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Eight samples of goethite ranging in surface area from 18 to 132 m2 g-' were mixed with phosphate at a range of pH values for periods which ranged from 0.5 h to 6 weeks. The sample with a surface area of 18 m2 g-' had been hydrothermally treated to improve its crystallinity. Its rate of reaction with phosphate depended on pH but was complete within a day. Its maximum observed reaction was close to the theoretical maximum for surface adsorption of 2.5 pmol m-2. For the other samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2.5 pmole mP2. The duration and extent of the reaction depended on the crystallinity of the goethite. The results were closely described by a model in which the phosphate ions were initially adsorbed on to charged external surfaces. The phosphate ions then diffused into the particles. This was closely described using equations for diffusion into a cylinder.Samples of goethite which had been loaded with phosphate dissolved more slowly in HC1, and had a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HC 1 removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it was necessary to dissolve much more of the iron to obtain a similar removal of phosphate. Brief treatment with NaOH removed most of the phosphate from the hydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite. These results are consistent with the idea that phosphate ions were not only bound on external surface sites but had also penetrated into meso-and micro-pores between the domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sites for phosphate adsorption are therefore caused mainly by their location on either external or internal sites. Models that ignore this are incomplete.

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A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH 50 ) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH 50 or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH 50 . For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH) 2þ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH) 2þ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.

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...