Goethite species distribution

To find the overall goethite-water distribution coefficient for benzoic acid (HBz), assume that only adsorption of the deprotonated species (benzoate, Bz") is important ... 

The complexation constants of the individual major seawater ions with otFeOOH determined in single salt solutions can be used to predict the titratable charge and surface species distribution of goethite in seawater. This prediction can then be compared with the experimentally determined charge of goethite in a mixed seawater type electrolyte. 

The simplified mass and proton balance model determined what the surface species distribution of goethite would be in a mixed, seawater type electrolyte. This surface species distribution was used to calculate a surface charge for goethite. 

Therefore, from a knowledge of the intrinsic acidity constants, Ns and the value of kj, the distribution of surface species, using data from Lumsdon and Evans (1994). 

With these data we can now calculate the distribution of the major species on the surface of goethite in seawater. This approach will form the basis for modeling trace metal adsorption in seawater and determining the competitive effects of the major ions with each other and with trace metals. 

Distribution of Surface Species on Goethite in Seawater as a Function of pH... 

Figure 10.25 Distribution plot of percent surface species on goethite at 25°C in 0.1 M NaNOj solutions for ZU(VI) = 10" M computed with the TL model. Cg = 1 g/L, = 45 m /g (Hsi 1981 Hsi and Langmuir 1985).

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

Well 16-17 has Fe(II) values near the detection limit of 0.05 mg/L and contains O2 at low levels, which may be the cause of the higher Pt electrode reading. The site is in a transition zone, out of redox equilibrium, and contains nitrite at ca. 0.5 mg N/L. However, the laboratory results discussed above indicate that Ehj at the WIG electrode may still provide an accurate representation of the distribution of ferric/ferrous species in the system. Thus, the pQ value (39.4) calculated from the Ehj at the WIG electrode may be reliable. This pQ indicates that a ferric oxyhydroxide with a solubility similar to that of colloid-sized goethite is present. 

As in the other surface complexation models, the chemical part consists in several adsorption reactions. In the application of the CD-MUSIC model, the choice of these reactions has been carefully based on spectroscopic evidence of course, that can be done also in other models such as TLM, BSM, and so on, but here it is rather essential for proper elucidation of the charge distribution. The original paper (Hiemstra and Van Riemsdijk 1996) applied the model to phosphate adsorption on goethite. Based on spectroscopic studies, they proposed the existence of monodentate-bound species (Reaction 12.44), an unprotonated bidentate-bound species (Reaction 12.45), and a protonated bidentate species, following ... 

FIGURE 12.15 The adsorption of PO4" by monodomainic goethite used by Hiemstra and Van Riemsdijk (1996) and that of Bowden et aL (1980). The lines are calculated with the charge distribution model and are consistent with the surface species observed by cylindrical internal reflection-Fourier transform infrared spectroscopy. (Reprinted from Journal of Colloid and Interface Science, 179, Hiemstra, T. and Van Riemsdijk, W. H. 488-508. Copyright 1996, with permission from Elsevier.)... 

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