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Environmental applications of nanotechnology

 
 

In groundwater remediation a variety of technologies may be used for decontamination. Nanotechnology, especially using zero-valent metals (ZVMs), represents just one of these approaches.

Remediation Process

Nanotechnology offers the ability to effectively enable contaminant treatment in situ. The process begins with the injection of nanoparticles into a contaminated aquifer via an injection well. The nanoparticles are then transported to the source of contamination by the groundwater flow where they then degrade the contaminant. Nanoparticles can sequester (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.[1]

Reaction Products

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. TCE (trichloroethylene), under reducing conditions by nanoiron, may sequentially dechlorinate to DCE (dichloroethene) and VC (vinyl chloride). VC is known to be more harmful than TCE, meaning this process would be undesirable.[1]

Delivery and Transport Issues

Using nanomaterials for groundwater remediation is promising due to the availability and effectiveness of many nanomaterials for degrading or sequestering contaminants. Their ability to be used in situ also makes them very attractive: there is no need for excavation or pumping them out of the ground. In order to realize the potential of nanomaterials the abilit to inject them to the subsurface and transport them to the contaminant source is imperative. Unless they are delivered to and stay at the contaminant source, they cannot be used efficiently.[1]

Injection Methods

Reactive nanoparticles can be injected into a well where they will then be allowed to transport down gradient to the contaminated area. Drilling and packing a well is quite expensive. Direct push wells are lower cost than drilled wells and are the most often used delivery tool for remediation with nanoiron. A nanoparticle slurry can be injected along the verticyal range of the probe to provide treatment to specific aquifer regions.[1]

Reasons for Success

There are two main reasons for the capabilities of nanoparticles as a remediation method. First is the small size, which grants them access to contaminants sorbed to soil. Also, their movement is largely governed by Brownian motion as compared to gravity. Thus, the flow of groundwater is sufficient to transport the particles. Nanoparticles then can remain in suspension to establish an in situ treatment zone.[2]

Preparation

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+ + Fe 0 → 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 of diameter 50-70 nm may be produced. The average specific surface area of Pd/Fe particles is about 35 m2/g. Ferrous iron salt has also been successfully used as the precursor.[2]

Titanium Oxide

TiO2 has been widely used for water treatment as it has a wide band gap energy (3.2 eV), it is inexpensive, non-toxic, highly photoactive and insoluble in water. A study of the degradation of Reactive Brilliant X-3B (a reactive dye) and catechol (a refractory organic compound) has been performed. The degradation occurred via oxidation with ozone under UV light under these conditions: a) without a catalyst, b) with TiO2 as a caalyst, and c) with carbon black-coated TiO2 catalyst on Al thin films. The reaction rate almost doubled with the use of TiO2 nanoparticles and attacking carbon black TiO2 made the separation from the pollutant easy and affordable.

In the degradation of sucrose and nitrate over titania-coated nano-hematite catalysts, the presence of Fe3+ in hematite results in trapping the electrons more efficiently than with TiO2 alone, reducing the degradation time from more than 60 min to about 45 min. TiO2 coupled with different metal oxides kept the e- trapped longer, reducing the recombination rate of the electrons to their hydrogen ions, and improved photocatalytic activity in degradation of methyl orange as an organic pollutant.[3]

Photodecontamination

Decontamination of wastewater is one of the most successful photochemical applications of photons. Solar photocatalysis using TiO2 nanocrystals removed various refractory pollutants, such as phonls, 4-chlorophenol, nitrobenzene, 2-chlorobenzoic acid and hydrobutanedioic acid. Solar light with TiO2 had maximum removal efficiency in comparison to both UV and solar radiation. However, some studie have shown that UV radiation in more effective in removing colored dye.[3]

Solid-phase microextraction

In environmental problems with trace contaminants, such as groundwater contamination, there is often the need for rapid, portable, cost-effective measurement systems for these trace contaminants. Unfortunately, instruments that can operate outside of a laboratory often are not sensitive enough for detection. One way around this is to separate the analyte from the sample and concentrate them to a smaller volume, easing detection and measurement. When this is done with small quantities of solid sorbents it is referred to as solid-phase microextraction (SPME). Typically, effective SPME materials require high affinities for the target analyte and large surface areas.[4]

Self-assembled monolayers on mesoporous supports

Self-assembled monolayers on mesoporous supports (SAMMS) have numerous properties that make them ideal sorbents for SPME. The mesoporous silica structure is made through a surfactant templated sol-gel process that endows SAMMS with high surface areas and a rigid open pore structure. SAMMS have been shown to be excellent sorbents for many targets, including heavy metals such as mercury, lead, and cadmium, chromate and arsenate, and radionuclides such as 99Tc, 137CS, uranium, and the actinides.[4]

References

  1. ^ a b c d 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.
  2. ^ a b 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.
  3. ^ a b Narr, J.; Viraraghavan, T.; Jin, Y. (2007). Applications of nanotechnology in water/wastewater treatment: a review. 16(5): 320-329.
  4. ^ a b 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.

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