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

Abstract

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 (pKa) 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 pKa 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.