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Preferential Emission of Microplastics from Biosolid-Applied Agricultural Soils: Field Evidence and Theoretical Framework
Summary
Field experiments showed that when farms spread treated sewage solids (biosolids) on fields, wind can blow the microplastics out of the soil much more easily than regular soil particles because microplastics are lighter and less sticky. Over 84% of wind events were strong enough to launch microplastics into the air, compared to only 23% for normal soil. This means people living near biosolid-treated farmland may be inhaling more airborne microplastics than current dust models predict.
Land application of wastewater biosolids on agricultural soils is suggested as a sustainable pathway to support the circular economy; however, this practice often enriches microplastics and associated contaminants in topsoil. Wind could transport these contaminated microplastics, thereby increasing their inhalation health risks. Analyzing wind-borne sediments collected from wind tunnel experiments on biosolid-applied agricultural fields, we show enrichment of microplastics in wind-blown sediments. We explain this preferential transport and enrichment of microplastics by using a theoretical framework. This framework reveals how the combined effects of the low density of microplastics and weakened wet-bonding interparticle forces between microplastics and soil particles lower their threshold velocity, the minimum wind velocity necessary for wind erosion to occur. Our calculations indicate that microplastics could be emitted at wind speeds lower than the characteristic threshold of background soil. Analyzing the windspeed distribution for 3 months of wind events over a bare soil surface, we showed that more than 84% of the wind events exceed the threshold velocity of microplastics of size 150 μm, while only 23% of the wind events exceed the threshold velocity of the background soil. Thus, current models for fugitive dust emissions may underestimate the microplastic emission potential of biosolid-amended soils.