Introduction

Coagulation/flocculation, and more generally speaking aggregation, represent physical processes in which initially dispersed colloidal units (solid inorganic particles, fulvic and humic acids and biopolymers) stick together, under the influence of various attractive forces, to build characteristic structures whose size increases with time. Aggregation phenomena are not only of great importance in environmental systems, but also in many areas of colloid science, chemistry and physics and industrial processes. In natural waters and wastewater treatment plants, the transport and fate of both nutrients and toxic compounds largely depend on their interactions with colloidal particles, biopolymers and the aggregates they form. The transport and elimination of vital or detrimental compounds then depends upon the kinetics of formation, structure, and sedimentation of these aggregates (Figure 4.1). Because of the complexity and large number of biophysical and chemical factors influencing these processes, as well as the fact that natural colloids include several components, no rigorous analytical theory or models have been derived and applied to describe aggregation in environmental systems.

On the other hand, owing to the development of computers and the introduction of scaling concepts, numerical and theoretical models have recently been applied to investigate the behaviour of colloidal suspensions. These models have proven to be important and convenient tools for the systematic investigation of some of the physi-cochemical factors (pH, temperature, solution ionic strength, colloidal concentration and chemical surface properties) that influence the morphology of colloidal structures

Biophysical Chemistry of Fractal Structures and Processes in Environmental Systems Edited by Nicola Senesi and Kevin J. Wilkinson © 2008, IUPAC

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