Do Ultrafine Bubbles Work as Oxygen Carriers?

Nanobubbles produced by nanopores to probe gas-liquid mass transfer characteristics

HYPOTHESIS

This study tested the hypothesis of how the nanopore size of membranes and how the surface charge of nanobubbles responds to its pinch-off from the nanopore. This study also tested the hypothesis that nanobubbles that remain in solution after production may increase the dissolved oxygen content in water.

EXPERIMENTS

The effect of membrane pore size, hydrodynamic conditions (gas and liquid flow rates), and physicochemical parameters (pH and temperature) on volumetric mass transfer coefficient (kLa) for oxygen nanobubbles formed by the nanopore diffusion technique was investigated. This study experimentally determined the kLa by carefully removing the dissolved oxygen by nitrogen purging from nanobubble suspension to examine the sole contribution of nanobubble dissolution in water to the reaeration.

RESULTS

Scaling estimates indicate that the nanobubble pinch-off radius and nanopore radius have a power-law correlation and that nanobubble size declines with the nanopore size. This is in line with our experimental results. The surface charge of nanobubbles delays its pinch-off at the gas-liquid interface. Nanobubbles offered 3-4 times higher kLa than microbubbles. Standard oxygen transfer efficiency in water was found to be 78%, significantly higher than that in microbubbles. However, dissolving stable nanobubbles in water does not considerably increase dissolved oxygen levels.

Fine bubble technology for the green synthesis of fairy chemicals

Fairy chemicals (FCs), such as 2-azahypoxanthine (AHX), are a potential new class of plant hormones that are naturally present in plants and produced via a novel purine metabolic pathway. FCs support plant resilience against various stresses and regulate plant growth. In this study, we developed a four-step method for synthesising AHX from 2-cyanoacetamide, achieving a good yield. Oxime was obtained from 2-cyanoacetamide via the oximation reaction. Cascade-type one-pot selective Pt/C-catalysed reduction of oxime, followed by a coupling reaction with formamidine acetate, yielded intermediate 5-amino-1H-imidazole-4-carboxamide (AICA). For the synthesis of AICA from oxime, we used modern fine bubble technology, affording AICA in 69% yield. Subsequently, we synthesised 4-diazo-4H-imidazole-5-carboxamide (DICA) from AICA via the diazotisation reaction. Notably, the synthesis of DICA from AICA was achieved, and the stability of previously known less stable DICA in the solid state was confirmed. Finally, PhI(OAc)2 (0.5 mol%) catalysed the intramolecular cyclisation of DICA in the green solvent water to yield AHX (overall yield of 47%). This study's innovative techniques and substantial discoveries highlight its potential influence and significance in FC science, thereby establishing a new standard for subsequent research.