## 3 Results and Discussion
### Gamma Radiolysis of PE Microplastics in Water

Table 1: Persistent compounds detected using the commercial PE stock in water or salt water after \(\sim\)2 MGy of gamma irradiation. Figure 1: GCMS chromatograms of VOCs and sVOCs in the original PE stock (red) and after \(\sim\)1 MGy gamma irradiation of original PE stock in water in a closed vessel (blue). not lead to the formation of significant quantities of smaller molecules/PE degradation products. Similar results were obtained for experiments performed at lower dose rates for an absorbed gamma dose of approximately 25 kGy. Previous research has shown that small molecule scission products are preferentially formed when polymers in solution are irradiated at low dose rates, as this reduces the localized concentration of carbon-centered radicals, which in turn minimizes crosslinking reactions [44]. We investigated the effects of lower dose rates (20 Gy min\({}^{-1}\)) and lower total irradiation doses on heat-treated PE microplastics in DI/Milli-Q water and simulated saltwater for low O\({}_{2}\) conditions. These low-dose samples were pre-saturated with nitrogen gas, and then subjected to 1 kGy of gamma irradiation, equivalent to \(\sim\)280 uM \({}^{\bullet}\)OH being generated in these aqueous solutions, far less than the equivalent amount of PE (100 mg) used in our 18 mL aqueous solutions. The products collected by SPME headspace extraction of mixtures irradiated to 1 kGy are listed in Table 2. The GCMS chromatograms (see Supplementary Information) for irradiated PE-water mixtures contained \(n\)-dodecane, 2-dodecanone, pentadecane, 2-tetradecanone, and 2-hexadecanone, of which \(n\)-dodecane was again the prominent product. Similar compounds were also identified in the complementary PE-salt water irradiations. These compounds, identified from the low irradiation dose PE-water mixtures, suggest that PE microplastics do undergo scission reactions to some extent in the absence of O\({}_{2}\). This contrasts with the reactivity of the PE microplastics exposed to higher radiation doses, which predominantly undergo crosslinking reactions. To better simulate the behavior of PE microplastics in natural water conditions, Lake Michigan (LM, USA) water was used as a solvent for analogous purified PE gamma irradiation experiments. The lake water pH was 7.7, slightly higher than the laboratory deionized water pH of 7.2. The concentrations of chloride, sulfate and nitrate were determined to be 12, 23 and 0.7 ppm, respectively. The typical dissolved organic carbon content of lake water is around 2 ppm [45]. Here, \(n\)-dodecane was again the only detected compound in the samples that were subjected to doses of \(\sim\)0.5 and 1.4 MGy. A smaller amount of \(n\)-dodecane and additional hydrocarbons (e.g., undecane and tridecane) were detected in samples irradiated to \(\sim\)2.0 MGy. A few scission products were also formed at these high \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**-2 MGy or 25 kGy Water or Saltwater**} & \multicolumn{1}{c}{**-1 kGy Water Or Saltwater**} & \multicolumn{1}{c}{**-0.5 and 1.4 MGy LM Water**} & \multicolumn{1}{c}{**-2 MGy LM Water**} \\ \hline trace amounts HCs & \(n\)-dodecane & \(n\)-dodecane & \(n\)-dodecane \\  & 2-dodecanone & undecane & tridecane \\  & pentadecane & & \\  & 2-tetradecanone & & \\  & 2-hexadecanone & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Compounds detected after gamma irradiation using heat-treated PE stock in Milli-Q water or salt water, or Lake Michigan (LM) water. (HCs = hydrocarbons). Figure 2: GCMS chromatogram of VOCs and sVOCs present in the commercial PE stock (red) and the absence of these compounds after 50 h of heating at 70 \({}^{\circ}\)C (blue). doses when the lake water was the solvent, in contrast to the irradiated pure water and saltwater solutions, as shown in Table 2.