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«THERMODYNAMIC MODELING OF METAL ADSORPTION AND MINERAL SOLUBILITY IN GEOCHEMICAL SYSTEMS A Dissertation Submitted to the Graduate School of the ...»

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Dissolved acetate, CH3COO ~, is used to determine the effect of aqueous metalorganic complexation on the adsorption of Cd to one- and three-sorbent systems. We conduct these experiments with 1 g l"1 sorbent in the presence of 0.3 M acetate, as sodium acetate. To predict the effects of aqueous Cd-acetate complexation on the Cd adsorption behavior in one-component systems with acetate, we use the stability constants for the Cd-sorbent complexes that we calculated from our one-component Cd adsorption experiments, along with the acetate acidity constant and Cd-acetate stability constants for the 1:1,1:2, 1 ;3, and 1:4 Cd:acetate complexes from the literature (Martell and Smith, 1977). Figure 4.5 compares the measured extents of Cd adsorption in the

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Figure 4.4: Adsorption of (A) 8.

9 x 10"5 M Cd(II) and (B) 8.9 x 10-6 M Cd(II) to mixtures of HFO and kaolinite. Dashed lines represent best-fit models to 1 g l"1 HFO and kaolinite end members from Figure 1. Darkened symbols and lines represent adsorption data and predicted adsorption for two-sorbent mixtures, including 0.75 g 1"' HFO + 0.25 g l'1 kaolinite (A, thin line), and 0.25 g T1 HFO + 0.25 g l"1 kaolinite ( •, thick line).

absence of acetate (solid squares) and in the presence of acetate (open circles). HFO experiments and models at 10 ppm and 1 ppm are depicted in Figures 4.5A and 4.5B, respectively. The predictive models that include Cd-acetate complexation (dashed lines) dramatically underestimate the extent of adsorption observed in the experiments at all pH values between 2 and 8. The misfit at low pH, between 2 and 5.5, is less severe than the misfit of the high pH data, and likely stemsfromthe failure of the basic HFO model to account for Cd adsorption at low pH. Shen et al. (2001) described the adsorption of Pb to soil in an acetate-bearing system by proposing that the aqueous Pb-acetate complex, Pb(CH3COO)+, adsorbs to surface sites as a ternary site-Pb-acetate complex.

Similarly, in our systems the 1:1 Cd-acetate complex, Cd(CH3COO)+, dominates the aqueous Cd speciation. To account for the observed adsorption behavior, we invoke ternary site-Cd-acetate complexation to account for the discrepancy between the

predicted extents of adsorption and the higher extents that were observed, according to:

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The equilibrium constant, K15, for the surface-Cd-acetate ternary complex is calculated using the previously-determined Cd-surface constants, and solving for the best-fit ternary surface complex constant. The calculated log K15 value from this approach is 0.9. The grey lines in Figures 4.5A and 4.5B represent these best-fit models.

Adsorption of the Cd-acetate complex improves the fits substantially at high pH, but the models continue to underpredict Cd adsorption below pH 7. This is likely due to the relatively poor fit of the one-component HFO model, and potentially because there are other ternary interactions that must be considered to model the data accurately. Our models suggest that in systems containing HFO and an organic ligand, the CA approach

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Figure 4.5: Adsorption of 8.

9 x 10"5 M Cd to 1 g l"1 A) HFO, C) B. subtilis, and E) kaolinite, and 8.9 x 10"6 M Cd to B) HFO, D) B. subtilis, and F) kaolinite. Darkened symbols and lines represent adsorption data and predicted adsorption for mixtures without acetate. Open symbols are adsorption data in the presence of 0.3 M acetate.

Dashed line indicates the predicted extent of adsorption in acetate experiments without the adsorption of a Cd-acetate complex, and grey line is the best-fit model including a Cd-acetate ternary surface complex.

may not be sufficient to describe Cd adsorption behavior unless ternary interactions between important aqueous complexes and the HFO surface are considered.

Figures 4.5C and 4.

5D depict the adsorption of 10 ppm and 1 ppm Cd to B.

subtilis cells, respectively. The model predictions of Cd adsorption in the presence of acetate (dashed lines) underestimate the actual extent of adsorption (open circles) across the entire pH range studied here. This result suggests that an aqueous Cd-acetate complex may be adsorbing to the bacterial surface. Models invoking a complexation reaction between the Cd(CH3COO)+ complex and either Site 2, 3, or 4 yield a better fit to the experimental data, but inclusion of this reaction at Site 2 only (Table 4.3) yields the best-fit (grey line). Models invoking Cd(CH3COO)+ adsorption at Sites 2 and 3, or Sites 2, 3, and 4 give fits that are equivalent to the Site 2-only model. We use the model ' invoking only Site 2 for its simplicity.

The adsorption of 10 ppm and 1 ppm Cd in the presence of kaolinite and acetate is displayed in Figures 4.5E and 4.5F, respectively. As with the HFO and B. subtilis, the predictive model that does not invoke a ternary complex under-predicts the extent of Cd • adsorption across the entire pH range. Poor, but improved fits are obtained when the aqueous Cd(CH3COO)+ ligand is complexed with either the amphoteric (XOH) or permanent structural {YO ~) sites. The model invoking only the XOH site underestimates adsorption below pH 5, and one invoking only the YO ~ site underestimates adsorption where pH is greater than 5. The best-fit (grey line) is obtained when Cd(CH3COO)+ adsorbs onto both sites simultaneously (Table 4.3). In the cases of the kaolinite and B.





subtilis cells, the adsorption of the Cd(CH3COO)+ complex from solution is sufficient to describe the observed data. Additionally, the stability constants of ternary surface-Cdacetate complexes calculated for an individual sorbent are similar whether modeling

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a. Log K values for each reaction are calculated from best-fit models of 1 ppm and 10 ppm Cd adsorption data.

adsorption datafromthe 1 ppm or 10 ppm experiments (see Table 4.3). For instance, the log lvalues for Cd(CH3COO)+ complexation at Site 2 of the B. subtilis cells is calculated to be 4.8 from both the 1 ppm and 10 ppm data. This provides further evidence that the Cd(CH3COO)+ complex is adsorbing to each sorbent, that we are using the correct ternary complexation stoichiometries (Table 4.3), and that these reactions are capable of accounting for adsorption over a range of Cd concentrations.

4.3.4 Cd adsorption to three-component mixtures The adsorption behavior of 1 ppm Cd to three-component mixtures of HFO, B.

subtilis, and kaolinite is illustrated in Figure 4.6. Four mixtures of the sorbents are investigated, one where each component comprises 33% of the 1 g l"1 sorbent present (Figure 4.6D), and three sets in which one of the sorbents is 80% of the total mass, and the remaining 2 sorbents represent 10% of the total mass, each. In the mixture where each component contributes 33% of the sorbent mass, the B. subtilis cells, HFO, and kaolinite contribute 66%, 18%, and 16% of the reactive sites, respectively. However, as in the two-component experiments containing HFO that are described in Section 3.3, the most Cd adsorption is observed in experiments that contain a greater fraction of HFO.

The mixture of 80% HFO + 10% B. subtilis cells + 10% kaolinite (Figure 4.6A) adsorbs Cd to a nearly identical extent as the 100% HFO system (see Figure 4. IB). Mixtures containing only 10% HFO and 80% B. subtilis or kaolinite (Figures 4.6B, 4.6C, respectively) adsorb Cd to a lesser extent. The lowest extent of Cd adsorption is observed in the single component kaolinite systems (Figure 4.1). Thus, the Cd adsorption behavior in the ternary mixtures of these sorbents is directly related to the extent of adsorption observed in the single-sorbent systems. All 3-component predictive

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Figure 4.6: Adsorption of 8.

9 x 10"6 M Cd to three-component mixtures of HFO, kaolinite, and B. subtilis, and in the presence of 0.3 M dissolved acetate. Darkened symbols and lines represent adsorption data and predicted adsorption for mixtures without acetate. Open symbols and dashed lines are adsorption data and predicted adsorption in the presence of 0.3 M acetate, respectively. Sorbent mixtures include A)

0.8 g f1 HFO + 0.1 g 1"' B. subtilis + 0.1 g l"1 kaolinite, B) 0.1 g T1 HFO + 0.8 g T1 B.

subtilis + 0.1 g l'1 kaolinite, C) 0.1 g l'1 HFO + 0.1 g l'1 B. subtilis + 0.8 g l'1 kaolinite, D) 0.33 g r1 HFO + 0.33 g l"1 B. subtilis + 0.33 g Pkaolinite.

models describe the adsorption data reasonably well. The predicted extent of adsorption in the system containing 80% HFO (Figure 4.6A) shows minor discrepancies due to the fit of the pure HFO end-member. However, in mixtures of HFO, B. subtilis cells, and kaolinite, the CA approach is generally successful in predicting Cd adsorption at our experimental conditions.

The results of adsorption experiments conducted with three sorbents in the presence of 0.3 M acetate are depicted in Figure 4.6 (open circles). The dashed lines represent the best-fit Cd adsorption models, including the ternary complexation reactions calculated in this study and listed in Table 4.3. Figure 4.6A depicts Cd adsorption for a mixure of 0.8 g l"1 HFO + 0.1 g l"1 B. subtilis cells + 0.1 g l"1 kaolinite in the presence of • acetate. Compared to the same mixture in the absence of acetate (solid squares), more adsorption is observed below pH 5, and less above pH 6. Similar trends with pH are observed in the other three-component mixtures, including: 0.1 g l"1 HFO + 0.8 g l"1 B.

subtilis cells + 0.1 g l"1 kaolinite (Figure 4.6B), 0.1 g l'1 HFO + 0.1 g l"1 B. subtilis cells + •

0.8 g l"1 kaolinite (Figure 4.6C), and 0.33 g l"1 HFO + 0.33 g l"1 B. subtilis cells + 0.33 g l'1 kaolinite (Figure 4.6D). The predictive models of these ternary systems may underestimate Cd adsorption below pH 5 (Figure 4.6) because of poor fit of the basis 1 ppm + HFO data (Figure 4.1B) is carried into these models. At pH greater than 6, all one-sorbent models of HFO, kaolinite, or B. subtilis cells in the presence of acetate (Figures 4.5B, 4.5D, 4.5F, respectively) fit the experimental data well. The predictive models of three-sorbent systems with acetate may overestimate adsorption at higher pH due to particle adhesion which causes a decrease in Cd adsorption as reactive sites are blocked. In particular, HFO, which has positively-charged sites under all experimental conditions, may aggregate with negatively charged kaolinite particles and/or B. subtilis cells.

4.4 Conclusions In this study, we test if the CA approach can account for Cd adsorption to mixtures including B. subtilis cells, HFO, kaolinite, and dissolved acetate. We first conduct Cd adsorption experiments onto each sorbent separately at two metal concentrations. Using acidity constants and site concentrations for each sorbent from the literature, we model these adsorption data to calculate Cd stability constants for each sorbent. Using these constants, we predict the extent of Cd adsorption in two- and threecomponent mixtures of the sorbents at various mass ratios. In the absence of dissolved acetate, the predictive models match experimental data well. However, in the systems that we test that contain dissolved acetate, the models predict a bigger decrease in adsorption than we observed. We account for this enhanced adsorption in the presence of' acetate by invoking the adsorption of the Cd(CH3COO)+ aqueous complex to each sorbent to form a ternary surface-Cd-acetate complex.

The component additivity approach represents a powerful means to extrapolate our understanding of metal adsorption in simple one-sorbent systems to predict metal distributions in complex multi-sorbent settings. Our results indicate that the CA approach is reasonably accurate under some circumstances. However, caution should be used when applying this approach to geologic systems, as site blockage and the formation of ternary complexes can significantly affect the accuracy of the CA predictions.

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THE EFFECT OF NP(V) INCORPORATION ON THE SOLUBILITY OF SODDYITE

5.1 Introduction Whereas some properties of materials change rapidly with grain size at the nanoscale (Hochella, 2002; Gilbert et al., 2004), similarly large responses of bulk properties to ppm levels of impurities are uncommon. Solubility in aqueous solutions greatly influences geochemical and environmental systems, yet the extent to which solubility is impacted by low-level impurities is largely unknown. No robust theory exists to explain such influences where they depart from ideal behavior. Here we examine a specific case where ppm-level substitution results in a dramatic reduction of the aqueous solubility of the recipient phase.

A system of particular environmental interest is the substitution of the Np(V)02+ neptunyl ion for the U(VI)022+ uranyl ion in a uranyl silicate phase. Commercial spent nuclear fuel consists primarily of the solid phase UO2, but also contains other actinides including Np, a potentially mobile radionuclide under moist oxidizing repository conditions such as those expected at the Yucca Mountain site, Nevada, USA. Under these conditions, U0 2 is unstable, and rapidly oxidizes to form a range of uranyl [U(VI)] phases (Wronkiewicz et al., 1992; Finch and Ewing, 1992; Finn et al., 1996; Finch and Murakami, 1999; Finch et al., 1999; McNamara et al., 2003). Uranyl silicates, including soddyite, (U02)2Si04(H20)2, are among the most important alteration phases to form (McNamara et al., 2003). Np in spent nuclear fuel is present predominantly as Np(IV), but under oxidizing conditions can transform into Np(V), which is present as the neptunyl cation, Np02+, in aqueous solutions. Due to the similarities in cation size and coordination geometry, the neptunyl moeity can substitute for the uranyl cation, U(VI)022+, in soddyite and other alteration phases (Klingensmith and Burns, 2007;

Klingensmith et al., 2007), but the effect of its incorporation on the solubility of these phases is unknown.



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