Precipitation of NH4UO2PO4·3H2O—Solubility and Structural Comparison with Alkali Uranyl(2 +) Phosphates

Precipitates formed in the system UO2(NO3)2-NH4OH-H3PO4-H2O, aged for 30 days at 298 K, were studied. The precipitates were characterized by chemical and thermogravimetric analyses, x-ray powder diffraction, infrared spectroscopy, polarized light microscopy, and by their fluorescent properties. The precipitation boundary was established tindallometrically and microscopically. On the basis of these measurements, the stability conditions, structural parameters, and solubility of the tetragonal polymorph of NH4[UO2PO4]·3H2O were determined. This compound shows a close structural relationship with H3O[UO2PO4]·3H2O (space group P4/ncc) and alkali uranyl(2+)phosphates polyhydrates M[UO2PO4]·nH2O (n =4 for M=Li; n =3 for M=Na, K, Rb and n =2.5 for M=Cs). The unit-cell dimensions determined for NH4UO2PO4·3H2O are: a=b=7.02 Å, c=18.08 Å (P4/ncc). The thermodynamic solubility product constant, Ks=a(NH4+)×a(UO22+)×a(PO43−), for NH4UO2PO4·3H2O was determined: log Ks= −26.50±0.09. The Ks values of M[UO2PO4]·n H2O (at ionic strength, I=0.23 mol dm−3) calculated from previously published experimental data by using correct stability constants of uranyl(2+)phosphate complexes are: log Ks=−22.61±0.08 for M=Na; log Ks= −23.92±0.12 for M=K; log Ks= −24.13±0.19 for M=Rb; log Ks= −23.80±0.20 for M=Cs; and log Ks= −24.74±0.10 for M=NH4,showing that NH4UO2PO4·3H2O is less soluble than corresponding alkali uranyl(2+)phosphates.

Precipitation conditions of ammonium uranyl(2 + )phosphate can be of interest for the separation of uranium as a secondary product in the production of monoammonium phosphate (additive of fertilizers) [9]. Three polymorphs of NH 4 U0 2 P0 4 ·3H 2 0(s) are known [10,11]. The solubility product of one of these compounds has been determined at undefined ionic strength [12,13] and at an ionic strength of 0.23 mol dm-3 [14] using inaccurate association and stability constants for phosphoric acid and uranyl phosphate complexes, respectively.
This paper describes the formation of different precipitates in the system U02(N03)2-NH40H-H 3 P0 4 -H 2 0 at 298 K. These precipitates were characterized by chemical and physical methods. The stability region for the precipitated tetragonal polymorph of NH 4 U0 2 P0 4 ·3H 2 0 was established as a function of reactant concentrations, and its solubility product constant was determined. The structure of this polymorph was compared to that of hydrogen uranyl(2 + )phosphate [2,15] and alkali uranyl(2+ )phosphates [3,4]. The solubility data for NH 4 U0 2 P0 4 ·3H 2 0 and MU0 2 P0 4 ·n H 2 0 obtained by Vesely, Pekarek, and Abbrent [14] were recalculated in this paper using a proper set of constants to obtain solubility products, and they were compared with our data.

Experimental Section
Stock solutions were prepared by dissolving the following P.A. chemicals in triply distilled water: U0 2 (N0 3 )2, H 3 P0 4 , and NH 4 0H (Merck, 2 Darmstadt). Standardization of solutions was performed by using classical analytical methods [16,17]. Precipitation in the system U02(N03)2-NH40H-H 3 P0 4 -H 2 0 (at 298 K) was performed at constant uranyl(2+ )nitrate concentration, 1 X 10-3 mol dm-3 ; the concentrations of NH 4 0H varied from 5 X 10-5 to 3.2 mol dm-3 and phosphoric acid from 5 X 10-3 to 1 mol dm-3 . The samples were prepared by mixing U0 2 (N0 3 )2 solution with an equal volume of NH 4 0H + H 3 P0 4 solution. Approximately 400 samples were prepared to define precipitation and phase boundaries. One day and 30 days after mixing the reactant solutions, the samples were examined in detail. The pH was measured with the Radiometer equipment: electrode GK 2302 C and pH-meter Mo 26. The precipitation boundary (the line that separates the region of precipitation from the region of clear solutions) was determined tyndallometrically and microscopically. The morphol-ogy of the precipitates was examined in white, polarized and UV light under an Orthoplan microscope (Leitz, Wetzlar). Selected precipitates were characterized by means of chemical and thermogravimetric analyses (TGA), x-ray powder diffraction patterns (XRD) and infrared (IR) spectra. The phase boundaries (lines that separate the regions in which different solid phases precipitate) were determined on the basis of these data.
The solid phase was chemically analyzed for uranium, phosphorus and nitrogen. Uranium was precipitated with (NH4)2HP04, heated at 1373 K and weighed as U 2 0 3 P 2 0 7 [18]. Phosphorus was determined gravimetrically by precipitation with ammonium molybdate [19] and spectrophotometrically as phosphovanadomolybdato complex [19]. Nitrogen was determined by chemical microanalysis. The water content was determined thermogravimetrically (Cahn RG recording electromicrobalance ).
X-ray diffraction patterns were recorded on a Phillips x-ray diffractometer with a proportional counter, using graphite monochromated CuKa radiation. The x-ray patterns were calibrated with graphite as the internal standard [10] with a unitcell a =2.463 A., c =6.714 A. (A.= 1.54178 A.). Relative intensities, I reb are given as peak heights. IR spectra (600 to 3600 cm-I ) were obtained using a Perkin-Elmer Mo-221 spectrophotometer and the standard KBr pellet technique.

K13XKI2xKIxa3(H+)
In table 2 are given the concentrations of all components in the solutions equilibrated with NH 4 U0 2 P0 4 ·3H 2 0(s) (points along the precipitation boundary). The ionic concentration product, K s =c(NH 4 +)Xc(Uol+)xc(POl-), expressed in greater detail form is In this equation C(U02)soln and c(NH 4 )soln are the total concentrations of uranyl and ammonium species in the solution, respectively. K 13 , K 12 , and Kl are the association constants of phosphoric acid [23][24][25] (table 3, equilibria 1-3) and {3ij are the stability constants of different uranyl phosphate complexes [5] (table 3, equilibria 4-7). The calculations were performed using a computer program de- signed on the basis of the procedure published earlier (ref. [5], eqs 1-5). The input data for the program were the concentrations of all components in the solution (table 2) and the values of thermodynamic equilibrium constants at 298 K (table 3, equilibria 1-7). The ionic strength, 1, defined as 1 =0.5 ~CZ2 (c and z are the concentration and valence charge of the ion, respectively) was calculated by an iterative procedure (iterations until the change was less than + 1 %). Consequently, the values of the equilibrium constants at 1 = 0 were calculated from thermodynamic equilibrium constants by using the values of the activity coefficients (y) of the ions at corresponding ionic strengths. Activity coefficients (at 298 K) of all ions (except UOl+) were calculated by using the _ Davies equation [26]: log Y= -0.509z 2 rV1/(Y1 + 1)-0.21]. For uranyl(2 +) ions the activity coefficients determined by Brusilovsky [27] were used. In figure 2 is presented the dependence of the activity coefficients on the ionic strength: for the ions with valence charge 2 the curve was calculated by using the Davies equation (curve 1) and for the uranyl(2 +) ions it was constructed by using the experimental values [27] (curve 2). The difference between these two curves is considerable.

Discussion
Ammonium uranyl(2 + )phosphate trihydrate precipitates as the only solid phase in a broad concentration range of the reactants ( fig. 1). On the contrary, in the presence of alkali ions, mixtures consisting of hydrogen and alkali uranyl(2+ )phosphates prevail [3,4]. These results can be explained by the greater sorption affinity of NH4 + on uranylhydrogen(2 + )phosphate tetrahydrate as compared to that of alkali cations [28].
The average values of ionic activity and concentration products are listed (table 3, equilibria 9-13). The solubilities of different uranyl phosphate compounds depend on the cationic species in the structure. The ionic product constants (Ks) increase as follows: Our experimentally determined Ks(1 =0) values of NH 4 U0 2 P0 4 ,3H 2 0 and KU0 2 P0 4 ·3H 2 0 [3] (table 3, eqUilibria 9 and 11) corrected to 1 =0.23 mol dm -3 shows an excellent agreement with the corresponding values recalculated from the data [14] originally determined at 1 =0.23 mol dm-3 • This confirms the accuracy of the stability constants of the uranyl phosphate complex species [5] and the experimental precision of the solubility data [3,14]. The Ks values determined and those recalculated in this work are in disagreement with the values given by Klygin et al. [13] and Muraveva et al. [29]. These authors did not consider the uranyl(2 + )phosphate complex formation. The value of Ks for KU0 2 P0 4 ·3H 2 0 determined by Chukhlantsev and Stepanov [12] is a hundred times higher than ours, but their solubility product constant of NH 4 U0 2 P0 4 ·3H 2 0 (at undefined ionic strength) is similar, (log Ks= -26.36) [12], to the value determined in this work (at 1 =0). It seems that (a) experimental uncertainties and (b) calculations which do not take into account complex species compensate each other, giving a value of Ks for NH 4 U0 2 P0 4 ·3H 2 0 similar to the one we determined. Recalculation of their data [12] is not possible because the analyses of the equilibrated solutions were incomplete. The x-ray powder pattern of NH4[U02P04}3H20 reveals a close structural relationship with the series M[U0 2 P0 4 ]·nH 2 0 (n =4 for M=Li; n =3 for M=H 3 0, Na, K, Rb; and n =2.5 for M=Cs). The crystal structure of M[U02P04]·n H 2 0 ( fig. 3, structure of H 3 0[U0 2 P0 4 ]·3H 2 0 [15]) reveals packing arrangements of infinite layers of octahedra and tetrahedra and water layers containing M (H30+, alkali or NH4 +) ions. Uranium exhibits an octahedral coordination. The P0 4 tetrahedron acts as a monodentate bridging group; each P0 4 group is coordinated to four UOl+ ions. A striking structural feature is the arrangement of the water molecules. The size of the hydrogen, alkali and ammonium ionic species in particular compounds affects the content of the crystalline water in the unit-cell. The unit-cell parameters of alkali uranyl(2+ )phosphates are calculated [21,22] from our previously published XRD data [4] and are compared with those of NH 4 [U0 2 P0 4 ]·3H 2 0 (table  4). Calculated unit-cell dimensions of these compounds are in very good agreement with the values obtained from single-crystal data [30]. The increasing values of the unit-cell volumes of the trihydrates M[U0 2 P0 4 ]·3H 2 0, M=Na, K, NH 4 , Rb (table 4), correlate with increasing ionic radii [31] of corresponding species: Na(ri=0.97 A), K(ri= 1.33 A), NH 4 (ri= 1.43 A), and Rb(ri= 1.47 A). The results of this work along with the recalculations of previously published experimental results [3,4,14] give a detailed and complete description of the formation, solubility and structural relationship of ammonium and alkali uranyl(2 + )phosphates.