Photodissolution of submillimeter-sized microplastics and its dependences on temperature and light composition

Photodissolution has the potential to efficiently remove microplastics from the surface ocean. Here, we examined the effects of temperature and incident sunlight composition on the photodissolution of submillimeter-sized microplastics of polypropylene (PP), polystyrene (PS), and thermoplastic polyurethane (TPU) in seawater. The photoproduction of dissolved organic carbon (DOC), chromophoric dissolved organic matter, and dissolved nitrogen (TPU only) was observed to increase exponentially within 7 days of full-spectrum irradiation. The temperature dependence of photodissolution increased with irradiation time for PP and PS but remained relatively constant for TPU. A 20 °C increase in temperature enhanced DOC photoproduction by 10 times for PP, three times for PS, and four times for TPU at 7-d irradiation, giving activation energies of 59.4-84.8 kJ mol-1. Photodissolution of all three polymers was exclusively driven by ultraviolet-B (UVB) radiation. PS-derived DOC was photomineralizable, while PP- and TPU-derived DOC appeared photo-resistant. Extrapolating the lab-based DOC photoproduction rates to warm surface oceans yields lifetimes of 6.5 years for PP, 3.6 years for PS, and 3.7 years for TPU. This study demonstrates that photodissolution of the tested microplastics is restricted to the thin UVB-penetrable surface ocean and that water temperature plays a critical role in controlling the photodissolution of these microplastics.