نانو مواد ضد میکروبی برای ضدعفونی سازی آب و کنترل میکروبی آن: کاربرد ها و پیامد های بالقوه Antimicrobial nanomaterials for water disinfection and microbial control: Potential applications and implications

۱٫ Introduction The use of sand filtration and chlorine disinfection marked the end of waterborne epidemics in the developed world more than a century ago. However, outbreaks of water borne diseases continue to occur at unexpected high levels. According to the data compiled from Center of Disease Control Morbidity and Mortality Weekly Report, there were 155 outbreaks and 431,846 cases of illness in public and individual U.S. water systems from 1991 to 2000 (Chlorine Chemistry Division of the American Chemistry Council, 2003). Worldwide, waterborne diseases remain the leading cause of death in many developing nations. According to the 2004 WHO report, at least one-sixth of the world population (1.1 billion people) lack access to safe water (WHO, 2004). The consequences are daunting: diarrhea kills about 2.2 million people every year, mostly children under the age of 5. The importance of water disinfection and microbial control cannot be overstated. Although disinfection methods currently used in drinking water treatment can effectively control microbial pathogens, research in the past few decades have revealed a dilemma between effective disinfection and formation of harmful disinfection byproducts (DBPs). Chemical disinfectants commonly used by the water industry such as free chlorine, chloramines and ozone can react with various constituents in natural water to form DBPs, many of which are carcinogens. More than 600 DBPs have been reported in the literature (Krasner et al., 2006). Considering the mechanisms of DBP formation, it has been predicted that DBPs will be formed any time chemical oxidants are used in water treatment (Trussell, 1993). Furthermore, the resistance of some pathogens, such as Cryptosporidium and Giardia, to conventional chemical disinfectants requires extremely high disinfectant dosage, leading to aggravated DBP formation. Therefore, there is an urgent need to reevaluate conventional disinfection methods and to consider innovative approaches that enhance the reliability and robustness of disinfection while avoiding DBP formation. The rapid growth in nanotechnology has spurred significant interest in the environmental applications of nanomaterials. In particular, its potential to revolutionize century-old conventional water treatment processes has been enunciated recently (USEPA, 2007; Shannon et al., 2008). Nanomaterials are excellent adsorbents, catalysts, and sensors due to their large specific surface area and high reactivity. More recently, several natural and engineered nanomaterials have also been shown to have strong antimicrobial properties, including chitosan (Qi et al., 2004), silver nanoparticles (nAg) (Morones et al., 2005), photocatalytic TiO2 (Cho et al., 2005; Wei et al., 1994), fullerol (Badireddy et al., 2007), aqueous fullerene nanoparticles (nC60) (Lyon et al., 2006), and carbon nanotubes (CNT) (Kang et al., 2007). Unlike conventional chemical disinfectants, these antimicrobial nanomaterials are not strong oxidants and are relatively inert in water. Therefore, they are not expected to produce harmful DBPs. If properly incorporated into treatment processes, they have the potential to replace or enhance conventional disinfection methods. Another potential application of antimicrobial nanomaterials is their use in decentralized or point-of-use water treatment and reuse systems. The concept of decentralized or distributed water treatment systems has attracted much attention in recent years due to concerns on water loss and quality deterioration associated with aging distribution networks and the increasing energy cost to transport water, as well as the increasing need for alternative water sources and wastewater reuse for areas with water shortage problems and national security issues (NRC, 2006; Haas, 2000). It is envisioned that functional nanomaterials, including those with antimicrobial properties, can be used to build high-performance, small-scale or point-of-use systems to increase the robustness of water supply networks, for water systems not connected to a central network, and for emergency response following catastrophic events. This paper reviews the antimicrobial mechanisms of several nanomaterials, discusses their applicability for water disinfection and microbial control as well as the limitations, and highlights critical research needs to realize nanotechnology-enabled disinfection and microbial control.