摘要：

Soil organic matter (SOM) is often considered the dominant sorptive phase for organic contaminants and pesticides in soilwater systems. This is evidenced by the widespread use of organic-matter-normalized sorption coefficients (KOM) to predict soilwater distribution of pesticides, an approach that ignores the potential contribution of soil minerals to sorption. To gain additional perspective on the potential contributions of clays and SOM to pesticide retention in soils, we measured sorption of seven pesticides by a K-saturated reference smectite clay (SWy-2) and SOM (represented by a muck soil). In addition, we measured the adsorption of atrazine by five different K-saturated smectites and Ca-saturated SWy-2. On a unit mass basis, the KSWy-2 clay was a more effective sorbent than SOM for 4,6-dinitro-o-cresol (DNOC), dichlobenil, and carbaryl of the seven pesticides evaluated, of which, DNOC was sorbed to the greatest extent. Atrazine was sorbed to a similar extent by KSWy-2 and SOM. Parathion, diuron, and biphenyl were sorbed to a greater extent by SOM than by KSWy-2. Atrazine was adsorbed by CaSWy-2 to a much lesser extent than by KSWy-2. This appears to be related to the larger hydration sphere of Ca2+ (compared to that of K+) which shrinks the effective size of the adsorption domains between exchangeable cations, and which expands the clay layers beyond the apparently optimal spacing of 12.2 for sorption of aromatic pesticide structures. Although a simple relation between atrazine adsorption by different K-smectites and charge properties of clay was not observed, the highest charge clay was the least effective sorbent; a higher charge density would result in a loss of adsorption domains. These results indicate that for certain pesticides, expandable soil clays have the potential to be an equal or dominant sorptive phase when compared to SOM for pesticide retention in soil. Keywords: Sorption; pesticide; clay; organic matter; smectite

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