Structural investigation on uranium mine water colloids and their interaction with UO22+

Flooding abandoned uranium mines normally involves mixing of shallow groundwater with acidic, iron-rich pore water. A simulation experiment in mesocosm scale showed that nanoparticles of 2-line ferrihydrite formed and took up about 97% of total uranium between pH 5-7. The molecular topology of ferrihydrite and the U(VI) surface complex structure were investigated by EXAFS and ATR-FTIR spectroscopy. The contribution of carbonate to surface sorption was studied by preparing reference samples at varied CO2 partial pressure and pH conditions, and by applying iterative target transformation factor analysis on EXAFS spectra. Surface sorbed UO22+ delayed the thermally induced aqueous phase transformation of ferrihydrite to hematite, shown by Mössbauer spec-troscopy. Desorption of U(VI) from the solid phase was less effective for original colloid and sediment samples from a uranium mine and for aged reference samples than for fresh U(VI)-ferrihydrite precipitates, suggesting that part of the uranium was fixed within the mineral lattice.