Methylsiloxanes and Their Brominated Products in One e-Waste Recycling Area in China: Emission, Environmental Distribution, and Elimination

The present study investigated the sources and fates of methylsiloxanes and their brominated products in one e-waste recycling area of China. During thermal (30–1000 °C) recycling experiments for printed wiring boards (PWBs), besides volatile methylsiloxanes (D4, D5, and D6), their monobrominated products, that is, D3D(CH 2 Br), D4D(CH 2 Br), and D5D(CH 2 Br), were also found by quadrupole time-of-flight gas chromatography–mass spectrometry to have 2–3 orders of magnitude lower emissions (0.31–1.3 μg/g) than those (18.1–866 μg/g) of parent methylsiloxanes. Overall, the fastest emissions of methylsiloxanes and bromo-methylsiloxanes occurred at 300–400 and 400–500 °C, respectively, accounting for 35.3–51.0 and 39.4–82.1% of their total emission. In the e-waste recycling area, concentrations of D4–D6 were 1.1–75.0 μg/g dw [detection frequency (df) = 100%] in 31 dusts from PWB treatment workshops, while limits of detection (LOD) < 683 ng/g dw (df = 69–100%) in 48 surrounding soils were up to 3 orders of magnitudes higher than those in reference areas. Meanwhile, D3D(CH 2 Br)–D5D(CH 2 Br) were detected in both dusts (<LOD—1.2 μg/g dw, df = 48–52%) and soils (<LOD—70.3 ng/g dw, df = 23–77%) from the e-waste recycling area, but they were not present in reference samples. Simulating experiments showed that hydrolysis (9.07–378 d) and volatilization (8.55–1007 d) half-lives of monobrominated D4–D6 in soils were 1.6–5.0 times longer than those of their parent methylsiloxanes.