Zerovalent iron

Zerovalent iron and other zerovalent metals (ZVI and ZVM, respectively) have a variety of applications ranging from filters to electrodes to trenches. One of the emerging uses for ZVI is iron wall remediation. This technology uses ZVIs to form a permeable reactive barrier (PRB) which filters out contaminants in groundwater, leaving only decontaminated groundwater and dissolved iron on the other side of the PRB..mw-parser-output .tocleft{float:left;clear:left;width:auto;background:none;padding:.5em .8em 1.4em 0;margin-bottom:.5em}.mw-parser-output .tocleft-clear-left{clear:left}.mw-parser-output .tocleft-clear-both{clear:both}.mw-parser-output .tocleft-clear-none{clear:none} Zerovalent iron and other zerovalent metals (ZVI and ZVM, respectively) have a variety of applications ranging from filters to electrodes to trenches. One of the emerging uses for ZVI is iron wall remediation. This technology uses ZVIs to form a permeable reactive barrier (PRB) which filters out contaminants in groundwater, leaving only decontaminated groundwater and dissolved iron on the other side of the PRB..mw-parser-output .tocleft{float:left;clear:left;width:auto;background:none;padding:.5em .8em 1.4em 0;margin-bottom:.5em}.mw-parser-output .tocleft-clear-left{clear:left}.mw-parser-output .tocleft-clear-both{clear:both}.mw-parser-output .tocleft-clear-none{clear:none} The development of granular iron PRB technology was reliant on two advances: that metallic irons breaks down chlorinated organic compounds, and that reactions can proceed in situ under normal groundwater conditions. Metals have been used as catalysts since the 20th century, with more literature available concerning the corrosion of metal shipping and storage containers. Because this literature concerned pure solvents rather than aqueous solutions and the processes often occurred at high temperatures and pressures, it was not looked at by the environmental community. In 1972, zerovalent metals were found to be effective in breaking down pesticides and other chlorinated organic compounds in aqueous solution. However, this finding was also overlooked, perhaps because it was only recorded in patents and it preceded awareness of chlorinated solvents in groundwater as an environmental problem. In the 1980s, a student at the University of Waterloo examined the possibility of sample bias caused by sorption of contaminants to well casings and other materials used in groundwater sampling. While contaminants were lost from solution as a result of diffusion into polymers, contaminant losses were also observed when solutions came into contact with certain metals, and these losses were not consistent with a diffusion process. Reductive dechlorination was considered the most likely cause. This was confirmed by tests that showed several transitional metals had the ability to degrade many chlorinated aliphatic compounds. The corrosion reaction involving water is slow, whereas the corrosion of Fe0 with dissolved oxygen is fast, presuming there is O2 present. These are the reactive processes:Anaerobic corrosion: Fe0 + 2H2O → Fe2+ + H2 + 2OH−Aerobic corrosion: 2Fe0 + O2 + 2H2O → 2Fe2+ + 4OH−The presence of a reducible contaminant can produce another reaction which can then contribute to the overall corrosion rate. The zerovalent metal (usually granular iron) is the bulk reducing agent in these systems. However, corrosion of iron metal yields Fe2+ and hydrogen, both of which are possible reducing agents for contaminants such as chlorinated solvents. A heuristic model consisting of three possible mechanisms has proven very useful. Pathway A represents direct electron transfer (ET) for Fe0 to the adsorbed halocarbon (RX) at the metal/water point of contact, resulting in dechlorination and production of Fe2+. Pathway B shows that Fe2+ (resulting from corrosion of Fe0) may also dechlorinate RX, producing Fe3+. Pathway C shows that H2 from the anaerobic corrosion of Fe2+ might react with RX if a catalyst is present. Hydrogenation also plays a minor role in most systems and iron surfaces will be covered with precipitates of oxides (or carbonates and sulfides) under most environmental conditions. Concern stemming from how the oxide layer mediates transfer of electrons from Fe0 to adsorbed RX led to the formulation of another heuristic model, again consisting of three mechanisms. In the second model, pathway I shows essentially direct ET from Fe0 to RX in a corrosion pit, or similar defect in the oxide film. Pathway II shows the oxide film mediating ET from Fe0 to RX by acting as a semiconductor. Pathway III shows the oxide film as a coordinating surface containing sites of Fe0 that complex and reduce RX. Sequestration of a contaminant refers to a removal process which does not involve contaminant degradation. Sequestration by Fe0 typically occurs via adsorption, reduction, and coprecipitation. Often, adsorption is only a prelude to other processes which do transform the contaminant in order to assure that the process cannot be reversed. However, there are cases where adsorption is the sequestration process of primary importance, especially with metals that occur as soluble cations which cannot be reduced to insoluble forms by Fe0. It also can be true in heavy metals, such as Cd, Cu, Hg, etc., which exist predominantly as soluble cations but could be reduced to insoluble species by Fe0.