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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. Selenium Weathering In Spoil. Postulated selenium weathering process: Selenide: Se -2 FeSe

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selenium weathering kinetics and at source control

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

selenium weathering in spoil
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

selenium forms determined by ph and redox
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

selenium leaching study coal refuse
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

how much selenium is mobile
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

slide6

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

West Virginia University

another quicker way to identify how much of the rock s se is mobile
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

slide8

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

64 week leaching study
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

does 5 g l really mean 5 g l
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

estimated 10 years to leach 90
Estimated 10 years to leach 90%

West Virginia University

ferrihydrite field trials hobet 21
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

cross section of ferrihydrite test cell
Cross-section of ferrihydrite test cell

Sandstone

Pit cleanings

FeOOH

4 ft.

3 ft.

24 ft. long x 16 ft. wide

West Virginia University

test cell application rates and plot layout
Test cell application rates and plot layout

Individual water

collection tanks

West Virginia University

Drainage collection tanks

Drainage collection tanks

plastic liner and underdrain
Plastic liner and underdrain

West Virginia University

ferrihydrite placement
Ferrihydrite placement

West Virginia University

water collection tanks
Water collection tanks

West Virginia University

overview 30 cells
Overview: 30 cells

West Virginia University

segregated pit cleaning cell
Segregated Pit Cleaning Cell

West Virginia University

special handling cell ferrihydrite application in unlined cell
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

questions
Questions?

Paul Ziemkiewicz, PhD, Director

West Virginia Water Research Institute

West Virginia University

West Virginia University