A Critical Review of Nanobubble Flotation for Seawater Treatment Process

Abstract

The growth in public demand for clean water is increasing due to the development of the population, triggering the decline in clean water resources. Seawater provides an unrestricted, consistent supply of high-quantity water from the water cycle. It is a solution to the public issue of limited clean water, which can be processed with desalination technology to get fresh and clean water. Seawater desalination removes salt and other impurities from seawater to produce fresh, potable water. Furthermore, to produce freshwater using nanobubbles, seawater desalination and nanobubble flotation are interconnected through their roles in the water treatment and purification process. It is necessary to modify the nanobubble flotation, which has unique properties (minimal nano gas), to separate the salt ions and suspended solids from water to get freshwater. This paper has reviewed the water treatment that was conducted for the nanobubble flotation, especially ion flotation, which is a formation of positively buoyant bubble particles that agglomerate mixed with a recycling stream to saturate with air or carbon dioxide at high pressure to generate nanobubbles. This review investigates effective and efficient nanobubble flotation for the water treatment process in the seawater desalination issue to get pure water. The review highlights the mechanism of NB flotation that can effectively separate the dissolved ions and suspended solids in the flotation column, which contains seawater with different salt concentrations. This review focuses on ion flotation and investigates three mechanisms in the flotation process, which consist of collisions, attachment, and detachment. This process can enhance the flotation performance in the flotation separation process. As a result, it has produced fresh, potable water.

1. Introduction

Nowadays, 1.2 billion people worldwide are affected by water shortages, which negatively affect their health, food, and energy [1]. Population growth, increased industrialization, and increased energy needs, on the one hand, and loss of snow melt, glacier shrinkage, and other circumstances, on the other hand, will exacerbate the situation in the coming years [2]. According to the World Water Council, the number of people affected will rise to 3.9 billion in the coming decades [3]. As one of the most promising approaches to alleviating water scarcity, desalination can increase water supply beyond what is available from the hydrological cycle. Seawater desalination provides an unlimited and stable supply of high-quality water that does not harm natural freshwater ecosystems [4]. Thus, freshwater scarcity is a pressing global problem and desalination—the extraction of freshwater from seawater—has proven to be a critical solution. With only 2.7% of the world’s water being freshwater and only 0.3% being directly usable by humans, freshwater scarcity is a growing problem. This issue has existed while freshwater consumption continually increased. It is predicted that three-quarters of the world’s population will suffer from freshwater shortages by 2050 [5]. Desalination plants, which remove salt and minerals from seawater, are crucial in combating water scarcity. Over 19,000 plants worldwide produce more than 100 million cubic meters of fresh water daily. These plants are primarily located in water-scarce regions with abundant energy resources, such as the United States and the Gulf States. China and India have also significantly progressed in seawater desalination research [6]. Classification of desalination technologies thermally driven, mechanically driven, and electrically driven. Thermally driven uses heat as driving energy, i.e., multi-effect distillation and solar desalination. Mechanically driven is based on mechanical energy, as in the Mechanical vapor compression (MVC) process. The electrically powered uses electrical energy to get the best performance in the desalination process as in Reverse osmosis (RO), Electro-Dialysis (ED), and the capacitive deionization process [7]. The challenges and gaps in desalination technology are generally related to energy consumption, high investments, environmental concerns, membrane performance, and innovative approaches. Traditional desalination technologies require significant energy input in energy consumption, resulting in high operating costs and environmental impact. However, high investments, including initial investments for desalination plants, remain challenging. The ecological challenge is that brine disposal (concentrated salt water) can harm marine ecosystems [8]. With the latest membrane technologies, membrane performance challenges remain, including critical challenges in improving membrane permeability, boron rejection, and chlorine resistance. In addition, there are challenges in innovative approaches, where research is currently underway to develop desalination methods based on renewable energy sources such as solar and marine thermal energy [9].

Micro- and nanobubbles (MNBs) have emerged as powerful tools in water purification and treatment [10]. Their unique properties make them valuable for various applications. Due to their small size and large surface area, MNBs have tiny dimensions and provide a large surface area for interactions with pollutants. Additionally, MNBs have a long residence time, meaning that MNBs remain suspended in water for extended periods, extending contact time with contaminants. In addition, MNBs have high mass transfer performance, so MNBs enable efficient transfer of gases and solutes. In addition, MNBS has good zeta interface potential and a high surface charge, which aids particle removal. What is unique is that MNBs generate hydroxyl radicals that oxidize organic pollutants. Therefore, applications of micro- and nanobubbles play quite an essential role in flotation. MNBs improve flotation processes by binding to particles, giving them buoyancy, and facilitating separation. Therefore, flotation with MNBS removes suspended solids, algae, and organic matter [11]. Applying flotation with MNBs also improves aeration, where MNBs improve oxygen transfer efficiency in aeration systems to improve the biological treatment and oxidation of pollutants. In addition, flotation with MNBS is also used in ionization, improving ozone dissolution, resulting in better disinfection, oxidation, and effective removal of color, taste, and odor compounds [9]. MNBs hold promises for sustainable development of seawater desalination. However, researchers continue to innovate, addressing challenges and pushing the boundaries of clean water production [12]. The best and most practical desalination plants offer a cost-effective solution for removing suspended salt or solids from the seawater to produce potable water while being environmentally friendly [13].

The most important phenomenon in the nanobubble flotation process is the interactions between bubbles and pollutant particles. These interactions include ion flotation for the seawater process in the exchange of ions [11] and the mechanism of ion flotation. Interestingly, nanobubbles possess several unique properties compared to MNBs, including high mass transfer [14], long-term stability [15], high zeta potential, high surface-to-volume ratio, and generation of free radicals upon collapse [16]. Due to their ability to generate highly reactive free radicals, one of the best applications of nanobubbles is the treatment of wastewater and drinking water [17]. Different types of nanobubbles have another application. Hydrogen nanobubble fuel mixes can further develop burning execution and customary gas [18]. Nitrogen nanobubble water expansion can upgrade the hydrolysis of waste-actuated slime and further develop methane creation during the time spent on anaerobic digestion, which assists and promotes hydrolysis [19]. Oxygen nanobubbles produce methane during the anaerobic assimilation of cellulose [20], and bulk carbon dioxide nanobubbles can be used in food processing [21]. Ozone micro-nanobubbles can increase ozone mass transfer to achieve high dissolved ozone concentration in the aqueous phase, prolong the reactivity of ozone in the aqueous phase, and be widely used to decompose organic contaminants [22]. The gas nanobubbles injected into the flotation tank interact with the coarse particles. The gas nanobubbles injected into the flotation tank interact with them [23]. The physicochemical phenomena for this process are observed in column flotation [24]. According to the purpose of this review article, based on their application and usefulness, flotation has been classified into different types, e.g., ore flotation [25], cell flotation [26], dissolved air flotation [27,28,29], ion flotation [30], and nanobubble flotation [31]. To the authors’ knowledge, there are no concerns regarding the experimental application of the nanobubble flotation column in the fully separating process of the dissolved ions and suspended solids during the seawater desalination process and the corresponding success mechanism. The objective of this review is to discuss the application of nanobubbles’ interaction, focusing on ion flotation in the water treatment process. The ion flotation perspective focuses on ion interactions and the interaction impact of different nanobubble gas types in the water treatment process to get drinking water, specifically within a flotation column. It emphasizes ion flotation, which involves interaction with salt ions, heavy metals, and nanobubbles gas within the flotation column. This process includes the aspects of ion flotation related to ion separation, attachment, and detachment.

2. Materials and Methods

This review sought to comprehend the effects and mechanisms of NB flotation on various nanobubble gas types, for example, water nanobubbles (Xiazhichun Co., Ltd., Kunming, China) used in seawater desalination for drinking water production, including ion flotation, especially for application in the salt ion, and heavy metals, such as Lithium-ion batteries (Company Rho motion, Shenzhen, China), pollutants, which used in this review are Sodium Dodecyl Sulfate (Raw Material, Shaanxi Dideu Medichem Co., Ltd., Baoji, China), and Tea Saponin (Shanghai Meetu Industry, Shanghai, China). To achieve this objective, a four-step systematic review was carried out. After locating the database in the first step, the title and abstract were used to filter the results. In addition, the selected papers in the full text were analyzed, and choosing a group of documents to include was the final step. The PRISMA guideline was also adopted during this review.

2.1. Objective

The main objective of this study was summarized in four research questions (RQ).

RQ1: 

What makes NB flotation technology more useful for seawater desalination?

RQ2: 

How do physical, chemical, electronic, and mechanical interactions between nanobubble flotation and seawater?

RQ3: 

What does nanobubble technology give in terms of efficiency for the seawater desalination process?

RQ4: 

How is the mechanism of salt ion interaction with bubbles and the water quality due to the desalination process?

The research questions were used to identify some keywords. The components that make up the keywords are the bubble (and its synonyms, such as gas nanobubbles and nanobubbles), seawater, and ion flotation. Using these keywords as a search expression, the Boolean operator was created. Science Direct and Google Scholar were used for the search. From the search, 429 documents were found, as shown in Table 1, and advanced to the screening step.

2.2. Screening

In this step, the extended time of distribution is not set in stone from the complete distinguished reports. Nevertheless, the year of publication was limited from 2013 to January 2024 in ScienceDirect, whereas the year was also limited from 2013 to January 2024 in Google Scholar. Consequently, 49 Science Direct documents and 380 Google Scholar documents were chosen. After that, the kind of paper screening is used to keep out documents other than research papers. The result in Science Direct using the “research article” type of paper filtering is 15 documents, whereas, in Google Scholar, the result is 224. The title and abstract were checked during the screening; if they contained one or more keywords or search queries, they were included in the full-text analysis. As a result, 369 documents were left out of this step. In November 2023, the screening step was not completed.

Table 1. Identification step results.

Search Local	Search Expression	Search Result	Types of Documents
Science Direct	(Seawater! OR Seawater desalination!) AND (with nanobubbles) (Ion flotation! OR salt ion!) AND (with nanobubbles)	246	85 Review articles 136 Research articles 13 Book Chapter 3 Encyclopedia 1 Conference abstracts 8 Others
Google Scholar	(Seawater! OR Seawater treatment!) AND (with nanobubbles) (Ion flotation! OR salt ion!) AND (with nanobubbles)	380	224 Research articles 146 Review articles 10 patents

Table 1. Identification step results.

Search Local	Search Expression	Search Result	Types of Documents
Science Direct	(Seawater! OR Seawater desalination!) AND (with nanobubbles) (Ion flotation! OR salt ion!) AND (with nanobubbles)	246	85 Review articles 136 Research articles 13 Book Chapter 3 Encyclopedia 1 Conference abstracts 8 Others
Google Scholar	(Seawater! OR Seawater treatment!) AND (with nanobubbles) (Ion flotation! OR salt ion!) AND (with nanobubbles)	380	224 Research articles 146 Review articles 10 patents

2.3. Eligibility

The first step in determining eligibility was downloading 75 documents from two search engines. The 75 articles were then analyzed to determine how seawater conversion interacts with nanobubbles and the mechanism behind the various gas nanobubbles. Five documents out of 75 were ruled out because they contained review articles, and five were ruled out because they contained similar articles. The 65 articles were, therefore, left out of this step.

2.4. Inclusion

This study included 22 documents after full-text analysis. These papers discussed how nanobubbles and graphene oxide interact with seawater conversion and the mechanisms behind the various gas nanobubbles and membranes. The distribution reports were reviewed in the reach season between 2013 and 2024. Figure 1 depicts how the steps of analysis followed the PRISMA diagram.

3. Types of Gas Nanobubbles and Flotation in the Seawater Desalination

In this review, the application of nanobubbles for the flotation process and the effect of gas types in the conversion of seawater to fresh, potable water were highlighted in terms of concern for the water molecule mobility, free radical formation, mass transfer, and degradation of waste chemicals [12]. This section describes the water molecule mobility, free radical formation, mass transfer, generation, and stability bubbles for nanobubbles technology [32]. However, different gas nanobubbles are used for seawater desalination based on nanobubble flotation and the atomic interaction between bubbles and particles [33]. Each gas-type bubble has unique properties and functions beyond seawater desalination, which will be explained in the next section [34].

3.1. Type of Gas Nanobubbles

Different kinds of gas-type bubbles have various potential applications, which consist of Hydrogen, Nitrogen, Oxygen, Carbon dioxide, etc. Hydrogen nanobubble gas mixes have the potential to significantly enhance burning execution compared to regular fuel, underscoring their practical applications [35]. Adding water containing nitrogen nanobubbles can improve the hydrolysis of waste-activated sludge and methane production during anaerobic digestion [36]. In the anaerobic digestion of cellulose, oxygen nanobubbles produce methane [19], and CO₂ bulk nanobubbles can be utilized in food processing [37]. Some parameters, such as bubble size and zeta potential (ZP) capability, made of a few gases in various arrangement conditions, were estimated to concentrate on nanobubble strength [20]. The bubble size is the critical boundary utilized to order the bubble [38]. Another significant boundary of nanobubbles is the electric charge on the bubble surface, which can be utilized to examine the dependability of a colloidal framework [39]. Consequently, the electric capability of the colloidal framework can be communicated concerning zeta potential, and subsequently, zeta potential estimations were utilized to make sense of the air pocket strength [40]. Other physical aspects of this technology are pressure and differences [41], high-speed cavitation [42], ultrasonic waves, and ultrafine pores [43].

High-speed cavitation, pressure difference with circulation [44], ultrasonic waves, and passing ultrafine pores are just a few of the bulk nanobubble preparation methods [45], Khan (2020) mentioned. In this review, the gear displayed in Figure 2 was used to create many nanobubbles in the fluid quickly. The bubble size, zeta potential [11], and interfacial characteristics all play a role in the nanobubble’s stability and reactivity [46]. The energy the system supplies to generate nanobubbles and solution properties also significantly impacts their characteristics [40]. The solution’s temperature, pressure, ion type, concentration, pH, surfactant presence, organic matter or impurities, and saturated gas concentration are important factors [47]. The properties of a bubble can also be affected by the kind of in-filled gas and its solubility and reactivity [48]. In addition, the size of the bubble, the formation of radicals, and the associated chemical reactions are significantly influenced by the generation mechanism and the energy provided to the system (e.g., hydrodynamic method, ultrasound) [49].

NBs can be produced by a few strategies, as displayed in Figure 2. The production of NBs through simple, inexpensive, stable, and scalable methods is one of the significant issues in the expanded market [50]. Several businesses in the United States, South Korea, Canada, and Japan have produced such bubbles using special techniques, including cavitation chambers, electrolysis [51], shear planes, pressurized dissolution, and swirling fluids in a mixing chamber [52]. A critical number of works zeroed in on bubble age and properties after 2000, initially revealed by Kim et al. Later, in 2007, Kikuchi and colleagues used electrolysis to make NBs [53]. According to Oeffinger and Wheatley’s findings, the addition of some surfactants to a perfluorocarbon gas resulted in the formation of NBs [54]. Ming Xu et al. (2022) investigated the temperature-dependent NB formation in a closed cuvette [55]. Ohgaki et al. (2010) said that the concentration of NBs was 1.9 × 10¹⁶ bubbles per dm³ and stayed the same for up to two weeks [56]. Etchepare et al. (2017) investigated to generation of NBs in a multiphase pump [41]. The findings demonstrated that the average size and concentration of the bulk NBs remained constant for more than 60 days [41]. Nazari et al. (2022) investigated NBs produced by hydrodynamic cavitation in water using various reagents [57]. Counter-flow hydro and bulk NBs made with oxygen and air in the water dynamic cavitation were studied [58].

3.2. Generation of Nanobubbles

Cavities are frequently used to generate nanobubbles in solutions [59]. Pressure drops below a particular critical value, causing cavitation [60]. Considering the strain decrease component, cavitation systems can be grouped into four distinct sorts [61], as follows:

• Hydrodynamics—system geometry-induced variation in the pressure of liquid flux [62].
• Acoustic—a sound made when ultrasound is applied to liquids [63].
• Particle—passing light photons with high intensity through liquids [64].
• Optical—lasers with short pulses focused on solutions with low absorption coefficients [65].

According to Tsuge (2019), nanobubbles’ hydrodynamic generation typically occurs by the following [66]:

• Compress gas flows in liquids to dissolve them and then release the resulting mixtures through nano-sized nozzles to form nanobubbles [67].
• Use focusing, fluid oscillation, or mechanical vibration to break up gas into bubbles by injecting low-pressure gases into liquids [37].

In addition, ultra-fine bubbles have been made by electrolysis, applying in the nano-porous membranes, sonochemistry with ultrasound, and mixing water and solvent [68]. Numerous factors, including pressure, temperature, and the type and concentration of the dissolved gas and electrolyte solution, influence the formation of nanobubbles [69]. There are currently many commercially available nanobubble generators, most intended for small pilot projects or the laboratory [50].

Investigation about the nanobubble generators that can produce nanobubbles in the flotation process for ion separation, especially [30]. The issue of waste chemical degradation and ion separation is investigated at the microscopic scale, the ion scale. The separation process for this flotation process is ion flotation, which will be discussed in the next section. Ion flotation has been proposed since the 1960s and has been a promising method for removing heavy metals.

3.3. Generation and Production of Different Gas Nanobubbles

The following section shows the generation of different gas nanobubbles with other methods. To create water nanobubbles, a Xiazhichun Co. Ltd. ultra-micro bubble generator (XZCP-K-1.1) with a range of 100 nm to 10 m was used [70]. In the 10 L glass container with a circulation between the inlet and outlet, as shown in Figure 3A, inlet and outlet pipes were immersed in deionized water/electrolyte solutions. The machine mixed the gas and liquid, and then high-density, uniform, and “milky” nanobubbles of water were produced (Figure 3B) through a nozzle using the hydrodynamic cavitation method. When the generator was turned on, damaging pressure gas was pumped into the machine from a gas. As the micro-bubbles rose and fell at the air-water interface, the cloudy and milky nanobubbles water gradually became clear (Figure 3C). This cycle took 2–3 min (Figure 3D). Using Nano Sight, NS300, Malvern (Worcestershire, UK) outsourcing, the machine produced many density nanobubbles of water with a density of more than 108 bubbles per milliliter after working for 15 min. Following the preparation of the above steps, the nanobubble water reached a temperature of 313 K and was left to cool to room temperature. The large bubble sizes of 1 to 10 m were then removed from the prepared nanobubble water using centrifugal treatment in the following manner. A centrifuge tube of 50 mL was used to store the nanobubbles water; following that, a 6 min centrifugal treatment at 6000 rpm, or 31.4 m/s peripheral velocity, was carried out to eliminate any potential impurities and giant bubbles. One sealed centrifuge tube containing nanobubbles of water was kept at room temperature (298 K) for continuous measurements. At the same time, the size distribution, zeta potential, Eh (Energy of Harvesting), and pH were all measured after centrifugation to record the first day’s data.

3.4. Nanobubble Application in the Water Treatment

According to the literature, by particle size, nanobubbles (NBs), also called micro-nanobubbles (MNBs), have been helpful in water treatment, aeration procedures [71], flotation [72], and disinfection [45]. Literature has also shown that the primary areas of MNB applications in water treatment are the reduction of system structure size, as shown above, the generation of nanobubbles, operating time, processing plant operating costs, and efficiency in removing pollutants, particularly seawater [73]. This review details the flotation process in water treatment. Accordingly, people worldwide are affected by the lack of freshwater due to organic and inorganic contaminants [74]. Many methods have been developed to remove contaminants from aqueous solutions, such as oxidation or reduction, chemical precipitation, adsorption, ion exchange, reverse osmosis (RO), electrochemical treatment, membrane technology, evaporation, and electroflotation. These methods have many disadvantages, such as high cost, generation of large amounts of sludge, high reagent or energy requirements, time consumption, incomplete removal of target ions, and difficulty treating large volumes of wastewater [75]. Therefore, ion flotation has been selected as an excellent alternative for wastewater treatment due to its low energy consumption in the water treatment process, efficient time in operation, maximum space requirements, and simplicity of model design in the water treatment process.

3.4.1. Aeration Process

Aeration is the process of introducing or penetrating oxygen into water. It is a crucial aspect in supplying oxygen for biochemical substrate reactions and aquatic life in water treatment [76]. Numerous studies have examined the effects of aeration processes on wastewater, biological water treatment, and groundwater recovery [34]. Enhancing the effectiveness of the factors that influence the speed of mass transfer was one of many studies’ primary goals. Dissolved oxygen (DO) plays a significant role in overcoming this inefficiency in typical aerobic systems [77]. Most of the contact equipment in these systems uses diffusers or mechanical aerators, both of which necessitate significant electrical input and high maintenance costs [78]. Research has primarily focused on optimizing conventional bubbles and aerator design to enhance mass transfer aeration [79]. However, there is little industrial-scale research on using high-mass transfer bubbles [50]. Felix Reichmann et al. (2021) concluded that MB aeration is better suited to bioreactors after describing the characteristics of the transfer rate of gas-liquid mass in stirred-tank reactors [80]. They examined how MB aeration affected the mesophilic filamentous fungus Trichoderma reesei’s fermentation, limited by oxygen mass transmission. This study showed that the concentration of broken-up oxygen was higher than the focus at a lower unsettling rate because of the utilization of MB air circulation [29].

In addition, compared to conventional bubbling, the concentration of cellular mass increased rapidly during the rapid growth phase, rising from 0.1 to 0.18 g/LH. Patel et al. (2021) reported that NBs in aerated water resulted in better seed germination than in regular water [34]. Similarly, Malik (2020) researched the growth of lettuce (Lactuca sativa) using MB aeration [81]. They discovered that the dry and fresh bulk of appropriately aerated MB lettuce were 1.7 and 2.1 times higher than those of macro bubble lettuce. The researchers hypothesized that the specific surface area of MNBs and their more remarkable ability to attract positive ions were related to higher germination and growth rates in these studies [82]. In addition, oxygen MNBs outperform air micro-NBs in terms of mass transfer efficiency by 126 times and dissolved oxygen (DO) by three times. The longer MBs remain in the water and the greater the mass transmission at the bubble interface, the more effective oxygen transmission becomes. Khan (2020) studied the use of NBs for the degradation of aerobic waste in wastewater treatment using MNBs [45]. The findings demonstrated that the volume transfer rate and oxygen utilization rate of the synthetic aerated NB treatment plants were nearly twice as high as those of conventional air bubbles [83].

3.4.2. Flotatixon Process

Flotation has also played a significant role in water purification as a separation method (Hopper and McCowen, 1952) [84]. Dust, chemicals (heavy metals especially), organic matter, metal ions, and oils are the most specific substances that must be removed from flotation (Azevedo et al., 2016) [71]. For instance, Dockko and Han (1998) discussed the possibility of improving flotation efficiency by altering the bubbles’ characteristics, i.e., the size of the surface and particle properties [27]. The efficiency of the separation process is closely linked to the size of the bubbles. NBs and MBs are frequently utilized in flotation to remove pollutants from the water more effectively. Subsequent experimental studies have demonstrated the effectiveness of this method for collecting bubbles and particles [85]. According to Ahmed and Jameson (1985), the rate of flotation increased 100 times when the bubble size decreased from 655 to 75 nm, indicating that the bubble size significantly reduced in this process [86].

In addition, Zhang (2021) stated that a strong correlation exists between the possibility of tiny particles colliding with small bubbles and the reduction in bubble size in flotation, which increases separation efficiency [87]. Unlike standing molecule sizes and little air pockets, surface charges play a massive part in buoyancy (Collins and Jameson, 1976) [88]. According to Maeng et al., (2021), positively charged MBs are expected to effectively remove algae from the water at 90% cell elimination and 92% chlorophyll reduction [89]. The MBs could achieve an elimination rate of more than 30% for organic substances, such as dissolved and organic carbon and aliphatic or aromatic mixtures [90]. Sumikura also investigated the possibility that the NBs increased surface hydrophobicity and expanded the area of flotation particles, both of which improved flotation efficiency (Sumikura et al., 2007) [91].

3.4.3. Disinfection Process

Ozone oxidation of pollutants and pathogens is a promising wastewater purification technique (Xie et al., 2021) [92]. According to studies, ozone gas bubbles successfully treat water entirely, even with a short contact time and low concentration, due to their potent disinfection properties. This method’s application for chemical-resistant spore-forming bacteria, such as Cryptosporidium and Bacillus subtilis, has frequently increased due to its effectiveness. Also, MBs make this process more efficient because kinetic disinfection reduces Escherichia coli (the type of bacteria) faster (by 99 percent) with a smaller water tank and less ozone (for applying MBs) than traditional ozone disinfection [91]. Using 490 Watt/Liter energy, another experiment to stop E. coli multiplication achieved a 75 percent reduction in just 3 min (Mezule et al., 2009) [93], which is the hydrodynamics cavitation results from various experiments, and it has demonstrated the efficacy of MNBs as a non-reagent method for water disinfection [92].

4. Results & Discussion

4.1. Types of Flotation

Masses of macro-scale experiments have improved and verified the flotation performance of various minerals in the presence of BNBs, as shown in

Table 2. Applications in the flotation for different types of process.

No	Flotation Type	Types of Mineral Separation	Application(s)
1	Flotation of two types of graphite: lithium-ion batteries graphite (LIBG) and natural ore graphite (NOG)	Lithium-ion batteries, graphite (LIBG), and natural ore graphite (NOG)	The flotation efficiency should be examined under two conditions: without nanobubbles (NBs) and with their presence [94]
2	Dissolved air flotation (DAF): DAF combines with the other flotation to do the material separation	Fine Minerals	Removal of sulfate ions; Zeta potential measurement of bubbles size; Improving Nickel Recovery in Froth Flotation by Purifying Concentrators Process Water [27]
3	Cyclonic-Static Micro-Bubble Flotation Column (FCSMC)		Incorporated and industrialized for all flotation steps circuits in the mineral separation in China [95]
4	Ion flotation	Ion particles such as Carbonate (CO3)	Iron, selenium, and gold ions can be removed, and the ion-flotation process can selectively remove specific ions from mixed ion solutions [96].

. Furthermore, flotation can be seen in different applications for different purposes. This section will show the type of flotation from the industrial and scientific perspectives.

Table 2 shows the differences in flotation based on its application; The process has highlighted the advantages of water treatment in the flotation process, particularly for ion flotation. Ion flotation is one potential technique to remove hazardous ions from drinking water at low-level concentrations. It is environmentally friendly and biodegradable [97]. The flotation techniques used in these processes are froth flotation [30], dissolved air flotation [98], precipitation flotation [99], and ion flotation [100]. The ion flotation derives from the mineral separation industry. This technique can remove organic and inorganic contaminants from wastewater in anionic or cationic forms. Today, ion flotation is used for recovering precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and a negligible amount of sludge [101].

4.2. Effect of the Nanobubble Flotation to Enhance the Water Treatment Process

Regarding extracting pollutants from water, ion flotation does not offer a notable method for separating heavy metals [23,66]. Ion flotation is capable of eliminating both organic and inorganic pollutants, whether they are anionic or cationic, from wastewater [102]. The ion flotation process involves the adsorption of surfactants onto the nanobubbles at the base of the column flotation, which will engage with the heavy metal ions for adequate collection. As the bubbles ascend, surfactant ions are captured in the froth. One of the most commonly used synthetic surfactants in various industries is sodium dodecyl sulfate (SDS) [103,104,105]. This surfactant is also known as sodium lauryl sulfate.

The SDS is a surfactant that is effective in removing heavy metals as shown in Figure 4. Kukizaki M. et al. reported that applying the SDS as the collector in an ion flotation process removes the Mn²⁺, Cu²⁺, and Zn²⁺ from water [106].

Table 3. Using sodium dodecyl sulfate and tea saponin surfactants in the process of ion flotation to remove heavy metal ions from water.

Surfactant	Pollutants	Condition	Results of Ion Flotation	Removal (%)	Ref.
Sodium Dodecyl Sulphides (SDS)	Zn (II), Mn (II), Cu (II)	Cmetal:CSDC:Caxillary ligand = 1:5:5; pH = 4	Water needs to be purified with the acids	90.5, 99.8, 73.4	[107]
Sodium Dodecyl Sulphides (SDS)	Cr (III)	Cmetal:CSDC = 2:1; pH = 8	Water must be infused with the oxygen	91.6	[108]
Sodium Dodecyl Sulphides (SDS)	Cu (II), Pb (II), Ni (II), Cd (II), Zn (II)	Cmetal:CSDC = 1:1; pH = 9	Water must be infused with the oxygen	97.5, 87.5, 87, 83, 92.5	[98,99]
Sodium Dodecyl Sulphides (SDS)	Cd (II)	CSDC:CCd = 3:1; pH = 4	Water needs to be added to the distribution of nanobubbles	94	[109]
Sodium Dodecyl Sulphides (SDS)	Ni (II), Zn (II)	Cmetal:CSDC = 1:13.5; pH = 9.7	Water must be infused with the oxygen	99.8, 90.4	[110]
Sodium Dodecyl Sulphides (SDS)	Cd (II)	Cmetal:CSDC = 1:2; pH = 10	Water must be infused with the oxygen	99.8	[109]
Tea Saponin	Cu (II)	Csurfactant:Cmetal = 3:1; pH = 4	Should added to the nanobubbles	81	[111]
Tea Saponin	Cd (II)	Csurfactant:Cmetal = 11:1; pH = 7.5	Need to add the oxygen through the infused nanobubbles	8	[112]
Tea Saponin	Pb (II)	Csurfactant:Cmetal = 11:1; pH = 4.8	Need to infuse the higher concentration of nanobubbles into it.	12	[113]

shows the results obtained from various studies of the various pollutants and removal using the SDS to remove different kinds of heavy metal ions through ion flotation.

According to Table 3, even though the axillary ligand metals are used, the removal rate percentage is relatively lower than the surfactant of SDS. Additionally, tea saponin surfactant was used to remove cadmium (Cd (II)), copper (Cu (II)), and lead ions in an aqueous solution [98,99]; the removal efficiency decreased rapidly with the increasing ionic strength of the sodium chloride (NaCl), which has an average concentration of about 0.001 to 0.004 M. Further research and methodological work are needed on how to treat other valuable ions, such as gold, etc., to reduce the significant costs of current refining processes. The surfactant also showed that the high efficiency is to remove relatively high concentrations of copper ions, and it could be used as a promising alternative for applying to treat different industrial and mining wastewater.

In addition, nanobubbles have enhanced the desalination process through the interaction of bubbles and seawater, which have a few types of interactions. The theory of buoyancy explains that when bubbles in water decrease in size, their rising speed decreases as well. Micro-nanobubbles (MNBs) are less likely to collapse rather than regular bubbles because they have a lower surface, a smaller volume, and less buoyancy. Studies suggest that MNBs with a diameter of less than 1 μm remain stable in water for a long time due to their significantly slower rising speed than Brownian motion [114].

Nanobubbles—those tiny, ephemeral spheres of gas suspended in liquid—have been making waves (also intended) in water desalination. This is their influence on enhancing the desalination process. Some factors are influenced to enhance the desalination process. The first factor is about the nanobubbles and membrane; researchers have explored the use of nanobubbles (NBs) to improve the performance of thin-film composite (TFC) polyamide membranes in forward osmosis (FO) desalination [115]. These NBs are generated by adding sodium bicarbonate (NaHCO₃) to the aqueous phase [116]. Secondly, by adjusting the Micro-Nano structure of the polyamide (PA) rejection layer, NBs alter the membrane’s roughness. With enhanced NBs, the PA layer exhibits more blade-like and band-like features. These features effectively reduce the reverse solute flux of the PA layer and improve salt rejection in the FO membrane [117]. Thirdly, the influence of the surface area and its interaction. One of the primary advantages of utilizing 30 nm nanobubbles is substantially increased surface area; this increased surface area allows for more effective interactions, facilitating better separation during desalination. NBs enhance the membrane’s ability to reject salt ions and impurities. The fourth aspect is about reducing chemical dependency; the efficiency of NBs in removing impurities minimizes the reliance on chemical additives commonly used in traditional desalination. This reduction in chemical dependency contributes to cost savings and lessens the environmental footprint of the water treatment process. In summary, these minuscule bubbles were pivotal in advancing desalination technology.

4.3. Interactions of NBs and Seawater: Physical, Chemical, Electronic, and Mechanical Interactions

The interactions between nanoparticles (NBs) and seawater can be diverse and multifaceted, involving physical, chemical, electronic, and mechanical aspects. Also, this interaction plays a role in conducting a deeper investigation into water treatment. The properties of interactions between NBs and seawater consist of the following: (a) physical interactions, (b) chemical interactions, (c) electronic interactions, and (d) mechanical interactions.

4.3.1. Physical Interactions

Nanobubbles (NBs) have tiny particles in particle size distribution (nanoscale). When nanoparticles (which means nanobubbles) interact with seawater, they may undergo dispersion, where they become evenly distributed throughout the water due to Brownian motion and other forces [118]. However, they can also agglomerate, forming larger clusters due to attractive forces such as van der Waals interactions or electrostatic forces. Another aspect of nanoparticles is to adsorb onto various surfaces in seawater, such as sediments, organic matter, or biological surfaces [119]. This adsorption can affect the behavior and fate of the nanoparticles in the marine environment [120]. NBs have been well-known due to their advantages, which include the small size distribution (nanoscale) [121], large specific surface area that has evidence of surface tension [121], long residence time in the water during the interaction [85], high mass transfer efficiency [85], high interface zeta potential that concludes the distribution of particle charge on the surface of the bubble [40], and the ability to generate hydroxyl radicals (which produces the free radical) [122]. These characteristics are significantly distinct from those of the traditional large bubbles.

4.3.2. Chemical Interactions

The surface of nanoparticles can undergo chemical reactions with any substance related to seawater, leading to changes in their properties and behavior. The nanoparticles may undergo oxidation, reduction, or dissolution processes depending on their composition and the chemical environment [123]. The presence of hydrated ion functional groups degrades the stability of GO based on water treatment [105]. The oxidized regions act as spacers to separate adjacent GO sheets and allow water molecules to intercalate between water molecules and GO sheets. The GO structures in water are the main challenge ahead of their application in aqueous media, as separation membranes disintegrate over time [124]. For application to water treatment, GO membranes should be stabilized by reduction or chemical crosslinking [125]. Moreover, nanoparticles may exchange ions with seawater constituents, change their surface charge, and more reactivity. This ion exchange can influence the stability and interactions of the nanoparticles in seawater [30,126].

4.3.3. Electronic Interactions

The electronic structure of nanoparticles can influence their interactions with seawater constituents. Electrochemical water treatment technology utilizes applied potential or current to drive processes such as electron transfer [127] or multiple proton-coupled electron transfer [128], leading to chemical processes such as oxidation, reduction, adsorption, and migration of pollutants [79]. Otherwise, it is applied in situ, which generates chemical processes to remove the contaminants [129]. Depending on the process principle, it could be divided into electrochemical oxidation, electrocatalytic reduction, electrodialysis, electro-coagulation, electrochemical advanced oxidation processes, and generated active chlorine [130], as seen in Figure 5. For example, nanoparticles with certain electronic properties may exhibit enhanced reactivity toward specific chemical species in seawater [50]. In addition, the surface potential of nanoparticles, which depends on factors such as surface charge and composition, can affect their interactions with charged species in seawater through electrostatic interactions [131].

4.3.4. Mechanical Interactions

Nanoparticles in seawater can experience mechanical forces such as sedimentation due to gravity or transport by water currents. The nanoparticle’s size, shape, and density influence sedimentation rates and transport behavior. The perception of hydrodynamic forces around particles, drops, or bubbles moving in Newtonian liquids is modestly mature. It is possible to get the predictions of the attractive–repulsive interaction among particles for moving ensembles of dispersed particulate objects. Driven forces, like gravity-driven flows, such as the rise or sedimentation of single spheroidal objects, pairs, and dispersions, are focused on mechanical interactions [132]. The effects of two main rheological attributes—viscoelasticity and shear-dependent viscosity—on the interaction and potential aggregation of particles [104], drops, and bubbles could be identified [50]. This interaction is influenced by the mechanical-driven force on salt ionic, such as Sodium Chlorine.

Sodium chlorine, for example, can significantly impact the ion flotation process with a high salt content [133]. The high salt concentrations can alter the pH and ionic strength of the pulp phase, as well as the bubbles’ properties. Moreover, dissolved salt ions can impact the wetting properties of salt crystal surfaces, directly influencing the interaction between the salt surfaces and the collectors in the flotation process, e.g., ion flotation [134].

4.4. Nanobubbles Technology for Various Applications

Bubbles are a fascinating hydrodynamic phenomenon that significantly impacts both mechanical and chemical interactions. Unfortunately, when bubbles collapse, it can create adverse effects such as high-speed jets and shock waves [16]. However, engineers have found ways to utilize the dynamic characteristics of bubbles in various applications, including ultrasonic cleaning [135], shock lithotripsy, and air-gun detection. Researchers are working hard to uncover the mechanisms behind this nonlinear multiphase interaction problem.

Nowadays, nanobubbles are used widely in several industries, including manufacturing [136], agriculture [137], and medicine [138]. Nanobubbles are also useful in medicine for drug delivery [139] and medical imaging [11]. Because of their superior contrast and material transport properties, by increasing water permeability, nanobubbles are well-known to encourage plant growth in agriculture [140]. In industry, nanobubbles boost a solution’s oxidation capacity and conduct chemical interactions with contaminants; thus, they are frequently employed in wastewater treatment [141] and surface cleaning [51] industries. In the seawater desalination process, nanobubbles can enhance dissolved oxygen concentration in water. Nanobubbles will be employed more frequently as preparation technology and research advance. Developing and optimizing processes for the stable and effective production of nanobubbles will be crucial in this research area.

4.4.1. Ion Separation in Seawater Desalination

The separation of ions is crucial for various applications such as resource recovery, water treatment, and energy production and storage [126]. While techniques like chemical precipitation, selective adsorption, and solvent extraction have proven effective, membranes offer a continuous separation of ions with minimal waste and lower energy costs [142].

Desalination of brackish water and seawater has become an increasingly popular solution to global water scarcity [143] as involved in ion separation, as shown in Figure 6. According to

Table 4. Type of water treatment process.

Type of Process Water Treatment Method	Application	Selectivity	Membranes	Installation of Water Treatment Technology
Reverse Osmosis (RO)	Water desalination	Salt removal	TFC membrane, cellulose acetate membrane
Nanofiltration	Water softening, food processing	Polyvalent ion removal, organic matter removal	Polyamide TFC membranes, cellulose acetate membranes, poly(piperazine-amide) membranes
Ultrafiltration	Water treatment, dairy processing	Removal of particulates and macromolecules when protein retention	Poly(vinylidene fluoride) hollow fiber membranes, polyether sulfone membranes, polyamide TFC membranes [144]
Reverse electrodialysis Electrodialysis	Energy conversion Water desalination		Swollen gel-type ion-exchange membranes that carry positive or negative charges, fluorinated ion- exchange membranes with sulfonic acid side groups
Gas separation	N2 production, waste gas stream treatment	N2 separation from air, CO2 capture from flue gas or natural gas	polymers: Polydimethylsiloxane, ethylene oxide/propylene oxide-amide copolymers

, reverse osmosis (RO) is the most widely utilized desalination technology due to its energy efficiency and space-saving design. Also, it shows us that in Table 4, the systems membranes have been established for industrial processes. In recent years, numerous efforts have been made to enhance membrane performance, specifically higher permeability, to improve RO’s energy efficiency and the performance of the ion separation.

4.4.2. Nanobubble Generations Methods

The formation of BNBs in a liquid can be achieved through various means, such as adjusting gas pressure, ultrasonic intensity, or stirring intensity. Preparing NBs typically involves mechanical stirring, gas dissolution release, pressure variation, and cavitation. Additionally, microfluidic and nanoporous membrane methods are also utilized for BNB preparation. This section offers an overview of the methods used for BNB preparation and concludes with a comprehensive summary of the pros and cons of each method, presented in the form of a table.

(a)

Mechanical Stirring Method

The preparation of BNBs involves mechanical agitation, which entails the iterative rotational stirring of a surfactant-containing liquid phase through a mechanized mechanism. This process promotes interactions between the gas and liquid phases, resulting in the formation of bubbles [45]. BNBs can be created with a pump and circular column under varying pressures and air-liquid interfacial tensions, and they can maintain their stability for up to 60 days [40]. Using nanobubbles generated via mechanical stirring can enhance heat transfer oil’s thermal conductivity and viscosity, as shown in Figure 7. Various hollow-shaped rotating mechanisms can be utilized to generate BNBs in pure water, and increasing the rotational speed, extending the operating time, and elevating the temperature can increase the concentration of bubbles generated.

• (b) Nanoscale Pore Membrane Method

BNBs can be created using the nanoporous membrane method by forcing gas into the nanoscale pores of the membrane. The diameters of the nanobubbles increase as they expand, and the drag force causes them to detach from the pore, creating BNBs larger than the pore diameter. The SPG membrane is a uniform and adjustable inorganic membrane that can prepare monodisperse nanobubbles. BNBs can be generated by adjusting the pore size of the membrane. BNBs can be prepared using tube ceramic membranes by injecting air at different pressures into the water through the tube, as shown in Figure 8. Finally, a membrane-based physical sieving method can adjust the size range of generated BNBs by controlling the gas filtration rate and membrane quality.

• (c) Microfluidic Method

The preparation of BNBs through microfluidics involves regulating mixed gas and liquid flow using microfluidic chips [132]. To create microbubbles, a gaseous mixture is introduced through a gas inlet and passes through the liquid phase, which exerts viscous forces on the gas to create micro-nanobubbles. Some of the gas within these microbubbles dissolves into the aqueous phase and eventually shrinks, giving rise to BNBs. Labarre et al. (2022) [145] were the first researchers to use a microfluidics-based approach to prepare BNBs, compared to using the experimental setup, as shown in Figure 9. They employed a mixed gas of water-soluble nitrogen and water-insoluble perfluorocarbon (PFC) as the gaseous phase for the microfluidic bubble generator. Initially, monodisperse microbubbles are generated, which gradually shrink as the water-soluble nitrogen dissolves, ultimately resulting in BNBs of a specific size. The degree of bubble contraction can be controlled by adjusting the ratio of water-soluble nitrogen and water-insoluble PFC. This method is advantageous because it precisely controls the size and uniformity of the resulting BNBs.

• (d) Acoustic Cavitation Method

The acoustic cavitation method is utilized for BNB preparation to create a localized negative pressure in the liquid medium. This can be achieved by either high-speed propeller rotation or high-intensity sound waves, forming micro- and nano-scale bubbles near small gas nuclei. Ziying Xu (2025) [146] conducted experiments on BNB preparation using this method, with the experimental setup illustrated in Figure 10. Their research found that BNBs exist in pure water but not in organic solvents, disappearing when a particular organic solvent-to-water ratio is reached. This is because of the electrostatic charge on the surface of the BNBs, stabilized by hydroxyl ion adsorption generated by water’s auto-ionization. Pure organic solvents do not undergo auto-ionization, resulting in this outcome.

• (e) Hydrodynamics Cavitation Method

The hydrodynamic cavitation technique has several advantages, such as being highly energy efficient, low cost, and scalable. Its primary aim is to create cavitation in a medium by altering its flow velocity, causing pressure fluctuations similar to those produced by acoustic cavitation techniques [54]. Therefore, hydrodynamic cavitation can replace acoustic cavitation for the generation of nanobubbles. Oliveira et al. (2018) experimented with generating nanobubbles via hydrodynamic cavitation [54]. They used a two-chambered swirling jet nozzle to produce nanobubbles in a saturated or supersaturated solution through a circulation system, as shown in Figure 11. The results indicated that the device successfully generated nanobubbles with diameters of less than 200 nm, and these nanobubbles carried a negative charge when present in water. Pourkarimi et al. (2021) refined the cavitation reactor, using numerical simulation to examine the impact of various geometric parameters on the flow field structure [147]. They identified the optimal design and fabricated a laboratory-scale vortex-type micro-nanobubble generator. Flow experiments were conducted, resulting in the production of bubbles with diameters as small as 301 nm. This undertaking provided valuable insights into exploring the methodologies of micro-nano bubble generation and the quest for their optimal structural configuration.

4.5. Effect of Gas Nanobubbles in the Water Treatment

Water’s bulk nanobubbles are compressed by a gas-liquid interface, which causes them to continuously shrink as they rise to the surface of the water and exhibit a self-pressurization effect. According to Tuziuti et al. (2018), the pressure gradient is inversely proportional to the rate of gas diffusion from the high-pressure region to the low-pressure region, and as the bubble shrinks, the mass transfer from the inside to the surrounding liquid increases [123]. Additionally, because the surface area of a bubble is inversely proportional to its radius (Shen et al., 2022), bulk nanobubbles exhibit a large specific surface area [121]. Due to the self-pressurization effect of the bulk nanobubbles, long residence times, and large specific surface areas, more gas can dissolve into water through the bubble interface, effectively improving the gas transfer efficiency to the liquid phase [37]. Bubbles eventually burst and vanish until their internal pressure reaches a specific limit. To consider various applications, the existence period, average size, zeta potential, and suspension pH and Eh as a function of time of five distinct kinds of nanobubbles of O₃, N₂, O₂, and 8% H₂ in Ar, CO₂, and air were compared in this review.

According to

Table 5. Advantages and disadvantages of the preparation methods BNBs.

Methods of BNBs	Advantages	Disadvantages	Recommendation
Mechanical Stirring method	The principle is simple and easy to implement	Only a tiny number of nanobubbles can be prepared	Using stirring motors to produce a tiny number of nanobubbles
The nanoscale pore membrane method	Enables control over bubble size and distribution	Requires specialized membranes with accurate pore sizes. Potential blockage or fouling of pores may reduce efficiency over time.	Reconstruct the blockage or fouling of pores to do the process time efficiently
Microfluidic method	Enables precise control of bubble size and distribution. Offers a high degree of automation and integration with other processes	Requires complex microfluidic devices and fabrication techniques	Make a simulation and model for the complex microfluidic devices and the fabrication techniques
Acoustic cavitation method	Efficient and rapid generation of nanobubbles	Requires specialized equipment and ultrasound sources. Control over bubble size and distribution may be limited.	Using the special tool for producing the ultrasound sources to cover and control the distribution bubble size
Hydrodynamic cavitation method	High energy efficiency, low cost, and scalability	Efficiency can be influenced by factors such as the flow rate and pressure.	Doing the variations of pressure and flow rate through the change of geometry factors to produce the nanobubbles
Dissolved gas release method	Easy and straightforward to implement. Low cost	Limited control over bubble size and distribution. This may result in larger bubble sizes compared to other methods	Make various or combinations of the methods to control the production of the bubble size distribution
Periodic pressure variation method	A more uniform bubble can be prepared, and the pressure and period to control the bubble size.	Only a tiny number of nanobubbles can be	This method has the same recommendation as the first type of BNB method
Hydraulic air compression method	Nanobubbles can be produced on a large scale at low cost and with high efficiency.	Limited control over bubble size and distribution	Control the bubble size by using the tools to measure the bubble size, and do another process to change the bubble size

, in the flotation result, the effect of different gases on heavy metal ion removal efficiency in the ion flotation process was also examined. Pure nitrogen and dry air were introduced separately to the bubble column to produce bubbles with an average of about 2 mm diameter [148]. The results presented in Table 4 show that air gas was slightly better for ion flotation than nitrogen, removing 99.9% of the arsenic compared with 99.4% for nitrogen. Mercury was found to have the highest removal rate in the presence of nitrogen gas, at 99.9%; with air, 99.6% was removed. The data indicates the results of removing arsenic, lead, and mercury from water using S-octanoyl-cys as the collector and N₂ and air as the inlet gases to produce bubbles. It is necessary to require the nanobubble for flotation results. [45]. Because it can impact the other contaminants in water treatment, to remove heavy metals through the rising bubble from the bottom of the flotation column.

4.6. Effect of Surfactant on the Ion Flotation for Seawater Desalination

Flotation technology is an effective method for treating industrial wastewater, which includes ion flotation, precipitation flotation, and adsorption flotation, as stated by Santander et al. (2011) [149]. Other physical phenomena in the flotation process are Precipitation and adsorption flotation. It can generate significant amounts of toxic sludge, which is hard to treat. But, ion flotation can help extract heavy metal ions with low concentrations (0.01 ppm) from wastewater solutions. It has multiple benefits, such as low energy consumption, large processing capacity, low cost, easy operation, and low sludge volume, as indicated by Deliyanni et al. (2017) [150]. The ion flotation process also combines metal ions and surfactants in an aqueous solution, which creates flocculated precipitates. These precipitates are then removed and recovered by a foam flotation process [103], where the surfactant acts as both an adsorbent and foaming agent, as stated by Saleem et al. (2020) [151].

The researchers propose to use biosurfactants with high chelating capacity in the flotation of heavy metal ions. Biosurfactants are chemically synthesized surfactants that have hydrophobic and hydrophilic ends. The hydrophilic group is generally polar and comprises peptides, amino acids, monosaccharides, or polysaccharides. Compared to traditional synthetic collectors, biosurfactants have lower toxicity, higher selectivity, and are biodegradable, as Hernández-Expósito et al. (2006) [152]. Also, biosurfactants can be made from inexpensive organic waste and remain active even at extreme pH and salinity levels, as demonstrated by Menezes et al. (2011) [153]. Furthermore, macromolecular biosurfactants have a strong chelating ability to heavy metal ions, forming stable complexes, and are widely used in the remediation of heavy metal-contaminated soil, as noted by Dongchan Kim et al. (2020) [154].

5. Author Outlook

The existence of bulk nanobubbles in water was initially controversial, but recent experimental research has proven their presence due to the exponential increase in research efforts in recent years. Nanobubbles have unique features such as high stability, long lifetimes, large surface-volume ratio, high mass transfer efficiency, and the ability to generate free radicals, which can improve conventional technologies in the water treatment field. To further exploit their usage and gain a better in-depth understanding of the parameters that have profound impacts, it is crucial to consider the broad applications of nanobubbles for future perspectives. With their small size and the existence of a surface charge, nanobubbles have significant potential as a new environmentally friendly method to remove organic compounds, as shown in Figure 12. They also effectively improve the air flotation process to separate suspensions due to their large specific surface area and durability, which enhances the oxygen mass transfer efficiency; furthermore, the ability to generate free radicals with strong oxidation capabilities is essential in decomposing organic substances. As a result, this technology can prove immensely valuable in purifying wastewater that contains organic pollutants. Nevertheless, additional investigation is necessary to overcome the obstacles bulk nanobubbles pose. Regarding the complex challenge, it is crucial for building the theoretical foundation of large-scale bulk nanobubbles in the industrial aspect. From an industrial perspective, heavy metal ions may be separated from contaminated water through the bulk nanobubbles process.

Heavy metal ions are contaminants that can be separated from water by the ion flotation process.

Table 6. Flotation results for 5 mg/L (ppm) of different heavy metal ions using s-octanoyl-cys, C (surfactant) = 0.01 M and pH = 8.

Contaminants	Inlet Gas	As (ppm) After 30 min	Removal (%) After 30 min	As (ppm) After 60 min	Removal (%) After 60 min	Ref.
Arsenic	Air	0.137	97.3	0.006	99.9	[50]
Arsenic	Nitrogen (N2)	0.032	99.4	0.029	99.4
Mercury	Air	0.024	99.5	0.020	99.6
Mercury	Nitrogen (N2)	0.022	99.6	0.002	99.9	[100]
Lead	Air	0.399	92.0	0.0467	99.1	[102]
Lead	Nitrogen (N2)	0.257	94.9	0.032	99.4

shows that several flotation techniques use froth, dissolved air, precipitation, and ion flotation [155]. Furthermore, salt ion is an important factor to consider in water treatment, especially for seawater treatment. Ion flotation derives from the mineral separation industry, which can remove organic and inorganic contaminants from wastewater in anionic or cationic forms [156]; it is also used for the recovery of precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and negligible amount of sludge [157]. The salt ion is an ion dissolved in water and, in certain types of seawater, bonded to heavy metals. The separation of ions uses membranes that are integrated with nanobubbles to produce fresh, potable water from seawater. For further investigation, a multi-membrane system can be used and integrated with ion flotation in water treatment to produce fresh, potable water from seawater.

6. Conclusions

The flotation nanobubble for seawater desalination has been reviewed. Seawater offers a continuous and reliable source of high-quality water, functioning within the natural freshwater ecosystems’ ongoing water cycle. Nevertheless, the challenge is vast. Only 2.7% of the planet’s water is freshwater, and a mere 0.3% is readily available for human use, making freshwater scarcity a critical global issue. This predicament has intensified due to rising freshwater usage. By 2050, three-quarters of the global population is expected to experience freshwater shortages. Extracting freshwater from seawater through a water treatment process has emerged as an essential solution. Water treatment facilities that can eliminate salt and minerals from seawater in addressing water scarcity. Over 19,000 plants worldwide produce more than 100 million cubic meters of fresh water daily, highlighting the situation’s urgency. A recent study on the water treatment process utilizing nanobubble flotation has been evaluated.

Various nanobubble traits like the free movement of water molecules, mass transfer, the electrical charge at the surface (zeta potential), and the chemical breakdown of wastewater. The key aspect of the nanobubble flotation process is the interaction between bubbles and surfactant particles. These interactions involve ion flotation in the water treatment process for seawater with the exchange of ions and the principles of ion flotation. Notably, nanobubbles exhibit distinctive characteristics when compared to micro-nano bubbles (MNBs), which include enhanced mass transfer, prolonged stability, elevated zeta potential, a high surface-to-volume ratio, and the production of free radicals upon their collapse. Various types of gas nanobubbles have been used for different gas nanobubbles. Hydrogen nanobubbles in fuel mixtures can improve combustion efficiency compared to conventional gas. Nitrogen nanobubbles can enhance the hydrolysis of waste-activated sludge and boost methane production during anaerobic digestion. Oxygen nanobubbles facilitate methane generation during the anaerobic digestion of cellulose, while bulk carbon dioxide nanobubbles find applications in the food processing industry. Ozone micro-nanobubbles can enhance ozone mass transfer, achieving high levels of dissolved ozone concentration in the water phase and extending the reactivity of ozone in that phase. They are also extensively utilized for breaking down organic contaminants.

Here are the insights into the comprehensive concept of nanobubble flotation for seawater desalination. The influence of salt ions on the characteristics of nanobubbles was analyzed. The extended DLVO theory was utilized to interpret and elaborate on our experimental results concerning the stability of nanobubbles. Nanobubbles improve the membrane’s capability to filter out salt and contaminants. The fourth point focuses on lowering chemical reliance; the effectiveness of nanobubbles in purification reduces the need for chemical additives typically utilized in conventional desalination. This decrease in chemical reliance leads to cost reductions and diminishes the environmental impact of the water treatment procedure. In conclusion, these tiny bubbles play a crucial role in the progress of desalination technology.