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Keywords:

  • carbon nanotubes;
  • gold nanoparticles;
  • green chemistry;
  • green nanoscience;
  • industrial ecology;
  • life cycle assessment (LCA)

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

Commercial and research interest in nanotechnology has exploded in recent years, with nearly US$9 billion in investment from public and private sources in 2005. While the list of potential applications for nanotechnologies continues to grow, there is increasing pressure from governments and researchers alike to understand the implications of this new class of materials. The emerging field of green nano applies green chemistry and engineering principles to the synthesis of nanomaterials. Here we outline several strategies for the development of green nano and review past policy and research activities in understanding nanotechnology's environmental implications. By means of the green chemistry metric of E-factor, an analysis is undertaken of the traditional syntheses of several specific nanomaterials, including carbon nanotubes, fullerenes, and metal nanoparticles. It was found that the E-factors of these production processes vary over several orders of magnitude, making it difficult to comment generally about the resource use efficiencies of nanomaterials production. For gold nanoparticles specifically, E-factors for six different production methods are found to range from 102 to 105, demonstrating that greener synthesis routes are possible and that environmental benefits can begin to be quantified. Expanding the analysis to include life-cycle stages upstream and downstream of production and to incorporate environmental health effects is encouraged, though significant data gaps exist.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

Richard Feynman's famous 1959 lecture “There's Plenty of Room at the Bottom” (Feynman 1960) began a variety of nanotechnology discussions, many of which continue today. As a strategic and distinct area of scientific inquiry, nanotechnology research began in the United States with the establishment of the National Nanotechnology Initiative (NNI). Recently, scientific journals that are focused specifically on nanotechnology have been started (including the International Journal of Nanotechnology, Nano Letters, the Journal of Nanoscience and Nanotechnology, and the Journal of Nanoparticle Research). This recent growth has been significantly supported through financial investment of the U.S. government of over $1 billion in 2006 (White House OSTP 2006) and an estimated $9 billion globally, including both public and private funds, in 2005 (Allen 2005).

Much of the interest and investment in nanoscience and nanotechnology stems from the ability to impart new properties and new capabilities to materials on the basis of their size and geometry. This complements the traditional route of controlling material properties through composition. With this potential, companies and research groups around the world have launched into aggressive investigations of the characteristics and applications of nanomaterials. Currently, there are hundreds of commercial products that claim to use nanotechnology, in areas ranging from clothing to medical devices to sports equipment (Project on Emerging Nanotechnologies 2007a). The global demand for nanoscale materials, tools, and devices is expected to reach $28.7 billion in 2008, increasing at an average annual growth rate of 30.6% (McWilliams 2004), with projections of a $1 trillion market by 2015 (Roco and Bainbridge 2001).

Although the significant majority of this cutting-edge science is focused on the applications of nanomaterials, there has been an increasing number of scientific investigations and analyses that are exploring the potential implications of these materials on human health and the environment. Several of these studies have raised concerns about the ability of nanomaterials to cross certain biological membranes in ways that may be harmful (Oberdorster et al. 2004); mobilization of the nanomaterials in porous media (Lecoanet et al. 2004); and the ability for nanomaterials to “escort” other toxic substances, analogous to drug delivery strategies that are currently under development (Kingsely et al. 2006). These concerns have prompted many to call for regulatory action by governments to extend their mission of ensuring public welfare to include regulating nanotechnology.

To begin this dialogue publicly in the United States, the federal Environmental Protection Agency (EPA) released a review draft of a white paper on nanotechnology in 2007 that discussed the potential environmental benefits of nanotechnology, risk management issues, risk assessment challenges, and research needs in both environmental applications and implications (Science Policy Council 2007). Comments submitted during the draft period by business and environmental advocacy communities expressed significant concerns related to whether and when EPA will pursue a strategic, scientific process to develop a regulatory mechanism, establish statutory authority, and provide a publicly available information basis for this emerging technology (Science Policy Council 2006).

Around the same time, several valuable initiatives emerged that raised issues associated with the implications of nanotechnology. These efforts include activities by the Royal Society of Chemistry and Royal Academy of Engineering of the United Kingdom as well as the formation of an NNI subcommittee. The report from the United Kingdom titled Nanoscience and Nanotechnologies: Opportunities and Uncertainties was published in 2004 and describes the present and future benefits of nanotechnologies while highlighting the immediate need for research to address uncertainties regarding the health and environmental effects of nanoparticles (Royal Society of Chemistry and Royal Academy of Engineering 2004). This report sparked significant discussion in the nanoscience and nanotechnology community. In the year following this report, the Nanoscale Science, Engineering, and Technology Subcommittee of the U.S. NNI formally established the Nanotechnology Environmental and Health Implications (NEHI) Working Group to provide for exchange of information among relevant agencies, to manage the national research agenda related to the responsible development of nanotechnology, and to promote communication about its environmental and health implications (National Nanotechnology Initiative 2007).

Although reports and committee infrastructure of this type are important, there is a large gap between studying the potential environmental and human health effects associated with nanotechnology and beginning to design nanotechnology such that potentially harmful implications are inherently reduced or eliminated.

Design Through the Principles of Green Chemistry and Green Engineering

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

The incorporation of environmental criteria in the design of new materials, products, processes, and systems can be extremely complex. The designer strives to reduce or eliminate the use and generation of hazardous substances while maximizing efficiencies throughout the life cycle and considering other environmental, economic, and social factors. To this end, it may be helpful to engage with successfully demonstrated design frameworks, such as the Principles of Green Chemistry and Green Engineering (Figure 1; Anastas and Warner 1998; Anastas and Zimmerman 2003). The principles were developed through identification examples of chemistry and engineering from various disciplines, sectors, and scales that have advanced sustainability. The goal has been to elucidate the underlying design principles embedded in these examples and then to apply these proactively to design new sustainable molecules, materials, products, processes, and systems.

image

Figure 1. Summary of the principles of green chemistry and green engineering.

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Just as the principles of green chemistry and green engineering have been successfully applied to many sectors for environmental and economic benefits, they can be used in the design of the emerging nanomaterials and nanotechnology. Although the nanotechnology field is developing rapidly, it is important to its success and long-term acceptability to consider environmental and human health impacts at the design phase. Dahl and colleagues (2007) provide an excellent discussion of the application of specific principles to nanoscience and nanotechnology.

As will be discussed below, many substances currently used to make nanomaterials are of concern and synthesis processes are often quite inefficient, leading to substantial waste generation, energy consumption, and water use. Furthermore, the toxicity and environmental impacts of nanomaterials themselves are not well understood and may in some cases prove to be significant. By adhering to the principles set forth, we can address these challenges from the outset at the design stage.

For example, the design of nanoparticles with benign toxicological effects on humans should follow much the same framework as described for designing safer chemicals (Anastas and Warner 1998), including the following:

  • • 
    modification or termination of biological pathways of action,
  • • 
    alteration of reactive functional groups,
  • • 
    reduction or elimination of bioavailability,
  • • 
    reduction or elimination of need for associated hazardous substances,
  • • 
    design for end of useful product life (or complete biodegradation on environmental trigger).

It is, of course, imperative that these designed characteristics are inherent to the nanomaterials and will not be modified during use or at end of life.

Green Nano

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

This nascent area of research is often referred to as green nanoscience/nanotechnology, or “green nano,” the design of nano-scale substances, materials, and processes through green chemistry and green engineering that results in the development of new performance without adverse consequences to humans and the biosphere (McKenzie and Hutchison 2004). It has been said that green nano in its simplest form is preserving the applications while minimizing or eliminating the negative implications of nanomaterials.

With all the benefits that the pursuit of green nano offers, it constitutes a very small area of the U.S. federal research budget as of 2007. Roughly, the budget breakdown for governmental research funding in the United States is 95% for nanomaterial applications, 4% for implications, and less than 1% for green nano broadly defined (Dunphy Guzman et al. 2006). Although this area may be growing slowly, there are leading groups that have enunciated and illustrated the concepts of green nano with recent conferences and symposia highlighting work that is currently being conducted. For example, the Woodrow Wilson International Center for Scholars, in partnership with the Pew Charitable Trusts, has been highly visible in its efforts to encourage the development of the nanotechnology industry with an eye toward implications and green nano.

In addition, green nano research and outreach is often cited as protecting the development of the field as a whole from public fears, real or perceived, and the legislative and/or regulatory backlash that might follow those fears. As discussed by Bullis (2005), there is a fear among the proponents of nanotechnology's benefits that health problems linked to just one “unsafe” product in the marketplace may lead to consumer backlash against all nanotechnology. This could lead not only to the economic loss of billions of dollars in profits and wasted research and development but also to the loss of significant potential health and environmental benefits, such as reducing the side effects from cancer treatment or quickly cleaning up toxic waste sites.

Risk perceptions of emerging industries and new technology can prove catastrophic, as was demonstrated by the introduction of genetically modified organisms (GMOs) into the marketplace. A strong movement in opposition to GMOs developed in the late 1990s in many countries, especially in Europe, although these technologies were presented from the outset as highly promising and their advantages often cited (Bonny 2003). Several risk perception studies have been conducted in the United States and Europe to explore the reasons for public opposition to biotechnology. Some early studies found that the risks of DNA technology were perceived as unknown, with possible negative consequences that were delayed in time and not directly observable (Slovic 1987). It has been concluded that governmental failure to address public concerns has had consequences for public trust, leading to significant negative impacts on the commercialization of these emerging technologies (Frewer et al. 2004).

Perceptions of risks posed by nanotechnology at this point indicate that Americans are fairly pro-nanotechnology, with 53% indicating that they believed nanotechnology's benefits would either “slightly” or “strongly” outweigh its risks, whereas 36% indicated that they believed that nanotechnology's risks would either “slightly” or “strongly” outweigh its benefits (Kahan et al. 2007).1 Although these risk perception results are fairly promising, they do suggest that there is a need, as learned from the GMO example, to continue to provide information from trusted sources and to incorporate new risk information into regulatory processes and activities. Perhaps even more important will be to address the issues of environmental and health concern by proactively reassuring the public that nanotechnology is being responsibly pursued. Actively engaging in the development of green nano can play a significant role in reassuring the public and maintaining the power and potential of nanotechnology to realize benefits for society, the economy, and the environment.

Whereas many researchers have focused on the fate of nanomaterials during use and their potential toxicities to human and ecosystem health, very few have produced quantitative work on the environmental implications of production (synthesis).2 As very limited toxicological information is available on these new compounds, it is extremely difficult to assess their potential risks to human and ecosystem health in any sort of comprehensive manner.

Current Examples of Green Nano

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

Nano-related research groups around the world are constantly inventing new synthetic pathways, some of which align with green chemistry and green engineering principles. In this section, we present examples of these green processes across several different classes of nanomaterials.

Carbon Nanotubes

Carbon nanotubes (CNTs) have received the most attention in the popular science media, in terms of both technological potential and possible negative health impacts (Lekas 2006).3 The three methods most widely used for commercial production of CNTs are arc discharge, pulsed-laser vaporization, and chemical vapor deposition (CVD). The former two methods require extremely high temperatures and produce CNTs with low purity and low yield, respectively. CVD produces high-purity CNTs at a reasonable yield but requires metal growth catalysts that can be expensive and time consuming to produce. CVD is by far the most common CNT production method in commercial systems, but there have been efforts to find green chemistry improvements, such as the substitution of gaseous hydrocarbon feedstock with camphor (Kumar and Ando 2007). Solvent-free techniques for the subsequent functionalization of carbon nanotubes have also been developed, including those using fluorination (Khabashesku et al. 2002), diazonium salts (Dyke and Tour 2003), and dithiols (Basiuk et al. 2006).

Gold and Silver Nanoparticles

The development of green chemistry techniques for the synthesis of gold (Au) and silver (Ag) nanoparticles has been an important accomplishment, as most traditional methods are both extremely hazardous and expensive (Betts 2005). The first widely used synthesis was that of Turkevich and colleagues (1951), using trisodium citrate both to reduce and to stabilize gold nanoparticles. It is a relatively benign procedure with no need for solvents or purification; however, the resulting colloidal gold solution is very sensitive to other factors, such as pH and ionic strength, making further chemical modification difficult (Dahl et al. 2007). Schmid and colleagues (1981) were among the first to create phosphine-stabilized gold nanoparticles, but their process used borane or diborane as a reducing agent and benzene to solvate the nanoparticles, both of which carry significant health and flammability risks. Brust and colleagues (1994) developed a widely used synthesis that substitutes borohydride for borane as a reducing agent and toluene for benzene as a solvent, greatly reducing these risks. Although this method used thiol for stabilization of the nanoparticles, Weare and colleagues (2000) took a very similar approach using phosphine instead. Moving away from organochemical solvents, researchers have developed pathways to produce gold nanoparticles based in water (Kim et al. 2004) or supercritical carbon dioxide (CO2; McLeod et al. 2004) that can produce particles of a tunable size. Early attempts at using CO2 as a solvent required fluorinated additives; Anand and colleagues (2006) rose to the challenge and eliminated fluorine from their process. Even greener, though less size-selective and with slower kinetics, are techniques that fall into the category of “bionanoscience”: producing nanoparticles using organisms as the catalysts. Researchers have exploited the mechanism of metals uptake by plants (Gardea-Torresdey et al. 2002), fungus (Sastry et al. 2003), and bacteria (Klaus et al. 1999; Ahmad et al. 2003). Perhaps the most environmentally benign of all techniques for synthesizing Ag and Au nanoparticles is that of Raveendran and colleagues (2006), who use water-based reactions with starch and glucose in a microwave oven. Although this simple technique is only appropriate for a certain size class of nanoparticles, it is extremely cheap and free of any potential hazards.

Metal Oxide Nanoparticles

Zinc oxide (ZnO) and titanium dioxide (TiO2) have been used in bulk and nano forms for a variety of applications. There are many different processes used in the generation of ZnO and TiO2 nanoparticles; among these is the widely explored sol-gel method, which makes use of significant quantities of different alcohols (Spanhel and Anderson 1991) and the commercialized hydrolysis-calcination method, which uses a solvent of hydrochloric acid and requires extensive energy inputs (Robichaud et al. 2005). One route for green synthesis is the spray pyrolysis technique described by Okuyama and Lenggoro (2003). This technique creates aerosols by spraying zinc solution through a tiny nozzle. These aerosols are quickly heated, which evaporates the solvent (easily recovered) and leaves the metal nanoparticle.

Ferritic Nanoparticles

Another bionanoscience application has been in the area of iron nanoparticles. Hosein and colleagues (2004), among others, have used the iron storage protein ferritin to make iron nanoparticles. Iron is added until the particles within the protein cage are of a specified size, and then the ferritin is washed away with distilled water. In many laboratories, iron nanoparticles manufactured in this way are subsequently used as seeds for carbon nanotube growth (Li et al. 2001).

E-factor Analysis of Nanomaterials Production

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

There have been many calls for research into the potential environmental effects of new nanomaterials, several specifying life cycle approaches (Roco and Bainbridge 2001; Reijnders 2006; Sweet and Strohm 2006). In October 2006, the Woodrow Wilson Center conducted a workshop titled Nanotechnology and Life Cycle Assessment. The participants concluded that existing life cycle assessment (LCA) protocols, such as the ISO 14040 series, were appropriate for evaluating nanomaterials but that both standards and data for toxicity assessments are lacking (Project on Emerging Nanotechnologies 2007b). To date, however, few studies have actually carried out comprehensive, quantitative analyses.

By incorporating risk assessment techniques from the insurance industry, Robichaud and colleagues (2005) present a life cycle treatment of the hazards of commercial production of five nanomaterials, including several analyzed in the present study. These are compared with risk assessments for the production of other materials, including plastics, petroleum, and lead-acid batteries. Lloyd and Lave (2003) examined the life cycle benefits of using nanomaterials in automotive applications. The electronics research community has seen perhaps the most activity in nanotechnology LCA, as disseminated through conference proceedings and associated journals of the Institute of Electrical and Electronic Engineers (IEEE). Recently, Khannaa and colleagues (2007) published an important early LCA of carbon nanofibers with a particular focus on life cycle energy use.

Although it does not consider material life cycle stages outside of production, E-factor is a measure of environmental impact and sustainability that has been commonly employed by chemists.4E-factor considers the ratio of product to the total inputs (or all materials used in the production process), given by the following equation (Sheldon 1997):

  • image

E-factor (or waste-to-product ratio) includes all chemicals involved in production. Energy and water inputs are generally not included in E-factor calculations, nor are products of combustion, such as water vapor or carbon dioxide. Another important metric of material efficiency in green chemistry is that of atom economy (Trost 1991). Atom economy calculates the efficiency

with which the atoms that are used as feedstocks in the chemical transformation are incorporated into the final desired product. Unlike E-factor, which is a measure of conditional efficiency—that is, efficiency based on the specific conditions and circumstances employed within a process—atom economy is an intrinsic efficiency metric that measures how inherently materially efficient a process can be if all of the conditions are perfect. This measure is most frequently applied to chemical transformations where substances of discrete molecular structure are transformed into new, homogeneous target products. Although atom economy calculations can be applied in some circumstances to the production of nanomaterials, in many cases the analysis cannot be done, as often in nano manufacturing systems either the feedstocks or the products do not necessarily have a discrete molecular structure but rather have a distribution of structures with a range of molecular weights. So, for most situations, E-factor will be generally more appropriate. Atom economy and E-factor have been used by many in the chemical industry and have the potential to serve as a bridge between chemistry measures of sustainability that focus just on production and those that employ more comprehensive life cycle analyses.

Methods and Analysis

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

On the basis of a review of the synthesis literature, we chose to examine conventional synthesis techniques for three nanomaterials: carbon nanotubes, fullerenes, and titanium dioxide. We then considered several traditional and green syntheses for a single nanomaterial, namely gold nanoparticles. Table 1 shows a list of synthesis techniques considered here.

Table 1.  Representative synthesis methods and associated references
NanomaterialSynthesis techniqueReference
Carbon nanotubes (CNT)Thermal chemical vapor depositionLekas (2006), Kong et al. (1998)
Fullerenes (C-60, C-70)Benzene–oxygen flameKratschmer and colleagues (1990), Zhennan and colleagues (1991), Howard and colleagues (1992)
Titanium dioxide (TiO2)Hydrolysis-calcinationVerhulst and colleagues (2003), Okuyama and Lenggoro (2003)
Gold nanoparticlesPhosphine-stabilizedSchmid and colleagues (1981)
Phosphine-stabilizedWeare and colleagues (2000)
Thiol-stabilized liquid–liquidBrust and colleagues (1994)
Thiol-functionalized ionic liquidsKim and colleagues (2004)
Uptake by alfalfaGardea-Torresdey and colleagues (2002)
Starch-glucoseRaveendran and colleagues (2006)

For each process, the mass inputs of the reagents were gathered from the original published syntheses described in experimental methods sections of the relevant articles. Mass balance and reaction stoichiometry were used to derive the mass flows of outputs and by-products for which data were not provided. In a few cases, the yield of the process was not reported in the literature; these were found through personal communications or assumed to be the industry best-case scenario of approximately 10%. The analysis of Robichaud and colleagues (2005) was also used as a data source for several nano production processes. Some reagents and catalysts used in nanomaterial production are not readily commercially available and so must be synthesized prior to their use (e.g., ionic liquids or metallic nanoparticles for CNT growth); the chemical inputs for producing these reagents are included in the present analysis and represent the only inclusion of upstream effects. An example of the E-factor calculation for the thiol-stabilized liquid–liquid synthesis of gold nanoparticles is shown in table 2 (Brust et al. 1994).

Table 2. E-factor measures for thiol-stabilized liquid–liquid synthesis of gold nanoparticles
CompoundReported amountUnit conversionInput mass (g)
  1. Note: M = molar units; mM = millimolar units; mwt = molar weight.

Reagents
 Sodium borohydride (NaBH4)25 mL, 0.4M37.8 mwt  0.38
 Hydrogen tetrachloroaurate (HAuCl4)30 mL, 30 mM339.8 mwt  0.31
 Tetraoctlyammonium bromide50 mM in toluene546.8 mwt  2.19
 Dodecanethiol (C12H25SH)170 mg  0.17
Total (R)  3.00
Solvent
 Toluene (solvent)80 mL0.865 g/mL 69.2
Total (S)69.2
Purification
 Toluene (solvent)80 mL0.865 g/mL 69.2
 Ethanol (first wash)400 mL0.787 g/mL314.8
 Toluene (purification)10 mL0.865 g/mL  8.65
 Ethanol (second wash)400 mL0.787 g/mL314.8
Total (P)638.3   
Total (R+S+P)710.5   
Product
 Gold nanoparticles (Prod)    0.21
E-factor with washing = (R+S+P)/(Prod) (710.5/0.21)3,320    
E-factor without washing = (R+S)/(Prod) (72.2/0.21)  338

Results and Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

Table 3 shows the results of the calculations of E-factor for all nanomaterial production processes under consideration. E-factors are shown both including and excluding solvents used for washing and purification.

Table 3. E-factor measures for selected nanomaterial syntheses
(Nanomaterial) Synthesis techniqueE-factor with purificationE-factor without purification
  1. Note: CNT = carbon nanotube; TiO2= titanium dioxide.

  2. aThe amount of toluene used for purification is negligible compared to the gaseous inputs.

  3. bGold nanoparticles were characterized in situ for this study, so that extraction from plant tissue (a purification step) did not take place.

  4. cThe resulting “starched” gold nanoparticles are not further purified.

(CNTs) Chemical vapor deposition  170  22
(Fullerenes) Benzene–oxygen flame   950 950a
(TiO2) Hydrolysis-calcination17,800  44
(Gold nanoparticles)
 Phosphine-stabilized (Schmid et al. 1981) 7,2001,760 
 Phosphine-stabilized (Weare et al. 2000)19,200 200
 Thiol-stabilized liquid–liquid 3,320 340
 Thiol-functionalized ionic liquids99,40096,400  
 Uptake by alfalfa   163b
 Starch-glucose29,60029,600c

Our analysis shows that the E-factors for nanomaterials production are highly variable (over three orders of magnitude) and are also quite high in comparison with those for other materials, as shown in table 4. The E-factors for gold nanoparticles varied widely with the different syntheses. As can be clearly seen from table 3, the E-factors for several syntheses are dominated by the contribution of solvents and washes for purification, which are used in much larger amounts than are direct product inputs. The difference in E-factors with and without purification steps is most marked in the hydrolysis-calcination technique to produce TiO2 (which uses large amounts of ethanol for washing of the nanoparticles), a difference of nearly 400-fold. For the phosphine-stabilized method of Weare and colleagues (2000), the difference is closer to 100-fold, whereas CVD and the methods of Brust and colleagues (1994) and Schmid and colleagues (1981) have differences of only 10-fold. For the remaining syntheses, there is not a significant difference between the two E-factors. This is because the quantities of solvents used for washing in these procedures are much smaller than for the actual synthesis reactions.

Table 4.  Annual throughput and E-factor comparison of different material classes
Industry segmentProduct throughput [log kg]E-factor
  1. Source: Adapted from Sheldon (1997).

Bulk chemicals7–9<1–5
Fine chemicals5–75–50
Pharmaceuticals4–625–>100
Nanomaterials2–3100–100,000

The synthetic chemistry community is well aware of the environmental and economic disadvantages that stem from the need for extensive purification. Currently, one of the most exciting areas of the field of green chemistry is the development of alternate purification techniques for nanoparticles or syntheses that eliminate purification steps entirely (e.g., Sweeney et al. 2006). Commercialization of nanomaterials production processes will also likely decrease E-factors as companies strive to increase yield and reduce expensive chemical waste.

There are a number of reasons for the exceptional E-factor score for thermal CVD of CNTs. The feedstocks for the process are mostly in gaseous form, obviating the need for liquid solvents. The yield is quite high compared with other synthesis processes, which reduces the effect of the hydrochloric acid bath used for CNT purification. The analysis does take into account the indirect material inputs associated with the ferritic nanoparticle growth catalysts, but this is mostly methanol and does not contribute significantly to the overall E-factor. CNTs produced via CVD also require purification (with hydrochloric acid to remove the metallic growth catalysts); again, it is clear from table 3 that this step is the main source of nonproduct waste.

Although it may be the most environmentally benign process in terms of chemical toxicity, the starch-glucose technique has such a large starch:gold ratio that the E-factor is fairly high. For the phosphine- and thiol-stabilized particles, the use of alcohols for washing and purification greatly increases the E-factors. In the case of thiol-functionalized ionic liquids, a large amount of methylene chloride is used for separation at two different stages, and toluene is used as a solvent. These are the drivers for the extremely high E-factor of that process.

Any interpretation of these results warrants a number of cautions. The present analysis is largely based on bench-scale syntheses; any commercialized production will certainly aim to increase yields and decrease solvent use, mainly for economic reasons. The inclusion of water inputs or water vapor and carbon dioxide by-products, as required under any consideration of mass balance, would change the relative E-factor scores, particularly those for the biological uptake and starch-glucose techniques. For the various syntheses of gold nanoparticles, it is important to note that the functional units (i.e., the nanoparticles themselves) are rarely equivalent; they will differ in average size and dispersion, they will have different functionality (or chemical reactivity), they may be stabilized with different compounds, and they will likely differ in purity. Finally, the E-factor calculations are based on synthesis procedures that are constantly being modified by different groups and so are subject to a high degree of uncertainty. The important result here is the range of values for E-factors of nanomaterials relative to other materials; the use of absolute values or ratios among nanomaterials should be avoided or taken up only qualitatively.

The analysis presented here is not a comprehensive treatment that might be offered by a full-scale LCA of process reagents; performing such a study would greatly aid the evaluation of environmental impact of nanomaterials themselves. Even within the production stage, the present focus on mass ignores toxicity associated with acute or chronic exposure, synergistic effects from mixtures, and any consideration of ecosystem impacts. The main reason for this omission is that many of the reagents used in nanomaterial synthesis have not been subjected to the battery of toxicological tests that would allow for a quantitative measure of these broad classes of risk to human and ecosystem health. In particular, the common input reagent for gold nanoparticles, hydrogen tetrachloroaurate trihydrate (HAu4Cl), lacks even basic toxicological data. Several other chemicals considered here are lacking in reported toxicological data in primary, peer-reviewed sources. Nonetheless, as toxicity measures are refined and hazard databases expand to include more substances, such an assessment will become possible.

With respect to nanomaterials themselves, most are so new and have been so little studied that there are not enough data for detailed LCAs or material flow analyses. Eventually, as the production capacity for nanomaterials increases, it is likely that comprehensive evaluations of environmental impact will become more common.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

The young field of nanotechnology has immense potential to provide a wide range of important advances for the benefit of society. It will be essential as we pursue these benefits that we do so through thoughtful design of nanomaterials and processes. It is clear from the initial review provided here that the early approaches to the production of nanomaterials have a number of areas of concern for human health and the environment. Some of these concerns are beginning to be addressed through the engagement of green chemistry and green engineering to make the next generation of designs as environmentally benign as possible. These initial efforts have largely been focused on improvements to the material efficiency of the manufacturing process. In the next phase of green chemistry and engineering engagement in the nano realm, scientists, engineers, and designers should seek to address all stages of the life cycle for improved energy and mass efficiency and to understand underlying hazards with a systems perspective. The use of design frameworks such as the principles of green chemistry and green engineering will be one essential element in ensuring that green nano provides the basis for safer innovations in this emerging field.

Notes
  • 1

    Editor's note: For a description and analysis of the risk perception study by Kahan and colleagues, see “Americans’ Nanotechnology Risk Perception: Assessing Opinion Change” (Smiley-Smith et al. 2008) in this issue.

  • 2

    Editor's note: A review of nanomanufacturing methods and some of their environmental impacts can be found in an article by Şengül and colleagues (2008) in this issue.

  • 3

    Editor's note: Environmental assessments of CNTs by Healy et al. (2008) and Khanna and colleagues (2008) can be found in this issue.

  • 4

    J.-P. Lange of Shell Co. produced an analysis for many different petrochemicals using E-factor over the entire production phase (Lange 2002). He mentions the need to extend the analysis to the use and disposal stages as well. In the present study, we attempt to apply the measure of E-factor to various synthesis processes for different nanomaterials.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors
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About the Authors

  1. Top of page
  2. Summary
  3. Introduction
  4. Design Through the Principles of Green Chemistry and Green Engineering
  5. Green Nano
  6. Current Examples of Green Nano
  7. E-factor Analysis of Nanomaterials Production
  8. Methods and Analysis
  9. Results and Discussion
  10. Conclusions
  11. References
  12. About the Authors

Matthew J. Eckelman is a doctoral student in environmental engineering at Yale University in New Haven, Connecticut. Julie B. Zimmerman is an assistant professor at Yale University jointly appointed to the Environmental Engineering Program in the Department of Chemical Engineering and the School of Forestry and Environmental Studies. Paul T. Anastas is Professor in the Practice of Green Chemistry and the director of the Center for Green Chemistry and Green Engineering at Yale University, which focuses on the design and implementation of sustainable products, processes, and systems.