Transmission electron microscopic observations of nanobubbles and their capture of impurities in wastewater

Abstract

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

Generation of micro- and nano-bubbles in water by dissociation of gas hydrates

“…As found previously [10], MNBs were identified as spherical or oval holes in the replica film. To estimate the size of the MNBs, we measured both the major and minor axes of the hole and calculated the equivalent diameter of a sphere having the same cross-section.…”

“…Thus the estimated D tends to be smaller. However, the TEM-cross-section method nevertheless yields a size distribution nearly the same as that obtained by a dynamic light scattering method [10]. Thus, to qualitatively compare between replica samples after the CH 4 hydrate dissociation this TEM-cross-section method should be adequate.…”

“…In this condition, the freezing rate exceeds 10 3 K min -1 . For the subsequent freeze-fracture replica formation, we followed the procedure in [10,26], and give only a brief description here. To reduce the formation of artifacts during the replica film formation, the frozen droplet was fractured under vacuum (10 -4 to 10 -5 Pa) and low temperature (about 100 K).…”

“…While the shrinkage of MBs in water can be observed in an optical microscope, it is impossible to determine visually whether the bubble finally disappears by dissolving in the water or remains as a MNB of diameter too small to resolve with a microscope. But recently, MNBs have been observed in bulk liquids by using electron microscopy with the freeze-fracture replica technique [7][8][9][10].…”

Collapse of Surface Nanobubbles

“…They can result from hydrophobic silanized surfaces after contact with water [20] or be obtained from flow lines as recently reported [27,28]. Also, solid contaminations originating from the solvent were reported in nanobubble research [21,32]. These contaminations have some properties such as nanoscopic size, shape, long lifetime (stability), and deformability [20,21,27,32] very much in common with nanobubbles.…”