Contaminant Desorption during Long-Term Leaching of Hydroxide-Weathered Hanford Sediments

Abstract

BRIEF:Leaching waste-altered Hanford sediments with neutral solutions desorbs Sr from de novo waste-generated minerals and desorbs Cs from high-affinity ion exchange sites. 2 ABSTRACTMineral sorption/co-precipitation is thought to be a principal sequestration mechanism for radioactive 90 Sr and 137 Cs in sediments impacted by hyperalkaline, high-level radioactive waste (HLRW) at the DOE's Hanford Site. However, the long-term persistence of neoformed, contaminant bearing phases after removal of the HLRW source is unknown. We subjected pristine Hanford sediments to hyperalkaline Na-Al-NO 3 -OH solutions containing Sr, Cs, and I at 10 -5 , 10 -5 , and 10 -7 molal, respectively, for 182 days with either <10 ppmv or 385 ppmv pCO 2 . This resulted in the formation of feldspathoid minerals.We leached these weathered sediments with dilute, neutral-pH solutions. After 500 pore volumes (PV), effluent Sr, Cs, NO 3 , Al, Si and pH reached a steady-state with concentrations elevated above those of feedwater. Reactive transport modeling suggests that even after 500 PV, Cs desorption can be explained by ion exchange reactions, whereas Sr desorption is best described by dissolution of Sr-substituted, neo-formed minerals. While, pCO 2 had no effect on Sr or Cs sorption, sediments weathered at <10ppmv pCO 2 did desorb more Sr (66% vs. 28%) and Cs (13% vs. 8%) during leaching than those weathered at 385 ppmv pCO 2 . Thus, the dissolution of neo-formed aluminosilicates may represent a long-term, low-level supply of 90 Sr at the Hanford site.

Strontium and Cesium Release Mechanisms during Unsaturated Flow through Waste-Weathered Hanford Sediments

“…After reaction with STWL, sediments were washed repeatedly using ethanol to remove entrained solution, and then freeze dried until further use. Minerals in the weathered sediment were identified using synchrotron x-ray diffraction (XRD) analysis [9,21].…”

“…In contrast to our prior study pertaining to saturated transport at long reaction times [16], the simulations in this study only focused on the more complex initial period of elapsed times (< 200 PV) in the leaching experiments. Specific thermodynamic parameters for the minerals were obtained from the literature and the EQ3 database if they were available [12,18,21,[26][27][28]. Unavailable thermodynamic and kinetic parameters were estimated by numerical optimization of the simulation results to the experimental data (see discussion below).…”

“…since the latter is only reliable when the specific surface areas of the reacting phases are known [21].…”

“…However, the intensities of the chabazite reflections in the HI-column decreased after BPW leaching, indicating potential mineral dissolution. The initially decreasing concentration trends of Na, Al, and Si in the column effluents could be an indication of dissolution of labile aluminosilicate secondary minerals formed during the STWL weathering process [12,15,21,29]. However, identification of the amorphous solid phase or secondary mineral undergoing dissolution is quite challenging because of the transient nature of the labile mineralogy.…”

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…The low carbonate BPW solution was representative of NO 3 -free Hanford porewater of pH 7.1-7.2 (Thompson et al, 2010). Carbonate salts were substituted for chloride salts to produce the high carbonate BPW solution (pH 8.0-8.2).…”