## 3 Results and Discussion  
### Gamma Radiolysis of PE Microplastics in Water  
Water mixtures containing the original commercial PE microplastics were found to contain numerous other chemicals, mostly volatile or semi-volatile hydrocarbons and ketones, as determined from SPME headspace extractions and GCMS analysis and shown in Figure 1 (red trace). Several mixtures of the original PE stock in water (Milli-Q) and salt water (0.5 M NaCl in Milli-Q) were prepared and subjected to gamma radiolysis. Beyond ~4 kGy of absorbed gamma dose, dissolved O\({}_{2}\) in the water was no longer present for chemical transformations. Therefore, since these PE stock microplastics were subjected to gamma doses between 0.5 and 2.0 MGy, O\({}_{2}\) was present only during the very early stages of these irradiations. A typical chromatogram for irradiated PE in water is also shown in Figure 1 (blue trace). After irradiation, most of the identified contaminants from the original PE stock were no longer detectable. This is attributed to radicals from the radiolysis of water in the absence of O\({}_{2}\), namely \({}^{\bullet}\)OH, hydrated electrons (e\({}_{\text{eq}}\)\({}^{-}\)), and hydrogen atoms (H\({}^{\bullet}\)), having reacted to degrade these organic chemicals, similar to aqueous solutions containing dissolved organic contaminants [32]. However, a few of the original contaminants were still detected even after 2 MGy of absorbed dose, equivalent to \(>\)0.5 M \({}^{\bullet}\)OH radicals generated in situ. The most prominent compounds identified from the irradiated PE-water samples were \(n\)-dodecane, 2-dodecanone, and some other hydrocarbons listed in Table 1.  
For a few of the irradiated PE microplastic mixtures, the water was separated from the microplastics using vacuum filtration, and headspace extractions were performed on the water. No compounds were detected in the headspace of the separated water, indicating that the compounds collected in the headspace extractions were adsorbed to the PE. While no specific absorption data for these particular compounds have been reported for PE microplastics, previous studies measured significant adsorption (\(\sim\)6 g oil/g PE) of machine lubrication oil onto microplastics [42]. Other researchers found that the adsorption of both light and heavy crude oil fractions onto PE powders or sheets was enhanced when the PE had previously received 30 kGy of \({}^{60}\)Co irradiation [43]. To further investigate the presence of the most prevalent impurity, \(n\)-dodecane, in these irradiated aqueous PE samples, the removal of the chemical contaminants from the original PE stock was necessary. This was achieved by heating a thin layer of the solid PE microplastics in open air at 70 \({}^{\circ}\)C for at least 50 h. The blue trace chromatogram in Figure 2, (flat baseline), shows that the heat-treated PE stock no longer contains any significant levels of trace chemical contaminants, relative to the initial material (red trace). Based on the success of this simple purification/pre-treatment, all subsequent experiments utilized heat-treated PE, or purified PE. Gamma irradiation experiments were then performed on the purified PE microplastics, in both Milli-Q and Milli-Q/0.50 M NaCl (saltwater) using similar radiation doses (\(\sim\)0.5-2 MGy) as the original PE stock. From the SPME headspace extractions of the irradiated, purified PE-water mixtures, only trace amounts of C\({}_{8}\)-C\({}_{12}\) hydrocarbons were extracted from a few samples. In approximately half of these analyzed samples, none of these hydrocarbon compounds were detected. This suggests that the reactions of PE microplastics with water radiolysis radicals at higher total absorbed gamma doses do \begin{table}
\begin{tabular}{c c} \hline \hline
**Water** & **Salt Water** \\ \hline \(n\)-dodecane & \(n\)-dodecane \\
2-dodecanone & tridecane \\  & 2-dodecanone \\  & hexadecane \\ \hline \hline \end{tabular}
\end{table}