Effect of pH and Ionic Strength on the Mechanism of Association of Rodenticides with Natural Organic Components of Soil

The mechanisms of association of hydrophobic organic contaminants with natural organic matter (for example humic acid, HA) in soil and sediment are a major objective of environmental and geochemical research. This paper discusses a general model for studying the process of association between a series of rodenticides and humic acid, by use of a C₁₈ stationary phase. An approach based on extended Langmuir distribution isotherms was used to study the effect of bulk solvent pH and ionic strength (adjusted by addition of sodium cation) on the mechanism of HA–rodenticide binding. The results demonstrated that: (i) HA can be adsorbed on the surface of the C₁₈ phase; (ii) the rodenticides can be associated with HA adsorbed on the C₁₈ surface; and (iii) ionic strength and bulk solvent pH both modify the conformation of HA and thus its mechanism of association with the rodenticide molecules.