Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2007
Publication Date: 9/8/2007
Citation: Pereira, T.R., Laird, D.A., Johnston, C., Teppen, B., Li, H., Boyd, S. 2007. Mechanism of dinitrophenol herbicide sorption by smectites in aqueous suspensions at varying pH. Soil Science Society of America Journal. 71:1476-1481. Interpretive Summary: An understanding of adsorption mechanisms helps scientists and engineers predict which pesticides and other organic compounds are likely to stick to soil particles and which are likely to move and end up as contaminants in the ground water. Organic acids have a negative charge at high pH and are neutral at low pH, therefore the soil pH has long been assumed to be an important factor in controlling the fate of organic acids in soils. Recent work has demonstrated that an organic acid known as dinitro-o-cresol (DNOC) is very strongly adsorbed by potassium-saturated clays at low pH. We discovered that DNOC is also very strongly adsorbed by potassium-clays at high pH and that potassium ions are co-adsorbed with the DNOC. We also discovered that DNOC is strongly adsorbed on some calcium-clays at low pH but not at high pH. Based on these observations we proposed that DNOC is adsorbed as a complex with potassium. This information will help scientist and engineers to better predict the fate of DNOC in soil environments, and is a significant advance in scientific understanding because we describe a new mechanism by which organic molecules can adsorb on clays.
Technical Abstract: 4, 6- dinitro-o-cresol (DNOC) is an important pesticide that is strongly adsorbed by smectite clays. Because DNOC is a weak acid with a pKa of about 4.4, pH was hypothesized to be a dominant state variable controlling sorption. In this study, 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 were quantified. The smectite-DNOC systems were pH adjusted from pH 3 to pH 7. Nearly 100% of added DNOC was adsorbed by K-SWy-2 at pH 3, and sorption decreased with increasing pH to about 60% at pH 7. Sorption of DNOC on K-SAz-1 decreased from about 90% at pH 3 to about 30% at pH 7. Ca-SWy-2 suspensions adsorbed about 80% of added DNOC at pH 3 but sorption decreased to about 15% for pH 4 and above. Over the entire pH range Ca-SAz-1 sorbed about 15% of added DNOC. Slightly larger amounts of DNOC were adsorbed by the never-dried smectites compared to the freeze-dried-rehydrated smectites. Analysis of supernatants from the K-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+ results in less K+ exchange by H+ compared to the control without DNOC. These mechanisms explain the minimal influence of pH on the adsorption of DNOC by the K-smectites.