Stormwater runoff carries microplastics from their source and deposits them on subsurface or stormwater treatment systems that are designed to filter the contaminants. Microplastics can accumulate in these systems over years and may pose groundwater contamination risks if they can move downward. These systems are naturally subjected to dry-wet and freeze-thaw cycles, which are known to remobilize soil colloids. The same process could enhance the downward transport of microplastics. Yet, no study to date has examined the effect of freeze-thaw cycles on the mobility of microplastics. This study examines the effect of freeze-thaw cycles on the downward migration of polypropylene microplastics in biofilters containing three different types of media: quartz sand and sand with compost or soil (25% by volume). After depositing 0.1 g of microplastics on each simulated biofilter to simulate deposition of microplastics in many years, the contaminated biofilters were subjected to intermittent infiltration of stormwater punctuated by freeze-thaw cycles (6 h freezing at -20 C and 17h thawing at 25 C). The biofilters were subjected to a total of 45 cycles. To compare the effect of freeze-thaw cycles on mobility, other sets of contaminated biofilters were subjected to intermittent infiltration of stormwater without freeze-thaw cycles. Results showed that more than 90% of microplastics are retained within the first 3 cm of filter media irrespective of the media type and the concentration of microplastics with depth decreased exponentially. Comparing the distribution of microplastics with respect to the depth of filter media with and without freeze-thaw cycles, we showed that freeze-thaw cycles increased the downward mobility of microplastics in all media, although the mobility is more prominent in sand biofilters. The results indicate that these natural weathering conditions could speed up the downward migration of microplastics in biofilters. We attributed these results to disruption of soil aggregate and release of microplastics from the soil into pore water along with soil particles when ice is formed during freezing and melted during freeze-thaw cycles. Overall results show that prediction of downward mobility of microplastics in soil or stormwater biofilters should account for enhanced mobility under natural weathering conditions such as freeze-thaw cycles.

Enhanced downward transport of microplastics in stormwater biofilters during freeze-thaw cycles

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.