Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy

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Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy Katja Schmeide *, Susanne Sachs, Marianne Bubner, Tobias Reich, Karl Heinz Heise, Gert Bernhard Forschungszentrum Rossendorf e.V., Institute of Radiochemistry, P.O. Box 510119, D-01314 Dresden, Germany Received 23 October 2002; accepted 29 January 2003

Abstract The complexation of uranium(VI) by humic acids (HAs) and fulvic acids (FAs) was studied to obtain information on the binding of uranium(VI) onto functional groups of humic substances. For this, various natural and synthetic HAs were chemically modified resulting in HAs with blocked phenolic OH groups. Both from the original and from the modified humic substances, solid uranyl humate complexes were prepared at pH 2. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to study the chemical modification process of humic substances, to study the structure of uranyl humate complexes and to evaluate the effect of individual functional groups of humic substances (carboxylic and phenolic OH groups) on the complexation of uranyl ions. The results confirmed the predominant blocking of phenolic OH groups in the modified HAs. These modified HAs are suitable model substances to study the role of phenolic OH groups of HAs in dependence on pH. By EXAFS spectroscopy, identical ˚ were determined. In the structural parameters were determined for all uranyl humates. Axial U
/O bond distances of 1.78 A ˚ . The blocking of phenolic OH groups of equatorial plane approximately five oxygen atoms were found at a mean distance of 2.39 A HAs did not change the near-neighbor surrounding of uranium(VI) in uranyl humate complexes. Thus, the results confirmed that predominantly HA carboxylate groups are responsible for binding of uranyl ions and that the influence of phenolic OH groups is insignificant under the applied experimental conditions. The carboxylate groups are monodentate coordinated to uranyl ions. # 2003 Elsevier B.V. All rights reserved. Keywords: Uranium; Humic acids; Fulvic acids; Complexation; EXAFS; FTIR

1. Introduction Humic substances, such as humic acids (HAs) and fulvic acids (FAs), are known to influence the speciation and thus, the mobility of radionuclides in the environment due to their strong complexing ability [1,2]. Therefore, risk assessments, predicting the fate and transport of actinides in the environment, require basic knowledge of the interaction of humic substances with metal ions. A lot of studies of the complexation of

* Corresponding author. Tel.: /49-351-260 2436; fax: /49-351-260 3553. E-mail address: [email protected] (K. Schmeide). 0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0020-1693(03)00184-1

radionuclides, such as uranium, by humic substances have been performed applying various methods (e.g., [3
/8]). However, due to the chemical and structural heterogeneity of humic substances, the nature of metal complexation sites in humic substances is still uncertain [9]. Carboxylic groups are often considered the only functional groups of humic substances responsible for the complexation of metal ions at pH values below 9 (e.g. [10,11]). Other authors suggest that further functional groups of humic substances, such as phenolic, enolic, and alcoholic OH groups [7,9,12
/14] as well as amino groups [15,16], can also contribute to the complexation of metal ions.

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For the contribution of phenolic and other acidic OH groups to the interaction between original HAs and metal ions there are in principle several possibilities. At low pH value, where the phenolic OH groups are protonated due to their high pKa values, intermolecular hydrogen bonds between hydrogen atoms of phenolic OH groups and oxygen atoms of uranyl ions can contribute to the complexation process in addition to carboxylic groups. Phenolic OH groups can also be deprotonated when coordinated to metal ions, thereby is their acidity influenced by mesomeric substituent effects. Furthermore, six- or five-membered chelate rings of high stability can be formed by phenolic OH groups that are ortho -positioned to carboxylic groups or by neighboring phenolic OH groups. For instance, Roßberg et al. [17] determined structural parameters for chelate complexes between uranium(VI) and catechol (1,2dihydroxybenzene) and pyrogallol (1,2,3-trihydroxybenzene) at pH 5. Bartusˇek and Ruzˇicˇkova´ [18] found that the complex formation between uranium(VI) and catechol, resorcin (1,3-dihydroxybenzene) and phenol starts at about pH 3. The ability to form chelates at low pH values is also reported for aliphatic OH groups in asubstituted carboxylic acids, which are less acidic than aromatic OH groups. For instance, Szabo´ and Grenthe [19] found for the uranium(VI) glycolate system that a chelate is formed by proton dissociation from the a-OH group at around pH 3, indicating a very large increase, a factor of at least 1013, of its dissociation constant on coordination to uranium(VI). In a recent study of uranium(VI) complexation by HA at pH 4 by time-resolved laser-induced fluorescence spectroscopy Pompe et al. [14] have shown that, already at pH 4, phenolic OH groups contribute to the interaction between HA and uranyl ions. This shows that the nature of HA functional groups, responsible for binding of metal ions, should be studied systematically in dependence on pH. In this paper, we studied the complexation of uranium(VI) by humic substances at pH 2. In addition to original natural and synthetic HAs and FAs, we employed chemically modified HAs with blocked phenolic OH groups as model substances. The comparison of the complexation behavior of the original and modified HAs enables the assessment whether or not phenolic OH groups of HAs contribute to the complexation of uranyl ions. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied for this study. FTIR spectroscopy was applied to verify the blocking of phenolic OH groups in the modified HAs and to study the binding of uranyl ions onto HA functional groups. EXAFS spectroscopy was used to determine structural parameters of the nearneighbor surrounding of uranium(VI) complexed by humic substances.

2. Experimental 2.1. Humic substances Three natural and two synthetic humic substances were applied. The natural humic substances were Kranichsee HA (KHA) and Kranichsee FA (KFA) that were isolated from surface water of the mountain bog ‘Kleiner Kranichsee’ (Johanngeorgenstadt, Saxony, Germany) [20] as well as the commercially available Aldrich HA (AHA; Aldrich, Steinheim, Germany). AHA was purified prior to use according to the literature [21]. The synthetic HA model substances were the HAs type M1 [22] and type M42 [5]. Modified HAs with blocked phenolic OH groups [14] were synthesized starting from the original HAs type KHA, AHA, M1 and M42. In a first step, HA functional groups (carboxylic and phenolic/acidic OH groups) were methylated. For this, methanolic suspensions of original HAs were stirred with diazomethane, which was previously prepared from Diazald† (SigmaAldrich, Steinheim, Germany), at /5 to 5 8C for 3 h. Then, the solvent was distilled off. The methylation procedure was repeated several times until no further diazomethane was incorporated into the HA. The solvent that was distilled from the reaction mixture was then yellow colored due to unreacted diazomethane. The resulting HAs were lyophilized. The methylated HAs are termed HA-B. In a second step, the methyl ester groups were hydrolyzed by reacting the methylated HAs with 2 M NaOH (Merck, Darmstadt, Germany) under inert gas at room temperature (r.t.) for 8 h. The alkali-insoluble residue was separated by centrifugation. The modified HAs were precipitated with 2 M HCl (Merck) and separated by centrifugation. The washed, dialyzed (Thomapor† , MWCO B/1000, Reichelt Chemietechnik, Heidelberg, Germany) and lyophilized products were applied for this study. The HAs with blocked phenolic OH groups are termed HA-PB. It is to note that in addition to phenolic OH groups also other acidic OH groups of the HA, i.e. OH groups substituted to five-membered heterocycles or enolic OH groups can be methylated with diazomethane leading to nonhydrolizable methyl ether groups [14]. All types of HAs (HA, HA-B, HA-PB) were characterized for their functional group content (cf. Table 1). 2.2. Preparation of uranyl humate complexes Uranyl humate complexes were prepared from natural and synthetic unmodified humic substances (KHA, KFA, AHA, M1, M42) and from the corresponding modified HAs with blocked phenolic OH groups (KHAPB, AHA-PB, M1-PB, M42-PB). Solid uranyl humate complexes were prepared by reacting aqueous HA suspensions that were previously

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Table 1 Functionality of humic substances and uranyl loading of humic substances in uranyl humates Humic substance Functionality

KHA KHA-B KHA-PB AHA AHA-B AHA-PB M1 M1-B M1-PB M42 M42-B M42-PB KFA a b c d

Uranyl humate

Uranyl loading

Phenolic OH a (mequiv. g 1)

COOH b (mequiv. g 1)

Phenolic OH/COOH

mg U g 1 HA % COOH c

3.99/0.5 0.3 1.69/0.4 3.49/0.5 0.69/0.3 1.19/0.4 2.49/0.1 0.3 0.99/0.3 2.39/0.4 d 0.69/0.1 d 0.69/0.3 d 4.89/0.7

4.209/0.17 B/0.1 a 2.839/0.05 4.419/0.11 B/0.1 a 3.259/0.05 1.349/0.05 B/0.1 a 1.169/0.03 4.109/0.10 B/0.1 a 3.219/0.08 6.059/0.31

0.93

UO2
/KHA

0.57 0.77

UO2
/KHA-PB 50.2 UO2
/AHA 99.8

15 19

0.34 1.79

UO2
/AHA-PB 70.4 UO2
/M1 28.9

18 18

0.78 0.56

UO2
/M1-PB UO2
/M42

24.1 89.3

17.5 18

0.19 0.79

UO2
/M42-PB UO2
/KFA

60.6 118.5

16 16.5

110.2

22

Radiometrically determined [26]. Determined by calcium acetate exchange [27]. Calculated on the assumption that one uranyl ion occupies two proton exchanging sites of the HA molecule. Acidic OH groups.

degassed under vacuum, with 0.1 M uranyl perchlorate solutions at pH 2. The pH value of 2 of the HA suspension was achieved by adjusting the pH in the supernatant repeatedly with 0.1 M HClO4 or 0.1 M NaOH. For these pH adjustments, the supernatant was separated from the humic material each time. Finally, the reaction products were isolated by centrifugation, washed with ultrapure water, dialyzed (Thomapor† , MWCOB/1000), and lyophilized. The uranium content of the uranyl humates was determined by ICP-MS ¨ berlingen, Ger(Mod. ELAN 5000, Perkin
/Elmer, U many) after digestion of the samples with HNO3 in a microwave oven. 2.3. FTIR measurements The samples were dispersed in KBr (Uvasol† , Merck) and pressed as 13 mm diameter pellets. The FTIR spectra were recorded using a FTIR spectrometer (Mod. SPECTRUM GX 2000, Perkin
/Elmer Ltd., Beaconsfield, Buckshire, UK) in the middle infrared region (4000
/400 cm1) at r.t.

collected from each sample and averaged to improve the signal to noise ratio. The first inflection point at the Zr K-edge at 17 998 eV was used for energy calibration. The U LIII ionization threshold, E0, was defined as 17 185 eV. Data analysis was performed according to standard procedures [23] using the EXAFSPAK software [24]. The program FEFF6 [25] was used to calculate theoretical scattering amplitudes and phase-shift functions. The EXAFS oscillations were fitted using a two-shell fit with axial and equatorial oxygen atoms (Oax, Oeq) as ˚ 1. backscatterers in the k -range between 2.8 and 16.7 A The multiple scattering along the uranyl unit (Oax
/U
/ ˚ was also included into the fit. The Oax) at 3.6 A coordination number of the axial oxygen atoms in the uranyl group (Nax) and the shift in threshold energy (DE0) were held constant at 2 and /13.6 eV, respectively.

3. Results and discussion

2.4. EXAFS measurements and data analysis

3.1. Characterization of humic substances and of uranyl humate complexes

The samples were dispersed in Teflon and pressed as 13 mm diameter pellets. The uranium content of the resulting pellets was 11
/22 mg uranium. The EXAFS measurements were carried out at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. U LIII-edge X-ray absorption spectra were collected in transmission mode at r.t. A Si(111) double-crystal monochromator was used in channel-cut mode. Four EXAFS scans were

The functional group contents of the various humic substances are compiled in Table 1. Due to methylation of HAs, methyl ester and methyl ether groups are formed from carboxylic groups and phenolic/acidic OH groups, respectively. As a result the carboxylic and phenolic OH groups of the methylated HAs (HA-B) are almost completely blocked. Due to saponification of the methylated HAs, the methyl ester groups are hydrolyzed whereas the phenolic OH groups should

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remain blocked. However, Table 1 shows that, compared with HAs type HA-B, the number of phenolic OH groups increased somewhat during the saponification step. This may be attributed to an unfolding of HA molecules thereby uncovering functional groups that were previously sterically hindered. The obtained modified HAs (HA-PB) have phenolic OH group contents that are 59
/74% lower than those of the unmodified HAs. That means that the phenolic OH groups cannot completely be blocked by methylation of the HAs. The carboxylic group contents of the modified HAs (HAPB) are somewhat lower than those of the unmodified HAs. Possible reasons are a partial decomposition of HA molecules in acid-soluble components and/or leaching of smaller HA molecules with a higher carboxylic group content from the HA mixture [14] or an incomplete hydrolysis of the methyl ester groups that were previously formed during methylation with diazomethane (cf. Section 3.2). Nevertheless, Table 1 shows that the molar ratio of phenolic OH to carboxylic groups becomes always significantly smaller due to the modification process. The loading of HAs with uranyl ions in the uranyl humates was between 15 and 22% of the carboxylic group capacity of the HAs (Table 1). The uranyl loading of HAs, expressed in (mg U g1 HA) or related to the carboxylic group content (% COOH), is always lower for the HAs with blocked phenolic OH groups (HA-PB) than for unmodified HAs. It mainly correlates with the carboxylic group content of the HAs, and to a small extent with their phenolic OH group content. 3.2. FTIR spectroscopy Exemplary for all natural and synthetic HAs the FTIR spectra of the original and modified HAs type

Fig. 1. FTIR spectra of HAs type KHA (original HA), KHA-B (methylated HA) and KHA-PB (HA with blocked phenolic OH groups). For clarity, the spectra have been shifted along the y -axis.

KHA are shown in Fig. 1. The spectrum of the original KHA is discussed in detail in Schmeide et al. [6,20]. Thus, in the following only the IR absorption bands pointing to the formation of methyl ester and/or methyl ether groups in the spectra of the modified HAs are discussed as well as absorption bands confirming the complexation of uranium. Compared with the spectrum of KHA the intensity of the absorption bands at 2950 and 2852 cm 1 (aliphatic C
/H stretching) as well as 1457 and 1439 cm 1 (C
/H deformation of CH3 groups) is increased in the spectrum of KHA-B. This is attributed to the formation of methyl ether and ester groups upon methylation of KHA with diazomethane (cf. Section 2.1). In addition, the broad band centered at about 3424 cm 1 (O
/H stretching vibrations) is smaller and its intensity is somewhat decreased. The band near 2630 cm 1 (OH stretching of COOH), present in the spectrum of KHA, is absent in the spectrum of KHA-B. The absorption band at 1720 cm 1 (C /O stretching of carboxylic groups), observed in the spectrum of KHA, is shifted to 1735 cm 1 and its intensity is significantly increased. This indicates the formation of ester groups that absorb at higher wave numbers than carboxylic groups [27]. The intensity of the absorption band caused by C
/O stretch at 1205 cm 1 is also increased. Furthermore, the intensity of the bands and shoulders at 1106, 1153 and 1260 cm 1 is enhanced which is attributed to the formation of ether groups. The hydrolysis of the methylated HA (KHA-B) leads to KHA-PB. The band near 2630 cm 1 (OH stretching of COOH) is again present in the spectrum. Furthermore, the absorption band at 1735 cm 1 (C /O stretching of ester groups), observed in the spectrum of KHAB, is shifted back to 1721 cm 1 (C /O stretch vibration of carboxylic groups) and its intensity is decreased in the spectrum of KHA-PB. This shows that the ester groups present in KHA-B are hydrolyzed by reacting the methylated HAs with NaOH. However, compared with KHA, the intensity of the absorption band at 1721 cm 1 relative to the intensity of the band at 1617 cm 1 as well as the intensity of the band at 1207 cm 1 is somewhat higher in the spectrum of KHA-PB. This could mean that the ester groups formed during the methylation are not completely hydrolyzed. This is supported by the somewhat lower COOH content of KHA-PB compared with KHA (cf. Table 1). The intensity of the bands at 2938, 2852, and 1457 cm 1 is lower in the spectrum of KHA-PB compared with the spectrum of KHA-B, but higher than in the spectrum of KHA. Furthermore, the bands and shoulders at 1110, 1153 and 1260 cm 1 are somewhat enhanced in the spectrum of KHA-PB compared with those in the spectrum of KHA. This shows that phenolic OH groups remained blocked during the hydrolysis.

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These FTIR spectroscopic results have verified the formation of methyl ester and methyl ether groups due to methylation of the original HA with diazomethane and furthermore, the hydrolysis of the methyl ester groups due to saponification of the methylated HA leading to HA with blocked phenolic OH groups. This result is supported by a 13C-CP/MAS NMR study [28] which showed in addition, that the applied modification procedure causes only the desired structural changes in the HA, i.e., that the overall structure of the HA is not significantly changed. The FTIR spectra of the uranyl humate complexes of KHA and KHA-PB are shown in Fig. 2. Compared with the corresponding free HAs (cf. Fig. 1), a new absorption band occurs at 934 cm1 which is attributed to the asymmetric stretching vibration of UO22. The intensity of the absorption band at about 1720 cm 1 (C /O stretch vibrations of COOH) relative to the band at about 1620 cm 1 is decreased since a part of the COOH groups is converted to the COO  form during the complexation process. Simultaneously, the intensity of the band between 1580 and 1510 cm 1 (COO  asymmetric stretching) and that of the band at 1384 cm 1 (COO  symmetric stretching) is increased. The lower uranyl loading of the HA with blocked phenolic OH groups (cf. Table 1) is visible in the lower intensity of the band at 934 cm1 in the spectrum of UO2
/KHA-PB compared with that of UO2
/KHA. The difference between the asymmetric and the symmetric stretching frequency of COO, which is about 150 cm 1 for both uranyl humate complexes, is comparable to that generally observed for monodentate carboxylate coordination [29]. The intensity differences of the bands and shoulders in the region between 1280 and 1070 cm 1, that were previously assigned to phenolic OH groups (mainly at 1260 and 1092 cm 1), in the spectra of uranyl humates

Fig. 2. FTIR spectra of uranyl complexes of KHA and KHA-PB. For clarity, the spectra have been shifted along the y -axis.

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compared with those in the spectra of the respective free HAs are too small to observe a contribution of the phenolic OH groups to the complexation of uranyl ions. An overlapping of bands of this region caused by different functional groups make interpretation difficult. The results obtained from the FTIR spectra of original and modified HAs type AHA, M1 and M42 and of their corresponding uranyl humates are comparable to those obtained for KHA.

3.3. EXAFS spectroscopy Representative of all uranyl humates, the U LIII-edge k3-weighted EXAFS spectra and the corresponding Fourier transforms of the uranyl humates of HA type KHA, KHA-PB, M1 and M1-PB are shown in Fig. 3. Both the EXAFS oscillations and the Fourier transforms of all uranyl humate complexes are similar.

Fig. 3. Raw U LIII-edge k3-weighted EXAFS spectra (a) and corresponding Fourier transforms (without phase corrections) (b) of uranyl complexes with KHA, KHA-PB, M1 and M1-PB. Solid lines represent experimental data and dashed lines the best fit of the data. For clarity, the spectra have been shifted along the y -axis.

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Table 2 EXAFS structural parameters of uranyl humate complexes Sample

UO2
/KHA UO2
/KHA-PB UO2
/AHA UO2
/AHA-PB UO2
/M1 UO2
/M1-PB UO2
/M42 UO2
/M42-PB UO2
/KFA

U
/Oax

U
/Oeq

Nax

˚) R (A

˚ 2) s2 (A

Neq

˚) R (A

˚ 2) s2 (A

2 2 2 2 2 2 2 2 2

1.78 1.78 1.78 1.77 1.78 1.78 1.78 1.78 1.78

0.001 0.002 0.001 0.001 0.002 0.001 0.001 0.001 0.002

5.2 5.4 5.3 5.1 5.2 5.0 5.4 5.4 5.3

2.39 2.40 2.40 2.40 2.38 2.38 2.40 2.40 2.39

0.012 0.013 0.012 0.011 0.014 0.014 0.014 0.013 0.012

˚. Errors: N9/10%, R9/0.02 A

In Table 2 the structural parameters of the uranyl humates are compiled such as coordination number (N ), bond distance (R ) and Debye
/Waller factor (s2) obtained from fits to the EXAFS equation. Within the experimental error, for all investigated uranyl humates identical structural parameters are determined. Axial ˚ . In the U
/O bond distances amount to 1.789/0.02 A equatorial plane approximately five oxygen atoms are ˚. found at a mean distance of 2.399/0.02 A The structural parameters of the synthetic, unmodified HAs (M1, M42) and of the natural, unmodified HAs (KHA, AHA) are identical although the HAs differ in their proportion of functional groups (cf. Table 1) and in the content of aromatic structural elements. This shows that the synthetic HAs model the complexation behavior of natural HAs very well. Of course, also for KHA and KFA, isolated from the same source, the same structural parameters are determined. The structural parameters, obtained for uranyl humates prepared from unmodified HAs at pH 2, are in good agreement with results of previous EXAFS studies [30,31] on uranyl humates prepared at pH B/1 to 4. Almost identical structural parameters are determined for solid uranyl humates prepared from modified HAs (HA-PB). Since in case of these modified HAs the phenolic OH groups are predominantly blocked, primarily carboxylic groups are available for complexation of uranyl ions (cf. Table 1). Thus, the identical structural parameters, determined for modified and unmodified HAs, imply that the short-range order surrounding of the uranyl ion in all humates is comparable although the HAs differ in their phenolic OH/COOH-group ratio. This shows that predominantly the HA carboxylate groups are responsible for binding of uranyl ions under the experimental conditions applied in this study. Thus, the results from Reich et al. [32] and Denecke et al. [30], obtained for uranyl complexes with original HAs under comparable experimental conditions, are confirmed. A monodentate coordination of the HA carboxylic groups to uranyl ions follows both from the relatively short

bond distances between uranium and equatorial oxygen ˚ ) and from the fact that no atoms (RU
Oeq /2.39 A ˚ could be carbonyl carbon atoms at about 2.90 A detected. This confirms the results of FTIR spectroscopy. Furthermore, the EXAFS results indicate that, compared with carboxylic groups, the phenolic OH groups have only a minor or no influence on the complexation of uranyl ions at pH 2. This is supported by the analytical results of the uranyl humates (cf. Table 1), which show that the uranyl loading of the unmodified and of the corresponding modified HAs is correlated stronger with the carboxylic group content of the HAs than with the phenolic OH group content. Comparable results were found by Denecke et al. [33] who studied the interaction of hafnium(IV) and thorium(IV) both with HA and with Bio-Rex70 at pH 1.6 by EXAFS spectroscopy. Bio-Rex70, a cation exchange resin having solely carboxylate groups, can serve as model substance for HAs and is comparable with our modified HAs with blocked phenolic OH groups. The authors found similar structural parameters for metal complexes of HA and of Bio-Rex70. From this it was concluded that carboxylate groups are responsible for cation binding in HA and that phenolic OH groups play a subordinate role. Contrary to the results obtained for the uranyl humate complexes where no influence of the blocking of the phenolic OH groups is detectable, an influence was found for low-molecular weight model compounds by Denecke et al. [34]. EXAFS structural parameters were determined for two crystalline uranyl carboxylate compounds: first, disalicylatodioxouranium(VI), UO2[C6H4(COO)(OH)]2, which contains both carboxylate and phenolic OH groups and second, di-o -methoxybenzoatodioxouranium(VI), UO2[C6H4(COO)(OCH3)]2, in which the phenolic OH group is blocked. Thus, containing the same primary functional groups as HAs, these substances in principle could serve as model compounds for our unmodified and modified HAs. The authors found that in UO2[C6H4(COO)(OH)]2 only one to two bidentate coordinated carboxylate groups of the salicylate ligands are bound to uranium and that uranyl units must be linked since the equatorial uranium shell consists of five ˚ . In to six Oeq atoms at a distance of 2.42 A UO2[C6H4(COO)(OCH3)]2 the carboxylate groups were found to be monodentate coordinated (RU
Oeq / ˚ ). That means the blocking of the phenolic OH 2.29 A group by a methyl group changed the mode of coordination of the carboxylic group from bidentate to monodentate. This example shows that it is not always possible to use simple organic compounds as structural models for such disordered and complex materials as HAs.

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The influence of phenolic OH groups on the complexation behavior of HAs compared with carboxylic groups should increase with pH as it was shown, for instance, by an EXAFS study of the uranium(VI) complexation with protocatechuic acid (3,4-dihydroxybenzoic acid) as a function of pH [35]. It was found that at pH 4.3 the carboxylic group in the 1:1 complex is bidentate coordinated to the uranyl cation (RU
Oeq / ˚ , RU
C /2.88 A ˚ ). With increasing pH value, 2.45 A eq the bond distance of the equatorial oxygen decreased. At pH values higher than 5, the ligands coordinate in an o -diphenolic bonding fashion to the uranyl cation, the carboxylic group is not longer involved in complexation ˚ ). Furthermore, for the ura(RU
Oeq /2.36
/2.38 A nium(VI) complexation by glycolic acid and a-hydroxyisobutyric acid [36] the formation of chelate complexes due to deprotonation of the a-OH group of the ligands at pH values ]/5 was shown (RU
Oeq /2.36
/ ˚ ). Further EXAFS analyses of uranium(VI) 2.37 A complexes formed by compounds having solely neighboring phenolic OH but no carboxylic groups such as catechol (1,2-dihydroxybenzene) and pyrogallol (1,2,3trihydroxybenzene) at pH 5 [17] have shown that phenolic OH groups are coordinated to the uranyl ion ˚ . These examples also show at a distance of 2.39
/2.40 A that bond distances determined for phenolic OH groups, that are coordinated to uranyl ions, are very similar to ˚ of the uranyl humates. Thus, the our RU
Oeq /2.39 A contributions of the carboxylate and phenolate groups cannot be distinguished by EXAFS spectroscopy alone. For the solid uranyl humates that were prepared at pH 2, we conclude that the complexation of uranyl ions is dominated by carboxylic groups of HAs. Phenolic OH groups seem to play a minor role although a contribution of HA phenolic OH groups to the complexation of uranyl ions cannot completely be excluded by EXAFS analysis. The reason is that structural parameters, determined by EXAFS analysis, always represent an average over all interactions between uranyl ions and HA molecules that have a complex and heterogeneous structure. The resulting broad distribution of U
/Oeq bond distances is evident in the large Debye
/Waller 2 ˚ 2) determined for RU
O (cf. factors (sav. /0.013 A eq Table 2). Thus, it is necessary to combine EXAFS spectroscopy with further structure-sensitive methods. This is supported by Korshin et al. [9] who concluded from the lack of prominent and easily distinguishable features in the Fourier transform magnitudes of Cu2
/HA complexes ˚ , that the EXAFS method alone cannot at R /2.5 A unambiguously determine the nature and structural properties of the atoms lying beyond the first coordination shell of metal
/HA complexes.

139

4. Conclusion This work has shown that chemically modified natural and synthetic HAs are useful model substances for detailed studies of the complexation of metal ions by humic substances. In addition to unmodified HAs they can be applied to improve the understanding of the nature of metal complexation sites in humic substances further and particularly to clarify the role of phenolic OH versus COOH groups. Since the influence of phenolic OH groups is expected to increase with pH due to increasing deprotonation of these potential binding sites, further EXAFS studies on uranyl humate complexation should be performed at higher pH values in combination with additional structure-sensitive methods. For this, HAs with a high phenolic OH/COOH-group ratio (e.g. HA type M1) are especially suitable. At higher pH values, however, the hydrolysis of uranium(VI) has to be taken into account.

Acknowledgements This work was supported by the EC Commission under contract No. FI4W-CT96-0027 and by the Bundesministerium fu¨r Bildung, Forschung und Technologie (BMBF) under contract No. 02 E88150. The authors thank M. Meyer and R. Ruske for their help in modification and characterization of HAs. Furthermore, the authors thank A. Roßberg, C. Hennig and H. Funke for their support during the EXAFS measurements, R. Nicolai for FTIR measurements and W. Wiesener for ICP-MS analyses.

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