How can nanomaterials remove pollutants from water

Main topic October 2012

Small particles with great potential?

It is no coincidence that nanoparticles have found a steadily growing interest in many areas of research and practical application in recent years. Nanoparticles offer a number of advantages - this also applies to the field of environmental technologies: Due to the relatively large outer particle surface, they have a higher specific activity as catalysts or reagents than coarser particles of these materials. In water, pollutants are transported to the small particles without significant resistance. This increases the reaction speed. Nanoparticles can also form stable colloidal suspensions. They can be injected easily into aquifers - an effective procedural approach to the treatment of contaminated groundwater. As tempting as all these advantages sound, it should not be forgotten to always check the use of nanoparticles for possible risks.

Development of nanoparticles for groundwater and wastewater purification

The BMBF-financed Fe-Nanosit project (www.nanopartikel.info/cms/Projekte/Fe-NANOSIT) aims to answer this complex question: How must nanoparticles be made so that they are both effective and safe in groundwater and wastewater treatment can be used? Scientists and practitioners call this "tailoring". This requires both basic research and applied research on a field scale.

The flow reactor contains magnetite nanoparticles. With the help of such reactive nanoparticles, UFZ scientists develop new technologies for groundwater and wastewater treatment.
Photo: André Künzelmann / UFZ

In the laboratory, UFZ researchers produce Carbo-Iron, which is to be tested for cleaning contaminated groundwater.
Photo: André Künzelmann / UFZ

As part of the Fe-Nanosit project, UFZ scientists are developing new technologies for groundwater and wastewater treatment in which reactive nanoparticles play an essential role. At the same time, researchers at the UFZ are carrying out a comprehensive risk assessment that focuses on possible environmental hazards. In this way, the scientists want to ensure that they develop a sustainable technology with which risks and side effects for the environment can be largely eliminated from the outset. Only if this succeeds will trust in the potential of nanoparticles and acceptance for these new technologies grow and ease the way into their application.

The following two approaches to particle design as well as examples of the ecotoxicological behavior of some selected nanoparticles are intended to show that research still has some tasks ahead of it:

1. Groundwater purification

Carbo-Iron® is a UFZ development and consists of carbon (Carbo) and iron (Iron). Iron nanoparticles have huge potential for groundwater purification, but due to certain other properties they show limitations for in-situ application. Because pure iron leads to blockages and thus prevents a wide reaction zone from building up in the aquifer. However, without a sufficiently large reaction zone, the reactive iron particles cannot effectively break down the dissolved pollutants. To overcome these disadvantages, scientists from the Department of Technical Environmental Chemistry have developed a composite material consisting of activated carbon and iron: Carbo-Iron.

The innovative basic idea is to use iron nanoclusters on very fine-grained activated carbon (dP < 1="" µm)="" aufzutragen="" und="" in="" kolloidaler="" form,="" also="" als="" suspension="" (aufschlämmung),="" einzusetzen.="" das="" eisen="" erhält="" auf="" diese="" weise="" durch="" das="" trägermaterial="" aktivkohle="" neue="" eigenschaften,="" beispielsweise="" das="" potenzial,="" chlorierte="" kohlenwasserstoffe="" (ckws)="" anzureichern,="" oder="" die="" mobilität="" in="" natürlichen="" sedimenten="" zu="">
Carbo-Iron combines the excellent properties of activated carbon for binding organic pollutants with the high chemical reactivity of nano-iron for reductive processes. This means that with Carbo-Iron it is possible to first concentrate the pollutants, which are mostly in highly diluted form in the groundwater, on the activated carbon part and then efficiently break them down in a concentrated form. With additional chemical tricks, the scientists have also succeeded in making Carbo-Iron (unlike nano-iron) handle in air and form stable suspensions in water.

Carbo-Iron has already proven in the laboratory that it is better than nano-iron. Its significantly improved transport behavior for the construction of the reaction zones, but also its high affinity for undissolved organic pollutant phases (NAPL - non aqueous phase liquid) make the material attractive both for the remediation of pollutant sources and pollutant plumes in the groundwater aquifer. The first pilot test is currently running in the field together with Golder Assoc. GmbH at a location near Celle. The groundwater at the site is contaminated with tetrachlorethylene (PCE). Tetrachlorethylene is an organic solvent that is used in chemical cleaning and for degreasing metal parts in industrial applications. The results on site are promising: Carbo-Iron has also proven in field tests that it can break down organic pollutants such as tetrachlorethylene (PCE) excellently.

2. Wastewater treatment

The objectives of wastewater treatment are different from those of groundwater remediation. This must also be reflected in the design of nanoparticles. The degradation processes in wastewater must also be effective. Above all, however, they have to run quickly and in a way that conserves resources and energy. Therefore, in addition to reagents, catalysts also play an important role in wastewater treatment.
The aim of the UFZ scientists is to develop magnetic nanocatalysts that, for example, catalyze oxidation reactions of pollutants or with which - combined with palladium as a catalytic component - the chlorine can be selectively removed from chlorine-containing organic pollutants. With the help of magnets, the magnetic nanocatalysts can then be removed from the purified water. In addition to the high reactivity and robustness of the nanomaterial, the main focus in the case of wastewater treatment is the safe separation of the nanoparticles from the water. This step enables the nanocatalysts to be regenerated and recycled.

Degradation processes in wastewater must run quickly and conserve resources and energy. UFZ scientists are currently developing magnetic nanocatalysts that - combined with the palladium catalyst - selectively remove chlorinated pollutants from wastewater. With the help of magnets, the nanocatalyst can then be removed from the wastewater and can be regenerated and recycled.
Photo: André Künzelmann / UFZ

If chlorinated organic pollutants are to be removed from wastewater, palladium comes into play alongside magnetic nanoparticles: It catalyzes the splitting off of chlorine by reducing chlorinated hydrocarbons in the aqueous phase, just like iron, but much faster than iron. For this reductive wastewater treatment, the palladium is applied to the magnetic nanoparticles. However, there are only a few successful practical examples of water treatment with palladium catalysts. The reason: unprotected palladium is extremely sensitive to numerous catalyst poisons. The palladium catalysts therefore lose their effectiveness very quickly under real operating conditions.
For this reason, when designing particles, it is very important to ensure that the palladium catalysts are protected reliably and over a long period of time. In return, the particles are given wafer-thin polymer layers, which both shield most of the substances contained in the water and prevent palladium from being released into the environment. Finally, the magnetic carrier enables the nanoparticles to be removed from the water and reused after the accumulated pollutants have been separated off.

The examples given show that the design and development of nanoparticles is a complex process. It not only requires knowledge of the objectives for which nanoparticles are to be developed. It is also necessary to have a thorough understanding of every single step within the process and the possible risks of the nanoparticles. That is why the design of nanoparticles for environmental applications always includes an ecotoxicological assessment. It is the basis for identifying risks and, if necessary, revising the design of the nanoparticles and the materials used in them. This is the only way to prevent nanoparticles from having undesirable side effects for people and the environment.

Risk assessment of nanoparticles for the environment

If Carbo-Iron or magnetic nanocatalysts are used to treat groundwater and wastewater, the question is obvious what will happen to them when they have done their job. Are they released into the environment or how are they distributed there? To what extent and with what consequences? First of all, it must be made clear that activated carbon - i.e. carbon - and iron also occur naturally in the environment and in the water cycle. The decisive factor for the environmental risk assessment is above all the tiny particle size of the substances additionally introduced into the cycle.

UFZ scientists use aquatic organisms such as daphnia (water fleas) to assess the environmental risks of nanomaterials. In doing so, they are based on the standard test protocols such as those of the OECD.
Photo: André Künzelmann / UFZ

Zebrafish are easy to breed and their embryos are therefore often used in the risk assessment of nanomaterials.
Photo: André Künzelmann / UFZ

Carbo-Iron, which was injected into groundwater aquifers, remains at the respective place of use. Its further distribution and exposure in the subsurface is locally very limited and can be estimated well based on the known amount of input. The magnetic and palladium-loaded nanocatalysts, on the other hand, are largely removed from the wastewater treatment process. When used properly, a minimal release into the environment can therefore be expected. In the event of an accident, for example if the magnetic separation fails completely, it can be released into the sewage treatment plant. This is why ecotoxicologists at the UFZ are investigating what effects released nanomaterials can have on environmental organisms, even before the developed technologies are used.

The UFZ scientists use various aquatic test organisms for which standard test protocols exist (e.g. OECD). Since these standard tests were established for testing environmental chemicals, they often cannot be directly transferred to the ecotoxicological testing of nanomaterials. Therefore, the tests must first be adapted to the requirements of nanomaterials. And test strategies and scenarios have to be developed depending on the application of the nanoparticles. An example: Carbo-Iron is used deep in the ground. There is extremely little oxygen there. None of the test organisms used in the general laboratory standard tests are at home in such a habitat.

As a first approximation of the real test conditions, the scientists therefore concentrate on the aquatic organisms algae, daphnia and zebrafish. Even if the common denominator of the catalytic wastewater treatment with nanoparticles and the bio tests is water (because both are carried out in this environment), there are differences between the two applications that the researchers must take into account in their investigations.

Different pH values, salt and oxygen contents influence the behavior and properties of the nanoparticles. Depending on the environmental conditions, they can range from severely isolated to severely agglomerated (clumped). It is therefore necessary to adapt the experimental approaches in the laboratory step by step to the conditions of a real renovation case. At the same time, the optimal growth conditions for the test organisms must be maintained. Possible impurities in the nanomaterials also play a role in the bio-tests. Residues from production, for example, may not affect the catalytic wastewater treatment, but the bio tests, namely when they are toxic.

It is also important to select the appropriate concentrations for testing. Because the dose makes the effect. This ranges from worst-case scenarios due to the unintentional release of larger quantities of nanoparticles to low doses, which can lead to long-term exposure if, for example, nanomaterials are used in soils to break down pollutants and remain there.

Another challenge for the scientists is to consider mixtures of substances instead of individual components when assessing the risk: Do toxic intermediate products arise in the catalytic wastewater treatment when the pollutants are broken down? What effects do mixtures of nanomaterials, pollutants, degradation products and naturally occurring organic substances such as humic acids have?

Not all questions have already been answered. The scientists are nevertheless able to present initial results: So far, they have not been able to determine any acute toxic effects for the tested nanomaterials - such as Carbo Iron - in the organisms examined. For selected mixtures of nanomaterials and pollutants, such as Carbo Iron and the pollutant tetrachloroethene (PCE), no additional toxicity was observed due to the combination with the nanomaterial. However, this finding says nothing about possible long-term effects of nanoparticles & Co.

Further information on possible toxic and ecotoxic effects of nanomaterials can be found at: www.nanopartikel.info/cms/Wissensbasis

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References (selection)

Mackenzie, K., Bleyl, S., Kopinke, F.-D. (2012):
Carbo-Iron - an Fe / AC composite as an alternative to nano-iron for groundwater treatment. Water Res. 46,3817-3826.

Navon, R., Eldad, S., Mackenzie, K., Kopinke, F.-D. (2012):
Protection of palladium catalysts for hydrodechlorination of chlorinated organic compounds in wastewaters. Appl. Catal. B. 119, 241-247.

Bleyl, S., Kopinke, F.-D., Mackenzie, K. (2012):
Carbo-Iron® -Synthesis and stabilization of Fe (0) -doped colloidal activated carbon for in situ groundwater treatment. Chem. Eng. J. 191, 588-595.

Hildebrand, H., Mackenzie, K., Kopinke, F.-D. (2009):
Highly active Pd-on-magnetite nanocatalysts for aqueous phase hydrodechlorination reactions. Environ. Sci. Technol. 43, 3254-3259.

Kühnel, D., Scheffler, K., Wellner, P., Meißner, T., Potthoff, A., Busch, W., Springer, A., Schirmer, K. (2012):
Comparative evaluation of particle properties, formation of reactive oxygen species and genotoxic potential of tungsten carbide based nanoparticles in vitro
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Internalization of engineered nanoparticles into mammalian cells in vitro: influence of cell type and particle properties
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Hildebrand, H., Kühnel, D., Potthoff, A., Mackenzie, K., Springer, A., Schirmer, K. (2010):
Evaluating the cytotoxicity of palladium / magnetite nano-catalysts intended for wastewater treatment
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Kühnel, D., Busch, W., Meissner, T., Springer, A., Potthoff, A., Richter, V., Gelinsky, M., Scholz, S., Schirmer, K. (2009):
Agglomeration of tungsten carbide nanoparticles in exposure medium does not prevent uptake and toxicity toward a rainbow trout gill cell line
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