Sorption of 2,4-dichlorophenoxyacetic acid by model particles simulating naturally occurring soil colloids

Binary and ternary model particles containing montmorillonite, ferrihydrite , and humic acid (HA) were used to determine the changes in the sorption be havior of the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) upon the int erassociation 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, f errihydrite and humic acid coatings on montmorillonite provided sorption si tes for 2,4-D, although the exclusion of the anionic form from highly charg ed 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 not very dif ferent from that on pure ferrihydrite be ca use most of the ferrihydrite su rface area was from micropores being not accessible to large humic macromol ecules and hence remained available for herbicide sorption. In ternary sorb ents HA coatings reduced the sorption of 2,4-D by the montmorillonite-ferri hydrite binary complex, indicating that HA blocked many of the sorption sit es provided by the Fe coatings. This work shows that the amount and nature of the surface that remains available after the interassociation of single soil constituents is a critical parameter in determining the sorptive behav ior of the resultant aggregate for ionizable contaminants such as 2,4-D. Th us, the use of calculated sorption parameters such as K-oc, K-ow, K-mineral , or K-Fe, for modeling contaminant behavior in aquatic or soil environment s may result in serious deviation from the reality.