The migration of actinides in the environment is affected considerably by their interactions with the mineral � water interface. In deep geologic repositories used for storing nuclear waste, actinides encounter more extreme environmental conditions than those normally studied in surface and ground water systems. In particular, these nuclear waste repositories have been found to possess elevated ionic strengths (> 1 M) and temperatures (80 oC). However, the vast majority of aqueous speciation data and sorption data for actinides has been collected for low ionic strength, low temperature conditions. Therefore, sorption experiments conducted at high ionic strengths and elevated temperatures are warranted.In this work, uranium and neptunium adsorption onto the iron oxide mineral hematite (a-Fe2O3) was studied over a range of concentrations (1 � 1000 �g/L), temperatures (25 � 80oC), and ionic strengths (0.01 and 1.0 M). Hematite was selected as a representative degradation product of steel canister corrosion within a spent nuclear fuel repository. While uranium has previously been determined spectroscopically to form inner-sphere surface complexes with hematite, adsorption of uranium in our study was observed to decrease considerably with increasing ionic strength, indicating that changes in double layer thickness, potential formation of outer-sphere complexes, ion competition, and/or ion activity are influencing uranium binding. In contrast, neptunium adsorption to hematite was not found to be affected by increasing ionic strength; which is intriguing given neptunium's weaker affinity for the mineral surface compared with that of uranium. Sorption of uranium at constant ionic strength increased with increasing temperature. This rise in sorption is hypothesized to be due to an entropy driven reaction in which hydrating waters of uranium and the mineral surface are displaced upon formation of the surface complex.

Influence of temperature and ionic strength on actinyl ion sorption to hematite

248th National Meeting (2014)

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