Revealing the long-term impact of photodegradation and fragmentation on HDPE in the marine environment: Origins of microplastics and dissolved organics
Mahyar Ghanadi, Lokesh P. Padhye
Abstract
The extensive usage of high-density polyethylene (HDPE) materials in marine environments raises concerns about their potential contribution to plastic pollution. Various factors contribute to the degradation of HDPE in marine environments, including UV radiation, seawater hydrolysis, biodegradation, and mechanical stress. Despite their supposed long lifespans, there is still a lack of understanding about the long-term degradation mechanisms that cause weathering of seawater-exposed HDPE products. In this research, the impact of UV radiation on the degradation of HDPE pile sleeves was studied in natural as well as laboratory settings to isolate the UV effect. After nine years of exposure to the marine environment in natural settings, the HDPE pile sleeves exhibited an increase in oxygen-containing surface functional groups and more morphological changes compared to accelerated UVB irradiation in the laboratory. This indicated that combined non-UV mechanisms may play a major role in HDPE degradation than UV irradiation alone. However, UVB irradiation was found to release dissolved organic carbon and total dissolved nitrogen from HDPE pile sleeves, reaching levels of up to 15 mg/L and 2 mg/L, respectively. Our findings underscore the significance of taking into account both UV and non-UV degradation mechanisms when evaluating the role of HDPE in contributing to marine plastic pollution. By investigating HDPE commissioned in actual marine conditions as a potential source of microplastics and dissolved organics, the study provides a more accurate assessment of its aging processes compared to pure laboratory-based studies. Although the literature suggests that photodegradation is the most dominant aging mechanism for HDPE, our results prove that such findings cannot be generalized as HDPE is rarely used in pure form commercially. The release of microplastics and dissolved organics depends on additives and composition of HDPE, and for commercial HDPE sleeves, in addition to photodegradation, hydrolysis, biodegradation, and mechanical stress play a significant role in their degradation.