Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Nanobubble Technologies Offer Opportunities To Improve Water Treatment Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Ariel J. Atkinson,† Onur G. Apul,‡ Orren Schneider,§ Sergi Garcia-Segura,*,† and Paul Westerhoff† Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/08/19. For personal use only.
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Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85287-5306, United States ‡ Department of Civil and Environmental Engineering, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States § Orren Schneider LLC, Plainsboro, New Jersey 08536, United States CONSPECTUS: Since first hypothesizing the existence of nanobubbles (NBs) in 1994, the empirical study of NB properties and commercialization of NB generators have rapidly evolved. NBs are stable spherical packages of gas within liquid and are operationally defined as having diameters less than 1000 nm, though they are typically in the range of 100 nm in one dimension. While theories still lack the ability to explain empirical evidence for formation of stable NBs in water, numerous NB applications have emerged in different fields, including water and wastewater purification where NBs offer the potential to replace or improve efficiency of current treatment processes. The United Nations identifies access to safe drinking water as a human right, and municipal and industrial wastewaters require purification before they enter water bodies. These protections require treatment technologies to remove naturally occurring (e.g., arsenic, chromium, fluoride, manganese, radionuclides, salts, selenium, natural organic matter, algal toxins), or anthropogenic (e.g., nitrate, phosphate, solvents, fuel additives, pharmaceuticals) chemicals and particles (e.g., virus, bacteria, oocysts, clays) that cause toxicity or aesthetic problems to make rivers, lakes, seawater, groundwater, or wastewater suitable for beneficial use or reuse in complex and evolving urban and rural water systems. NBs raise opportunities to improve current or enable new technologies for producing fewer byproducts and achieving safer water. This account explores the potential to exploit the unique properties of NBs for improving water treatment by answering key questions and proposing research opportunities regarding (1) observational versus theoretical existence of NBs, (2) ability of NBs to improve gas transfer into water or influence gas trapped on particle surfaces, (3) ability to produce quasi-stable reactive oxygen species (ROS) on the surface of NBs to oxidize pollutants and pathogens in water, (4) ability to improve particle aggregation through intraparticle NB bridging, and (5) ability to mitigate fouling on surfaces. We conclude with key insights and knowledge gaps requiring research to advance the use of NBs for water purification. Among the highest priorities is to develop techniques that measure NB size and surface properties in complex drinking and wastewater chemistries, which contain salts, organics, and a wide variety of inorganic and organic colloids. In the authors’ opinion, ROS production by NB may hold the greatest promise for usage in water treatment because it allows movement away from chemical-based oxidants (chlorine, ozone) that are costly, dangerous to handle, and produce harmful byproducts while helping achieve important treatment goals (e.g., destruction of organic pollutants, pathogens, biofilms). Because of the low chemical requirements to form NBs, NB technologies could be distributed throughout rapidly changing and increasingly decentralized water treatment systems in both developed and developing countries.
1. HOW CAN NANOBUBBLES ENHANCE CONVENTIONAL WATER TREATMENT? Hundreds of water treatment technologies exist and are used in many configurations in increasingly complex and evolving urban and rural water systems.1 Figure 1 summarizes key processes enabling these technologies including particle aggregation, foulant mitigation, disinfection and chemical oxidation, adsorption, or biological transformation. Nanobubbles (NBs) raise opportunities to improve current or © XXXX American Chemical Society
enable new technologies for producing fewer byproducts and achieving safer water.1,2 NBs are stable spherical packages of gas within liquid and are operationally defined as having diameters less than 1000 nm, though typically in the range of 100 nm in one dimension (Table 1).3 NBs with diameters < 13 nm rapidly (hours) and continuous production of •
OH could be advantageous in water distribution systems to
oxidize DBPs, reduce biofilms and pathogens, and substitute for chlorine. D
DOI: 10.1021/acs.accounts.8b00606 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 4. Time (t) dependent •OH oxidation of a fluorescence compound (APF; C0 = 1 μM) by NBs, which decrease in number concentration by 75% over 48 h and correspond with a decrease in dissolved oxygen. Data obtained from ref 25.
Figure 5. Green arrows show increased removal of total organic carbon (TOC)ad color in a simulated dyestuff wastewater by ozone-NBs versus macrobubbles. Data obtained from ref 47.
thus facilitating biodegradation.44 In an unpublished study, we added NBs at a wastewater treatment plant in Missouri; the organics and turbidity rapidly decreased, but ammonia removal was significantly reduced. Therefore, potential impacts of oxidative stress by NBs or ROS on the selective microbial community (e.g., nitrifying bacteria) should be investigated. Similarly, recent work demonstrates opportunities for oxygen delivery to hypoxic/anoxic zones in natural waters using nanobubbles loaded onto particles.45,46 However, few longterm wastewater aeration studies exist using NBs, and it would be important to quantify oxygen delivery, organic pollutant removal, and shifts in microbial ecology of the bacteria, which are critical to treat wastewater. Ozone, oxygen, or other low solubility gases are added to water for disinfection or oxidation purposes, and the capital cost (i.e., size of tanks, compressors) and energy required are highly dependent on gas transfer efficiencies. Compared with conventional diffusers, ozone-NB delivery devices doubled the
in bioreactors produce 5−10 mm-sized bubbles, achieving ∼30% oxygen transfer efficiency 4.5 m above the diffusers that are at the bottom of reactors.41,42 Aeration efficiency is limited by oxygen (1) transfer rate from gas to liquid and (2) uptake rate of microbes. As db decreases, the gas mass transfer increases due to greater total interfacial area, and rise velocity decreases due to less buoyancy. This results in longer persistence of NBs in the water column and gas from NBs going into solution as NBs burst.22 NBs bursting, dissolving, or coalescing in the water column instead of bursting at the surface may have secondary benefits. We found cholesterol, personal care products, and endocrine disruptors on aerosols generated at the surface of conventional aeration basins; the aerosols appear to pass through air/odor control and transmit these pollutants to the atmosphere.43 These hydrophobic pollutants associate with the gas−water interface. Presumably this also occurs for NBs and may be an important mechanism for lowering the molecular weight of organics in wastewater, E
DOI: 10.1021/acs.accounts.8b00606 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 6. Arrows represent time-dependent formation of gas bridges between solid surfaces coated with nanobubbles. Adapted with permission from ref 59. Copyright 2005 Elsevier.
kL, and the bubbles had a 2-fold longer t1/2 in the water column, improving ozone gas transfer efficiency.47 In one study, NBs reduced the ozone dose required to attain a 2-log coliform inactivation by >70%.48 Additionally, Figure 5 shows accelerated decolorization and ∼50% improved mineralization efficiency of organics (g TOC/g ozone) with the use of ozone NBs versus macrobubbles, across all doses, in a simulated wastewater.41 Oxygen transfer is also important in emerging water purification processes. We demonstrated in situ electrogeneration of H2O2 using carbonaceous electrodes instead of off-site delivery or on-site storage of H2O2 to enable AOP use.49,50 However, cathodic reduction of oxygen is limited by the mass transfer of O2 from the bulk to the cathode surface, limiting the Faradaic efficiency and thus •OH production in the presence of ultraviolet light or ferrous ion chemical addition.51,52 Addition of air-NBs into electrogeneration systems would lead to higher O2 mass transfer, thus enhancing H2O2 yield. A consensus exists that NBs improve gas transfer efficiency and enable additional beneficial reactions in water. The energy requirements for large-scale NB devices and potentially antagonistic effects on subsequent downstream processes (e.g., particle removal, shifts in microbial communities) at treatment facilities are less documented and will only be answered using longer-duration pilot testing.
Gas bridging between NBs and adsorbent media may also enable superior removal of organic molecules from water (Figure 1). For example, a significant increase in perfluorooctanesulfonate (PFOS) removal by carbon-based adsorbents was reported when NBs formed on the adsorbent surface.65 The polar sulfonic head of PFOS was speculated to orient toward the water phase while the fluorinated alkene chain aligned toward air-NBs on the hydrophobic carbon surfaces.65 If attractive forces are assumed to be uniform between NBs and the adsorbent surface, then hemispherical gas-NB formation on the surface is thermodynamically most favorable because the largest number of gas molecules would interact with the adsorbent surface. The net negative charge on NBs at the pH of drinking or wastewaters (pH 5−9)9 results in electrostatic attraction to positively charged adsorbent surfaces. NBs are simultaneously and paradoxically hydrophobic and negatively charged.65 This paradox could be considered from the viewpoint of Pickering Emulsions, where we found that solid nanoparticles (NPs) accumulate at the interface between water and nonpolar solvents.66 Because NPs and NBs have low mass, their movement is dominated by Brownian motion and may not overcome the Helmholtz surface tension at boundaries between different phases. To better understand and control NB formation on an adsorbent surface, the interactions between the π-electrons of graphitic surface, Hbond accepting/donating polar functional groups, and negatively charged NBs must be systematically investigated. The π-electron density of the graphitic surfaces may promote induced polar interactions between the available electrons of gas molecules (e.g., oxygen) in NBs. Similarly, permanent dipole moment of oxygen-containing functional groups may promote cluster formation around them via Lewis acid/base interactions.67,68 Considering the heterogeneity of carbon surfaces, multiple molecular interactions may govern NBs formation concurrently.69,70 Although NBs appear to enhance particle removal, additional research is needed to understand how NBs bridge surfaces and how bridging potentially enhances sorption of pollutants by media like activated carbon.
5. CAN NANOBUBBLES BUILD (GAS) BRIDGES TO AID IN PARTICULATE OR SURFACTANT REMOVAL? Particle removal is achieved by aggregating particles to enhance their separation through settling, filtration, or flotation.53 Particle−particle interactions are controlled by electrostatic, van der Waals, and hydrophobic forces. Most particles in drinking and wastewater are negatively charged. Metal-salt and cationic polymeric coagulants are added to destabilize particles and reduce electrostatic repulsion, aiding in aggregate-floc formation enhancing particle separations from water.54−58 Because NBs are similarly negatively charged, they can form gas bridges that enhance particle−particle aggregation.20 Figure 6 illustrates how NBs on surfaces coalesce and form a “bridge” between particles59 or between particles and filtration media,60 thus acting as a chemical-free means of enabling particle removal. In conventional dissolved air flotation (DAF), also used to separate particles in water, gases are supersaturated in pressurized water and then released into atmospherically pressurized water, spontaneously forming microbubbles (30− 70 μm diameter).61 This process also produces NBs, which may explain the improved particle removal of DAF compared with conventional microbubble-diffusors.62 DAF using NBs (250−1000 nm) clarified chemical mechanical planarization (CMP) wastewater twice as rapidly as coagulation and flocculation63 due to enhanced gas-bridging aggregation of CMP particles.64
6. CAN NANOBUBBLES INHIBIT AND REMOVE SURFACE FOULING? Biological, organic, and inorganic substances adhere onto wetted surfaces (e.g., sand, activated carbon, or metal oxide filter media; membranes; quartz-sleeved UV lamps; heat exchangers; pipes, sensors, or plumbing fixtures) and reduce efficiency of water treatment. Common strategies to mitigate or remove these foulants involve chemicals (surfactants, acids, bases, oxidants) or inducing localized high-energy hydrodynamic shear (e.g., increase turbulence, change flow direction). NBs’ high surface area and interfacial free energy could replace or augment the current fouling mitigation strategies. Ushida et al.71 demonstrated that water containing 80 nm NBs cleaned textiles soiled with hydrophobic organics (fats, proteins, and coloring agents) 5% better than water without F
DOI: 10.1021/acs.accounts.8b00606 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 7. BSA fouling of hydrophilic and hydrophobic silicon wafers over time at different BSA concentrations in water with and without NBs. Data obtained from ref 9.
nanoparticles (ENPs) accumulate at the interface between water and nonpolar cell membranes (i.e., lipid bilayers), and ENP loading at the interface is proportional to ENP surface area.75,76 Furthermore, high ENP surface loadings caused “holes” at the interface of the two phases, and chemicals partitioned across lipid bilayers. NBs, because of their similar size to ENPs, could similarly interact and inactivate bacteria that often foul membrane or other surfaces through disruption of cell structure. The vibrational motion77 of nanoparticles may induce shear forces that disrupt biofilms. Evidence supports the use of NBs to mitigate fouling for a wide variety of surfaces, leading to reduced chemical usage and energy-intensive washing. Additional research is needed to differentiate NB fouling mitigation mechanisms, physical (shear induced by turbulence, coalescence of NBs on surfaces increasing buoyancy of gas and dislodging biofilms) versus chemical (ROS or electrochemically evolved gases (O3, Cl2)).
NBs. Highly ordered pyrolytic graphite fouled with a common biofoulant (bovine serum albumin (BSA)) was shown, via atomic force microscopy, to be cleaned by in situ electrochemically produced NBs.72 Small defouled islands observed were thought to be caused by formation of ∼40 nm NBs, and the evolved NBs contained BSA. Liu and Craig reported that NBs and surfactants acted synergistically, improving the cleaning of hydrophilic surfaces fouled with lysozymes through the promotion of SDS/lysozyme complex formation.73 We showed a similar self-cleaning outcome on boron-doped, diamond-coated electrodes fouled with biofilms. Although NBs were not directly monitored during the experiments, the physical dislodging of biofilms observed was attributed to enhanced ROS production and scour on the surface.74 Similar scouring capabilities of NBs produced by electrolysis using a commercial cell (ec-H2O Nanoclean) were reported.9 NBs (112 ± 2 nm, 7.5 × 108 NBs/mL) removed 1 nm thick lysozyme films on silicon and achieved an 80% reduction of a 2.5 nm thick BSA film.9 Figure 7 shows how NBs inhibit the fouling of hydrophilic and hydrophobic silicon. At lower BSA concentrations (
