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Selenium weathering kinetics and at-source control

This presentation discusses the selenium weathering process, the leachability of selenium from coal refuse, and methods to immobilize leached selenium. It presents findings from laboratory leaching studies and field trials using ferrihydrite.

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Selenium weathering kinetics and at-source control

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  1. Selenium weathering kinetics and at-source control Paul Ziemkiewicz, PhD Director, West Virginia Water Research Institute West Virginia Acid Mine Drainage Task Force Annual Symposium 30 Mar 10 West Virginia University

  2. Selenium Weathering In Spoil • Postulated selenium weathering process: • Selenide: Se-2 FeSe Oxidizes to: • Selenite: Se+4 SeO32- Sorbs to FeO(OH) Oxidizes to: • Selenate: Se+6 SeO42- Highly Mobile !! • The trick is to catch selenium at step #2 West Virginia University

  3. Selenium forms determined by pH and redox The pH range that we care about Selenate Elemental Se (solid) Electron loss (oxidation) Selenite Electron gain (reduction) Selenide West Virginia University

  4. Selenium leaching study: Coal Refuse • Begun in November 2008 at WRI lab • Objective, to Identify: • What proportion of selenium is leachable • How rapidly selenium will leach from coal refuse • How to immobilize leached selenium • Method: • Samples of coal refuse were placed in humidity cells • Treatments: ferrihydrite, steel wool, FGD cake • Leachate collected every two weeks West Virginia University

  5. How much selenium is mobile? Selenium concentrations in rock vary between non detect and 10 mg/kg The actionable level is generally 1.0 mg/kg Most of the problematic rock is between 2 and 5 mg/kg The extractable fraction is between 20% and 40%, averaging about 33% That includes the organically bound selenium fraction which may not, in fact, be mobile West Virginia University

  6. Distribution of Selenium in Kanawha Formation (from Roy and Vesper sequential extraction study): Refuse values are averages of shale and coal West Virginia University

  7. Another quicker way to identify how much of the rock’s Se is mobile Pumure, J.J. Renton, R.B. Smart, 2010 Ultrasonic extraction of arsenic and selenium from rocks associated with mountaintop removal/valley fills coal mining: Estimation of bioaccessible concentrations Chemosphere 78 (2010) 1295–1300 West Virginia University

  8. Fig. 7. Comparison of total Se obtained using UE and sequential extractiontechniques. Combined water, exchangeable and NaOH soluble extracts are considered bioaccesible. West Virginia University

  9. 64 Week Leaching Study The initial selenium leaching rate was about 0.07%/day Declines gradually after about 30 weeks After 64 weeks of laboratory leachingFeO(OH) kept [Se] near to or below 5 μg/L West Virginia University

  10. Ferrihydrite has kept [Se] under control for 64 weeks West Virginia University

  11. Does 5 μg/L really mean 5 μg/L? • The refuse pile in question leaches the conservative anion chloride as well as selenium • The liquid to solid ratio for each leach cycle was about 2:1 • The resulting concentration was adjusted mathematically such that chloride concentration in the leachate matched that for the field site (a several million ton refuse pile) • So, it’s a reasonable assumption for the time being • But, it’s unlikely that a field site will leach with the efficiency of a humidity cell so let’s not get carried away West Virginia University

  12. Assume 33% of Se is leachable:leaching rate ~ 0.07%/day West Virginia University

  13. Estimated 10 years to leach 90% West Virginia University

  14. Ferrihydrite Field Trials: Hobet 21 Installed in March 2010 Each cells consists of about 60 tons of selenium rich pit cleanings Pit cleanings were placed over different thicknesses of ferrihydrite Ferrihydrite originated at a passive treatment system Supplied by Hedin Environmental Services Each cell is lined with plastic and drains to a 330 gal tank Chemistry and volume will be taken bi weekly West Virginia University

  15. Cross-section of ferrihydrite test cell Sandstone Pit cleanings FeOOH 4 ft. 3 ft. 24 ft. long x 16 ft. wide West Virginia University

  16. Test cell application rates and plot layout Individual water collection tanks West Virginia University Drainage collection tanks Drainage collection tanks

  17. Plastic liner and underdrain West Virginia University

  18. Ferrihydrite placement West Virginia University

  19. Ferrihydrite prior to addition of pit cleanings: ~20% moisture West Virginia University

  20. Placement of pit cleanings over ferrihydrite West Virginia University

  21. Water collection tanks West Virginia University

  22. Overview: 30 cells West Virginia University

  23. Segregated Pit Cleaning Cell West Virginia University

  24. Special Handling Cell:Ferrihydrite application in unlined cell Soil/spoil Pit cleanings Ferrihydrite/pit cleanings blend Pit cleanings Ferrihydrite/pit cleanings blend West Virginia University

  25. Questions? Paul Ziemkiewicz, PhD, Director West Virginia Water Research Institute West Virginia University West Virginia University

  26. West Virginia University

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