Improvement of Diuron dissipation in soil with hydroxypropil-beta-cyclodextrin solutions

Abstract

Diuron is a biologically active pollutant in the environment, and both, diuron and its principal metabolite, 3,4-dichloroaniline, exhibit a high toxicity and persistence in soil, and are potential poising environmental contaminants. Hence, determining the main factors influencing diuron degradation in the soil-water system could prove a basis for development of bioremediation tools. In this sense, only biodegradation studies should not be considered sufficient to manage risk assessments, and mineralization experiments ought to be planned to assert that the environmental risk has disappeared.

The bioavailability of organic pollutants governs their toxicology and degradation in contaminated soils, especially in those with high organic matter content. Environmental microbiologist defines bioavailability as the contaminant fraction which represents its accessibility to a living organism for assimilation, degradation and ecotoxicology expression. The influence of sorption on soils with different properties the biodegradation of organic compounds has been recognized as important, albeit poorly understood phenomenon, in bioremediation.

In environmental chemistry, cyclodextrins (CDs) have been used to enhance desorption of pollutants from soil contributing to increase their bioavailability. In fact, the inclusion of Diuron in hydroxypropil-beta-CD (HPBCD) in solution gave as result up to 24-fold increase in its aqueous solubility.

Consequently, as a first step to investigate the behaviour on this herbicide in soil, diuron adsorption-desorption experiments on two selected soils, with a significant difference in their organic matter contents (1 and 14%), were carried out employing 0.01 M HPBCD and aqueous solution as extractants. Positive hysteresis was observed in the soil with higher organic matter content when aqueous solution was used, indicating that diuron desorption from this soil was not completely reversible. On the contrary, the application of HPBCD solution increased very much its desorption, and a negative hysteresis was obtained, that is, more diuron was desorbed with respect to diuron adsorption isotherm. In the case, of the soil with lower organic matter content, all the herbicide could be desorbed with both extractant solutions. From these results it could be concluded that HPBCD solutions are able to enhance the bioavailability of this hydrophobic herbicide in soil, allowing its further biodegradation.

Respirometry methodology was used for mineralization experiments in the investigated soils with the aim of assessing the effect of bioremediation processes. For this purpose, radiolabelled [ring-U-14C]-diuron was employed in order to determine the 14CO2 from biotic degradation (endogenous flora). HPBCD solution was added to the respirometer after mineralization plateau was reached, in order to observe an increasing of the degradation rate. The use of CD solutions contributed to increase the diurón hydrosolubility and improved its desorption and mineralization in soil, which makes that the use of these solutions can be considered as an effective in situ soil bioremediation tool for this herbicide.