Iron Catalyzed Halogenation Processes in Saline Soils

Abstract:

Within upcoming years the extent of salt deserts and salt lakes will probably increase due to climate change.

It is known that volatile organic halogens (VOX) are released from saline soils and thus higher emissions from these environments are likely expected in the future.

The origin of some organohalogens is not reasonably constrained by established natural halogenation processes.

Therefore detailed biogeochemical investigations of these environments are necessary to identify the specific halogenation pathways.

Redox-sensitive metals like iron are already known as triggers of chemical reactions via so called Fenton and Fenton-like reactions requiring H₂O₂ which is photochemically produced in water.

In this study we collected soil samples from several salt lakes in Western Australia with pH values ranging from 2 to 8.

The high pH variability was considered useful to study the impact of iron mobility and availability on halogenation processes.

Iron was found to mainly occur as oxides and sulfides within the alkaline soils and acidic soils, respectively.

All soil samples were lyophilised and finely ground prior to incubation at 40 °C for 24 h in aqueous solutions.

Formation of volatile organic compounds (VOC) and VOX from these soils was observed using GC-FID and GC-MS. When H₂O₂ was added to the samples much higher concentrations of VOC and VOX were observed.

Furthermore, when the pH of the soils was changed towards lower values higher emissions of VOC were also observed.

Based on C-H activation processes we delineate a halide containing iron complex as a provider of anions reacting with previously generated hydrocarbon radicals.

We suggest iron sulfate derivatives as those complexes which are generated if the above-mentioned natural H₂O₂ addition to iron sulfates and sulfides occurs. The origin of these complexes is able to explain the halogenation of chemically unreactive alkanes.