From hydrolysis to the formation of colloids: polymerization of tetravalent actinide ions

Abstract

Polymerization reactions of tetravalent metal ions in solution gained considerable renewed interest in recent years but are still not fully understood. The relvance of their complex chemistry spans from industrial applications in the case of zirconium salts to the nuclear fuel cycle but also environmental research for the actinides thorium and plutonium. Their interactions in the environment must be known in order to understand potential migration pathways of the radiotoxic elements and in particular to perform safety assessments of prospective sites for high level nuclear waste repositories. The relevance of polymer and colloid formation, in particular of tetravalent plutonium, became obvious when unpredicted high levels of plutonium contaminations were found many kilometers off nuclear weapons test sites in Nevada (USA) and close to a nuclear reprocessing plant at Mayak (Russia). In these cases, plutonium transport was strongly enhanced by formation of so called pseudo colloids, i.e. plutonium ions chemically sorbed to natural colloids. In the present work, hydrolysis, polymerization and colloid formation are investigated by the spectroscopic techniques LIBD (laser-induced breakdown detection), TRLFS (time-resolved laser-fluorescence-spectroscopy), UV-Vis (absorption spectroscopy) and XAFS (X-ray absorption fine structure spectroscopy). Observation of polynuclear complexes by ESI-TOF-MS (nano-electrospray mass-spectrometry) fills the gap in the reaction chain from the hydrolysis of monuclear ionic solution species to the formation of nm-sized aquatic colloids. Combining these to some extent especially taylored and improved methods allows making a step forward toward a molecular level understanding of the complex interplay of redox chemistry, hydrolysis reactions and polymer formation of tetravalent metal ions in solution.