From sediment sinks to ice sinks: Differential phase partitioning of PAH derivatives in a cold-climate aquatic ecosystem

The environmental risks of PAHs are well-documented, yet far less is known about their more toxic and persistent derivatives, MePAHs and NPAH, especially in the unique context of seasonally frozen wetlands. This study provides the first evidence of MePAHs and NPAHs enrichment in ice within a temperate wetland, based on their distribution behavior, migration behavior, and associated environmental risks in the Songhua River. Using GC-MS/MS quantification combined with ice enrichment factors (IEFs) and fugacity modeling, we found that ice-covered water exhibited significantly higher concentrations of _ΣMePAHs than non-ice water (ice periods: 258 ± 125 ng/L; non-ice periods: 36.6 ± 22.8 ng/L). Through a systematic analysis of the enrichment behavior and property influence of PAHs derivatives in ice and sediments, it was found that MePAHs have the characteristic of preferential enrichment, and their MePAHs levels are 6–76 times higher than those of NPAHs. During ice phases, concentrations were further elevated (MePAHs: 412 ± 421 ng/L; NPAHs: 11.2 ± 8.24 ng/L), with severe pollution observed at industrially impacted sites (e.g., Ash River site: MePAHs = 1249 ng/L). Hydrophobic compounds, such as 2-NNAP (IEF = 8.53) and low-ring MePAHs (IEF >1), were notably enriched in ice, while low temperatures hindered photolysis and microbial degradation, leading to prolonged pollutant retention. Fugacity analysis revealed divergent phase partitioning: low-ring MePAHs (e.g., 1-MNAP, ff > 0.7) tended to migrate from sediment to water, whereas mid/high-ring compounds (e.g., 4-NBP, ff < 0.3) preferentially settled in sediment. Strong correlations between MePAHs and NPAHs (∗∗∗p ≤ 0.001) suggested common fossil fuel and industrial emission source.