Chlorine disinfection enhances the degradation of biodegradable microplastics into nanoplastics and dissolved organic carbon in a simulated disinfection process

Abstract

The extensive global utilization of biodegradable plastics has given rise to the growing presence of biodegradable microplastics (BMPs) in wastewater treatment plants (WWTPs). Although a majority of microplastics can settle in sludge, some still make their way into disinfection tanks. Hence, it is imperative to comprehend the fate of BMPs in disinfection tanks. Polylactic acid (PLA) and polybutylene adipate (PBAT) are the two most prevalently used biodegradable plastics. Consequently, we simulated the disinfection conditions of WWTPs to investigate the aging pathways of granular PLA (PLAg), sheet PLA (PLAs), granular PBAT (PBATg) and sheet PBAT (PBATs) under the influence of the disinfectant sodium hypochlorite (NaClO). Scanning electron microscopy (SEM) results demonstrated that NaClO notably enhanced the surface roughness of BMPs and triggered the formation of holes and cracks, and this roughness tended to intensify with the elevation of NaClO concentration and the prolongation of aging time. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the carbonyl index (CI) and the oxygen-to-carbon ratio (O/C) continuously increased in both PBATg and PBATs throughout the aging process. Nevertheless, PLAg and PLAs presented an initial increase followed by a decrease, which could be attributed to the combined effect of ester bond hydrolysis and NaClO-driven oxidation. Moreover, the concentration of dissolved organic carbon (DOC) increased with both extended aging time and higher NaClO concentration. The detection of biodegradable nanoplastics (BNPs) observed by transmission electron microscopy (TEM) further verified that NaClO aging induced the fragmentation of the parent plastics into BNPs. In summary, our results emphasize that NaClO-driven oxidation results in both physical structural damage and chemical transformation of BMPs. These findings offer a vital theoretical foundation for enhancing wastewater treatment procedures to control BMP pollution while also providing essential empirical data for future sustainability assessments.