Kinetics of particle deposition and detachment

The packed column technique serves as a useful tool in the investigations of the deposition and removal mechanisms of particles dispersed in a liquid in contact with a solid substrate. A model is presented which can be used to estimate explicitly the effects of multilayer deposition, which is based on three phenomenological parameters: two mass transfer coefficients, and one blocking parameter. The latter accounts for the area occupied by each adhered particle. The method can be used to evaluate the experimental results in terms of the particle-collector and particle-particle interactions. The former are related to heterocoagulation, and the latter to homocoagulation. It is shown that the rate of deposition of hematite particles onto glass beads, in the absence of a repulsion barrier, is governed by a convective diffusion mechanism. If a repulsion barrier exists, the deposition rate depends on the ionic strength in a manner consistent with colloid stability. The adhered 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 onto the glass beads, irreversible deposition of hematite particles takes place, which is caused by chemical bonds between the collector beads and the metal oxide. The effect of external force fields can also be analyzed using the packed column technique. It is shown how the deposition of hematite on steel is affected by a magnetic field. It is possible to induce deposition under conditions where particles and beads bear the same sign of charge, and thus repel each other. Under such conditions, a deep secondary minimum is generated, which can be controlled by the strength of the magnetic field.