Organic contaminants in soil : desorption kinetics and microbial degradation

The availability of organic contaminants in soils or sediments for microbial degradation or removal by physical means (e.g.) soil washing or soil venting) depends on the desorption kinetics of these contaminants from the soil matrix. When the organic contaminants desorb very slow from the soil matrix, remediation of polluted sites may be hampered. In this thesis, sorption kinetics of organic contaminants in soil was investigated with emphasis on the influence of natural organic matter on slow desorption rates, and the influence of sorption kinetics on the microbial degradation rates of these contaminants. Two organic matter fractions, humic acids and humin, were shown not only to differ in their affinity for an organic contaminant, but also in the kinetics of their interactions with the contaminant.The results showed that humin can play an important role in the sorption behaviour of a soil. Interaction between humic acids and an organic contaminant were shown to be non-linear. Non-linear sorption behaviour was also observed for soil. The effects of non-linear sorption behaviour were demonstrated in simulations. Non-linear sorption may be caused by a limited number of sites with a high sorption energy that are present in humin. Microbial degradation of an organic contaminant in soil was predicted using measured desorption kinetics and measured intrinsic microbial degradation kinetics. It was shown that only for the slow-desorbing contaminant fraction, the desorption rate was the rate-determining step for the degradation in soil.When microorganisms are able to degrade contaminants that become available by a slow desorption process, the natural degradation of organic contaminants in soil can be an efficient remediation method in the last stages of the in situ remediation of a contaminated soil. Continuous monitoring of such a contaminated soil however will be needed.