Geochemical Controls on Limestone Utilization in Abandoned Mine Land Reclamation Poonam Giri Tracy Branam Dr. Greg Olyphant
ht t p:/ / anr.ext .wvu.edu/ land-reclamation Project Scope Overview of Acid Mine Drainage I. (AMD), Remediation Options, and Practices Armoring Process and II. Associated Concerns Conceptual Model III. Experimental Design and IV. Methods S imulation Results and Insights V. Critical Research Questions VI.
1 kg CaCO3 (369.34 cubic cm) -50 100 g CaCO3 (36.934 cubic cm) Potential Alkalinity (mg/L as CaCO 3 ) pH mg/L as CaCO 3 ) 9 8 7 6 5 50 0 2 FeS 2 (s) + 7 O 2 + 2 H 2 O → 2 Fe 2+ +4 SO 4 2- + 2 H + 2 Fe 2+ + 0.5 O 2 + 2H + → 2 Fe 3+ + H 2 O FeS 2 (s) + 14 Fe 3+ +8 H 2 O → 15 Fe 2+ +2 SO 4 2- + 16 H + + Al, Mn + Trace metals : Zn, Cr, Cu, Ni, Pb Carbonate treatments offer significant neutralization potential : 1 m 3 CaCO 3 can produce 2.64 X10 4 mg/L alkalinity * ! * complete reaction at 25°C, 1 atm, pH 7 www.unit edaggregates.net / knox-county-sand-and-gravel/ CaCO 3 (s) + H + → Ca 2+ + HCO 3 - CaCO 3 (s) + H 2 CO 3 * → Ca 2+ + 2HCO 3 - CaCO 3 (s) + H 2 O → Ca 2+ + HCO 3 - + OH-
Limestone-Based Treatments Open (Oxic) Limestone Drain Buried (Anoxic) Limestone Drain Natural – no caustic chemical additives (passive system) Inexpensive– limestone is readily available, low maintenance Multiple well-established options for design and application Easy to apply … ? Limestone dissolves in the AMD and adds alkalinity However, acidity and alkalinity co-exist …Some of the produced alkalinity causes metal oxidation and hydrolysis! Formation of precipitates limit lifetime of the system
Concerns Regarding Limestone S ystems Armored limestone is only ~60% as effective in generating alkalinity as fresh stone Formation of metal solids leads to : (Pearson and McDonnell, 1975; Ziemkiewicz et al., 1997) Coating (“ armoring” ) of grain surfaces Mineral Name Reaction Pore space plugged (A) Oxides and Hydroxides Fe 3+ + 3H 2 O ↔ Fe(OH) 3 + 3H + Iron Oxide Al 3+ + 3H 2 O ↔ Al(OH) 3 + 3H + Aluminum Oxide Thus, Al 3+ + 3H 2 O ↔ Al(OH) 3 + 3H + Gibbsite Fe 3 + + 2 H 2 O ↔ FeO(OH) + 3H + Goethite Unreacted Fe 3 + + 2 H 2 O ↔ FeO(OH) + 3H + Lepidocrocit e 2Fe 3+ + 3 H 2 O ↔ Fe 2 O 3 +6 H + Hematite limestone is sealed Mn 2+ + 2 H 2 O ↔ e- + MnOOH +2 H + Manganite CaCO 3 (s) + H + ↔ Ca 2+ + HCO 3 Calcite - off from acidic (B) Sorbates >(s)FeOH + Mn 2+ + → >(s)FeOMn + + H + Manganese solution >(w)FeOH + Mn 2+ + → >(w)FeOMn + + H + >(s)FeOH + Zn 2+ + → >(s) FeOZn + + H + Zinc >(w)FeOH + Zn 2+ + → >(w)FeOZn + + H + Neutralization >(s)FeOH + Pb 2+ + → >(s)FeOPb + + H + Lead >(w)FeOH + Pb 2+ + → >(w)FeOPb + + H + process is retarded Hammarst rom et al., 2003. Applied Geochemistry, v. 18 (11) >(s)FeOH + Cu 2+ + → >(s)FeOCu + + H + Copper >(w)FeOH + Cu 2+ + → >(w)FeOCu + + H +
Addressing Details And Mechanics Of Armoring Literature shows wide range (48-96% ) in efficiency (Ziemkiewicz et al. , 1997; Cravot t a and Trahan, 1999; Wat zlaf et al., 2000) Key Questions: X Assume 60% efficiency? Which elementary reactions occur? What is their spatial distribution? How quickly do reactions proceed? How does the system evolve through time? ht t p:/ / www.facst aff.bucknell.edu/ kirby/ ALDOLD.html
O 2 H 2 O Acid Mine Drainage (AMD) Treatment in an Oxic Limestone Drain (OLD) Refuse ↓ Reactive Surface Area, ↓ Reaction Rate ↓ Reaction Rate 3 . Remediation 2 . Buffering & Neutralization Water Discharge: Chemistry High pH, 5 . Sorption & Data: Low E.C. & Low pH, high Coprecipitation TDS, E.C. & TDS, Low metal E high metal content, ↑ pH, ↑ n content, residual sulfate rich F + ions ions S (A) Oxides and F (B) Sorbates AMD ↑ pH E Hydroxides L 1 . Dissolution ↓ Reactive E transport Surface U Area P Physical Data: Temperature, E ↑ pH Discharge, 6 . Armoring Precipitation - ions N T (C) Sulfate n →0 Mineral 4 . Precipitation Minerals Formation 7 . Clogging - H 2 O Channel Parameters: Dimensions (L, W, H), + H 2 O slope, ↓ n composition (CaCO 3) , (D) Hydrated grain size, packing , (E) Sulfate Sulfate K, k, n, D Salts Minerals
Investigation Methods Transient numerical modeling provides a quantitative examination of the simultaneously occurring geochemical reactions between limestone drains and AMD. “ how fast, and to what extent, do elementary and coupled reactions occur over time? ” S uite of Transport Reactions Advection (Darcy’ s law) Time-dependent Dispersion (Fick’ s Law) Interactions between AMD and rock Develop a model which allows reaction coupling and feedback loops…
Reaction Kinetics Overall Mineral Reaction Rate: specific reaction Thermodynamic term (Chemical) drive surface area React ion mechanics, t endency t oward Grain size and shape cat alyst s/ inhibitors equilibrium wit h solut ion Precipitate Limestone
Key Minerals and Rates Fe(OH) 3 (a) and Al(OH) 3 (a): highly soluble FAST reactions (transport control) Equilibrium phases Limestone, Oxides, Stable Minerals : less soluble reaction rate is variable (surface control) Kinetic Phases Oxidation : variable rate pH, biological controls Kinetics Specific Reaction Rate Equation (k), at 25 ° C Phase Reference Palandri and Limestone Kharaka, 2004 Aqueous Iron Mineral GFW Density Molar A BET Grain A 0 /V S inger and Oxidation (g/mol) (g/cm 3 ) (m 2 /g) Size (cm 2 /L) Volume S tumm, 1970 (Abiotic) (cm 3 /mol) Aqueous Iron Limestone 36.93 Kirby et al., 100.09 2.71 3.45 × 10 -5 6.4 cm 14.02 Oxidation 1999 Goethite 21.51 (Biotic) 88.85 4.13 32 a 0.5 µm a 1.1×10 -5 Palandri and Gibbsite 32.23 78.00 2.42 50 b 0.15µm c 1.5×10 -5 Goethite Kharaka, 2004 Gypsum 73.90 172.18 2.33 1.1 d 20 µm d 7.3×10 -7 Palandri and Gibbsite Notes: GFW = Gram Formula Weight Kharaka, 2004 A BET = Brunauer– Emmett– Teller (BET) Surface Area Miller et al., A0/ V = Mineral surface in contact with solution Alunite 2016 Palandri and Gypsum Kharaka, 2004
Midwestern Anoxic Limestone Drain S urface and underground coal mining operations between 1895 and 1983, leaving coarse-grained refuse piles and fine-grained tailings deposits Perennial acidic discharge from a flooded underground mine working In 1996, installed a 250 foot ALD followed by a settling pond 973.39 m 3 of # 2 grade limestone Depth of 5 feet S ealed with a low-permeability soil cap and plastic liner Discharge through the drain is 54 gpm
Initial S urface Area
Water Quality at the Midwestern AML S ite before after pH 3.7 – 5.1 6.0 – 7.3 Acidity (mg/ l) 367 236 Alkalinity (mg/ l) 11 267 Iron (mg/ l) 76 86 Aluminum 4 <2 S ulfate (mg/ l) 1,380 1,463
ALD Model S imulations Initial pore ALD water influent Model Design Temp 15° C 19° C pH 6.5 5.1 1D model (slice through ALD) Initial Eh 227 mV 230 mV Mineral Chemical Al Volume 0.6 mg/ L 3.5 mg/ L Assemblage Formula 5 m cells, containing ultra-pure #2 limestone and negligible ) (% oxides Primary - HCO 3 535.6 mg/ L 16.4 mg/ L Limestone 40% Porosity CaCO 3 60 Gravel Ca 101.8 mg/ L 312.5 mg/ L Secondary 15 year simulation at a time step of 5.2 hours Amorphous Iron Fe(OH) 3 (a) - Amorphous iron and aluminum phases at equilibrium Oxide Cl 8.1 mg/ L 14.5 mg/ L Goethite FeO(OH) 0.08 Kinetic Abiotic Iron Oxidation and limestone dissolution Amorphous Fe +2 4.1 mg/ L 70.3 mg/ L Al(OH) 3 (a) - Aluminum Oxide Kinetic reactions for gibbsite, goethite, and gypsum Gibbsite Al(OH) 3 0.07 Fe +3 0.2 mg/ L 10.1 mg/ L Gypsum O 4 • 2H 2 O 0.16 Precipitates are 1-3 mm thick (Hammarstrom et al. ,2003 CaS Ziemkiewicz et al. ,1994, others) K 3.3 mg/ L 4.9 mg/ L Mg 68.5 mg/ L 79.5 mg/ L Boundary Conditions Unidirectional Flow at 23 m/ day (constant) Mn 0.6 mg/ L 5.1 mg/ L Thickness of precipitates is uniform and constant (non-selective, impermeable armors) Na 15.6 mg/ L 15.6 mg/ L Influent water a mix of spring, mine and spoil water (17:3:1) -2 SO 4 142.2 mg/ L 1268.8 Cauchy-type flux boundaries (discharge prescribed) mg/ L Diffusion coefficient of 3.0 x 10 -10 m 2 s -1 Newton-Raphson Iteration with convergence tolerance of 10 -12
Actual Simulated pH 6.0 – 7.3 6.2 Acidity (mg/l) 236 198 Alkalinity (mg/l) 267 236 Midwestern ALD 13-year Model Deviation Net 31 38 Performance Iron (mg/l) 86 79 Standard Mean Median Deviation Variance RMSE NRMSE Aluminum (mg/l) 1 0 Temp. 14.52 14.05 1.13 1.28 4.64 0.32 ° C Sulfate (mg/l) 1,463 1390 pH 6.50 6.50 0.23 0.05 0.41 0.06 Eh 0.17 0.16 0.03 0.61 0.16 0.98 V Net mg/ L 41.75 25.00 55.41 3070.59 14.71 0.35 Alkalinity CaCO 3 - 325.24 0.19 HCO 3 330.00 28.39 805.99 61.12 85.20 8.06 0.10 Total Fe 83.00 11.77 138.60 Fe +2 82.81 6.18 0.07 86.00 13.13 172.43 Fe +3 4.19 8.59 0.25 1.50 7.60 57.82 Al +3 1.60 0.47 0.26 1.90 0.88 0.78 Ca +2 472.60 51.78 0.11 470.00 25.30 640.19 mg/ L Cl - 10.33 2.34 0.23 10.00 4.83 23.36 K + 8.42 0.20 8.00 3.74 13.96 1.66 Mg +2 97.45 9.46 0.29 98.00 12.57 157.90 Total Mn 8.25 2.56 0.31 8.00 2.20 4.85 Na + 16.10 3.97 0.25 17.00 2.14 4.60 -2 SO 4 1458.19 1481.00 184.84 34165.68 138.68 0.14
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