reaction pathways for the chemical formation of alkyl iodides are discussed. As humic acids and iron(III) are widespread in the terrestrial environment, and the concentration of iodide in soil is strongly enriched (compared to seawater), this soil source of naturally occurring organoiodides is suggested to contribute significantly to the input of iodine into the troposphere.

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.