Rare earth elements cycling across salinity and redox gradients
This dissertation combines laboratory experiments with analysis of field samples and geochemical modeling to examine rare earth elements (REEs) geochemistry. The Mississippi River estuary, Louisiana and the Pettaquamscutt River estuary, Rhode Island provided ideal study sites to investigate the effects of salinity and redox gradients, respectively, on the cycling of the REEs in natural environments. Similar to the REE behavior in major estuaries such as the Amazon estuary, the REEs in the Mississippi River undergo salt-induced coagulation removal during mixing with the saline Gulf of Mexico seawater. However, unlike the Amazon estuary in which dissolved REE removal of up to 90% has been reported, only ca. 50% removal is observed in the Mississippi River estuary. The closed-system batch reaction experiment which followed showed that interactions with the Mississippi River particulate material substantially alter the dissolved REE concentrations of the Gulf of Mexico seawater. Combined effects of dissolution of the labile phases on the riverine particles and secondary mineral precipitation of likely REE phosphate phases result in a 24 ± 12 folds (mean ± 1σ) net increase in the REE concentrations of the seawater. Less than 1% of the REE contents in the operationally defined “exchangeable” phase of the sediments was mobilized at the maximum REE concentrations in the reacted seawater. The behavior of the REEs in the Pettaquamscutt River estuary is coupled with the cycling of Fe and Mn oxides/oxyhydroxides in the oxic surface waters and across the chemocline. Reaction path modeling suggests that the REE content of the oxic surface waters depicts a combined effect of mixing of 3 water masses and surface complexation with hydrous manganese oxides to achieve the cerium depleted pattern that characterizes the entire water column.