Complex Formation of Uranium(IV) with Phosphate and Arsenate

Heavy metals in the aquatic environment are normally transported as a complexed species. Knowledge about the complex formation is therefore an essential constituent in prediction of the migration of these elements.
Especially in Saxony and Thuringia the intense uranium mining and milling causes a wide variety of contaminated waters. These seepage and mine waters contain several inorganic and organic complex forming agents [1, 2].
We studied the complex formation of uranium(IV) with phosphoric acid and arsenic acid in strong acid solution by UV-vis Spectroscopy and Laser-induced Photoacoustic Spectroscopy (LIPAS). In both systems the formation of a one to one complex was detected. The dependency of the complex formation on the concentration of hydrogen ions in the solution leads to the conclusion, that one proton is released during the complex formation reaction. Studying the complex formation as function of ionic strength we can extrapolate the formation constant at infinite dilution using the SIT theory.
Including the protonation constants for phosphoric and arsenic acid, respectively, we obtained the formation constants according to the complex formation reaction

U4+ + 2H+ + XO43- ? UH2XO43+ (1)

for X = P log ß0 = 25.23 ± 0.13 and for X = As log ß0 = 23.94 ± 0.08.
Comparing these data with the corresponding uranium(VI) systems [3,4] it can be seen that the binding tendency of the dihydrogenarsenate is lower than that of dihydrogenphosphate. Also the binding tendency of the uranium(VI) is lower than that of uranium(IV). This is an expected behavior.
The specific ionic interaction coefficients for the uranium(IV) phosphate and arsenate system are calculated to be e(UH2XO43+ - ClO4-) = 0.42 and 0.46 in the phosphate and in the arsenate system, respectively.

[1] G. Geipel, G. Bernhard, M. Rutsch, V. Brendler, H. Nitsche; Speciation in Water Released from Mining and Milling Facilities In T.E. Baca and T. Florkowski (eds.), The environmental Challenges of Nuclear Disarmament, Kluwer Academic Publishers, 2000, p. 323-332
[2] G. Bernhard, G. Geipel, V. Brendler, H. Nitsche; Speciation of Uranium in Seepage Waters from a Mine Tailing Pile Studied by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS); Radiochimica Acta, 74, 87, (1996)
[3] I. Grenthe, J. Fuger, R. J. Lemire, A. B. Muller, C. Nguyen-Trung and H. Wanner, Chemical Thermodynamics of Uranium, 1st ed. , Elsevier Science Publishers, Amsterdam, 1992.
[4] M. Rutsch, G. Geipel, V. Brendler, G. Bernhard, H. Nitsche; Interaction of uranium (VI) with arsenate (V) in aqueous solution studied by time-resolved laser-induced fluorescence spectroscopy, Radiochimica Acta 86, 135-141 (1999)