Binary and ternary model particles containing montmorillonite, ferrihydrite, and humic acid (HA) were used to determine the changes in the sorption behavior of the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) upon the interassociation of these three major soil constituents. On single sorbents, 2,4-D sorption was high with S-type isotherms on ferrihydrite, moderate with L-type isotherms on HA, and zero on montmorillonite. In binary sorbents, ferrihydrite and humic acid coatings onmontmorillonite provided sorption sites for 2,4-D, although the exclusion of the anionic form from highly charged clay surfaces partially obscured the role of Fe and HA as sorbents of 2,4-D. Sorption of 2,4-D on ferrihydrite-HA binary particles was notvery different from that on pure ferrihydrite because most of the ferrihydrite surface area was from micropores being not accessible to large humic macromolecules and hence remained available for herbicide sorption. In ternary sorbents HA coatings reduced the sorption of 2,4-D by the montmorillonite-ferrihydrite binary complex, indicating that HA blocked many of the sorption sites provided by the Fe coatings. This work shows that the amount and nature of the surface that remains available after theinterassociation of single soil constituents is a critical parameter in determining the sorptive behavior of the resultant aggregate for ionizable contaminants such as 2,4-D. Thus, the use of calculated sorption parameters such as K{sub}(oc), K{sub}(ow),K{sub}(mineral), or K{sub}(Fe), for modeling contaminant behavior in aquatic or soil environments may result in serious deviation from the reality.
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