1 / 33

John J. Mahoney Hydrologic Consultants, Inc. of Colorado Lakewood, Colorado

Coprecipitation Reactions - Verification of Computational Methods in Geochemical Models. John J. Mahoney Hydrologic Consultants, Inc. of Colorado Lakewood, Colorado. Updated and Expanded Version for Website Viewing. Original Presentation was included in EPA/525/C-00/004,

barrett-morse
Download Presentation

John J. Mahoney Hydrologic Consultants, Inc. of Colorado Lakewood, Colorado

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Coprecipitation Reactions - Verification of Computational Methods in Geochemical Models John J. Mahoney Hydrologic Consultants, Inc. of Colorado Lakewood, Colorado

  2. Updated and Expanded Version for Website Viewing Original Presentation was included in EPA/525/C-00/004, January 2001 Mining Impacted Pit Lakes 2000 Workshop Proceedings: A Multimedia CD Presentation Additional slides in italics have been added to the presentation to clarify certain items Updated again in 2006 as part of Geochemical Modeling Course - University of Alberta

  3. Fate of Metals in Ground-Water Systems 1. Precipitation 2. Adsorption 3. Coprecipitation or Solid Solution Reactions

  4. Ra in BaSO4 (Barite) Sr in CaCO3 (Calcite) Cd in CaCO3 (Calcite) Sr in CaSO4 2H2O (Gypsum) AsO4 in Ca5(PO4)3OH (Apatite) Al in FeOOH (Goethite) MoO4 in Ca6Al2(SO4)3(OH)12 24H2O (Ettringite) Cr(III) in Fe(OH)3 (Ferrihydrite) Cr(VI) in BaSO4 (Barite) Clay Minerals Coprecipitation Reactions ControlConcentrations in Ground Water

  5. Coprecipitation Reactions Under-Utilized in Applied Models 1. Emphasis on Precipitation and Adsorption Reactions 2. Belief that Standard Models (MINTEQA2, PHREEQE and PHREEQC) Cannot Perform Coprecipitation Calculations 3. Data Bases for Coprecipitation Reactions Not Provided in Standard Models 4. Belief That Data Describing Solid Solution Reactions is Too Limited or Lacking for User’s Problem

  6. Coprecipitation of Cadmium in Calcite 1. Well Documented Process 2. Several Referenced Articles and Numerous Textbooks Discuss Process 3. Values for Distribution Coefficients (D) Are Large 4. D Values Range from 70 to 1,500 5. Ideal Solution - D Values of 680 or Greater Than 4,000 Reported

  7. Requirements of Calculation 1. Initial Concentration of Cd (Before Coprecipritation Reactions) 2. Amount of Calcite That Will Precipitate During Each Step of Model (Initial and Final Calcium Concentration) 3. Value for Distribution Coefficient 4. Appropriate Equation

  8. Doerner and Hoskins Equation For coprecipitation in a calcium-bearing mineral æ ö æ ö Cai Mei ç ÷ ç ÷ log = l log ç ÷ ç ÷ Caf Mef è ø è ø Where Mei = initial quantity of trace metal in solution, Mef = final quantity of metal, Cai = initial quantity of calcium in solution, Caf= final quantity of calcium in solution, and l = distribution coefficient (D).

  9. Doerner and Hoskins Equation 1. Typically Described in Textbooks 2. Heterogeneous System 3. Limited to Small Values of D

  10. C iTr C 0Cr C 0Tr = l ( C iCr - C 0Cr ) + C iCr Riehl Equation Where C 0Tr = Concentration of Trace Component (Cd) in Boundary Layer, C iTr = Initial Concentration of Trace Component in Solution, C iCr= Initial Concentrations of Carrier (Calcium) in Solution, and C 0Cr = Concentration of Carrier in Boundary Layer.

  11. C lCr S = C 0Cr Riehl Equation 1. Assumes Where C lCr = Concentration of the Carrier in Bulk Solution, and S = 1.0 when Precipitation Stops. 2. Homogeneous System 3. Works for a Wide Range of D Values

  12. Properties of Equations 1. Both Equations Calculate Same Concentrations for D = 1 2. For Small Amounts of Precipitate and Low D Values - Similar Results for Both Equations 3. At D > 1, Riehl Equation Produces More Reasonable Concentrations 4. Homogeneous Model (Riehl Equation) also Produces More Reasonable Concentrations at Different Masses of Precipitate

  13. Application of Method 1. Use PHREEQC to Estimate Mass of Calcite that will Precipitate 2. Comparison of Concentrations from Step 2 to Step 3 of Model 3. Estimate Concentrations of Cd in Solution 4. Cd Sorbed onto Hydrous Ferric Oxide Before Coprecipitation Reactions 5. Solve Equation for Cd Concentrations Using Spreadsheet Program or Calculator 6. Repeat Process for Step 4 (Evapoconcentration)

  14. Introduction to Thermodynamic Model 1. Above method, the Bulk Kd Approach, can be applied using output from various geochemical models (MINTEQA2, PHREEQE, and PHREEQC) see Mahoney (1998) for more details using Bulk Kd approach Calculations could be performed using a spreadsheet or even a programmable calculator if amount of precipitated phase was known 2. After completion of work summarized in Mahoney (1998) the USGS released version 2.0 of PHREEQC (Parkhurst and Appelo, 1999) with a thermodynamic approach to modeling solid solution reactions The rest of the presentation evaluated the thermodynamic approach used in PHREEQC (version 2)

  15. Ca 2 + + CO32 - CaCO3 (Ca 2 +)(CO32 -) = Kcc(CaCO3) Cd 2 + + CO32 - CdCO3 (Cd 2 +)(CO32 -) = Kota(CdCO3) Where (CdCO3 ) and (CaCO3 ) represent the activities of the solids (no solid solution), Kcc = solubility product constant for calcite, Kota = solubility product constant for otavite, and (Ca 2 +), (Cd 2 +) and (CO32-) represent activities in solution. Thermodynamic Model

  16. Cd 2 + + CaCO3(s) CdCO3(s) + Ca 2 + (CdCO 3)s (CaCO3)s (Cd 2 +) (Ca 2 +) = Kx Where (CdCO3 )S and (CaCO3 )S = the activities of the components in the solid solution, and Kx = Kcc / Kota Exchange Reaction

  17. [Cd 2 + ] XCdCO3 = D XCaCO3[Ca 2 + ] Distribution Coefficient Where XCdCO3 = mole fraction of cadmium in solid solution XCaCO3 = mole fraction of calcium in solid solution [Cd 2 +] and [Ca 2 +] = molalities of cadmium and cadmium in solution, and D = the Distribution Coefficient

  18. Activity Coefficient in Solid lCdCO3XCdCO3 = (CdCO3 )S lCaCO3XCaCO3 = (CaCO3 )S Where lCaCO3 and lCdCO3 represent the rational activity coefficients in the solid

  19. Ideal Solid Solution lCdCO3 = 1 and lCdCO3D = Kx

  20. 1nlCdCO3 = X2CaCO3 [a0 - a1 (3XCdCO3 - XCaCO3) + …] 1nlCaCO3 = X2CdCO3[a0 - a1 (3XCaCO3 - XCdCO3 ) + …] Where a0 and a1 are the Guggenheim Nondimensional Parameters Non Ideal Solid Solutions

  21. Regular Solid Solution HM = XCdCO3XCaCO3W Where HM= enthalpy of mixing W= the ion interaction parameter D = Kxexp[ - (1 - (2XCdCO3) W /(2.303RT)], and W /(2.303RT) = a0

  22. Additional Issues - Calculation of a0 Mole fraction term (XCdCO3) in following equation needs to be kept in mind when calculating a0 D = Kxexp[ - (1 - (2XCdCO3) W /(2.303RT)], In general the value for XCdCO3 will be small because the amount of trace metal in the initial solution will be a small fraction of amount of carrier that will precipitate. 112µg/L of cadmium in solution and a net removal of 40 mg/L of calcium by calcite precipitation results in XCdCO3 of 0.001 for the solid solution if all cadmium is removed from solution

  23. Additional Issues - Solubility Products and Distribution Coefficients Various values for the Kota have been presented in the literature and these values have found their way into different databases Users should assure that the values for Kota or any other Ksp value for the trace element phase produce an internally consistent value for D and hence a0

  24. 1.00E+00 1.00E-03 Riehl Equation Cadmium Concentration (ppm) 1.00E-06 PHREEQC Model (Red dashes) 1.00E-09 Doerner and Hoskins Equation 1.00E-12 0.1 1 10 100 1000 10000 Distribution Coefficient (D) Comparison of Solid Solution Models

  25. ConclusionsCoprecipitation Reactions 1. Important Process to Control Metals in Aqueous Systems 2. Numerous Methods Available to Estimate Effect 3. Results in Significant Decreases in Concentrations 4. Failure to Consider Can Produce Concentrations That are Unrealistically High and May Cause Regulatory Scrutiny

  26. ConclusionsBulk Methods 1. Bulk Methods Easy to Apply 2. Textbook Examples 3. Iterative Approach Requires Shifting Between Model Concentrations and Spreadsheet 4. Distribution Coefficients Available for Many Systems 5. Riehl Equation Most Appropriate for Predicting Concentrations for Systems with Large Distribution Coefficients

  27. ConclusionsThermodynamic Approach 1. Uses Single Program - Does Not Require Calculations Outside of Program 2. Usually Requires Detailed Evaluation and Understanding of System to Estimate Value for a0 3. PHREEQC (v2) Method Produces Final Concentrations Comparable to Riehl Equation Values

  28. SEE ALSO • 1. Mahoney, J.J., 1998, Incorporation of coprecipitation reactions in • predictive geochemical models: in Proceedings of Tailings and Mine • Waste '98, Fort Collins, Colorado, p. 689-697. A.A. Balkema pubs. • 2. Mahoney, J.J., 2001, Coprecipitation reactions – verification of • computational methods in geochemical models: in Mining Impacted • Pit Lakes 2000 Workshop Proceedings: a Multimedia CD Presentation. • (Workshop held April 4–6, 2000 Reno, NV) United States • Environmental Protection Agency Office of Research and • Development. EPA/625/C-00/004. Session 4.

  29. Example Input File for Kd = 70 TITLE Example 10.--Solid solution of otavite and calcite. PHASES # Fix-H+ not included because of space limits Otavite CdCO3 = CO3-2 + Cd+2 log_k -11.31 Calcite CaCO3 = CO3-2 + Ca+2 log_k -8.48 SOLUTION 1 -units mmol/kgw pH 8.0 Ca 3.45 Cd 0.002 END USE SOLUTION 1 EQUILIBRIUM_PHASES 1 CO2(g) -2.0 10 Calcite 0.0 0.0 Fix_H+ -8.0 Na(OH) 20.0 SOLID_SOLUTIONS 1 Ca(x)Cd(1-x)CO3 -comp Calcite 0.00 -comp Otavite 0.00 -Gugg_nondim 2.28 REACTION 2 Ca 0.01 END

  30. Example Output File for Kd = 70(portion) -------------------------------Phase assemblage-------------------------------- Moles in assemblage Phase SI log IAP log KT Initial Final Delta Calcite 0.00 -8.48 -8.48 0.000e+00 0.000e+00 CO2(g) -2.00 -20.15 -18.15 1.000e+01 9.988e+00 -1.250e-02 Fix_H+ -8.00 -8.00 0.00 Na(OH) is reactant 2.000e+01 2.000e+01 -9.843e-07 --------------------------------Solid solutions-------------------------------- Solid solution Component Moles Delta moles Mole fract Ca(x)Cd(1-x)CO3 1.00e-02 Calcite 1.00e-02 1.00e-02 1.00e+00 Otavite 2.00e-06 2.00e-06 2.00e-04

  31. For Further Information Contact John Mahoney, Ph.D. Senior Geochemist Hydrologic Consultants, Inc. of Colorado 303 969 8033 jmahoney@hcico.com

More Related