In 2011-12, the European Soil Data Centre of the European Commission conducted a project aimed to collect data on contaminated sites in Europe using the European Environment Information and Observation Network for soil (EIONET-SOIL). According to this project there are around 342.000 identified polluted sites . Common contaminants include heavy metals, mineral oil, explosives, hydrocarbons, chlorinated solvents, which derive from many sectors and activities. Particularly, contaminations of soil and groundwater with dense non-aqueous phase compounds (DNAPL), such as chlorinated solvents, represent a significant environmental challenge because pose a serious threat to human health and to terrestrial and aquatic ecosystems.
Chlorinated solvents are organic compounds characterized by the presence of at least one chlorine atom bonded to a carbon atom. They are volatile and tend to be colorless at room temperatures. These compounds are characterized by a higher density than water one, so, once they are in contact with the soil they can easily penetrate the groundwater (the distribution of the different contaminants is similar in the liquid and the solid matrices ). Moreover, their meandering penetration into an aquifer makes them difficult to locate and remediate.
Some of these compounds are produced naturally in the environment: they are emitted by marine algae or volcanoes and they play an important role in communication between plants ; but most of them are made in the laboratory. They are numerous and ubiquitous, since they have been heavily used for several decades for a variety of applications: extraction, washing, degreasing of metals, cleaning agents, paint removers (figure 1). We can say that they are in all living things, it is very common find them in many building materials such as paints, adhesives, wall boards, ceiling tiles and protective coatings, but also in the indoor environment, they can result from new furnishings and office equipment such as photocopy machines, printers, permanent markers or correction fluids.
Figure 1: Sources of Volatile Organic Compounds (source: epa.gov)
Most of these compounds (mostly synthetic ones) cause harm to the environment and pose risk to humans due to their carcinogenic and toxic characteristics. The effects on human and environmental health depend on the physical and chemical properties of the contaminants which determine the remedial technology: long-term exposure to volatile organic compounds (VOCs), or short-term exposure to high levels of VOC, can cause damage to the liver, breast, kidneys, and central nervous system . For these reasons, anthropogenic chlorinated compounds are adjusted by regulations which assess their maximum allowable concentrations in waters intended for human consumption. In Europe, the VOC Solvents Emissions Directive is the main policy instrument for the reduction of industrial emissions of volatile organic compounds (VOCs). In the USA, they are regulated by Environmental Protection Agency (EPA). What is came out from their reports is that the most hazardous VOCs include Carbon Tetrachloride (CT), Tetrachloroethene (also called Tetrachloroethylene or Perchloroethylene, PCE), Trichloroethene (also called Trichloroethylene, TCE) and Chloroethene (also called Vinyl Chloride, VC).
In the last decades, big efforts are addressed to minimize the releasing of these kind of contaminants into the environment, but a vast number of sites are still polluted. The most recent remediation technologies are based on the destroying chlorinated compounds. In fact, have been proved that in favorable (anaerobic) conditions, chlorinated solvents can undergo reduction [4-8]. The remediation technologies of chlorinated solvents working with reactive minerals (magnetite, pyrite, sulfide, ZVI) are based on different abiotic degradation mechanisms. One of these it is hydrogenolysis, this is a reaction in which a carbon-chlorine bond is broken and hydrogen replaces chlorine atom, with the simultaneous addition of two electrons. Unfortunately, in the most of the cases, hydrogenolysis of chloroethylenes leads to the formation of more toxic and recalcitrant chlorinated byproducts . The most important and common abiotic reaction is the reductive elimination (α-elimination and β-elimination). It is a process involving a two-electron transfer to the target molecule and the elimination of two chlorine atoms . Reductive elimination results in the formation of relatively benign products (such as acetylene or ethene) , which are the non-toxic end-products that everybody would like to see after reaction (figure 2).
Figure 2: Transformation of Tetrachloroethene and Trichloroethene into less toxic end-product via reductive elimination (green arrow) and via hydrogenolysis (grey arrow)
Green rusts (GRs) have drawn our attention because they are capable to reduce chlorinated solvents via reductive elimination, transforming them into less toxic compounds (under anaerobic conditions) . The characteristics of green rusts depend on the type of anion present within these interlayers. According to the interlayer anions, X-ray diffraction analysis distinguishes two types of GRs : GR1, containing planar or spherical anions (e.g., CO32-, Cl–) [13-14] and GR 2, containing three-dimensional anions (e.g., SeO42-, SO42-) [15-17]. No significant research about reactivity of GR reactants against chlorinated solvents has been conducted. So, our task will be figure out which are the best reactants for each VOC cited above.
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