Method to Attenuate U(VI) Mobility in Acidic Waste Plumes Using Humic Acids

Abstract

Abstract. Acidic uranium (U) contaminated plumes have resulted from acid-extraction of plutonium during the Cold War and from U mining and milling operations. A sustainable method for in-situ immobilization of U under acidic conditions is not yet available. Here, we propose to use humic acids (HAs) for in-situ U immobilization in acidic waste plumes. Our laboratory batch experiments show that HA can adsorb onto aquifer sediments rapidly, strongly and practically irreversibly. Adding HA greatly enhanced U adsorption capacity to sediments at pH below 5.0. Our column experiments using historically contaminated sediments from the Savannah River Site under slow flow rates (120 and 12 m/y) show that desorption of U and HA were non-detectable over 100 pore-volumes of leaching with simulated acidic groundwaters.Upon HA-treatment, 99% of the contaminant [U] was immobilized at pH < 4.5, compared to 5% and 58% immobilized in the control columns at pH 3.5 and 4.5, respectively. These results demonstrated that HA-treatment is a promising in-situ remediation method for acidic U waste plumes. As a remediation reagent, HAs are resistant to biodegradation, cost effective, nontoxic, and easily introducible to the subsurface.2

Retention and chemical speciation of uranium in an oxidized wetland sediment from the Savannah River Site

“…In general, the U sorption profiles with the wetland sediment were similar to those with other pure phase minerals (Cheng et al, 2004;Hsi and Langmuir, 1985;Payne et al, 1998;Waite et al, 1994;Wan et al, 2011) and sediments (Dong et al, 2012;Wan et al, 2011). However, this wetland sediment had two distinguishing aspects with its U sorption trends.…”

“…The retained U was associated with acid soluble and organic fractions of the wetland sediments, and the U L 3 -edge XANES spectra of the retained U are nearly identical to that of uranyl acetate (U(VI)). These results might indicate that the molecular mechanisms for the high U retention onto the SRS wetland sediments would be either chemical sorption or bonding with NOM-like humate colloids (Wan et al 2011) or accumulation in the wetland plants (Hinton et al, 2005;Knox et al, 2008) that constituted a significant source of the NOM in the SRS wetland sediments. n/a = not available.…”

“…Uranyl and NOM form strong bonds, including at low pH levels (Wan et al, 2011). This may partially explain why at pH < 4, the U sorption was much higher onto the fraction of <53 µm sediment than onto the 53-250 µm fraction, because the former may contain much more NOM than the latter, in addition to the particle size effect.…”

“…long term U immobilization in wetlands include the chemical and/or microbial reduction of soluble U(VI) ions to fairly insoluble U(IV) species (Kalin et al, 2005;Lovley et al, 1991;Noubactep et al, 2006;Turick et al, 2008), mineral precipitation, sorption onto NOM (Wan et al, 2011), ion exchange or sorption to sediment minerals (Akber et al, 1992;Dong et al, 2012;Grybos et al, 2007), and accumulation in plants (Hinton et al, 2005;Knox et al, 2008).…”

“…Bertsch et al (1994) reported the in situ characterization of chemical speciation of U in the sand fraction of a SRS sediment sample using micro X-ray absorption spectroscopy, which appeared to indicate the presence of U(VI). Given these initial observation that much of the U may naturally exist in the U(VI) form, the remediation technologies of the contaminated SRS sediments with apatite (Ca 10 (PO 4 ) 6 (OH, F) 2 ) (Arey et al, 1999;Kaplan et al, 2004;Seaman et al, 2001), humic acid (Wan et al, 2011), native trees (Hinton et al, 2005, natural hyperaccumulators, for example, the netted chain fern (Woodwardia areolata) (Knox et al, 2007), and microbial metabolites (Turick et al, 2008) have been evaluated. However, the tendency of U to enter the mobile phase and become available to local biota or be transported from the contaminated area has not been well quantified.…”