Nanobubble Reactivity: Evaluating Hydroxyl Radical Generation (or Lack Thereof) under Ambient Conditions

A critical review on the properties and applications of bulk micro and nanobubbles for the degradation of organic pollutants in wastewater treatment

The presence of persistent organic pollutants in wastewater streams has presented significant challenges towards their removal. In the recent decade, bulk micro (1-100 μm) and nanobubble (50-150 nm) (MNB) technology has exhibited technological advancements via integration of MNB technology in degrading organic pollutants from wastewater streams. The present review critically analyses the physico-chemical properties such as stability, zeta potential, mass transfer rates, rising velocity and size distribution of MNBs. The paradigm shift from conventional wastewater treatment to more sustainable solution is initiated by the production of OH- ions and free radicals for the degradation of organic pollutants by the MNB technology. Applications of MNBs are also explored in various wastewater treatment processes such as floatation, membrane cleaning, adsorption, aeration, and advanced oxidation processes. Future researches highlighting the challenges in the development of efficient and robust MNB technology and its real-time applications have also been highlighted. It is anticipated that MNBs could be a sustainable and economic solution for wastewater treatment.

Characterization of the effects of micro-bubble treatment on the cleanliness of intestinal sludge, physicochemical properties, and textural quality of shrimp (Litopenaeus vannamei)

This study assessed the effects of Micro-bubble (MB; average size of 36.50 μm) on the physicochemical properties of live shrimp (Litopenaeus vannamei) and cooked ones. This study also examined the visual appearance, intestinal sludge's presence, microbial decontamination efficacy, and shrimp meat's textural properties. In addition, sensory evaluation was conducted to appraise the impact of MB on the gustatory and aromatic attributes of shrimp. This study showed that MB treatment was substantially better than traditional aeration (TA) treatment in terms of appearance, color, and intestinal sludge cleanliness. Specifically, all the above indicators exhibited optimal performance by the MB 30 group. Moreover, the MB treatment effectively decontamination microbes, reduced total volatile bases nitrogen (TVB-N; from 15.27 to 10.30 mg/100 g) and improved the textural indicators, while the best performed with the MB30 group. The sensory evaluation showed that the MB treatment groups exhibited enhanced aroma and taste profiles (7.45 to 8.00 points) of the shrimp compared to the TA (6.80 and 7.80 points) and control (7.15 and 7.25 points) groups, favored by the evaluation panels. Therefore, the findings of this study reveal that MB treatment provides a practical and invaluable theoretical framework to effectively address concerns about the quality and safety of live shrimp. Refraining from contamination during initial production would benefit stakeholders by reducing food and economic losses while ensuring longer shelf-life and safety of shrimp.

Development of a 10-litre pilot scale micro-nano bubble (MNB)-enhanced photocatalytic system for wastewater treatment

A 10-litre pilot scale micro-nano bubble (MNB)-enhanced photocatalytic degradation system was developed using ZnO as the photocatalyst and salicylic acid (SA) as the model pollutant. The effectiveness of the MNB/ZnO/UV system was systematically compared with those of MNB, UV, MNB/UV, MNB/ZnO and ZnO/UV degradation systems. The effects of process parameters, including catalyst dosage, pollutant concentration, air-intake rate, pH and salt content on the degradation of SA, were comprehensively investigated. Optimum performance was obtained at neutral conditions with a catalyst dosage of 0.3 g/L and an air-intake rate of 0.1 L/min. For the degradation of SA, a kinetic constant of 0.04126/min was achieved in the MNB/ZnO/UV system, which is 4.5 times greater than that obtained in the conventional ZnO/UV system. The substantial increase in the degradation rate can be attributed to that the air MNB not only enhanced the gas-liquid mass transfer efficiency but also elevated the concentration of dissolved oxygen. A 10-litre pilot scale MNB/ZnO/UV system was successfully applied to the purification of lake water and river water, demonstrating great application potential for wastewater treatment.

Progress and limitations in reactive oxygen species quantitation

Reactive oxygen species (ROS) are a set of oxygen- and nitrogen-containing radicals. They are produced from a wide range of sources. In biological contexts, cellular stress leads to an overproduction of ROS, which can lead to genetic damage and disease development. In industry, ROS are often productively used for water purification or for analyzing the possible toxicity of an industrial process. Because of their ubiquity, detection of ROS has been an analytical goal across a range of fields. To understand complicated systems and origins of ROS production, it is necessary to move from qualitative detection to quantitation. Analytical techniques that combine quantitation, high spatial and temporal resolution, and good specificity represent detection methods that can fill critical gaps in ROS research. Herein, we discuss the continued progress and limitations of fluorescence, electrochemical, and electron paramagnetic resonance detection of ROS over the last ten years, giving suggestions for the future of the field.

Advanced nanobubble flotation for enhanced removal of sub-10 µm microplastics from wastewater

Sub-10 µm microplastics (MPs) in aquatic environments pose significant ecological and health risks due to their mobility and potential to carry harmful microcontaminants. Our effluent analysis from a Hong Kong Sewage Treatment Works shows that traditional treatment often fails to effectively remove these MPs. These small-sized MPs are commonly neglected due to challenges in accurate quantification, analysis, and removal. This study introduces a nanobubble-assisted flotation process that enhances the removal efficiency of both regular and irregular small-sized MPs from wastewater. The proposed process outperforms the traditional flotation process by fostering a more effective interaction between bubbles and MPs, increasing removal rates of MPs from 1 µm to 10 µm by up to 12% and providing a total efficiency boost of up to 17% for various particle sizes. Improvements are attributed to enhanced collision and adhesion probabilities, hydrophobic interactions, as well as better floc flotation. Supported by empirical evidence, mathematical models, and Molecular Dynamics simulations, this research elucidates the nanoscale mechanisms at play. The findings confirm the nanobubble-assisted flotation technique as an innovative and practical approach to removing sub-10 µm MPs in water treatment processes.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability

The origin of the stability of bulk Nanobubbles (NBs) has been the object of scrutiny in recent years. The interplay between the surface charge on the NBs and the Laplace pressure resulting from the surface tension at the solvent-NB interface has often been evoked to explain the stability of the dispersed NBs. While the Laplace pressure is well understood in the community, the nature of the surface charge on the NBs has remained obscure. In this work, we aim to show that the solvent and the present ions can effectively polarize the NB surface by inducing a dipole moment, which in turn controls the NB stability. We show that the polarizability of the dispersed gas and the polarity of the dispersing solvent control the dipole-induced dipole interactions between the solvent and the NBs, and that, in turn, determines their stability in solution.

Influence of static pressure on toluene oxidation efficiency in groundwater by micro-nano bubble ozonation

Micro-nano bubble ozonation has been widely applied in the purification of drinking water due to its superior characteristics such as high mass transfer rate and long resistance time. However, its application in groundwater remediation is limited, partially due to the unclear effect of static water pressure on the oxidation efficiency. This study constructed a batch reactor to investigate the influence of static pressure on toluene oxidation by ozone micro-nano bubble water. To achieve constant pressure, weight was added above the mobile reactor roof, and the initial concentrations of toluene and dissolved ozone were 1.00 mg L-1 and 0.68 mg L-1 respectively. Experimental results demonstrated that as the static water pressure increased from 0.0 to 2.5 m, the average microbubble diameter decreased significantly from 62.3 to 36.0 μm. Simultaneously, the oxidation percentage of toluene increased from 40.3% to 58.7%, and the reaction rate between toluene and hydroxyl radical (OH·) increased from 9.3 × 109 to 1.39 × 1010 M-1 s-1, indicating that the shrinkage of micro-nano bubbles generated an abundance of OH· that quickly oxidized toluene adsorbed at the bubble interface. A greater enhancement of oxidation efficiency for nitrobenzene, as compared to p-xylene, was observed after the addition of 2.5 m water pressure, which verified the larger contribution of OH· under static pressure. Although the improvement of oxidation efficiency was reduced under acid and alkaline environments, as well as in practical groundwater matrices, the overall results still demonstrated the promising application of micro-nano bubble ozonation in groundwater remediation.