This study investigated some sources and elimination mechanisms of phenylmethylsiloxanes in landfill leachates. During a 20-day leaching experiment for electronic wastes collected from one Chinese landfill, significant release (4.9 ng/L to 1.3 μg/L) of cis-/trans-2,4,6-triphenyl-2,4,6-trimethylcyclotrisiloxanes (cis-P3 and trans-P3) and cis-/trans-2,4,6,8-tetraphenyl-2,4,6,8-tetramethylcyclotetrasiloxanes (cis-P4 and trans-P4a,b,c) in simulated leachates was found. From January 2017 to December 2018, P3 and P4 isomers were detected in raw leachates from active cells [<LOQ-990 ng/L, detection frequency (df) = 67-100%] and closed cells (<LOQ-282 ng/L, df = 2.1-98%) of this landfill. Generally, mean mass loads of total phenylmethylsiloxanes in raw leachates were larger in the summer (380 mg/d) and winter (295 mg/d) for active cells, while they decreased from 36.1 mg/d to <LOQ for closed cells during the entire period. During leachate treatment processes, sorption to sludge was responsible for major removal (64-84%) of phenylmethylsiloxanes, while Fenton treatment accounted for 8.5-25% removal. Simulated Fenton experiments indicated that hydroxylation half-lives of P3 (1.3-1.5 h) and P4 (0.65-0.86 h) were 15-91 times faster than their hydrolysis half-lives (22-59 h, pH 3.5). Furthermore, monohydroxylated P4 isomers with a hydroxyphenyl group had larger (3.2-3.9 times) concentrations than those with a hydroxymethyl group, meaning that a phenyl group may be more likely to be hydroxylated than a methyl group.
Keywords: hydroxylation; landfill leachate; phenylmethylsiloxanes; temporal trends.