Transmission electron microscopic observations of nanobubbles and their capture of impurities in wastewater
© Uchida et al; licensee Springer. 2011
Received: 17 December 2010
Accepted: 5 April 2011
Published: 5 April 2011
Unique properties of micro- and nanobubbles (MNBs), such as a high adsorption of impurities on their surface, are difficult to verify because MNBs are too small to observe directly. We thus used a transmission electron microscope (TEM) with the freeze-fractured replica method to observe oxygen (O2) MNBs in solutions. MNBs in pure water and in 1% NaCl solutions were spherical or oval. Their size distribution estimated from TEM images close to that of the original solution is measured by light-scattered methods. When we applied this technique to the observation of O2 MNBs formed in the wastewater of a sewage plant, we found the characteristic features of spherical MNBs that adsorbed surrounding impurity particles on their surface.
PACS: 68.03.-g, 81.07.-b, 92.40.qc
Small bubbles, such as microbubbles (MBs; usually range from 10-4 to 10-6 m in diameter) and nanobubbles (NBs; less than 10-6 m in diameter), have various properties that differ from macroscopic bubbles (greater than 10-3 m in diameter). For example, smaller bubbles have lower buoyancies, so they take longer to reach the liquid surface and thus they have longer residence times. Also micro- and nanobubbles (MNBs) have either negative or positive zeta potentials [1, 2]. This property inhibits the easy agglomeration or coalescence of bubbles and results in the relatively uniform size distribution of MNBs. Additionally, the smaller the bubble, the larger the specific interfacial area. Thus, the efficient physical adsorption of impurities included in the solutions on the bubble surface is expected. MNBs have now attracted attention for applications in engineering areas such as the sewage treatment of wastewater by air flotation [3, 4, 5, 6] detergent-free cleaning of adsorbed proteins [7, 8].
Moreover, as expected from the Young-Laplace equation, the smaller the bubble, the higher the pressure inside it. Therefore, the driving force for mass transfer from gas phase to surrounding liquid increases with decreasing bubble size. The gas solubility and the chemical reactions at the gas-liquid boundary are thought to be enhanced injecting the MNBs instead of normal aeration of macroscopic bubbles. MNBs have thus also attracted much attention as a functional material in the biological area, such as accelerating metabolism in vegetables , aerobic cultivation of yeast , and sterilization by a mixture of ozone MBs .
MBs have been observed by an optical microscope [12, 13] to shrink in water with dissolving gas molecules in surrounding water and with increasing internal gas pressures. However, when bubbles become smaller than the spatial resolution of the optical microscope, it is difficult to recognize whether the bubble finally disappears by dissolving in water or it remains in water as a NB. The lifetime of MNB is also not agreed upon. Early studies suggested that the life time of NBs (10 to 100 nm in radius) in water was between 10-6 and 10-4 s (estimated by the simulation ), or that no evidence of carbon dioxide NB existence was found in ethanol solution by static and dynamic light scattering and infrared spectroscopy . These conclusions are inconsistent with those observed in the engineering or biological investigations reported previously. In order to use MNBs for such practical applications, it is necessary to observe them directly and to reveal their fundamental properties.
The present study focused on finding evidence of existing MNBs and their functions, especially NBs, in the liquid phase using a transmission electron microscope (TEM) along with the freeze-fractured replica technique. This technique has usually been applied for biological investigations but is also useful for investigating the microstructures and the dynamic features of MNBs in solution when a small droplet is quenched at liquid nitrogen temperature [16, 17, 18]. To verify the effectiveness of this technique, we first observed oxygen (O2) MNBs formed in pure water. We then applied this technique to a commercially obtained MNB solution containing 1% NaCl, and finally to a wastewater solution from a sewage plant.
We prepared a pure MNB solution by introducing pure O2 gas (Nissan Tanaka Co., Saitama, Japan; purity of 99.999%) into the ultra-high purity water (Kanto Chem. Co., Inc., Tokyo, Japan) with a MNB generator (Aura Tec Co. Ltd., Fukuoka, Japan, OM4-MDG-045) operating for 120 min at 293 K. Since this sample preparation procedure was similar to that used in the previous work , the average bubble size was estimated as 140 nm, and the zeta potential of bubbles to be -40 mV. Based on dynamic light scattering (DLS) measurement (Quantum Design Japan Inc., Tokyo, Japan, Nanosight-LM10), the number concentration of MNBs was estimated to be on the order of 107 cm-3 of solution under the same sample preparation conditions.
The details of the replica sample preparation were mentioned elsewhere , so we explain them just briefly here. A small amount of this solution (10 to 20 mm3) was put on an Au-coated Cu sample holder and was rapidly frozen by immersing it into a liquid nitrogen bath. In this condition, the freezing rate ranged from 102 to 103 K min-1. The frozen droplet was then fractured under vacuum (10-4 to 10-5 Pa) and low temperature (approximately 100 K) to reduce the formation of artifacts. The replica film of this fractured surface was prepared by evaporating platinum and carbon (JEOL Ltd., Tokyo, Japan, JFD-9010) prior to removing the replica film from the ice body by melting. We used a field-emission gun-type TEM (JEOL Ltd., Tokyo, Japan, JEM-2010) to observe the replica film at a 200-kV acceleration voltage. An imaging plate (Fujifilm Co., Tokyo, Japan, FDL-UR-V) was used for acquiring the observed image.
The same processes were used for MNBs in the dilute salt solution to investigate the effect of solutes on MNB existence in solutions. The O2 MNBs in water containing 1% NaCl were donated by REO Research Institute (Miyagi, Japan). We prepared the replica sample for this solution just after its delivery, when it took more than one week after the MNB formation.
Based on the above fundamental investigations for observing MNBs in solutions by the present experimental method, we observed the features of MNBs in the polluted water that was actually used for an engineering application. The polluted solution was sampled from a sewage plant as the wastewater of inositol extraction from defatted rice bran at Tsuno Rice Fine Chemicals Co., Ltd. (Wakayama, Japan). The polluted solution was expected to include several water-soluble impurities, such as glucide derived from rice starch (approximately 2 wt%) and calcium sulfate (almost saturated at room temperature), as well as insoluble micro particles. The original wastewater sample was milky-white with no macroscopic impurities. In this prototype plant manufactured by Mayekawa MFG. Co., Ltd., Ibaraki, Japan, pure O2 gas was aerated through the MNB generator (Nikuni Co., Ltd., Kanagawa, Japan, MBG20ND04Z-1GB) for 5 min. After aeration, some amounts of macroscopic insoluble impurities were observed in the bulk wastewater, which could have come from the grime in the plant system. However, the volume of sampled solutions used for the replica preparation was so small that we could exclude such macroscopic impurities easily. Solution droplets for the replica preparation were quenched just after the 5-min aeration at the plant site. The replica of the quenched sample was then prepared in the laboratory after transportation while maintaining the cryogenic temperature.
Results and discussion
We performed the TEM observation of the freeze-fracture replica to investigate the morphological features of MNBs in solutions. The MNBs in pure water were spherical or oval, and their size distribution ranged from 10-6 to 10-7 m, which was close to those obtained by the usual method for the MNB characterization (DLS measurement). Similar MNB features were observed in the TEM images of the 1% NaCl solution system, although the interaction between MNBs and the precipitated solute particles was not obvious. These results confirmed the feasibility of applying TEM observation with the freeze-fracture replica method for investigating MNBs in solutions.
When we applied this method to MNBs aerated in the wastewater of a sewage plant, we observed the special features of MNBs that collected surrounding impurities on their surfaces. The detailed investigation of obtained TEM images of the same wastewater suggested that the sweep area of a MNB in the solution was limited. Therefore, it is conceivable that the application of MNBs to engineering aspects is effective, but its total effectiveness would strongly depend on the number concentration of MNBs and on their lifetime.
micro- and nanobubbles
transmission electron microscope
dynamic light scattering.
A part of this study was financially supported by the Society for Techno-innovation of Agriculture, Forestry and Fishers (the Green project), organized by Dr. A. Iwamoto and Dr. K. Koide. TEM observations were financially supported by the Hokkaido Innovation through Nano Technology Support and technically supported by Dr. N. Sakaguchi and Dr. T. Shibayama (Hokkaido Univ.). The replica sample preparations were technically supported by Prof. K. Gohara and Dr. M. Nagayama (Hokkaido Univ.), and Dr. S. Okutomi (JEOL Ltd.). DLS measurement data was partly provided by Ms. A. Irie (Quantum Design Japan, Inc.) and I. Otsuka (Ohu Univ.).
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