Adsorption and interactions of Diquat and Paraquat with montmorillonite

The purpose of this study is to elucidate details of the interactions between the divalent cationic herbicides diquat (DQ) and paraquat (PQ) and montmorillonite. Interactions were studied by ultraviolet absorption (UV) and linear dichroism infrared (LDIR) spectroscopies, x-ray diffraction, and adsorption isotherm measurements. For added herbicide amounts below the cation-exchange capacity (CEC) of the clay, adsorption was almost complete. At saturation, the adsorption of DQ was 125% of the CEC, whereas PQ adsorbed up to the CEC. In competitive adsorption experiments, when total cationic charges exceeded the CEC, monovalent organic cations were preferentially adsorbed to the clay at the expense of the divalent cations, in accord with the expected larger exclusion of divalent cations from positively charged surfaces. Adsorption of the divalent herbicides reduced the basal spacing of the clay from 1.45 nm to ≈ 1.30 nm. This result implies the desorption of interlayer water and can be explained in terms of the flat orientation of the adsorbed PQ cation and the compact size and keying of the DQ cation into the ditrigonal cavity of the basal surface of the clay. This explanation is also supported by LDIR results. For adsorption up to the CEC, the main UV absorption bands of the herbicides around 300 nm were red shifted. The absorption spectrum of DQ adsorbed beyond the CEC suggests that some of the molecules were attached to the clay via weaker interactions, similar to the mechanisms found for monovalent organic cations. In view of the low basal spacing of the clay, the observed red shift cannot be due to the existence of DQ or PQ dimers.