Site density of goethite

In this problem you are to compare the predicted adsorption of phosphate by goethite as computed using the constant capacitance and diffuse-layer models. Assume the same general system conditions as given in Problem 12. In other words, there are 0,6 g/L of goethite in suspension. The surface-site density of goethite equals 1.35 x 10 mol sites/g, and its surface area is 45 m /g. Total phosphate is 1.0 x 10 M and SNa = 3.0 X 10-3 M. 

Lutzerrkirchen. J. et al. Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces, Geochim. Cosmochim. Acta. 66. 3389. 2002. 

At first the data of the column experiments were compared with model predictions achieved by using the site density of amorphous iron hydroxide (0.2 mol sites/mol Fe( ,oii(i)) and by using the site density of goethite (0.016 mol sites/mol F6(soiid))- The PHREEQC2 model runs with a site density typical for amorphous iron hydroxide overestimated the retention of the three tested oxoanions by a factor of 2.1 to 4.7 and the model runs with the site density of goethite underestimated the retention by a factor of 2.2 to 4.6. The relative deviation between the observed breakthrough of the oxoanions and the model predictions is quite similar for all three oxoanions. Best approximations of the model predictions were achieved with site densities (mol sites/mol Fe( ,oii(j)) of 0.054 (chromate), 0.044 (phosphate) and 0.074 (arsenate). 

In this problem we use the diffuse-layer model to compute the adsorption of orthophosphate species by goethite between pH 3 and 10. Intrinsic constants for the adsorption reactions are available in the MINTF.QA2 file,/ o-d/w.f/6r. Similar calculations have been discussed and performed by Hohl etal. (1980). (a) Assume 0.6 g/L of goethite in suspension, with a surface-site density of 1.35 x 10 moles of sites per... 

Model Runs Using the Site Density of Amorphous Iron Hydroxide and Goethite... 

Column experiments were conducted to simulate the transport of oxoanion aquifers containing iron hydroxides. The Riedel de Haen Hfo material used in these column experiments exhibits a low specific surface area and contains a significant proportion of goethite. PHREEQC2 is equipped to model surface complexation of oxoanions onto iron hydroxides but a consistent data set of surface complexation constants is only available for amorphous hydrous ferric oxides. Tests were conducted to determine whether it is possible to use PHREEQC2 with this data set to model the oxoanion transport in the columns. If the data set of surface complexation constants is also suitable for the Riedel de Haen Hfo material than it should be sufficient to adjust the site density of the iron hydroxide surface in the model. 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

A CA approach described zinc adsorption on aquifer sand material from Cape Cod by assuming that the aluminum and iron phases present in the quartz grain coatings have a surface area and site density similar to those of poorly crystalline materials (Davis et al., 1998). A similar approach provided only a semiquantitative prediction of uranium adsorption on an alluvial aquifer sediment from Naturita, Colorado, depending on the assumptions made about the relative amounts of surface area of quartz, ferrihydrite, and goethite (Davis et al., 2004). In both of these studies, the surface complexation model considered... 

The most obvious practical use of modeling results (i.e., stability constants, site densities, and capacitance values) is in the prediction of the behavior of solutes outside the laboratory conditions (e.g., for environmental or industrial purposes). This is, in the first place, hampered by the variety of experimental results obtained with one solid (e.g., goethite, cf. Fig. 1). It is further complicated by the different models used (see Sec. in.A and lll.E). 

Dithionite-citrate and HCl extractions of the W1 sample dissolved 70.4 mg Fe g" of sample, probably present as goethite and hematite. Assuming a composition of Fc203, this corresponds to 100.7 mg of Fe203 g of sample. Submicron sized Fe oxide particulates typically have surface areas from 20 to 50 m g a 50 m g-i value is equivalent to a crystalline Fe oxide surface area of 5.04 m g within the W1 soil sample, roughly 20% of the total surface area of the sample. The predicted U(VI) adsorption on the crystalline Fe oxide component of the schist material using this surface area is shown in Fig. 4, with the assumption that the U(VI) binding constants and the site densities are the same as observed for ferrihydrite (Table 4-4). As was found for ferrihydrite (Fig. 4-3), the calculated U(VI) adsorption is less than that observed experimentally for the W1 sample, but the prediction is much improved. 

Any iron present in the leach solution is removed by precipitation as goethite (FeOOH). The solution is then purified by cementation with lead powder to remove copper, silver, bismuth, arsenic and antimony, leaving impurities such as zinc, which do not co-deposit with lead. The purified solution is electrolysed in a diaphragm cell, which uses a coated copper cathode and a titanium mesh anode coated with ruthenium and iridium oxides. An ion permeable membrane separates the cathode and anode compartments. The cathode is fabricated from a dimpled copper sheet coated with an inert adhesive sheet between the dimples, leaving numerous sites of high current density to promote dendritic growth of the lead deposit. The crystalline lead falls from the cathode and is collected in the base of the cell. 

Acid-base constants/proton-affinity constants These parameters cannot, at present, be established experimentally. Because there are probably several distinct sites and electrostatic effects, the overall surface-charge curves do not give any hint as to the nature and reactivity of these sites. Experimental back-titration data by Schulthess and Sparks [103], which indicate steps in the proton adsorption isotherm (i.e., the surface-charge density versus pH curve), were interpreted by these authors to be indicative of individual sites with individual reactivities. However, such data are not reproduced by the bulk of research groups. In the previous subsection, the back-titration technique has been discussed in some detail and, in particular, the results on goethite obtained with base titrations at low pH were found to show significant scatter compared to data obtained with the coulometric approach. 

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