Foam Delivery of Calcium Polysulfide for Cr(VI) Remediation in Unsaturated Porous Media - 2-D Flow Cell Experimental Study
EMSL Project ID
30205
Abstract
Deep vadose zone chromium [Cr(VI)] remediation at the U.S. Department of Energy (DOE) Hanford Site is critical to end ongoing sources of groundwater contamination and to prevent Cr(VI) migration to the Columbia River. Due to the high cost and technological limitations, it is not practical to excavate and remove these contaminants. Alternatively, in-situ stabilization is expected to be one of the most important remediation strategies. Calcium polysulfide, a strong reducing chemical, precipitates highly soluble metals and radionuclides to less soluble sulfides and hydroxides. Calcium polysulfide reduces Cr(VI) to Cr(III), which is then precipitated readily as chromium hydroxide and/or iron-chromium hydroxide. Calcium polysulfide has been proposed as a potential reductant for Hanford deep vadose zone Cr(VI). However, water-based calcium polysulfide delivery to the DVZ is a concern because of the high mobility of Cr(VI). The wetting front of a water-based delivery will easily mobilize these pollutants and thus potentially enhance transport to the underlying aquifer prior to reaction with the reductant. Furthermore, preferential flow paths of the calcium polysulfide solution in the vadose zone are very difficult to overcome, resulting in bypassing of contamination in less permeable zones during remediation.
In contrast to water-based delivery systems, foam flow in the vadose zone is not dominated by gravity but can be directed by differences of pressure. The injection of foam, a shear thinning fluid, can achieve more uniform sweeping over heterogeneous systems. The foam-aided calcium polysulfide delivery technology has the potential to resolve both the uneven remedial fluid distribution and the massive Cr(VI) mobilization front issues.
In a on-going PNNL Laboratory Directed Research and Development (LDRD), it was demonstrated that foam-delivered calcium polysulfide could mitigate Cr(VI) mobilization in one-dimensional column experiments, thus considerably increase the in-situ immobilization of Cr(VI) in the sediments. To further develop this foam-delivery concept for application as a vadose zone remediation technology, the influence of heterogeneity on foam sweeping efficiency in the vadose zone has to be investigated. Controlled two-dimensional (2-D) flow cell experiments will be the best approach for this investigation.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2008-10-01
End Date
2010-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Zhong L, JE Szecsody, M Oostrom, MJ Truex, X Shen, and X Li. 2011. "Enhanced Remedial Amendment Delivery to Subsurface Using Shear Thinning Fluid and Aqueous Foam." Journal of Hazardous Materials 191(1-3):249-257. doi:10.1016/j.jhazmat.2011.04.074