Soil Mineral Coatings on Biodegradable Microplastics: Formation and Control of Enzymatic Degradation

Abstract

In agriculture, biodegradable plastics weather and fragment into microplastics that inevitably acquire mineral coatings. However, the impact of mineral coatings on microplastic degradation remains poorly understood. We examined coating formation and enzymatic degradation of pristine and photoaged poly(butylene adipate-co-terephthalate) microplastics (PBAT-MPs) in loessial soil and red soil under drought, moist, alternate wetting-drying, and flooded regimes during 64 days. Soil type, moisture, and photoaging governed coating accumulation. Loessial soil produced greater Al and Fe loading on PBAT-MPs than red soil, while unsaturated regimes increased Al and Fe by 394%–1109% relative to flooding (p < 0.05). Synchrotron micro-X-ray fluorescence combined with absorption near-edge structure spectroscopy analyses showed heterogeneous elemental distributions and mineral speciation on PBAT-MPs that resembled those of the source soils. Unexpectedly, loessial-soil coatings increased PBAT-MPs’ terephthalic acid and adipic acid release by 9.1%–17.4% and reduced residual PBAT mass by 6.5%–23.5% (p < 0.05). Kinetic analyses showed that although mineral coatings slowed enzyme adsorption via diffusion limitations, localized leaching of Ca2+ from the coatings enhanced cutinase activity and outweighed this physical constraint. These findings demonstrate that mineral coatings can regulate the enzymatic degradation of PBAT, providing new insights into the environmental fate of biodegradable microplastics in agricultural soils.