Selenium In-situ Selenium Treatment

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1 IMWA Sydney, Nova Scotia Mine Water & Innovative Thinking 7/11/11 At source control of Selenium Paul Ziemkiewicz, PhD Director, West Virginia Sept Objectives: Identify practical methods for immobilizing Se in source rock Identify yse oxidation/leaching kinetics Estimate duration of Se production 1 Selenium In-situ Selenium Treatment Postulated selenium weathering process: 1. Selenide: Se - FeSe Rapidly oxidizes to:. Selenite: Se + SeO - Sorbs to FeOOH Slowly oxidizes to:. Selenate: Se + SeO - Highly Mobile!! The goal is to catch selenium at step # Selenium occurs in two important oxidized forms: Selenite Selenate Eh HSeO - (+) H SeO (+) HSeO - SeO - (+) (+) Se(elem) () SeO - (+) H Higher ph Se (-) -. HSe favors selenate - -. (-) selenate selenite Selenium leaching study: Coal Refuse Begun in November 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 ph 1

2 IMWA Sydney, Nova Scotia Mine Water & Innovative Thinking 7/11/11 Treatments Steel wool or zero valent iron (ZVI) If there was significant selenate in the leachate, ZVI might reduce it to an immobile form Ferrihydrite, FeOOH, AMD sludge Donated by Bob Hedin, Hedin Environmental, Inc. Low moisture content If most of the leachate is selenite, ferrihydrite will adsorb it FGD cake or scrubber sludge If there was significant selenate then the sulfite in FGD cake might reduce it to selenite or elemental selenium Proved to be a significant net source of selenium Selenium Content of Test Refuse Sample [Se] mg/kg Mean 1.55 Std dev..1 7 Treatment Application Rates Distribution of Selenium in Kanawha Formation (from Roy and Vesper sequential extraction study): Refuse values are averages of shale and coal grams appl. Treatment dry refuse rate FeOOH * % Steel wool 555 5% * % Fe = 5 g of Fe Org shale Coal Tailings Soluble/Exchange 1% % 11% Fe, Mn Oxides % % % Sulfides % % 11% Organic 1% 5% 11% total extracted % 5% % Residual % 75% % Total % % % 9 Estimated Se in each humidity cell assuming % extractable Results after 7 weeks of leaching Total Extractable 1. The selenium leaching rate is about.%/day%/day [Se] mg/kg. FeOOH kept [Se] near to or below.5 μg/l Se/humidity cell.. mg 11 1

3 7/11/11 [Se] μg/l [Se] remains near 5 μg/l with FeOOH control FeOOH Does 5 μg/l really mean 5 μg/l? The refuse pile in question leaches the conservative anion chloride The liquid to solid ratio for each leach cycle was about :1 The resulting concentration was adjusted mathematically such that t chloride concentration ti in the leachate matched that t for the field site (a several million ton tailings pile) Observed [Se] x 7.=estimated field [Se] Very close to field [Se] observations Nonetheless, it s unlikely that a field site will leach with the efficiency of a humidity cell so let s not get carried away 1 7% of mobile Se Leached by week 7 Se Leaching Rate is about.%/day % 5% % % % 5% day.5%.%.%.%.5% % IMWA Sydney, Nova Scotia Mine Water & Innovative Thinking % Se/d.% Weeks of leaching Weeks of leaching 1 5% % 5% % % % 5% Assume % of Se is leachable: leaching rate ~.%/day % predicted observed Estimated years to leach 9% predicted observed % 9% % 7% % 5% % % % % %

4 IMWA Sydney, Nova Scotia Mine Water & Innovative Thinking 7/11/11 Y=cumulative % Se leached X= weeks % % 1% y =.ln(x) -.97 R² =.91 predicted observed Log. (predicted) Compared to Sulfide Oxidation Rates Iron sulfide oxidation rates:.% /day-coal iron sulfides to.7 %/day-hydrothermal pyrite Iron selenide in coal:.%/day Or, to times faster However: Unlike AMD, there is nothing to neutralize selenium and, Field leaching will likely be slower And, there just isn t that much selenium to start with Why Selenite Sticks to Ferrihydrite and Selenate does not Water molecules Typical high total sulfur level in coal associated rock: 5.% An exceptionally high selenium concentration in coal associated rock would be: 5. mg/kg or:.5% Or,, times more S than Se ace Ferrihydrite Surfa Selenate (+) Selenite (+) Hydration sheath limits sorption Weak bond No hydration sheath, strong, bidentate bonds to FeO(OH) Hayes et al. (7) Science 1 What is Ferrihydrite? By product of AMD treatment: AMD sludge Starts out at about 9% water Contains Iron, Aluminum, Manganese hydroxides Plus gypsum, silicate, calcite depending on AMD treatment process The useful components are the iron oxyhydroxides: Ferrihydrite-fresh AMD treatment sludge Iron oxyhydroxides undergo dehydration: 1. Ferric hydroxide: Fe(OH) minus H O=. Ferrihydrite/Goethite: FeOOH

5 IMWA Sydney, Nova Scotia Mine Water & Innovative Thinking 7/11/11 Ferrihydrite-dried out, turning into goethite Application: Lining a Pit Floor with Ash 5 Segregated Pit Cleaning Cell Special Handling Cell: Ferrihydrite application in unlined cell Soil/spoil Pit cleanings Ferrihydrite/pit cleanings blend Pit cleanings Ferrihydrite/pit cleanings blend 7 Questions? At Source Control of Selenium Paul Ziemkiewicz, PhD, Director West Virginia pziemkie@wvu.edu 9 5