Soil–water interfacial adsorption of phenanthrene along a Chinese climatic gradient of soils with and without the addition of black carbon Original Research Article

Sorption isotherms for a hydrophobic solute probe, phenanthrene, were determined in 16 Chinese soils. They were sampled along a climatic gradient, and amended, or not, with charcoal (0.2%, 0.5%, and 1%), a form of black carbon (BC). Within the concentration range of added phenanthrene (0.2–0.8 mg l− 1), most of the adsorption isotherms of the unamended soils were non-linear. Both the Freundlich equation and the Dual Reactive Domain Model (DRDM) model closely fitted the data, indicating that phenanthrene sorption in these soils was site-specific and demonstrated capacity-limited adsorption in a condensed organic domain. Correlations between the Freundlich model capacity factor (KF) and soil physico-chemical properties showed that the total soil organic C (TOC) concentrations, cation exchange capacities and silt had a cumulative effect on phenanthrene sorption, indicating that organic and inorganic components interacted in this process. The soils studied also indicated that humic acid carbon (HAC) concentration may be a further relevant factor that should be considered. The soils covered a wide range of physical and chemical properties, in particular organic C and the organic carbon-normalized distribution coefficients (KOC) demonstrated a large range of variation. Therefore, KOC values may be poor predictive parameters for phenanthrene sorption by soils. Addition of BC not only enhanced the sorption of phenanthrene but also altered the sorptive characteristics of the soils studied.