Gas Bubbles In Solutions Research Articles

A Two-Dimensional Transport Model of Bipolar Membrane Electrodialysis for the Electro-Regeneration of Carbon Capture Solvents

Bipolar membrane electrodialysis (BPMED) is a promising technology that uses an electric field to split water into H+ and OH- ions. A repeating stack of cation selective, anion selective, and bipolar membranes are placed between two electrodes, creating a repeating pattern of acid, base, and diluting flow chambers. A major application of BPMED is the electrochemical solvent regeneration of capture solutions used in CO2 point source (PS) or direct air capture (DAC). Protons generated using BPMED can be used to shift the acid-base equilibria back to CO2, providing a more sustainable alternative to traditional thermal regeneration methods when integrated with renewable electricity sources. This method combines electrosynthesis and separation, producing a pure stream of CO2 for sequestration and a concentrated base stream.Despite its potential, current limitations to BPMED implementation for this application stem from a large power consumption and high membrane costs. In addition, gas bubbles in solution can cause high electrical resistances, and experimental studies of this phenomenon are challenging due to the small scale on which it occurs. To address these challenges, we present a two-dimensional model of BPMED for CO2 capture solvent recovery. This model depicts a single repeating cell triplet and was constructed using COMSOL Multiphysics version 6.1. The Navier-Stokes equations were implemented to compute a convection field, and the ionic species concentration was found by solving the Nernst-Planck equation. A source term in the concentration partial differential equations was implemented to account for ionic speciation and water splitting reactions, captured through kinetic rate expressions.Experimental validation was conducted on a PC Cell BED 1-4 recirculating batch system with a stack of seven cell pairs of PC acid 60 (anion selective), PC SK (cation selective), and PC Bip (bipolar) membranes. Good agreement between the model and experimental results across a range of conditions and variables demonstrates accuracy and robustness. The calculated pH and concentration profiles revealed that the majority of the speciation reaction takes place inside, or directly adjacent to, the bipolar membrane, including bubble nucleation. Bubbles form inside membrane pores and displace the electrolyte, greatly increasing the electrical resistance of the membrane, particularly when membrane porosity is low. However, the reaction plane location where bubbles are generated can be manipulated by increasing the channel width or reducing the applied voltage.Future research will focus on expanding to multiphase CFD simulations to investigate the effect on flow. Our two-dimensional model provides valuable insights into the complex and multifaceted nature of BPMED, offering a powerful tool for investigating small-scale processes and advancing the use of BPMED for CO2 capture solvent recovery. Figure 1

Feasibility Study of Tetracycline Removal by Ozonation Equipped with an Ultrafine-Bubble Compressor

In this study, a combination of an ozone gas producer and an ultrafine-bubble compressor was used to degrade tetracycline, which is a well-known antibiotic and medicine commonly used in human and animal care, and effects of varying the reaction parameters were studied. Experiments indicate that each gram of introducing ozone can degrade 2.72 g of tetracycline at pH 3 and 1.48 g at pH 11. However, basic conditions contribute to increased mineralization of tetracycline because of the ·OH radical oxidation mechanism. Higher reaction temperatures and higher ozone dosages enhance the reactivity between the ozone molecules, ·OH radicals, and tetracycline, resulting in a decline in the toxicity of the tetracycline solution as measured by cell viability. The mineralization of organic compounds is the key to decreasing the toxicity of the solution. Ultrafine-bubble ozonation can provide homogeneity of gas bubbles in solution hence it not only reduces the requirement of ozone and thus the operational cost of the reaction, but also extends the efficacy of the method to the treatment of solutions with high tetracycline concentrations.

Effect of Mechanical Shaking on the Physicochemical Properties of Aqueous Solutions.

Long-lived luminescence in the blue region was found to occur in deionized water saturated with atmospheric gases following mechanical shaking. Luminescence intensity decreased exponentially after the cessation of stress. During vigorous mechanical shaking, we observed gas bubbles in solution, and the liquid–gas interface area increased noticeably. At the same time, the concentration of molecular oxygen decreased, which could not be attributed to the water warming up with exposure to mechanical stress. However, deaerated water rapidly became saturated with gases following mechanical stress. The recommendation that cell culture media should be mixed after they are removed from the fridge in order to allow saturation with oxygen is probably misleading. It was shown that gases existed in water both in the form of individual molecules and nanobubbles. Mechanical stress did not influence the number or size of nanobubbles. While gas nanobubbles were absent in freshly prepared deaerated water, they appeared following exposure to mechanical stress. In addition, in mechanically treated gas-saturated water, there was seemingly an equilibrium shift towards the decomposition of carbonic acid to water and carbon dioxide. At the same time, the pH of water tended to increase immediately after mechanical stress. It was demonstrated that reactive oxygen species (ROS) form in gas-saturated water under mechanical stress (30 Hz, amplitude of 5 mm). The relative generation rate of hydrogen peroxide and of the hydroxyl radical was 1 nM/min and 0.5 nM/min, respectively. It was found that with an increase in the frequency of mechanical action (f), the rate of ROS generation increased in proportion to f 2. The major pathways for hydrogen peroxide generation are probably associated with the formation of singlet oxygen and its further reduction, and the alternative pathway is the formation of hydrogen peroxide as a result of hydroxyl radical recombination.

An alternative treatment method for fluorosurfactant-containing wastewater by aerosol-mediated separation

The treatment of fluorosurfactant-containing wastewater is still challenging nowadays. Here, a method is presented to remove fluorosurfactants from water, amongst others from electroplating wastewater. This elimination technique is based on the generation of gas bubbles in solution, enrichment and scavenging of fluorosurfactants by transport of the gas bubbles to the water surface. Finally the bubbles collapse and release an aerosol which is enriched with fluorosurfactants. By sampling of the released aerosols a mass balance was established for 6:2 fluorotelomer sulfonic acid (6:2 FTSA). Thereby 99.8% of the initial amount was revocered in the collected aerosols. Fluorosurfactant concentration in solution decreased exponentially with half-lives ranging from 2 to 6 min for 6:2 FTSA as well as perfluorooctane carboxylate (PFOA) and perfluorooctane sulfonate (PFOS). Elimination rate in defined matrix (0.2 M H2SO4) within 60 min was 99.6, 99.9 and 99.8% for 6:2 FTSA, PFOA and PFOS, respectively. The removal rate of 6:2 FTSA increased in solutions with higher ionic strength. Different wastewater from an electroplating industry containing 6:2 FTSA was treated with the described method without any sample pre-treatment and elimination of 6:2 FTSA took place with the same effectiveness as in synthetic matrices.

Innovative processes for electropolishing of medical devices made of stainless steels

Currently, many medical devices are made of implantable metals such as 316LVM stainless steel. Electropolishing is a common process to obtain a smooth surface, free of contaminants and more passive, which allows for minimizing the foreign body response and cell adhesion. However, polishing of small implants with a highly complicated geometry and nonuniform metallurgy might result in unsatisfactory results. The objective of this work was to develop an electropolishing process effective for complex metallic implants such as artificial heart valve frames and miniature glaucoma implants. Polishing in an ultrasonic bath and pulsed voltage polishing processes were studied and compared to the standard ASTM process. Current-voltage curves were constructed for different solutions and bath temperatures. The polished parts were evaluated by stereomicroscopy, optical microscopy, atomic force microscopy, noncontact surface profilometry, and X-ray diffraction. Pulse polishing was found useful in eliminating the erosion effects of gas bubbles in solution. Electropolishing in an ultrasonic bath was found useful when a rough, patterned surface is needed, e.g. for osseointegration purposes. Preliminary animal studies followed by histopathology indicated that the polished surfaces stimulated only a moderate body reaction, as desired in such applications. The pronounced dependence of the measured roughness values on both the measurement technique and scanned area should inspire the preparation of a new test-method standard.

ζ potential measurement for air bubbles in protein solutions

Protein adsorption at gas–liquid interfaces is important in a number of processes including foam formation in bioreactors, foam fractionation for protein recovery, and production of protein based food and drinks. The physical properties of the gas–liquid interface will influence foam stability; important properties will include both surface rheological and electrokinetic properties. While surface rheological properties of gas–protein solution interfaces have been reported, there are no published values for electrokinetic properties at such interfaces. In this paper, ζ potential values of gas bubbles in solutions of three proteins, measured using a microelectrophoresis technique, are reported. The three proteins chosen were BSA, β-casein, and lysozyme; these proteins have all been used previously in protein foaming studies. The effect of protein concentration and ionic strength is considered. For BSA and β-casein, ζ potential was found to increase with increasing protein concentration and ionic strength. For air bubbles in lysozyme solutions, measured ζ potential was zero. ζ potential values for air bubbles in some binary protein mixtures are also presented.

Droplet fractionation of hexavalent chromium from bubbles bursting at liquid surfaces of chromic acid solutions

The aim of this study was to evaluate the effects of electrolyte concentration and gassing rate (in terms of gas flow rate) on the droplet fractionation of Cr(VI) from bubbles bursting at liquid surfaces of chromic acid solutions. An experimental system was used to generate gas bubbles in solutions, and the droplets thus generated were measured by an aerodynamic particle sizer (APS). A Marple personal impactor was used simultaneously to collect droplet samples for chemical analysis. Ratios of the Cr(VI) concentration in droplets to that in the bulk solution were calculated to evaluate the droplet fractionation effect. The results showed that the concentration ratios had a range of about 5 to less than unity for the experimental conditions used in this study, indicating that the droplets can be either enriched or depleted with Cr(VI) relative to the bulk solution. The concentration ratio decreased exponentially with increasing electrolyte concentration, but decreased approximately linearly with increasing gas flow rate. In addition, surface tension also played an important role in chemical fractionation by droplet formation from bursting bubbles. The results should have applications in assessing the health risk of chromium electroplating plants where chromium-containing droplets are generated as a result of electrochemical reactions and gassing.

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