Nanotechnology and in Situ Remediation: A Review of the Benefits and Potential Risks

Objective Although industrial sectors involving semiconductors; memory and storage technologies; display, optical, and photonic technologies; energy; biotechnology; and health care produce the most products that contain nanomaterials, nanotechnology is also used as an environmental technology to protect the environment through pollution prevention, treatment, and cleanup. In this review, we focus on environmental cleanup and provide a background and overview of current practice; research findings; societal issues; potential environment, health, and safety implications; and future directions for nanoremediation. We do not present an exhaustive review of chemistry/engineering methods of the technology but rather an introduction and summary of the applications of nanotechnology in remediation. We also discuss nanoscale zerovalent iron in detail. Data sources We searched the Web of Science for research studies and accessed recent publicly available reports from the U.S. Environmental Protection Agency and other agencies and organizations that addressed the applications and implications associated with nanoremediation techniques. We also conducted personal interviews with practitioners about specific site remediations. Data synthesis We aggregated information from 45 sites, a representative portion of the total projects under way, to show nanomaterials used, types of pollutants addressed, and organizations responsible for each site. Conclusions Nanoremediation has the potential not only to reduce the overall costs of cleaning up large-scale contaminated sites but also to reduce cleanup time, eliminate the need for treatment and disposal of contaminated soil, and reduce some contaminant concentrations to near zero—all in situ. Proper evaluation of nanoremediation, particularly full-scale ecosystem-wide studies, needs to be conducted to prevent any potential adverse environmental impacts.

To date, only a small fraction of site reme diation has been conducted by the U.S. EPA. Most cleanup is funded by public and private property owners who are potentially respon sible for the contamination (U.S. EPA 2004). The U.S. EPA (2004) estimated that it will take 30-35 years and cost up to $250 bil lion to clean up the nation's hazardous waste sites. The U.S. EPA (2004) anticipates that these high costs will provide an incentive to develop and implement cleanup approaches and technologies that will result in "better, cheaper, and faster site cleanups." Developing costeffective, in situ groundwater treatment technologies could save billions of dollars in cleanup costs. Figure 1A shows U.S. EPA estimates of the total number of hazardous waste sites in the United States (U.S. EPA 2004). State and private property owners make up the larg est percentage (51%), followed by sites with underground storage tanks (43%). Together, they account for nearly 94% of all hazard ous waste sites in the United States. Most of these sites have similar contaminants, such as solvents and other organics, metals, and petro leum products. Although DOD and DOE sites constitute < 4% of the total number of sites, they tend to be larger and more complex. Figure 1B shows estimates for the total cleanup costs associated with these sites. Although state and privateparty sites make up 51% of the total hazardous waste sites, they represent only 14% of the total costs. Most of the estimated costs to remediate U.S. hazardous wastes sites are borne by RCRACA (21%), DOE (17%), and DOD (16%) sites. Together, these add up to $89 billion, or 54% of the total market.
More than 80% of NPL sites have con taminated groundwater. This is particularly important considering that more than half of the U.S. population relies on groundwater for drinking. Once groundwater is polluted, its remediation is often protracted, costly, and sometimes infeasible.
Pump and treat. Early treatment rem edies for groundwater contamination were primarily pumpandtreat operations. This method involves extracting contaminated groundwater via wells or trenches and treat ing the groundwater above ground (ex situ) using processes such as air stripping, carbon adsorption, biological reactors, or chemical precipitation (U.S. EPA 2001). Many of these processes produce highly contaminated wastes that then have to be disposed.
The U.S. EPA (2001) studied the aver age operating costs of pumpandtreat sys tems at 32 Superfundfinanced sites and found the annual cost to be approximately $767,000/site. The average pumpandtreat system operated for 5 years, treating an aver age 118 million gallons of water per site for an average cost of $9.4 million to clean up a single site (U.S. EPA 2001). Many of these sites have ongoing monitoring, which con tinues to incur annual operating costs. Sites contaminated with nonaqueousphase liquids (NAPLs) tend to operate for longer periods of time, incurring even higher average costs (U.S. EPA 2004).
Pumpandtreat projects represent the largest number of treatments at Superfund sites, 38% (725 of 1,915). Of the 1,915 treatment remedies tracked by the U.S. EPA, 36% (687 projects) have been completed or shut down. However, only 11% of these 687 projects are pumpand treat projects (U.S. EPA 2008a).
In situ remediation. A common type of in situ, or belowground, remediation method currently used to clean up contaminated groundwater is the permeable reactive barrier (PRB). PRBs are treatment zones composed of materials that degrade or immobilize con taminants as the groundwater passes through the barrier. They can be installed as perma nent, semipermanent, or replaceable barriers within the flow path of a contaminant plume. The material chosen for the barrier is based on the contaminant(s) of concern (U.S. EPA 2001). One drawback of PRBs is that they can only remediate contaminant plumes that pass through them; they do not address dense NAPLs (DNAPLs) or contaminated ground water that is beyond the barrier.
Other in situ treatment technologies include thermal treatment (steamenhanced extraction, electrical resistive heating, or thermal conduc tive heating), chemical oxidation, surfactant cosolvent flushing, and bio remediation.
Because of the high cost and lengthy oper ating periods for pumpandtreat remedies, use of in situ groundwater treatment technolo gies is increasing. Remedies selected for NPL sites are documented in records of decision (RODs). A ROD provides the justification for the remedial action (treatment) chosen at a Superfund site. The percentage of RODs that selected in situ groundwater treatment went from zero in fiscal years [1982][1983][1984][1985][1986]

Nanoremediation
Nanoremediation methods entail the appli cation of reactive nano materials for trans formation and detoxification of pollutants. These nano materials have properties that enable both chemical reduction and cataly sis to mitigate the pollutants of concern. For nano remediation in situ, no ground water is pumped out for aboveground treatment, and no soil is transported to other places for treat ment and disposal (Otto et al. 2008).
Nanomaterials have highly desired prop erties for in situ applications. Because of their minute size and innovative surface coatings, nano particles may be able to pervade very small spaces in the subsurface and remain sus pended in ground water, allowing the particles to travel farther than larger, macrosized parti cles and achieve wider distribution. However, in practice, current nano materials used for remediation do not move very far from their injection point (Tratnyek and Johnson 2006 Table 1, and used for remediation, see Theron et al. (2008) and Zhang (2003).] nZVI. nZVI particles range from 10 to 100 nm in diameter, although some vendors sell micrometerscale iron powders as "nano particles." Typically, a noble metal (e.g., palla dium, silver, copper) can be added as a catalyst. The second metal creates a catalytic synergy between itself and Fe and also aids in the nano particles' distribution and mobility once injected into the ground Tratnyek and Johnson 2006;U.S. EPA 2008b). These BNPs may contain more than two dif ferent metals. The second metal is usually less reactive and is believed to promote Fe oxidation or electron transfer (U.S. EPA 2008b). Some noble metals, particularly palladium, catalyze dechlorination and hydrogenation and can make the remediation more efficient (U.S. EPA 2008b; Zhang and Elliott 2006).
The underlying chemistry of the reaction of Fe with environmental pollutants (par ticularly chlorinated solvents) has been exten sively studied and applied in micrometerscale ZVI PRBs (Matheson and Tratnyek 1994). There are two main degradation pathways for chlorinated solvents: beta elimination and reductive chlorination. Beta elimination occurs most frequently when the contaminant comes into direct contact with the Fe particle. The following example shows the pathway of trichloro ethene (TCE): TCE + Fe 0 → Hydrocarbon products + Cl -+ Fe 2+ /Fe 3+ .
Under reducing conditions fostered by nZVI in groundwater, the following reaction takes place: where PCE is perchloroethylene, DCE is dichloroethylene, and VC is vinyl chloride (Tratnyek 2003, U.S. EPA 2008b. In the 1990s, Fe at the nano scale was synthesized from Fe(II) and Fe(III) to pro duce particles ranging from 10 to 100 nm, initially using boro hydride as the reductant, and examined in laboratory studies. Zhang (2003) tested nZVI for the transformation of a large number of pollutants, most notably halogenated organic compounds commonly detected in contaminated soil and groundwa ter. The author reported that nano scale Fe par ticles are very effective for the transformation and detoxification of a variety of common environmental pollutants, including chlori nated organic solvents, organochlorine pesti cides, and polychlorinated biphenyls (PCBs). According to Zhang (2003), Femediated reactions should produce an increase in pH and a decrease in the solution redox potential created by the rapid consumption of oxygen, other potential oxidants, and the production of hydrogen. Although batch reactors pro duce pH increases of 2-3 and an oxidationreduction potential (ORP) range of -500 to -900 mV, it is expected that the pH and ORP would be less dramatic in field applications where other mechanisms reduce the chemical changes (Zhang 2003). Previous work show ing an increase of pH by 1 and an ORP in the range of -300 to -500 mV supports this assess ment (Elliott and Zhang 2001;Glazier et al. 2003). Zhang (2003) also showed that modify ing Fe nano particles could enhance the speed and efficiency of the remediation process.
The first field application was reported in 2000 (Zhang 2005). Nano particles have been shown to remain reactive in soil and water for up to 8 weeks and can flow with the ground water for > 20 m. In one study, Zhang (2003) produced a 99% reduction of TCE within a few days of injection.
Because nano scale particles are so small, Brownian movement or random motion, rather than wall effects, dominates their physi cal movement or transport in water. The move ment of micrometerscale particles, especially micro scale metal particles, is largely controlled by gravityinduced sedimentation because of their size and high density. In the absence of significant surface electro static forces, nano sized particles can be easily suspended in water during the design and manufacturing stages, thus providing a versatile remediation tool that allows direct injection as a liquid into the subsurface where contaminants are present. Coating the Fe particles to improve mobility and catalytic reaction rates is important. Some of the particles flow with the ground water and remain in suspension for various amounts of time, whereas others are filtered out and bind to soil particles, providing an in situ treatment zone that could hold back emanating plumes (Henn and Waddill 2006).
The high reactivity of nZVI particles is in part a direct result of their high specific surface area. For example, nZVI produced by the boro hydride method has surface areas in the range of 20-40 m 2 /g, which can yield 10-1,000 times greater reactivity compared with granular Fe, which has a surface area < 1 m 2 /g (Wang and Zhang 1997). nZVI's small particle size also allows more of the material to penetrate into soil pores, and it can be more easily injected into shallow and deep aquifers, a property that is particularly beneficial when contamination lies underneath a building.
Initially, Fe nano particles have a core of ZVI and an outer shell of Fe oxides, which suggest the following redox reactions: where s is solid, aq is aqueous, g is gas, and l is liquid (Matheson and Tratnyek 1994).
Although Fe nano particles have been shown to have a strong tendency to form micro scale aggregates, possibly because of their weak surface charges, coatings can be applied to change the surface properties. These different forms of Fe could be useful for the separation and transformation of a variety of contaminants, such as chlorinated organic solvents, organochlorine pesticides, PCBs, organic dyes, various inorganic compounds, and the metals As(III) (trivalent arsenic), Pb(II) (bivalent lead), copper [Cu(II) (bivalent copper)], Ni(II) (bivalent nickel), and Cr(VI) (hexa valent chromium) (Sun et al. 2006).
Nanoremediation, particularly use of nZVI, has sitespecific requirements that must be met in order for it to be effective. Adequate site characterization is essential, including information about site location, geologic con ditions, and the concentration and types of contaminants. Geologic, hydro geologic, and sub surface conditions include composition of the soil matrix, porosity, hydraulic conductiv ity, ground water gradient and flow velocity, depth to water table, and geochemical prop erties (pH, ionic strength, dissolved oxygen, ORP, and concentrations of nitrate, nitrite, and sulfate). All of these variables need to be evaluated before nano particles are injected to determine whether the particles can infiltrate the remedia tion source zone, and whether the conditions are favorable for reductive transformation of contaminants. The sorp tion or attachment of nano particles to soil and aquifer materials depends on the surface chemistry (i.e., electrical charge) of soil and nano particles, ground water chemistry (e.g., ionic strength, pH, and presence of natural organic matter), and hydro dynamic condi tions (pore size, porosity, flow velocity, and degree of mixing or turbulence). The reac tions between the contaminants and the nZVI depend on contact or probability of contact between the pollutant and nano particles (U.S. EPA , 2008b. In field tests, Henn and Waddill (2006) found that, with the use of nZVI, decreases in parent pollutant compound concentrations (TCE and trichloroethane) were accompa nied by increases and subsequent decreases in daughter product concentrations (cis1,2DCE, 1,1dichloro acetic acid, 1,1DCE, and vinyl chloride). Longterm observations indicated volume 117 | number 12 | December 2009 • Environmental Health Perspectives that although the degradation was continuous, it was at a much slower rate for the daughter products. Their study concluded that there was overall reduction in contaminants, a reduced plume size, and reduction in the contaminant mass flux emanating from the source. The nano scale Fe created conditions for abiotic degradation for about 6-9 months, followed by biological degradation as the primary degra dation process. Both processes had significant impacts on the degradation of contaminants (Henn and Waddill 2006). Cao et al. (2005) found that nZVI particles in an aqueous solu tion reduced perchlorate to chloride almost completely without producing intermediate degradation products.
Fe oxide nano particles have been shown to bind As irreversibly up to 10 times more effec tively than micrometersized particles. Based on their superparamagnetic properties, the Fe particles and bound As can be separated from the water with a magnetic field. Laboratory tests have shown 99% removal of As using 12nmdiameter Fe oxide nano particles (Rickerby and Morrison 2007). Kanel et al. (2006) concluded that nZVI can reduce As(V) to As(III) in a short period of time at neu tral pH. They also found that a high amount of nZVI was needed to completely remove As(V), possibly because of the presence of dis solved organic carbon, sulfate, and phosphate.
The hydrophilic properties of nZVI enable the remediation of aqueousphase con taminants, including DNAPLs. Because the addition of nZVI in the source zone reduces contaminants, it increases the concentra tion gradient between the aqueous phase and DNAPLs, thereby increasing the mass transfer of contaminants from DNAPLs to the dissolved aqueous phase, where they are then treated (Watlington 2005). To address DNAPLs directly, emulsified ZVI (eZVI) has been used. When the emulsion droplets come into contact with dissolved TCE, the contaminant diffuses into the interior of the emulsion droplet, where it comes into contact with the ZVI and is degraded. A concentra tion gradient is established by migration of the TCE molecules into the interior aqueous phase of the emulsion droplet and by migration of the byproducts out of the droplet and into the surrounding water phase, further driving the degradation reactions (O'Hara et al. 2006).
In other field test research conducted between 2003 and 2005 in North America and Europe, nZVI was effective in treating various compounds in groundwater, including chlorinated solvents and Cr(VI) (Macé et al. 2006). These field tests showed that the con centrations of chlorinated solvents decreased dramatically during the first few hours and days after injection and remained low in con junction with the mass balance of nZVI ver sus the mass of chlorinated hydrocarbons in the remediation area. When present, sulfates and nitrates also decreased in parallel with the chlorinated solvents, but diminished the effect of nZVI on other solvents. Macé et al. (2006) found that nZVI moved with groundwater away from the injection site. Based on this, they hypothe sized that nZVI could treat larger areas of the affected aquifers. They found dramatic but shortlived reductions of volatile organic com pounds (VOCs) in fractured bedrock and a slower, steadier decrease of VOCs in primary porosity aquifers. The same study suggested that the degradation of VOCs and travel velocity are indirectly proportional to the hydraulic conductivity. BNPs reacted more quickly and were spent more rapidly than nZVI, whereas nZVI reacted more slowly but had a longer effect. Macé et al. (2006) noted minor but inconclusive changes to the micro bial community due to the addition of nano particles. These changes could affect parallel bio remediation.
In an extensive study, the Navy con ducted field tests using nZVI to remedi ate two of its contaminated sites (Naval Air Engineering Station, Lakehurst, NJ, and Naval Air Station, Jacksonville, FL) and using micrometersized ZVI powder at a third site, Hunters Point Shipyard, Hunters Point, California (Gavaskar et al. 2005). In the Jacksonville study, TCE concentrations in a well approximately 20 ft from the source zone were reduced up to 99%, suggesting that some of the nZVI migrated outside of the treatment zone through preferential pathways. Longterm monitoring of the treatment zone was recommended to demonstrate that the decline in parent compounds (e.g., TCE) and byproducts (e.g., cis1,2DCE) persists after the ZVI is depleted, which will permit deter mination of how much, if any, DNAPL mass truly remains in the treatment zone (Gavaskar et al. 2005).   In addition to groundwater remediation, nano technology holds promise in reducing the presence of NAPLs. Recently, a material using nanosized oxides (mostly calcium) was used in situ to clean up heating oil spills from underground oil tanks. Preliminary results from this redoxbased technology suggest faster, cheaper methods and, ultimately, lower overall contaminant levels compared with pre vious remediation methods. Most of these sites have been in New Jersey, with cleanup con ducted in consultation with the New Jersey Department of Environmental Protection (see Continental Remediation LLC 2009).
The state of the practice. The number of actual applications of nZVI is increasing rap idly. Only a fraction of the projects has been reported, and new projects show up regularly. Figure 2 and Supplemental Material, Table 2 (doi:10.1289/ehp.0900793.S1) describe 44 sites where nano remediation methods have been tested for site remediation. These sites are in seven countries (including the United States) and in 12 U.S. states. All of the sites have some form of chlorinated compounds of concern, such as PCE, TCE, or PCBs. Other pollutants include Cr(VI) and nitrate. The sites include oil fields, manufacturing sites, military installations, private properties, and residences. Figure 3A shows the types of manufac tured nano particles used for remediation at the sites. More than twothirds of the sites are treated with a form of ZVI, and most of the BNPs contain Fe. Figure 3B shows the types of media treated at these sites. More than threefourths of the sites treated contain contaminated groundwater.
Supplemental Material, Table 2 (doi:10. 1289/ ehp.0900793.S1) lists details of the 44 sites treated with nano materials and the results of the treatment. Because data from most of these sites were not collected as part of a research project, the information is general and, in most cases, incomplete. For example, site 2 is a BP Global site in Alaska contami nated with trichloro ethane; when BNPs were used, practitioners saw reductions of 60% and 90% for shallow test and deep test con centrations, respectively. At site 11, in the Czech Republic, nZVI was used to reduce chlorinated solvents. Levels were reduced to an order of magnitude lower than origi nal concentrations and were maintained for 6 months. Supplemental Material, Table 2, provides an overview of the current state of the practice using nano materials, mainly ZVI, for site remediation.
Because many of the remediation projects using nano particles are just beginning or are ongoing, cost and performance data are lim ited. However, as the technology is applied at an increasing number of sites with varying geologies, more data will become available on performance, cost, and environmental aspects, thereby providing site managers and other stakeholders with additional information to determine whether the technology might be applicable to their specific sites. PARS Environmental Inc. (2004) con ducted a casestudy cost comparison of a manufacturing site in New Jersey where the primary contaminants of concern were TCE and PCE. They estimated that using the pumpandtreat method would cost approxi mately $4,160,000 and PRB approximately $2,200,000. nZVI would cost approxi mately $450,000, representing a cost savings of 80-90% over the pumpandtreat method. Table 1 indicates the relative magnitude of the media and contaminant group at four types of remediation sites. Using Table 1 and Figure 1B, the cost savings to remedi ate groundwater can be estimated for NPL, RCRA, DOD, and DOE sites. Using nano remediation, potential savings of $87 billion to $98 billion can be realized to clean up the nation's hazardous waste sites over the next 30 years. Although this estimate is based on publicly available data and assumes use of nZVI or a variation of nano remediation for all sites with contaminated ground water, it is a reasonable estimate of the magnitude of cost savings achievable using this technol ogy. Increased manufacturing capacity to supply the amount of nano materials needed could lead to lower costs from economies of scale. It should be noted, however, that not all sites have conditions suitable for nano scale remediation methods.
In addition to the potential cost savings associated with using nano technology for site remediation, the amount of time required to clean up a site could be greatly reduced. The average pumpandtreat system operates for about 18 years (U.S. EPA 2001). In a study using nZVI, Zhang (2003) observed a 99% reduction in TCE levels within days of injection. This shortened time interval not only reduces operating costs but also reduces the time that workers are exposed to a con taminated site during cleanup. Environmental disturbances that can affect the local eco system's flora, fauna, and micro organisms are reduced because nZVI is injected using small wells instead of excavating soils or removing ground water using pumpandtreat methods; the time of site disturbance is also shorter.

Potential Implications
Fate and transport. When released into the environment, manufactured nano particles aggregate to some degree and behave like natural nano materials. However, to be effec tive, nZVI needs to form stable dispersions in water so it can be delivered to watersaturated porous material in the contaminated area. Yet, its rapid aggregation limits its mobility ). The rapid aggregation of the nano scale Fe particles supports the need for polymer or other coatings to modify the nZVI surface in order to improve mobility .
Depending on the composition of ground water and the hydrologic conditions, certain nano scale colloids have the ability to travel unexpectedly large distances in the environ ment (Kersting et al. 1999;Novikov et al. 2006;Vilks et al. 1997). They could form stable nano clusters in ground water that are likely to be highly mobile, carrying with them surfacesorbed contaminants. These natural particles can carry materials between redox zones and facilitate or inhibit contaminant transport (Waite et al. 1999).
The mobility of natural or synthetic nano particles in the natural environment will strongly depend on whether the nano particles remain completely dispersed, aggregate and settle, or form mobile nanoclusters. Gilbert et al. (2007) suggested that many manufac tured metal oxide and other inorganic nano particles will exhibit clusterforming behavior similar to that of natural nano particles. Despite numerous observations that nano scale minerals represent an important fraction of the environ mental colloids, the fundamental aggregation and transport properties of nano particles have not been extensively studied.
In addition to selfaggregation, nano particles could associate with suspended solids or sediment, where they could bioaccumulate and enter the food chain or drinking water sources. These fate processes depend on both the charac teristics of the particle and the char acteristics of the environmental system (Boxall et al. 2007).
The use of nano particles in environmental remediation will inevitably lead to the release of nano particles into the environment and subsequent ecosystems. To understand and quantify the potential risks, the mobility, bioavailability, toxicity, and persistence of  (Nowack 2008). To pose a risk, nano particles must be hazardous and have a route for exposure. Although aggregated and/or absorbed nano particles are usually less mobile, they still have the potential to be taken up by filter feeders and other sedimentdwelling organisms. The U.S. EPA has raised the pos sibility of biomagnification of nano particles; however, no data currently exist proving or disproving this hypothesis (Biswas and Wu 2005;. To be able to quan tify the stability of nano particles in the envi ronment, the stability of their suspensions and their tendency to aggregate and interact with other particles must first be determined (Mackay et al. 2006). Potential toxicity. Naturally occurring nano scale Fe oxide particles with metals (such as copper) bound to their surface have been found many kilometers downstream from mining sites, indicating the ability of these colloidal nano particles to move and transport sorbed contaminants (Hochella et al. 2005). These binding properties and processes may show sizedependent reactivity on crystal line Fe oxide nano particles, and each process might occur with different thermo chemical and kinetic relationships as a function of size (Madden et al. 2006). Thus, whereas the nano particles themselves may not possess toxic properties, the pollutants they could carry with them may. Fe nano materials may bind with and carry copper, which has a toxicity threshold for algae, flowering plants, fungi, and phyto plankton that is surpassed only by mercury and sometimes silver (Sposito 1989). Handy et al. (2008) suggested that despite the environment containing many natural par ticles at the nano scale, manufactured nano particles may act differently. These materials are designed to have specific surface properties and chemistries that are not likely to be found in natural particles. The properties of manu factured nano particles enhance novel physico chemical and possibly toxicologic properties compared with natural particles. A range of eco toxicologic effects of various manufactured nano materials has been reported, including effects on microbes, plants, invertebrates, and fish (Boxall et al. 2007). Laboratory stud ies using fish, Daphnia, copepods, and other organisms (Adams et al. 2006;Fortner et al. 2005;Lovern et al. 2007;Oberdörster et al. 2006) have shown that these organisms can take up some manufactured nano particles.
The factors and processes affecting eco toxicity are complex, and the impact of manufactured nano particles on organisms is determined by a range of properties, including dissolution potential, aggregation potential, particle surface properties, the characteristics of the exposure environment, and the bio chemical, physiological, and behavioral traits of the organism being exposed (Dhawan et al. 2006). Although available data indicate that current risks of manufactured nano particles in the environment to environmental and human health are probably low (see Table 3 in Boxall et al. 2007), knowledge of their poten tial impact in the environment and on human health is still limited.
Research on ultrafine particulates (< 100 nm in one dimension) has shown that as particle size decreases, potential for pul monary toxicity tends to increase even if the material's larger form is inert. nZVI is typically between tens and hundreds of nanometers in size at the time of production. Under labora tory conditions, these particles tend to aggre gate and produce clusters that can build up to the micrometer size. If this occurs, they will not take on the properties that apply to actual nano sized particles and will behave similarly to larger environmental colloids (Tratnyek and Johnson 2006).
Inhalation exposure to Fe 0 (s) nano particles could result in the release of Fe(III), followed by oxidative damage due to generation of Fe(IV) (Keenan and Sedlak 2008). In vitro studies examining the response of the cen tral nervous system to low concentrations of nanoFe and nanomagnetite showed that these nano particles are taken up into cells and pro duce an oxidative stress response (Wiesner et al. 2006). These studies indicate a potential for adverse health effects from exposure and uptake of Fe oxide nano particles into mam malian cells. The authors caution, however, that these tests were conducted at much higher dosages than would be encountered normally (Wiesner et al. 2006).
In some cases, Fe oxide nano particles (a potential end product from redox reactions of nZVI) can be internalized by cells and cause cell death. Low solubility of Fe oxide nano particles enables them to persist in biological systems and could potentially induce long term effects involving mutagenic influence on organisms (Auffan et al. 2006). However, there are limited data on the interactions of Fe oxide nano particles with cells and the effect that coatings can have on cell adhesion, inter nalization, and interaction.
Mineral nano particles are common com ponents of natural aqueous systems. Several natural inorganic and biologically mediated processes produce mineral nano particles, such as metal sulfides and metal oxides (Labrenz et al. 2000;Villalobos et al. 2003). Nanoscale Fe (oxy)hydroxide phases are among the most common natural mineral nano particles formed by precipitation from solution after oxidation of aqueous ferrous Fe ( Van der Zee et al. 2003). Although Fe is an essen tial element for growth in nearly all species, an abundance of free chelating Fe has been linked to DNA damage, lipid peroxidation, and oxidative protein damage in vivo (Valko et al. 2005).
Particle coating, surface treatments, surface excitation by ultraviolet radiation, and particle aggregation can modify the effects of particle size, suggesting that some nano particles could exert their toxic effects as aggregates or through the release of toxic chemicals (Nel et al. 2006). Although the aggregates are fractallike, they may exhibit some of the properties of the dis crete nano particles, including specific surface area and reactivity, particularly because these particles have been manufactured at the nano scale in order to harness particular nano scale properties.
Generally, little concern has been raised about the toxicity of nZVI because Fe oxides formed during remediation are already pres ent in the form of rust and because the nano Fe particles have not been found to produce radically new properties, compared with microscalesized Fe particles (Watlington 2005). Whether the addition of catalytic coatings changes these properties or presents another hazard has yet to be determined. Oberdörster et al. (2006) suggested that toxic ity studies should not simply focus on human and wildlife but should also examine benthic and soil flora and fauna, because they make up the basis of food chains. Biological systems did not evolve alongside the nano particles that are now being manufactured and released (Moore 2006). Different reactions to nZVI may be found in some lower organisms.
The Royal Commission on Environmental Pollution (2008) summed up the current approach to potential implications from nano materials: While there have been no significant events that would lead us to suppose that the contemporary introduction of novel materials is a source of environmental hazard, we are acutely aware of past instances where new chemicals and products, originally thought to be entirely benign, turned out to have very high environmental and public health costs.

Societal Issues
Most societal issues are based on the unknown risks of using nano scale materials for site reme diation. At one end of the spectrum, some non governmental groups invoked the precautionary principle in an attempt to halt all use of the technology until proven safe. In early 2003, the ETC Group called for the precautionary principle to be applied to nano technology (ETC Group 2003). They based their concerns on Eric Drexler's concept of multiple nano scale machines that might selfreplicate and change matter into "gray goo" (Drexler 1986). Drexler later clarified this image (Phoenix and Drexler 2004), but not before Prince Charles of England became concerned enough about the risks of nano technology to ask the Royal Society to examine the implications of nano technology. In one part of their report (Royal Society and Royal Academy of Engineering 2004), the Royal Society came out strongly against the use of nano materials for remediation.
We recommend that the use of free (that is, not fixed in the matrix) manufactured nano particles and environmental applications such as remedia tion be prohibited until appropriate research has been undertaken and it can be demon strated that the potential benefits outweigh the potential risks.
In contrast, the European Commission's Scientific Committee on Emerging and Newly Identified Health Risks in 2005 (European Commission 2005) listed environ mental reme diation technology as one of nano technology's benefits. This group also called for riskrelated research.
In a position paper, the Québec Commission (Commission de l'Ethique de la Science et la Technologie 2006) indicated that [T]he biggest source of potential environmental exposure is the use of nano particles in sanitizing contaminated groundwater and soil; concerns have been raised about the impact the high reactivity of nano particles might have on plants, animals, microorganisms, and ecosystems.
The report noted "the importance of increasing the amount of research on the potential envi ronmental consequences of nano technology in order to determine which substances may be hazardous." Other risk framework documents have recommended research into the toxicity, fate and transport, and bioaccumulation of released nano materials (Maynard and Aitken 2006). A U.S. EPA white paper (U.S. EPA 2007) pointed out the positive aspects of using nano materials in environmental remediation while also calling for research on the possible negative effects.
In June 2007, DuPont and Environmental Defense released their nano risk framework (Medley and Walsh 2007). They chose ZVI nano particles as a case study. After going through the steps in the framework to assess the potential risk of using this technology, DuPont (2007) decided it "would not con sider using this technology at a DuPont site until the end products of the reactions follow ing injection, or following a spill, are deter mined and adequately assessed." DuPont did not use their full output worksheet in this case study because of the lack of environmental, health, and safety data.
Although there is no consensus among these various reports on nano technology risk management, no doubt is expressed about the potential efficacy of the technology. However, the concerns over safety may limit the wide spread deployment of nano remediation. The reports cited above, as well as other published reports, consistently call for research specific to the possible risks of using nano technology in environmental remediation applications.
The consensus is caution, not precaution, and, in the absence of definitive risk data, the tech nology is generally viewed as more beneficial than harmful.

Recommendations
Develop analytical tools to measure and monitor manufactured nano particles in the environment. Currently, standard meth ods to readily detect and monitor nano particles in the environment do not exist. There are only a few quantitative analytical techniques for measuring nano particles in environmental systems, and most of these are timeconsuming and require expensive equipment and expertise. Because there is no regulatory requirement to monitor environ mental nano particles, or other particles such as those in drinking water, there is a critical lack of data and information about the occur rence and fate of nano particles once they are released into the environment. Some models and extrapolations attempt to quantify the amount of nano particles in various environ mental systems. However, these models are based on estimates of nano particles released into the environment and have not been cali brated with actual measurements in the field (Mueller and Nowack 2008). It is difficult, if not impossible, to extrapolate the toxicity and pathology of nano particles at the eco system level until sufficient baseline data on these particles are gathered (Moore 2006). There are also no biomarkers that can be used to track nano particles as part of a biological monitoring program; although existing regu latory toxicity tests could be appropriate for nano particles, a risk analysis would not be possible without proper measurement of the concentrations of nano particles in the envi ronment (Handy et al. 2008).
Increase research to evaluate the effects of nano particles on the full ecosystem. Nowack and Bucheli (2007) concluded that results from ecotoxicologic studies show that organisms are affected by certain nano particles under certain environmental conditions. However, the stud ies were conducted using elevated concentra tions of pristine nano particles. The authors recommended that future studies estimate the exposure to functionalized nano particles, because most manufactured nano particles are functionalized, which changes their behavior. Changes by environmental factors such as light, oxidants, and micro organisms-which result in chemical or biological modifications or degradation of the functionalized surface or coating of the surface with natural com pounds-are important processes that have not been studied thoroughly (Nowack and Bucheli 2007). In addition, most nano particles are released embedded in a matrix and not as single nano particles (Koehler et al. 2007). It is important to study nano particles in the form in which organisms in the ecosystem and humans might be exposed to them.
The properties that can be harmful to the environment are the very same properties that are advantageous and exploited during treat ment and remediation regimes. For instance, the catalytic properties of nano particles that induce the degradation of pollutants can also induce a toxic response when taken up by cells. In addition, the high sorption capacity of nano particles that is used to remove organic and inorganic pollutants from ground water may also sequester and transport other pol lutants in the environment (Nowack 2008). As such, more work is needed on transfers in environmental systems, for example, from the environment to the organism and throughout the trophic structure.
Further research is needed to develop and understand the mechanisms affecting the fate and transport of manufactured nano particles in water, soil, and sediments; their interac tions with each other, other manufactured nano particles, suspended solids, and dissolved organic material; and how these inter actions are influenced by different environmental variables. The potential for manufactured nano particles to act as carriers for other envi ronmental contaminants also requires further examination.
Improve engineering applications using nano technology for in situ remediation. There is a need to develop "smarter" nano materials for remediation. For example, new coatings or functional groups could enhance mobil ity in groundwater. More sophisticated nano materials may have the ability to perform several functions, such as catalyzing several different pollutant reactions on the same par ticle or interacting with both hydro phobic and hydro philic pollutants. We can build in self termination for active nano particles so they become benign after their remediation function is finished; design nano particles that destroy a wide spectrum of pollutants; and improve delivery systems for injecting nano particles into contaminated groundwater plumes.
All these engineering improvements can increase the ability of this technology to reme diate more of the world's hazardous waste sites. Engineering more effective particles can improve the ability to reach and remediate pol lutant plumes and minimize potential harm.

Conclusions
In situ nanoremediation methods entail the application of reactive nano materials for transformation and detoxification of pollut ants in situ. These nano materials have proper ties that enable both chemical reduction and catalysis to mitigate the pollutants of concern. No groundwater is pumped out for above ground treatment, and no soil is transported to other places for treatment and disposal. volume 117 | number 12 | December 2009 • Environmental Health Perspectives Nano scale Fe particles are effective for the remediation and transformation of a variety of environmental contaminants. Because of the high cost and lengthy operating periods for pumpandtreat remedies, in situ ground water treatment technologies are increasing. The number of actual applications of nZVI is increasing rapidly. Only a fraction of the projects have been reported, and new projects show up regularly. Although the technology is likely a beneficial replacement of current prac tices for site remediation, potential risks are poorly understood. The factors and processes affecting ecotoxicity are complex, and knowl edge of the potential impacts of manufactured nano particles in the environment on human health is still limited. Most societal issues are based on these unknown risks of using nano scale materials for site remediation.
Nanoremediation has the potential to reduce the overall costs of cleaning up large scale contaminated sites, reduce cleanup time, eliminate the need for treatment and disposal of contaminated dredged soil, and reduce some contaminant concentrations to near zero, and it can be done in situ. In order to prevent any potential adverse environmental impacts, proper evaluation, including fullscale ecosystemwide studies, of these nano particles needs to be addressed before this technique is used on a mass scale.