Methylsiloxanes Release from One Landfill through Yearly Cycle and Their Removal Mechanisms (Especially Hydroxylation) In Leachates

Abstract

In one yearly cycle (2016), D4 and D5 were detected in biogas samples (n = 36, 0.105-2.33 mg/m) from a Chinese municipal landfill, while D4-D6 were detected in influents/effluents of leachate storage pond (n = 72, < LOQ-30.5 μg/L). Mass loads of cVMS in both biogas (591-6575 mg/d) and leachate influents (659-5760 mg/d) increased from January to July (summer), and then decreased from July to December (winter). Removal experiments indicated that 1) hydrolysis and volatilization were predominant removal mechanism for D4 and D5, respectively, in leachate storage pond, responsible for their more significant removal (94.5-100%) in August; 2) indirect phototransformation (t = 25.5-87.0 days), such as hydroxylation by OH radical generated in leachates, was the predominant (50.0-75.5%) removal pathway for D6, which led to the largest removal efficiencies (65.2-73.7%) in June, the month with the largest sun light intensity and highest photosensitizer (e.g., Fe and NO) concentrations. Monohydroxylated products of D5 and D6, D4TOH and D5TOH, were detected in leachate effluents (39.6-187 ng/L) during May-July. Compared to D5 and D6, volatilization half-lives of D4TOH (86.3 days) and D5TOH (177 days) in leachates were 2.9 and 1.4 times longer, while their hydrolysis half-lives (7.50 days for D4TOH and 21.5 days for D5TOH) were 7.1 and 10 times shorter, respectively.

Cyclic Phenylmethylsiloxane Oligomers in Municipal Landfills and Their Elimination Mechanisms in Leachate Treatment Processes

“…Figure showed that mass loads of ∑(P3 + P4) in the active cell (A) had peaks (217–588 mg/d, mean = 380 mg/d) in the summer (June–August), especially July and August, of both years 2017 and 2018. This result was similar to one previous study, which reported that emission rates of dimethylsiloxanes in one landfill of China were largest in the summer . In these 2 months, both higher leachate temperatures (25.7–32.6 °C, mean = 29.6 °C) and higher leachate TOC (2675–3312 mg/L, mean = 2877 mg/L) would accelerate desorption rates of phenylmethylsiloxanes from solid waste to leachate, which were consistent with the results of the simulated leaching experiments (section ).…”

“…Based on the flow volume of the leachates and the served population of the landfill, mean mass loadings of ∑P3 and ∑P4 in the leachates were estimated to be 0.51 (in range 0.12–1.49) and 0.46 (in range 0.09–1.21) μg/d per capita , respectively, and 2–3 orders of magnitude lower than those (225 μg/d for ∑P3 and 77.0 μg/d for ∑P4 per capita ) in influents of WWTP with the same catchment reported previously . In addition, mean per capita mass loadings of ∑P3 and ∑P4 in leachates were 10.8–14.9 times lower than those of cyclic dimethylsiloxanes (6.85 μg/d for D4 and 5.40 μg/d for D5) in another Chinese landfill, which were consistent with the lower global usage of phenylmethylsiloxanes than the use of dimethylsiloxanes…”

“…For decades, organosiloxanes have been widely utilized in both manufacturing processes and daily life as lubricants, adhesives, defoamers, protective coatings, and electrical insulating agents. − The world production volume of polyorganosiloxanes increased year by year (e.g., from 2 million metric tons in 2002 to 8 million metric tons in 2015) . Due to widespread use of polydimethylsiloxanes (accounting for 80% of total siloxane consumption), some oligodimethylsiloxanes have been found in various matrixes from atmospheric, terrestrial, and aquatic environments. − Besides polydimethylsiloxanes, modified polyorganosiloxanes with other functional groups (such as trifluoropropyl, phenyl, etc.) also had vast usage in processes and consumer products, and hence, their oligomers (as impurities of polymers) have been reported to be detected in some wastewater streams …”

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

“…Following previous studies, , some measures were taken to reduce the background levels of siloxanes: (1) the analyst did not use personal care products containing siloxanes during sample collection, pretreatment, and analysis; (2) the sample pretreatments and analysis were performed in the dedicated clean room, where the air recycling system was equipped with a particulate filter and activated carbon sorbents; (3) prior to use, glass vials and quartz tubes were precleaned with n -hexane and then heated at 300 °C; (4) prior to use, both PUF/XAD-2/PUF cartridges and anhydrous sodium sulfate cartridges were precleaned by being immersed in hexane for 4 h. Subsequently, after rinsing with 10 mL of hexane, the cartridges were dried using purified nitrogen and sealed in glass tubes; (5) only steel pipes, not silicone tubing, were used for the nitrogen blowing process; and (6) during sampling and pretreatment events, both field and procedural blanks were prepared to assess potential ambient contamination.…”

Chlorinated-Methylsiloxanes in Shengli Oilfield: Their Generation in Oil-Production Wastewater Treatment Plant and Presence in the Surrounding Soils

“…Following previous studies, ,, some measures were taken to avoid the siloxanes contamination during sample collection and analysis: (1) the analyst did not use hand lotions or other consumer products containing siloxanes; (2) prior to use, all glass tubes/pipettes were cleaned with n -hexane, and then heated at 300 °C; (3) prior to use, anhydrous sodium sulfate cartridges were immersed in n -hexane for 4 h, and subsequently rinsed with 10 mL of n -hexane. After rinsing, the cartridges were dried using purified nitrogen and stored in capped glass tubes; (4) the sample pretreatments were performed in dedicated clean room with recycled air system; (5) in nitrogen blowing process, only steel pipes, not silicone tubing, were used; (6) during sampling events, field blanks were collected to assess potential ambient contamination.…”