Physical and Chemical Kinetics in Humic Dispersions

abstrakt

angličtina

The very high affinity of humic substances is well known. In this study, the interaction of Cu2+ ions with humic solid (HS), sols and gels is studied. In all of these cases, the complexation between metal ions and phenolic or carboxylic groups takes place. According to results of quantum chemistry, these functional groups are the most convenient for binding with transition metals.1 In general, HS are partially soluble in aqueous solutions. In natural systems, reactions of HS with metal ions are proceeding in liquid phase or on liquid/solid interface. In case of solid HS, Langmuir model is frequently used for description of sorption equilibrium. Although experimental data obtained for sorption of Cu2+ ions on solid HS can be fitted relatively precisely by this model, its utilization is connected with some inaccuracies. Above all, the forming of Cu-HS complexes causes the liberation of H+ ions (from phenolic or carboxylic groups), which is not taken into account (consideration?). In addition, Langmuir model is deduced (derived?) on the basis of kinetics of sorption, but the experimental kinetic data measured for Cu2+ and solid HS do not correspond to this model.2 Therefore colloidal solutions (sols) of HS were prepared and used for study of chemical interaction between Cu2+ ions and HS by means of conductometry and potentiometry. We assumed the simplest mechanism of complexation, in which one Cu2+ ion displaces two H+ ions. Thus the conductivity would increase (mainly because of the increase of acidity) as long as the studied system is in equilibrium. But the kinetic curves, obtained using potentiometry, show the maximum (typical for the reaction intermediate in consecutive reactions), which does not correspond with simple complexation mechanism. On the basis of obtained kinetics data, the complex reaction mechanism was proposed.3 In natural systems, the complexation of HS with metal ions is accompanied by its diffusion into solid particles. Because the study of diffusion in solid HS is experimentally difficult (dá se to tak napsat?), HS were prepared in form of gel, which is more suitable for experimental measurements. Humic gel (in opposite to solid HS) can be formed in precise size and shape therefore the definition of boundary conditions is very easy. Using the value of effective diffusion coefficient of Cu2+ (determined in previous 4), the diffusion of Cu2+ in humic gel in dependence of initial concentration of Cu2+and time was described in detail. In conclusion the kinetics of interaction of copper ions with HS in solid, sols and gels is compared.

humic acids, colloids, kinetics, adsorption, complexation, diffusion

KLUČÁKOVÁ, M.; PEKAŘ, M.

2004

17. 3. 2004

Northeastern University

Boston

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