Potential disinfection byproducts-related risks to drinking water? Molecular insights into the dissolved organic matter from photodegradation of polyethylene microplastics

Microplastics (MPs) are constantly exposed to sunlight and release dissolved organic matter (DOM) from photodegradation. However, the molecular composition and transformation of such DOM remain unclear, particularly in process engineering like drinking water treatment. We deciphered the molecular characteristics of the DOM leached from MPs and, further, the potential contribution to the formation of disinfection byproducts (DBPs). The leached DOM was dominantly composed of the low molecular weight fraction (<1k Da), which contributed the most to DBP formation, challenging water treatment employing UV and chlorine disinfection. Chain scission and oxidation led to oligomer structures in leachates that were mainly distributed in the range C10-C21 with an incorporated number of oxygen atoms between 1 and 10. The DOM exhibited low O/Cwa (0.31-0.39) and high H/Cwa (1.54-1.69) ratios and became more saturated with increased exposure time. The leached DOM with such characteristics may not be efficiently removed by activated carbon or coagulation, which enriches the formation of DBPs. Information on the molecular composition of MP-derived DOM and the potential of the individual molecular weight fractions as precursors of DBPs is valuable for assessing the potential contribution of MPs to DBP-related risks in drinking water and identifying potential mitigation strategies.