Paul Ziemkiewicz, PhDDirector, West Virginia Water Research Institute

West Virginia Acid Mine Drainage Task Force Annual Symposium

30 Mar 10

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Selenium weathering kinetics and at-source control

Selenium Weathering In Spoil

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Postulated selenium weathering process:

1. Selenide: Se-2 FeSe Oxidizes to:

2. Selenite: Se+4 SeO32- Sorbs to FeO(OH)

Oxidizes to:3. Selenate: Se+6 SeO4

2- Highly Mobile !!

The trick is to catch selenium at step #2

Selenium forms determined by pH and redox

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The pH range that we care

about

Selenate

Selenite

Selenide

Elemental Se (solid)

Electron gain(reduction)

Electron loss(oxidation)

Selenium leaching study: Coal Refuse

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Begun in November 2008 at WRI lab Objective, to Identify:

What proportion of selenium is leachableHow rapidly selenium will leach from coal refuseHow to immobilize leached selenium

Method:Samples of coal refuse were placed in humidity cellsTreatments: ferrihydrite, steel wool, FGD cakeLeachate collected every two weeks

How much selenium is mobile?

1. Selenium concentrations in rock vary between non detect and 10 mg/kg

2. The actionable level is generally 1.0 mg/kg3. Most of the problematic rock is between 2 and 5 mg/kg4. The extractable fraction is between 20% and 40%,

averaging about 33%5. That includes the organically bound selenium fraction

which may not, in fact, be mobile

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Distribution of Selenium in Kanawha Formation (from Roy and Vesper sequential extraction study):

Refuse values are averages of shale and coal

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Org shale Coal Refuse

Soluble/Exchange 12% 10% 11%

Fe, Mn Oxides 0% 0% 0%

Sulfides 10% 10% 11%

Organic 18% 5% 11%

total extracted 40% 25% 33%

Residual 60% 75% 68%

Total 100% 100% 100%

Another quicker way to identify how much of the rock’s Se is mobile

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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

Rock Sequential UltrasonicSample # extraction extraction

BT 61 19% 17%BT 241 26% 24%BT 572 38% 38%BT 701 60% 48%

average 36% 32%

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

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64 Week Leaching Study

1. The initial selenium leaching rate was about 0.07%/day

2. Declines gradually after about 30 weeks

3. After 64 weeks of laboratory leachingFeO(OH) kept [Se] near to or below 5 μg/L

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Ferrihydrite has kept [Se] under control for 64 weeks

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Does 5 μg/L really mean 5 μg/L?

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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 2:1The 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 beingBut, it’s unlikely that a field site will leach with the

efficiency of a humidity cell so let’s not get carried away

Assume 33% of Se is leachable:leaching rate ~ 0.07%/day

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Estimated 10 years to leach 90%

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Ferrihydrite Field Trials: Hobet 21

1. Installed in March 20102. Each cells consists of about 60 tons of selenium rich pit

cleanings3. Pit cleanings were placed over different thicknesses of

ferrihydrite4. Ferrihydrite originated at a passive treatment system Supplied

by Hedin Environmental Services5. Each cell is lined with plastic and drains to a 330 gal tank6. Chemistry and volume will be taken bi weekly

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Cross-section of ferrihydrite test cell

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Pit cleanings

FeOOH

Sandstone

24 ft. long x 16 ft. wide

3 ft.4 ft.

Test cell application rates and plot layout

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1 16

2 173 184 195 206 217 228 239 24

10 2511 2612 2713 2814 2915 30

Individual water collection tanks

Ferrihydriteapplication rate

% tons in.0.0% 0.0 0.000.4% 0.2 0.252.8% 1.7 2.25

11.3% 6.8 9.0022.5% 13.6 18.00

Plastic liner and underdrain

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Ferrihydrite placement

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Ferrihydrite prior to addition of pit cleanings: ~20% moisture

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Placement of pit cleanings over ferrihydrite

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Water collection tanks

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Overview: 30 cells

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Segregated Pit Cleaning Cell

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Special Handling Cell:Ferrihydrite application in unlined cell

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Ferrihydrite/pit cleanings blend

Pit cleanings

Ferrihydrite/pit cleanings blend

Pit cleanings

Soil/spoil

Questions?

Paul Ziemkiewicz, PhD, DirectorWest Virginia Water Research Institute

West Virginia University

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