Perturbed angular correlation study of binding of hafnium to humic acid and its model compounds

In the present work we have used Time Differential Perturbed Angular Correlation (TDPAC) technique to study the binding of hafnium with mandelic acid and compared the results with hafnium humate complex. Three BaF2 detectors of dimensions 2' x 2' (2 Nos) and 1.1/2' x 1.1/2'(1 No) coupled to a fast slow coincidence and multi-parameter data acquisition system were used for TDPAC counting. The time resolution for 133-482 KeV cascade of 181Hf was 0.8 nsec. Humic acid (HA) in the sodium form procured from ACROS, Belgium, was used in the experiment without further purification. The proton exchange capacity (PEC) for HA was determined by potentiometric titration (4.6 ± 0.1 meq/g) and by the Baryta method (4.8 ± 0.1 meq/g). 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 1M HCl under hot condition. The complex was precipitated, filtered and washed with distilled water. In the case of Hf-humate, 181Hf stock solution was added into humic acid solution, having concentration of 20 mg/l (volume = 10 ml, pH = 2) and equilibrated for 48 hrs. The solid residue was centrifuged and washed with distilled water. TDPAC spectra were analysed by DPAC software to obtain the quadrupole interaction frequency (w) and asymmetry parameter (h). The Fourier transform of TDPAC spectrum of Hf-mandelate complex shows two w values, namely, 663 and 728 Mrad/s, which are in reasonable agreement with the literature data. The asymmetry parameters for the two sites were found to be 0.7 and 0.3, respectively. In the case of humate complex, the Fourier transform of the TDPAC spectrum showed a broad distribution of quadrupole interaction frequencies. This indicates absence of specific binding site in HA. Metal ions may bind to the humic acid by direct association with the donor groups (specific site binding model) or they may be associated with anionic macromolecule as a whole (polycondensate model). While in the case of specific site binding one expects 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-HA 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. The fraction of high affinity binding sites is expected to be small, which cannot selectively bind the metal ion at concentration levels of μg/ml, as in the present case (0.9 μg/ml). It would, therefore, be interesting to carry out these studies at concentration level of ng/ml which is possible with carrier-free radioisotopes. (author)