Sequestration of Subsurface Elemental Mercury

Batch Experiments:    

Batch studies are being conducted to elucidate the key physical, thermodynamic, and kinetic parameters controlling the partitioning of Hg to select sulfide minerals. The goal is to select the most promising minerals for more detailed study and to measure kinetic and thermodynamic parameters for incorporation into reactive transport models. Because sulfide-containing minerals are thermodynamically unstable in the presence of atmospheric O2, all batch experiments are initially being performed in the absence of oxygen, except for those experiments that are specifically designed to investigate the role of O2 in the performance of the technology. Reactors are set up in a glove box under a N2:CO2 (98:2) atmosphere (pCO2 of 10-1.7 atm) at ambient temperature (22±0.1°C). All reagents are prepared from reagent grade chemicals. Adjustments to pH are made with HNO3 or NaOH. All solutions are heated and sparged with N2 to drive off any dissolved O2 prior to reaction. Mercury has been shown to sorb to container walls and volatilize from reactors, causing significant mass balance problems. To address these issues, Teflon containers and connections are used to minimize losses of Hg to container surface. All suspensions are prepared in a constant ionic strength background matrix of NaCl (0.1 M) because Hg forms very strong associations with chloride ions. After filtration, samples are preserved in 1% BrCl to prevent Hg volatilization and/or losses to sample containers. Samples intended for spectroscopic investigations remain undisturbed to prevent a change in character that could affect spectroscopic results. Batch studies of Hg uptake by potential immobilizing materials are conducted for a range of solution conditions and materials to evaluate those that have the greatest promise. Batch studies are used to quantify relevant thermodynamic and kinetic parameters. For rate studies, aliquots from the suspensions are taken as function of time, filtered through a 0.22 µm membrane, and preserved in BrCl. Equilibrium uptake and capacity studies are conducted by measuring Hg uptake at various fixed pH values. Suspensions containing potential barrier material are spiked with increasing initial Hg concentrations and then gently agitated until a steady-state distribution has been achieved between the solid and the aqueous phase as indicated by a constant Hg concentration in the aqueous and solid phase.

Column Experiments:

While batch systems can provide important information with respect to the kinetic and thermodynamic properties of a system, the effects of physical factors such as the structure, porosity, the distribution of the reactive media, and the effects of residence time and fluid velocity on the efficiency of the system are not detected in such studies. It is therefore necessary for a study that aims to provide insight into the environmental behavior of a compound to consider the behavior of the compound under dynamic (flow) scenarios. In addition, column studies are ideally suited to investigations that explore the effects of complex (time-dependent, multisolute) chemical conditions on pollutant flux. This information is critical for determining the degree to which equilibrium and kinetic data apply to systems under environmentally relevant pore flux conditions so that accurate reactive transport models can be developed and tested. Potential candidates are evaluated in batch terms of removal efficiency and capacity as discussed above. After this screening, column studies are limited to no more than 2-4 minerals or materials. A number of important physical and chemical variables have been identified that may affect the performance of a particular metal-sulfide barriers. Columns are packed with quartz sand and one of the reactive media discussed above. The columns are attached to pulseless pumps and eluted with a 0.1 M NaCl background matrix, from the base of the column upward, at a constant rate. Columns containing barrier materials are eluted with a 0.1 M NaCl solution (N2 sparge to remove O2, pH = 4), and effluent pH is monitored. The columns continue to be eluted until effluent characteristics reach a steady state as indicated by a consistent breakthrough curve. The eluent solution is then switched to an identical solution containing a known concentration of Hg. Column effluent is monitored for Hg and pH. After a steady-state Hg concentration is achieved, the eluent is switched back to the eluent without Hg. Aqueous parameters continue to be monitored until a steady-state level is reached. The effect of the following variables are being investigated: hydrodynamics (pore water velocity), long-term stability (column studies are well suited for investigating the release of the immobilized pollutant after changes in the chemical composition of the aqueous phase, because the equivalent of years of flow in the field can be flushed through the columns in reasonable time frames in the laboratory), redox conditions, and potential column regeneration.

Advanced Spectroscopic Techniques:

Synchrotron-generated X-ray absorption spectroscopy (XAS) is a powerful tool for probing the local atomic environment of target elements in samples with environmentally relevant (low) concentrations. Previous studies have shown that XAS can provide semi-quantitative information describing Hg partitioning among several phases. XAS is being utilized to determine the oxidation state (from near-edge structure, XANES) and atomic coordination environment (from extended fine structure, EXAFS) of Hg in representative reacted samples. Data are collected at a U.S. Department of Energy national user facility, the Advanced Photon Source at Argonne National Laboratory. Batch and flow-through experiments are conducted several weeks in advance of synchrotron analysis as well as on site at the Advanced Photon Source, to test whether redox cycling and/or time affect Hg adsorption.  Differences in redox conditions are also investigated by using minerals that were pre-rinsed and left untreated with nitric acid.  All batch, flow-through, and spectroscopic measurements are conducted under anaerobic conditions using nitrogen and helium gases. Samples of natural cinnabar (HgS) and Hg(NO₃)₂ are analyzed as a benchmarks. The computer program EXAFSPAK is then used to apply least-squares fitting algorithms of the EXAFS function χ(k) to determine nearest and second-nearest neighbor atomic identities, coordination numbers, and distances for samples in which Hg concentration is >200 ppm. Comparison with theoretical models generated using the ab initio computer code FEFF6 with spectra from relevant model compounds enable distinction between adsorption and substitution/coprecipitation modes of Hg attenuation by secondary minerals. Microbeam EXAFS is employed to target individual grains identified (by microbeam XRF) as having elevated Hg content. Samples of the materials from both batch and column experiments are analyzed to clarify the mechanism responsible for immobilization. The microscopic information determined in this task is used to provide a mechanistic base for the kinetic and thermodynamic parameters determined from the batch and column experiments.