Micro–nanobubble technology and water-related application

Currently, there is a growing demand for water treatment technologies considering global environmental challenges such as degradation and depletion of water resources. Microand nanobubble (MNB) technology and its application for wastewater treatment has emerged as a problem-solving alternative for such challenges. This paper reviews the important studies on water treatment in the areas of MNBs and discusses their fundamental properties, such as bubble stability (as tiny entities in water solutions), generation methods, and various chemical and physical features. The paper further overviews the current status of MNB application in water treatment processes such as flotation, aeration, and disinfection and its uses in various sectors, including agriculture, aquaculture, medical, and industry. Based on this review, studies regarding MNBs’ basic properties, generation, and application are identified and recognized for future research. This study concludes that despite the promising role of MNBs in water-related application, the current status of research has not reached its true potential. Specifically, there is a need to enhance MNB application at a


INTRODUCTION
Global economic development cannot be separated from water resources. Currently, in many developing countries, water resources are facing extreme challenges such as water scarcity, imbalances in the water distribution and production model, low efficiency of water use, droughts, and other environmental problems (Khan et al. ). Further, the increasing amount of wastewater caused by rapid urbanization and industrialization has significantly increased the challenges of both water availability and quality (Kivaisi ). Therefore, the treatment and recycling of wastewater are increasingly needed to ensure the sufficient availability of water. Generally, biological methods, such as activated sludge, have been used to treat the pollutants from both industrial and domestic waste water (Terasaka et al. ). However, such methods have disadvantages such as high energy costs and result in an abundant amount of solid waste, creating an additional cost to dispose of the produced waste (Orel et al. ). Therefore, there is dire need to develop a water treatment technology that can effectively address the increasing challenges of water scarcity sustainably.
In such a scenario, micro-and nanobubbles (MNBs) have emerged as a useful technology to be used in water treatment.
MNBs are tiny bubbles with diameters of nanometres and micrometres having several unique physical properties that make them useful for water treatments (Xiaoli et al. ).
For instance, their unique property of having large surface area enables an efficient mass transfer process between the liquid and gas phases, which helps to facilitate chemical reactions (Bouaifi et al. ). Such processes lead to the collapse of the MNB, which produces shock waves in the water, generating OH radicals as a result (Khuntia et al. ). The water treatment processes, such as electroflotation and dissolved air flotation, generally make use of MNBs (Miettinen et al. ). In recent years, the use of such methods has been widely adopted for decontamination of domestic and industrial water treatment, due to the higher bioactivity of MNBs (Kulkarni & Joshi ). Further, due to the enormous feasibility, MNB-based water treatments are being applied in various sectors, including industry and agriculture (Serizawa ).
During recent years, research on MNBs for waterrelated applications has significantly increased, considering their feasibility as a sustainable technology for water treatment (Tsai et al. ). Despite a wide range of studies and experimental evidence, the use and application of MNBs for water treatment is still limited. This study, in this regard, is designed to present an overview of existing literature on micro-nanobubbles and their use in water-related applications. This study will provide insights into recent developments and the status of research on MNBs and highlight future trends regarding the use of MNBs.
The paper is divided into seven sections, beginning with the introduction as the first section. The second section explains the basic properties of MNBs. The third section discusses the MNB generation mechanism and overviews the development timeline of MNBs. Similarly, major water treatment processes and MNB use and applications in various sectors are discussed in the fourth and fifth sections, respectively. Sections six and seven provide future scope and conclusions, respectively.

FUNDAMENTAL PROPERTIES OF MNBS
Generally, water bubbles can be categorized into three major types, i.e., ordinary or macrobubbles, microbubbles (MBs), and nanobubbles (NBs). The diameter of macrobubbles ranges from 100 μm to 2 mm. These bubbles quickly rise to the surface of a liquid and collapse.
While microbubbles are smaller than macro-bubbles, with a diameter range of 1 μm-100 μm (Azevedo et al. ), these bubbles shrink in the water and then dissolve into it.
In contrast, NBs are extremely small gas bubbles that have several unique physical properties that make them very different from normal bubbles. Generally, NBs range According to classical thermodynamics theory, NBs cannot exist or be stable due to the limitation of radius curvature (Ljunggren & Eriksson ; Ushikubo et al. ). For instance, the small radius of curvature gives NBs a higher internal pressure relative to the external pressure, which leads to rapid dissolution (Holmberg et al. ). For example, an NB with radius 100 nm (atmospheric pressure in the surrounding water ¼ 10 5 N m À2 and surface tension ¼ 72 mN/m) gives an internal pressure of 1.5 MPa (Attard ). The basic theory of this phenomenon is that high internal pressure contained in the NB does not achieve balance with the atmosphere, which would inevitably lead to the bubbles bursting in a very short time (Ljunggren & Eriksson ). Lou et al. () first reported the existence of NBs as bright and stable spheres on a flat solid surface, through AFM (atomic force microscopy). However, in recent years various other methods have also been adopted for NB investigation, such as neutron reflectometry, X-ray reflectivity and quartz crystal microbalance (Zhang ). One of the unique properties of NBs is stated to be their higher longevity, which gives them a larger contact angle and longer existence time in the liquids (Lou et al. ; Takahashi et al. ; Ying et al. ).

Zeta potential of MNBs
Zeta potential is a physical characteristic used to measure the magnitude of the attraction between particles and bubbles or electrostatic repulsion. It is an important property that determines the longevity of MNBs in a colloidal system (Jia et al. ). For the calculation of zeta potential, the Smulochowski equation is used (Han & Dockko ; Ushikubo et al. ). Generally, MBs and NBs have a negative charge in the pH range of 2-12, depending on the kind of gas introduced, and the zeta potential ranges from À50 to À40 mV and À30 to À20 mV at a neutral pH. The negative zeta potential value is determined by the excess of hydroxyl ions (OHÀ) relative to hydrogen ions (Hþ) at the gas-water interface (Ohgaki et al. ). The charging mechanism of NBs is also associated with the preferential adsorption of hydroxyl ions in the electrolytic solution. Further, the stability of the NBs is highly dependent on hydrogen bonding at the water-gas interface, due to which the NB has an impermeable, kinetically stable surface making it more diffusion-resistant (Wang et al. ).

Mass transfer properties
In various gas-liquid phase operations, the efficiency of the process is generally determined by the gas to liquid transfer rate; hence mass transfer is a system's critical property (Wilkinson & van Dierendonck ). The mass transfer efficiency relies on the bubbles' size distribution, rising velocity, gas-liquid hydrodynamics, coalescence, and break-up surface-to-volume ratio, and physical properties (Bouaifi et al. ). According to gas absorption two-film theory, the mass transfer rate of two phases is determined by the coefficient of liquid-gas mass transfer, surface area to volume ratio, and concentration gradient within these phases (Whitman ; Bouaifi et al. ). The unsteadystate method is used to calculate the gas-liquid mass transmission rate (Terasaka et al. ).  smaller-volume bulk liquid. They calculated the mass transfer rate employing a temperature correlation factor at 20 C.

MNB GENERATION AND TECHNOLOGY MNB generation mechanism
MNBs are generated on the surface of hydrophobic particles (Fan et al. a, b). Depending on internal or external factors, the formation of MNBs can be induced in several possible ways (Demangeat ). Generally, water bubbles are generated by dissolving gas with pressure and releasing gas while reducing pressure bubbles, which is considered a conventional method. In this method, the device is mainly composed of a water pump, air compressor, and air tank.
The water pump provides a certain pressure to send the circulating water to the dissolved gas tank, and the air compressor presses the air into the dissolved gas tank. The high-pressure gas-water mixing state formed in the dissolved gas tank makes the gas supersaturated and dissolved, and then the gas is precipitated out of the water in the form of MNBs by sudden decompression (Deng et al. ). Figure 2 shows a schematic diagram of the commonly used MNB generator (Takahashi et al. ). A pump circulates water in a transparent acrylic tank through a gas-dissolution tank and MNB generating nozzle. The air is injected into the circulating water on the suction side of the pump and is dissolved by a high-pressure system. A gaseous phase of MNB is then produced from the water supersaturated with air due to the pressure reduction at the nozzle. There are variations in the methods related to MNB generation, and a comparison of various bubble-generation principles and apparatus is shown in Table 1

Effects of various operational conditions on bubble size
The literature shows that the distribution and size of MNBs depend on the system design and various operational conditions. For example, MBs' fraction and size are generally At high pressure, the gas is highly soluble, so the number of bubbles is large and more bubbles rise stably.
The efficiency is low since the whole process is continuous.
Dissolving gas with pressure and releasing gas with impeller Fine bubbles are produced directly by the gas dispersed by the impeller, or in combination with the pressurized gas, performing three processes of mixing gas and water, releasing gas and gas dissolving.
Simple principle, efficiently produced in the combination of gas-water mixtures, more dissolution and release of the gas into the pump.

Dispersing gas method
High-speed rotational flow The gas-water mixture flows into the circulating hollow, forming a pressurized gas. The gas cuts into ultra-fine bubbles and rotates as the water passes.
Generally high quality and efficient.
The flow path is difficult to design and produce.

Flow-path section change
The flow cross-section slowly decreases and then quickly increases; the water collapses violently and vortices are formed.
Repeating the process leads to stronger turbulence and finer bubbles.
A large flow path makes it easy to repair and less likely to block up.
Difficult to adjust oxygen content during streaming significant changes in conditions.

Fine porous materials
The pressure gas forms fine bubbles through the strength of small porous materials.
The quality of the bubbles mainly depends on the porous material. The method is simple.
associated with the pressure changes across the nozzle system. The more the pressure, the smaller the bubble size due to the increase in air density; however, the size of MBs remains constant at a pressure above 3.5 atm (de Rijk An earlier overview study on MNBs showed that the rate of use of oxygen and the volume transfer rate in the synthetic aerated NB treatment plants was almost twice as high as with conventional air bubbles. The degradation time of organic waste production in NB-aerated units was less than half compared with the conventional systems. Lastly, the decay and growth rate in the NB aerated unit was also much faster than the conventional system (Temesgen ). Due to such properties, there is further need for experimental studies in this area.

Flotation process
Flotation has also been a major separation process in the area of water purification (Hopper & McCowen ).
The most specific substances that need to be removed

APPLICATION OF MNBS IN VARIOUS SECTORS
During recent years, due to the promising features of MNBs, their application has been extended to various fields such as agriculture, medical, industry, aquaculture and domestic use.  In such treatment, high time-cost efficiency is predicted in terms of higher organic pollutant separation, flotation, aeration efficiency for biological treatments, and advanced oxidation using OH radicals. The chemical-free radical generation property has vast potential in ozone-based MNB uses for the oxidation process. Further, the higher bursting energy of MNBs and higher aeration efficiency also have subsequent potential in terms of reducing membrane fouling and sludge formation in membrane bio-reactors.  Agriculture MNB-treated water in agriculture improves the soil's physiological and biological conditions by encouraging aerobic microorganisms, which improves soil particle structure, water absorption and oxygen dissolution, levels of the rhizosphere, microbial species and phosphate and urease, which positively impact plant growth ( Takahashi  . MNB application through a drip kills bacteria, removes harmful substances and odors from water, and improves freshness and taste and yield of fruits and vegetables (Takahashi et al. ). MNB use for biological and weed control (i.e., facilitating Triopsidae growth, which stops weed growth in rice fields and also decreases chemical and fertilizer usage) (Serizawa ). MNB water improves the rate of seed germination (Liu et al. b).

Aquaculture and fisheries
MNBs improves blood flow and branchial respiration of fish (Serizawa ).
MNBs can be applied to the purification of sludge at the sea bottom (the air is supplied to sludge in the form of nanobubbles, which can recover the poor oxygen condition at the sea bottom, activate marine life and decompose organic substances) (Serizawa ). MNB-treated water application on aquatic plants and fisheries significantly increases growth by improving nutrient uptake (Cho et al. ).

Cellular biological
MNBs are used in fermentation (Marui ).

Medical
MNBs are used to diagnose tumors in the human body by ultrasonic imaging (

Industry energy systems
MNBs eliminate mixed oil and carbon from water and provide economic benefits for water reuse (dissolving air with polyelectrolytes with hydrogen MNBs) (Tansel & Pascual ). MNBs are used in solar energy (solar vapor nanobubble generation as a result of the complex interaction of several phenomena that occur at the nanoscale and can be used in a variety of applications, i.e., solar steam energy NB generation) (Polman ).

Domestic uses
Oxygen produced by ozone decomposition generates ions that can oxidize pollutants in drinking water (Batagoda et al. ). MNBs are used in laundry and tableware. Their use in swimming pools, showers, and bathtubs for strong antioxidant effects have subsequent health benefits such as removal of aging skin, deep-rooted dust particles, bacteria, and chemical residue, which accelerate skin metabolism, and blood circulation (Nessbert ; Nikusystec ).
water-related applications in major primary sectors such as agriculture and aquaculture; however, expansion is needed for sectors such as medical science.
Further literature shows ambiguity in terms of fundamental properties, such as stability of bulk NBs, which have not been well explained until now. Only a few experimental studies deal with the stability and longevity of NBs, with no considerable consensus. The studies have yet to decide whether properties such as separation and stability follow scientific guidelines (classical thermodynamic principles).
Although progress has been made and several hypotheses have been proposed explaining the long-term stability of NBs, none of them describe these experimental observations.
Lastly, this study seeks further investigation regarding improvements in performance techniques, bubble growth, and size under various physical and chemical conditions, bubble generation methods, and automated optimization.