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Nanobubble technology applications in environmental and agricultural systems: Opportunities and challenges
Abstract and Figures
In recent years, there has been growing interest in the varied applications of nanobubble technology. Given their unique physicochemical properties, such as minuscule size (< 1 µm), surface charge, and high internal pressure, nanobubbles (NB) could provide new opportunities in the fields of environmental engineering (including environmental remediation, water treatment, aerobic fermentation, anaerobic digestion, and algal biomass production), and agriculture (including agronomy, horticulture, aquaculture, aquaponics, bioponics, and hydroponics). In addition, applying NB-derived reactive oxygen species (ROS) can inactivate pathogens in water treatment, remove harmful microorganisms on food, and remove persistent organic pollutants from wastewater (removal efficacies > 60%). NB technology can also maintain high aqueous phase dissolved oxygen levels compared to conventional aeration, as demonstrated in hydroponics and intensive crop farming, where NB-treated water led to increases in plant yields (10–40%). However, a concise and comprehensive source of information on the fundamental mechanisms involving NB technology is lacking. As NB applications advance into the biological frontier, these mechanisms serve as critical knowledge areas toward understanding the NB–biomolecular and cellular mechanisms of action. In addition, mass transfer performance is not stringently assessed. To advance and summarize current understanding, this review provides an updated, in-depth discussion of the fundamental mechanisms and performance of NB technologies for various applications in environmental and agricultural fields. Mechanistic details focusing on electrostatic and hydrophobic attachment, the formation of ROS, and gas–liquid mass transfer are discussed. This review further outlines the opportunities and challenges and concludes with important research needs in NB technology.
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... 14−16 MBs, despite having a shorter lifespan, boost gas−liquid mass transfer due to their smaller size compared with millimeter-sized bubbles (i.e., macrobubbles). 17 Thus, MNB aeration technology finds numerous applications, including in wastewater remediation and in agriculture. 17 Based on our recent review of MNB technology, 17 there are few studies exploring its application in aquaponic systems. ...
... 17 Thus, MNB aeration technology finds numerous applications, including in wastewater remediation and in agriculture. 17 Based on our recent review of MNB technology, 17 there are few studies exploring its application in aquaponic systems. However, research on hydroponics, aquaculture, and biological wastewater treatment provides valuable insights into the potential benefits of MNBs in aquaponics. ...
... 17 Thus, MNB aeration technology finds numerous applications, including in wastewater remediation and in agriculture. 17 Based on our recent review of MNB technology, 17 there are few studies exploring its application in aquaponic systems. However, research on hydroponics, aquaculture, and biological wastewater treatment provides valuable insights into the potential benefits of MNBs in aquaponics. ...
Aquaponics is a climate-smart food production system that relies on nutrient recovery from aquaculture effluent. The availability of dissolved oxygen (DO) limits the performance of aquaponic systems. To address this issue, air micronanobubbles (MNBs) were tested as an alternative aeration method. Due to their small size (<50 μm) and long-term stability, MNBs can enhance the gas–liquid mass transfer of oxygen and maintain DO concentrations above saturation, enhancing aerobic biochemical processes. Through 16S rRNA gene metabarcoding and water quality analyses, we investigated the influence of continuous MNB aeration on DO concentrations and carbon–nitrogen cycling. Our results indicated that continuous MNB aeration (KLa20 = 7.76 ± 0.38 h–1) in the grow bed of an aquaponic system increased DO concentrations to 9.31 ± 1.57 mg L–1 without affecting other system components. In comparison, aeration with a conventional diffuser (KLa20 = 1.73 ± 0.54 h–1) maintained lower DO concentrations of 6.32 ± 0.32 mg L–1 on average. Further benefits of MNBs include higher reductions in total solids (16 ± 2%) and volatile solids (32 ± 2%) and an increase in nitrate concentrations over time. Notably, MNB aeration enhanced the total butterhead lettuce yield and root biomass per plant by 35 ± 8% and 20 ± 5%, respectively. While MNB aeration reshaped the microbial community and potentially disturbed certain microbial symbiotic relationships (e.g., Denitratisoma, Thermomonas, and Nitrospira), the overall metabolic pathways remained largely intact. Overall, our study provides insights into the application of MNB aeration to enhance plant biomass production in floating raft aquaponic systems and increase nitrification and remineralization.
... Nanobubble technology involves introducing nanobubbles (<100 nm) containing a selected gas into water (Agarwal et al., 2011;Atkinson et al., 2019), where these structures exhibit neutral buoyancy and high solubility due to their considerable surface-to-volume ratio (Gurung et al., 2016). Nanobubbles with their minuscule size and negative surface charge enable better dispersion in water due to mutual repulsion (Marcelino et al., 2023). Ozone, when present in nanobubble form, remains stable for a longer time and allows rapid reactions to degrade pollutants while increasing the concentration of dissolved oxygen in water (Seridou and Kalogerakis, 2021;Pal et al., 2022). ...
... Moreover, produced oxygen-free radicals have higher reactivity and oxidation potential than widely used disinfectants such as chlorine (Takahashi et al., 2007). As a result, ozone nanobubbles (NB-O 3 ) can improve water quality by reducing pathogen loads, optimizing dissolved oxygen levels, and restricting harmful disinfection byproducts (Atkinson et al., 2019;Marcelino et al., 2023). The use of NB-O 3 , therefore, may serve as a promising approach to deal with mycobacterial infections in betta fish, although its potential has not yet been explored. ...
Betta splendens, a valuable ornamental fish species, is particularly susceptible to mycobacteriosis which poses a challenge to the sustainability of its culture and trade. Since there are no effective treatments for the disease, betta fish farms must take rigorous preventive measures. This study investigated the efficacy of ozone nanobub-bles (NB-O 3) to disinfect water for mitigating the risk of mycobacteriosis in betta fish. Laboratory tests showed a significant disinfecting effect of NB-O 3 against a highly pathogenic, multidrug-resistant strain of Mycobacterium chelonae by destroying bacterial cells. After incubation in NB-O 3 water for 60 min, the concentration of M. che-lonae in distilled water was reduced by 96.71 to 99.92%. In practice, treatment of reserved and cultured water from betta farms with direct NB-O 3 for a single 10 min significantly reduced total bacterial counts and pre-sumptive mycobacteria by over 90%. In experimental infection, sub-adult betta fish (2-month-old) cultured for 14 days in water spiked with M. chelonae (~10 6 CFU/mL), which served as a positive control group, had a low percent survival of 38.33%. In contrast, fish reared in the same water as the positive control group but treated with NB-O 3 three times for 10 min at 20 min intervals had a significantly higher percent survival of 93.93%. Moreover, the relative percent survival (RPS) of the treatment group showed a statistically significant difference , with an RPS of 89.19% compared to only 67.57% in the negative control group (neither exposure to M. chelonae nor treatment with NB-O 3). In conclusion, this study demonstrated that NB-O 3 effectively reduces bacterial concentrations in water, mitigates the risk of mycobacteriosis, and enhances the survivability of betta fish exposed to multidrug-resistant M. chelonae. This non-chemical and non-antibiotic approach offers a promising solution for disease control in the betta fish industry.
... Bulk nanobubbles represent spherical bubbles uniformly dispersed in water or other liquids, which have diameters smaller than 1000 nm [4]. Currently, nanobubbles have widespread applications in various fields, including medicine [5], agriculture [6], and industry [7]. In the medical field, nanobubbles are often utilized in medical imaging [8] and drug delivery [9] due to their excellent contrast and material transport capabilities. ...
... The stability of membrane nanobubbles primarily depends on the shell layer, wherein a dense molecular layer can protect the gas core and slow down its diffusion [14]. Currently, nanobubbles have widespread applications in various fields, including medicine [5], agriculture [6], and industry [7]. In the medical field, nanobubbles are often utilized in medical imaging [8] and drug delivery [9] due to their excellent contrast and material transport capabilities. ...
Nanobubbles represent a special colloidal system, as they have high stability and large specific surface areas. The preparation of nanobubbles is currently a hot research topic, as it crucial to investigate their characteristics and expand their applications. This article explains the mechanism of generating nanobubbles based on chemical and physical methods, introduces their basic composition’s structure and properties, summarizes the methods of preparing bulk nanobubbles (BNBs) and surface nanobubbles (SNBs), and clarifies the preparation principles and techniques. Seven practical applications of nanobubbles are cited in this paper, including their use as ultrasonic contrast agents in medical imaging, drug delivery systems in drug transportation, promoters of plant growth by affecting plant respiration and water absorption at the roots, tools to remove dirt from surfaces by generating energy during nanobubble bursting, producers of high-density negative ions and free radicals to react with pollutants in wastewater, tools to reduce the resistance of the fluid flow through channels by lowering the internal friction, and means of improving the mineral flotation recovery rate by enhancing the absorption capacity of bubbles to minerals. Finally, the future development of nanobubble preparation technology is discussed, including their roles in optimizing equipment and preparation methods; improving the quantity, efficiency, stability, controllability, and homogeneity of nanobubble generation; and promoting the industrial production of nanobubbles.
... Nanobubbles have found increasing use in various industrial applications [1,2], agriculture [3], wastewater treatment [4], sonochemistry [5], seabed remediation [6], and as contrast agents [7,8]. Because of their small size they can stay suspended in water for prolonged time as random Brownian motion compensates for the effect of buoyancy. ...
Principles of laser-induced nanobubble formation in water are studied and presented. Nanobubbles were generated by laser light at intensities below threshold for laser-induced breakdown and subsequently expanded by a rarefaction wave to facilitate their observation and analysis. Different methods were used to study nanobubble formation and characteristics. Firstly, probability of nanobubble formation as a function of water sample purity was examined. Secondly, relation between laser fluence at different wavelengths and the number of generated nanobubbles was investigated. Thirdly, measurements of nanobubble lifetime were conducted indicating a contradiction to the Epstein-Plesset equation-based prediction of free bubble dissociation. Accumulated evidence suggests that the presence of physical impurities is a prerequisite for nanobubble formation. Consequently, a lack of impurities results in the absence of nanobubbles in contrast to assumptions by existing studies. The findings presented in this paper provide new insights into the fundamental properties of laser-induced nanobubbles in water.
... Nanobubble-assisted extraction is a new and innovative approach supporting the extraction technology along ultrasound. Compared to conventional extraction methods, using NBs has many benefits, including the NBs gentle agitation does not harm the matrix or the target compounds [55] . The use of NBs can speed up the extraction process; this is one of the prime benefits for commercial applications. ...
Nanobubble technology is one of the latest green technologies in food industry applications. Nanobubbles (NBs) are gas-filled nanoscopic bubbles with a diameter of < 500 nm. The mass production of bulk nano-bubbles raises growing interest due to their high stability, internal pressure, and an enormous surface-to-volume ratio. Also, they can increase surface area, alter the physicochemical characteristics of the medium, and facilitate mass transfer. Along with their size and stability, air, nitrogen, and carbon dioxide, NBs are widely recognized for their significance in food processing. The potential existence and ability of NBs to remain stable in liquids under ambient conditions have been a contentious issue for a long time due to the conventional thermodynamic theory. In this review, fundamental properties, and several generation methods of NBs have been described along with their mechanism. Moreover, NBs generation methods can produce fine and undeviating gas diffusion characteristics, which can be used to control the uniformity and textural qualities of various creamy and gel-based foods. Thus, we also described the possible applications of NBs along with their mechanism of action in extraction, freezing, foams, and film formation. The ability of NBs to impart health benefits makes them new, improved, and environmentally safe green techniques.
Sonochemistry, although established in various fields, is still an emerging field finding new effects of ultrasound on chemical systems and are of particular interest for the biomedical field. This interdisciplinary area of research explores the use of acoustic waves with frequencies ranging from 20 kHz to 1 MHz to induce physical and chemical changes. By subjecting liquids to ultrasonic waves, sonochemistry has demonstrated the ability to accelerate reaction rates, alter chemical reaction pathways, and change physical properties of the system while operating under mild reaction conditions. It has found its way into diverse industries including food processing, pharmaceuticals, material science, and environmental remediation. This review provides an overview of the principles, advancements, and applications of sonochemistry with a particular focus on the domain of (bio-)medicine. Despite the numerous benefits sonochemistry has to offer, most of the research in the (bio-)medical field remains in the laboratory stage. Translation of these systems into clinical practice is complex as parameters used for medical ultrasound are limited and toxic side effects must be minimized in order to meet regulatory approval. However, directing attention towards the applicability of the system in clinical practice from the early stages of research holds significant potential to further amplify the role of sonochemistry in clinical applications.
Nanobubble (NB) technology has demonstrated the potential to enhance or substitute for current treatment processes in various areas. However, research employing it as a novel advanced oxidation process has thus far been relatively limited. Herein, we focused on the oxidative capacity of oxygen NBs and investigated the feasibility of utilizing their enhanced oxidation of ferrous ions (Fe2+) in a sulfuric acid medium when using copper as a catalyst and their effect mechanism. It was demonstrated that oxygen NBs could collapse to produce hydroxyl radicals (·OH) in the absence of dynamic stimuli using electron spin resonance spectroscopy, and methylene blue was used as a molecular probe for ·OH to illustrate that NB stability, determined by their properties, is the critical factor affecting ·OH release. In subsequent Fe2+ oxidation experiments, it was discovered that both strong acidity and copper ions (Cu2+) contribute to accelerating the collapse of NBs to produce ·OH. While ·OH derived from the collapse of NBs acts on Fe2+, the molecular oxygen generated homologously with ·OH will further activate the catalytic oxidation of Fe2+ by interacting with Cu2+. With the synergistic effect of the above two oxidation-driven mechanisms, the oxidation rate of Fe2+ can be significantly increased up to 88% due to the exceptional properties of oxygen NBs, which facilitate the formation of an atmosphere with persistent oxygen supersaturation and the generation of oxidation radicals. This study provides significant insight into applying NBs as a prospective technology for enhanced oxidation processes.
Municipal wastewater treatment which is associated with high energy consumption and excessive greenhouse gas (GHG) emissions, has been facing severe challenges toward carbon emissions. In this study, a high-rate activated sludge-two-stage vertical up-flow constructed wetland (HRAS-TVUCW) system was developed to reduce carbon emissions during municipal wastewater treatment. Through carbon management, optimized mass and energy flows were achieved, resulting in high treatment efficiency and low operational energy consumption. The carbon emission of the HRAS-TVUCW system (i.e., 0.21 kg carbon dioxide equivalent/m3 wastewater) was 4.1-folds lower than that of the conventional anaerobic/anoxic/aerobic (A2O) process. Meanwhile, the recovered energy from the HRAS-TVUCW system increased its contribution to carbon neutrality to 40.2%, 4.6-folds higher than that of the A2O process. Results of functional microbial community analysis at the genus level revealed that the controlled dissolved oxygen allocation led to distinctive microbial communities in each unit of HRAS-TVUCW system, which facilitated denitrification efficiency increase and carbon emissions reduction. Overall, the HRAS-TVUCW system could be considered as a cost-effective and sustainable low-carbon technology for municipal wastewater treatment.
Deductive arguments regarding the unexpected stability of nanobubbles in water include the excessive internal pressure of minuscule gas pockets. In this study, the derivation assumptions of the Young–Laplace equation are evaluated closely to discuss the possible modifications towards making conclusive remarks about the predictive power of the equation at the nano-scale.
Mineral scaling is a phenomenon that occurs on submerged surfaces in contact with saline solutions. In membrane desalination, heat exchangers, and marine structures, mineral scaling reduces process efficiency and eventually leads to process failure. Therefore, achieving long-term scaling resistance is beneficial to enhancing process performance and reducing operating and maintenance costs. While evidence shows that superhydrophobic surfaces may reduce mineral scaling kinetics, prolonged scaling resistance is limited due to the finite stability of the entrained gas layer present in a Cassie-Baxter wetting state. Additionally, superhydrophobic surfaces are not always feasible for all applications, but strategies for long-term scaling resistance with smooth or even hydrophilic surfaces are often overlooked. In this study, we elucidate the role of interfacial nanobubbles on the scaling kinetics of submerged surfaces of varied wetting properties, including those that do not entrain a gas layer. We show that both solution conditions and surface wetting properties that promote interfacial bubble formation enhances scaling resistance. In the absence of interfacial bubbles, scaling kinetics decrease as surface energy decreases, while the presence of bulk nanobubbles enhances the scaling resistance of the surface with any wetting property. The findings in this study allude to scaling mitigation strategies that are enabled by solution and surface properties that promote the formation and stability of interfacial gas layers and provide insights to surface and process design for greater scaling resistance.
The application of microbubbles for water treatment is an emerging technology which has been shown to significantly enhance gas-liquid contacting processes. When applied to ozonation, microbubble technology has been shown to enhance mass transfer and the speed and extent of compound removal compared with conventional bubbling techniques. One explanation as to why microbubble systems outperform conventional systems is that microbubbles shrink, collapse and spontaneously generate hydroxyl radicals which is thought to enhance the speed of compound removal. This study compared microbubble (mean diameter 37 μm) and conventional bubble (mean diameter 5.4 mm) ozonation systems under identical conditions. The experiments were normalised for effective ozone dose to determine whether microbubble ozonation generated significantly more hydroxyl radicals than conventional bubble ozonation. 4-chlorobenzoic was used as the hydroxyl radical probe and the proportion of hydroxyl radicals generated for a given effective ozone dose was quantified. The •OH-exposure to O3-exposure (the Rct) was used to compare the systems. The ratio of the mean RctMicrobubble to mean RctConventional was 0.73, 0.84 and 1.12 at pH 6, 7 and 8 respectively. Statistical assessment of the Rct showed that there was no significant difference between the bubble systems. No evidence was found to support the hypothesis that microbubble systems generate more •OH. Instead, the level of •OH-exposure is linked to the effective dose and pH of the system and future designs should focus on those factors to deliver •OH based benefits.
Micro- and nanobubbles (MNBs) are microscopic gas bodies sized at micro (<100 μm) and nanoscale (<1 μm), that have a long lifetime in aqueous solutions and large specific surface area due to their small size. Recently, scientific interest has been focused on ozone micro- and nanobubbles (OMNBs) used in disinfection processes since research findings support the idea that ozone micro and nanosized bubbles can significantly improve the disinfection capacity and the residual activity of ozone. The aim of this critical review is to present recent studies which investigate the feasibility of ozone-based disinfection processes by exploiting the strong oxidizing ability of ozone and the noteworthy longevity of MNBs in aqueous solutions. Properties of MNBs and generation techniques are briefly discussed besides the monitoring methods for their characterization in terms of size and number. In this critical review, we provide recent research related to the application of OMNBs in disinfection of drinking water, as well as in aquaculture, agriculture, and wastewater treatment. Finally, research gaps and limitations of this technology are highlighted and directions for future studies are suggested.
In this study, the influence of nanobubbles (NBs) application in ozone (O3) based advanced oxidation processes (AOP) is investigated. The results demonstrate the potential of NBs application to O3 – based AOP. It was observed that NBs suppress the negative influence of pH and operating temperatures on the efficiency of ozonation. In addition, the application of NBs tends to improve the solubility of O3 and the rate of mass transfer under the influence of a broad range of temperature and pH conditions. The results of this research indicate that application of NBs minimized the reduction in concentration of dissolved O3 with an increase in temperature. Furthermore, application of NBs highly improved the OH radical formation in acidic conditions. The results of this research depicted for first time that the application of NBs strongly encourages the initiation of reactions involving OH radicals. It was found by this research that NBs can boost the concentration of OH radicals up to 3.5 fold compared to equivalent MB-supported ozonation systems. This is assumed to improve the efficiency of currently existing conventional bubble supported O3 – based AOP systems. HIGHLIGHTS
MB nanosizing reduces the negative influence of pH on the efficiency of ozonation.;
MB nanosizing reduces the negative influence of temperature on ozonation efficiency.;
Nanobubbles enhance ̇OH concentration by up to 3.5 fold compared to microbubbles.;
Nanobubbles improve the mass transfer of O3 during ozonation compared to MBs.;
Methane is an abundant and low-cost gas with high global warming potential and its use as a feedstock can help mitigate climate change. Variety of valuable products can be produced from methane by methanotrophs in gas fermentation processes. By using methane as a sole carbon source, methanotrophic bacteria can produce bioplastics, biofuels, feed additives, ectoine and variety of other high-value chemical compounds. A lot of studies have been conducted through the years for natural methanotrophs and engineered strains as well as methanotrophic consortia. These have focused on increasing yields of native products as well as proof of concept for the synthesis of new range of chemicals by metabolic engineering. This review shows trends in the research on key methanotrophic bioproducts since 2015. Despite certain limitations of the known production strategies that makes commercialization of methane-based products challenging, there is currently much attention placed on the promising further development.
Previous research into the effects of ultrafine bubbles (UFB) on plant growth have been contradictory. To facilitate the resolution of these contradictions, the aim of the present study was to clarify the interspecific differences in growth responses among cereal/leguminous species under different levels of UFB concentrations. Seedlings of six species were grown hydroponically with three different UFB concentrations and two levels of plant nutrition to evaluate biomass and elongation growth. UFB growth promotion under zero-nutrition occurred in all species. Interspecific differences were noted in response to differing UFB concentration levels. Rice and soybean had higher above-ground biomass production at both low and high concentrations. Conversely, other crops exhibited promoted growth at only one of the concentrations. Negative effects occurred in full nutrient conditions except for root elongation. This study demonstrated that growth-promoting effects with UFB depended on the crop species being tested and the concentration of UFB used.
Microbubbles are very fine bubbles that shrink and collapse underwater within several minutes, leading to the generation of free radicals. Electron spin resonance spectroscopy (ESR) confirmed the generation of hydroxyl radicals under strongly acidic conditions. The drastic environmental change caused by the collapse of the microbubbles may trigger radical generation via the dispersion of the elevated chemical potential that had accumulated around the gas−water interface. The present study also confirmed the generation of ESR signals from the microbubble-treated waters even after several months had elapsed following the dispersion of the microbubbles. Bulk nanobubbles were expected to be the source of the spin-adducts of hydroxyl radicals. Such microbubble stabilization and conversion might be caused by the formation of solid microbubble shells generated by iron ions in the condensed ionic cloud around the microbubble. Therefore, the addition of a strong acid might cause drastic changes in the environment and destroy the stabilized condition. This would restart the collapsing process, leading to hydroxyl radical generation.
Nanobubbles have many potential applications depending on their types. The long-term stability of different gas nanobubbles is necessary to be studied considering their applications. In the present study, five kinds of nanobubbles (N2, O2, Ar + 8%H2, air and CO2) in deionized water and a salt aqueous solution were prepared by the hydrodynamic cavitation method. The mean size and zeta potential of the nanobubbles were measured by a light scattering system, while the pH and Eh of the nanobubble suspensions were measured as a function of time. The nanobubble stability was predicted and discussed by the total potential energies between two bubbles by the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The nanobubbles, except CO2, in deionized water showed a long-term stability for 60 days, while they were not stable in the 1 mM (milli mol/L) salt aqueous solution. During the 60 days, the bubble size gradually increased and decreased in deionized water. This size change was discussed by the Ostwald ripening effect coupled with the bubble interaction evaluated by the extended DLVO theory. On the other hand, CO2 nanobubbles in deionized water were not stable and disappeared after 5 days, while the CO2 nanobubbles in 1 mM of NaCl and CaCl2 aqueous solution became stable for 2 weeks. The floating and disappearing phenomena of nanobubbles were estimated and discussed by calculating the relationship between the terminal velocity of the floating bubble and bubble size.
Bulk nanobubbles (NBs) generated electrochemically by short voltage pulses of alternating polarity behave differently from those produced by regular methods. Only bubbles smaller than 200 nm are formed in the process and their concentration is very high. Moreover, the bubbles containing both H2 and O2 gases disappear fast via the combustion reaction, although the reaction in such a small volume cannot happen according to the classical combustion theory. Experimental facts about these unusual NBs are reviewed and current understanding of the observed phenomena is provided. Visualisation methods of a cloud of NBs above the electrodes are briefly discussed. Experimental signatures demonstrating the reaction between the gases in NBs are considered. A surface-assisted mechanism proposed for the combustion reactions in restricted volumes with a high surface-to-volume ratio is discussed. It it explained how the same mechanism is able to describe explosion of microbubbles that is observed at special conditions.
High concentrations of certain pathogenic bacteria in water usually results in outbreaks of bacterial diseases in farmed fish. Here, we explore the potential application of an emerging nanobubble technology in freshwater aquaculture, specifically aimed to reduce the concentrations of pathogenic fish bacteria in freshwater, and assess whether nanobubbles are safe for Nile tilapia (Oreochromis niloticus). An ozone nanobubble (NB-O 3) treatment protocol was established, based on examination of nanobubble size, concentration, disinfection property, and impact on fish health. A 10-min treatment with NB-O 3 in 50 L water generated approximately 2-3 × 10 7 bubbles/mL, with the majority of bubbles being less than 130 nm in diameter and an ozone level of 834 ± 22 mV oxidation-reduction potential (ORP). A single treatment with water spiked with either Streptococcus agalactiae or Aeromonas veronii effectively reduced the bacterial load by 26-48 fold or 96.11-97.92 %. This same protocol was repeated three times. The result was a 22058 to 109978 fold reduction in bacteria or 99.93-99.99 % decrease. In comparison, bacterial concentrations in the control tanks remained unchanged during the experiments. In Nile tilapia-cultured water with the presence of organic matter (e.g. mucus, feces, bacterial flora, feed, etc.), the disinfection property of NB-O 3 was reduced; however, we still observe a reduction of 59.63 %, 87.25 %, and 99.29 % after the first, second, and third consecutive treatments, respectively. To evaluate the safety of NB-O 3 on fish, juvenile Nile tilapia were exposed to NB-O 3 treatment for 10 min. No mortality was observed during the treatment or 48 h post treatment. Gill histology examination revealed that a single NB-O 3 treatment caused no alteration in cell morphology. However, damage in the gill filaments, such as blood congestion, aggregates of basal cells at the secondary lamellae or loss of the secondary lamella was noticed in the fish receiving two or three consecutive exposures within the same day. Results of the experiments conducted in this study suggest that NB-O 3 technology is promising for reducing pathogenic bacteria in aquaculture systems and may be useful at reducing the risk of bacterial disease outbreaks in farmed fish.
Nanobubble technology, as an emerging and sustainable approach, has been used for remediation of eutrophication. However, the influence of nanobubbles on the restoration of aquatic vegetation and the mechanisms are unclear. In this study, the effect of nanobubbles at different concentrations on the growth of Iris pseudacorus (Iris) and Echinodorus amazonicus (Echinodorus) was investigated. The results demonstrated that nanobubbles can enhance the delivery of oxygen to plants, while appropriate nanobubble levels will promote plant growth, excess nanobubbles could inhibit plant growth and photosynthesis. The nanobubble concentration thresholds for this switch from growth promotion to growth inhibition were 3.45 × 10 7 and 1.23 × 10 7 particles/mL for Iris and Echinodorus, respectively. Below the threshold, an increase in nanobubble concentration enhanced plant aerobic respiration and ROS generations in plants, resulting in superior plant growth. However, above the threshold, high nanobubble concentrations induced hyperoxia stress, particularly in submergent plants, which result in collapse of the antioxidant system and the inhibition of plant physiological activity. The expression of genes involved in modulating redox potential and the oxidative stress response, as well as the generation of relevant hormones, were also altered. Overall, this study provides an evidence-based strategy to guide the future application of nanobubble technology for sustainable management of natural waters.
Membrane distillation (MD) has been garnering increasing attention in research and development, since it has been proposed as a promising technology for desalinating hypersaline brine from various industries using low-grade thermal energy. However, depending on the application context, MD faces several important technical challenges that would lead to compromised performance or even process failure. These challenges include pore wetting, mineral scaling, and membrane fouling. This review is devoted to providing a state-of-the-art understanding of fundamental mechanisms and mitigation strategies regarding these three challenges. Guided by the fundamental understanding of each membrane failure mechanism, we discuss both operational and material strategies that can potentially address the three technical challenges. In particular, the material strategies involve the development of MD membranes with tailored special wetting properties to impart resistance against different types of membrane failure. Lastly, we also discuss research needs and best practices in future studies to further enhance our ability to overcome technical challenges toward the practical, sustainable, and scalable applications of MD.
Technology for aquaculture is growing with the high demand of fish. Micro / nano bubble is one of technology that is developed to increase dissolved oxygen in water. This technology is used to increase Koi fish production in high density via RAS. The purpose of this study was to determine the characteristics of micro / nano bubble technology and its effect on biology of koi fish (Cyprinus carpio). Lutor was used to introduce micro / nano bubble in the fish tank by using fish length (e.g. 7.49 ± 0.29 cm). This research used completely randomized design with one factor (fish density) and three replications. This factor was three densities of fish at 15 fish.60 L-1 (A), 30 fish.60 L -1 (B), and 45 fish. 60 L -1 (C). The result showed that there was the absolute length of each treatments was (0.52 ± 0.03a) cm for A, treatment B (0.36 ± 0.07ab) cm and C treatment (0.29 ± 0.08b) cm. The best treatment is A treatment (15 fish. 60 L -1) and different significant in statistic with C treatment (45 fish. 60 L-1) but not significant in statistic with B treatment (30 fish. 60 L -1).
The antimicrobial efficacy of novel photodynamic inactivation and nanobubble technologies
was evaluated against Vibrio parahaemolyticus and Aeromonas hydrophila as two important aquatic
microbial pathogens. Photodynamic inactivation results showed that LED (470 nm) and
UV-A (400 nm)-activated curcumin caused a complete reduction in V. parahaemolyticus at 4
and 22 C, and a greater than 2 log cfu/mL reduction in A. hydrophila, which was curcumin
concentration-dependent (p < 0.05). Furthermore, the photodynamic approach caused a greater
than 6 log cfu/mL V. parahaemolyticus reduction and more than 4 log cfu/mL of A. hydrophila reduction
in aquaponic water samples (p < 0.05). Our results with the nanobubble technology showed that the
nanobubbles alone did not significantly reduce bacteria (p > 0.05). However, a greater than 6 log cfu/mL
A. hydrophila reduction and a greater than 3 log cfu/mL of V. parahaemolyticus reduction were achieved
when nanobubble technology was combined with ultrasound (p < 0.05). The findings described in
this study illustrate the potential of applying photodynamic inactivation and nanobubble–ultrasound
antimicrobial approaches as alternative novel methods for inactivating fish and shellfish pathogens.
Hypothesis
Water electrolysis performed by short (≲5μs) voltage pulses of alternating polarity generates a dense cloud of H2 and O2 nanobubbles. Platinum electrodes turn black in this process, while they behave differently when the polarity is not altered. We prove that the modification of Pt is associated with highly energetic impact of nanobubbles rather than with any electrochemical process.
Experiments
Nanobubbles are generated by planar Pt or Ti microelectrodes. The process is driven by a series of alternating or single polarity pulses. In the case of Ti electrodes a Pt plate is separated by a gap from the electrodes. Nanoparticles on the surface of platinum are investigated with a scanning electron microscope and elemental composition is analysed using an energy-dispersive X-ray spectrometer.
Findings
Vigorous formation of Pt nanoparticles with a size of 10;nm is observed when the process is driven by the alternating polarity pulses. The effects of Pt corrosion have different character and cannot explain the phenomenon. Similar nanoparticles are observed when the Pt plate is exposed to a stream of nanobubbles. The process is explained by spontaneous combustion of hydrogen and oxygen nanobubbles on Pt surface. The phenomenon can be used to remove strongly adhered particles from solids.
Biofouling poses considerable technical challenges to agricultural irrigation systems. Controlling biofouling with strong chemical biocides is not only expensive and sometimes ineffective, but also contributes to environmental pollution. This study investigated the application of nanobubbles (NBs) on minimizing biofouling in agricultural irrigation water pipelines. Treatment performances were assessed using low concentration bubbles (LCB) and high concentration bubbles (HCB) together with a negative control (CK: no-NBs). 16 s rRNA gene sequencing and X-ray diffraction were used to characterize the microbial community and mineral compositions of biofilms in water emitters. Results demonstrated that NBs effectively mitigated biofouling through reducing fixed-biomass by 31.3−52.1%. A significantly different microbial composition was found in the biofilm community with reduced biodiversity. Molecular ecological network analysis revealed that NBs were detrimental to the mutualistic interactions among microbial species – destabilizing the network complexity and size, which was expressed as decreasing in extracellular polymers and biofilm biomass. Furthermore, NBs significantly decreased the deposition of carbonate, silicate, phosphate, and quartz on the pipe surfaces, leading to reductions of total content of minerals in biofilms. Therefore, this study demonstrated that NBs treatment could be an effective, and eco-friendly solution for biofouling control in agricultural water distribution systems.
We show that the mixing of organic solvents with pure water leads to the spontaneous formation of suspended nano-entities which exhibit long-term stability on the scale of months. A wide range of solvents representing different functional groups are studied: methanol, ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical analytical techniques to provide compounded evidence that the nano-entities observed in all these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to solvent nanodroplets, impurities or contamination. The nanobubble suspensions are characterized in terms of their bubble size distribution, bubble number density and zeta potential. The bubble number density achieved is a function of the type of solvent. It increases sharply with solvent content, reaching a maximum at an intermediate solvent concentration, before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, they cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio depending on the type of solvent. The gas solubility of the solvent relative to water as well as the molecular structure of the solvent are determining factors in the formation and stability of bulk nanobubbles. These phenomena are discussed and interpreted in the light of the experimental results obtained.
The long-term stability of nanobubbles with high internal gas pressure is a puzzling question for nanobubble researchers. The classical Molecular Dynamics simulation based on LAMMPS software was used to evaluate the performance of an oxygen nanobubble with high gas density. This research consists of two main cases. In the first case, bubble behavior was examined with 3,247 O2 molecules embedded in a 4.5 nm radius spherical volume to represent the bubble and was surrounded by 438,490 H2O molecules with 1 g/cm3 density. In the second case, the parametric study was conducted while maintaining the same parameters where the amount of O2 molecules changed at the initial configuration. Hence six different O2 bubble configurations were simulated under two different temperature settings, 20℃ and 30℃. The Lennard Jones potentials are used for molecular interactions. The simulations were run first under the NVT and then changed to the NPT ensemble. Simulation results were analyzed for bubble size, pressure, surface tension, gas diffusion, the influence of internal initial gas densities, gas concentration, and temperature conditions. The high initial gas concentration with high initial internal gas density causes a more stable bubble condition under both NVT and NPT ensembles. This stability can be attributed to the gas supersaturation conditions. The systems with low initial internal gas densities transferred to a smaller radius gas cluster with a high internal density as they shifted to the NPT ensemble. The system with a higher temperature causes elevated system pressures at NVT and volume expansion during NPT. Under NVT simulation, bubble size was higher at lower temperatures. Under NPT conditions, bubble size increases for larger initial density cases and decreases for lower density cases. Further increased temperature causes faster gas diffusion, and higher internal bubble pressure leads to unstable conditions.
We present a molecular dynamics simulation study on the effects of sodium chloride addition on stability of a nitrogen bulk nanobubble in water. We find that the lifetime of the bulk nanobubble is extended in the presence of NaCl and reveal the underlying mechanisms. We do not observe spontaneous accumulation or specific arrangement of ions/charges around the nanobubble. Importantly, we quantitatively show that the N2 molecule selectively diffuses through water molecules rather than pass by any ions after it leaves the nanobubble due to the much weaker water-water interactions than ion-water interactions. The strong ion-water interactions cause hydration effects and disrupt hydrogen bond networks in water, which leave fewer favorable paths for the diffusion of N2 molecules, and by that reduce the degree of freedom in the dissolution of the nanobubble and prolong its lifetime. These results demonstrate that the hydration of ions plays an important role in stability of the bulk nanobubble by affecting the dynamics of hydrogen bonds and the diffusion properties of the system, which further confirm and interpret the selective diffusion path of N2 molecules and the extension of lifetime of the nanobubble. The new atomistic insights obtained from the present research could potentially benefit the practical application of bulk nanobubbles.
Nanobubbles have the potential to curtail loss of oxygen during activated sludge aeration due to their extensive surface areas and lack of buoyance in solution. In this study, nanobubbles aeration was explored as a novel approach to enhance aerobic activated sludge treatment and benchmarked against coarse bubble aeration at the lab scale. Nanobubble aerated activated sludge reactors achieved greater dissolved oxygen levels at faster rates. Higher soluble chemical oxygen demand removal by 10% was observed when compared to coarse bubble aeration with the same amount of air. The activated sludge produced compact sludge yielding easier waste sludge for subsequent sludge handling. The samples showed fewer filamentous bacteria with a lower relative abundance of floc forming Corynebacterium, Pseudomonas, and Zoogloea in the sludge. The microbiome of the nanobubble-treated activated sludge showed significant shifts in the abundance of community members at the genus level and significantly lower alpha and beta diversities.
Injection of ozone nanobubbles into water reduces bacterial load, improves dissolved oxygen, and modulates the fish innate immune system. Little is known about the effect that nanobubble treatment has on the concentration of viruses in water. This study, investigated the disinfection impact of oxygen and ozone nanobubbles (NB-O2 and NB-O3) on an Aeromonas hydrophila-specific phage, pAh6.2TG, a virus lab model. After 5-, 10- and 15-min treatments with NB-O2, the concentration of phage remained the same, while the same treatment with NB-O3 eradicated 99.99 to 100% of the phage in the water. Since this phage has been shown to control bacterial infections in fish, we further investigated whether NB-O2 improved the adherence of the phage to the body surface of the fish (i.e. skin mucus, and gills) and phage penetration into fish internal organs, specifically the liver. Nile tilapia, Oreochromis niloticus were used as experimental fish in this study. The results indicated that the number of phages adhered to the skin mucus and gills in NB-O2 treatment group was 1.07 to 15.0 times higher than in the untreated control group without gas nanobubbles. The phage uptake into fish liver after NB-O2 treatment increased 1.29 to 4.75 fold compared to untreated control. These findings suggested a plausible application of NB-O2 treatment for improving efficacy of phage therapy in aquaculture. On the other hand, NB-O3 application may be useful for disinfection of harmful viruses in culture water, but the application would need to be omitted during phage treatment. This study provides preliminary information on potential applications of nanobubble technology in aquaculture to reduce viral load in the water.
Fine bubble (FB) and ultrafine bubble (UFB) generators have been installed in hydroponic systems to promote plant growth. However, their application is limited because the mechanism of the effect of UFBs is not well understood. We aimed to evaluate the effect of UFBs on promoting leaf lettuce growth in hydroponics using nutrient solutions prepared with tap water (Control) and two UFB water—tap water (standard, STD) and calcium nitrate solution (original, ORL). We then compared the differences among the three treatments under the same nutrient concentration and environmental conditions. The shoot fresh weights, dry weights, nutrient element (Zn, K, Mn, Fe, Cu, Ca, and Mg) concentrations, and total nutrient element quantity were measured after harvesting. The shoot fresh and dry weights were higher in the STD and ORL treatments than in the control, but no difference was noted between the STD and ORL treatments. Plant nutrient element uptake was higher in the STD and ORL treatments than in the Control treatment. Additionally, Mn uptake differed between the STD and ORL treatments. The results indicated that plant growth promotion in the STD and ORL treatments was affected by reactive oxygen species generated by UFBs. UFB properties also allowed for the supply of sufficient nutrients enabling rapid growth. Furthermore, we suggest that the type of solution used altered the properties of the FBs and UFBs, which in turn influenced the uptake of nutrients by the plants.
The use of nanobubbles (NBs) has gained significant attention in various applications (e.g., aeration in biological water treatment, water disinfection, membrane defouling, and ground water and sediment remediation) in recent decades because of their superior characteristics such as the improved mass transfer at the gas-liquid interfaces, their lifetime up to a couple of weeks, the formation of reactive oxygen species (ROS) with high oxidative potential. However, there is a lack of information about the effect of various factors on the stability of NBs for a long storage period under freshwater conditions. In this study, a comprehensive investigation was conducted to systematically examine the stability of oxygen NBs in water under various conditions which are closely related to a typical freshwater or the drinking water treatment. The oxygen NB stability in water was evaluated by monitoring the change in the bubble concentrations, size distribution, average diameter, and zeta potential for 60 days of storage time under different pH, hardness, ionic strength, natural organic matter (NOM), chlorine, and temperature conditions. In addition, the formation of hydroxyl radical (•OH) was investigated using disodium terephthalate which form fluorescent adducts with •OH in the presence of oxygen NBs. Among the parameters investigated, the impacts of cations, low pH, and high SUVA254 NOM on the stability of oxygen NBs were more significant than other conditions. The half-lives of oxygen NBs under various conditions follow the order Ca²⁺ < Na⁺ < pH 3 < high SUVA254 NOM < pH 5 < 30°C. Oxygen NBs were more stable in softwater than hardwater. Oxygen NBs were relatively stable for 3 days regardless of pH. For a longer storage period, oxygen NBs disappeared faster at pH 3 than at high pH. High SUVA254 NOM destabilized NBs more than low SUVA254 NOM, indicating the impact of hydrophobicity on the NB stability. The temperature effect on the NB stability was negligible for a short storage time, while higher temperature destabilized oxygen NBs for a longer storage time. One of the main disappearance pathways of oxygen NBs in water was found to be coalescing, rising, and leaving the containers, which would be promoted greatly by cations, low pH and NOM with high aromaticity. The formation of hydroxyl radical in NB solutions was detected at pH 3 by a florescent probe molecule. When oxygen NBs are released in water bodies, high calcium, high SUVA254 NOM, and low pH would significantly reduce the availability of NBs and their residence time in freshwater.
Hypothesis
Carbon dioxide nanobubbles can increase effective gas-transfer to solution and enhance buffering capacity given the stable suspension in water of CO2 gas within nanobubbles and the existence of larger gas/water interface.
Experiments
The physico-chemical properties and responses of CO2 nanobubbles were recorded at different generation times (10, 30, 50, and 70 min) and benchmarked against traditional macrobubbles of CO2 for the same amount of delivered gas. Effective concentration of CO2 was evaluated by measuring the buffer capacity (β). The size distribution of nanobubbles during the experiments was measured by Nanoparticle Track Analysis.
Findings
The mass transfer coefficient (KLa) showed a dramatically increase by 11-fold for the same volume of gas delivered when using nanobubbles. The β values obtained for nanobubbles were 7 times higher than that of traditional bubbles which can lead to significant source of CO2 availability by using the nanobubble method. Nanobubbles, consequently, undergo mass loss at higher pH corresponding to mass transfer process due to concentration gradient at the surrounding nanobubbles. This is the first report of CO2 nanobubbles buffer capacity evaluation.
The mechanism leading to the extraordinary stability of bulk nanobubbles in aqueous solutions remains an outstanding problem in soft matter, modern surface science, and physical chemistry science. In this work, the stability of bulk nanobubbles in electrolyte solutions under different pH levels and ionic strengths is studied. Nanobubbles are generated via the technique of ultrasonic cavitation, and characterized for size, number concentration and zeta potential under ambient conditions. Experimental results show that nanobubbles can survive in both acidic and basic solutions with pH values far away from the isoelectric point. We attribute the enhanced stability with increasing acidity or alkalinity of the aqueous solutions to the effective accumulation of net charges, regardless of their sign. The kinetic stability of the nanobubbles in various aqueous solutions is evaluated within the classic DLVO framework. Further, by combining a modified Poisson-Boltzmann equation with a modified Langmuir adsorption model, we describe a simple model that captures the influence of ion species and bulk concentration and reproduce the dependence of the nanobubble’s surface potential on pH. We also discuss the apparent contradiction between quantitative calculation by ion stabilization model and experimental results. This essentially requires insight into the structure and dynamics of interfacial water on the atomic-scale.
Biogas is mainly produced from the anaerobic fermentation of biomass, containing methane with an extensive range between about 50% and 70%. Higher methane content biogas has higher energy and heat value, which needs biogas upgrading. There are mainly two types of biogas upgrading technologies (ex-situ and in-situ). This manuscript presents a review of technologies on in-situ biogas upgrading. These technologies comprise H2 addition technology (e.g., continuous stirring tank reactor (CSTR), hollow fiber membrane (HFM), nano-bubble (NB) technology, upflow anaerobic sludge blanket (UASB)), high-pressure anaerobic digestion (HPAD), bioelectrochemical system (BES), and additives (e.g., ash, biochar, and iron powder). The results confirm the excellence of H2-addition technology, with the highest average CH4 content obtained (HFM: 92.5%) and one of the few full-scale cases reported (Danish GasMix ejector system: 1110 m³). Meanwhile, newly pop-up technology such as HPAD delivers appropriate CH4 content (an average of 87%) and is close to the full-scale application (https://bareau.nl/en/for-professionals/). More importantly, the combo between HPAD and H2-addition technology is prominent as the former improves the low gas-to-liquid obstacle confronted by the latter. Additionally, recently emerging BES can't stand out yet because of limited efficiency on CH4 content or constraint full-scale application behaviors (disability to operate at high current density). However, its combination with H2-addition technology to form the Power to Gas (PtG) concept is promising, and its commercial application is available (http://www.electrochaea.com/). Hydrogenotrophic methanogens are imperative players in all reviewed technologies for the generation of upgraded CH4.
Poor mass transfer in gas–liquid systems is still a major bottleneck in many biological processes which limits effective bioconversions. Nano bubble technology (NBT) is an emerging platform which offers an immense boost in several biochemical processes. Tailored application of micro and nanobubbles (MNBs) with precise tuning with gas types and dosing rates were important for biological growth enhancement as well as growth control. From recent studies, nano gas bubbles with a diameter <200 nm was promising in the interface of gas–liquid mixing systems to improve gaseous mass-transfer and associated bioprocessing. Likewise, bubbles in micro range also showed improvement depending on size, however application of nanoparticle found promising in their stabilization. Typically, nano size gas bubbles showed lifespan over a month and offered 2–30-folds higher gas solubility depending on the gas in the aqueous system. Nano bubbles in water can persist for longer duration and privileged with adequate dissolved gases hence could promote better growth and thereby productivity of microbes. At present, there is no comprehensive review published on NBT covering its main focus on bioprocess enhancement. In this review, we aimed to provide recent updates on versatile NBT for its present role as well as potential and emerging scope in various application areas to improve the efficiency of biological processes towards better bioprocessing, economic and societal benefits.
Gas saturated solutions have attracted great attention in the past two decades with reports of stable nanobubbles in solutions. The fundamental interest focus arises from the surprising stability which opens up a wide range of potential applications where the interactions between particles and nanobubbles is important. Here we review the current state of knowledge on systems involving both nanobubbles and nanoparticles. As nanoparticles and nanobubbles are found together in many circumstances, particularly those involving applications of nanobubbles, knowledge of these systems is important. This includes examining the formation of nanoparticles from nanobubbles, the nucleation of nanobubbles from nanoparticles, and the interactions between nanobubbles and nanoparticles. It is clear that further work is required to more fully understand these systems in particular on the problem of nanobubble nucleation and nanobubble-nanoparticle interactions at the submicron scale.
Bulk nanobubbles which are usually observed in pure water have a mean diameter typically around 100 nm. We use a combination of physical and chemical techniques to prove the hypothesis that the nanoentities observed in pure water are stable clusters of much smaller stable nanobubbles. The stability of bulk nanobubble clusters is affected by factors such as ionic strength or internal energy of the system. We show that bulk nanobubbles on the order of 100 nm exist in a stable cluster form in neutral or basic media, and dissociate into tiny primary nanobubbles on the order of 1 nm in acidic media, or in the presence of small amounts of salt. These new findings suggest that bulk nanobubbles which have a high surface energy unsurprisingly tend to behave in a similar manner to solid nanoparticles in terms of their agglomeration tendency, which is confirmed by the DLVO theory. The results will have important implications for our understanding and interpretation of the behaviour of bulk nanobubbles, in particular their interfacial and colloidal stability.
The present study has two primary goals, the first goal is to investigate a bibliometric analysis and assess the trends to evaluate the global scientific production of microbubbles and nanobubbles from 2000 to 2020.
The aim is to elucidate the cornucopia of benefits the two technologies (micro and nanobubbles) can offer in environmental sciences and environmental amelioration such as wastewater treatment, seed germination, separation processes, etc. The second goal is to explicate the reason behind every chart and trend through environmental engineering perspectives, which can confer value to each analysis. The data was acquired from the Web of Science and was delineated by VOS viewer software and GraphPad Prism. Considering 1034 publications in the area of micro-and nanobubbles, this study was conducted on four major aspects, including publication growth trend, countries contribution assessment, categories, journals and productivity, and keywords co-occurrence network analysis. This article revealed a notable growth in microbubbles and nanobubbles-related publications and a general growth trend in published articles in a 20-year period. China had the most significant collaboration with other countries, followed by the USA and Japan. The most dominant categories for microbubbles were environmental sciences and environmental engineering comprising 22.5% of the total publications, while multidisciplinary subjects such as nanotechnology and nanosciences (8%) were among the dominant categories for nanobubbles. Keyword's analysis results showed that microbubbles had reached the apex since their discovery.
Consequently, they are being used mostly in water/wastewater treatment or environmental improvement. On the other hand, nanobubbles are still in their infancy, and their pervasive use is yet to be fully materialized. Most of the publications are still striving to understand the nature of nanobubbles and their stability; however, a critical analysis showed that during the past two years, the trend of using nanobubbles as a cost-effective and environmentally friendly approach has already begun.
Hypothesis
Bulk nanobubbles are nanoscopic gaseous domains in an aqueous solution. Their surprising long-term stability remains controversial due to the widespread assumption that spherical bubbles cannot achieve stable equilibrium. To uncover the intrinsic mechanisms underlying stabilization, the thermodynamic behavior of nanobubbles in water over a wide range of temperatures is explored.
Experiments
Bulk nanobubbles with a typical radius of 50 – 200 nm are generated using acoustic cavitation. Increasing temperature significantly narrows the bubble-size distribution and their mean radius shrinks to a minimum of approx. 50 nm at 45°C. For higher temperatures a slight increase is observed. The thermal induced shrinkage is reversible: upon cooling they return to the original state.
Findings
The observation can be explained with a charge-stabilization mechanism. The intricate balance of competing interactions between water self-ionization and mobility of ions on the surface gives rise to this non-monotonic dependency. Nanobubbles consequently undergo charge loss at lower temperatures and charge conservation at higher temperatures, corresponding to their shrinkage and slight expansion. With theoretical calculations, we further quantity the equilibrium properties of nanobubbles and their zeta potential under various initial conditions. The temperature-sensitive nature of bulk nanobubbles offers a vital step forward exploring and industrializing their stability.
Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.
Ozone (O3) has been widely used for the elimination of recalcitrant micropollutants in aqueous environments, due to its strong oxidation ability. However, the utilization efficiency of O3 is constrained by its low solubility and short half-life during the treatment process. Herein, an integrated approach, using nanobubble technology and micro-environmental chemistry within cyclodextrin inclusion cavities, was studied in order to enhance the reactivity of ozonisation. Compared with traditional macrobubble aeration with O3 in water, nanobubble aeration achieved 1.7 times higher solubility of O3, and increased the mass transfer coefficient 4.7 times. Moreover, the addition of hydroxypropyl-β-cyclodextrin (HPβCD) further increased the stability of O3 through formation of an inclusion complex in its molecule-specific cavity. At a HPβCD:O3 molar ratio of 10:1, the lifespan of O3 reached 18 times longer than in a HPβCD-free O3 solution. Such approach accelerated the removal efficiency of the model micropollutant, 4-chlorophenol by 6.9 times, compared with conventional macrobubble ozonation. Examination of the HPβCD inclusion complex by UV-visible spectroscopy and Nuclear Magnetic Resonance analyses revealed that both O3 and 4-chlorophenol entered the HPβCD cavity, and Benesi-Hildebrand plots indicated a 1:1 stoichiometry of the host and guest compounds. Additionally, molecular docking simulations were conducted in order to confirm the formation of a ternary complex of HPβCD:4-chlorophenol:O3 and to determine the optimal inclusion mode. With these results, our study highlights the viability of the proposed integrated approach to enhance the ozonation of organic micropollutants.
Nanobubble technology has significant potential to improve the anaerobic digestion (AD) process by ameliorating the rate-limiting steps of hydrolysis and methanogenesis, as well as providing process stability by reducing sulfide and volatile fatty acid (VFA) levels. Nanobubbles (NB) can enhance substrate accessibility, digestibility, and enzymatic activity due to their minuscule size, high electrostatic interaction, and ability to generate reactive oxygen species. Air- and O2-NB can create a microaerobic environment for higher efficiency of the electron transport system, thereby reducing VFAs through enhanced facultative bacterial activity. Additionally, H2- and CO2-NB can improve hydrogenotrophic methanogenesis. Recently, several studies have employed NB technology in the AD process. There is, however, a lack of concise, synthesized information on NB applications to the AD process. This review provides an in-depth discussion on the NB-integrated AD process and the putative mechanisms involved. General discussions on other potential applications and future research directions are also provided.
Bulk nanobubbles (BNBs) are submicron gaseous domains dispersed in solutions which are supposed to survive for several hours or even days. In recent years, there has been a rapid growth on the research and extraordinary applications of BNBs. However, conventional theories based on gas diffusion and Laplace pressure predicted that nanoscale gas bubbles in water should dissolve within microseconds, presenting a modern-day paradox in current nanobubbles researches. Also, it is still challenging to efficiently produce BNBs and determine their gaseous nature with the available techniques. In this review, we start from a general introduction and brief history of nanobubbles researches and revisit the current progress on the generation methods and detection techniques. Two possible formation mechanisms are suggested and the plausibility of the proposed theories on BNBs stability is discussed with some suggestions for future studies on bulk nanobubbles.
Nitrogen, a critically important nutrient that boosts yields in agriculture and food production, is currently overused to meet the rising demand for food. Surplus nitrogen ends up in the environment in excess of the capacity of natural nitrogen cycle, thereby leading to serious environmental pollutions, such as eutrophication of water bodies and emission of nitrous oxide (a highly potent greenhouse gas) to atmosphere. Aquaponics–bioponics is an emerging soilless technology for nitrogen recovery that links organic vegetable production to aquaculture effluent remediation (aquaponics) or organic waste recycling (bioponics). This Review presents the concept of aquaponics–bioponics for nitrogen recovery. Nitrogen transformations and nitrogen mass distributions in aquaponic–bioponic systems are critically discussed, along with the nutrient availability of several organic composts that can be integrated with the systems, and the microbial communities involved. This discussion is followed by a dynamic nitrogen modeling for managing nitrogen from different wastes in aquaponics–bioponics. Various emerging engineering technologies that could improve aquaponics–bioponics are presented, including aeration with microbubbles and/or nanobubbles, cocultivation with algae, process automation with Internet of Things, and integration with indoor vertical farming (plant factory with artificial light). Overall, the Review lays out the state-of-art in aquaponics–bioponics and highlights potential approaches for developing highly efficient nitrogen recovery technologies from diverse organic waste streams.
Nanobubbles (NBs) in liquid exhibit many intriguing properties such as low buoyancy and high mass transfer efficiency and reactivity as compared to large bulk bubbles. However, it remains elusive why or how bulk NBs are stabilized in water, and particularly, the states of internal pressures of NBs are difficult to measure due to the lack of proper methodologies or instruments. This study employed the injection of high-pressure gases through a hydrophobized ceramic membrane to produce different gaseous NBs (e.g., N2, O2, H2, and CO2) in water, which is different from cavitation bubbles with potential internal low pressure and noncondensed gases. The results indicate that increasing the injection gas pressure (60-80 psi) and solution temperatures (6-40 °C) both reduced bubble sizes from approximately 400 to 200 nm, which are validated by two independent models developed from the Young-Laplace equation and contact mechanics. Particularly, the colloidal force model can explain the effects of surface tension and surface charge repulsion on bubble sizes and internal pressures. The contact mechanics model incorporates the measurement of the tip-bubble interaction forces by atomic force microscopy to determine the internal pressures and the hardness of NBs (e.g., Young's modulus). Both the colloidal force balance model and our contact mechanics model yielded consistent predictions of the internal pressures of various NBs (120-240 psi). The developed methods and model framework will be useful to unravel properties of NBs and support engineering applications of NBs (e.g., aeration or ozonation). Finally, the bulk NBs under sealed storage could be stable for around a week and progressively reduce in concentrations over the next 30-60 days.
Nano-bubble water (NBW) has been proven to be effective in promoting organics utilization and CH4 production during anaerobic digestion (AD) process, suggesting its potential in improving the stability of the AD process and thereby alleviating acidic inhibition. In this work, the effect of NBW on digestion stability and CH4 production was investigated to evaluate the ability of NBW on AD recovery from acidic inhibition. Results showed that NBW supplementation increased the total alkalinity (TA) and partial alkalinity (PA), and reduced the ratio of VFA/TA, thus maintained the stability of the AD process. Generation/consumption of VFAs was also enhanced with NBW supplementation under acidic inhibition with pH values of 5.5, 6.0 and 6.5. The cumulative CH4 production was 246–257 mL/g-VS in NBW groups, which was 12.1-17.2% higher than the control. Moreover, with NBW supplementation, the maximum CH4 production rate was raised according to the modeling results.
Herein an investigation on the performance and structural properties with aspects of stability, composition, functional group, and three-dimensional distribution were approached to evaluate the influence of nanobubble aeration to the two most common microbial aggregates, activated sludge and biofilm. This study found that applying nanobubble effectively provided extra oxygen for microbial aggregates and achieved a 10.58% improvement in total nitrogen removal. The structure of microbial aggregates was enhanced, where extracellular protein and polysaccharides respectively increased as maximum as 3.40 and 1.70 times in biofilm and activated sludge, accompanied by the development of activated sludge floc size and the thickness of biofilm. Further investigation on extracellular polymeric substance and surface of microbial aggregates showed the composition of functional substances of microbial aggregates were shifted by the application of nanobubble, especially the oxygen-sensitive ones. Confocal laser scanning microscopy imaging visualized that the nanobubble changed the morphology of biofilm to a more evenly one. However, an adaptive process was more needed for activated sludge rather than biofilm, it suggested application of NB optimized the distribution of functional microorganisms in-depth and the metabolism pathway of them by accelerating the structure development of microbial aggregates, especially for biofilm.
The stability of nanobubbles in electrolyte solutions under different ion valence values was studied using deionized water, NaCl, Na2SO4, Na3PO4, CaCl2, and FeCl3. Nanobubbles were generated using hydrodynamic cavitation, and bubbles were tested for size and zeta potential. All the samples were stable for one week with no significant deviation in either bubble size or zeta potential values. The variation of size and zeta potential among six samples can be attributed to the solution properties and was mainly dependent on solution pH and the cation valency. The ion profiles revealed that the cation concentration at the bubble surface was higher than that of bulk, confirming that the bubbles were negatively charged for neutral and high pH values (≥4) under low valency cation adsorption. The high valency cations have the potential to neutralize or completely reverse the bubble charge. Anions or co-ions have minimal effect on the surface potential or the surface charge. The calculated internal pressures of bubble were unrealistically high, suggesting that the surface tension should be lower than that of water for nanobubble solutions. The interaction energy profile shows no significant energy barrier that overcomes the attractive van der Waals forces for all the solutions, except NaCl which had a 1.87 × 10⁻²⁰ J barrier at a 5 nm separation distance. However, with the recorded stable bubbles, the calculation of the attractive van der Waals forces produced unrealistic values indicating that the Hamaker constant used for the calculation may not be valid at the nanobubble gas-liquid interface. This revealed that nanobubbles should contain exceptional interfacial properties that need to be carefully investigated and evaluated.
In this study, high solid anaerobic digestion of pig manure (PM) under nano-bubble water (NBW) addition was investigated with focus on digestion stability, methanogenesis performance and related mechanisms. Volatile fatty acids (VFAs) inhibition occurred when total solids (TS) was about 8% without NBW addition, which was alleviated with improved digestion stability under NBW addition, facilitating the process of high solids anaerobic digestion (HSAD). The cumulative CH4 yield, on the other hand, was 201-230 mL/g-VS in the NBW reactors at TS of 3-6%, about 20.3-25.0% higher than the control reactors. At the same time, with higher water mobility and zeta potential, NBW was found to promote the consumption of soluble proteins/carbohydrates during the above AD process.
Lignin with highly complex aromatic hydrocarbons is recalcitrant to biodegradation. Hydroxyl radical (OH) plays a vital role in lignin depolymerization, which can be generated from nanobubble water (NBW). This study focused on evaluating the effect of nitrogen gas NBW (N2-NBW) on lignin degradation and methane production via anaerobic co-digestion (AcoD) of waste activated sludge (WAS) and alkaline lignin. The results showed that total methane yield was obviously increased by 17% under N2-NBW addition, which followed a dose-dependent manner. Volatile fatty acids (VFAs) production and consumption in addition to volatile solids (VS) reduction were enhanced during the AcoD process supplemented with N2-NBW. Lignin degradation rate was improved by 10% under N2-NBW addition condition compared to the control at an initial lignin concentration of 50 mg/L. It's hypothesized that OH generated by NBW can facilitate lignin degradation and bioenergy conversion.
Nanobubble water (NBW) contains 10⁶–10⁸ gas bubbles per milliliter with diameter <1 µm. These fine bubbles possess high mass transfer efficiency and hydrophobic property beneficial for agricultural and biomedical application. Nowadays, much attention has been paid to the secure and sustainable management of lignocellulosic wastes. Cellulose is regarded as one kind of slowly biodegradable organic components owing to its complex crystalline structure. This study investigated the effect of O2-containing gas NBW on anaerobic digestion (AD) of cellulose for methane production. Results show that the cumulative methane yields from the reactors with Air-NBW (193 NmL/g-VSreduced), N2-NBW (196 NmL/g-VSreduced) and O2-NBW (233 NmL/g-VSreduced) addition were increased by 8–30% in comparison to the control (with the same amount of deionized water addition) (179 NmL/g-VSreduced). Under NBW addition, the reductions in cellulose content and cellulose crystallinity were respectively enhanced by 8–14% and 9–21% during AD, in which cellulase activity was elevated by 10–38%. The O2-NBW reactor was found to have the highest electron transport system activity, increasing by 1.7 times compared to the control, most probably due to the collapse of O2-nanobubbles and release of O2 resulting in micro-oxygen environment under the test condition. Besides, microbial community analysis suggests that the direct interspecies electron transfer could be quickly established with the addition of NBW. Results from this study also shed light on the mechanisms involved in the bioconversion of cellulose to methane via AD supplemented with NBW.
This paper proposes the use of hydrogen oxidizing bacteria (HOB) for the removal of orthophosphate from surface water as treatment step to prevent cyanobacterial blooms. To be effective as an orthophosphate removal strategy, an efficient transfer of hydrogen to the HOB is essential. A trickling filter was selected for this purpose. Using this system, a removal rate of 11.32 ± 0.43 mg PO4-3-P/L.d was achieved. The HOB biomass, developed on the trickling filter, is composed of 1.25% phosphorus on dry matter, which suggests that the orthophosphate removal principle is based on HOB growth. Cyanobacterial growth assays of the untreated and treated water showed that Synechocystis sp was only able to grow in the untreated water. Orthophosphate was removed to average residual values of 0.008 mg/L. In this proof of principle study, it is shown that HOB are able to remove orthophosphate from water to concentrations that prevent cyanobacterial growth.
This article presents the review of the research papers concerning nanobubble generation. In main section of this article, numerous methods of nanobubble generation have been discussed pinpointing the differences in results obtained in similar experimental setups. Different generation methods have been tabularized to present the composition of phases, the diameter of generated bubbles as well as the commentary concerning discrepancies within one method. The number of nanobubble applications in environmental processes is increasing in the last years, however the thorough investigation of their generation methods is not covered in literature. This review article is gathering knowledge about nanobubble generation methods and is comparing results obtained by different research teams. It should lay foundation for future research concerning nanobubbles, what will lead to increasing the efficiency of various environmental processes, including wastewater flotation, metal recovery and soil and groundwater remediation.