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 natu ral 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 m onovalent electrolytes and by analyzing the colloids in the effluent over t ypically 1000 pore volumes. The concentration of released particles cannot be modeled with simple first-order kinetics but can be rationalized in term s of a distribution of release rate coefficients k. The experimentally obse rved effluent concentration often decays with time as a power law c proport ional to t(-(alpha +1)) for long times, suggesting a distribution of releas e rate coefficients p (k) proportional to k(alpha -1) for small k. The obse rved values of exponent alpha range between 0.01 and 0.8. The composition o f the pore water is found to have a profound influence on the particle rele ase characteristics. With decreasing salt concentration the rate for partic le release increases. Anionic organic and inorganic ligands have a major ef fect on the release process. For the ligands studied, the amount of release d particles decreases in the sequence malonate, chloride, phtalate, and azi de.