Mobility of polypropylene microplastics in stormwater biofilters under freeze-thaw cycles

Stormwater biofilters naturally experience dry-wet and freeze-thaw cycles, which could remobilize deposited particulate pollutants including microplastics. Yet, the effect of these natural weathering conditions on the mobility of deposited microplastics has not been evaluated. We deposited microplastics on columns packed with sand or a mixture of sand with soil (25% by volume) to simulate biofilter media, subjected them to intermittent infiltration events punctuated by either freeze-thaw cycles or drying cycles. Comparing the vertical distribution of microplastics in biofilters after both treatments, we showed that more than 90% of microplastics were retained within the first 3 cm of filter media, but the distribution in deeper layers varied with media type and treatment conditions. Freeze-thaw cycles were more effective than dry-wet cycles in increasing the downward mobility of deposited microplastics. We attributed these results to the disruption of filter media by expanding ice crystals, which could release deposited colloids and associated microplastics. An increase in natural colloid concentration in the effluent following freeze-thaw treatments confirmed the hypothesis. The results are useful in predicting microplastic transport in the root zone in stormwater biofilters or contaminated land experiencing natural freeze-thaw cycles.