1. GEOCHEMICAL METHODS FOR THE DETERMINATION OF DIFFERENT METAL SPECIES
Environmental Biogeochemistry of Trace Metals
(CWR6252)

2. PART -1 pE-pH Phase Diagrams and Metal Speciation

3. CASE STUDY: The IRON-WATER SystempE pH
Fe2+
Fe3+
Fe(OH)2+
Reduction of water H2(g)
Oxidation of water O2(g)
pE=20.8-pH
pE=-pH
Preliminary: Set boundaries where water is stable

4. Constructing a pE-pH diagram
Step 1: Identify the most common species of the element in the given system (example system: Fe – H2O)
Step 2: Collect the tabulated ΔGof data for each of the above species
Step 3: Write and balance reactions between species
Step 4: Calculate Gibbs free energy (ΔGoR) for these reactions
Step 5: Calculate logKeq for each reaction
Step 6: Calculate pE0 for redox reactions
Step 7: Write the expression for equilibrium constant (keq) for each reaction
Step 8: Derive the boundary equation between species from Keq expressions by rearranging and substituting known values
Step 9: Plot the boundary equation on the diagram

5. Constructing the pE-pH diagram for Fe-H20 system
Step 1: Identify the most common species of the element in the given system
Step 2. Look up the thermodynamic data for each specie (ΔGof) link

6. (a) Vertical Line Fe3+/FeOH2+
Step 3. Write and balance reactions between species starting with oxidized species
Fe3++H2O=FeOH2++H+
Step 4. Calculate the standard Gibbs free energy of rxn (ΔGoR)
= ( )-( )=12.54 kJ/mol
Step 5. Calculate logKeq
= /5.708= -2.2 kJ/mol at 25oC

7. (a) Example of a Vertical Line Fe3+/FeOH2+
Step 6: Skip for this reaction (not a redox)
Step 7: Write the expression for the equilibrium constant (keq)
Step 8: Derive the boundary equation by taking the log of both sides, re-arranging, and then substituting with known values
since [FeOH2+]=[Fe3+] at equilibrium and pH=-log[H+], then
a vertical line
Step 9: Plot the Fe3+/FeOH2+ boundary equation
pH=2.2

8. (b) Example of a Horizontal Line Fe3+/Fe2+
Step 3:
Fe3++ e- = Fe2+
= (-82.88)-( )= kJ/mol
= 74.32/5.708= 13
, n is the number of electrons
Step 4:
Step 5:
Step 6:

9. (b) Example of a Horizontal Line Fe3+/Fe2+
Step 7:
Step 8: Take the log of the equation and divide by the number of electrons (in this case n=1)
Since [Fe2+]=[Fe3+], pE=-log[e-], and
then pE = pE0 = 13
 horizontal line
Step 9: Plot the Fe3+/Fe2+ boundary equation
pE = 13

10. (c) Example of a Diagonal Line FeOH2+/ Fe2+
Step 3: FeOH2++e- +H+= Fe2++H2O
Step 4:
=( )-( )= kJ/mol
Step 5:
=86.86/5.708=15.2
Step 6:
where n is the number of electrons transferred in the redox reaction

11. Diagonal Line Cont’d Step 7:
Step 8: Take the log of the equation and divide by the number of electrons (in this case n =1)
Since pE=-log[e-], pH=-log[H+], [Fe2+]=[FeOH2+], [H2O]=1, and
diagonal line
Step 9: Plot the Fe2+/FeOH2+ boundary equation
pE=15.2-pH

12. Equations of the Different Boundary Lines for the Fe-H2O systempE pH
Fe2+
Fe3+
Fe(OH)2+
Reduction of water H2(g)
Oxidation of water O2(g)

13. Example pE-pH Diagrams Trace metals in Simple Aqueous Systems 1
Example pE-pH Diagrams Trace metals in Simple Aqueous Systems 1. Zinc, Cadmium, and Lead at 250C and 1 atm
Pb-CO2-S-O-H2O
system
Zn-CO2-S-O-H2O
system
Cd-CO2-S-O-H2O
system

14. 2. Arsenic and Selenium at 250C and 1 atm
As-O-S-H2O system
Se-O-H2O system

15. Common species of Fe in Water and thermodynamic data
link