Short-term evolution of hydration effects on soil organic matter properties and resulting implications for sorption of naphthalene-2-ol

PURPOSE: Sorption of xenobiotics in soils and especially to soil organic matter (SOM) determines their mobility and bioavailability in ecosystems. However, SOM as the major sorbent may be altered in its physicochemical properties upon changes in boundary conditions such as hydration. Hence, the goal of this study was to determine the influence of soil hydration on physicochemical properties of SOM and the resulting effects on sorption of xenobiotics. MATERIALS AND METHODS: Samples of a Histosol with 51 % SOM were adjusted to five water contents from 10 to 75 % (w/w based on dry soil mass) and aged for water contact times of 0 weeks to 3 years. The hydrated samples were characterized with respect to thermal properties of SOM and of the incorporated water via differential scanning calorimetry and with respect to hydration-induced swelling via 1H-NMR relaxometry, and the sessile drop method was applied to determine their soil–water contact angle. Sorption kinetics and isotherms of naphthalene-2-ol in the pre-treated peat samples were determined in batch experiments. RESULTS AND DISCUSSION: SOM matrix rigidity varied with the water content and increased with water contact time. An initial minimum in SOM rigidity at ~30 % water content became maximum after ~20 weeks, also resulting in the strongest resistance towards water infiltration. We argue that the anomalies at 30 % water content are related to the critical water content for the formation of freezable water w crit in the peat samples, which was 26.2 ± 0.3 %. Conditions for water-assisted molecular bridging were assumably optimal at 30 % water content. Whereas parameters of naphthalene-2-ol sorption reflecting the sorbed amount were mainly altered by the wetting properties of SOM, sorption linearity and hysteresis were influenced by the anomalies in peat matrix properties at a water content around 30 %. CONCLUSIONS: The study revealed that the interplay of SOM and water led to highly variable and complex changes in SOM physicochemical properties. These properties may serve as a predictor for sorption of xenobiotics in soil at varying hydration conditions enabling a more precise assessment of the environmental fate of xenobiotics.