Hydrolysis contributions in U(VI) spectroscopic speciation in acetate media

Inorganica Chimica Acta 426 (2015) 113–118

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Hydrolysis contributions in U(VI) spectroscopic speciation in acetate media D. Kwiatek a,1, G. Meinrath b,⇑, S. Lis a,1 a b

Department of Rare Earth, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland RER Consultants, Fuchsbauerweg 50, D-94036 Passau, Germany

a r t i c l e

i n f o

Article history: Received 1 September 2014 Received in revised form 28 October 2014 Accepted 2 November 2014 Available online 10 December 2014 Dedicated to the memory of Prof. Heino Nitsche (Berlin, Berkeley, Dresden). Keywords: Uranyl species UV–Vis spectroscopy Stability constants Speciation Spectral deconvolution

a b s t r a c t An analysis of 17 UV–Vis absorption spectra is presented, collected in a region of the pH/lg(acetate) diagram where the UO2CH3COO+ species prevails. The study was conducted (a) to test a previously determined single component of the UO2CH3COO+ species, (b) to assess the impact of (UO2)2(OH)2+ 2 hydrolysis species into numerical peak deconvolution and (c) to study statistical properties of peak deconvolution procedures as a basis for measurement uncertainty assessment. Comparative speciation was performed for a two-species and three-species model, which resulted in different values of U(VI)-acetato formation quotients. The numerical analysis corroborated a detectability of (UO2)2(OH)2+ 2 at relative amounts >1%. Lower relative contributions remain ambiguous due to the large Pearson correlation of U(VI) UV–Vis single component spectra. The results of the study show the quantitative deconvolution of U(VI) spectra into + 2+ single components of UO2+ 2 , UO2(CH3COO) and (UO2)2(OH)2 species. Theoretical U(VI) concentrations are reproduced within a few percent. Ó 2014 Published by Elsevier B.V.

1. Introduction The complexation of U(VI) by acetate is a potentially relevant reaction of U(VI) in natural aqueous systems. This potential relevance has motivated a considerable number of studies not only but also in the past decade [1–4]. Our interest in the U(VI) acetate reaction has been mainly for spectroscopic speciation reasons, partly caused by the correlations observed between the coordination geometry of the equatorial sphere of the linear UO2+ 2 entity and its characteristic UV–Vis absorption band in the range 340 nm to 520 nm [5,6]. This interest has motivated us to provide a complete direct spectroscopic speciation of the U(VI) acetate reaction in the region pH 2 to pH 5 [7]. This pH range covers the relevant reaction steps in the U(VI)–acetate system and avoids possible interference, e.g. due to complexation by atmospheric carbonate at higher pH. The speciation effort should provide a set of UV–Vis spectra of relevant U(VI) acetato species and explore their applicability to quantitative direct speciation also in presence of unavoidable competing reaction, especially hydrolysis. Such an assessment is ⇑ Corresponding author. Tel.: +49 171 8744181. E-mail addresses: [email protected] (D. Kwiatek), [email protected] (G. Meinrath), [email protected] (S. Lis). 1 Tel.: +48 61 8291345. http://dx.doi.org/10.1016/j.ica.2014.11.027 0020-1693/Ó 2014 Published by Elsevier B.V.

currently available not or at best in a lacunary fashion. Thus this study is motivated by the following four objectives: (a) validating a previously derived single component spectrum of UO2CH3COO+, (b) apply this spectrum to the direct speciation of U(VI) aqueous solutions under conditions where theoretical speciation predicts both acetato complexation and hydrolysis of U(VI), (c) compare the UO2CH3COO+ single component spectrum to U(VI) spectra of other species (e.g. sulfato and carboxylato species) and (d) investigate the applicability of statistical tests to decide for the optimum model for data interpretation. 2. Experimental 2.1. Reagents The UV–Vis spectra were registered for the solutions with total concentrations of U(VI) in the range 9  104 mol dm3 and 1  102 mol dm3 and pH 1.9 to pH 5.0. Total acetate concentrations were studied between 0.003 mol dm3 and 0.14 mol dm3. From numerical speciation free acetate concentrations were calculated to vary between 7.7  105 mol dm3 and 5.8  102 mol dm3. The U(VI) perchlorate stock solutions were prepared from UO2(CH3COO)22H2O solid (CHMAPOL/LACHEMA Co. Warsaw/Poland). After dissolution in water and re-precipitation with H2O2 (20%), the yellow precipitate was filtered, washed and heated in a furnace at 200 °C

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(4 h) and 400 °C (8 h) giving the UO3 solid. For the next step this solid was dissolved in a stoichiometric amount of perchloric acid (70%, Fluka Co./Switzerland). A detailed procedure is given in Ref. [8]. Acetate concentrations were adjusted by stock solutions of 0.3 M NH4CH3COO (POCH Co./Gliwice/Poland). Numerical modeling of the sample solutions indicates that ionic strength of the samples varies between I = 0.01 and I = 0.3, hence is found outside the range of diluted solutions where the thermodynamic parameters become highly sensitive to small changes of composition. All experimentation, if not stated otherwise, was made at room temperature.

2.2. Apparatus and data collection A double-beam UV–Vis spectrometer (UV-2401 PC, Shimadzu Co./Japan) was used for collecting absorptions in the UV–Vis range. Spectra were recorded quadruply and averaged for noise reduction. Samples were placed in quartz cells with 20 mm path length and recorded digitally in the wavelength range 345–570 nm in 0.1 nm steps with a slit width of 1 nm. Determination of pH was made by glass combination electrodes (ELMETRON pH-meter Cp-315 Co. Zabrze/Poland) following the 5-point calibration scheme described by IUPAC [9,10]. The calibration pH standard solutions were traceable material (Merck Co Darmstadt/Germany).

2.3. Data analysis Speciation was made with the geochemical code PREEEQC [11] and the probabilistic speciation code LJUNGSKILE [12]. Thermodynamic data, if not given otherwise, are taken from the JESS Thermodynamic Database [13]. Spectral deconvolutions are made with custom-made codes based on the sequential Simplex [14] using least-squares criteria. Variance–covariance matrices and uncertainties of the spectral curves are estimated from quadratic forms in the minimum of the numerical optimum [15]. If not stated otherwise, uncertainties are given as 68% confidence intervals. For data derived from small sample sizes, the necessary corrections have been made to transform standard deviations into confidence regions. The uncertainties in spectral decompositions (cf. Figs. 3–5) are given on the .95 percentile range. None of the uncertainties represents a complete measurement uncertainty budget since only statistical contributions to uncertainty (e.g. misfit) are considered.

3. Results and discussion 17 UV–Vis spectra have been collected in the range pH 1.9 to pH 5.0 and total acetate concentrations in a range 1.5  101 mol dm3 to 3  103 mol dm3. Total U(VI) concentrations were varied between 9  104 mol dm3 and 102 mol dm3. The physical conditions of the samples are summarized in Fig. 1, which is calculated for an average U(VI) total concentration of 102 mol dm3. These 17 spectra are given by circles. A group of 14 spectra has been identified previously [7] as samples holding UO2CH3COO+ as only U(VI) species next to the ‘free’ UO2+ 2 ion which is generally assumed to be present as UO2(H2O)2+ 5 . These 14 spectra are given as open diamond-shaped symbols in Fig. 1. The theoretical speciation indicates that some of the newly collected 17 spectra are found in a region where hydrolysis of U(VI) may be detected. The hydrolysis of U(VI) with respect to its UV–Vis spectroscopic speciation is described in detail in [16–18] (and references herein) for both saturated and undersaturated systems. Interference by hydrolysis is easily neglected in U(VI) speciation studies because of its apparently marginal contributions in the lower percent regions. But because of the high molar absorption of (UO2)2(OH)2+ 2 , these small contributions may nevertheless have a notable effect on the observed UV–Vis absorption spectrum. For illustration the single component spectra of UO2+ 2 , UO2CH3 COO+ and (UO2)2(OH)2+ 2 are given in Fig. 2. Note the different right-hand-side (r.h.s.) and left-hand-side (l.h.s.) ordinate scaling. The molar absorption of the species in the characteristic UV–Vis absorption band of U(VI) are e = 9.7 dm3 mol1 cm1 at 413.8 nm 3 1 for UO2+ cm1 at 418 nm for UO2CH3COO+ 2 , e = 17.8 ± 1.0 dm mol [7], and e = 101 ± 2 at 421.8 dm3 mol1 cm1 for (UO2)2(OH)2+ 2 [16–18]. Fig. 2 illustrates that minor contributions below a relative concentration of 1% of (UO2)2(OH)2+ 2 could be interpreted numeri+ cally either as UO2+ 2 or UO2CH3COO without a major impact on the sum of residuals. Thus the numerical peak deconvolution would be biased slightly. On the other hand, inclusion of the (UO2)2(OH)2+ 2 single component spectrum will result in a bias if a (small) amount is considered, e.g. due to correlation, where no hydrolysis occurs. According to the law of mass action, a constant b11 can be defined as given in Eq. (1):

lg b11 ¼ lg R  lg½CH3 COO free

ð1Þ

where R = [UO2CH3COO+]/[(UO2+ 2 ] and [A] giving molar concentrations of species A. For a given spectrum, differences in R may result when comparing deconvolutions with two and three single

Table 1 Results of spectral deconvolution of 17 UV–Vis absorption spectra of U(VI) samples holding varying amounts of U(VI), acetate and H+. pH

lg[CH3COO]free

[U(VI)]total

[U(VI)]calc

D

[UO2+ 2 ] (A)

1.93 2.25 2.94 3.33 1.92 2.42 3.43 2.44 3.55 3.49 3.53 2.65 3.00 3.33 3.38 2.98 3.33

3.745 3.730 3.746 3.477 3.802 2.783 2.960 3.212 3.142 3.020 2.859 3.157 2.665 2.567 2.816 2.732 2.448

4.85  103 4.87  103 2.43  103 2.61  103 9.8  104 1.03  103 4.93  103 2.44  103 2.44  103 2.43  103 2.43  103 9.9  104 2.44  103 2.48  103 1.03  103 9.8  104 9.9  104

4.99  103 5.18  103 2.56  103 2.62  103 1.01  103 1.00  103 5.30  103 2.60  103 2.57  103 2.58  103 2.61  103 1.03  103 2.52  103 2.46  103 1.06  103 1.01  103 9.48  104

2.9 6.3 5.3 0.3 3.0 2.9 7.5 6.5 5.3 6.1 7.4 4.0 3.2 0.8 2.9 3.0 4.2

4.62  103 4.98  103 2.43  103 2.51  103 9.29  104 5.97  104 3.50  103 2.16  103 1.79  103 1.74  103 1.57  103 8.69  104 1.19  103 7.63  104 4.81  104 6.94  104 3.06  104

(93) (97) (96) (99) (81) (60) (67) (84) (71) (69) (61) (86) (48) (31) (46) (72) (33)

[UO2CH3COO+] (A)

[(UO2)2(OH)2+ 2 ] (A)

lg b11

3.27  104 (7) 9.63  105 (2) 8.49  105 (3)

2.02  105 2.45  105 1.18  105 5.70  105

2.59 2.02 2.29



1.15  104 3.73  104 1.52  103 3.88  104 6.87  104 7.44  104 9.27  104 1.35  104 1.28  103 1.67  103 5.43  104 2.59  104 6.23  104

(19) (38) (30) (15) (27) (30) (36) (13) (51) (69) (53) (27) (67)

(0.4) (0.5) (0.5) (1.1)

 5

1.74  10 (1.8) 7.15  105 (1.4) 1.27  105 (0.5) 5.23  105 (2.0) 2.40  105 (1.0) 6.14  105 (2.4) 1.30  105 (1.2) 2.62  105 (1.0) 1.16  106 (0.4) 1.9  105 (1.7) 2.88  105 (1.5) 9.5  106 (1.0)

lg K22

5.71 (a) 2.91 2.58 2.60 2.47 2.73 2.65 2.63 2.35 2.69 2.91 2.87 3.09 2.76

5.80 (b) 5.89 (c) 5.78 (d) 5.66 (e)

All concentrations in mol dm3; D: difference between the theoretical and the calculated U(VI) concentration in%; [X]: concentration of species X; (A): relative (percent) amount of species A (differences to 100 due to rounding).

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0.08

[U(VI)] th : 4.9 . 10-3 mol dm-3

(b)

[U(VI)]calc: 5.3 . 10-3 mol dm-3 pH 3.43 lg [CH3COO-]free: -2.96

Fig. 1. Constraints in the system U(VI) acetate in the range pH 1–5 and total acetate concentrations varying from 104 mol dm3 and 1 mol dm3. Diamond-shaped symbols indicate locations of experimental samples from a previous study [7]. Circles show additional evidence from this work. The filled circles annotated a–e correspond to those samples where hydrolysis has been speciated.

absorption / [cm-1]

0.06 A : UO2+ 2 B : UO2CH3COO+ C : (UO2)2(OH)22+

0.04 A lg R : -0.36 lg ß11 : 2.60 lg K22 : -5.79

0.02

B

C

120 0.00

20

A2049

350

400

450

500

wavelength / [nm] 18

B

14

C

80

12 60

10

8 40

A

6 4

molar absorption / [L mol -1 cm-1]

16

molar absorption / [L mol -1 cm-1]

350

100

20

2 0

0 350

400

450

500

wavelength / [nm] Fig. 2. Single component spectra of the three species of interest: UO2+ (A), 2 UO2CH3COO+ (B), (UO2)2(OH)2+ 2 (C). Note the different ordinate scales.

component spectra. It may be argued that the difference between the two-species and the three-species model might be suitable for a statistical analysis using an F-test. The both models are nested, because the two-species model might be interpreted as a reduction of the three-species model. Thus, the three-species model would be accepted if the respective F ratio is larger than the critical value tabulated for a given confidence level and given degrees of freedom [19]. In case of UV–Vis spectra, however, the requirements for an

400

450

500

0.003 0.000 -0.003 Fig. 3. Experimental UV–Vis spectrum at pH 3.43 numerically deconvoluted into single components. The total acetate concentration is 0.02 mol dm3. Dotted lines give 0.95 percentiles confidence limits. Residuals are given in the lower window. The spectrum is denoted (b) in Table 1.

F-test are not fulfilled. The F-test is based on the assumption of independently and identically distributed (i.i.d.) residuals. The parameters (here the single component spectra) must be uncorrelated. It has been shown for U(VI) spectra previously in detail that correlation coefficients q are high (|q| > 0.8) and residuals are not i.i.d [20]. In contrary, the spectral residuals explicitly depend on their neighbors. This has been shown by appropriate statistical tests and is the rational of threshold bootstrap methods to simulate UV–Vis spectra [20,21]. When non-linearity, non-normality and correlation are taken into account, the uncertainty regions in n-dimensional parameter space are not ellipsoids any longer but irregular shaped confidence regions will result [22]. The respective statistical analyses have been repeated with present experimental spectra and the results will not be reiterated here. The 17 UV–Vis spectra given as circles in Fig. 1 serve multiple purposes. First, the capability of previously derived single component spectrum of UO2CH3COO+ to interpret appropriate multicomponent UV–Vis spectra is demonstrated using experimental data not applied for its derivation. Second, the speciation diagram is critically analyzed for its predictive capabilities by searching for a minor component, here (UO2)2(OH)2+ 2 species. In Fig. 1 the hatched region is characterized by relative amounts above 1% (UO2)2(OH)2+ 2 . Towards higher pH, thermodynamic modeling suggests a joint region where both UO2(CH3COO)2° and UO2(CH3COO) 3 may have relative amounts above 10% each. Thus, on basis of a formation constants lg b12 = 4.7 for UO2(CH3COO)2 and lg b13 = 5.1 for UO2(CH3COO) 3 [13], a species UO2(CH3COO)2° would occur in

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0.04

0.04 [U(VI)]th : 2.44 . 10-3 mol dm-3

(c)

.

-3

[U(VI)] th : 2.43 . 10-3 mol dm-3

(d)

-3

[U(VI)]calc : 2.57 10 mol dm

[U(VI)] calc : 2.58 . 10-3 mol dm-3

pH 3.55 lg [CH3COO-]free : -3.14

pH 3.49 lg [CH3COO-]free : -3.02

0.03 A : UO2+ 2 +

B : UO2CH3COO

C : (UO2)2(OH)22+

0.02 A lg R : -0.42 lg ß11 : 2.73

absorption / [cm-1]

absorption / [cm-1]

0.03

A : UO2+ 2 B : UO2CH3COO+ C : (UO2)2(OH)22+

0.02 A lg R : -0.37 lg ß11: 2.73 lg K 22: -5.78

lg K22 : -5.89

0.01

B

0.01

B

C

C

0.00

A2703

350

0.00 400

450

500

350

wavelength / [nm] 350

400

450

400

450

500

wavelength / [nm] 500

350

0.0010

0.0010

0.0005

0.0005

0.0000

0.0000

-0.0005

-0.0005

-0.0010

-0.0010

400

450

500

Fig. 4. Experimental UV–Vis spectrum at pH 3.43 numerically deconvoluted into single components. The total acetate concentration is 0.02 mol dm3. Dotted lines give 0.95 percentiles confidence limits. Residuals are given in the lower window. The spectrum is denoted (c) in Table 1.

Fig. 5. Experimental UV–Vis spectrum at pH 3.55 numerically deconvoluted into single components. The total acetate concentration is 0.02 mol dm3. Dotted lines give 0.95 percentiles confidence limits. Residuals are given in the lower window. The spectrum is denoted (d) in Table 1.

solution only together with considerable amounts of UO2CH3COO+ and UO2(CH3COO) 3 . To avoid complications at this stage of the study, conditions have been selected where the higher acetato species of U(VI) could be assumed to be negligible. The UV–Vis spectra of these samples, however, had to be quantitatively interpreted by the three single components given in Fig. 2. Results are given in Tables 1 and 2. Table 1 specifies solution compositions, the percentual difference D between analytical and calculated U(VI) total concentrations, deconvoluted concentrations of single components together with the percentual contribution of each species in brackets, the respective experimental formation constants b11 for the three species model according to Eq. (1). In the last column, the formation constant K22 is given for five samples where the calculated value indicated a realistic estimate of the hydrolysis dimer. In all other cases, K22 was calculated one or two orders of magnitude to high – never too low. For illustration purposes, three of the five solutions spectra and their respective quantitative deconvolution into single components are given in Figs. 3–5. All three samples hold small relative amounts of the hydrolysis species that nevertheless contributes considerably to the absorption. These spectra would be interpretable by a two-species model with considerable misfit only. While at occasions a more or less acceptable interpretation of the experimental spectral curve by the two-component model might be possible, the parameters calculated from the fitting results would be inacceptable. Both the calculated total U(VI) concentration and the concentration quotient R are not fitted but calculated from the fitted spectral curves. In many cases the misfit would

be large enough to cause the fitting routine, a Simplex algorithm [14], to fail convergence. The three-component model results in an excellent agreement between theoretical and calculated U(VI) total concentrations and a homogeneous value of lg b11 given in Table 2. Using chemical arguments, we realize that the formation constant lg K22 for the formation of (UO2)2(OH)2+ 2 derived from the spectra is much too high as long as the relative amount of this species is below 1% The spectra at higher dimer concentration give 5.6 < lg K22 < 5.9. Given the scatter of values for lg K22 in literature, this is a quite reasonable range (cf. Ref. [39] Fig. 8). A chemist may conclude that a relative concentration > 1% is required for (UO2)2(OH)2+ 2 to be detectable in spectral deconvolution while the lower concentrations are numerical artifacts due to the high correlation and the increased number of degrees of freedom. Table 2 summarizes the obtained values for lg b11 for the three species model. The first group of 14 values has been calculated for the previous data [7]. For this group of data a pooled lg b11 of 2.67 ± 0.16 has been obtained. For comparison, the respective results for the two component model (species UO2+ 2 and UO2CH3COO+) are given at the right side. The observed bias due to model differences is 0.18 and thus within the magnitude of the overall uncertainty. It illustrates that model selection is a relevant factor contributing to the complete measurement uncertainty budget. It underscores a previous statement that formation constants from spectroscopic speciation of f-elements should be associated with a measurement uncertainty u of at least u = 0.15 (k = 1) [23].

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D. Kwiatek et al. / Inorganica Chimica Acta 426 (2015) 113–118 Table 2 Formation quotients lg b11 for UO2CH3COO+ calculated from concentration ratios R of +  UO2+ 2 and UO2CH3COO species and the free CH3COO concentrations for 31 samples (right-side data gives results for two-component analysis [7]; left-side data shows results with consideration of acetate complexation and hydrolysis). lg b11

lg R

lg[Ac]free

lg b11

lg R

1.90 2.24 2.42 2.52 2.54 2.56 2.65 2.73 2.80 2.88 2.89 2.96 3.24 3.43

2.48 2.57 2.57 2.39 2.81 2.64 2.61 2.86 2.56 2.66 2.77 2.89 – 2.88 2.67 ± 0.16 2.59 2.02 2.29 – 2.91 2.58 2.60 2.47 2.73 2.65 2.63 2.35 2.69 2.91 2.87 2.30 2.76 2.62 ± 0.21

1.15 0.96 0.52 1.59 1.32 1.60 0.40 +0.10 1.02 0.30 0.72 +0.19 – +0.32

3.629 3.656 3.090 3.979 4.127 4.119 3.006 2.759 3.582 2.996 3.492 2.709 3.952 2.562

2.67 2.72 2.69 2.78 2.93 2.66 2.74 2.99 2.83 2.74 2.89 3.02 3.11 3.19 2.85 ± 0.17

0.96 0.93 0.397 1.20 1.20 1.46 0.26 +0.23 0.75 2.53 0.60 +0.31 0.84 +0.66

1.15 1.71 1.46 – 0.89 0.20 0.36 0.75 0.42 0.37 0.23 0.81 +0.03 +0.34 +0.05 0.43 +0.31

3.745 3.730 3.746 3.477 3.803 2.782 2.960 3.212 3.142 3.020 2.859 3.157 2.665 2.567 2.816 2.732 2.448

1.93 2.25 2.94 3.33 1.92 2.42 3.43 2.44 3.55 3.49 3.53 2.65 3.00 3.33 3.38 2.98 3.33

lg ([UO 2(CH3COO)+]/[UO 2+ 2 ])

B: UO2 (IsoNicNO)+ C: UO2 (CH3COO)+ [5] D: UO2 (CH3COO)+ [7] E : UO2 SO4°

25

E 20

D 15

C B

10

A 5

0 350

400

450

500

wavelength [nm] Fig. 7. Comparison of UV–Vis single component spectra for various U(VI) species with monoligands in aqueous solution. The spectrum C suggested in Ref. [5] for UO2CH3COO+ clearly differs from the spectrum D derived in Ref. [7] for the same species. The spectrum B derived for U(VI)-isonicotinic acid N-oxide [39] also agrees with spectrum D. The difference in the absorption band to the UV region below 375 nm shows the important information in this region of the spectrum. The spectrum E given for the U(VI) monosulfato species [40], however, clearly differs because sulfate cannot link to U(VI) as a bidentate.

-1.5 -2.0

A: UO22+

molar absorbance / [l mol-1 cm-1]

pH

30

lg ß11 = 2.62 ± 0.04

-2.5 -3.0

the numerical values are quite similar but the estimates of uncertainty differ considerably. This is mainly caused by fixing the slope of the regression line to its theoretical value.

-3.5 -4.0

: experimental data : mean (slope fixed at unity) : .95 confidence limits of slope : .95 confidene limit of prediction

-4.5 -5.0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

lg [CH 3COO ]free

Fig. 6. Interpretation of the numerically determined species ratios R by the calculated free acetate (CH3COO) concentration. The double-logarithmic plot yields formation constant lg b11 as the y-intercept with the theoretical slope of 1. Dashed lines give 0.95 percentile uncertainty of the slope. The dotted lines give 0.95 percentile uncertainties of predicting a further point.

Pooling all 29 data pairs, a value lg b11 = 2.62 ± 0.21 is found. In all cases, already the first decimal is uncertain. In Fig. 6 the calculated concentration ratios R are shown as a function of the calculated free acetate concentration for all available 29 samples. These data are interpreted by a straight line of unit slope whose y-axis intercept is an estimate of lg b11. Comparing the values for lg b11 from Table 2 with the estimate from Fig. 6,

4. Conclusions The large quantity of publications directed to the U(VI)–acetate interaction is surprising when compared to the rather minor field of predominance indicated by Fig. 1 and the rather low formation constant lg b11 = 2.8. Taking into account that natural aqueous systems are defined by range between pH 4 and pH 9, and total acetate concentrations below 106 mol dm3 [24–26], acetic acid is quite unlikely to coordinate to U(VI) in natural waters. This does not preclude that under exceptional circumstances very high acetate concentrations may occur, especially under human influence. Extreme acetate concentrations are, e.g., reported from oil field brines [27,28]. These are, nevertheless, rather special local spots. The interest in U(VI) acetate complexation therefore is almost always motivated by its likely influence on U(VI) transport in the geosphere. Uranium in nature is often associated with organic materials, e.g. humic and fulvic acids and other naturally occurring organic materials [29]. The carboxylic groups in these heterogeneous materials are generally considered as relevant functional

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groups for uranium bonding [29–31]. The direct study of uranyl humate complexation is associated with several difficulties requiring ancillary assumptions. Proposed models are not free from contradictions [31]. Here, acetate is considered as a model substance. The coordination of the U(VI) entity in aqueous solution is rather well understood. U(VI) is exclusively coordinated in the plane equatorial to the axial uranyl oxygens by four, five or six ligands. The large number of structural studies in solid and in aqueous phase (e.g. [27,32–35]) confirms that the uranyl entity allows little variation in the bond lengths and bonding angles for forming a stable bond. In natural organic materials, the bonding sites will rarely be arranged in a way suitable to satisfy these requirements. Hence, the interaction of primary interest is the formation of the U(VI) monoacetato species. For this reason, special attention has been directed towards this interaction. In Fig. 7, the single component spectrum of UO2CH3COO+ (spectrum D) is compared to a spectrum (spectrum C) assigned to this species from Ref. [5]. In fact, the UV–Vis absorption spectrum C is the only absorption spectrum given for this species covering the complete wavelength range of interest to our knowledge. To allow comparison, the spectrum originally given in wavenumbers has been digitized and transformed to a wavelength scale. The comparison shows that the spectrum published 40 years ago has a comparable molar absorption. The spectral resolution, however, is not adequate any more. Unfortunately, no information could be retrieved providing further information on its determination. Spectrum C has been forwarded in the framework of studies correlating characteristics of the UV–Vis absorption spectra, mainly of solid U(VI) compounds, with the coordination geometry about the uranyl entity [5,36–38]. The U(VI) monoacetato species was considered to have a Cs geometry assuming a bidentate coordination of the acetato ligand. The single component spectrum of the U(VI) isonicotinic acid N-oxide species is given [39] in Fig. 7 as curve B for comparison. The formation constant is similar with lg b11 = 2.1. The spectral curve of the characteristic U(VI) absorption band are also quite similar for both carboxylato species. The major difference is in the absorption band to the UV-region. The bathochromic shift is stronger for the isonicotinic acid N-oxide ligand. Hence the wavelength region below 400 nm is an essential component of a UV–Vis absorption spectrum of a U(VI) species. The similarity between the carboxylato species B and D is not granted. For illustration, the single component spectrum of the monosulfato species UO2SO4° [40] is shown. While the bond angle for the coordinating acetato oxygens in the carboxylato groups is <60° (‘short bite’) and bidentate coordination is possible, the oxygens in the sulfate ligand does not allow for a ‘short bite’ coordination. The resulting UV–Vis absorption spectrum is quite different from the carboxylato species B and D. The direct spectroscopic speciation of the U(VI) acetate interaction is in satisfactory agreement with the numerical speciation shown in Fig. 1. The 31 spectra discussed in this work cover the complete range of the region where UO2CH3COO+ is the predominant species. Nevertheless, the highest relative amount of the monoacetato species is short below 70%. It seems unlikely that samples (almost) pure in UO2CH3COO+ can be fabricated in aqueous solution. Samples free of hydrolysis species can be produced at high total acetate concentrations with [CH3COO(H)] > 102 mol dm3. Formation of the higher acetato species, e.g. UO2(CH3COO)2 and UO2(CH3COO) 3 , have a still smaller field of predominance because total acetate concentrations with [CH3COO(H)] > 101 mol are

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