In soils, organic matter and minerals are often associated such that it is unclear how the presence of the former component influences the sorptive properties of the latter one. In this study, sorption and desorption of the herbicides 4,6-dinitro-o-cresol (C7H6N2O5) and dichlobenil (C7H3Cl2N) by Ca2+-, K+-smectites, and humic acid-smectite complexes, was investigated using batch-equilibrations and x-ray diffraction (XRD). Greater sorption of 4,6-dinitro-o-cresol compared with dichlobenil was observed for both smectites and humic acid-smectite complexes. For both pesticides, K+ smectites were more effective sorbents than Ca2+ smectites, with the lower charge-density clay (SWy-2) displaying a greater sorption capacity than the higher charge-density clay (SAz-1). The presence of humic acid did not impact pesticide sorption by K+ clays, but could enhance or suppress pesticide sorption by Ca2+-clays. A composite model for estimating pesticide sorption, which assumes mineral and organic matter function individually as sorbent phases, predicted sorption within a factor of 0.8 to 1.5 times the measured values. Humic acid did not contribute to pesticide desorption hysteresis in K+-humic acid-clay complexes, but was a source of hysteresis in the corresponding Ca2+ complexes. The basal spacings of K-SWy-2 and humic acid-modified K-SWy-2 increased gradually from approximately 10.4 to 12.2 Å with increasing 4,6-dinitro-o-cresol loadings. Also, XRD patterns of humic acid-modified and unmodified K-SWy-2 smectite clays were found identical. These results demonstrate the intercalation of 4,6-dinitro-o-cresol and suggest that humic acids are restricted to the external surfaces of clay tactoids. Together, these results indicate that clay mineral fractions in soils, including those with organic coatings, may play an important role in the retention of certain pesticides.
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