Selenate sorption

Figure 13 illustrates the fit of the model to the effects of time and solution concentration for selenite and for selenate sorption by a soil. 

Wu et al. (2001) found that molybdate sorption was affected insignifically in the presence of equimolar amounts of selenate however, selenate sorption was significantly reduced in the presence of molybdate at pH < 7.0, where a 30% decrease in sorption was noticed (Figure 5.7). 

Selenate. Harrison and Berkheiser (20) examined selenate sorption onto freshly precipitated hydrous ferric oxide using dispersive IR spectroscopy on air dried samples. They concluded that selenate forms a bidentate bridging complex, replacing both protonated and unprotonated hydroxyls. 

Hayes et al. (27) and Manceau and Charlet (22) examined selenate sorption onto goethite, and goethite and hydrous ferric oxide, respectively using EXAFS. Hayes et al. (27) concluded that the surface complex was outer-sphere while Manceau and Charlet (22) concluded that the complex was inner-sphere. This discrepancy demonstrates how spectroscopic studies are not always conclusive. The reasons for these differences are speculative but could... 

Since we can write no surface reactions with Se04 (using neutral or positively charged surface sites) which release less than one hydroxyl, these results suggest that some selenate sorption may be non-specific or outer-sphere. Alternative surface complexes can be written using the sites defined by the MUSIC model (25), FeOrf, FejO, FejOH, and FejO. Consideration of alternative sites does not change the interpretation of the titration and EM data. 

We conclude that selenate sorption is mostly inner-sphere and possibly bidentate. 

Fig. 9.1. Sorption of selenate (SeO ) to a loamy soil, showing mass sorbed per gram of dry soil, as a function of concentration in solution. Symbols show results of batch experiments by Alemi et al. (1991 their Fig. 1) and lines are fits to the data using the reaction KA, reaction Freundlich, and Langmuir approaches.

We consider as a first example sorption of selenate (SeO ), as predicted by the reaction K(, reaction Freundlich, and Langmuir approaches (Sections 9.1-9.3). Alemi et al (1991) observed the partitioning of selenate in batch experiments between 10 g of a loamy soil and 20 ml of a pH 7.5 solution containing small amounts of Na2Se04 their results are shown in Figure 9.1. 

Su, C. Suarez, D.L. (2000) Selenate and selenite sorption on iron oxides An infrared and electrophoretic study. Soil Sci. Soc. Am. J.101-111... 

Baur, I. Johnson, C. A. 2003a. Sorption of selenite and selenate to cement minerals. Environmental Science and Technology, 37, 3442-3447. 

Figure 9. Logarithmic plots of sorption of phosphate, selenite and selenate by a soil at the indicated pH values. To permit plotting on a common scale, the concentrations of selenite and selenate have been multiplied by factors to account for their lower affinity. For example, the values at pH=6 for selenite were 0.05 and for selenate 0.008. The lines indicate the fit of a model based on the heterogeneity of the reacting surfaces [44].

While the sorption curves are almost linear on a log log scale, the model fits a gentle curve as this is consistent with a bigger body of information (Fig. 9.). At any given level of sorption, the concentration of selenite in solution decreases with time and with increasing temperature. It is this decrease that is modelled as due to diffusive penetration. Selenate differs in that the sorption curves are steeper (as also shown in Fig. 9.) and, importantly, that the effects of time, though detectable, are much smaller. These two species therefore provide a test for the argument that apparent non-reversibility of sorption occurs because of the continuing reaction. 

Figure 14. Desorption of selenite (a) and selenate (b), after incubation for 10 days at 60°C with selenite or selenate respectively at the levels indicated by the arrows on the vertical axis. Different levels were used so that the concentration range would be similar. The broken lines show the sorption curves [80].

Figure 14. shows that selenite desorption did not follow the same track as sorption. This is because it takes time to reverse the diffusive penetration. This was tested from the fit of the model. When the model which had been fitted to sorption data, some of which is shown in Fig. 13., was used to predict desorption, the prediction was close to the observation. In contrast, desorption of selenate was greater. This is consistent with its much slower continuing sorption reaction. The greater desorption was also predicted... 

Barrow reports common intersection points of uptake (pH) curves obtained for sorption of phosphates (constant total concentration) by soils at different NaCl concentrations [25]. The slope of these curves decreases when the ionic strength increases, and the uptake of phosphates from 1 mol dm NaCl is almost independent of pH. The position of these intersection points on the pH scale is a function of the initial phosphate concentration. Similar effect was reported for selenates (IV). Interestingly for borates [26] whose uptake increased with pH over the pH range of interest, a common intersection point (pH of zero salt effect) was also observed but in this case the slope was higher for higher ionic strengths. This type of behavior has not been reported for well defined adsorbents,... 

The correlation of the shape of uptake curves with pK i (sigmoidal uptake curves for relatively strong acids, uptake curves with a maximum for very weak acids) is limited, e.g. selenic IV acid (sigmoidal curves with a few exceptions) is weaker than arsenic V or phosphoric add (for which adsorption maxima are often observed). The discrepancies in sorption behavior of anions can be due to dissolution of the adsorbents. 

Sulfate was poor at preventing arsenate and molybdate sorption onto metal oxides and soils (Wu et al., 2001 Violante et al., 2005b), but reduced the sorption of selenate significantly (Wu et al., 2001). The alleviating effect of sulfate application on arsenic toxicity has also been reported (review by Kitagishi and Yamane, 1981). 

The sorption of TcOj by roots of hydroponically grown soybean seedlings (Glycine max.) was shown to be linear from 10 M pertechnetate solutions for at least 6 h and to exhibit characteristics of carrier-mediated transport commonly associated with the sorption of nutrient ions in higher plants. Analyses of TcO uptake in the presence of individual nutrient anions revealed the sorption to be competitively inhibited by sulphate, phosphate, selenate. and molybdate indicating the use of common transport mechanisms [54],... 

Vlll. The effects of time and temperature of incubation on the sorption and subsec]uenl desorption of selenite and selenate by a soil. /. Soil Sci. 40, 29-37. 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

Identification of the specific species of the adsorbed oxyanion as well as mode of bonding to the oxide surface is often possible using a combination of Fourier Transform Infrared (FTIR) spectroscopy, electrophoretic mobility (EM) and sorption-proton balance data. This information is required for selection of realistic surface species when using surface complexation models and prediction of oxyanion transport. Earlier, limited IR research on surface speciation was conducted under dry conditions, thus results may not correspond to those for natural systems where surface species may be hydrated. In this study we review adsorbed phosphate, carbonate, borate, selenate, selenite, and molybdate species on aluminum and iron oxides using FTIR spectroscopy in both Attenuated Total Reflectance (ATR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) modes. We present new FTIR, EM, and titration information on adsorbed arsenate and arsenite. Using these techniques we... 

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