Redox Geochemistry

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Redox Geochemistry
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WHY?

• Redox gradients drive life processes!
• The transfer of electrons between oxidants and
  reactants is harnessed as the battery, the source
  of metabolic energy for organisms
• Metal mobility ? redox state of metals and
  ligands that may complex them is the critical
  factor in the solubility of many metals
• Contaminant transport
• Ore deposit formation

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J. Willard Gibbs

• Gibbs realized that for a reaction, a certain
  amount of energy goes to an increase in entropy
  of a system.
• G H TS or DG0R DH0R TDS0R
• Gibbs Free Energy (G) is a state variable,
  measured in KJ/mol or Cal/mol
• Tabulated values of DG0R available

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Equilibrium Constant

• for aA bB ? cC dD
• Restate the equation as
• DGR DG0R RT ln Q
• DGR available metabolic energy (when negative
  exergonic process as opposed to endergonic
  process for energy) for a particular reaction
  whose components exist in a particular
  concentration

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Activity

• Activity, a, is the term which relates Gibbs Free
  Energy to chemical potential
• mi-G0i RT ln ai
• Why is there now a correction term you might ask
• Has to do with how things mix together
• Relates an ideal solution to a non-ideal solution

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Ions in solution

• Ions in solutions are obviously nonideal states!
  
• Use activities (ai) to apply thermodynamics and
  law of mass action
• ai gimi
• The activity coefficient, gi, is found via some
  empirical foundations

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Activity Coefficients

• Extended Debye-Huckel approximation (valid for I
  up to 0.5 M)
• Where A and B are constants (tabulated), and a is
  a measure of the effective diameter of the ion
  (tabulated)

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Speciation

• Any element exists in a solution, solid, or gas
  as 1 to n ions, molecules, or solids
• Example Ca2 can exist in solution as
• Ca CaCl
  CaNO3
• Ca(H3SiO4)2 CaF CaOH
  
• Ca(O-phth) CaH2SiO4 CaPO4-
• CaB(OH)4 CaH3SiO4 CaSO4
• CaCH3COO CaHCO3 CaHPO40
• CaCO30
• Plus more species ? gases and minerals!!

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Mass Action Mass Balance

• mCa2mCa2MCaCl mCaCl20 CaCL3- CaHCO3
  CaCO30 CaF CaSO40 CaHSO4 CaOH
• Final equation to solve the problem sees the mass
  action for each complex substituted into the mass
  balance equation

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Geochemical models

• Hundreds of equations solved iteratively for
  speciation, solve for DGR
• All programs work on same concept for speciation
  thermodynamics and calculations of mineral
  equilibrium lots of variation in output,
  specific info

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Oxidation Reduction Reactions

• Oxidation - a process involving loss of
  electrons.
• Reduction - a process involving gain of
  electrons.
• Reductant - a species that loses electrons.
• Oxidant - a species that gains electrons.
• Free electrons do not exist in solution. Any
  electron lost from one species in solution must
  be immediately gained by another.
• Ox1 Red2 ? Red1 Ox2

LEO says GER
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Half Reactions

• Often split redox reactions in two
• oxidation half rxn ? e- leaves left, goes right
• Fe2 ? Fe3 e-
• Reduction half rxn ? e- leaves left, goes right
• O2 4 e- ? 2 H2O
• SUM of the half reactions yields the total redox
  reaction
• 4 Fe2 ? 4 Fe3 4 e-
• O2 4 e- ? 2 H2O
• 4 Fe2 O2 ? 4 Fe3 2 H2O

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Half-reaction vocabulary part II

• Anodic Reaction an oxidation reaction
• Cathodic Reaction a reduction reaction
• Relates the direction of the half reaction
• A ? A e- anodic
• B e- ? B- cathodic

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ELECTRON ACTIVITY

• Although no free electrons exist in solution, it
  is useful to define a quantity called the
  electron activity
• The pe indicates the tendency of a solution to
  donate or accept a proton.
• If pe is low, there is a strong tendency for the
  solution to donate protons - the solution is
  reducing.
• If pe is high, there is a strong tendency for the
  solution to accept protons - the solution is
  oxidizing.

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THE pe OF A HALF REACTION - I

• Consider the half reaction
• MnO2(s) 4H 2e- ? Mn2 2H2O(l)
• The equilibrium constant is
• Solving for the electron activity

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DEFINITION OF Eh

• Eh - the potential of a solution relative to the
  SHE.
• Both pe and Eh measure essentially the same
  thing. They may be converted via the
  relationship
• Where ? 96.42 kJ volt-1 eq-1 (Faradays
  constant).
• At 25C, this becomes
• or

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Free Energy and Electropotential

• Talked about electropotential (aka emf, Eh) ?
  driving force for e- transfer
• How does this relate to driving force for any
  reaction defined by DGr ??
• DGr - n?E
• Where n is the of e-s in the rxn, ? is
  Faradays constant (23.06 cal V-1), and E is
  electropotential (V)
• pe for an electron transfer between a redox
  couple analagous to pK between conjugate
  acid-base pair

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Electropotentials

• E0 is standard electropotential, also standard
  reduction potential (write rxn as a reduction ½
  rxn) EH is relative to SHE (Std Hydrogen
  Electrode)
• At non-standard conditions

At 25 C
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Electromotive Series

• When we put two redox species together, they will
  react towards equilibrium, i.e., e- will move ?
  which ones move electrons from others better is
  the electromotive series
• Measurement of this is through the
  electropotential for half-reactions of any redox
  couple (like Fe2 and Fe3)
• Because DGr -n?E, combining two half reactions
  in a certain way will yield either a or
  electropotential (additive, remember to switch
  sign when reversing a rxn)
• E ? - DGr, therefore ? spontaneous
• In order of decreasing strength as a reducing
  agent ? strong reducing agents are better e-
  donors

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• Redox reactions with more negative reduction
  potentials will donate electrons to redox
  reactions with more positive potentials.
• NADP 2H 2e- ? NADPH H -0.32
• O2 4H 4e- ? 2H2O 0.81
• NADPH H ? NADP 2H 2e- 0.32
• O2 4H 4e- ? 2H2O 0.81
• 2 NADPH O2 2H ? 2 NADP 2 H2O 1.13

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ELECTRON TOWER
more negative
oxidized/reduced forms potential acceptor/donor
more positive
BOM Figure 5.9
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Microbes, e- flow

• Catabolism breakdown of any compound for energy
• Anabolism consumption of that energy for
  biosynthesis
• Transfer of e- facilitated by e- carriers, some
  bound to the membrane, some freely diffusible

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NAD/NADH and NADP/NADPH

• Oxidation-reduction reactions use NAD or FADH
  (nicotinamide adenine dinucleotide, flavin
  adenine dinucleotide).
• When a metabolite is oxidized, NAD accepts two
  electrons plus a hydrogen ion (H) and NADH
  results.
• NADH then carries
• energy to cell for other uses

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• transport of
• electrons coupled
• to pumping protons

CH2O ? CO2 4 e- H 0.5 O2 4e- 4H ?
H2O
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Proton Motive Force (PMF)

• Enzymatic reactions pump H outside the cell,
  there are a number of membrane-bound enzymes
  which transfer e-s and pump H out of the cell
• Develop a strong gradient of H across the
  membrane (remember this is 8 nm thick)
• This gradient is CRITICAL to cell function
  because of how ATP is generated

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HOW IS THE PMF USED TO SYNTHESIZE ATP?

• catalyzed by ATP synthase

BOM Figure 5.21
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ATP generation II

• Alternative methods to form ATP
• Phosphorylation ? coupled to fermentation, low
  yield of ATP

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ATP

• Your book says ATP Drives thermodynamically
  unfavorable reactions ? BULLSHIT, this is
  impossible
• The de-phosphorylation of ATP into ADP provides
  free energy to drive reactions!

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Minimum Free Energy for growth

• Minimun free energy for growth energy to make
  ATP?
• What factors go into the energy budget of an
  organism??

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REDOX CLASSIFICATION OF NATURAL WATERS

• Oxic waters - waters that contain measurable
  dissolved oxygen.
• Suboxic waters - waters that lack measurable
  oxygen or sulfide, but do contain significant
  dissolved iron (gt 0.1 mg L-1).
• Reducing waters (anoxic) - waters that contain
  both dissolved iron and sulfide.

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The Redox ladder
The redox-couples are shown on each stair-step,
where the most energy is gained at the top step
and the least at the bottom step. (Gibbs free
energy becomes more positive going down the
steps)