Novel insights into photoaging mechanisms and environmental persistence risks of polylactic acid (PLA) microplastics: Direct and indirect photolysis

• Published in: The Science of the total environment 2024-12, Vol.954, p.176350, Article 176350
• Main Authors: Li, Qianyu, Cao, Jiachun, Li, Juan, Li, Didi, Jing, Binghua, Zhou, Junhui, Ao, Zhimin
• Format: Article
• Language: English
• Subjects: Biodegradable plastic; Persistent organic pollutants; Polylactic acid; Reactive oxygen species; Time-dependent density functional theory
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.176350

Polylactic acid (PLA), as a biodegradable plastic, exhibits high sensitivity to ultraviolet (UV) radiation, yet the mechanisms and environmental risks of its photoaging remain unclear. This study uses quantum chemical calculations (DFT and TD-DFT) and kinetic simulations to explore the direct and indirect photoaging of PLA. Direct photoaging indicates that the highest oscillator intensity absorption peaks occurred at 172 and 246 nm, corresponding to the 13th singlet (S13) and 48th triplet (T48) states, thereby initiating the Norrish I and Norrish II mechanisms. The innovative “electron-hole” technology effectively clarifies the variations in photoaging mechanisms under different wavelengths. Indirect photoaging involves multiple reactive oxygen species (ROS) like •OH, 1O2, •O2−, and •HO2. The study confirms the anhydride production hypothesis and proposes two novel •OH-induced mechanisms: carbonyl carbon addition and branched methyl hydrogen dehydrogenation. Both mechanisms are thermodynamically advantageous, but their products pose potential environmental risks. ROS species and concentrations impact both PLA's photoaging mechanisms and environmental persistence. Low •OH concentration in northeast China, especially in winter, suggests a significant photoaging risk. This study offers pioneering insights into photoaging mechanisms and emphasizes the pivotal role of ROS, offering recommendations for managing PLA environmental impacts and fates in China. Polylactic acid (PLA) can become a source of persistent organic pollutants upon photoaging, driven by reactive oxygen species (ROS). [Display omitted] •Variation in electron transition leads to distinct photoaging mechanisms across different wavelengths.••OH and •HO2 can oxides PLA into oligomer but not 1O2 and •O2−.•Tertiary carbon dehydrogenation induced by •OH emerges as the most favorable mechanism.•Two novel indirect photoaging mechanisms pose significant environmental risks.•PLA photoaging products may transform into POPs, particularly in Northeast China.