Abiotic behaviour of organic micropollutants in soils and the aquatic environment. A review: I. Partitioning

Recent research has confirmed the significance of organic carbon (OC) as the key sorbent for hydrophobic organic chemicals (HOC), as well as for many polar compounds. However, the triazine herbicides exhibit a variable affinity for soil organic matter (SOM) which is attributed to the extent of humification of the organic fraction. Charge transfer mechanisms are important for triazine sorption to OC and either proton or electron transfer may account for the reaction mechanism with humic acids. For many uron herbicides (e.g, chlorotoluron, metabromuron chloroxuron, defenoxuron), sorption correlates with SOM. However, specific interactions between uron herbicides and a limited quantity of active constituents within SOM have also been proposed to explain deviations from sorption linearity at low herbicide relative concentration. Other studies indicate that isoproturon sorbs to organic colloids in solution and that a sorption threshold to SOM may be operative. Below the threshold, isoproturon appears to sorb predominantly to clays, indicating the presence of a limited number of 'active' sorptive sites within clay minerals. Research suggests that pesticide interactions with clay minerals may be influenced by near-surface day geometry; the accessibility of the sorbing region of the sorbate to the active site of the clay; the identity of exchangeable cations on the clay and solution electrolytes. These recent studies indicate that interactions between micropollutants and soils and sediments often need to be evaluated on a compound-specific basis. This is especially the case for polar compounds which may partition to these environmental phases by diverse mechanisms.