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{{Nanotechnology implications}}
{{Nanotech}}
'''Nanoremediation '''is the use of [[nanoparticle]]s for [[environmental remediation]]. It is being explored to treat [[groundwater|ground water]], [[wastewater]], [[soil]], [[sediment]], or other contaminated environmental materials.<ref name=Crane2012>{{Cite journal
| doi = 10.1016/j.jhazmat.2011.11.073
| pmid = 22305041
| issn = 0304-3894
| volume = 211–212
| pages = 112–125
| last = Crane
| first = R. A.
|author2=T. B. Scott
 | title = Nanoscale zero-valent iron: Future prospects for an emerging water treatment technology
| journal = Journal of Hazardous Materials
| series = Nanotechnologies for the Treatment of Water, Air and Soil
| date = 2012-04-15
}}</ref><ref name=EPA1>{{Cite web
| last = U.S. EPA
| title = Nanotechnologies for environmental cleanup
| accessdate = 2014-07-29
| date = 2012-11-14
| url = http://cluin.org/techfocus/default.focus/sec/nanotechnology:_applications_for_environmental_remediation/cat/application/
}}</ref>
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.<ref name = Karn2009>{{Cite journal
| issn = 0091-6765
| volume = 117
| issue = 12
| pages = 1823–1831
| last = Karn
| first = Barbara
|author2=Todd Kuiken |author3=Martha Otto
 | title = Nanotechnology and in Situ Remediation: A Review of the Benefits and Potential Risks
| journal = Environmental Health Perspectives
| date = 2009-12-01
| jstor = 30249860
 | doi=10.1289/ehp.0900793
| pmid = 20049198
| pmc = 2799454
}}</ref><ref>{{Cite web
| last = Project on Emerging Nanotechnologies
| title = Nanoremediation Map
| accessdate = 2013-11-19
| url = http://www.nanotechproject.org/inventories/remediation_map/
}}</ref><ref name=Mueller2012>{{Cite journal
| doi = 10.1007/s11356-011-0576-3
| pmid = 21850484
| issn = 1614-7499
| volume = 19
| issue = 2
| pages = 550–558
| last = Mueller
| first = Nicole C.
|author2=Jürgen Braun |author3=Johannes Bruns |author4=Miroslav Černík |author5=Peter Rissing |author6=David Rickerby |author7=Bernd Nowack
 | title = Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe
| journal = Environmental Science and Pollution Research
| date = 2012-02-01
| url = http://doc.rero.ch/record/311158/files/11356_2011_Article_576.pdf
}}</ref> In Europe, nanoremediation is being investigated by the EC funded NanoRem Project.<ref>{{cite web|title=Nanotechnology for Contaminated Land Remediation|url=http://www.nanorem.eu/|accessdate=3 December 2014}}</ref> A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale.<ref name=Bardos2014>{{cite web|last1=Bardos|first1=P.|last2=Bone|first2=B.|last3=Daly|first3=P.|last4=Elliott|first4=D.|last5=Jones|first5=S.|last6=Lowry|first6=G.|last7=Merly|first7=C.|title=A Risk/Benefit Appraisal for the Application of Nano-Scale Zero Valent Iron (nZVI) for the Remediation of Contaminated Sites|url=http://www.nanorem.eu/Stream.aspx?p=/App_Data/docs/user7Gallery/NANOREM%20NZVI%20risk%20benefit%20issues%20paper%20FINAL.pdf|website=www.nanorem.eu|accessdate=3 December 2014}}</ref> 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 or ''in situ'' application.

Some nanoremediation methods, particularly the use of nano zero-valent iron for groundwater cleanup, have been deployed at full-scale cleanup sites.<ref name=EPA1 /> Other methods remain in research phases.

==Applications==
Nanoremediation has been most widely used for groundwater treatment, with additional extensive research in [[Nanotechnology in water treatment|wastewater treatment]].<ref name = Mueller2012/><ref name=":0">{{Cite web
| last = U.S. EPA
| title = Remediation: Selected Sites Using or Testing Nanoparticles for Remediation
| accessdate = 2014-07-29
| url = http://cluin.org/products/nanozvi/
}}</ref><ref name = Theron2008>{{Cite journal
| doi = 10.1080/10408410701710442
| pmid = 18259980
| issn = 1040-841X
| volume = 34
| issue = 1
| pages = 43–69
| last = Theron
| first = J.
|author2=J. A. Walker |author3=T. E. Cloete
 | title = Nanotechnology and Water Treatment: Applications and Emerging Opportunities
| journal = Critical Reviews in Microbiology
| date = 2008-01-01
}}</ref><ref name =Chong2010>{{Cite journal
| doi = 10.1016/j.watres.2010.02.039
| pmid = 20378145
| issn = 0043-1354
| volume = 44
| issue = 10
| pages = 2997–3027
| last = Chong
| first = Meng Nan
|author2=Bo Jin |author3=Christopher W. K. Chow |author4=Chris Saint
 | title = Recent developments in photocatalytic water treatment technology: A review
| journal = Water Research
| date = May 2010
}}</ref> Nanoremediation has also been tested for soil and sediment cleanup.<ref name=Gomes2013>{{Cite journal
| doi = 10.1016/j.scitotenv.2012.11.098
| pmid = 23334318
| issn = 0048-9697
| volume = 445–446
| pages = 237–260
| last = Gomes
| first = Helena I.
|author2=Celia Dias-Ferreira |author3=Alexandra B. Ribeiro
 | title = Overview of in situ and ex situ remediation technologies for PCB-contaminated soils and sediments and obstacles for full-scale application
| journal = Science of the Total Environment
| date = 2013-02-15
}}</ref> Even more preliminary research is exploring the use of nanoparticles to remove toxic materials from [[gases]].<ref name=Sanchez2011>{{Cite journal
| doi = 10.1016/j.trac.2010.11.011
| issn = 0165-9936
| volume = 30
| issue = 3
| pages = 507–516
| last = Sánchez
| first = Antoni
|author2=Sonia Recillas |author3=Xavier Font |author4=Eudald Casals |author5=Edgar González |author6=Víctor Puntes
 | title = Ecotoxicity of, and remediation with, engineered inorganic nanoparticles in the environment
| journal = TrAC Trends in Analytical Chemistry
| series = Characterization, Analysis and Risks of Nanomaterials in Environmental and Food Samples II
| date = March 2011
| url = https://ddd.uab.cat/pub/artpub/2011/163741/treanache_a2011m3v30n3p507.pdf
}}</ref>

=== Groundwater remediation ===
Currently, [[groundwater remediation]] is the most common commercial application of nanoremediation technologies.<ref name=Bardos2014 /><ref name=":0" />
Using [[nanotechnology|nanomaterials]], 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.<ref name="Environ Nanotech2">Lowry, G. V. (2007). Nanomaterials for groundwater remediation. In: Wiesner, M.R.; Bottero, J. (eds.), "Environmental Nanotechnology". The McGraw-Hill Companies, New York, NY, pp. 297-336.</ref>

Nanotechnology offers the potential to effectively treat contaminants ''in 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 (via [[adsorption]] or [[complexation]]), immobilizing them, or they can degrade the contaminants to less harmful compounds. Contaminant transformations are typically [[redox]] reactions. When the nanoparticle is the oxidant or reductant, it is considered reactive.<ref name="Environ Nanotech2" />

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.<ref name="Environ Nanotech2" />

=== Surface water treatment ===
The use of various nanomaterials, including carbon nanotubes and TiO<sub>2</sub>, shows promise for treatment of surface water, including for purification, disinfection, and desalination.<ref name=Theron2008 /> 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 for [[nanofiltration]].{{Cn|date=January 2021}}

=== Trace contaminant detection ===
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 as [[solid-phase microextraction]].<ref name="Nanostructured Sorbents">Addleman, R. S.; Egorov, O. B.; O'Hara, M.; Zemaninan, T. S.; Fryxell, G.; Kuenzi, D. (2005). Nanostructured sorbents for solid phase microextraction and environmental assay. In: Karn, B.; Masciangioli, T.; Zhang, W.; Colvin, V.; Alivisatos, P. (eds.), Nanotechnology and the Environment: Applications and Implications. Oxford University Press, Washington, DC, pp. 186-199.</ref>

With their high reactivity and large surface area, nanoparticles may be effective [[sorbent]]s to help concentrate target contaminants for solid-phase microextraction, particularly in the form of [[self-assembled monolayers]] on mesoporous supports. The [[mesoporous silica]] structure, made through a surfactant templated [[sol-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 as <sup>99</sup>Tc, <sup>137</sup>CS, uranium, and the actinides.<ref name="Nanostructured Sorbents" />

==Mechanism==
The small size of nanoparticles leads to several characteristics that may enhance remediation. Nanomaterials are highly reactive because of their high [[surface area]] per unit mass.<ref name = Karn2009/> Their small particle size also allows nanoparticles to enter small pores in [[soil]] or [[sediment]] that larger particles might not penetrate, granting them access to contaminants sorbed to soil and increasing the likelihood of contact with the target contaminant.<ref name = Karn2009/>

Because nanomaterials are so tiny, their movement is largely governed by [[Brownian 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 an ''in situ'' treatment zone.<ref name="Iron Nanoparticles">Zhang, W.; Cao, J.; Elliot, D. (2005). Iron nanoparticles for site remediation. In: Karn, B.; Masciangioli, T.; Zhang, W.; Colvin, V.; Alivisatos, P. (eds.), Nanotechnology and the Environment: Applications and Implications. Oxford University Press, Washington, DC, pp. 248-261.</ref>

Once a nanoparticle contacts the contaminant, it may degrade the contaminant, typically through a [[redox]] reaction, or [[adsorption|adsorb]] 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.<ref name=Sanchez2011/> Target contaminants include organic molecules such as [[pesticide]]s or [[organic solvent]]s and metals such as [[arsenic]] or [[lead]]. Some research is also exploring the use of nanoparticles to remove excessive nutrients such as nitrogen and phosphorus.<ref name=Sanchez2011/>

==Materials==
A variety of compounds, including some that are used as macro-sized particles for remediation, are being studied for use in nanoremediation.<ref name=EPA1/> These materials include zero-valent metals like [[zerovalent iron|zero-valent iron]], [[calcium carbonate]], carbon-based compounds such as [[graphene]] or [[carbon nanotubes]], and metal oxides such as [[titanium dioxide]] and [[iron oxide]].<ref name = Karn2009/><ref name=Sanchez2011/><ref>{{Cite journal
| doi = 10.1016/j.cej.2013.04.070
| issn = 1385-8947
| volume = 226
| pages = 336–347
| last = Wang
| first = Shaobin
|author2=Hongqi Sun |author3=H. M. Ang |author4=M. O. Tadé
 | title = Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials
| journal = Chemical Engineering Journal
| date = 2013-06-15
| hdl = 20.500.11937/35439
| hdl-access = free
}}</ref>

===Nano zero-valent iron===
As of 2012, nano [[zerovalent iron|zero-valent iron]] (nZVI) was the nanoscale material most commonly used in bench and field remediation tests.<ref name=EPA1/> nZVI may be mixed or coated with another metal, such as [[palladium]], [[silver]], or [[copper]], that acts as a [[Nanomaterial-based catalyst|catalyst]] in what is called a bimetallic nanoparticle.<ref name = Karn2009/> nZVI may also be [[emulsion|emulsified]] 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.<ref name=Crane2012 /><ref name=EPA1/> Commercial nZVI particle sizes may sometimes exceed true “nano” dimensions (100&nbsp;nm or less in diameter).<ref name = Karn2009/>

nZVI appears to be useful for degrading organic contaminants, including [[organochloride|chlorinated organic compounds]] such as [[polychlorinated biphenyl]]s (PCBs) and [[trichloroethene]] (TCE), as well as immobilizing or removing metals.<ref name = Karn2009/><ref name = Theron2008 /> nZVI and other nanoparticles that do not require light can be injected belowground into the contaminated zone for ''in situ'' [[groundwater remediation]] and, potentially, soil remediation.

nZVI nanoparticles can be prepared by using sodium borohydride as the key reductant. NaBH<sub>4</sub> (0.2 M) is added into FeCl<sub>3</sub>•6H<sub>2</sub> (0.05 M) solution (~1:1 volume ratio). Ferric iron is reduced via the following reaction:

4Fe<sup>3+</sup> + 3B{{chem|H|4|-}} + 9H<sub>2</sub>O → 4Fe<sup>0</sup> + 3H<sub>2</sub>B{{chem|O|3|-}} + 12H<sup>+</sup> + 6H<sub>2</sub>

Palladized Fe particles are prepared by soaking the nanoscale iron 
particles with an ethanol solution of 1wt% of palladium acetate ([Pd(C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>)2]<sub>3</sub>). This causes the reduction and deposition of Pd on the Fe surface:

Pd<sup>2+</sup> + Fe <sup>0</sup> → Pd<sup>0</sup> + Fe<sup>2+</sup>

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 of
diameter 50-70&nbsp;nm may be produced. The average [[specific surface area]] of Pd/Fe particles is about 35 m<sup>2</sup>/g. Ferrous iron salt has also been successfully used as the precursor.<ref name="Iron Nanoparticles" />

===Titanium dioxide===
[[Titanium dioxide]] (TiO<sub>2</sub>) 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.<ref name =Chong2010 /> When exposed to [[ultraviolet light]], such as in [[sunlight]], titanium dioxide produces [[hydroxyl radical]]s, which are highly reactive and can [[oxidizing agent|oxidize]] contaminants. Hydroxyl radicals are used for water treatment in methods generally termed [[advanced oxidation process]]es. Because light is required for this reaction, TiO<sub>2</sub> is not appropriate for underground ''in situ'' remediation, but it may be used for wastewater treatment or pump-and-treat groundwater remediation.{{Cn|date=January 2021}}

TiO<sub>2</sub> is inexpensive, chemically stable, and insoluble in water. TiO<sub>2</sub> has a wide [[band 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 TiO<sub>2</sub> or alternative [[photocatalyst]]s that might use a greater portion of [[photon]]s in the [[visible spectrum|visible light spectrum]].<ref name = Theron2008 /><ref name = DiPaola2012>{{Cite journal
| doi = 10.1016/j.jhazmat.2011.11.050
| pmid = 22169148
| issn = 0304-3894
| volume = 211–212
| pages = 3–29
| last = Di Paola
| first = Agatino
|author2=Elisa García-López |author3=Giuseppe Marcì |author4=Leonardo Palmisano
 | title = A survey of photocatalytic materials for environmental remediation
| journal = Journal of Hazardous Materials
| series = Nanotechnologies for the Treatment of Water, Air and Soil
| date = 2012-04-15
| hdl = 10447/74239
| hdl-access = free
}}</ref> Potential modifications include doping TiO<sub>2</sub> with metals, nitrogen, or carbon.{{Cn|date=January 2021}}

==Challenges==
When using ''in situ''
remediation 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 affect
the effectiveness and/or cost of remediation. [[Trichloroethylene|TCE]] (trichloroethylene), under reducing conditions by nanoiron, may sequentially dechlorinate to [[Dichloroethene|DCE]] (dichloroethene) and [[Vinyl chloride|VC]] (vinyl chloride). VC is known to be more harmful than TCE, meaning this process would be undesirable.<ref name="Environ Nanotech2" />

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.<ref>{{Cite journal
| doi = 10.1023/A:1025520116015
| issn = 1572-896X
| volume = 5
| issue = 3–4
| pages = 323–332
| last = Zhang
| first = Wei-xian
| title = Nanoscale Iron Particles for Environmental Remediation: An Overview
| journal = Journal of Nanoparticle Research
| date = 2003-08-01
}}</ref> For ''in 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 include [[surfactants]], [[polyelectrolyte]] coatings, emulsification layers, and protective shells made from [[silica]] or [[carbon]].<ref name=Crane2012 />

Such designs may also affect the nanoparticles’ ability to react with contaminants, their uptake by organisms, and their [[Nanotoxicology|toxicity]].<ref>{{Cite journal
| doi = 10.1021/es087058e
| issn = 0013-936X
| volume = 42
| issue = 6
| pages = 1821–1824
| last = Lubick
| first = Naomi
| title = Risks of Nanotechnology Remain Uncertain
| journal = Environmental Science & Technology
| date = 2008-03-01
| doi-access = free
}}</ref> A continuing area of research involves the potential for nanoparticles used for remediation to disperse widely and harm wildlife, plants, or people.<ref>{{Cite journal
| doi = 10.1021/es062726m
| issn = 0013-936X
| volume = 40
| issue = 14
| pages = 4336–4345
| last = Wiesner
| first = Mark R.
|author2=Greg V. Lowry |author3=Pedro Alvarez |author4=Dianysios Dionysiou |author5=Pratim Biswas
 | title = Assessing the Risks of Manufactured Nanomaterials
| journal = Environmental Science & Technology
| date = 2006-07-01
| doi-access = free
}}</ref>

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.<ref>{{Cite journal
| doi = 10.1264/jsme2.ME11126
| volume = 26
| issue = 4
| pages = 271–281
| last1 = Ševců
| first1 = Alena
| last2 = El-Temsah
| first2 = Yehia S.
| last3 = Joner 
| first3 = Erik J.
| last4 = Černík
| first4 = Miroslav
| title = Oxidative Stress Induced in Microorganisms by Zero-valent Iron Nanoparticles
| journal = Microbes and Environments
| date = 2011
| pmc = 4036022
}}</ref>

==See also==
{{portal|Technology|Environment}}
*[[Green nanotechnology]]
*[[Nanofiltration]]

==References==
{{reflist|30em}}

[[Category:Nanotechnology and the environment]]