Coagulation in continuous particle size distributions; theory and experimental verification

Abstract

Previous theories for particle coagulation are not readily applicable to the continuous particle size distributions encountered in natural waters. By extending concepts developed in the analysis of aerosol dynamics, predictions of continuous particle size distributions were obtained dimensionally for size intervals dominated by Brownian, shear, differential sedimentation coagulation and gravitational settling. A dynamic steady state size distribution was assumed to exist, maintained by a constant flux of particle volume through the distribution. Predictions have been successfully compared with the shapes of particle size distributions measured in oceanic waters. An experimental program was designed to test the predictions with cleaned clay and silica minerals in artificial seawater. A series of batch experiments was conducted at fluid shear rates of 1/2 to 32 sec^(-1) in a rotating cylinder apparatus. During the experiments, total suspended volumes were determined from suspension optical absorbance, and particle size distributions were measured with a Coulter Counter-multichannel analyzer system. The volume flux through the distribution was estimated from the rate of suspended particle volume removal, which was second order in suspended volume and depended on the fluid shear rate. The Brownian and shear coagulation predictions were verified for the kaolinite, illite, and montmorillonite clays. The three clays were significantly different in the regions of Brownian and shear coagulation dominance and in the volume removal rates at low fluid shear rates. At higher shear rates the volume removal rates declined because of aggregate breakup by fluid shear in the rotating cylinder apparatus. Differences in the clay coagulation and breakup characteristics were explained by variations in clay aggregate porosities. Experiments with silica did not agree with predictions because the silica particles were not destabilized in seawater. Predictions for differential sedimentation coagulation and gravitational settling could not be tested because of larger aggregate breakup by the Coulter Counter. The theoretical predictions have direct application to particle coagulation in oceanic waters and possible application to more complex systems, such as estuarine waters and water and wastewater treatment operations.

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