Field and laboratory evaluation of the mobility of cobalt-60/EDTA in an arid environment

The ability of the organic complexant EDTA to enhance the mobility of cobalt-60 was investigated in both laboratory and field experiments. Laboratory tests consisted of short term (approximately 7 day) column and batch adsorption tests using soil from the Hanford site as well as long term (approximately 70 day) batch tests with Hanford soil and soils from Oak Ridge and Savannah River. In addition, two large scale tracer tests were conducted using Hanford soil. One used a large (1.6 m) laboratory column, spiked with cobalt-60/EDTA and the other was a field test conducted in an 8 m deep lysimeter. Enhanced mobility decreased sorption were observed in both column and batch tests when the cobalt-60/EDTA solutions contacted Hanford and Oak Ridge soil for only a few days. When long contact times were allowed (months) the Hanford soil showed large increases in sorption with time. The low sorption exhibited initially by the Oak Ridge soil increased slightly over time, however, the high sorption observed with the Savannah River soil remained constant with time. The reduced mobility, with time, observed in Hanford soils was confirmed in both the large scale laboratory and breaking down when contacted with Hanford and Savannah River soil and to a lesser extent, the Oak Ridge soil. It is not known at this time why the complex is breaking down or why the kinetics are different among the soils tested. The implication to waste management is that the potential for transport of cobalt by EDTA complexation may not be as serious as once thought