and Research Needs
The relatively high potency of some antimicrobial nanoparticles and their increasing availability and aff ordability make them attractive for water
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disinfection applications. Some antimicrobial nanomaterials off er potential advantages over traditional chemical disinfectants that are prone to generating harmful disinfection by-products and experience short-lived reactivity.
Nanomaterial disinfectants do not provide a disinfection residual in distribution systems; consequently, they might not be suitable alone in municipal water treatment facilities. Most likely, nanoparticles will be used to enhance water disinfection applications, such as UV inactivation of viruses, solar disinfection of bacteria, and membrane fi ltration processes that are more resistant to biofouling. Although potential economic and logistic limitations currently preclude the widespread application of such nanoparticles, a potential paradigm shift toward decentralized water treatment and reuse systems will likely stimulate research activity in nanotechnology-enabled microbial control. The future research discussed here is likely to overcome many of the current technical limitations and help discern viable applications for nanotechnology to enhance disinfection and sustainable water management.
Signifi cant limitations exist for the use of nanomaterials for microbial control.
For nanomaterials in suspension, design obstacles include loss of antimicrobial activity in the presence of high NOM and salt concentrations, nanoparticle aggregation, and loss of nanoparticles from the system if not retained or recycled.
For example, although the buckminsterfullerene water suspension (THF/nC 60 ) is noted for its strong antibacterial activity [ 30 ], it is an unlikely water treatment candidate not only due to its potential human cytotoxicity [ 19 ], but also because of the loss of antimicrobial eff ect due to interactions with NOM that decrease bioavailability or the presence of salts that promote coagulation and precipitation [ 31 ]. These limitations are likely to be shared by other nanoparticle suspensions. Furthermore, good dispersion of nanoparticles in water is required for full utilization of the reactive surfaces, and an effi cient separation process is required downstream to retain the nanoparticles. One approach that might enhance retention and recycling of suspended nanoparticles is to mount them onto magnetic platforms such as the magnetite nanoparticles that were recently used to remove arsenic from potable water [ 32 ]. Since magnetite nanoparticles can be separated from water by a relatively low magnetic fi eld, they could be used as a platform to develop multifunctional nanocomposite materials ( Fig.
12.1 ) that are subject to magnetic separation. This would enable both chemical disinfection and photocatalytic destruction of waterborne pathogens while ensuring retention of the nanomaterials.
Immobilization of nanomaterials on reactor surfaces or water fi ltration membranes eliminates the need for separation, but the effi ciency of disinfection may be compromised by the lower eff ective nanomaterial dose, reduced access to light source, and sometimes loss of reactive surface area. When coated on surfaces in contact with potable water to prevent microbial attachment and biofi lm formation, antimicrobial nanoparticle coatings are likely to rapidly lose their eff ectiveness due to adsorption of extracellular polymeric material and occlusion by precipitating debris. In addition, nanoparticles can escape the reactor and enter drinking water if not properly immobilized, which is not
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desirable for both economic and human or environmental health concerns.
Research on methods to anchor nanoparticles to reactor surfaces or to the selective layer of fi ltration membranes, and to separate and retain suspended nanoparticles, will be of paramount importance to decrease costs associated with premature loss and potential environmental impacts.
From an economic perspective, research needs to be conducted on the scalability and competitiveness of using antimicrobial nanoparticles for disinfection and microbial control, especially in comparison to established methods such as chlorination, ozonation, and UV treatment. Such economic analysis should consider relevant externalities associated with the potential environmental impacts in the event that the nanoparticles escape disinfection reactors. These concerns and the design diffi culties make it premature to recommend the broad application of nanomaterials as disinfectants in water treatment. Nevertheless, it is likely that future research will overcome many of the current technical limitations and help discern viable applications for nanotechnology to enhance disinfection and sustainable water management.
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© 2009 William Andrew Inc.
Fullerene Nanomaterials in Water Treatment and Reuse
So-Ryong Chae , Ernest M. Hotze , and Mark R. Wiesner Department of Civil and Environmental Engineering, School of Engineering,
Duke University, Durham, NC, USA
13.1 Introduction 168