Miljøprojekt, 902 – The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

Summary and conclusions

It has often been assumed that chemicals in soil are degraded according to simple first-order kinetics from which a half-life, a DT₅₀-value, is deduced. However, it has emerged from many cases that the degradation does not follow simple first-order kinetics. The cause for these reported deviations from simple first-order kinetics may probably be found partly in the microbial environment in the soil, partly in the fact that the sorption over longer time (and the formation of bound residues) take place according to more complicated processes than generally assumed.

All known models used for describing the fate, transport or risk evaluation of chemicals in soil include terms describing binding and degradation, respectively. Thus, an understanding of the dynamics of these parameters and the mutual connection between them is unavoidable if the models are to be used successfully.

Even though it till now has not been possible to give a complete description of the binding of xenobiotic chemicals to soil, a vast number of studies of this subject have been carried out just as a large number of processes have been described because the binding of the substances in the soil is of great importance to their fate. The sorption of chemicals to soil may vary from being completely reversible to being completely irreversible.

A very complex connection between the sorption of xenobiotic chemicals to the soil organic matter and the microbial degradation of the substances in the soil environment is more the rule than the exception. In order to throw light upon this connection, the development in the past few years has been in the direction of investigating the microbial degradation of chemicals in soil under circumstances as close to the real environment as possible. It has often been seen that the sorption has a limiting effect on the degradation rate of pesticides, which can be explained by the fact that first of all it is the amounts of pesticide available in the aqueous phase of the soil that can be microbially degraded. Even though this factor plays a part, a degradation of sorbed chemicals may still take place to a certain extent. If the desorption is dependent on time and takes place at a rate that is on a level with or lower than the degradation rate, the dependence of the degradation on the sorption leads to complex kinetics even though the degradation kinetics itself is simple first-order kinetics. Recent studies are dominated by the view that two-compartment first-order kinetics provides a better description of the degradation of chemicals in soil when systems are used that simulate the natural circumstances. The two-compartment kinetics expresses a distribution of the added chemical between the aqeous phase and the soil phase and indicates a first-order rate constant for the degradation in the fluid phase and the soil phase, respectively. The two-compartment kinetics is often preferred, both for chemicals that are mainly bound to the soil organic matter and for chemicals that are mainly bound to the clay minerals.

A half-life cannot be deduced from the two-compartment model for which reason it cannot be used without adaptation in the commonly applied dynamic leaching models, such as PRZM-2, PRZM, PELMO, GLEAMS, PESTLA, VARLEACH, LEACHM, MACRO, PLM, and MIKE-SHE, which are used for predicting leaching of pesticides to the groundwater. Both in connection with approvals and in model runs, it would be relevant to take into account that the degradation follows more complex kinetics than simple first-order kinetics, just as it would be relevant to take into account the problem concerning bound residues, which has been extremely much in f_OC us in the last 10 years.

There are already a large number of studies of binding mechanisms for both desorbable xenobiotics and bound residues. However, it is necessary to couple these studies to studies of bioavailability to determine to which extent the substances can be reactivated in the natural system.

It is important to emphasize that laboratory experiments that are intended to be used for describing the complex connection between the binding and the degradation must allow for the extreme variation of the processes. There is already a large variation, which is merely caused by the heterogeneity of the soil. The part of the variation, which may be caused by the method, must be eliminated. Here, the first step must be to carry out the studies under conditions that resemble the natural conditions as much as possible. Furthermore, it is necessary to develop chemical methods of analysis able to extract the amounts of the xenobiotics from the soil that can be correlated with the bioavailable amounts that are the results of biological studies of bioavailability.

Both from a legal and a scientific point of view it is important to differentiate between free pesticide residues and bound pesticide residues just as it is important to distinguish between biological persistence and chemical persistence when persistence is in question, as these do not always go together.