Particle dynamics of nanoplastics suspended in water with soil microparticles: Insights from small angle neutron scattering (SANS) and ultra-SANS

Small-angle neutron scattering (SANS) and Ultra-SANS (USANS) were employed to understand the agglomeration behavior of nanoplastics (NPs) formed from a biodegradable mulch film, and microparticles of vermiculite (V), an artificial soil, suspended in water in the presence of low convective shear (ex-situ stirring) prior to measurements. Neutron contrast matching was employed to minimize the signal of V (by 100-fold) and thereby isolate the signal due to NPs in the neutron beam, as the contrast match point (CMP) for V (67 vol% deuteration in water) differed from that of NPs by more than 20%. The original NPs’ size distribution was bimodal: < 200 nm and 500-1200 nm, referred to as small and large NPs, i.e., SNPs and LNPs, respectively. In the absence of V, SNPs formed agglomerates at higher concentrations, with size decreasing slightly with stirring time to 40-50 nm, while the size of LNPs remained unchanged. The presence of V at 2-fold lower concentration than NPs did not change the size of SNPs but reduced the size of LNPs by nearly 2-fold as stirring time increased. Because the size of SNPs and LNPs did not differ substantially between solvents, both at CMP and 100% D2O, even with nanosized V particles contributing toward scattered intensity for the latter solvent, it is evident that SNPs and LNPs are mainly composed of NPs and not V. The results suggest that LNPs are susceptible to size reduction through collisions with soil microparticles via convection, yielding SNPs near soil-water interfaces within vadose zones.

Methods
Data for Fig 1 (nanoplastic recovery suspended in water and settling out) was collected in the laboratory and the results were recorded in a Microsoft Excel file. Other data was collected on the small-angle neutron scattering (SANS) and ultra-SANS instruments at Oak Ridge National Laboratory, specifically, the Bio-SANS (high-flux isotope reactor) and Beamline 1A (spallation neutron source), respectively (downloaded into Microsoft Excel files and displayed in Figs 2 and S1-S5). Data for Figs 3 and S6 include form factor-structure factor modeling of merged SANS and USANS data, after subtraction of a power law relationship.  Modeling was done using Igor Pro-based software written by National Institute of Standards scientists and the model fits to the data and resultant parameters were downloaded to Microsoft Excel files. The models' parameters allowed for determination of box plots and histograms of nanoplastic size and size distribution under several different conditions, Figs 4 and S7, respectively. The latter two figures were generated using JMP software, and were subsequently downloaded to Microsoft Excel files.