Release of colloidal particles from natural porous media saturated with monovalent cations is studied with saturated laboratory column experiments packed with a noncalcareous soil material. The column outflow was monitored over 1000 pore volumes.The composition of released particles remains constant over the course of the experiment. The major finding of the present study is that the release process cannot be modeled with simple first-order kinetics, but the effluent concentration decays in anonexponential fashion. The experimentally observed effluent concentration decays as t{sup}(-(1+α)) where the exponent typically has values between of 0.01 and 0.3. Such a powerlaw decay can be rationalized in terms of an exponential distribution of theactivation energies of release rate coefficients, and the exponentαturns out to be inversely proportional to the average activation energy. We observe a strong dependence of the release on the ionic strength, which can be interpreted as a systematicincrease of the average activation energy for particle release with increasing ionic strength. In the present system pH was accurately controlled, and the release pattern was surprisingly insensitive to pH variations.
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