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

Qianyu Li; Jiachun Cao; Juan Li; Didi Li; Binghua Jing; Junhui Zhou; Zhimin Ao

doi:10.1016/j.scitotenv.2024.176350

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

Sci Total Environ. 2024 Sep 18:176350. doi: 10.1016/j.scitotenv.2024.176350. Online ahead of print.

Authors

Qianyu Li¹, Jiachun Cao², Juan Li³, Didi Li⁴, Binghua Jing³, Junhui Zhou⁵, Zhimin Ao⁶

Affiliations

¹ Advanced Interdisciplinary Institute of Environment and Ecology, Guangdong Provincial Key Laboratory of Wastewater Information Analysis and Early Warning, Beijing Normal University, Zhuhai 519087, PR China; School of Environment, Beijing Normal University, Beijing 100875, PR China.
² Advanced Interdisciplinary Institute of Environment and Ecology, Guangdong Provincial Key Laboratory of Wastewater Information Analysis and Early Warning, Beijing Normal University, Zhuhai 519087, PR China; Guangdong-Hong Kong-Macao Joint Laboratory for Contaminants Exposure and Health, Guangzhou Key Laboratory Environmental Catalysis and Pollution Control, Institute of Environmental Health and Pollution Control, Guangzhou 510006, PR China; Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, Key Laboratory for City Cluster Environmental Safety and Green Development of the Ministry of Education, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China.
³ Advanced Interdisciplinary Institute of Environment and Ecology, Guangdong Provincial Key Laboratory of Wastewater Information Analysis and Early Warning, Beijing Normal University, Zhuhai 519087, PR China.
⁴ Guangdong-Hong Kong-Macao Joint Laboratory for Contaminants Exposure and Health, Guangzhou Key Laboratory Environmental Catalysis and Pollution Control, Institute of Environmental Health and Pollution Control, Guangzhou 510006, PR China; Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, Key Laboratory for City Cluster Environmental Safety and Green Development of the Ministry of Education, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China.
⁵ School of Environmental and Chemical Engineering, Wuyi University, Jiangmen 529020, PR China.
⁶ Advanced Interdisciplinary Institute of Environment and Ecology, Guangdong Provincial Key Laboratory of Wastewater Information Analysis and Early Warning, Beijing Normal University, Zhuhai 519087, PR China. Electronic address: Zhimin.ao@bnu.edu.cn.

PMID: 39304142
DOI: 10.1016/j.scitotenv.2024.176350

Abstract

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.

Keywords: Biodegradable plastic; Persistent organic pollutants; Polylactic acid; Reactive oxygen species; Time-dependent density functional theory.