Photocatalytic Degradation of Microplastics in Aquatic Environments Using Solar-Activated Bismuth Oxychloride

Abstract

Microplastic particles have now been reported in freshwater and marine systems and their ability to withstand traditional water treatment has put their destruction, not their removal to the top of the engineering agenda. Bismuth oxychloride has been of particular interest in this context, due to its structure containing a series of layers of [Bi₂O₂] ²⁺ which generates an internal electric field that separates the carriers generated by the photons, but also due to its surface chemistry which favours the formation of the hydroxyl radicals which are the most commonly implicated in polymer chain scission. Its large band gap, however, (3.19 – 3.60 eV) restricts the light absorption to almost exclusively the ultraviolet part of the sun's spectrum, which accounts for only 3-5% of the solar spectrum, and thus is not a property of the pure material but rather a materials design challenge. This paper discusses the structure and electronic mechanism of BiOCl photocatalysis of microplastics in water and reviews the strategies such as facet control, vacancy engineering, heterojunction construction and surface hydroxylation to broaden its response to visible light and enhance charge utilization. The degradation performance of polyethylene, polyethylene terephthalate, polystyrene and polypropylene targets is reported and the analytical data to support the presence of true chain scission and mineralisation as opposed to simple fragmentation is evaluated. Operating parameters that influence the activity such as pH, temperature, particle size, catalyst ratio to polymer and light source are discussed, along with reactor configurations that can utilize natural sunlight. The paper concludes that solar-activated BiOCl is a viable pre-treatment or polishing method for microplastic-containing waters, but quantitative mineralisation data and standardised metrics and outdoor testing are still the main challenges to scale-up.