Characterization of air submicrobubbles for water treatment under different generation conditions - Scientific ReportsAbstractMicro-nanobubble (...

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Characterization of air submicrobubbles for water treatment under different generation conditions - Scientific ReportsAbstractMicro-nanobubble (...
Characterization of air submicrobubbles for water treatment under different generation conditions - Scientific Reports
Abstract
Micro-nanobubble (MNB) technology has been proven to be effective in water and wastewater treatment. Submicrobubbles (SMBs) are considered to be a subgroup of MNBs ranging from 1 to 10 μm with limited studies related to their fundamental properties. This study focused on the fundamental characteristics of SMBs and the effect of generation conditions such as temperature, aeration time, and water volume on their properties. SMBs were generated under high pressure using shear force and their size and distribution were measured using the dynamic light scattering method. Dissolved oxygen (DO) concentrations were monitored during and after the generation. The zeta potential of the generated bubbles was also measured to assess bubbles stability. SMBs with a median diameter of 2 μm persisted in water even after the generator was stopped, indicating the high longevity of SMBs in water. Regardless of the aeration time or water volume used, the zeta potential of SMBs was highly negative with average values ranged between − 28 and − 30 mV, indicating high stability in water. DO concentration increased by up to 1.5 folds within a few minutes of generation and slightly decreased over 1 h. Results demonstrate that air SMBs are stable with long lifespan and high DO concentration.

Introduction
Micro-nanobubbles (MNBs) are micro/nanometer diameter-sized bubbles. The potential use of MNBs in several scientific and technological fields has gained attention in recent years, particularly in environmental engineering1. MNBs have proven to be a promising technology in applications such as groundwater remediation2,3, wastewater treatment4,5, surface water treatment6, biochemical process enhancement7, environmental pollution control8, ecological restoration9, food process10, agricultural processes11,12, aquaculture, and medical applications13. Specifically, MNBs have proven to be effective in water/wastewater treatment for the removal of organics14,15 and oils16, disinfection and sterilization processes17,18, and surface cleaning19. In addition, MNBs have been applied in various processes, including flotation20, aeration21, and membrane processes22.

The increase in MNB application in water treatment is related to the following advantages, including (1) reduced chemical usage; (2) significant potential for cost reduction in operations and design23; (3) low cost, convenience, and environmental friendliness for cleaning of conducting surfaces24; (4) slow rise velocity; (5) high mass-transfer efficiency; (6) high specific surface area; (7) generation of free radicals; and (8) longevity4,23. In contrast, conventional macrobubbles (millibubbles) rise rapidly to the liquid surface and burst at the air–liquid interface. Therefore, macrobubbles have different physicochemical properties than small-sized bubbles25.

In several studies, the unique characteristics of MNBs enabled them to outperform conventional macrobubbles. Nam et al. (2019) showed that microbubble (MB) ozonation is superior to conventional macrobubble ozonation in terms of generating higher concentrations of hydroxyl radicals and ozone in aqueous solutions26. Another study reported better performance for the degradation of bio-refractory organic compounds for MB ozonation compared to macrobubble ozonation27. Sun et al. (2020) showed that the reaction rate constant of MB ozonation in removing petroleum hydrocarbons from oily sludge was twice that of macrobubble ozonation28.

The efficiency of MNBs in treating polluted effluents in terms of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) was evaluated in a systematic review and meta-analysis29. The diameters of the MNBs used ranged from 0.01 to 70 μm. The review concluded that MNBs could remove BOD5 and COD at efficiencies ranging from 68% to 100%. In addition, the review found that MNBs removed up to 99.9% of inorganic and microbiological pollutants. Several studies have investigated the effect of using MNBs to reduce solids or pollutants in different water sources. All studies demonstrated high efficiencies of NBs or MBs in the reduction of total dissolved solids (TDS)30, turbidity, BOD, and COD31.

Air MNBs have been utilized in several studies to enhance treatment process performance. For example, Li et al. (2022) confirmed that air MNBs provide enhanced normalized fluxes of natural organic matter during the ultrafiltration separation of feeds containing various contaminants32. Under similar experimental conditions, the results with MNBs were significantly better than those obtained without MNBs. Air MNBs have been successfully applied in reverse osmosis desalination processes to improve membrane performance, and control gypsum scaling22.

Air MNBs have been used in biological wastewater treatment to improve aeration process. Ahmadi et al. (2022) determined that a nanobubble (NB) aeration system could significantly enhance the measured treatment efficacy parameters for a sequential batch reactor33. NB and MB aeration also enhanced gas mass transfer compared with conventional aeration in the activated sludge process. Therefore, they could be considered efficient upgrades to the current activated sludge process using conventional macrobubble aeration. This is reflected in the reduced energy consumption owing to the improvement in oxygen transfer, as well as easier organic and nutrient removal21. A recent study confirmed that MNBs are a great choice for treating blackening and odorization in rivers when combining activated sludge and biofilms. They demonstrated that MNB aeration was better than macrobubble aeration, with a 12-fold higher oxygen-transfer efficiency and 52.6% lower oxygen decline rate34. In addition, MNB aeration resulted in 50% less energy consumption compared to macrobubbles and showed higher oxygenation performance35. Therefore, MNB aeration has greater potential for biological wastewater treatment than macrobubble aeration. Furthermore, the oxygen-transfer efficiency was improved by decreasing the bubble size.

Air MNBs have shown potential for use in various wastewater treatments, either alone or in combination with other processes. One study applied MNBs and activated hydrogen peroxide to study the degradation of tetracycline in wastewater. The degradation rate of tetracycline hydrochloride reached 92.43%, and the main reactive oxygen radical was •OH, followed by HO2•/•O2-36. Another study applied MNB aeration to enhance the efficiency of Rhodamine B degradation during heat-activated persulfate oxidation. The MNBs successfully accelerated the reaction rate and increased the DO concentration. In addition, the combined system stably generated the radicals •SO4− and •OH, enhancing Rhodamine B degradation37.

Because the size of the bubbles plays a crucial role in their fundamental properties, different sizes will lead to different properties, which will be reflected in the treatment. Although some researchers describe MNBs as small-sized bubbles with diameters on the nano- and micrometer scale1, others specify MNBs as bubbles in the range of 200 nm to 10 μm25,38. The MNBs generated in this research will be referred to as submicrobubbles (SMBs), which lie in the range of 1–10 μm23.

Most of previous studies on MNBs characterization in water focused on MBs (10–100 μm) or NBs (< 1000 nm), whereas research on bubbles in the 1–10 μm range (SMBs) is limited. Temesgen et al. (2017)23 highlighted that bubbles with diameters ranging from 10 to 100 μm burst in liquids and take minutes to rise to the surface. This behavior differs from bubbles ≤ 1 μm, which swell, burst in the liquid, and take hours or even weeks to rise to the surface due to Brownian motion. Temesgen et al. (2017)23 indicated that the properties of SMBs lie between those of MBs and NBs. MBs are suitable for several applications including dissolved air flotation especially within the range of 30–50 μm because their micro-size promotes the adhesion of bubbles to water particles and gives suitable bubble collision efficiency. NBs within the range 700–900 nm are close to being stationary in water making them suitable for several applications such as aeration and advanced oxidation processes due to increased mass transfer rates because mass transfer depends on bubbles surface area and rising velocity39. As a result, it is expected that SMBs, with their size lying between MBs and NBs, will remain for a reasonable time, enabling efficient treatment. SMBs also have large interfacial area, slow rising velocity and high gas solubility for enhanced aeration process. In addition, when using SMBs it is expected to avoid problems related to over-aeration or very fast rising velocity that can affect water treatment process. Although some studies used air SMBs in their treatment process4, and some studies examined some of SMBs characteristics40; up to our knowledge, the investigation of SMBs basic properties in relation to their experimental conditions such as aeration time and water volume remains underexplored. This study focuses solely on, the under-investigated, SMBs properties to understand their behavior and identify their suitable applications in water and wastewater treatment.

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The main aim of this research is to investigate the properties of air SMBs generated using shear force and high pressure under different experimental conditions. The investigated properties include longevity, stability, and DO concentration levels, and the experimental conditions include aeration time and water volume. To achieve this aim, this paper investigates the impact of the experimental conditions on (1) the size and distribution, (2) the longevity and DO concentration, and (3) the stability of SMBs. Accordingly, the research aims to fill in the gap in bubble characteristics between NBs and MBs. Future applications in water and wastewater treatment are also considered. Subsequently, the zeta potential, which plays a crucial role in bubble stability, was measured. In addition, DO enhancement was studied for future applications of submicrosized bubbles. Finally, the main challenges encountered during bubble generation are highlighted.

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