DNOC a nitroaromatic pesticide has several uses in the fields of agriculture and horticulture. Recent research has shown that smectites adsorb surprisingly large amounts of DNOC (Sheng et al. 2003). Because DNOC is a weak acid (pK_a=4.4), we hypothesized that pH would be the dominant state variable controlling sorption. We quantified the effect of pH, saturating cation (K⁺ and Ca²⁺) and freeze drying on adsorption of DNOC by reference smectites (SWy-2 and SAz-1) in dilute aqueous suspensions using a batch equilibration technique. HCl was added to the suspensions to adjust pH (range 3 to 7) before adding DNOC. Systems were equilibrated by shaking for 24 hrs at 25°C then centrifuged and filtered. The amount of DNOC remaining in aqueous phase was determined with a spectrophotometer at 268 nm in a KPO₄ buffer (pH=7) matrix. Nearly 100% of added DNOC was adsorbed by K-SWy-2 at pH=3 and at pH=7 sorption decreased to about 60%. Sorption of DNOC on K-SAz-1 decreased from about 90% at pH=3 to 30% at pH=7. About 80% of added DNOC was sorbed by Ca-SWy-2 at pH=3 but sorption decreased to about 15% by pH=4 and remained stable at about 15% to pH=7. Ca-SAz-1 sorbed about 15% of added DNOC over the entire pH range. Slightly larger amounts of DNOC were adsorbed by the never dried smectites compared to the freeze dried-rehydrated smectites. X-ray diffraction analysis of the clay suspensions did not reveal a significant shift in d spacing upon DNOC sorption. This finding suggests that DNOC is primarily adsorbed on the external surfaces of suspended smectite colloids. Results also suggest some unknown mechanism allows the anionic form of DNOC to be adsorbed,as adsorption was observed at pH values well above the pK_a.

Ref:Sheng et al. 2003. Soil Sci. Soc. Am. J. 67:122–131.