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.

微塑料 (MP) 的老化（即降解和风化）在环境中无处不在。MP 老化过程被认为仅限于光亮和静态区域，而在黑暗和波动的缺氧区老化则知之甚少。在这里，我们为沉积物中不同缺氧/有氧条件下聚苯乙烯微塑料 (PS-MPs) 的老化提供了初步证据，并使用沉积物柱实验和纯培养实验进一步探索了这些机制。结果表明，交替缺氧-好氧沉积物中的 PS-MPs 显示出最高程度的老化。在原位实验中，在 2 个月的实验中，在交替缺氧 - 好氧条件下老化的 PS-MPs 的重量损失和 O/C 比比在静态有氧和静态缺氧条件下老化的高约 2 倍。在为期 2 个月的柱实验中，交替缺氧-好氧组中的 PS-MP 显示出重量损失和 O/C 比，分别是静态好氧和静态缺氧组相应值的三倍和两倍。柱和纯培养实验表明，与Fe氧化还原过程呈正相关的·OH的暗产生可以解释交替缺氧条件下MP老化的增强。这些发现为自然环境（例如潮间带和稻田）中 MP 的风险评估和管理提供了基础，也对优化 MP 降解过程的工程设计具有意义。柱和纯培养实验表明，与Fe氧化还原过程呈正相关的·OH的暗产生可以解释交替缺氧条件下MP老化的增强。这些发现为自然环境（例如潮间带和稻田）中 MP 的风险评估和管理提供了基础，也对优化 MP 降解过程的工程设计具有意义。柱和纯培养实验表明，与Fe氧化还原过程呈正相关的·OH的暗产生可以解释交替缺氧条件下MP老化的增强。这些发现为自然环境（例如潮间带和稻田）中 MP 的风险评估和管理提供了基础，也对优化 MP 降解过程的工程设计具有意义。