The Effects of Ultrasound on the Electro-Oxidation of Sulfate Solutions at Low pH

Enhanced ultrasonic degradation of methylene blue using a catalyst-free dual-frequency treatment

Methylene blue is one of the most common pollutants found in wastewater, primarily due to its widespread use in the dye industry. Consequently, it is imperative to explore environmentally friendly and efficient methods for degrading this pollutant into non-toxic byproducts. While ultrasonic degradation methods in combination with additives or catalysts have proven effective, such additives or catalysts may inadvertently contribute to secondary pollution. Moreover, the preparation of these catalysts imposes an additional burden in terms of effort and cost. To address these issues, this paper introduces a catalyst-free dual-frequency ultrasound degradation approach for methylene blue. The sonochemical quality of the cavitation bubbles is improved using this technique because the bulk solution is populated with two types of bubbles, whose mean sizes are determined by the dual ultrasound frequencies. The findings demonstrate that, under identical acoustic power density conditions, dual-frequency ultrasound consistently outperforms single-frequency modes across all investigated parameters. Furthermore, the larger the difference between the dual frequencies used, the more effective the degradation of methylene blue. Finally, after just 20 min of sonication, a degradation efficiency of 91% was achieved with dual frequencies of 20 and 80 kHz at an acoustic power density of 209.63 ± 6.94 W/L. Consequently, this technique offers an environmentally friendly, catalyst-free, and highly effective method for degrading methylene blue.

Sonochemical Hydrogen Production as a Potential Interference in Light-Driven Hydrogen Evolution Catalysis

The use of sonication to dissolve or disperse solids in solvents is a common practice in catalytic studies. However, the frequency and power of commercial ultrasonic baths are within the right range to trigger unwanted sonochemical reactions that can be a source of interference. Based on our own experience, we have noted that sonication used to disperse heterogeneous photocatalysts in water-alcohol mixtures as the first step in light-driven hydrogen evolution can lead to hydrogen evolution, which is not related to any photochemical or photocatalytic process. Furthermore, "dark" hydrogen evolution continues even when sonication is stopped, which can cause significant problems for time-dependent studies. To the best of our knowledge, this phenomenon has not been described as a potential issue for light-driven hydrogen evolution studies, and it should, therefore, be brought to the attention of the energy conversion research community to avoid errors in ongoing and future hydrogen evolution studies.

The Effects of Ultrasound on the Electro-Oxidation of Sulfate Solutions at Low pH

The electro‐oxidation of sulfate solutions is a well‐established route for the generation of powerful oxidants such as persulfate. Despite this, the effects of simultaneous ultrasound irradiation during this process has attracted little attention. Herein, we investigate the effects of a low‐intensity ultrasonic field on the generation of solution‐phase oxidants during the electro‐oxidation of sulfate solutions. Our results show that at high current densities and high sulfate concentrations, ultrasound has little effect on the Faradaic and absolute yields of solution‐phase oxidants. However, at lower current densities and sulfate concentrations, the amount of these oxidants in solution appears to decrease under ultrasonic irradiation. A mechanism explaining these results is proposed (and validated), whereby anodically‐generated sulfate and hydroxyl radicals are more effectively transported into bulk solution (where they are quenched) during sonication, whereas in the absence of an ultrasonic field these radicals combine with one another to form more persistent species (such as persulfate) that can be detected by iodometry.