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 (p K 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 Ca 2+ ), 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 Ca 2+ –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, Ca 2+ –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 p K 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.