In situ chemical reduction

In situ chemical reduction (ISCR) is a new type of environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. It is the mirror process of In Situ Chemical Oxidation (ISCO). ISCR is usually applied in the environment by injecting chemically reductive additives in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation. In situ chemical reduction (ISCR) is a new type of environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. It is the mirror process of In Situ Chemical Oxidation (ISCO). ISCR is usually applied in the environment by injecting chemically reductive additives in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation. The in situ in ISCR is just Latin for 'in place', signifying that ISCR is a chemical reduction reaction that occurs at the site of the contamination. Like ISCO, it is able to decontaminate many compounds, and, in theory, ISCR could be more effective in ground water remediation than ISCO. Chemical reduction is one half of a redox reaction, which results in the gain of electrons. One of the reactants in the reaction becomes oxidized, or loses electrons, while the other reactant becomes reduced, or gains electrons. In ISCR, reducing compounds, compounds that accept electrons given by other compounds in a reaction, are used to change the contaminants into harmless compounds. ISCR is a relatively new type of ground water remediation technology. The most work on this method of remediation has been done in the past 10–15 years, so there are still many gaps in the understanding of the chemistry behind this process. The development of ISCR started out when K.H. Sweeny conducted research with zero-valent copper and iron in the late 1970s. He was able to treat a number of different chlorinated substances such as DDT, endrin, chloroform, and hexachlorocyclopentadiene to name a few. His work has been the basis of ISCR today. In the 1990s, Gillham, Tratnyek, Kriegman, Zhang, and Batchelor all made significant contributions in testing different metals and oxides for the use of ISCR. Gillham and Tratnyek in particular applied the reductive chemistry to groundwater treatment with the emplacement of ZVI barriers. Although it has been shown that other metals like aluminum and magnesium can produce the same effect in the laboratory, ground water treatment most generally focuses on the use of iron. Other major contributions in this field includes Zhang, who researched nanoscale iron, and Batchelor, who researched zero-valent iron clay (ZVI Clay). This past decade, more aspects of ISCR have been researched and new methods of implementation, such as ZVI clay and emulsified ZVI (EZVI), have been created. Scientists have also found that certain iron minerals, like green rust, magnetite, and pyrite, also have reductive capabilities although they contain ferrous iron rather than ZVI. Zero Valent Metals are the main reductants used in ISCR. The most common metal used is iron, in the form of ZVI (zero valent iron), and it is also the metal longest in use. However, some studies show that zero valent zinc (ZVZ) could actually be up to ten times more effective at eradicating the contaminants than ZVI. Some applications of ZVMs are to clean up Trichloroethylene (TCE) and Hexavalent chromium (Cr(VI)). ZVMs are usually implemented by a permeable reactive barrier. For example, iron that has been embedded in a swellable, organically modified silica creates a permanent soft barrier underground to capture and reduce small, organic compounds as groundwater passes through it. There are also many iron minerals that can actively be used in dechlorination. These minerals use Fe2+. Particular minerals that can be used include green rust, magnetite, pyrite, and glauconite. The most reactive of the iron minerals are the iron sulfides and oxides. Pyrite, an iron sulfide, is able to dechlorinate carbon tetrachloride in suspension. These substances are very interesting because they are naturally present, and learning about how they produce reductive zones could lead to the development of better reductants for ISCR. Polysulfides are compounds that have chains of sulfur atoms. This is a relatively new reactant, but it has been tested on the field in treating TCE and in comparison to EHC. The use of Polysulfides is a type of abiotic reduction and works best in anaerobic conditions where iron (III) is available. The benefit of using polysulfides is that they do not produce any biological waste products; however, the reaction rates are slow and they require more time to create the DVI (dual valent iron) minerals that are needed for the reduction to occur. Dithionite (S2O2−4) can also be used as a reductant. It is usually used in addition to iron reduce contaminants. A number of reactions take place and eventually the contaminant is removed. In the process, ditionite is consumed and the final product of all the reactions is 2 sulfur dioxide anions. The dithionite is not stable for a long period of time.