Abstract
The performance of photocatalytic advanced oxidation must be improved in order for the technology to make the jump from academic research to widespread use. Research is needed on the factors that cause photocatalysis to become self-limiting. In this study, we introduced, for the first time, nanobubbles continuously into a running photocatalytic reactor. Synthetic air, O2, and N2 bubbles in the size range of 40 to 700 nm were added to a reaction system comprising P25 TiO2 photocatalyst in stirred aqueous solution excited by UV-A lamps, with methyl orange as a target contaminant. The removal of methyl orange was tested under conditions of changing pH and with the addition of different radical scavengers. Results indicated that the oxygen and air nanobubbles improved the photocatalytic degradation of methyl orange—the removal efficiency of methyl orange increased from 58.2 ± 3.5% (N2 aeration) to 71.9 ± 0.6% (O2 aeration). Dissolved oxygen (DO) of 14.93 ± 0.13 mg/L was achieved using O2 nanobubbles in comparison to 8.43 ± 0.34 mg/L without aeration. The photodegradation of methyl orange decreased from 70.8 ± 0.4% to 53.9 ± 0.5% as pH increased from 2 to 10. Experiments using the scavengers showed that O2− was the main reactive species in photocatalytic degradation under highly dissolved oxygen conditions, which also accounted for the observation that the removal efficiency for methyl orange decreased at higher pH. However, without photocatalyst, nanobubbles alone did not improve the removal of methyl orange, and nanobubbles also did not increase the degradation of methyl orange by only photolysis. These experiments show that oxygen and air nanobubbles can act as environmentally friendly catalysts for boosting the performance of photocatalytic water treatment systems.
Highlights
Despite decades of research, photocatalysis has not made the jump from academic research to widespread use in water treatment and consumer products
Several methods are available for making nanobubbles including hydrodynamic cavitation, particle cavitation, acoustic sonication, electrochemical cavitation, and mechanical agitation
It should be noted that most previous studies have prepared nanobubble solutions via generators that require circulation of the solution within the system, which can run only in batch-mode
Summary
Photocatalysis has not made the jump from academic research to widespread use in water treatment and consumer products. Photocatalytic pollution control relies on the irradiation of a photocatalyst resulting in the production of an electron–hole pair. These primary products react to produce a cascade of secondary reactive species. Many of the details concerning these species, such as their rate of production, concentration, lifetime, and mutual interactions, are only known in broad outline. In the idealized oxidative view, conduction band electrons (e− cb ) react with O2 to form O2 − , and valence band holes (h+ )
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