Sorption and Desorption of Pesticides by Clay Minerals and Humic Acid‐Clay Complexes

Abstract

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.

Thermodynamics of Nitroaromatic Compound Adsorption from Water by Smectite Clay

“…The measured basal spacings of the air-dried and rewetted K-SWy-2 clay films were plotted against their corresponding concentrations of adsorbed NACs (Figure 5). The data for 4,6-dinitro-o-cresol were adapted from a previous study (26) for comparison. Basal spacings of air-dried films increased progressively from 10.2 to 12.2 Å as the sorbed NAC concentrations increased, demonstrating the intercalation of NACs into the smectite interlayers.…”

Probing the microscopic hydrophobicity of smectite surfaces. A vibrational spectroscopic study of dibenzo-p-dioxin sorption to smectite

“…[11][12][13] In prior sorption studies, the nature of the exchangeable cation was shown to significantly influence the uptake of aromatic hydrocarbons as well as semi-polar, planar aromatic compounds including nitroaromatics, triazines and carbamates. 11,12,[14][15][16][17][18][19][20][21][22][23][24][25][26] Sorption of these compounds possessing moderately polar functional groups on smectites from aqueous was influenced by several key parameters. First, the interaction between the compound and exchangeable cation influences sorption.…”

Herbicide Selectivity in Preemergence of Sweet Sorghum Cultivated in Different Soil Texture

“…Since no herbicide affected the quality of harvested biomass (assessed using Brix), the main evaluated variable that correlates with productivity is BMP. In this case, of the 16 treatments evaluated in this type of soil, nine did not affect this variable and can be considered selective for sweet sorghum: atrazine (2000, 1500 and 1000 g ha -1 ), amicarbazone (280 g ha -1 ), S-metolachlor (480 g ha -1 ), pendimethalin Generally, organic matter and clay minerals are the main factors in the retention of herbicides applied to the soil (Li et al, 2003).…”