Spatial Distribution and Historical Trends of Modified Methylsiloxanes in Core Sediments from the Dongzhaigang Mangrove Wetland, Hainan Island, China

Modified methylsiloxanes (MMSs) are emerging contaminants with growing industrial applications and potential environmental risks, yet their occurrence in sensitive mangrove ecosystems remains poorly understood. This study investigated the occurrence, spatial distribution, historical trends, and ecological risks of 14 MMSs in core sediment from the Dongzhaigang mangrove wetland, Hainan Island, China. Among the target compounds, cis-D4F and trans-D4Fb had the highest detection frequency (23% each), with total MMSs concentrations ranging from <LOQ to 15.4 ng/g dry weight (dw, mean: 0.892 ng/g dw), indicating slight contamination. Trifluoropropylmethylsiloxanes (mean: 0.422 ng/g dw) and phenylmethylsiloxanes (mean: 0.462 ng/g dw) dominated, showing a significant positive correlation suggestive of common sources (industrial emissions, domestic sewage, shipping, and tourism). Spatially, the highest concentration (3.07 ng/g dw) was near a sewage outlet (C1), while temporal trends revealed continuous concentration increases peaking in 2024 (max: 15.4 ng/g dw), linked to Hainan′s industrial expansion. Composition profiles were dominated by trans-D4Fb and cis-D4F (61%–84% in key cores) due to their hydrophobicity. Ecological risk assessment showed D4F posed significant risks (HQ: 1.2–16.7) and P4 potential risks at specific sites, with MMSs exhibiting higher toxicity and persistence than parent siloxanes. This study provides baseline data for MMSs in tropical mangroves, emphasizing the need for continuous monitoring amid rising MMSs demand as restricted siloxane alternatives.