Enhanced aging of polystyrene microplastics in sediments under alternating anoxic-oxic conditions

Aging of microplastics (MPs) (i.e., degradation and weathering) is ubiquitous in the environment. The MP aging process is thought to be limited to light and static areas, while aging in dark and fluctuating anoxic-oxic areas is poorly understood. Here, we provide initial evidence for aging of polystyrene microplastics (PS-MPs) under different anoxic/oxic conditions in sediments, and we further explored these mechanisms using sediment column experiments and pure-culture experiments. The results showed that PS-MPs in alternating anoxic-oxic sediments displayed the highest degree of aging. In the in-situ experiment, both the weight losses and O/C ratios of PS-MPs aged under alternating anoxic-oxic conditions were ∼2 times higher than those aged under static oxic and static anoxic conditions during 2-month experiments. In a 2-month column experiment, the PS-MPs in the alternating anoxic-oxic group showed weight losses and O/C ratios that were, respectively, triple and double the corresponding values for the static oxic and static anoxic groups. Column and pure-culture experiments demonstrated that dark production of ·OH which showed a positive correlation with a Fe redox process could explain enhanced MP aging under the alternating anoxic-oxic conditions. These findings provide a basis for risk assessment and management of MPs in the natural environment, such as in intertidal zones and paddy fields, and also have implications for engineering of optimized MP degradation processes.