An Injectable Apatite Permeable Reactive Barrier for In Situ ⁹⁰Sr Immobilization

Abstract

An injectable permeable reactive barrier (PRB) technology was developed to sequester 90Sr in groundwater through the in situ formation of calcium‐phosphate mineral phases, specifically apatite that incorporates 90Sr into the chemical structure. This injectable barrier technology extends the PRB concept to sites where groundwater contaminants are too deep or where site conditions otherwise preclude the application of more traditional trench‐emplaced barriers. An integrated, multiscale development and testing approach was used that included laboratory bench‐scale experiments, an initial pilot‐scale field test, and the emplacement and evaluation of a 300‐feet‐long treatability‐test‐scale PRB. The apatite amendment formulation uses two separate precursor solutions, one containing a Ca‐citrate complex and the other a Na‐phosphate solution, to form apatite precipitate in situ. Citrate is needed to keep calcium in solution long enough to achieve a more uniform and areally extensive distribution of precipitate formation. In the summer of 2008, the apatite PRB technology was applied as a 91‐m‐long (300 feet) PRB on the downgradient edge of a 90Sr plume beneath the Hanford Site in Washington State. The technology was deployed to reduce 90Sr flux discharging to the Columbia River. Performance assessment monitoring data collected to date indicate that the barrier is meeting treatment objectives (i.e., 90% reduction in 90Sr concentration). The average reduction in 90Sr concentrations at four downgradient compliance monitoring locations was 95% relative to the high end of the baseline range approximately 1 year after treatment, and continues to meet remedial objectives more than 4 years after treatment.

Injectable Apatite for the Sequestration of Sr-90 in Groundwater

“…It is known that apatite minerals are very stable in water; the solubility product of hydroxy- term, the precipitate formed in the conditions similar to this study is amorphous and poorly crystalline monocalcium phosphate (Moore et al, 2004;Vermeul et al, 2014). The formation of amorphous and poorly crystalline monocalcium phosphate occurs within a week, while crystalline apatite forms within a few weeks (Vermeul et al, 2014). Therefore, the migration of the Sr concentration profile may be due to the dissolution of amorphous calcium phosphate upon supply of the groundwater with low ionic strength.…”

“…Emplacement of apatite precipitate by solution injection has significant advantages over other apatite emplacement methods (e.g., minimal disturbance of the subsurface). This method extends the PRB concept to sites where groundwater contaminants are too deep or where site conditions otherwise preclude the application of more traditional trench-emplaced barriers (Vermeul et al, 2014).…”

“…Citrate is needed to keep calcium and phosphate in solution long enough (days) to be injected into the subsurface. Relatively slow biodegradation of the Cacitrate complex allows sufficient time for injection and transport of the reagents to the areas of the aquifer where treatment is required (Vermeul et al, 2014). As Ca-citrate is degraded by naturally-occurring microbes, the free calcium and phosphate combine to form amorphous apatite.…”

Identification of Promising Remediation Technologies for Iodine in the UP-1 Operable Unit

“…Implementation of an apatite permeable reactive barrier (PRB) technology for in situ sequestration of strontium-90 ( 90 Sr) has been previously demonstrated on the Hanford Site (Vermeul et al 2014). Fullfield-scale application of this technology as a remedial alternative addressing a groundwater contaminant plume beneath the Hanford 100-N Area provides some advantages to this technology over other less welldeveloped technologies, because currently available technical expertise and regulatory/stakeholder familiarity with the apatite technology can be leveraged.…”

Desalination and Water Treatment Research at Sandia National Laboratories.

“…To demonstrate its efficacy as a PRB, an experimental apatite barrier was deployed in the subsurface in 2005 (Figure 7-2) along a 300-foot-long section of Columbia River shoreline to prevent radioactive strontium from reaching the river. The barrier was then field-tested by Pacific Northwest National Laboratories (PNNL) and Fluor Hanford Inc. After six years, (2005)(2006)(2007)(2008)(2009)(2010)(2011) monitoring of wells drilled between the barrier and the Columbia River demonstrated that the barrier sequestered an average of 95 percent of the strontium before it could reach the river (Vermuel et al, 2014).…”