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Nanoremediationis the use ofnanoparticles forenvironmental remediation. It is being explored to treatground water,wastewater,soil,sediment, or other contaminated environmental materials.[1][2]Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States.[3][4][5] In Europe, nanoremediation is being investigated by the EC funded NanoRem Project.[6] A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale.[7] During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process orin situ application.
Some nanoremediation methods, particularly the use of nano zero-valent iron for groundwater cleanup, have been deployed at full-scale cleanup sites.[2] Other methods remain in research phases.
Nanoremediation has been most widely used for groundwater treatment, with additional extensive research inwastewater treatment.[5][8][9][10] Nanoremediation has also been tested for soil and sediment cleanup.[11] Even more preliminary research is exploring the use of nanoparticles to remove toxic materials fromgases.[12]
Currently,groundwater remediation is the most common commercial application of nanoremediation technologies.[7][8]Usingnanomaterials, especially zero-valent metals (ZVMs), for groundwater remediation is an emerging approach that is promising due to the availability and effectiveness of many nanomaterials for degrading or sequestering contaminants.[13]
Nanotechnology offers the potential to effectively treat contaminantsin situ, avoiding excavation or the need to pump contaminated water out of the ground. The process begins with nanoparticles being injected into a contaminated aquifer via an injection well. The nanoparticles are then transported by groundwater flow to the source of contamination. Upon contact, nanoparticles can sequester contaminants (viaadsorption orcomplexation), immobilizing them, or they can degrade the contaminants to less harmful compounds. Contaminant transformations are typicallyredox reactions. When the nanoparticle is the oxidant or reductant, it is considered reactive.[13]
The ability to inject nanoparticles to the subsurface and transport them to the contaminant source is imperative for successful treatment. Reactive nanoparticles can be injected into a well where they will then be transported down gradient to the contaminated area. Drilling and packing a well is quite expensive. Direct push wells cost less than drilled wells and are the most often used delivery tool for remediation with nanoiron. A nanoparticle slurry can be injected along the vertical range of the probe to provide treatment to specific aquifer regions.[13]
The use of various nanomaterials, including carbon nanotubes and TiO2, shows promise for treatment of surface water, including for purification, disinfection, and desalination.[9] Target contaminants in surface waters include heavy metals, organic contaminants, and pathogens. In this context, nanoparticles may be used as sorbents, as reactive agents (photocatalysts or redox agents), or in membranes used fornanofiltration.[citation needed]
Nanoparticles may assist in detecting trace levels of contaminants in field settings, contributing to effective remediation. Instruments that can operate outside of a laboratory often are not sensitive enough to detect trace contaminants. Rapid, portable, and cost-effective measurement systems for trace contaminants in groundwater and other environmental media would thus enhance contaminant detection and cleanup. One potential method is to separate the analyte from the sample and concentrate them to a smaller volume, easing detection and measurement. When small quantities of solid sorbents are used to absorb the target for concentration, this method is referred to assolid-phase microextraction.[14]
With their high reactivity and large surface area, nanoparticles may be effectivesorbents to help concentrate target contaminants for solid-phase microextraction, particularly in the form ofself-assembled monolayers on mesoporous supports. Themesoporous silica structure, made through a surfactant templatedsol-gel process, gives these self-assembled monolayers high surface area and a rigid open pore structure. This material may be an effective sorbent for many targets, including heavy metals such as mercury, lead, and cadmium, chromate and arsenate, and radionuclides such as99Tc,137CS, uranium, and the actinides.[14]
The small size of nanoparticles leads to several characteristics that may enhance remediation. Nanomaterials are highly reactive because of their highsurface area per unit mass.[3] Their small particle size also allows nanoparticles to enter small pores insoil orsediment that larger particles might not penetrate, granting them access to contaminants sorbed to soil and increasing the likelihood of contact with the target contaminant.[3]
Because nanomaterials are so tiny, their movement is largely governed byBrownian motion as compared to gravity. Thus, the flow of groundwater can be sufficient to transport the particles. Nanoparticles then can remain suspended in solution longer to establish anin situ treatment zone.[15]
Once a nanoparticle contacts the contaminant, it may degrade the contaminant, typically through aredox reaction, oradsorb to the contaminant to immobilize it. In some cases, such as with magnetic nano-iron, adsorbed complexes may be separated from the treated substrate, removing the contaminant.[12] Target contaminants include organic molecules such aspesticides ororganic solvents and metals such asarsenic orlead. Some research is also exploring the use of nanoparticles to remove excessive nutrients such as nitrogen and phosphorus.[12]
A variety of compounds, including some that are used as macro-sized particles for remediation, are being studied for use in nanoremediation.[2] These materials include zero-valent metals likezero-valent iron,calcium carbonate, carbon-based compounds such asgraphene orcarbon nanotubes, and metal oxides such astitanium dioxide andiron oxide.[3][12][16]
As of 2012, nanozero-valent iron (nZVI) was the nanoscale material most commonly used in bench and field remediation tests.[2] nZVI may be mixed or coated with another metal, such aspalladium,silver, orcopper, that acts as acatalyst in what is called a bimetallic nanoparticle.[3] nZVI may also beemulsified with a surfactant and an oil, creating a membrane that enhances the nanoparticle's ability to interact with hydrophobic liquids and protects it against reactions with materials dissolved in water.[1][2] Commercial nZVI particle sizes may sometimes exceed true “nano” dimensions (100 nm or less in diameter).[3]
nZVI appears to be useful for degrading organic contaminants, includingchlorinated organic compounds such aspolychlorinated biphenyls (PCBs) andtrichloroethene (TCE), as well as immobilizing or removing metals.[3][9] nZVI and other nanoparticles that do not require light can be injected belowground into the contaminated zone forin situgroundwater remediation and, potentially, soil remediation.
nZVI nanoparticles can be prepared by using sodium borohydride as the key reductant. NaBH4 (0.2 M) is added into FeCl3•6H2 (0.05 M) solution (~1:1 volume ratio). Ferric iron is reduced via the following reaction:
4Fe3+ + 3BH−
4 + 9H2O → 4Fe0 + 3H2BO−
3 + 12H+ + 6H2
Palladized Fe particles are prepared by soaking the nanoscale iron particles with an ethanol solution of 1wt% of palladium acetate ([Pd(C2H3O2)2]3). This causes the reduction and deposition of Pd on the Fe surface:
Pd2+ + Fe0 → Pd0 + Fe2+
Similar methods may be used to prepared Fe/Pt, Fe/Ag, Fe/Ni, Fe/Co, and Fe/Cu bimetallic particles. With the above methods, nanoparticles ofdiameter 50-70 nm may be produced. The averagespecific surface area of Pd/Fe particles is about 35 m2/g. Ferrous iron salt has also been successfully used as the precursor.[15]
Titanium dioxide (TiO2) is also a leading candidate for nanoremediation and wastewater treatment, although as of 2010 it is reported to have not yet been expanded to full-scale commercialization.[10] When exposed toultraviolet light, such as insunlight, titanium dioxide produceshydroxyl radicals, which are highly reactive and canoxidize contaminants. Hydroxyl radicals are used for water treatment in methods generally termedadvanced oxidation processes. Because light is required for this reaction, TiO2 is not appropriate for undergroundin situ remediation, but it may be used for wastewater treatment or pump-and-treat groundwater remediation.[citation needed]
TiO2 is inexpensive, chemically stable, and insoluble in water. TiO2 has a wideband gap energy (3.2 eV) that requires the use of UV light, as opposed to visible light only, for photocatalytic activation. To enhance the efficiency of its photocatalysis, research has investigated modifications to TiO2 or alternativephotocatalysts that might use a greater portion ofphotons in thevisible light spectrum.[9][17] Potential modifications include doping TiO2 with metals, nitrogen, or carbon.[citation needed]
When usingin situremediation the reactive products must be considered for two reasons. One reason is that a reactive product might be more harmful or mobile than the parent compound. Another reason is that the products can affectthe effectiveness and/or cost of remediation.TCE (trichloroethylene), under reducing conditions by nanoiron, may sequentially dechlorinate toDCE (dichloroethene) andVC (vinyl chloride). VC is known to be more harmful than TCE, meaning this process would be undesirable.[13]
Nanoparticles also react with non-target compounds. Bare nanoparticles tend to clump together and also react rapidly with soil, sediment, or other material in ground water.[18] Forin situ remediation, this action inhibits the particles from dispersing in the contaminated area, reducing their effectiveness for remediation. Coatings or other treatment may allow nanoparticles to disperse farther and potentially reach a greater portion of the contaminated zone. Coatings for nZVI includesurfactants,polyelectrolyte coatings, emulsification layers, and protective shells made fromsilica orcarbon.[1]
Such designs may also affect the nanoparticles’ ability to react with contaminants, their uptake by organisms, and theirtoxicity.[19] A continuing area of research involves the potential for nanoparticles used for remediation to disperse widely and harm wildlife, plants, or people.[20]
In some cases,bioremediation may be used deliberately at the same site or with the same material as nanoremediation. Ongoing research is investigating how nanoparticles may interact with simultaneous biological remediation.[21]
