Stability and Mobility of Biodegradable Nanoplastics in the Subsurface

In agriculture, biodegradable plastic mulch has gained significant attention due to its in-situ degradability and satisfying  agronomic performance. However, these mulches do not degrade instantaneously; instead, they fragment into micro- and nanoplastics, which can persist in soils or migrate off-site via surface runoff or subsurface water flow.  Here, we studied the stability and mobility of biodegradable nanoplastics made from a polybutylene adipate co-terephthalate (PBAT) mulch in both pristine and weathered forms under various environmental conditions. Stability was assessed with aggregation kinetics in NaCl and CaCl2 solutions, and mobility was evaluated under unsaturated flow conditions in sand columns. Additionally, we examined the effects of proteins, i.e., negatively charged bovine serum albumin (BSA) and positively charged lysozyme (LSZ), on the stability and mobility of PBAT nanoplastics.  Results show that pristine PBAT nanoplastics exhibited greater aggregation in CaCl2 compared to NaCl, with critical coagulation concentrations of 20 mM in CaCl2 and 325 mM in NaCl. In contrast, weathered PBAT nanoplastics remained stable in both NaCl and CaCl2 solutions. Unsaturated column experiments revealed high mobility for both pristine and weathered PBAT nanoplastics, consistent with their high stability observed under low ionic strength conditions (i.e., 10 mM NaCl). Protein interactions affected stability and mobility: both BSA and LSZ promoted aggregation of pristine PBAT nanoplastics, with LSZ having a more pronounced effect. Correspondingly, LSZ reduced the mobility of pristine PBAT nanoplastics due to its destabilizing effect.  Our findings suggest that biodegradable nanoplastics derived from plastic mulch are stable and mobile under environmental conditions, posing potential risks of migration within and beyond agricultural systems.