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SOIL CHEMISTRY

Mechanism of Dinitrophenol Herbicide Sorption by Smectites in Aqueous Suspensions at Varying pH

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
^b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
^c Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907
^d Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824

^* Corresponding author (david.laird{at}ars.usda.gov).

The compound 4,6-dinitro-o-cresol (DNOC) is an important pesticide that is strongly adsorbed by smectite clays. Because DNOC is a weak acid with an acid dissociation constant (pK_a) of about 4.4, pH was hypothesized to be a dominant state variable controlling sorption. In this study, we quantified the effect of pH, saturating cation (K⁺ and Ca²⁺), and freeze-drying on adsorption of DNOC by two reference smectites with different charge densities (SWy-2 and SAz-1) in dilute aqueous suspensions. The smectite–DNOC systems were adjusted from pH 3 to 7. Nearly 100% of added DNOC was adsorbed by K⁺–saturated SWy-2 at pH 3, and sorption decreased with increasing pH to 62% at pH 7. Sorption of DNOC on K⁺–saturated SAz-1 decreased from 94% at pH 3 to 31% at pH 7. Suspended Ca²⁺–saturated SWy-2 adsorbed 82% of added DNOC at pH 3 but sorption decreased to 18% for pH 4 and above. Across the entire pH range, Ca²⁺–saturated SAz-1 sorbed about 12% of added DNOC. Slightly larger amounts of DNOC were adsorbed by the "never dried" (not freeze-dried) smectites compared with the freeze-dried and rehydrated smectites. Analysis of supernatants from the K⁺–saturated SAz-1–DNOC systems indicated co-adsorption of K⁺ with DNOC in the phenolate form at pH values above the pK_a of DNOC. At lower pH values, DNOC adsorption and complexation with interlayer K⁺ resulted in less K⁺ exchange by H⁺ compared with the control without DNOC. These mechanisms explain the minimal influence of pH on the adsorption of DNOC by the K⁺–saturated smectites.

Abbreviations: Ca-SAz-1, calcium-saturated Arizona montmorillonite reference clay (smectite) • Ca-SWy-2, calcium-saturated Wyoming montmorillonite reference clay (smectite) • DNOC, 4,6-dinitro-o-cresol • K-SAz-1, potassium-saturated Arizona montmorillonite reference clay (smectite) • K-SWy-2, potassium-saturated Wyoming montmorillonite reference clay (smectite)

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