No abstract

This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.

Adsorption-desorption of the herbicide glyphosate (GPS) onto the clay mineral montmorillonite has been studied in presence of copper, in order to clarify the effects that this strongly complexing metal could have on the availability and mobility of GPS in soil environment. In contrast with previous findings, GPS adsorption on montmorillonite decreases in the presence of Cu, in solution or adsorbed on the mineral, due to the formation of Cu-GPS complexes that have a lower tendency to be adsorbed on montmorillonite than the free GPS. GPS adsorption is higher at pH 4.2 than at pH 6.8, both in the presence and in the absence of Cu, but the decrease in GPS adsorption in the presence of Cu is lower at pH 4.2 than at pH 6.8, since at this latter pH the concentration of Cu-GPS complexes in relation to free GPS in solution is high. It is commonly believed that GPS remains almost permanently adsorbed onto soils and has very limited mobility, but GPS desorption experiments show that the amount of GPS desorbed from montmorillonite is not negligible and increases in the presence of Cu. These experiments lead to the conclusion that, in relation to GPS release from soil, it is necessary to take into account not only the type of soil to which it is applied (e.g., whether it has high clay minerals content) but also whether there is any element in the soil solution or adsorbed on the soil capable of forming strong complexes with GPS.

The aim of this study was to understand the interactions between alkylammonium cations present as monomers and micelles and a clay mineral, montmorillonite, to develop slow release formulations of anionic herbicides, such as sulfometuron (SFM) whose leaching in soils is an environmental and economic problem. In the proposed formulation the herbicide is incorporated in positively charged micelles of quaternary amine cations, which in turn adsorb on the negatively charged clay. The adsorption of hexadecyltrimethylammonium (HDTMA) and octadecyltrimethylammonium (ODTMA) on montmorillonite was studied above and below their critical micelle concentrations (CMC). At concentrations above the CMC, the loading exceeded the clay's cation exchange capacity (CEC) and indicated higher affinity of the cation with the longer alkyl chain. An adsorption model could adequately simulate adsorption at concentrations below the CMC, and yield fair predictions for the effect of ionic strength. The model indicated that above the CMC adsorbed micelles contributed significantly to the amount of ODTMA adsorbed. Evidence for adsorption of ODTMA micelles on montmorillonite was provided by X-ray diffraction, freeze-fracture electron microscopy, and dialysis bag measurements. SFM was not adsorbed directly on the clay mineral, and adsorbed at low levels, when the organic cation was adsorbed as monomers. In contrast, a large fraction of SFM adsorbed on the clay mineral when incorporated in micelles that adsorbed on the clay.