Vinylmethylsiloxanes in Municipal Wastewater Treatment Plant and Biosolid-Amended Soils: Their Distribution and Backbone/Vinyl Branch Degradation

This study is the first to investigate the emission and environmental fate of one type of modified methylsiloxane with double-bond (vinyl) groups. During 2018-2020, 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane (V3), 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4), and 2,4,6,8,10-pentavinyl-2,4,6,8,10-pentamethylcyclopentasiloxane (V5) were found in aqueous (< LOD-72.9 ng/L) and solid [13.0-371 ng/g dw (dry weight)] phases of wastewater samples from one Chinese municipal wastewater treatment plant (WWTP) as well as the corresponding biosolid-amended soils [< LOD-36.9 ng/g dw, df (detection frequency) = 37.5-41.7%, n = 48]. Based on the measure of environmental samples, simulated experiment, and product analysis by ESI-FT-ICR-MS, it was found that (1) in addition to sorption to sludge, abiotic degradation of vinylmethylsiloxanes (especially V3, t(1/2) = 0.5-1.9 h at pH = 5.2-9.2) should have an important contribution to their sufficient removal in WWTP; (2) different from siloxane analogues with saturated branches and aromatic branches, abiotic degradation pathways of vinylmethylsiloxane might include both the hydrolysis of Si-O backbone and the oxidation/addition reactions of vinyl branches; (3) although vinylmethylsiloxanes in wastewater could be transferred to soil by biosolids application, these compounds had no accumulation in soil, which should arise from their fast elimination, such as volatilization (t(1/2) = 3.2 h-20.9 days) and degradation (t(1/2) =9.1 h-96.3 days); and (4) degradation of the Si-O backbone and vinyl branches had slowing trends with the increase in the soil organic matter.