Is hydrogen gas in water present as bubbles or hydrated form?

Development of a mathematical model for a microbial denitrification co-culture system comprising acetogenic bacterium Sporomusa ovata and denitrifying bacterium Pseudomonas stutzeri

Previous study has shown that co-culturing acetogenic bacterium Sporomusa ovata (SO), with denitrifying bacterium Pseudomonas stutzeri (PS), is a promising strategy to enhance the microbial denitrification for nitrate-contaminated groundwater remediation. However, the mutual effects and reaction kinetics of these two bacteria in the co-culture system are poorly understood. In this study, a mathematical model for this co-culture system was established to fill this knowledge gap. Model simulation demonstrated that SO had a significant effect on the kinetics of denitrification by PS, while PS slightly affected the kinetics of acetate production by SO. The optimal initial HCO₃^-/NO₃^- ratio and SO/PS inoculation ratio were 0.77-1.48 and 67 for the co-culture system to achieve satisfied denitrification performance with less acetate accumulation. Finally, the minimum hydrogen supply was recommended when the initial bicarbonate and nitrate concentrations were assigned in the range of 2-20 mM and 2-4 mM for simulating the natural nitrate-contaminated groundwater treatment. These findings could provide useful insights to guide the operation and optimization of the denitrification co-culture system.

Electric Migration of Hydrogen Ion in Pore-Voltammetry Suppressed by Nafion Film

Micro-hole voltammetry exhibiting rectified current-voltage curves was performed in hydrochloric acid by varying the lengths and the diameters of the micro-holes on one end of which a Nafion film was mounted. Some voltammetric properties were compared with those in NaCl solution. The voltammograms were composed of two line-segments, the slope of one segment being larger than the other. They were controlled by electric migration partly because of the linearity of the voltammograms and partly the independence of the scan rates. Since the low conductance which appeared in the current from the hole to the Nafion film was proportional to the cross section area of the hole and the inverse of the length of the hole, it should be controlled by the geometry of the hole. The conductance of the hydrogen ion in the Nafion film was observed to be smaller than that in the bulk, because the transport rate of hydrogen ion by the Grotthuss mechanism was hindered by the destruction of hydrogen bonds in the film. In contrast, the conductance for the current from the Nafion to the hole, enhancing by up to 30 times in magnitude from the opposite current, was controlled by the cell geometry rather than the hole geometry except for very small holes. A reason for the enhancement is a supply of hydrogen ions from the Nafion to increase the concentration in the hole. The concentration of the hydrogen ion was five times smaller than that of sodium ion because of the blocking of transport of the hydrogen ion in the Nafion film. However, the rectification ratio of H+ was twice as large as that of Na+.

(Non)Existence of Bulk Nanobubbles: The Role of Ultrasonic Cavitation and Organic Solutes in Water

A drawback of studies on bulk nanobubbles is the absence of direct proof that the nano-objects reported are really nanobubbles. The aim of our work was to provide such a proof or disproof. We focused on two effects (processes) commonly considered in research on nanobubbles: ultrasonic cavitation and addition of organic compounds to water, which could create in principle a barrier at the gas/water interface contributing to the stability of nanobubbles. We found that both of these processes lead to the generation of nano-objects, which are, however, not bulk nanobubbles. Ultrasonication leads to the formation of fine metal nanoparticles originating from the disintegration of the surface of the metal ultrasonic probe. Addition of organic solutes to water leads to the formation of a population of nanoparticles/nanodroplets originating from the so-called mesoscale solubilization of hydrophobic compounds present in the added solute as molecularly dissolved impurities. Subsequent ultrasonication of such mixtures adds metal nanoparticles and only slightly modifies the size distribution of mesoscale particles. While our results do not dismiss existence of nanobubbles in general, described effects must be seriously taken into account, especially in the case of biomedical applications where they can result in serious side effects.

Electrolysis-Driven and Pressure-Controlled Diffusive Growth of Successive Bubbles on Microstructured Surfaces

Control over the bubble growth rates forming on the electrodes of water-splitting cells or chemical reactors is critical with respect to the attainment of higher energy efficiencies within these devices. This study focuses on the diffusion-driven growth dynamics of a succession of H₂ bubbles generated at a flat silicon electrode substrate. Controlled nucleation is achieved by means of a single nucleation site consisting of a hydrophobic micropit etched within a micrometer-sized pillar. In our experimental configuration of constant-current electrolysis, we identify gas depletion from (i) previous bubbles in the succession, (ii) unwanted bubbles forming on the sidewalls, and (iii) the mere presence of the circular cavity where the electrode is being held. The impact of these effects on bubble growth is discussed with support from numerical simulations. The time evolution of the dimensionless bubble growth coefficient, which is a measure of the overall growth rate of a particular bubble, of electrolysis-generated bubbles is compared to that of CO₂ bubbles growing on a similar surface in the presence of a supersaturated solution of carbonated water. For electrolytic bubbles and under the range of current densities considered here (5-15 A/m²), it is observed that H₂ bubble successions at large gas-evolving substrates first experience a stagnation regime, followed by a fast increase in the growth coefficient before a steady state is reached. This clearly contradicts the common assumption that constant current densities must yield time-invariant growth rates. Conversely, for the case of CO₂ bubbles, the growth coefficient successively decreases for every subsequent bubble as a result of the persistent depletion of dissolved CO₂.

An analytical model of hydrogen evolution and oxidation reactions on electrodes partially covered with a catalyst

Our previous theoretical study on the performance limits of the platinum (Pt) nanoparticle catalyst for the hydrogen evolution reaction (HER) had shown that the mass transport losses at a partially catalyst-covered planar electrode are independent of the catalyst loading. This suggests that the two-dimensional (2D) numerical model used could be simplified to a one-dimensional (1D) model to provide an easier but equally accurate description of the operation of these HER electrodes. In this article, we derive an analytical 1D model and show that it indeed gives results that are practically identical to the 2D numerical simulations. We discuss the general principles of the model and how it can be used to extend the applicability of existing electrochemical models of planar electrodes to low catalyst loadings suitable for operating photoelectrochemical devices under unconcentrated sunlight. Since the mass transport losses of the HER are often very sensitive to the H2 concentration, we also discuss the limiting current density of the hydrogen oxidation reaction (HOR) and how it is not necessarily independent of the reaction kinetics. The results give insight into the interplay of kinetic and mass-transport limitations at HER/HOR electrodes with implications for the design of kinetic experiments and the optimization of catalyst loadings in the photoelectrochemical cells.

Diverse nanostructures and gel behaviours contained in a thermo- and dual-pH-sensitive ABC (PNIPAM–PAA–P4VP) terpolymer in an aqueous solution

PNIPAM-b-PAA-b-P4VP (NAV), a thermo- and dual-pH-sensitive ABC triblock copolymer, was synthesized via sequential reversible addition–fragmentation chain transfer (RAFT) polymerization and subsequent hydrolysis.