Sustainability of humic acids in the presence of magnesium oxide

Abstract

The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy (DOE) repository for the disposal of transuranic waste generated from the U.S. defense program. Humic acids (HA), known to strongly bind to actinides and which may be generated by the degradation of organic materials present in the waste, may influence the performance of the WIPP. This work presents experimental results on the effect of WIPP brines, ERDA-6 and GWB, and of the WIPP engineered barrier, Premier Chemicals magnesium oxide, on the solubility of HA. In the absence of MgO, a portion of HA initially precipitates, but a constant concentration of HA remains in solution: over a 60 day period of time, at least 290 ± 10 mg/L HA can remain soluble in de-ionized (DI) water, 30 ± 4 mg/L in 95% ERDA-6, and 31 ± 4 mg/L in 95% GWB. For solutions initially containing from 0 to 400 mg/L HA, the ratio of the initial HA concentration and the soluble HA concentration is 1.3 in DI water, 13 in 95% ERDA-6, and 11 in 95% GWB. In the presence of MgO, all HA precipitate within 60 days in systems with a high liquid-to-solid ratio (10.0 g/g) or a low ratio (2.4 g/g). This phenomenon is due to HA precipitation and/or sorption on the surface of MgO.