Sequestration of Sr-90 Subsurface Contamination in the Hanford 100-N Area by Surface Infiltration of a Ca-Citrate-Phosphate Solution

Abstract

SummaryThe objective of this project is to develop a method to emplace apatite precipitate in the 100-N Area vadose zone, resulting in sorption and ultimately incorporation of Sr-90 into the apatite structure. The Ca-citrate-phosphate (Ca-citrate-PO 4 ) solution can be infiltrated into unsaturated sediments to result in apatite precipitate to provide effective treatment of Sr-90 contamination. Microbial redistribution during solution infiltration and a high rate of citrate biodegradation for river water microbes (water used for solution infiltration) produces a relatively even spatial distribution of the citrate biodegradation rate and ultimately, apatite precipitate in the sediment. Manipulation of the Ca-citrate-PO 4 solution infiltration strategy can be used to induce apatite precipitation in the lower half of the vadose zone (where most of the Sr-90 is located) and within low-K layers (which may have higher Sr-90 concentrations):• infiltration rate: more rapid Ca-citrate-PO 4 solution infiltration resulted in greater depth of apatite precipitate, but less lateral spreading• decrease in infiltration rate: rapid then slow solution infiltration resulted in greater lateral spreading of the apatite precipitate at depth• sequential solution then water infiltration: water infiltration after solution infiltration effectively moved the apatite mass to depth in homogeneous sediment systems• solution concentration: infiltration of higher Ca-citrate-PO 4 solution concentrations resulted in a greater depth of apatite precipitate• infiltration cycles: repeated infiltration of the Ca-citrate-PO 4 solution with time between cycles to allow for water drainage increased depth and greater lateral spread of apatite• low-K zones had a higher apatite precipitate due to higher residual water content• solution then water infiltration into a complex heterogeneous system with low-K and high-K discontinuous zones (8 ft high) showed high apatite precipitate in low-K and medium-grained sediment, but low treatment in high-K zones due to low water contentThe most effective infiltration strategy to precipitate apatite at depth (and with sufficient lateral spread) was to infiltrate a high concentration solution (6 mM Ca, 15 mM citrate, 60 mM PO 4 ) at a rapid rate (near ponded conditions), followed by rapid, then slow water infiltration. Repeated infiltration events, with sufficient time between events to allow water drainage in the sediment profile can be used to build up the mass of apatite precipitate at greater depth. Low-K heterogeneities were effectively treated, as the higher residual water content maintained in these zones resulted in higher apatite precipitate concentration. High-K zones did not receive sufficient treatment by infiltration, although an alternative strategy of air/surfactant (foam) was demonstrated effective for targeting high-K zones. The flow rate manipulation used in this study to treat specific depths and heterogeneities is not as easy to implement at field scale due to the lack of characterization of heterogene...

100-NR-2 Apatite Treatability Test: High-Concentration Calcium-Citrate-Phosphate Solution Injection for In Situ Strontium-90 Immobilization

“…Synthetic apatites have been made with up to 40% strontium substitution for calcium (Heslop et al 2005). The mechanism (solid-state ion exchange) of strontium substitution for calcium in the apatite structure has been previously studied at elevated temperatures (Rendon-Angeles et al 2000), and low-temperature aqueous rates under Hanford Site groundwater conditions (i.e., calcium/strontium ratio of 220/1) have also been studied (Szecsody et al , 2009). …”

“…The formation of amorphous apatite occurs within a week, and crystalline apatite forms within a few weeks. Citrate biodegradation rates in Hanford 100-N Area sediments (water saturated) at temperatures from 10°C to 21°C (aquifer temperature 15 to 17°C) over the range of citrate concentrations to be used (10 to 100 mM) have been determined experimentally (Szecsody et al 2009) and simulated with a first-order model (Bailey andOllis 1986, Bynhildsen andRosswall 1997). In addition, the microbial biomass has been characterized with depth and position along the Columbia River shoreline, and the relationship between biomass and the citrate biodegradation rate has been determined ).…”

“…This calculation assumes an average groundwater flow rate of 0.3 m/day (1 ft/day) and a 10-m (32-ft) apatite PRB thickness. Electron microprobe analysis of the strontium substitution in apatite precipitate (Szecsody et al 2009) showed that after 1.3 years, there was a 16.1% substitution of strontium for calcium in microcrystalline (20-to 40-micron particles composed of <1-micron crystals) apatite, and a 9.1% substitution of strontium for calcium in crystalline apatite (20 to 40 microns, single crystal). The majority of the apatite precipitating in sediment was microcrystalline, so the assumption of a 10% substitution is achievable.…”

“…The first bullet has been addressed by analyzing sediment samples collected from boreholes drilled within the treatment zone (Szecsody et al 2009(Szecsody et al , 2010 and performance-assessment groundwater monitoring following the low-concentration (Williams et al 2008) and high-concentration Ca-citrate-PO 4 injections.…”

“…Although the apatite contents were small, they were sufficient to demonstrate that phosphate mineral phases had been formed with the overlap zone between adjacent wells receiving an average treatment of 110% of the targeted apatite content within the Hanford formation and 30% of the targeted apatite content within the Ringold Formation (Szecsody et al 2009). The Ringold apatite content data, in addition to amendment arrival responses observed in available Ringold Formation monitoring wells, support the decision to install Ringold-only injection wells that were used during subsequent highconcentration treatments (and will be used in all future saturated-zone injection operations).…”