Abstract
In soils organic matter and clay minerals are often so associated that it is not clear that how the presence of organic matter influences the sorptive process of clay minerals. So, in order to factor and quantify component contributions of mineral fractions and humic acid phase to the sorption of pesticides on clay minerals, adsorption and desorption of three carbamate pesticides oxamyl (I), S-Ethyl-N-(methyl carbamoyl)-oxythio acetimidate (II) and N-Phenyl (ethyl cabamoyl) propylcarbamate (III) by Na+, Ca2+ saturated illite, kaolinite and humic acid-clay complexes were investigated by batch experiments. The adsorption was more on illite than kaolinite and followed the order pesticide III > I > II, adsorption on Na-saturated clay samples was more than Ca-saturated. The presence of humic acid enhances the pesticide adsorption. A composite model for estimating pesticide adsorption, which assumes mineral and organic matter fraction individually as adsorbent phases, predicted sorption within a factor of 0.75-1.4 times the measured value. The desorption data indicated that humic acid retains more pesticide than clay mineral. XRD patterns showed that humic acid is restricted to the external surfaces of clay tactoids, denoting that clay mineral fractions in soils including those with organic coatings play an important role in the retention of polar carbamate pesticides.
Highlights
Carbamate pesticides, a new horizon, agrochemicals are widely used as insecticides and herbicides in home, garden and agriculture, as they are less persistent, possess less mammalian toxicity and may be used against those pests which have acquired immunity against organochlorine and organophosphate pesticides
The value of 1/n during adsorption of three carbamate pesticides on Na- and Casaturated illite and kaolinite were less than unity indicating a convex or ‘L’ type of isotherm [17,18] [Fig 1a and 1b] and were higher for illite than kaolinite
The values of 1/n and K (Table 1) indicate that the intensity of the carbamate pesticides adsorption was in the order pesticide III > I > II
Summary
A new horizon, agrochemicals are widely used as insecticides and herbicides in home, garden and agriculture, as they are less persistent, possess less mammalian toxicity and may be used against those pests which have acquired immunity against organochlorine and organophosphate pesticides. Clay mineral can strongly adsorb certain aqueous phase organic compounds containing polar functional groups, suggesting the potential contributions of clay minerals to the retention of organic contaminants and pesticides in soils and sub soils [1,2,3]. The addition of organic amendments in soil might lead to an increase in pesticide adsorption [9], decreasing the pesticide leaching [10]. Several previous Studies [11,12] have attempted to identify the critical ratio of clay minerals/ soil organic matter at which sorption by clay mineral phase plays an important role. Organic matter and clay minerals are the two most important soil components which affects retention of soil applied pesticides [13,14]
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