Weathering of microplastics and their enhancement on the retention of cadmium in coastal soil saturated with seawater

Abstract

The plastic contamination and its associated ecological impacts in coastal areas have received great concern. The environmental conditions of varying aquatic-terrestrial ecosystems and physicochemical properties of existing plastics may further affect the transport and fate of subsequent entering contaminants in coastal areas. In this study, a coastal soil accumulated with five commonly-used micro-sized plastics (MPs) and saturated with four seawaters collected along the China coastlines from north to south were applied to investigate their effects on the mobility of Cd. The existing MPs in soil greatly enhanced Cd accumulation, with Cd retention increasing to 23.1 – 78.6%, compared to 18.1 – 26.6% in the soil without MPs. The five MPs showed the increasing trend of Cd retention: polystyrene < polyethylene < polypropylene < polyvinyl chloride < polyethylene terephthalate. The highest Cd retention by polyethylene terephthalate was attributed to its rich oxygen-containing groups which could enhance Cd sorption. The aged MPs was characterized by more oxygen-containing functional groups with higher surface charge, which enhanced the electrostatic interaction with Cd. Modeling showed that the depths for the maximum concentrations of Cd downward migration in the MPs soil are only down to 9 cm after 10-year seawater intrusion, while the depths go down to 26 cm in the soil alone. Overall, MPs existing in soil can retain more Cd in the surface soil, which impel our great attention of the combined pollution of existing microplastics and subsequent entering heavy metals in the coastal topsoil.