Quantitative understanding of nanoparticle flocculation in water treatment
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Flocculation is critical in drinking water treatment; in flocculation, the particle size distribution changes from a large number of small particles to a small number of larger particles. Larger particles are effectively removed by settling and filtration processes that follow flocculation. In recent years, manufacturing of engineered nanoparticles has skyrocketed, and these nanoparticles can enter our water supplies, but knowledge of their fate in water treatment is limited. The objective of this research was to update knowledge of flocculation by extending previous work at the microscale to the nanoscale. Flocculation involves transport of particles to the vicinity of one another and subsequent attachment if interactions at close distances are favorable. Transport and possible collisions are brought about by Brownian motion, differential sedimentation, and fluid shear; these processes, even at the nanoscale, are well understood. Whether collisions and attachment actually occur, however, depend on a balance of hydrodynamic interactions, van der Waals attraction, and electrostatic repulsion; this research quantitatively assessed, for the first time, this balance for collisions of nanoparticles by all three collision mechanisms using a well-established trajectory analysis approach. The collision efficiency (α) is the ratio of the number of successful collisions (attachment) to the number of collisions predicted by the transport equations. The analysis was performed with and without electrostatic repulsion, which occurs if particles are charged. In all cases, Brownian motion was the dominant flocculation mechanism. However, without electrostatic repulsion, differential sedimentation and fluid shear were found to be far more important than heretofore expected because the α value can be substantially higher than one, contrary to all previous understanding. With electrostatic repulsion, collisions by these two mechanisms were found to occur only if the particles are substantially different in size. Experiments in which the changes in particle size distributions of nanoparticles were carefully monitored were also performed, and the results compared to the mathematical predictions. Although not perfect, excellent agreement between the measured and predicted particle size distributions was found. The conclusion is optimistic: if nanoparticles are properly destabilized to reduce or eliminate surface charge, they will be well removed in conventional water treatment plants.