Speciation and fractionation of Ca and the REE in fresh and marine waters

Abstract: This study is concerned with speciation and fractionation of the rare earth elements (REE) and calcium (Ca) in aqueous solutions. The aim is to investigate the chemical states and physical sizes in which these elements can be present. The REE (including neodymium) and Ca have contrasting geochemical behavior in aqueous solutions. Ca is a major dissolved element, while the REE are trace components and highly reactive with aquatic particles.The major interests of the five papers included in this thesis are the following:· Papers I and V deal with the behavior of neodymium (Nd) and its isotopes in the Kalix River and some marine waters.· The diffusive gradients in thin-films (DGT) method is developed for measuring Ca and Mg in Paper II.· Paper III presents a speciation and fractionation study of Ca in the Kalix and Amazonian rivers.· The rare earth elements and their carrier phases are investigated in the Kalix river in Paper IV.For most elements a detailed study of speciation and fractionation can not be performed using only one method. This is due to the overall heterogeneity of the material, considering both size and chemical composition, which is present in aquatic solutions. During this project the aquatic geochemistry of the REE and Ca has been studied using mainly three methods; cross-flow filtration (CFF), field-flow fractionation (FFF) and diffusive gradients in thin-films (DGT). Field work has to a large part been conducted in the Kalix River, in northern Sweden, which is one of the last pristine river systems in Europe. Some field work has also been conducted in the Baltic Sea and the Arctic Ocean. Results from Amazonian rivers are also presented.These are the main conclusions from this work:The DGT technique works equally well for measuring Ca and Mg in natural waters as previously reported for trace metal.A significant colloidal phase for Ca could be detected in the Kalix River and in different Amazonian rivers. This was concluded independently using both CFF and FFF.Variations in REE signatures in the Kalix River suggests two different pathways for the REE during weathering and release form soil profiles and transport in the river.No significant variation in Nd-isotopic composition could be detected in the Kalix River although concentrations varied by a factor of ~10. This suggests that there is one major source for Nd in the river although different pathways for the REE may exist.A study of Nd in the Kalix River, the Baltic Sea and the Arctic Ocean showed that the isotopic compositions in the diffusible fractions were similar to water samples. However, the relative amount of diffusible Nd increased with salinity, probably reflecting the lower concentration of colloidal and particulate material in marine waters.