Evaluation of Experimental Procedures and Discussion of Two Different Modelling Approaches with Respect to Long-term Kinetics of Metal Cation Sorption onto (Hydr)oxide Surfaces
Author: Lützenkirchen J.
Source: Aquatic Geochemistry, Volume 7, Number 3, September 2001 , pp. 217-235(19)
Experimental procedures as well as literature data and modelling of long-term kinetics of metal ion adsorption processes onto thermodynamically stable (hydr)oxide surfaces from aqueous solution are evaluated. It is concluded that when experimental uptake data are obtained by the solution depletion method precise information on experimental procedures is crucial. This is true both for preliminary kinetics studies (which intend to fix an equilibration time used for subsequent thermodynamic studies) and for true kinetic studies. For the usually given adsorption versus time curves the corresponding (possibly changing) values of pH should be added. The use of sorption pH edges measured at different times to obtain kinetics curves at constant pH may not correspond to the actual behaviour of the system but be rather an artificial construct.
Concerning the models two basically different mechanisms have been used to account for the slow sorption step, which occurs in long-term studies: intraparticle diffusion of the adsorbing metal cation and slow redistribution of surface species. It is shown that the two mechanisms cannot be distinguished using macroscopic uptake data alone: data, which have been interpreted using one approach, may equally well be interpreted using the other one.
Document Type: Regular paper
Publication date: 2001-09-01