SELENIUM MOBILITY IN COAL AND OVERBURDEN IN CENTRAL APPALACHIA

R.R. Maggard

INTRODUCTION___________________________________

• Passage of SMRCA prioritizes issues such as AMD (Acid Mine Drainage)

– Methods to predict, prevent, and remediate typical AMD have since been developed

However…– Methods to deal with issues concerning

selenium have taken a different path

www.streamrestorationinc.org

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• EXAMPLE:– The analysis of iron for the entire geologic column never led

to the conclusion that the “special handling” of all iron-bearing strata would prevent the formation of AMD.

• Further study determined that pyritic iron was most problematic — not all iron present in the strata

• This is the approach needed for selenium

INTRODUCTION (cont’d)

http://www.agen.ufl.edu/~chyn/age4660/lect/lect_18/coalstruc.gif

OBJECTIVES________________________________________

• Determine the actual mobility of selenium in the strata– Which stratagraphic units are more problematic?

• Concentrate analytical work on using typical acid base analysis in conjunction with selenium analysis– Incorporate several leaching techniques of

potentially problematic strata in regard to Se mobility

METHODOLOGY FOR SELENIUM ANALYSIS

_____________________________________________• Solid composite samples were collected using methods

approved for coals and soils• Prepared for analysis using SW-846 Method 3050B

– Acid digestion

• Resulting extract then analyzed for Se using method SD 7740– Graphite Furnace Atomic Absorption

Spectroscopy• Minimum detection level: 0.5 mg/kg

– Hydride AF methodwww.seleniumspeciation.com

METHODOLOGY FOR SELENIUM LEACHABILITY________________________________________

Trends noted during previous studies:1. Se solubility and occurrences in mine discharges

appear to be related to neutral—higher pH’s with water of moderate to high alkalinity

2. Se has been known to be associated with sulfur in overburden and coal samples

3. Se occasionally detected in shale samples that exceed the 1 mg/kg established by WVDEP for special handling that contain undetectable/very low levels of pyritic sulfur

METHODOLOGY FOR SELENIUM LEACHABILITY (cont’d)

_______________________________

• As a result of these trends:

– Several methods were developed to determine circumstances that may facilitate maximum Se leachability under somewhat natural conditions

– Methods were designed to be a reproducible and simple expedited procedure

______________________________• Samples from a single core hole

were selected:

– Selections were based on varying combinations of conditions found to exist in different rock types

– Five (5) varying samples were selected• Sample Nos. 5, 8, 14, 12C, 16C

PROCEDURES

http://nc.water.usgs.gov/ccp/2005Cove/photos/core910.jpg

PROCEDURES_________________________________________________________• Sample #5 was selected with barely detectable pyritic sulfur of 0.01

percent with a net neutralization potential of 4.24 and a paste pH of 7.6 and a selenium value of 2.19 mg/kg by the Hydride AF Procedure.

• Sample #8 was selected with below detectable value of pyritic sulfur <0.01 percent with a net neutralization potential of 13.73 and a paste pH of 7.7 and a selenium value of 1.02 mg/kg by the Hydride AF Procedure right at the threshold of the WVDEP special handling criteria of 1 mg/kg.

• Sample #14 was selected with a 0.07 value for pyritic sulfur with a net neutralization potential of 1.86 and a paste pH of 7.6 and a selenium value of 0.75 mg/kg which is below the WVDEP threshold and was analyzed by the Hydride AF Procedure,

PROCEDURES (cont’d)________________________________________________• Sample #12C was selected with a below detectable value of

pyritic sulfur <0.01 percent with a net neutralization potential of 0.91 and a paste pH of 6.2 and a selenium value of 4.92 mg/kg. This sample was analyzed by the GFAA Procedure.

• Sample #16C was selected with an easily detectable value of pyritic sulfur of 0.82 percent with a deficiency of neutralization of 36.74 and a paste pH of 3.4 and a selenium value of 9.46 mg/kg. This sample was analyzed by the GFAA Procedure.

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• Exposed for one hour to leaching solution while shaken

• Sample size: 2 grams with 50 ml of leaching solution– Leaching solution ranged from pH of 2.0 (H2SO4)

to 10.0 (CaO)

• pH measured after one hour– Analyzed for: Total Se, Al, Fe, Mn, Ca, and Mg

LEACHING METHOD NO. 1

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ABA Sample ID pH Final pH

Selenium (ug/l) Aluminum Iron Manganese Calcium Magnesium

2.0 2.18 17.3 3.03 20.1 0.337 29.0 10.2

3.0 3.98 18.1 0.185 2.26 <0.10 14.9 5.23

4.0 8.56 17.5 <0.10 <0.10 <0.10 1.01 0.411

5.0 8.85 19.2 <0.10 <0.10 <0.10 0.681 0.290

6.0 9.01 19.6 <0.10 <0.10 <0.10 0.689 0.278

7.0 8.78 20.1 <0.10 <0.10 <0.10 0.652 0.289

8.0 8.91 21.2 <0.10 <0.10 <0.10 0.674 0.292

9.0 9.07 22.5 <0.10 <0.10 <0.10 0.934 0.388

10.0 9.45 20.4 <0.10 <0.10 <0.10 1.27 0.515

Sample No. 5

0.0

5.0

10.0

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25.0

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2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Sample No. 5

% FeS2: 0.01

NP: 4.24

pH: 7.6

Se (mg/kg): 2.19

Sample No. 8

ABA Sample ID pH

Final pH

Selenium (ug/l) Aluminum Iron Manganese Calcium Magnesium

2.0 2.32 5.6 6.29 124 5.9 63.9 17.1

3.0 5.32 6.4 0.147 1.98 0.684 21.0 6.33

4.0 9.30 6.7 <0.10 <0.10 <0.10 2.98 1.11

5.0 9.70 7.6 <0.10 <0.10 <0.10 2.61 0.945

6.0 9.67 7.6 <0.10 <0.10 <0.10 1.98 0.734

7.0 9.73 6.9 <0.10 <0.10 <0.10 1.73 0.596

8.0 9.59 8.1 <0.10 <0.10 <0.10 1.76 0.621

9.0 9.50 9.0 <0.10 <0.10 <0.10 3.25 1.13

10.0 10.0 8.6 <0.10 <0.10 <0.10 2.18 0.727

0.0

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9.0

Sel

eniu

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2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Sample No. 8

% FeS2: <0.01

NP: 13.73

pH: 7.7

Se (mg/kg): 1.02

Sample No. 14

ABA Sample ID pH

Final pH

Selenium (ug/l) Aluminum Iron Manganese Calcium Magnesium

2.0 2.01 5.5 6.66 43.4 0.699 20.9 8.48

3.0 3.74 4.2 0.320 10.2 0.123 14.2 5.57

4.0 7.30 5.9 <0.10 <0.10 <0.10 0.990 0.459

5.0 8.37 6.3 <0.10 <0.10 <0.10 0.655 0.306

6.0 8.48 6.6 <0.10 <0.10 <0.10 1.04 0.46

7.0 8.45 6.8 <0.10 <0.10 <0.10 0.707 0.331

8.0 8.45 7.1 <0.10 <0.10 <0.10 0.789 0.337

9.0 8.71 7.2 <0.10 <0.10 <0.10 1.14 0.482

10.0 9.34 7.0 <0.10 <0.10 <0.10 1.21 0.490

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2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Sample No. 14

% FeS2: 0.07

NP: 1.86

pH: 7.6

Se (mg/kg): 0.75

Sample No. 12C

ABA Sample ID pH

Final pH

Selenium (ug/l)

Aluminum Iron Manganese Calcium Magnesium

2.0 2.01 24.1 0.346 3.26 <0.10 3.85 1.08

3.0 3.04 18.9 <0.10 1.62 <0.10 3.13 0.889

4.0 4.60 13.6 <0.10 0.285 <0.10 1.69 0.553

5.0 5.03 13.6 <0.10 <0.10 <0.10 1.34 0.440

6.0 5.24 13.6 <0.10 <0.10 <0.10 1.23 0.405

7.0 5.04 14.1 <0.10 <0.10 <0.10 1.62 0.509

8.0 5.23 13.6 <0.10 <0.10 <0.10 1.57 0.416

9.0 5.91 15.5 <0.10 <0.10 <0.10 1.68 0.436

10.0 6.54 14.0 <0.10 <0.10 <0.10 2.63 0.547

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eniu

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2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Sample No. 12C

% FeS2: <0.01*

NP: 0.91

pH: 6.2

Se (mg/kg): 4.92

Sample No. 16C

ABA Sample ID pH

Final pH

Selenium (ug/l) Aluminum Iron Manganese Calcium Magnesium

2.0 1.99 81.8 2.65 95.7 <0.10 2.32 0.691

3.0 2.93 39.0 2.27 85.7 <0.10 2.20 0.641

4.0 3.35 26.2 2.31 85.8 <0.10 2.27 0.672

5.0 3.38 27.0 2.27 83.9 <0.10 2.23 0.659

6.0 3.32 26.7 2.24 82.0 <0.10 2.30 0.653

7.0 3.37 27.4 2.25 81.9 <0.10 2.42 0.651

8.0 3.38 26.3 2.27 82.8 <0.10 2.61 0.669

9.0 3.40 12.9 2.17 78.5 <0.10 3.19 0.646

10.0 3.41 25.1 2.26 82.5 <0.10 4.46 0.705

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Sel

eniu

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2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

Sample No. 16C

% FeS2: 0.82*

NP: -36.74

pH: 3.4

Se (mg/kg): 9.46

BlankABA Sample

ID pHFinal

pHSelenium

(ug/l) Aluminum Iron Manganese Calcium Magnesium

2.0 - <0.2 <0.10 <0.10 <0.10 <0.10 <0.10

3.0 - <0.2 <0.10 <0.10 <0.10 <0.10 <0.10

4.0 - <0.2 <0.10 <0.10 <0.10 <0.10 <0.10

5.0 - <0.2 <0.10 <0.10 <0.10 <0.10 <0.10

6.0 - <0.2 <0.10 <0.10 <0.10 <0.10 <0.10

7.0 - <0.2 <0.10 <0.10 <0.10 0.202 <0.10

8.0 - <0.2 <0.10 <0.10 <0.10 0.363 <0.10

9.0 - <0.2 <0.10 0.123 <0.10 1.09 <0.10

10.0 - <0.2 <0.10 <0.10 <0.10 2.29 <0.10

Se analyzed on 11-11-08 by method SW7742

Other metals analyzed 11-7-08 by method SW6010B,

Extraction solutions prepared using H2SO4 and hydrated lime.

Samples extracted for 1 hour and then filtered through 0.45 um filter.

Extraction ratio: 2 g sample / 50 mL extractant

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• Review of previous tables:

– All samples extracted measurable Se from 4 ug/l to 82 ug/l

– Sample No. 5 had same levels as Sample No. 16C (4x the Se in the solid sample)

– Maximum solubility of shales occurred at a pH of 9

– Maximum solubility of coals occurred at a pH of 2– Soluble iron appeared to be related to selenium

solubility in shales

LEACHING METHOD NO. 1 (cont’d)

LEACHING METHOD NO. 2___________________________________________

• Method was developed to determine relationship between sample size and volume of extract

http://www.chemistryexplained.com/elements/images/chel_0001_0003_0_img0389.jpg

•Sample No. 14 chosen because it had the lowest measurable Se in original sample (0.75 mg/kg)

•Sample size ranged from 0.5 g to 50 ml of extract at pH 9 to 2 g to 50 ml

•To prevent contamination of extract by solids each run was duplicated with a 0.45 um filter and 0.20 um filter

LEACHING METHOD NO. 2 (cont’d)___________________________________

• Values of resultant extract:

– Ranged from ~ 2 ug/l to 8 ug/l

• Last runs of 2.0 g to 50 ml repeat values from previous leaching, yielding 7 and 8 ug/l in the extraction

– Sample size and extract values appear to be approximately linear

Sample 14 Selenium Concentration Comparison as Determined with Increasing Sample to Extractant Ratios

and Two Filter Sizes

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NA 0.5 1.0 2.0Sample to Extractant Ratio

(g to 50 mL)

Sel

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pb

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0.45 um Filter

0.20 um Filter

PREVENTION AND TREATMENT_______________________________________

• Does the iron in AMD sludge prevent or reduce the mobility of selenium?– Dried AMD sludge was added to various

overburden and coal samples• Idea proposed by Dr. P. Ziemkiewicz

– Same leaching procedure was used as previously described with a 10 % addition of dried AMD sludge (by weight)

– Soluble Se levels reduced 17 to 33 %

Comparison of Selenium concentrations of normal leaching procedure and the addition of 0.1 g of dried

AMD sludge

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Sample ID

Sel

eniu

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pp

b)

Normal leaching procedure at 1 gto 50 mL of extract

Plus the addition of 0.1 g driedAMD sludge

CONCLUSIONS_______________________________________• Solubility and mobility of Se as a result of

various coal mining methods can be highly variable– Dependent on geochemical characteristics of parent rock

source– Pyritic sulfur content may inhibit the solubility/mobility of

Se in the water column• Lovett and Ziemkiewicz previously noted this possibility

• Additional leaching studies are being conducted– GOAL: to better understand the circumstances of Se

solubility and mobility and its prevention in Southern Appalachia coalfields