Sulfate Complexation of Trivalent Lanthanides Probed by Nanoelectrospray Mass Spectrometry and Time-Resolved Laser-Induced Luminescence

摘要

Sulfate complexation of lanthanides is of great interest for predicting speciation of radionuclides in natural environments.The formation of LaSO_4~+(aq)in HNO3/H2SO4 aqueous solutions of low ionic strength(I)was studied by nanoelectrospray ionization mass spectrometry(nanoESI-MS).Several gaseous species containing LaSO_4~+ were detected.The formation constant of LaSO_4~+(aq)was determined and extrapolated to I=0(log beta_1 deg=3.5 +-0.3)by using a simple specific ion interaction theory(SIT)formula.This value supports the potential of nanoESI-MS for the study of kinetically labile species.The species La(SO_4)_2~-was also detected.In addition,time-resolved laser-induced luminescence(TRLIL)was used to study Eu(III)speciation under ionic conditions of 0.02-0.05 M H~+(H_2SO_4/HClO_4)and 0.4-2.0 M Na~+(Na_2SO_4/NaClO_4).The data were interpreted with the species EuSO_4~+(log beta_1 deg=3.7_8 +-0.1)and Eu(SO_4)_2~-(log K_2 deg=1.5 +-0.2).For extrapolating to I=0,all of the major ions were taken into account through several SIT ion-pair parameters,epsilon.Most of the e values were estimated by analogy to known parameters for similar ion-pair interactions using linear correlations,while epsilon_(Eu~(3+),SO_4~(2-))=0.8_6 +-0.5 was fitted to the experimental data because,to date,SIT coefficients between multicharged species are not reported.The formation constants obtained here confirm some of those previously measured for Ln(III)and An(III)by various experimental techniques,and conversely do not give credit to the idea that in equilibrium conditions TRLIL and other spectroscopic techniques would provide stability constants of only inner-sphere complexes.The fluorescence lifetimes measured for EuSO_4~+ and Eu(SO_4)_2~-were consistent with the replacement of one H_2O molecule in the first coordination sphere of Eu~(3+)for each added SO_4~(2~)ligand,suggesting a monodentate SO_4~(2~)coordination.