Colloid chemical processes, kinetics

O Melia, Ch. R. (1990), "Kinetics of Colloid Chemical Processes in Aquatic Systems", in W. Stumm, Ed., Aquatic Chemical Kinetics, Reaction Rates of Processes in Natural Waters, Wiley-lnterscience, New York, pp. 447-474. 

KINETICS OF COLLOID CHEMICAL PROCESSES IN AQUATIC SYSTEMS... 

Kinetics of colloid chemical processes in aquatic systems... 

Discussion of these results is directed at the following question. How do these field results compare with present theories for the kinetics of aggregation and sedimentation in aquatic systems In answering this question, some laboratory determinations of attachment probabilities will be used in modelling simulations of the kinetics and effects of colloid chemical processes in lakes. 

An essential beginning in a comprehensive and quantitative assessment of the kinetics of colloid chemical processes is the speciation at a surface that results from the reactions of solutes with surface sites. In some cases this formulation has been accomplished by a description of surface charge, but the diversity of solids and solutes in aquatic systems necessitates a broader assessment. An example is the adsorption of selenite on t lie surface of goclhite (Hayes et al., 1987) in which a... 

As illustrated by the results presented in Figure 2 and in Table 2 at high ionic strength and high Ca2 + for favorable particle-particle interactions (e.g., in the deposition of non-Brownian particles, F = F%Taviiy + Fdrag +FlVDW Fchem = 0), transport models based on physical and hydrodynamic characteristics of a system can predict the initial kinetics of aggregation and deposition processes in aquatic systems quantitatively. In the presence of repulsive chemical interactions, however, quantitative theoretical predictions of such kinetics are very inaccurate and even many qualitative predictions are not observed. The determination of Fchem in aquatic systems merits study and development,- it is necessary for the quantitative prediction of the kinetics of colloid chemical processes in these systems. 

Until such theories can be developed, laboratory experiments can be performed to determine chemical effects in aquatic colloid chemical processes for actual situations. This is suggested by the analysis presented in this chapter of the aquifer study by Harvey et al. (1989) and is illustrated for Lake Zurich by the study of Weilenmann et al. (1989). Since mass transport can be described with some success [e.g., p, c),heor and 2(r,y )slhcor], this knowledge can be combined with laboratory determinations of attachment probabilities such as those illustrated in Table 2 for a(p, c)exp and listed in Table 5 for ci(i,j)s exp to describe the kinetics of deposition and aggregation (e.g., Eqs. 5 and 6) in aquatic systems. 

Finally, we demonstrate in discussions on weathering rates in the field, on the kinetics of colloid chemical processes, and on the role of surficial transport processes in geochemical and biogeochemical processes that spatial and temporal heterogeneities and chemical versus transport time scales need to be assessed in order to treat the dynamics of real systems. 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

The fast reactions of ions between aqueous and mineral phases have been studied extensively in a variety of fields including colloidal chemistry, geochemistry, environmental engineering, soil science, and catalysis (1-6). Various experimental approaches and techniques have been utilized to address the questions of interest in any given field as this volume exemplifies. Recently, chemical relaxation techniques have been applied to study the kinetics of interaction of ions with minerals in aqueous suspension (2). These methods allow mechanistic information to be obtained for elementary processes which occur rapidly, e.g., for processes which occur within seconds to as fast as nanoseconds (j0. Many important phenomena can be studied including adsorption/desorption reactions of ions at electri fied interfaces and intercalation/deintercalation of ions with minerals having unique interlayer structure. 

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

In discussing the mechanisms of the formation of monodispersed colloids by precipitation in homogeneous solutions, it is necessary to consider both the chemical and physical aspects of the processes involved. The former require information on the composition of all species in solution, and especially of those that directly lead to the solid phase formation, while the latter deal with the nucleation, particle growth, and/or aggregation stages of the systems under investigation. In both instances, the kinetics of these processes play an essential role in defining the properties of the final products. 

---