Department of Civil and Environmental Engineering, University of California, Los Angeles, CA, USA.
Moore Institute for Plastic Pollution Research, Long Beach, CA, USA.
Water Res. 2022 Aug 15;222:118950. doi: 10.1016/j.watres.2022.118950. Epub 2022 Aug 5.
Stormwater treatment systems remove and accumulate microplastics from surface runoff, but some of them can be moved downward to groundwater by natural freeze-thaw cycles. Yet, it is unclear whether or how microplastic properties such as density could affect the extent to which freeze-thaw cycles would move microplastics in the subsurface. To examine the transport and redistribution of microplastics in the subsurface by freeze-thaw cycles, three types of microplastics, with density smaller than (polypropylene or PP), similar to (polystyrene or PS), or greater than (polyethylene terephthalate or PET) water, were first deposited on the top of packed sand-the most common filter media used in infiltration-based stormwater treatment systems. Then the columns were subjected to either 23 h of drying at 22 ⁰C (control) or freeze-thaw treatment (freezing at -20 ⁰C for 6 h and thawing at 22 ⁰C for 17 h) followed by a wetting event. The cycle was repeated 36 times, and the effluents were analyzed for microplastics. Microplastics were observed in effluents from the columns that were contaminated with PET and subjected to freeze-thaw cycles. Comparison of the distribution of microplastics in sand columns at the end of 36 cycles confirmed that freeze-thaw cycles could disproportionally accelerate the downward mobility of denser microplastics. Using a force balance model, we show that smaller microplastics (<50 µm) can be pushed at higher velocity by the ice-water interface, irrespective of the density of microplastics. However, plastic density becomes critical when the size of microplastics is larger than 50 µm. The coupled experimental studies and theoretical framework improved the understanding of why denser microplastics such as PET and PVC may move deeper into the subsurface in the stormwater treatment systems and consequently elevate groundwater pollution risk.
雨水处理系统可以从地表径流中去除和积累微塑料，但其中一些微塑料可能会在自然冻融循环的作用下向下迁移到地下水。然而，目前尚不清楚微塑料的密度等特性是否以及如何影响冻融循环在地下移动微塑料的程度。为了研究冻融循环对地下微塑料的迁移和再分布，首先将三种密度小于（聚丙烯或 PP）、相似（聚苯乙烯或 PS）或大于（聚对苯二甲酸乙二醇酯或 PET）水的微塑料沉积在填充砂的顶部-最常见的渗透式雨水处理系统中使用的过滤介质。然后，将柱子在 22°C 下干燥 23 小时（对照）或进行冻融处理（在-20°C 下冷冻 6 小时，在 22°C 下解冻 17 小时），然后进行润湿事件。该循环重复 36 次，分析流出物中的微塑料。在受到 PET 污染并经历冻融循环的柱子的流出物中观察到微塑料。在 36 个循环结束时，对沙柱中微塑料的分布进行比较，证实冻融循环可以不成比例地加速密度较大的微塑料的向下迁移。使用力平衡模型，我们表明，无论微塑料的密度如何，较小的微塑料（<50 µm）都可以被冰-水界面以更高的速度推动。然而，当微塑料的尺寸大于 50 µm 时，塑料密度就变得至关重要。这项耦合的实验研究和理论框架提高了对为什么密度较大的微塑料（如 PET 和 PVC）可能在雨水处理系统中更深地迁移到地下，从而增加地下水污染风险的理解。
