Uranium interactions with reduced iron species: electron transfer between uranium and Fe(0)-Fe(II)-Fe(III) in natural clays and nanoscale zerovalent iron

摘要

Iron has the potential to be an effective remediation agent for uranium-contaminated subsurface environments. Remediation techniques based on reductive precipitation can be especially important when prevailing environmental conditions favour the formation of thermodynamically stable aqueous uranium (U(VI)) complexes such as Ca2(UO2)(CO3)30 and Ca(UO2)(CO3)32-. In this study it was observed that these two species dominated aqueous uranium speciation, as determined by cryogenic Time-Resolved Laser Fluorescence Spectroscopy (TRLFS), in a range of potable groundwater sources in the Northern Territory of Australia that contain elevated uranium concentrations. Based on these results, research was undertaken to investigate U(VI) uptake on nanoscale zero-valent iron (nZVI) and natural (Fe-rich)smectites, in the presence of aqueous Fe(II), under chemical conditions encompassing those observed in the Northern Territory water samples. It was observed that both Fe(II) sorbed onto the clays and nZVI could effectively immobilise uranium through the formation of reduced uranium precipitates. nZVI removed nearly 100% of uranium under all the conditions examined with reduction proceeding via the formation of carbonate-stabilised U(V) solid phase(s), U(IV) surface complexes (which were stable at low surface uranium loadings) andnano-sized uraninite at high uranium surface loading concentrations. Evidence for the reduction of uranium via electron transfer from the nZVI core was obtained by means of kinetic studies, X-ray photoelectron and X-ray absorption spectroscopies. In the case of the smectites examined (2 nontronites and 1 montmorillonite), U(VI) reduction and the nature of its end products was highly dependent on pH and time. Fe(II) sorption enhanced uranium removal by the smectites and the presence of both stable U(VI) and U(IV) surface complexes were detected. Above the Fe(II) sorption edge, however, these complexes eventually formed uraninite. Based on the quantity of Fe(II) that could be ‘sorbed’ by the smectites as well as X-ray diffraction and transmission electron microscopy evidence, it is proposed that interlayer Fe(II) is a primary redox-reactive specie driving the reduction of uranium. This thesis furthers knowledge on the redox, chemical and mineralogical evolution of Fe-U systems and contributes to understanding on the controls of uranium mobility in natural environments.