Trace metal speciation by capillary electrophoresis hyphenated to inductively coupled plasma mass spectrometry: sulfate and chloride complexes of Np(V) and Pu(V).

In the framework of nuclear waste disposal, it is very important to well understand the behavior of actinides in the presence of the common environmental inorganic ligands such as sulfate and chloride. In this work, the AnO2SO4(-) and AnO2Cl 1-1 complexes have been evidenced by capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICPMS) in perchlorate/chloride and in perchlorate/sulfate media for An = Np and Pu. Their binding constants have been measured: log beta(PuO2SO4(-))(0) = 1.30 +/- 0.11, log beta(PuO2Cl)(1 M NaCl) = -(0.40 +/- 0.07), log beta(NpO2SO4(-))(0) = 1.34 +/- 0.12, and log beta(NpO2Cl)(1 M NaCl) = -(0.40 +/- 0.07). These results are consistent with published values for Np(V). They confirm the expected analogy between Np(V) and Pu(V) for the weak bonding with chloride ligand, log10 beta(PuO2Cl) approximately = log10 beta(NpO2Cl), attributed to mainly electrostatic interactions. Conversely, a slight shift is observed for the bonding with sulfate ligand, log10 beta(NpO2SO4(-)) > log10 beta(PuO2SO4(-)), indicating that some covalency might stabilize the sulfate complexes.