Sorption of organic chemicals to soil organic matter: influence of soil variability and pH dependence.

For modeling the sorption of organic compounds in soils it is typically assumed that the organic carbon/water partitioning coefficient (Koc) of neutral organic chemicals can be treated as a constant property that remains unaffected by the type of soil organic matter as well as pH in the soil solution. Here the validity of these assumptions is evaluated with a large and diverse experimental data set of our own and literature data. To this end sorption experiments with 3 different soils and one peat were carried out using a column method. Differences in log Koc at pH values of 4.5 and 7.2 were on average <0.06 log units for 60 chemicals on Pahokee Peat. This result indicates that protonation/deprotonation of carboxylic groups in humic matter has no significant influence on sorption. The soil-to-soil variability of Koc was within factor of 3 regardless of the type of chemicals. The Pahokee Peat standard from the International Humic Substances Society appears to represent Koc in solid soil organic matter from different origins with an accuracy of ±0.23 log units (root-mean-square error). The comparison of the sorption data in Pahokee Peat with literature sorption data from the air in hydrated humic and fulvic acids suggested that the thermodynamic cycle for converting sorption data between air and water is applicable provided that for small, highly polar chemicals an additional water phase in the hydrated organic matter is considered.