Reactive mineral removal relative to soil organic matter heterogeneity and implications for organic contaminant sorption

- Lipids and black carbon-like structure are preserved in acid treated soils.
- Carbohydrates and other polar structures are released after acid treatment.
- Hydrophobic organics sorption depends on SOM more strongly after acid treatment.
- Ionic organic chemical sorption is less dependent on SOM after acid treatment.
- The fate and risk as affected by SOM dynamics are chemical-property dependent.

Soil organic matter (SOM) is generally treated as a static compartment of soil in pollutant fate studies. However, SOM might be altered or fractionated in soil systems, and the details of SOM property/composition changes when coupled with contaminant behavior are unknown. In this study, a mild acid treatment was adopted to remove reactive minerals and partially remove SOM components. After acid treatment, biomarker signatures showed that lignin-derived phenols were released and black carbon (as suggested by benzene-polycarboxylic acids) and lipids were enriched. The biomarker information was consistent with common bulk chemical characterization. The sorption coefficient Kd for PHE was two times higher after acid treatment, whereas Kd for OFL was three times lower. The organic carbon normalized sorption coefficient KOC values for PHE were higher for soils after acid treatment, indicating stronger interactions between PHE and SOM. The linear regression line between Kd and fOC for OFL showed lower intercepts and slopes after reactive mineral removal, suggesting a decreased contribution of minerals and reduced dependence on SOM. These results were attributed to the release of polar compositions in SOM accompanied by reactive mineral removal. Our results suggest that the mobility of ionic organic contaminants increases, whereas that of hydrophobic organic contaminants decreases after acid treatment with respect to reactive mineral depletion. This study emphasized that new insights into the coupling of SOM dynamics should be incorporated into organic contaminant behavior studies. SOM molecular biomarkers offer a useful technique for correlating SOM composition and sorption property changes.

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