Molecular Insights into Naturally Terrestrial-Derived Halogenated Organic Compounds: Chemodiversity, Speciation, and Comparison with Aquatic Disinfection Byproducts

Halogenated organic compounds (HOC) are ubiquitous in soil-water environments and are potentially persistent and toxic. While their occurrence has historically been attributed primarily to anthropogenic sources, growing evidence indicates that substantial natural HOC are stored in terrestrial ecosystems. As an enigmatic pool of soil organic matter, the molecular diversity of soil HOC remains largely unknown. Here, we establish a method to differentiate the speciation and molecular characteristics of soil extractable HOC using (ultra)high-resolution mass spectrometry, optimized through sample extraction solvents and cleanup strategies. Analyzing soil samples from six forest sites across eastern China, we revealed that natural HOC were present in both organic- and water-extractable fractions in all samples, comprising 0.2-14.4% of the total signal abundance. Lipid- and protein-like compounds were more abundant in organic-extractable HOC, while lignin- and lipid-like compounds dominated water-extractable HOC. Furthermore, distinct molecular characteristics were found among the two terrestrial-derived HOC fractions and disinfection byproducts (DBP) commonly detected in aquatic systems, with increasing oxidation and aromaticity, along with decreasing saturation and biolability. We propose a conceptual model describing the divergent molecular properties of natural terrestrial HOC and anthropogenic DBP across the soil-water interface, providing new insights into the different origins and fate of these HOC pools.