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

In one yearly cycle (2016), D4 and D5 were detected in biogas samples (n = 36, 0.105-2.33 mg/m(3)) from a Chinese municipal landfill, while D4-D6 were detected in influents/effluents of leachate storage pond (n = 72, < LOQ-30.5 mu g/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(1/2) = 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., Fe2+ and NO3-) 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.