GEOCHEMICAL KINETICS MODULES FOR AMDTreat 5.0+ Charles A. Cravotta - PDF document

4/10/2017 GEOCHEMICAL KINETICS MODULES FOR “AMDTreat 5.0+” Charles A. Cravotta III U.S. Geological Survey In collaboration with Brent P. Means U.S. Office of Surface Mining Reclamation and Enforcement “PHREEQ-N-AMDTREAT” http://amd.osmre.gov/default.htm AMDTreat 1

4/10/2017 Objective • Incorporate PHREEQC “kinetics tools” to AMDTreat 5.0+  FeII oxidation tool that utilizes established rate equations for gas exchange and pH-dependent iron oxidation and that can be associated with commonly used aeration devices; and  Limestone dissolution tool that utilizes established rate equation for calcite dissolution and that can be adjusted for surface area of commonly used aggregate particle sizes. BIMODAL pH FREQUENCY DISTRIBUTION 40 Anthracite AMD A. Anthracite Mine Discharges pH, field 35 Frequency in percent, N=41 pH, lab (aged) 30 pH increases after 25 “oxidation” of net alkaline water (CO 2 outgassing): 20 - = CO 2 (gas) + OH - HCO 3 15 10 5 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 40 B. Bituminous Mine Discharges Bituminous AMD pH, field 35 Frequency in percent, N=99 pH, lab (aged) pH decreases after “oxidation” 30 of net acidic water (Fe 25 oxidation and hydrolysis): 20 Fe 2+ + 0.25 O 2 + 2.5 H 2 O  Fe(OH) 3 + 2 H + 15 10 5 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 pH 2

4/10/2017 TREATMENT OF COAL MINE DRAINAGE Al 3+ Fe 2+ / Fe 3+ Passive Active Mn 2+ Increase pH/oxidation Increase pH/oxidation with natural substrates & with aeration &/or microbial activity industrial chemicals Reactions slow Reactions fast, efficient Large area footprint Moderate area footprint Low maintenance High maintenance Iron Oxidation Kinetics are pH Dependent (abiotic and microbial processes can be involved) (1996) (Kirby et al., 1999) ** C bact is concentration of iron ‐ oxidizing bacteria, in mg/L, expressed as dry weight of bacteria (2.8E ‐ 13 g/cell or 2.8E ‐ 10 mg/cell ). The AMDTreat FeII oxidation kinetic model uses most probable number of iron ‐ oxidizing bacteria per liter (MPNbact). C bact = 150 mg/L is equivalent to MPNbact = 5.3E11, where Cbact = MPNbact ∙ (2.8E ‐ 10). 3

4/10/2017 Abiotic Homogeneous Fe(II) Oxidation Rate (model emphasizes pH) Between pH 5 and 8 the Fe(II) Minutes oxidation rate increases by Hours 100x for each pH unit increase.* At a given pH, the rate Days increases by 10x for a 15 °C increase. Using the activation Months energy of 23 kcal/ mol with the Arrhenius equation, the rate can be adjusted for Years temperature. Fe(OH) 1+ Fe(OH) 2 0 Fe 2+ log k T1 = log k T2 + Ea /(2.303 * R) · (1/T 2 - 1/T 1 ) At [O 2 ] = 0.26 mM (pO 2 = 0.21 atm) and 25  C. Open circles (o) from Singer & Stumm (1970), and solid circles (  ) from Millero et al. (1987). *Extrapolation of homogeneous rate law: -d[Fe(II)]/dt = k 1 ·[Fe(II)]·[O 2 ]·[H + ] -2 Dashed lines are estimated rates for the various k 1 = 3 x 10 -12 mol/L/min dissolved Fe(II) species. Effects of O 2 Ingassing and CO 2 Outgassing on pH and Fe(II) Oxidation Rates Batch Aeration Tests at Oak Hill Boreholes (summer 2013) Control Not Aerated Aerated H 2 O 2 Addition 4

4/10/2017 PHREEQC Coupled Kinetic Model of CO 2 Outgassing & Homogeneous Fe(II) Oxidation—Oak Hill Boreholes pH FeII Dissolved CO 2 Dissolved O 2 k L,CO2 a = 0.00001 s -1 k L,O2 a = 0.0007 s -1 k L,O2 a = 0.0012 s -1 k L,CO2 a = 0.00011 s -1 k L,O2 a = 0.00023 s -1 k L,O2 a = 0.00002 s -1 k L,CO2 a = 0.00022 s -1 k L,CO2 a = 0.00056 s -1 CO 2 Outgassing is Proportional to O 2 Ingassing (model specifies first-order rates for out/in gassing) ‐ d[C]/dt = k L , C a ∙ ([C] ‐ [C] S ) exponential, asymptotic approach to steady state k L,CO2 a = 0.00001 s -1 0.0014 k L,CO2 a = 0.00011 s -1 0.0012 1st Order O 2 ingassing rate constant (1/s) y = 2.43x + 0.00 k L,CO2 a = 0.00022 s -1 R² = 0.96 0.0010 k L,CO2 a = 0.00056 s -1 Atmospheric equilibrium 0.0008 0.0006 Atmospheric equilibrium Aerated 0.0004 Not Aerated 0.0002 kLa [O2] vs. kLa [CO2] 0.0000 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 1st Order CO 2 outgassing rate constant (1/s) 5

4/10/2017 Estimated CO 2 Outgassing & O 2 Ingassing Rate Constants for Various Treatment Technologies kL,a_20 = (LN((C 1 ‐ C S )/(C 2 ‐ C S ))/t) / (1.0241 (TEMPC ‐ 20) ), where C is CO 2 or O 2 . Dissolved O 2 , temperature, and pH were measured using submersible electrodes. Dissolved CO 2 was computed from alkalinity, pH, and temperature data. New Iron Oxidation Rate Model for “AMDTreat” (combines abiotic and microbial oxidation kinetics) The homogeneous oxidation rate law (Stumm and Lee, 1961; Stumm and Morgan, 1996), expressed in terms of [O 2 ] and {H + } (=10 -pH ), describes the abiotic oxidation of dissolved Fe(II): -d[Fe(II)]/dt = k 1 ·[Fe(II)]·[O 2 ]·{H + } -2 The heterogeneous oxidation rate law describes the catalytic abiotic oxidation of sorbed Fe(II) on Fe(III) oxyhydroxide surfaces at pH > 5, where (Fe(III)) is the Fe(III) oxyhydroxide concentration expressed as Fe in mg/L (Dempsey et al., 2001; Dietz and Dempsey, 2002): -d[Fe(II)]/dt = k 2 (Fe(III)) ·[Fe(II)]·[O 2 ]·{H + } -1 The microbial oxidation rate law describes the catalytic biological oxidation of Fe(II) by acidophilic microbes at pH < 5 (Pesic et al., 1989; Kirby et al., 1999): -d[Fe(II)]/dt = k bio · C bact ·[Fe(II)]·[O 2 ]·{H + } where k bio is the rate constant in L 3 /mg/mol 2 /s, C bact is the concentration of iron-oxidizing bacteria in mg/L (dry weight), [ ] indicates aqueous concentration in mol/L. 6

4/10/2017 New Iron Oxidation Rate Model for “AMDTreat”— PHREEQC Coupled Kinetic Models of CO 2 Outgassing & Fe(II) Oxidation Kinetic variables can be adjusted, including CO 2 outgassing and O 2 ingassing rates plus abiotic and Duration of aeration (time for reaction) microbial FeII oxidation rates. TimeSecs : 28800 is 8 hrs Aer3: k L,CO2 a = 0.00056 s -1 CO 2 outgassing rate in sec ‐ 1 Aer2: k L,CO2 a = 0.00022 s -1 Adjustment CO 2 outgassing rate Aer1: k L,CO2 a = 0.00011 s -1 Adjustment O 2 ingassing rate (x kLaCO2) FeII.exe Aer0: k L,CO2 a = 0.00001 s -1 Adjustment abiotic homogeneous rate Adjustment abiotic heterogeneous rate Iron oxidizing bacteria, microbial rate Calcite saturation limit Addition of H 2 O 2 and recirculation of Hydrogen peroxide added* FeIII simulated. Constants temperature Adjustment to H2O2 rate corrected. Options to estimate Fe2 from Option to specify FeIII recirculation Fe and pH plus TIC from alkalinity and pH. Computes net acidity, TDS, SC, and precipitated solids. *multiply Fe.mg by 0.0090 to get [H2O2] Revised AMDTreat Chemical Cost Module — Caustic Titration with Pre-Aeration (Decarbonation) PHREEQC Coupled Kinetic Models of CO 2 Outgassing & Fe(II) Oxidation Original option for no aeration, plus new option for kinetic pre-aeration (w/wo hydrogen peroxide) that replaces original equilibrium aeration. Duration of pre-aeration in sec CO 2 outgassing rate constant in sec -1 PHREEQTitration_StMichaels.exe Adjustment CO 2 outgassing rate (x kLaCO2) Adjustment O 2 ingassing rate (x kLaCO2) Hydrogen peroxide added* Allows selection and evaluation of key Adjustment to H 2 O 2 rate variables that affect chemical usage Calcite saturation limit efficiency. *multiply Fe.mg by 0.0090 to get [H2O2] 7

4/10/2017 New Module For AMDTreat — PHREEQC Coupled Kinetic Models of CO 2 Outgassing & Fe(II) Oxidation, with Caustic Pre-Treatment Variable CO 2 outgassing and O 2 Option to adjust initial pH with caustic ingassing rates apply. Can choose to adjust initial pH with caustic. The required quantity of caustic is reported in units used by AMDTreat. CO 2 outgassing rate Adjustment CO 2 outgassing rate Adjustment O 2 ingassing rate (x kLaCO2) Caustic+FeII.exe Adjustment abiotic homogeneous rate Adjustment abiotic heterogeneous rate Kinetic variables, including CO 2 Iron oxidizing bacteria Calcite saturation limit outgassing and O 2 ingassing rates plus Hydrogen peroxide added abiotic and microbial FeII oxidation Adjustment to H2O2 rate rates, can be adjusted by user. In Option to specify FeIII recirculation addition to caustic chemicals, hydrogen peroxide and recirculation of FeIII solids can be simulated. *multiply Fe.mg by 0.0090 to get [H2O2] Limestone Dissolution Rate Model for AMDTreat (“PWP” model emphasizes pH and CO 2 ) r = ( k 1 • a H+ + k 2 • a H2CO3* + k 3 • a H2O ) According to Plummer, Wigley, and - k 4 • a Ca2+ • a HCO3- Parkhurst (1978), the rate of CaCO 3 dissolution is a function of three forward (dissolution) reactions: CaCO 3 + H + → Ca 2 + + HCO 3 - k 1 CaCO 3 + H 2 CO 3 * → Ca 2 + + 2 HCO 3 - k 2 CaCO 3 + H 2 O → Ca 2 + + HCO 3 - + OH - k 3 and the backward (precipitation) reaction: Ca 2 + + HCO 3 - → CaCO 3 + H + k 4 Although H + , H 2 CO 3 * , and H 2 O reaction with calcite occur simultaneously, the forward rate is dominated by a single species in the fields shown. More than one species contributes significantly to the forward rate in the gray stippled area. Along the lines labeled 1, 2, and 3, the forward rate attributable to one species balances that of the other two. 8