Zerovalent iron
Encyclopedia
Zerovalent iron and zero.valent metals (ZVI and ZVM, respectively) have a variety of applications ranging from filters to electrode
s 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. The small size of the iron particles means that this method of ZVI groundwater remediation also overlaps with the field of nanotechnology
.
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 compound
s.
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.
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.
, 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.
, consisting of scrap iron of construction grade, has been used as a reactive material for permeable reactive barriers for groundwater remediation
. Reactions are generally believed to occur on the Fe (oxide) surface; however, graphite inclusions have been shown can also serve as a reaction site for 2,4-Dinitrotoluene (DNT)
, etc.
than iron. This indicates that zinc may be used as a replacement for ZVI in dechlorinating chlorinated phenols. Chlorinated phenols are sequentially dechlorinated and thus less chlorinated phenols have been identified as a reduction product.
. The treatment system can contain a 2nd solvent, such as d-limonene, toluene
, or hexane
. The treatment system can contain a thickener, such as calcium stearate
or starch
. The treatment system can contain a stabilizer, such as glycerin, mineral oil
, or vegetable oil. The catalyst consists of zerovalent metal particles, esp. magnesium
or iron
, coated with a catalytic metal, such as palladium
, nickel
, or zinc
. The treatment system provides a paste-like system that is preferably applied to natural media and ex-situ structures.
, nanoparticles (1-100 nm diameter) containing zerovalent iron (nZVI) may be used. There are uncertainties regarding the features of this technology which have made it difficult to engineer applications for optimal performance or to assess the risk to human or ecological health. However, several key facts have been identified related to nZVIs which, if ignored, may lead to misconceptions of the technology. These are as follows: 1) nZVIs used in groundwater remediation
are larger than particles that exhibit "true" nano-size effects 2) the high reactivity of nZVI is mainly due to its high specific surface area
and 3) the mobility of the nZVI particles will, in almost all relevant scenarios, be less than a few meters (one implication of this limited mobility is that human exposure due to remediation
applications of nZVI is likely to be minimal). However, there are still many characteristics of this technology about which very little is known: e.g., how quickly nZVI will be transformed and to what products, whether this residue will be detectable in the environment, and how surface modifications of nZVI will alter its long-term environmental fate and effectiveness for remediation.
The top-down approach for manufacturing nano
-ZVMs starts with a large metal particle (granular or microscale) and breaks it down. To do this, either mechanical or chemical means may be used which include but are not limited to milling, etching, and machining.
The bottom up approach involves "growing" nanoparticle
s via chemical synthesis, self-assembling, positional assembling, etc.
Chromate
is a significant component of many radioactive contaminant plumes. One removal technique involves the addition o scrap metal iron. The reactive surface of ZVI in alkaline waters is a ferrous
coating such as green rust.
This still needs to be cited, comes from the laminate-cover booklet called "Advances in PRBs for Groundwater Treatment"
When selecting the reactive material of the PRB, one must consider the properties of the contaminant. In the case of treating arsenic-contaminated water, materials with a high potential for anion adsorption
and coprecipitation
are desirable. It is also beneficial to have a high potential to reduce the contaminant to an insoluble solid. For the removal of arsenic, the following is the chemical process involved:
FeCl3
+ 3H2O
+ [As
] → [As]Fe(OH)3 + 3H+
+ Cl-
Technetium
, a radioactive material, is of great concern due to its long half-life
and abundance in commercial nuclear waste. Technetium
in lower valency states has a lower solubility and sorbs more strongly to soils than does Te
7+. ZVI has the potential to remove technetium in groundwater.
To remediate
groundwater contaminated by uranium
a treatment has been developed to minimize the mobility of uranyl
by transferring it to a stable phase. Because the exact mechanism of the decontamination process is unknown, numerous forms of ZVI-materials have been synthesized, including carbon steel, low alloy steel, cast iron, and all other iron-containing alloys.
Chloramines pose a threat to aquatic ecosystems that are exposed to discharges of treated wastewater. Dechlorination with sulfite
reduces the most refractory organic chloramines too slowly to produce wastewater effluents that meet current ecosystem protection criteria in the United States. The aqueous-phase reducing agents that offer speed advantages over sulfite alone include dithionite
, thiosulfate
, and iodide-mediated sulfite. Ascorbic acid was even more reactive but was found to be slow relative to sulfite. The potential biological oxygen demand might constrain the choice of aqueous reductants. Metallic iron is shown to reduce inorganic and organic chloramines effectively.
Rapid nitrate reduction by iron powder is observed only at pH
≤4. pH
control with sulfuric acid prolongs nitrate reduction and increases the amount removed. Ammonia is the end product of nitrate reduction and accounts for all nitrate transformed under certain experimental conditions. However, by using nanoscale iron N2 gas is the product instead. Ferrous
species, Fe2+ and Fe(OH)2 are probably not involved in the reaction.
, DDD
, and DDE
. The rates of dechlorination was independent of the amount of iron; however, the rates with a surfactant
(Triton X-114) present were much higher. The rate of dechlorination is limited by the rate of dissolution into the aqueous phase.
. Additionally, viruses can interact with iron in different ways, and thus can be deactivated to different extents.
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s to trenches. One of the emerging uses for ZVI is iron wall remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
. This technology uses ZVIs to form a permeable reactive barrier
Permeable reactive barrier
A permeable reactive barrier , also referred to as a permeable reactive treatment zone , is a developing technology that has been recognized as being a cost-effective technology for in situ groundwater remediation. PRBs are barriers which allow some—but not all—materials to pass through...
(PRB) which filters out contaminants in groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
, leaving only decontaminated groundwater and dissolved iron on the other side of the PRB. The small size of the iron particles means that this method of ZVI groundwater remediation also overlaps with the field of nanotechnology
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
.
History
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 groundwaterGroundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
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
University of Waterloo
The University of Waterloo is a comprehensive public university in the city of Waterloo, Ontario, Canada. The school was founded in 1957 by Drs. Gerry Hagey and Ira G. Needles, and has since grown to an institution of more than 30,000 students, faculty, and staff...
examined the possibility of sample bias caused by sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
of contaminants to well casings and other materials used in groundwater sampling. While contaminants were lost from solution as a result of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
into polymers, contaminant losses were also observed when solutions came into contact with certain metals, and these losses were not consistent with a diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
process. Reductive
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
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 compound
Aliphatic compound
In organic chemistry, aliphatic compounds are acyclic or cyclic, non-aromatic carbon compounds.Thus, aliphatic compounds are opposite to aromatic compounds.- Structure :...
s.
Basic chemistry of iron corrosion
The corrosionRust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
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
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
corrosion: Fe0 + 2H2O → Fe2+ + H2 + 2OH-
Aerobic
Aerobic
Aerobic is a word that means "requiring air", where "air" usually means oxygen.Aerobic may also refer to:* Aerobic exercise, prolonged exercise of moderate intensity* Aerobics, a form of aerobic exercise...
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.
Pathways for contaminant reduction
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 heuristicHeuristic
Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...
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
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
corrosion of Fe2+ might react with RX if a catalyst is present.
Hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
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
Sequestration of a contaminant refers to a removal process which does not involve contaminant degradation. Sequestration by Fe0 typically occurs via adsorptionAdsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
, reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, and coprecipitation
Coprecipitation
In chemistry, coprecipitation or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed...
. 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
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, Hg
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, etc., which exist predominantly as soluble cations but could be reduced to insoluble species by Fe0.
Cast iron
Cast ironCast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
, consisting of scrap iron of construction grade, has been used as a reactive material for permeable reactive barriers for groundwater remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
. Reactions are generally believed to occur on the Fe (oxide) surface; however, graphite inclusions have been shown can also serve as a reaction site for 2,4-Dinitrotoluene (DNT)
High purity
High purity iron indicates commercial, granular iron for powder metallurgyPowder metallurgy
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape , and then heating the compressed material in a controlled atmosphere to bond the material . The powder metallurgy process generally consists of four basic steps: powder manufacture, powder...
, etc.
Zinc and other metals
Zinc has showed much higher reactivity toward pentachlorophenolPentachlorophenol
Pentachlorophenol is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names...
than iron. This indicates that zinc may be used as a replacement for ZVI in dechlorinating chlorinated phenols. Chlorinated phenols are sequentially dechlorinated and thus less chlorinated phenols have been identified as a reduction product.
Bimetallic combinations
Polychlorinated biphenyls (PCBs) are decontaminated using a treatment system including catalytic ZVM particles and an organic hydrogen donating solvent. The solvent includes water and an alcoholAlcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
. The treatment system can contain a 2nd solvent, such as d-limonene, toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
, or hexane
Hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
. The treatment system can contain a thickener, such as calcium stearate
Calcium stearate
Calcium stearate is carboxylate of calcium that is found in some lubricants and surfactants. It is a white waxy powder.-Production and occurrence:Calcium stearate is produced by heating stearic acid, a fatty acid, and calcium oxide:...
or starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
. The treatment system can contain a stabilizer, such as glycerin, mineral oil
Mineral oil
A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....
, or vegetable oil. The catalyst consists of zerovalent metal particles, esp. magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
or iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, coated with a catalytic metal, such as palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, or zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. The treatment system provides a paste-like system that is preferably applied to natural media and ex-situ structures.
Nano aspects of this technology
In addition to using "typical" scale iron in PRBsPermeable reactive barrier
A permeable reactive barrier , also referred to as a permeable reactive treatment zone , is a developing technology that has been recognized as being a cost-effective technology for in situ groundwater remediation. PRBs are barriers which allow some—but not all—materials to pass through...
, nanoparticles (1-100 nm diameter) containing zerovalent iron (nZVI) may be used. There are uncertainties regarding the features of this technology which have made it difficult to engineer applications for optimal performance or to assess the risk to human or ecological health. However, several key facts have been identified related to nZVIs which, if ignored, may lead to misconceptions of the technology. These are as follows: 1) nZVIs used in groundwater remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
are larger than particles that exhibit "true" nano-size effects 2) the high reactivity of nZVI is mainly due to its high specific surface area
Specific surface area
Specific surface area is a material property of solids which measures the total surface area per unit of mass, solid or bulk volume, or cross-sectional area...
and 3) the mobility of the nZVI particles will, in almost all relevant scenarios, be less than a few meters (one implication of this limited mobility is that human exposure due to remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
applications of nZVI is likely to be minimal). However, there are still many characteristics of this technology about which very little is known: e.g., how quickly nZVI will be transformed and to what products, whether this residue will be detectable in the environment, and how surface modifications of nZVI will alter its long-term environmental fate and effectiveness for remediation.
Top-down
The top-down approach for manufacturing nano
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
-ZVMs starts with a large metal particle (granular or microscale) and breaks it down. To do this, either mechanical or chemical means may be used which include but are not limited to milling, etching, and machining.
Bottom-up
The bottom up approach involves "growing" nanoparticle
Nanoparticle
In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...
s via chemical synthesis, self-assembling, positional assembling, etc.
Chromate
Chromate
Chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...
is a significant component of many radioactive contaminant plumes. One removal technique involves the addition o scrap metal iron. The reactive surface of ZVI in alkaline waters is a ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
coating such as green rust.
Arsenic
This still needs to be cited, comes from the laminate-cover booklet called "Advances in PRBs for Groundwater Treatment"
When selecting the reactive material of the PRB, one must consider the properties of the contaminant. In the case of treating arsenic-contaminated water, materials with a high potential for anion adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
and coprecipitation
Coprecipitation
In chemistry, coprecipitation or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed...
are desirable. It is also beneficial to have a high potential to reduce the contaminant to an insoluble solid. For the removal of arsenic, the following is the chemical process involved:
FeCl3
Iron(III) chloride
Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...
+ 3H2O
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
+ [As
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
] → [As]Fe(OH)3 + 3H+
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
+ Cl-
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
Technetium
Technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
, a radioactive material, is of great concern due to its long half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
and abundance in commercial nuclear waste. Technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
in lower valency states has a lower solubility and sorbs more strongly to soils than does Te
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
7+. ZVI has the potential to remove technetium in groundwater.
Uranium
To remediate
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
groundwater contaminated by uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
a treatment has been developed to minimize the mobility of uranyl
Uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
by transferring it to a stable phase. Because the exact mechanism of the decontamination process is unknown, numerous forms of ZVI-materials have been synthesized, including carbon steel, low alloy steel, cast iron, and all other iron-containing alloys.
Chlorine
Chloramines pose a threat to aquatic ecosystems that are exposed to discharges of treated wastewater. Dechlorination with sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
reduces the most refractory organic chloramines too slowly to produce wastewater effluents that meet current ecosystem protection criteria in the United States. The aqueous-phase reducing agents that offer speed advantages over sulfite alone include dithionite
Dithionite
The dithionite anion , is an oxoanion of sulfur formally derived from dithionous acid, H2S2O4.-Chemistry:Dithionous acid has not been detected either as a pure compound or in solution....
, thiosulfate
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
, and iodide-mediated sulfite. Ascorbic acid was even more reactive but was found to be slow relative to sulfite. The potential biological oxygen demand might constrain the choice of aqueous reductants. Metallic iron is shown to reduce inorganic and organic chloramines effectively.
Nitrate
Rapid nitrate reduction by iron powder is observed only at pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
≤4. pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
control with sulfuric acid prolongs nitrate reduction and increases the amount removed. Ammonia is the end product of nitrate reduction and accounts for all nitrate transformed under certain experimental conditions. However, by using nanoscale iron N2 gas is the product instead. Ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
species, Fe2+ and Fe(OH)2 are probably not involved in the reaction.
Pesticides
Iron has been shown to be successful at dechlorinating DDTDDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
, DDD
Dichlorodiphenyldichloroethane
Dichlorodiphenyldichloroethane is an organochlorine insecticide that is slightly irritating to the skin. DDD is a metabolite of DDT. DDD is colorless and crystalline; it is closely related chemically and is similar in properties to DDT, but it is considered to be less toxic to animals than is DDT...
, and DDE
Dichlorodiphenyldichloroethylene
Dichlorodiphenyldichloroethylene is a chemical compound formed by the loss of hydrogen chloride from DDT, of which it is one of the more common breakdown products. DDE is fat soluble which tends to build up in the fat of animals. Due to its stability in fat, DDE is rarely excreted from the body,...
. The rates of dechlorination was independent of the amount of iron; however, the rates with a surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
(Triton X-114) present were much higher. The rate of dechlorination is limited by the rate of dissolution into the aqueous phase.
Viruses
Viruses are especially problematic in the environment because they are more mobile and more resistant to chlorination and filtration than are bacteria. Removal of viruses from groundwater involve several different forms of iron, including Fe0, Fe2+, and magnetiteMagnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
. Additionally, viruses can interact with iron in different ways, and thus can be deactivated to different extents.