Title : Uranium ( VI ) Mobility in an Evolving Alkaline and Saline Plume Speaker : Matthew

Little is known about uranium, U(VI), transport in alkaline and saline solutions. However understanding U(VI) mobility in these conditions is critical due to the storage of U(VI) at high pH and ionic strengths (I). To this end we have studied the geochemical processes taking place in a reservoir, and during transport through quartz sand packed columns. The results show that at pH 10.5 and 11.5 with 10ppm U(VI) and 0.1M NaCl, 80% and 90% of the U(VI) precipitates in the reservoir respectively as 10’s nm sized Na6U7O24 crystals. These aggregate as 1um particles. When this solution is passed through the sand column, the particles are filtered out. The U(VI) particles visibly penetrate 4-5mm into the column. In the 4 to 5 hours representative of U(VI) injection into the columns, 50% of the U(VI) precipitates out in the reservoir, while 90% and 97% precipitation occurs in the columns at pH 11.5 and 10.5 respectively. This means that transport through the column enhances precipitation. Reducing U(VI) concentration to 1ppm causes 20% U(VI) and 100% to precipitate at pH 11.5 and pH 10.5 respectively. At pH 12.5, 0.1M NaCl and 10ppm U(VI), 100% of the U(VI) passes through the column with little retardation. Ionic strength was tested at 10ppm U(VI) with no added, 0.1M and 0.3M NaCl while keeping pH constant at 11.5. The results show that there is an increasing rate and amount of precipitation in the reservoir as I increases. When no NaCl is added, the U(VI) does not precipitate. When 0.1M NaCl is added, 90% precipitates out within 24 hours. Finally when 0.3M NaCl is added, 95% precipitates out within 4 hours. These results show that U(VI) mobility is controlled by a complex mix of homogenous and heterogeneous precipitation, and depends on [U(VI)], pH and I, and the rate depends on I. This implies that U(VI) mobility will vary significantly with plume evolution. Mobility will start off very high due to the effects of pH (12.5), and will decrease significantly as pH drops due to mixing with groundwater, as long as the ionic strength remains high. Biography 2010-2013 Studied Geology at the University of Birmingham (BSc) 2013-2014 Studied Nuclear Decommissioning and Waste Management at the University of Birmingham (MSc) In May 2015 I started my PhD in the Environmental Geochemistry Group, under the Supervision of Professor Dominik Weiss and in Porous Media Flow and Reaction group under the supervision of Dr Sam Krevor. My PhD is part of the Hydroframe consortium, led by Professor Robert Zimmerman, and is part of the NERC Radioactivity and the Environment (RATE) project. My PhD title is: Do microbes and natural organic matter lead to increased actinide mobility in fractured rocks? We are interested in how inorganic chemical controls (pH and I), and organic ligands affect U(VI) mobility in quartz sand.