Emission, Nonindustrial Sources, and Multiple Degradation Pathways of Chlorinated-Methylsiloxanes in Municipal Wastewater Treatment Plants: One National Study in China

Abstract

This is the first study to investigate national emission characteristics and multiple degradation pathways of chlorinated-methylsiloxanes in municipal wastewater treatment plants (WWTPs) of China. Monochlorinated products of three primary methylsiloxane oligomers (D4, D5, and D6), namely, D3D(CH₂Cl), D4D(CH₂Cl), and D5D(CH₂Cl), were detected (2.1–9.9 ng/L, df = 72.4–89.7%) in raw municipal wastewater from 29 WWTPs of 25 Chinese cities. Among these cities, the total emissions (1.1–19.9 μg·day^–1·person^–1) of chloromethylsiloxanes (∑chloromethylsiloxanes) and the chloromethylsiloxanes/methylsiloxanes ratios were positively related with per capita gross domestic product (r = 0.78 and 0.79, p < 0.01). Meanwhile, ∑chloromethylsiloxanes were also found in residential wastewater, swimming pool water, and medical wastewater treated by free chlorine or advanced oxidation, with national emission from these nonindustrial sources being about 5.7 tonnes, 2.7 kg, and 38.4 kg each year, respectively. Unlike their poor elimination (7.7–10.8%) during wastewater biotreatment process, both methylsiloxanes and chloromethylsiloxanes exhibited more obvious biodegradation (10.9–20.6%) during sludge digestion. Simulation experiments indicated that chloromethylsiloxanes were degraded 1.3–1.7 times faster (t_1/2 = 74.3–137 days) than parent methylsiloxanes. Hydrolysis and dechlorination, respectively, contributed 54.5% and 27.7% to chloromethylsiloxanes degradation. Additionally, the hydrolysis product (CH₂Cl)MeSi(OH)₂ had a 1.5–2.8 times faster dechlorination rate than chloromethylsiloxanes, suggesting that Si–O hydrolysis may accelerate dechlorination of chloromethyl.