Redox Geochemistry

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Redox Geochemistry
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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 electron.
• If pe is low, there is a strong tendency for the
  solution to donate electron - the solution is
  reducing.
• If pe is high, there is a strong tendency for the
  solution to accept electron - 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

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

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Growth Efficiency

• How much energy does it take to grow a new
  microbe?
• How much energy does a microbe gain from any
  metabolic reaction?
• How much energy is wasted, i.e., how much
  energy does it cost the microbe to hang out in
  its environment that is not directly attributed
  to the energy required for growth and division