The time differential perturbed angular correlation study of binding of hafnium to humic acid and its model compound

Polyhedron 28 (2009) 1399–1402

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The time differential perturbed angular correlation study of binding of hafnium to humic acid and its model compound N. Rawat, S. Kumar, B. Tomar * Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

a r t i c l e

i n f o

Article history: Received 31 October 2008 Accepted 27 February 2009 Available online 11 March 2009 Keywords: Time differential perturbed angular correlation 181 Hf Mandelic acid Humic acid Electric field gradient

a b s t r a c t Time differential perturbed angular correlation (TDPAC) spectroscopy has been used to study the binding of hafnium to mandelic acid, which is a model compound of humic acid. The Fourier transform of TDPAC spectrum of Hf–mandelate complex showed two x values, namely, xQ1 = 77 (±0.4) Mrad/s and xQ2 = 111 (±3.1) Mrad/s. The asymmetry parameters for the two sites were found to be 0.7 and 0.3, respectively. The electric field gradient (EFG) around the metal ion was theoretically calculated using the computer code GAMESS. The calculated EFG value was found to be in reasonable agreement with that deduced from experiment. Contrary to Hf–mandelate, the TDPAC spectrum of Hf–humate system did not show characteristic features of discrete binding sites. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Humic substances are the common organic matter produced by degradation of the plants and animals and are known to play an important role in the migration of long-lived radionuclides in the aquatic environment [1–4]. These substances are classified into fulvic acid (soluble in water at all pH), humic acid (soluble in water at pH > 2) and humins (insoluble at all pH) [5]. These compounds have strong tendency to get adsorbed on the colloidal particles, particularly at lower pH values, wherein the colloidal particles are positively charged. Further they can form strong complexes with the metal ions present in the natural water, thereby influencing the sorption behavior of these metal ions by the colloidal particles [2]. Owing to these characteristic properties, humic substances have attracted considerable attention during the past few decades. Though extensive studies have been carried out on the complexation of actinides and long-lived fission products with humic acid as well as its effect on their sorption on colloids, little is known about the nature of binding of metal ions with humic acid. Carboxylates and phenolic groups are known to be the most predominant functional groups in humic acids, though other functional groups such as amines and amides have also been identified. Owing to complex structure of humic acid, model compounds, such as salicylic acid, phthalic acid and mandelic acid, con-

* Corresponding author. Tel.: +91 22 25595006; fax: +91 22 25505151. E-mail address: [email protected] (B. Tomar). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.02.036

taining these functional groups are used to study the binding of metal ions with these macromolecules [6]. Spectroscopic techniques, such as time resolved-fluorescence spectroscopy (TRFS) [7–9], extended X-ray absorption fine structure (EXAFS) [10–12] and X-ray photoelectron spectroscopy [13], have been carried out to investigate the structure as well as binding sites of humic acid. Time differential perturbed angular correlation spectroscopy (TDPAC) is a nuclear hyperfine technique which provides information about the chemical environment around the probe nucleus in a compound [14]. TDPAC technique has been used by Kupsch et al., to investigate the binding of 111Cd and 199Hg (divalent metal ion) to humic acid [15]. However the literature reports on TDPAC study of the binding of higher valent metal ions (Pu, Am, Cm, etc.) to humic acid are scarcely available. In the present work we have used the TDPAC technique to study the binding of hafnium with mandelic acid and compared the results with hafnium - humate complex. 181Hf has been used as a probe atom. Though hafnium has higher ionic potential than plutonium, it can be used as a probe nucleus for binding of plutonium to humic acid. Mandelic acid was used as a model compound of humic acid as it contains the carboxylic and hydroxyl group. These functional groups are present as dominant in humic acid. The TDPAC spectra were used to deduce the EFG around the probe atom. Theoretical calculation of the EFG around the probe atom was carried out using the computer code GAMESS [16], and was compared with the EFG deduced from TDPAC spectra. The TDPAC study of hafnium complex with humic acid was also carried out and the data was compared with that for hafnium mandelate complex.

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G22 ðtÞ ¼ Sk0 þ

3 X

Skn cos xn t

ð2Þ

n¼1

where Skn are the amplitudes of the corresponding frequencies (xn). The data were analyzed using the software DEPAC to obtain the interaction frequency (x1) and asymmetry parameter (g) of the EFG [17]. The quadrupole interaction frequency (xq) was obtained from x1 using the following relation [18]:

xQ ¼ 2. Experimental Reagent grade mandelic acid and hafnium oxychloride were used for the present experiment. Humic acid (ACROSS) was used. Ten milligrams of hafnium oxychloride was irradiated in Dhruva reactor of Bhabha Atomic Research Centre, Trombay (flux  1014 n cm2 s1) for a time period of 7 days to obtain high specific activity 181Hf. For the preparation of mandelate complex, zirconium was used as a carrier for hafnium. Zirconyl chloride solution containing 181Hf activity was mixed with a mandelic acid solution in 1 M HCl under hot condition. The complex was precipitated, filtered and washed with distilled water. In the case of Hf– humate complex, a small aliquot from the 181Hf stock solution was evaporated to dryness and humic acid solution, having pH 2 (20 mg/l) was added to it. Hafnium concentration in the solution was around 107 M, which is small compared to the humic acid concentration (104 M). The solution, equilibrated for 48 h, was subsequently centrifuged. The solid residue was found to contain 99% of the Hf activity. Three BaF2 detectors of a fast-slow coincidence setup, coupled to multi-parameter data acquisition system, were used for TDPAC counting. The time resolution for 133– 482 keV cascade of 181Hf was 0.8 ns. 3. Results and discussion

x1 2 Þ1=2

4½7ð3 þ g

sin

1 3

ð3Þ

 ðcos1 bÞ

The EFG can be deduced from quadrupole interaction frequency (xQ) by the equation

xQ ¼ eQV ZZ =4Ið2I  1Þh

ð4Þ

where Q is the quadrupole moment of the intermediate level and I is its spin. The asymmetry parameter (g) is given by

g ¼ ðV XX  V YY Þ=V ZZ

ð5Þ

where |VZZ| > |VXX| > |VYY|. The Fourier transform of TDPAC spectrum (Fig. 1) of Hf–mandelate complex shows two xQ values. The data for quadrupole interaction frequencies for two sites are given in Table 1. The existence of two sites was also observed by Baussaha et al. [19] while Barbieri et al. [20] observed only one quadrupole frequency. The two xQ values indicate that in Hf–mandelate, Hf is complexed in two types of well defined environments. The IR spectroscopy of Hf–mandelate shows bands at 2600, 2300 and 1850 cm1 which are typical of compounds with hydrogen bonds and these bonds link alcoholic oxygen of one bridging mandelate and carboxylic oxygen of second bridging mandelate [21]. These results show the possible existence of monomeric and dimeric species which could be present in Hf–mandelate. The existence of other complexes, like, Hf(L)3OH or Hf(L)2OH (L = mandelate) could also lead to different electronic environments around Hf nuclei and thus in different quadrupole frequencies.

Fig. 1 shows the TDPAC spectrum of the Hf–mandelate complex calculated using the formula,

A2 G2 ðtÞ ¼ 2

Wð180 ; tÞ  Wð90 ; tÞ Wð180 ; tÞ þ 2Wð90 ; tÞ

ð1Þ

where W(180°, t) and W(90°, t) are the coincidence counts between 133 and 482 keV gamma rays at 180° and 90°, respectively. The attenuation factor (G22(t)) contains the information about the interaction frequencies (xn) and the asymmetry parameter g:

Table 1 TDPAC experimental data for two Hf sites in Hf–mandelate complex. Abundance (%)

xQ (Mrad/s)

g

d

VZZ (1018) (V/cm2)

26.9 73.0

77 ± 0.4 111 ± 3.1

0.7 0.3

0.054 ± 0.005 0.57 ± 0.04

0.80 ± 0.004 1.16 ± 0.03

0.20

1

ω1

1.2

Fourier transform 2

ω1

1.0

0.15

0.10

G 0.4

Intensity

0.6

22

(t)

0.8

0.05

0.2 0.0 0

5

10

15

Delay (ns)

20

25

30 0

1000

2000

3000

4000

0.00 5000

ω , Mrad/s

Fig. 1. TDPAC spectrum of Hf–mandelate complex at 300 K and its Fourier transform showing interactions frequencies x11 and x12 for the site 1 and 2, respectively.

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0.12

1.2

1.0

Intensity

G22 (t)

0.08 0.8 0.04 0.6

0.4 0

5

10

15

20 0.0

0.5

1.0

1.5

2.0

2.5

0.00 3.0

ω , (Mrad/s)

Delay (ns)

Fig. 2. TDPAC spectrum of Hf–humate complex and its Fourier transform.

In the case of humate complex, the Fourier transform of the TDPAC spectrum showed a broad distribution of quadrupole interaction frequencies (Fig. 2). This indicates absence of specific binding site in HA. Metal ions may be bound to the humic acid by direct association with the donor groups (specific site binding model) [22] or they may be associated with anionic macromolecule as a whole (polycondensate model) [23]. While in the case of specific site binding, we expect well defined quadrupole interaction frequencies, in the case of polycondensate model any such specific coordination site may be absent. The present observations point towards the polyconsendate model. Stability constants of metal– humate complexes are known to decrease with increasing metal ion concentration, indicating the presence of different binding sites with widely varying affinity for the metal ions [24]. The fraction of high affinity binding sites is expected to be small. These sites can-

Table 2 Experimental and theoretical values of EFG (VZZ) and asymmetry parameter (g). Species

L3 Hf (l-L)2Hf L3

Theoretical values

Experimental values

g

EFG (au)

g

EFG (au)

0.38

0.35

0.3

0.19 ± 0.01

not selectively bind the metal ion at concentration levels of lg/ml, as in the present case (0.9 lg/ml). Similar results were obtained in the case of 111mCd–humate complex by Kupsch et al. [15]. Therefore, it would be interesting to carry out these studies at concentration level of ng/ml. It is possible with carrier free radioisotopes.

4. Theoretical calculations To find out the present species exactly, we independently calculated the EFG on the Hf nuclei using ab initio molecular orbital methods. Theoretical calculations were done using GAMESS. The Gaussian Orbital Valence basis sets were used for Hf. The 60 core electrons of Hf were described by the effective core potential of Hay et al. [25] which incorporates mass velocity and Darwin relativistic effects into the potential and the valence electrons by a (5s 6p 3d)/[3s 3p 2d] Gaussian function basis set. To reduce the number of atoms for ab initio calculation of Hf with large ligand, the phenyl groups were replaced by hydrogen. Table 2 gives the theoretically calculated EFG for dimeric species of Hf–mandelate and is compared with experimentally obtained EFG. The structure of dimeric species used for EFG calculation is given in Fig. 3. The reasonably good agreement between theoretical and experimental values confirms the presence of dimeric Hf–mandelate species.

Fig. 3. Structure of L3 Hf (l-L)2Hf L3 used for EFG calculation.

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5. Conclusion The TDPAC spectrum of Hf–mandelate showed two well defined chemical environments around Hf nuclei. The experimentally obtained perturbation parameters show good agreement with the theoretically obtained parameters using GAMESS code. The TDPAC data confirm that Hf–mandelate exist predominantly as dimeric species while the minor species could be a mixed hydroxy mandelate complex of hafnium. The TDPAC of Hf–humate, on the other hand, indicates non specific site binding of metal ion. Acknowledgement Authors thank A. Bhattacharya for his help in the theoretical calculations of EFG. References [1] G.R. Choppin, in: G.R. Choppin, M.K. Khankhasayev (Eds.), Chemical Separation Technologies and Related Methods of Nuclear Waste Management, Kluwer Academic Publishers, 1999, p. 247. [2] R. Artinger, B. Kienzler, W. Schubler, J.I. Kim, J. Contam. Hydrol. 35 (1998) 261. [3] M. Samadfam, T. Jintoku, S. Sato, H. Ohashi, T. Mitsugashira, M. Hara, Y. Suzuki, Radiochim. Acta 88 (2000) 717. [4] R. Artinger, B. Kienzler, W. Schubler, J.I. Kim, Radiochim. Acta 91 (2003) 743. [5] F.J. Stevenson, Humus Chemistry, Genesis, Composition, Reactions, WileyInterscience Publication, 1982.

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