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

Selenium Weathering In Spoil Water Research Institute West Virginia University 2 Postulated selenium weathering process: 1. Selenide:Se -2 FeSe Oxidizes to: 2. Selenite:Se +4 SeO 3 2- Sorbs to FeO(OH) Oxidizes to: 3. Selenate:Se +6 SeO 4 2- Highly Mobile !! The trick is to catch selenium at step #2

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

Selenium leaching study: Coal Refuse Water Research Institute West Virginia University 4 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

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/kg 3. Most of the problematic rock is between 2 and 5 mg/kg 4. 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 Water Research Institute West Virginia University 5

Distribution of Selenium in Kanawha Formation (from Roy and Vesper sequential extraction study): Refuse values are averages of shale and coal Water Research Institute West Virginia University 6 Org shaleCoalRefuse Soluble/Exchange12%10%11% Fe, Mn Oxides0% Sulfides10% 11% Organic18%5%11% total extracted40%25%33% Residual60%75%68% Total100%

Another quicker way to identify how much of the rocks Se is mobile Water Research Institute West Virginia University 7 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 RockSequentialUltrasonic Sample #extraction BT 6119%17% BT 24126%24% BT 57238% BT 70160%48% average36%32%

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

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 leaching FeO(OH) kept [Se] near to or below 5 μg/L Water Research Institute West Virginia University 9

Ferrihydrite has kept [Se] under control for 64 weeks Water Research Institute West Virginia University 10

Does 5 μg/L really mean 5 μg/L? Water Research Institute West Virginia University 11 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, its a reasonable assumption for the time being But, its unlikely that a field site will leach with the efficiency of a humidity cell so lets not get carried away

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

Estimated 10 years to leach 90% Water Research Institute West Virginia University 13

Ferrihydrite Field Trials: Hobet Installed in March Each cells consists of about 60 tons of selenium rich pit cleanings 3. Pit cleanings were placed over different thicknesses of ferrihydrite 4. Ferrihydrite originated at a passive treatment system Supplied by Hedin Environmental Services 5. Each cell is lined with plastic and drains to a 330 gal tank 6. Chemistry and volume will be taken bi weekly Water Research Institute West Virginia University 14

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

Test cell application rates and plot layout Water Research Institute West Virginia University Drainage collection tanks Individual water collection tanks Ferrihydrite application rate %tonsin. 0.0% % % % %

Plastic liner and underdrain Water Research Institute West Virginia University 17

Ferrihydrite placement Water Research Institute West Virginia University 18

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

Placement of pit cleanings over ferrihydrite Water Research Institute West Virginia University 20

Water collection tanks Water Research Institute West Virginia University 21

Overview: 30 cells Water Research Institute West Virginia University 22

Segregated Pit Cleaning Cell Water Research Institute West Virginia University 23

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

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

Water Research Institute West Virginia University 26