Nanomaterials are growing in influence in a number of distinct areas of technology. An increasing number of novel products are being introduced into the market in fields such as medical equipment, chemical processing, energy generation and information technology.
Nanomaterials have begun to be widely used in environmental remediation, which is based on using highly reactive or absorbent nanomaterials to remove pollutants. The characteristics of these nanomaterials enable efficient chemical transformation or degradation of the contaminants.
Examples of nanomaterials which have been studied for use in environmental remediation include carbon nanotubes, nanoscale zeolites, nanofibers, and titanium dioxide.
Nanomaterials in Water Treatment
Nanosorbents are nanoscale particles of inorganic or organic materials that are capable of absorbing other substances. Most environmental applications of nanosorbents are in the field of wastewater treatment and drinking water production, with other applications focused on air pollutants or ground water contamination.
Nanosorbents have been shown to have better properties than traditional sorbents, such as a large surface area and a high substance specificity. They also provide the ability to combine a number of reactive agents together, and allow fine control over mass transport properties.
Because of these advantageous properties, nanosorbents can quickly and specifically remove or recover target contaminants.
Key applications of nanosorbents are:
- Groundwater/soil remediation by carbo-iron
- Nanoclays for adsorbing phosphorus and organic contaminants
- Nano-aerogels for removing uranium from groundwater
- Nano-iron oxides for the adsorption of hormones and pharmaceuticals from waste water
- Dendrimers, nano-metal oxides and polymer nanofibres for removing heavy metals and arsenic.
The properties of many nanomaterials make them ideal for absorbing environmental pollutants in waste water treatment.
Examples of Nanosorbents
The company AquaNano is developing a nanostructured sorbent called Captymer, which comprises branched macromolecules with chemically or physically tunable sites that are combined to form globular microparticles. Due to the high density of the adsorbing sites, the product is said to have twice as much adsorption capacity than traditional materials.
AquaNano has stated that the product is disposable or regenerable, and is available as beads or powders. It may be applied for selectively removing contaminants such as nitrate, bromide, uranium and perchlorate from potable groundwater sources in order to recover nutrients such as phosphates and nitrate, or for the removal of borate/boron from wastewater streams.
Hong Kong Polytechnic University has developed a polymeric nanosorbent that has been successfully applied in wastewater treatment by the Dunwell Group. It is described as an effective adsorbent for a number of inorganic and organic components in wastewater. The saturated nanoparticles containing the adsorbed contaminant can be separated from the wastewater by a membrane system, after which they can be regenerated.
In another approach, Idaho National Laboratory used metal oxides embedded in a polymer matrix for arsenic adsorption.
In most cases, it is essential that water treatment systems that use nanosorbents ensure that the nanosorbent is retained and it is not released into the drinking water or the environment. This is possible by encapsulating the nanosorbents in larger particles by filtration or by fixing of the sorbent on a support material. Alternatively, magnetic nanosorbents can be removed with electromagnets, along with the adsorbed contaminants.
Nanoscale iron hydroxide ion exchange beads have been used to remove arsenic from drinking water. The beads serve as a support medium, and an alkaline regeneration process facilitates the removal of the arsenic from the nanosorbent iron.
Nanomaterials for Pollution Remediation
Nanomaterials have been used for remediating contaminated groundwater and subsurface source areas of contamination at hazardous waste sites. Early treatment remedies for groundwater contamination were mainly pump-and-treat operations. Due to the considerably high cost and long operating periods, the use of in-situ treatment technologies is considerably increasing.
From the early 1990s, site project managers have taken advantage of metallic substance properties such as elemental iron for the degradation of chlorinated solvent plumes in groundwater. One instance of an in situ treatment technology for chlorinated solvent plumes is installing a trench with macroscale zero-valent iron to form a permeable reactive barrier.
Zero-Valent Iron Nanoparticles
According to recent research, nanoscale zerovalent iron (nZVI) may prove highly effective and less expensive when compared to macroscale ZVI under similar environmental conditions. According to research done by injecting nZVI particles aquifiers that are contaminated with chlorinated hydrocarbons, faster and more effective groundwater cleanups take place when compared to conventional pump-and-treat methods or PRBs.
Researchers are developing a range of nanoparticles to destroy or adsorb contaminants as part of ex situ or in situ processes. These particles include dendrimers, ferritin, metalloporphyrinogens and SAAMS. Certain materials can be made with surface functional groups that serve as adsorbents for the scavenging of specific contaminants from waste streams.
SAMMS Particles
SAMMS particles include a nanoporous ceramic substrate with a functional group monolayer that is tailored to attach to the target contaminant. Covalent bonding of the functional molecules to the silica surface takes place so that the other end group binds to several contaminants. Researchers state that SAAMS particles maintain good thermal and chemical stability and can be readily restored or reused. Contaminants that can be successfully sorbed to SAAMS include mercury, radionuclides, arsenate, chromate, selenite and pertechnate.
Photocatalytic Nanotubes
Nanotubes are engineered molecules that are most commonly made from carbon. They are highly electronegative, electrically insulating and polymerized easily. Titanium dioxide nanotubes are also available and these can be used as a photocatalytic degrader of chlorinated compounds. Research has shown that titanium dioxide nanotubes are effective at high temperatures - in one study, their use resulted in a 50% decrease in contaminant concentration in just 3 hours.
Other nanomaterials wich have been used for environmental remediation include:
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Ferritin - an iron storage protein, can minimize toxicity of contaminants such as technetium and chromium in groundwater and surface water to enable remediation.
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Dendrimers - neatly organized, hyper-branched polymer molecules that have end groups, core and branches. FeO/FeS nanocomposites that are synthesized with dendrimers as templates can be used for the construction of permeable reactive barriers for remediation of ground water.
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Metalloporphyrinogens - metal complexes having naturally occurring organic porphyrin molecules. Biological metalloporphyrinogens are vitamin B12 and hemoglobin.
According to experiments, metalloporphyrins can reduce chlorinated hydrocarbons like PCE, TCE and carbon tetrachloride under anoxic conditions to remediate contaminated groundwater and soil.
Nanopollution and Health Concerns
It is believed that the current boom in nanotechnology will bring a lot of positive changes to the world. However, the increased levels of nanomaterials in waste which this will inevitably result in may give rise to an increase in "nano-pollution", which is very difficult to detect or contain, and which has health consequences which are largely unknown.
One of the major concerns which must be addressed before nanomaterials begin to be used more extensively for water treatment and pollution remediation is their own effect on the environment. We need to understand how well we can control the materials, how effectively we can recover them, and what effect their use will have on the ecosystem.
Conclusion
Nanomaterials such as nanoparticles, nanofibers, and porous materials can function as catalysts and adsorbents, or be used to remove harmful gases, organic pollutants, contaminated chemicals and biological substances.
Nanomaterials have proved to be better than conventional techniques in environmental remediation due to their high reactivity and high surface area.
The benefits of nanomaterials in environmental remediation can be summarized as follows:
- Increased surface area or sorption capacity
- High reactivity
- Readily tailored for application in several environments
- Easy dispersability
Sources and Further Reading