Nanomaterials with similar or higher disinfection effi cacies than conventional water treatment would make excellent alternative treatments or could be used in conjunction with existing technologies such as ultraviolet (UV) disinfection. They could also be used for biofouling control for water fi ltration membranes and other surfaces in water treatment reactors and distribution pipelines. The antimicrobial mechanisms as well as current and potential applications of several nanomaterials in microbial control are summarized in Table 12.1 . Their merits and limitations relevant to water disinfection are discussed later.
12.2.1 Nanosilver
Silver is the most commonly used nanomaterial for microbial control. Several antimicrobial mechanisms of nanosilver (nAg) have been postulated, such as adhesion to cell surface altering the membrane properties, penetration inside bacteria resulting in DNA damage, and the release of antimicrobial Ag + ions [ 6 , 7 ]. Currently, there are over one hundred consumer products that contain nAg as an antimicrobial agent including nutrition supplements, food storage containers, kitchenware, refrigerators, textiles, laundry additives, washing machines, paints, faucets, sanitizers, contact lens solutions, catheters, and wound dressings [ 8–10 ]. Several home water purifi cation systems utilizing nAg are available on the market, for example, Aquapure ® , Kinetico ® , and
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QSI-Nano ® . These systems can remove 99.99 percent of pathogenic bacteria, viruses, protozoa, and cysts. The leaching of nAg particles or Ag + ions in polished water is undetectable. Future applications of nAg include coating of pipes in distributions systems to prevent regrowth of pathogens, and incorporation into membranes for large-scale water fi ltration [ 11 , 12 ].
12.2.2 Titanium Oxide
The antibacterial activity of titanium oxide (TiO 2 ) is related to ROS production, especially peroxide and hydroxyl radicals under UV-A (320–400 nm) irradiation via both oxidative and reductive pathways [ 13 ]. However, bactericidal activity of TiO 2 (330 nm average aggregate size) has also been observed in the dark, indicating that other mechanisms may be involved [ 14 ]. Commercial water purifi cation systems based on TiO 2 photocatalysis already exist (e.g., Purifi cs ® ). Studies on the photocatalytic disinfection effi ciency of TiO 2 are relatively few, but have demonstrated the potential benefi ts of using TiO 2 for drinking water disinfection (Table 12.1). The most promising property of TiO 2 - based disinfection is probably its photoactivation by sunlight. Complete inactivation of fecal coliforms was achieved in 15 minutes at an initial bacterial concentration of 3000 cfu/100 mL in a study using water stored in a plastic container that was coated inside with TiO 2 and exposed to sunlight [ 15 ].
Disinfection systems such as this will be especially useful in developing countries
Table 12.1 Applications of Nanomaterials Utilizing Antimicrobial Properties Nanomaterial Antimicrobial
mechanism
Current applications Potential future applications nAg Release of Ag +
ions, disruption of cell membrane and electron transport
Potable water fi lters, clothing, medical devices, coatings, washing machines, refrigerators, food storage
Surface coatings, membranes
TiO 2 Production
of ROS, cell membrane and cell wall damage
Air purifi ers, water purifi ers
Solar and UV disinfection of water and wastewater, reactive membranes, hollow fi bers, biofouling-resistant surfaces
CNT Physically
compromise cell envelope
None Biofouling-resistant
membranes, carbon hollow fi bers, packing in fi xed bed columns CNT: Carbon nanotubes; ROS: Reactive oxygen species; UV: Ultraviolet.
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where infrastructure and electricity for water treatment are not available.
However, TiO 2 -based solar disinfection is in general a very slow process due to the small fraction of UV-A in solar radiation. Success in research on metal or nitrogen doping to improve visible light absorbance of TiO 2 is critical to the application of TiO 2 solar disinfection. Recently, it was demonstrated that doping TiO 2 with silver greatly improved UV-A photocatalytic bacterial inactivation by TiO 2 (30 nm) [ 16 ]. In another study, 1 percent nAg in P-25 TiO 2 (w/w) reduced the reaction time required for complete removal of 10 7 cells/mL E. coli from 65 to 16 minutes [ 17 ]. Silver is believed to enhance photoactivity by facilitating electron-hole separation and/or providing more surface area for adsorption. Therefore, silver doping is expected to enhance disinfection by TiO 2 (e.g., 17 nm) under all wavelengths of UV as well as solar radiation [ 18 ].
12.2.3 Fullerenes
Fullerenes are not currently used in water disinfection, but certain types of fullerenes have potential applicability. Hydroxylated C 60 or fullerol, which is relatively nontoxic [ 19 ], exhibits photochemical activity that could be exploited for disinfection or degradation [ 20 ]. However, it is neither an inexpensive nor readily available alternative to TiO 2 , which is a more established and stronger photo-oxidant. Compared to TiO 2 , fullerenes produce signifi cantly lower amounts of hydroxyl radicals, which are the strongest oxy-radicals. Another obstacle to the use of fullerol in water treatment is the diffi culty in immobilizing, separating, and recycling fullerol nanoparticles. No method currently exists to easily and cost-effi ciently remove these small, light nanomaterials. However, grafting functionalized fullerenes to a surface could be considered. C 60 encapsulated in polyvinyl pyrrolidone (PVP) exhibits antibacterial activity [ 21 ] and photoactivity, albeit with undetermined toxicity to humans. The functional groups on its organic cage might facilitate its anchorage to a surface without losing its antibacterial properties, a property desirable in disinfection applications involving fi xed beds, membranes, or surfaces.
Carbon nanotubes (CNTs) represent another class of fullerenes that have been reported to exhibit antimicrobial properties. Knowing that a physical contact might be needed to kill bacteria [ 22 , 23 ], CNTs can be exploited in several ways for disinfection applications. First, single-walled nanotubes (SWNT) could be coated and immobilized on fi lters [ 22 ]. Additionally, multi- walled CNTs could be made into hollow fi bers [ 24 ]. These nanotube fi lters were able to remove microbial contaminants such as E. coli and poliovirus, and they also off er several advantages including increased mechanical strength, heat resistance, and easy cleaning. Bundles of nonaligned single- or multi-walled nanotubes contained within a fi lter, could also be considered as packing in a fi xed-bed fi lter (Table 12.1).
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12.2.4 Combining Current Technologies with Nanotechnology
The most economic and, therefore, most likely use of nanotechnology for water treatment would involve incorporating it into existing treatment strategies.
The use of nanomaterials could enhance the performance of chlorination, advanced oxidation processes, and membrane fi ltration systems, especially in large centralized water treatment systems. In contrast, smaller point-of-use systems in the near future could be based entirely on nanotechnology.
While chlorination and ozonation are eff ective for the removal of bacteria and viruses, they are ineff ective against cyst-forming protozoa such as Giardia and Cryptosporidium . UV-C can kill these organisms, but UV alone is relatively ineff ective against viruses unless the contact time and energy output are signifi cantly increased. This suggests an opportunity to exploit the photosensitivity of nanomaterials, such as some fullerenes [ 20 ] and TiO 2 that produce ROS to enhance UV disinfection. Large-scale UV reactors internally coated with TiO 2 have already been shown to enhance water disinfection rate [ 25 ]. Additionally, TiO 2 (e.g., P-25) can degrade a wide range of organic contaminants including natural organic matter, a major membrane foulant [ 26 ].
The increasing application of membranes for drinking water and wastewater treatment [ 27 ] promises another attractive application of nanomaterials in water treatment. In spite of the advantages membrane systems off er, the inherent problem of organic fouling and biofouling poses the biggest obstacle to their broader application. Nanomaterials can be incorporated into membranes to enhance their mechanical strength and anti-fouling capacities. Polymeric and ceramic membranes containing TiO 2 (8–10 nm) were found to be highly effi cient in destroying a number of organic contaminants and pathogenic microorganisms in the presence of UV-A irradiation; these membranes are hence less vulnerable to organic and biological fouling [ 28 , 29 ]. Similarly, nanocomposite membranes incorporating other functional (e.g., catalytic, photocatalytic, and antimicrobial) nanoparticles into water treatment membranes can be developed.
The nanoparticles immobilized on membranes will reduce membrane fouling by degrading organic and biological foulants, as well as remove contaminants that are not rejected by membranes. For photocatalysts, an outside-in submerged microfi ltration (MF) or ultrafi ltration (UF) membrane reactor confi guration can be utilized to allow introduction of light using submerged sources such as optical fi bers.