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

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.