Downward transport of naturally-aged light microplastics in natural loamy sand and the implication to the dissemination of antibiotic resistance genes

Current understanding on the fate and behavior of microplastics (MPs) in complex soil media remains inadequate. We characterized the aging and hetero-aggregation of a MP sampled in farmland soil, and explored its vertical downward transport in natural loamy sand. The MP was identified with FTIR spectrum as polypropylene, a plastic lighter than water. FTIR spectrum combined with SEM imaging confirmed the MP was highly aged, generating colloidal plastic fibers and carbonyl groups. SEM imaging coupled with EDX analysis suggested hetero-aggregation of the MP with soil minerals. Soil leaching tests performed with the clean MP (without soil minerals) (CMP), the raw MP (RMP) (with soil minerals), and the RMP with humic acid (HA) (RMP + HA) demonstrated that the mobility was insignificant for the CMP, moderate for the RMP and highest for the RMP + HA, resulting in a maximal downward traveling distance of 0 cm, 3–4 cm, and 9–10 cm, respectively. Correlation between the maximal traveling distance and zeta potential of the CMP, RMP, and RMP + HA confirmed surface charge as a dominant control on the MP mobility; while the increasing density of the MP, due to hetero-aggregation with soil minerals, was identified as a driving mechanism for its downward transport, despite its intrinsic density lower than water. Occurrence of only the lower-sized rod-shaped plastic fibers at the maximal traveling distance suggested the natural aging, a process leading to plastic vibration and fragmentation, was conducive to plastic translocation. The three explored classes of antibiotic resistance genes (ARGs) (tetracycline, beta-lactam and sulfonamide) were all detected in the plastic surface, suggesting the MP may function as a potential pathway for the dissemination of ARGs to the deeper soil layer. These findings are important to understand the concentration distribution of both the MPs and ARGs in agriculture impacted soils, a natural reservoir of both emerging contaminants.