Effects of Brownian motion and structured water on aggregation of charged particles

Coagulation processes are integral to water treatment for removal of colloids and colloid-associated pollutants and therefore must be well understood. However, current research shows a disparity between experimental results and theoretical predictions. In some conditions, colloids that are predicted to be stable exhibit significantly less stability in experiments. To date, reasons offered for this disparity have not completely resolved the problem. In this paper, three issues that have previously been ignored are proposed as contributors to the disparity. The first is a model for estimating Brownian jump length which is anisotropic, with which it is shown that particles take longer to move away than once thought, and therefore, have a greater opportunity for attachment. The second issue develops a method for estimating the impact on the calculation of electrostatic repulsion due to variation in dielectric constant of water near surfaces. It is shown that including the low dielectric at the surface will greatly decrease the predicted electrostatic repulsion. and therefore, will decrease stability. The third issue concerns the impact of surface-induced water structure on the effective kinetic energy of particles near another surface. A model is developed for estimating the impact of kinetic energy on Brownian jump length and with the model it is shown that the impact can be great. Particles will have more energy to jump toward the interacting surface, than away, thus increasing the probability of aggregation. (C) 2001 Elsevier Science B.V. All rights reserved.