Release and transport of colloidal particles in natural porous media 2. Experimental results and effects of ligands

We present an extensive experimental data set of particle release from natural porous media saturated with monovalent cations. The generation process of mobile colloidal particles is studied by means of leaching of saturated laboratory columns packed with a noncalcareous soil material with various monovalent electrolytes and by analyzing the colloids in the effluent over typically 1000 pore volumes. The concentration of released particles cannot be modeled with simple first-order kinetics but can be rationalized in terms of a distribution of release rate coefficients k. The experimentally observed effluent concentration often decays with time as a power law c proportional to t(-(alpha +1)) for long times, suggesting a distribution of release rate coefficients p (k) proportional to k(alpha -1) for small k. The observed values of exponent alpha range between 0.01 and 0.8. The composition of the pore water is found to have a profound influence on the particle release characteristics. With decreasing salt concentration the rate for particle release increases. Anionic organic and inorganic ligands have a major effect on the release process. For the ligands studied, the amount of released particles decreases in the sequence malonate, chloride, phtalate, and azide.