Promotion effect of foam formation on the degradation of polyvinyl alcohol by ozone microbubble

Polyvinyl alcohol (PVA), extensively utilized in diverse industries, presents a challenge in terms of degradation. The combination of ozone (O3) with microbubbles enhances ozone's residence time and oxidation efficiency, offering a promising avenue for effective PVA degradation. This study investigated PVA degradation using ozone microbubble (MB/O3) and oxygen microbubble (MB/O2). Bubbles evolving into remarkably resistant foam was observed for the first time. PVA removal via MB/O3 and MB/O2 followed a first-order kinetic model, exhibiting removal rates of 98.58 % and 48.68 %, respectively, within the initial 20 min. The physicochemical reaction of PVA degradation by MB/O3 and the physical removal reaction by MB/O2 were demonstrated. The microscale changes of bubbles and foam were captured, uncovering the phenomenon of self-pressurized dissolution of MB/O3 and the creation of larger foam structures through intact microbubbles, ultimately leading to their rupture. A comprehensive four-stage theory was proposed to explain PVA degradation by MB/O3, where physical separation, foam film degradation, and bubble coalescence within the foam play crucial roles. This theory was validated under varying pH and salt concentration conditions. Moreover, when the pH was decreased from 8.69 to 3.3, there was a 33.2 % increase in the COD removal rate. Mechanism analysis further confirmed the theory and revealed that, during PVA degradation, MB/O3 underwent stochastic cleavage, yielding small molecule remnants comprising aldehyde, carboxyl, and ester moieties. Finally, by comparison with other PVA degradation processes, the trial of MB/O3 was proven to be a straightforward and effective treatment approach for PVA degradation.