In Situ Remediation of Multi-Metal Groundwater Plumes (Arsenic, Cobalt, Lithium, and Molybdenum)- ​ Treatability Studies

In Situ Remediation of Multi-Metal Groundwater Plumes (Arsenic, Cobalt, Lithium, and Molybdenum): Treatability Studies Authors Dr. Dimitri Vlassopoulos - United States - Anchor QEA Dr. Masa Kanematsu - United States - Anchor QEA Dr. Jason Stuckey - United States - Anchor QEA Ms. Grace Weatherford - United States - Anchor QEA Ms. Minna Carey - United States - Anchor QEA Abstract In situ injection for groundwater remediation at CCP sites could save millions of dollars compared to conventional technologies. Anchor QEA performed successful treatability studies with a wide range of reagents on groundwater samples from multiple sites with multiple constituents of interest (COIs) including arsenic, cobalt, lithium, and/or molybdenum. Studies included aquifer soil and groundwater sampling and geochemical characterization, batch tests to screen and optimize reagents, column tests to assess treatment effectiveness and reversibility, and selective sequential extractions to document sequestration mechanisms and stability. Column tests were conducted to evaluate the COI removal performance of reagent-treated aquifer soils under flow conditions and to confirm treatments do not release other constituents from the aquifer/soil matrix. At the end of the column tests, treatment stability was determined by pumping background site groundwater through the columns. Several reagent solutions were successful in removing COIs below standards, depending upon the site-specific COI mix and groundwater geochemistry. The most robust solutions for removing COI mixes precipitated (hydr)oxides of iron and/or manganese, as well as layered double hydroxides. Some treatments, however, require a two-stage injection. Depending on the target COIs, treatment could also be achieved by single-stage injections, which are easier to implement at some sites

Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50 years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15 m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment–water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment–water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions

Mechanism of Hg(II) immobilization in sediments by sulfate-cement amendment

Reactive amendments such as Portland and super-sulfate cements offer a promising technology for immobilizing metalloid contaminants such as mercury (Hg) in soils and sediments through sequestration in less bioavailable solid forms. Tidal marsh sediments were reacted with dissolved Hg(II) in synthetic seawater and fresh water solutions, treated with Portland cement and FeSO(4) amendment, and aged for up to 90 days. Reacted solids were analyzed with bulk sequential extraction methods and characterized by powder X-ray diffraction (XRD), electron microscopy, and synchrotron X-ray absorption spectroscopy at the Hg L(III)- and S K-edge. In amended sediments, XRD, SEM and sulfur K-edge XANES indicated formation of gypsum in seawater experiments or ettringite-type (Ca(6)Al(2)(SO(4))(3)(OH)(12)(.)26H(2)O) phases in fresh water experiments, depending on the final solution pH (seawater ∼8.5; freshwater ∼10.5). Analysis of Hg EXAFS spectra showed Cl and Hg ligands in the first- and second-coordination shells at distances characteristic of a polynuclear chloromercury(II) salt, perhaps as a nanoparticulate phase, in both seawater and fresh water experiments. In addition to the chloromercury species, a smaller fraction (∼20-25%) of Hg was bonded to O atoms in fresh water sample spectra, suggesting the presence of a minor sorbed Hg fraction. In the absence of amendment treatment, Hg sorption and resistance to extraction can be accounted for by relatively strong binding by reduced S species present in the marsh sediment detected by S XANES. Thermodynamic calculations predict stable aqueous Hg-Cl species at seawater final pH, but higher final pH in fresh water favors aqueous Hg-hydroxide species. The difference in Hg coordination between aqueous and solid phases suggests that the initial Hg-Cl coordination was stabilized in the cement hydration products and did not re-equilibrate with the bulk solution with aging. Collectively, results suggest physical encapsulation of Hg as a polynuclear chloromercury(II) salt as the primary immobilization mechanism

Linear Melt Rheology of Pom-Pom Polystyrenes with Unentangled Branches

We measured the linear viscoelastic properties of a series of polystyrene melts with pom-pom architecture consisting of backbones ranging from marginally to well-entangled (of molecular weight Mb), endgrafted with q unentangled branches (Mbr) per backbone end. In this case, the branches relax very fast and act primarily as solvents for the backbone. Using a time-marching tube-based model, we showed that, in such a case only, the pom-pom polymer is equivalent to a blend of long (ML Mb + 2Mbr) and short (MS Mbr) linear chains with respective proportion (Mb + 2Mbr)/2(q - 1)Mbr