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

________________________________________ 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)

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 method

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

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 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 and a paste pH of 3.4 and a selenium value of 9.46 mg/kg. This sample was analyzed by the GFAA Procedure.

_______________________________________ 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 (H 2 SO 4 ) to 10.0 (CaO) pH measured after one hour –Analyzed for: Total Se, Al, Fe, Mn, Ca, and Mg LEACHING METHOD NO. 1 periodictable.com/Samples/025.7/s9s.JPG

ABA Sample ID pHFinal pH Selenium (ug/l)AluminumIronManganeseCalciumMagnesium < < < < < < < < Sample No. 5

% FeS 2 : 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)AluminumIronManganeseCalciumMagnesium < < < < < < <

% FeS 2 : <0.01 NP: pH: 7.7 Se (mg/kg): 1.02

Sample No. 14 ABA Sample ID pH Final pH Selenium (ug/l)AluminumIronManganeseCalciumMagnesium < < < < < < <

% FeS 2 : 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) Aluminu mIronManganeseCalciumMagnesium < < < < < < < < < < <

% FeS 2 : <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)AluminumIronManganeseCalciumMagnesium < < < < < < < < <

% FeS 2 : 0.82* NP: pH: 3.4 Se (mg/kg): 9.46

Blank ABA Sample ID pH Final pH Selenium (ug/l)AluminumIronManganeseCalciumMagnesium 2.0-<0.2< <0.2< <0.2< <0.2< <0.2< <0.2< < <0.2< < <0.2< < < <0.2< <0.10 Se analyzed on by method SW7742 Other metals analyzed 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

_________________________________________________ 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 _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

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 %

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