Competitive Complexation of Nitrates and Chlorides to Uranyl in a Room Temperature Ionic Liquid

By coupling EXAFS, UV-vis spectroscopy, and molecular dynamics and quantum mechanical calculations, we studied the competitive complexation of uranyl cations with nitrate and chloride ions in a water immiscible ionic liquid (IL), C(4)mimTf(2)N (C(4)mim(+) : 1-butyl-3-methyl-imidazolium; Tf2N- = (CF3SO2)(2)N)(-): bis(trifluoromethylsulfonyl)imide). Both nitrate and chloride are stronger ligands for uranyl than the IL Tf2N- or triflate anions and when those anions are simultaneously present, neither the limiting complex UO2(NO3)(3)(-) nor UO2Cl42- alone could be observed. Ala U/NO3/ Cl ratio of 1/2/2, the dominant species is likely UO2Cl(NO3)(2)(-). When chloride is in excess over uranyl with different nitrate concentrations (U/NO3/Cl ratio of 1/2/6, 1/4/4, and 1/12/4) the solution contains a mixture of UO2Cl42- and UO2Cl3(NO3)(2-) species. Furthermore, it is shown that the experimental protocol for introducing these anions to the solution (either as uranyl counterion, as added salt, or as IL component) influences the UV vis spectra, pointing to the formation of different kinetically equilibrated complexes in the IL.