Aquatic colloids: Concepts, definitions, and current challenges

Colloidal phases in natural waters may be important to various environmental questions, especially those concerning the cycling of vital and toxic trace chemicals. Current treatments of the role of colloids in chemical speciation largely rely on operational definitions of phases such as 1,000-Da ultrafilter and 0.45-µm filter cut-offs. Defining chemical phases exclusively by a physical parameter such as size is contributing to a situation where the observed filterable vs. unfilterable distribution coefficients, D, are not well predicted from thermodynamically derived sorbed vs. solute equilibrium constants, K. Achieving the goal of relating the natural distributions of chemicals to theoretical expectations is contingent upon progress in development of a functionally meaningful colloid definition and interpretation of observed distributions of trace substances in terms of the relevant physicochemical properties of the system. We assess the phase status of typical components in natural waters from a “chemcentric” point of view (i.e. one whose motivation is to understand the cycling of trace chemicals in the environment). As a result, we define colloids so as to provide a thermodynamic grounding for evaluating chemical speciation and a hydrodynamic framework distinguishing phases that are transported with the solution from those that are not. These constraints lead one to define an aquatic colloid as any constituent that provides a molecular milieu into and onto which chemicals can escape from the aqueous solution, and whose movement is not significantly affected by gravitational settling. Such a definition allows development of mass balance equations, suited to assessing chemical fates, that reflect processes uniquely acting on dissolved, colloidal, or settling particle phases.