Inclusion complexes of α- and γ-cyclodextrins and the herbicide norflurazon: I. Preparation and characterisation. II. Enhanced solubilisation and removal from soils

Abstract

The interaction of norflurazon with α- and γ-cyclodextrins (CDs) yielded the formation of inclusion complexes at a 1:1 stoichiometric ratio in solution and in the solid state. Apparent stability constants of 50.7 ± 1.6 and 37 ± 1.7 M-1 and an increase in herbicide solubility by up to five and fourfold for α- and γ-CD, respectively, were determined from the phase solubility diagrams at 25°C in water. Three processing methods (kneading, spray-drying and vacuum evaporation) were used to prepare norflurazon-CD solid inclusion complexes, which were characterised by infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy. A high increase in the norflurazon dissolution rate was obtained with all the solid complexes with γ-CD, but when α-CD was used, only the solid system obtained after the vacuum evaporation process showed a higher dissolution rate. This finding is a first step in the development of new, environmentally sound formulations of norflurazon (NFL), due to the capacity for increasing its dissolution rate and hydrosolubility, and thus diminishing the use of organic solvents. On the other hand, the effect of α- and γ-cyclodextrin on the solubility of norflurazon in solution was also considered as a way of modifying its behaviour in the soil environment. Desorption studies of NFL from soils in the presence of α- and γ-cyclodextrin were carried out using a batch equilibration method. The results obtained showed that α- and γ-cyclodextrin greatly increased the removal of norflurazon previously adsorbed, proving the potential use of these CDs for in situ remediation of pesticide-contaminated soils.