Insight into the degradation process of functional groups modified polystyrene microplastics with dissolvable BiOBr-OH semiconductor-organic framework

The difficult separation of products after photocatalytic degradation of microplastics (MPs) limits the in-depth analysis of the MPs fragmentation process and degradation mechanism, and there are no studies about the photocatalytic degradation of functional groups modified MPs. In this work, we synthesized a “dissolvable” BiOBr-OH semiconductor-organic framework (SOF) to degrade polystyrene (PS) MPs modified by different functional groups, including –OH, –NH2, -SO3H, –COOH and -epoxy. First, a series of characterization confirmed that small molecules were intercalated into BiOBr layers to construct BiOBr-OH SOF. The BiOB-OH SOF system has significantly enhanced the photocatalytic degradation performance of microplastics, and BiOB-OH-0.75 has the strongest catalytic performance. The photocatalytic degradation performance of different functional groups modified PS-MPs differed significantly, with the weight loss order being -epoxy > –NH2 > –OH > –COOH > -SO3H. After dissolving the catalyst, clean nano plastic particles were obtained after degradation, and it was found that six types of PS-MPs exhibited three different types of fragmentation processes. The surface potential of and fragmentation mode of different PS-MPs dominated the adsorption and photocatalytic degradation of BiOBr-OH SOF. The degradation products of PS-MPs are mainly composed of styrene and functional group-modified monomer molecules and multimolecular complexes. In addition, by analyzing the toxicity of the degradation products of microplastics, it was found that solid products were the main reason for their enhanced toxicity. Finally, based on the photoelectrochemical characterization and free radical analysis, the mechanism of photocatalytic degradation of different functional groups modified PS-MPs by BiOBr-OH SOF was proposed.