GEOCHEMICAL INVESTIGATIONS OF METALS RELEASE FROM SUBMERGED COAL FLY ASH USING EXTENDED ELUTRIATE TESTS
A.J. Bednar, M.A. Chappell, J.M. Seiter, J.K. Stanley, D.E. Averett, W.T. Jones, B.A. Pettway, A.J. Kennedy, S.H. Hendrix, and J.A. Steevens
U.S. Army Engineer Research and Development Center, Environmental Laboratory
On December 22, 2008, over 5 million cubic yards of fly ash were released to the Emory River, TN from the Tennessee Valley Authority (TVA) Kingston Fossil Plant. Remediation and recovery efforts for this event are currently underway. This study was conducted in order to determine concentration and metal species in the fly ash, and to predict potential for significant metals release from fly ash particles during remediation strategies.
Total Dissolved Metals in Elutriates
RESULTS AND DISCUSSION:
- Field Collection:
- All field samples collected on June 12, 2009
- Fly ash was collected from the source pile and from the Emory River at five discrete locations approximately 2.2 miles upstream of the confluence of the Emory and Clinch Rivers and composited
- Fly ash and water also collected from the Ash Recovery Ditch (Sluice Channel) on site at the TVA Kingston Fossil Plant
- Reference water for use in preparing elutriates was collected from the Emory River approximately 12 miles upstream of the Kingston Fossil Plant
- Figure 3 A-C. Geochemical Measurements
- Eh values measured over the course of the 10 day elutriate experiment.
- pH values measured over the course of the 10 day elutriate experiment.
- TOC concentrations in elutriate waters over the 10 day preparation period.
Figure 2. Expanded view of dissolved metals concentrations in aerated Emory River and Sluice Channel elutriates as a function of time.
Table 1. Total elemental composition of the fly ash
- As, Se, Ba, and Cu are above the background levels in the reference sediment
- There was no noticeable difference in total dissolved metals between N and aerated elutriate systems.
Arsenic Speciation in Fly Ash Elutriates
Figure 1. Sampling locations at the TVA site in Kingston, TN
Selenium Speciation in Fly Ash Elutriates
- Elutriate Studies:
- Elutriates prepared with Emory River (EMR-EL) and Sluice Channel fly ash (SLC-EL) 10 g/L of fly ash added to respective site water, aerated or bubbled with N for 10 days
- This extended elutriate mixing time (10 d versus the normal 1 h) was usedto conservatively represent extreme conditions for the release, dissolution, and changes in metal speciation that might occur during dredging
- Both solid and liquid samples were taken at time points throughout the 10 day period and analyzed for total metal and metal speciation.
- Redox potential in elutriate samples was measured using a platinum electrode with a silver-silver chloride reference and a field portable meter.
- A combination electrode was used to measure pH.
- Metal Speciation Techniques:
- Metal concentrations for all solid and aqueous samples were analyzed using ICP–AES or ICP–MS as appropriate for the concentration ranges observed for major and trace using a Perkin Elmer (Wellesley, MA) Optima 5300DV ICP-AES or Elan DRC-II ICP-MS, respectively.
- Selenium and arsenic speciation was determined by High Performance Liquid Chromatography-ICP-MS (HPLC-ICP-MS) using an Agilent (Palo Alto, CA) 1100 HPLC interfaced to the Perkin Elmer Elan DRC-II ICP–MS with a MiraMist pneumatic nebulizer.
- Mercury was determined using a PS Analytical (Deerfield, FL) Millennium Merlin cold vapor atomic fluorescence spectrometer.
- Total organic carbon was measured using a Shimadzu TOC-V.
- Synchrotron-based X-ray Absobsorption Near Edge Structure (XANES) spectroscopy was run at beamline 11-2 at the Stanford Synchrotron Lightsource.
Figure 5 A and B. Arsenic speciation in the oxic and anoxic regime Emory River and Sluice Channel elutriate experiments. Non-detect concentrations of arsenic species (<2 µg/L) are plotted as one-half the detection limit (i.e. 1 µg/L).
Figure 4 A and B. Selenium speciation in the oxic and anoxic regime Emory River and Sluice Channel elutriate experiments. Non-detect concentrations of selenium species (<2 µg/L) are plotted as one-half the detection limit (i.e. 1 µg/L).
- Both liquid and solid state speciation data indicates that As(V) is the dominant form of arsenic in all fly ash sediments and elutriates
- Selenium is primarily in the selenite (Se(IV)) form
- Submersion of the fly ash affects Se speciation. Reduced Se species and/or organo-selenium are found in river and sluice channel samples
- Bubbling with air or N did not greatly affect Se speciation in the elutriate studies when compared to the Emory River or Sluice Channel sediments
- The results show that the fly ash exhibits geochemical stability, but it is undergoing kinetically slow transformations while submerged in the Emory River
- Since these experiments overestimate natural field conditions, it is not likely that dredging submerged sediments will cause Se oxidation to Se(VI)
- The major elements of concern are found in the less toxic forms: Se(IV) and As(V)
- FUTURE STUDIES:
- We just returned to the site to collect additional fly ash samples to look at Se and As speciation after long-term dredging. Studies will include: photo-oxidation and stability of Se, and As speciation of dredged materials.
- Permission was granted by the Chief of Engineers to publish this material. The authors thank Jonathan Burr (TDEC) for assistance in field sampling.
Figure 6 A and B. XANES scans and Linear Combination fits of Se in fly ash sediments (SED) and elutriate (EL) samples for both the Emory River (ER) and Sluice Channel (SL)