KINETICS OF PARTICLE DEPOSITION AND DETACHMENT

The packed column technique serves as a useful tool in the investigati ons of the deposition and removal mechanisms of particles dispersed in a liquid in contact with a solid substrate. A model is presented whic h can be used to estimate explicitly the effects of multilayer deposit ion, which is based on three phenomenological parameters: two mass tra nsfer coefficients, and one blocking parameter. The latter accounts fo r the area occupied by each adhered particle. The method can be used t o evaluate the experimental results in terms of the particle-collector and particle-particle interactions. The former are related to heteroc oagulation, and the latter to homocoagulation. It is shown that the ra te of deposition of hematite particles onto glass beads, in the absenc e of a repulsion barrier, is governed by a convective diffusion mechan ism. If a repulsion barrier exists, the deposition rate depends on the ionic strength in a manner consistent with colloid stability. The adh ered particles can be removed by rinsing the column with a solution of an appropriate pH, which renders both surfaces sufficiently strongly charged, if the ionic strength is kept low. When gelatin is adsorbed o nto the glass beads, irreversible deposition of hematite particles tak es place, which is caused by chemical bonds between the collector bead s and the metal oxide. The effect of external force fields can also be analyzed using the packed column technique. It is shown how the depos ition of hematite on steel is affected by a magnetic field. It is poss ible to induce deposition under conditions where particles and beads b ear the same sign of charge, and thus repel each other. Under such con ditions, a deep secondary minimum is generated, which can be controlle d by the strength of the magnetic field.