Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. II Modelling the extent and rate of reaction

Abstract

A model of the reaction of metal ions with a variable charge surface was modified and applied to goethite that had been formed in the presence of silicate. The data included the effects of initial concentrations of Ni, Zn and Cd ranging from 1 to 100 p~, of periods from 2 h to 42 d, of temperatures from 5 to 35"C, of pH from about 4 to about 8, and of background electrolyte from 0.01 to 1.0 M.The observed effects of concentration and pH were explained by assuming that the reacting surface sites were heterogeneous and that they reacted with the MOH+ ions in solution. The heterogeneity was described by assuming that the affinity of the sites for MOH+ ions decreased logarithmically as the log of the amount adsorbed increased. This gave a Freundlich relation between sorption and surface activity. The effects of increasing the concentration of the background electrolyte were explained as being caused by decreased activity of MOH+ ions in solution counteracted by decreases in the electric potential of the surface. To explain the effects of time and temperature, it was assumed that an initial rapid adsorption reaction was followed by slow diffusion of metal ions into the goethite. Using these assumptions, a comprehensive description of the data was obtained. I N T R O D U C T I O NIn soils, the oxides of iron, aluminium, and titanium provide sites for reactions with both nutrients and pollutants. However, extraction of these oxides from soil is difficult and many studies of their reactions have, therefore, used synthesized samples. If we are to test whether we understand these reactions, we need to be able to test whether we can describe them precisely by using equations. Unfortunately the sets of equations required to describe different reaction schemes are fairly complex and computer programs are needed to solve them. These sets of equations are commonly called models. The purpose of this work is to test one such model. The work differs from previous studies in two ways. One was that the goethite used was associated with appreciable silicon. Fordham & Norrish (1979) found that iron oxide pellets from a range of soils contained between 1 % and 4% of silicon by weight. They emphasized the difficulties in avoiding contamination in such work but considered that their values were for pellets that were free of contamination. A high silicon content was also reported for a fibrous goethite by Fordham et al. (1984).The other way in which this work differs is that the model was tested over a wide range of conditions. This is because, we argue, it is important for any model, or for any hypothesis, to be comprehensive. The conditions varied were: initial concentrations of metal in solution from 1 to 100 p~; temperatures from 5 to 35°C; periods from 2 h to 6 weeks; pH from about 4 to about 8; and concentration of background electrolyte from 0.0 1 to 1 M. This comprehensive testing contrasts with other work in which at best two, and often only one of these conditions were varied. For example, N , J. Burrow et ul. aqueous solutio...

Nickel adsorption by soils in relation to pH, organic matter, and iron oxides

“…This was due to the reduction in the metal repulsion by the positively charged surface, in consequence of the oxide removal (Silveira et al, 2002). For pH close to 7.0, practically all added Ni was adsorbed, corroborating results obtained by several authors that also observed drastic increase in the metal adsorption when soil pH were raised above 7.0 (Harter, 1979;Barrow et al, 1989;McBride, 1989;Schulthess & Huang, 1990;Scheidegger et al, 1996;Mellis & Casagrande, 2001 (Rodela et al, 2002). Therefore, even for the higher pH values tested in this research, the possibility of finding Ni in the form Ni(OH) 2 0 was very low.…”

An Analysis of Zinc Sorption to Amorphous versus Crystalline Iron Oxides Using XAS

“…On the other hand, thermodynamic analyses suggest that while transition metals like Zn may be chemically sorbed to goethite (13,14), adsorption to HFO involves physical forces (9,10,13). Long-term studies reveal that intraparticle diffusion is the ratelimiting mechanism for sorption to the microporous oxides, and thus zinc sorption may require from a few months to a few years to reach equilibrium (10,(15)(16)(17). Similar studies with goethite indicate microporosity is not significant (13).…”

Geochemical partitioning and bioavailability of copper to aquatic plants in an artificial oxide-organic sediment

“…Because of the complexity of metal-particle interactions, two common features of this type of study are single metal-substrate experimental designs, and the use of syntheticallyderived goethite (a-FeOOH), a crystalline Fe oxide (e.g. Ankomah, 1992;Barrow et al, 1989). In natural sediments, however, other oxides such as ferrihydrite and lepidocrocite have been identified (Fortin et al, 1993, which are more amorphous than goethite and consequently may have quite different sorption characteristics (Schwertmann & Cornell, 1991).…”