Microplastics and Metals: Microplastics Generated from Biodegradable Polylactic Acid Mulch Reduce Bioaccumulation of Cadmium in Earthworms Compared to Those Generated from Polyethylene

Abstract

Biodegradable polylactic acid (PLA) mulch has been developed to replace conventional polyethylene (PE) mulch in agriculture as a response to growing concerns about recalcitrant plastic pollution and the accumulation of microplastics (MPs) in soil. Cadmium is a significant soil pollutant in China. MPs have been shown to adsorb metals. In this study the earthworm Lumbricus terrestris was exposed to either Cd (1.0 - 100 mg / kg) or MPs (PE and PLA, 0.1 - 3 % w / w), or a combination of the two, for 28 days. Cd bioavailability significantly decreased in the presence of MPs. In particular, at the end of the experiment, PLA treatments had lower measured Cd concentrations in both earthworms (2.127 - 29.24 mg / kg) and pore water (below detection limits - 0.1384 mg /L) relative to PE treatments (2.720 - 33.77 mg / kg and below detection limits - 0.2489 mg / L). In our adsorption experiment PLA MPs adsorbed significantly more Cd than PE MPs with maximum adsorption capacities of 126.0 and 23.2 mg / kg respectively. These results suggest that the PLA MPs reduce earthworm exposure to Cd relative to PE by removing it from solution and reducing its bioavailability.

Keywords: Lumbricus terrestris, soil pore water, adsorption, bioavailability