Hydrolysis and chemical speciation of dioxouranium(VI) ion in aqueous media simulating the major ion composition of seawater

Abstract

The hydrolysis and chemical speciation of the dioxouranium(VI) ion at 25 °C was studied in a number of binary electrolytes (LiCl, NaCl, MgCl 2, CaCl 2, Na 2SO 4) and some mixtures (NaCl–Na 2SO 4, NaNO 3–Na 2SO 4, CaCl 2–MgCl 2) as well as artificial seawater (SSWE) as a function of ionic strength. The results in LiCl, CaCl 2 and MgCl 2 solutions confirmed the formation of (UO 2) 2(OH) 2 2+, (UO 2) 3(OH) 4 2+, (UO 2) 3(OH) 5 + and (UO 2) 3(OH) 7 − species (at I=0 mol l −1: log T β 22=−5.76, log T β 34=−11.82, log T β 35=−15.89 and log T β 37=−29.26). For NaNO 3, NaCl and artificial seawater the hydrolysis constant for the formation of the UO 2(OH) + species was also determined (at I=0 mol l −1: log T β 11=−5.19). The results in Na 2SO 4, Na 2SO 4/NaNO 3 and Na 2SO 4/NaCl required the formation of UO 2(SO 4) 0, UO 2(SO 4) 2 2−, UO 2(OH)SO 4 −, (UO 2) 2(OH) 2SO 4 0, (UO 2) 3(OH) 4SO 4 0 and (UO 2) 3(OH) 5SO 4 −, whose estimated values of each complex formation constant at I=0 mol (kg H 2O) −1 are (log T β pqr ±standard deviation, species in parenthesis): 3.32±0.02 [UO 2SO 4 0], 4.26±0.04 [UO 2(SO 4) 2 2−], −2.30±0.01 [UO 2(OH)(SO 4) −], −2.64±0.04 [(UO 2) 2(OH) 2(SO 4) 0], −8.45±0.04 [(UO 2) 3(OH) 4(SO 4) 0], −13.58±0.04 [(UO 2) 3(OH) 5(SO 4) −]. All the results were examined using the Pitzer model by considering the interactions of the cation hydrolytic species with Cl − and NO 3 − and anion hydrolytic species with Li +, Na +, Ca 2+ and Mg 2+ and, in addition, the “same sign” and “triple” interaction parameters. The resulting Pitzer parameters give an adequate representation of all the hydrolysis constants measured in the binary, ternary and artificial seawater solutions. Alternatively to the interpretation of the dependence of uranyl hydrolysis constants on ionic strength and on ionic medium in terms of variations of activity coefficients of ions, the formation of ion pairs was considered and some complex formation constants among dioxouranium(VI) species and different ions of background salts were calculated. Interactions of uranyl with major components of seawater were taken into account using the “single salt” BA approximation according to which SSWE is considered as a single sea salt (BA) where cation B and anion A, having charge z=± I/C BA =±1.117 , are representative of all major cations (Na +, K +, Ca 2+, Mg 2+) and anions (Cl − and SO 4 2−) of seawater, respectively. Pitzer parameters were also calculated for both the interactions of uranyl with B z+ and A z− ions and for the internal BA interactions. The last ones are: β (0)=0.1081 and β (1)=0.4238 for B 1.117+–A 1.117− interactions, and β (0)=−0.3134 and β (1)=1.5375 for H +–A 1.117− interactions. Literature data were collected carefully, and a critical analysis and accurate comparisons with results presented here were made.