Redox Geochemistry 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 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 DG ni G ( products) ni G (reactants ) 0 R 0 i i 0 i i • Tabulated values of DG0R available… Equilibrium Constant aCc aDd RT ln a b RT ln Q a A aB • 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 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 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 Activity Coefficients • Extended Debye-Huckel approximation (valid for I up to 0.5 M): log g Az 2 I 1 2 I aBI 1 2 0.2 I • Where A and B are constants (tabulated), and a is a measure of the effective diameter of the ion (tabulated) 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++ Ca(H3SiO4)2 Ca(O-phth) CaB(OH)4+ CaCH3COO+ CaCO30 CaCl+ CaF+ CaH2SiO4 CaH3SiO4+ CaHCO3+ CaNO3+ CaOH+ CaPO4CaSO4 CaHPO40 • Plus more species gases and minerals!! Mass Action & Mass Balance c n [CL] [ H ] i c l [C ] [ HL ] mCa mCa L 2 2 n x • mCa2+=mCa2++MCaCl+ + 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 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… 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 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 eO2 + 4 e- 2 H2O 4 Fe2+ + O2 4 Fe3+ + 2 H2O 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 ELECTRON ACTIVITY • Although no free electrons exist in solution, it is useful to define a quantity called the electron activity: pe log ae • 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. THE pe OF A HALF REACTION - I Consider the half reaction MnO2(s) + 4H+ + 2e- Mn2+ + 2H2O(l) The equilibrium constant is K aMn2 4 H a a 2 e Solving for the electron activity aMn2 ae 4 Ka H 1 2 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: pe Eh 2.303RT Where = 96.42 kJ volt-1 eq-1 (Faraday’s constant). At 25°C, this becomes pe 16.9 Eh or Eh 0.059 pe 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 = - nE – Where n is the # of e-’s in the rxn, is Faraday’s 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 acidbase pair 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: RT 0 EH EH nF At 25° C: a b a AaB ln c d aC aD a b 0.0592V a A aB 0 EH EH log c d n aC aD 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 =-nE, 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 • Redox reactions with more negative reduction potentials will donate electrons to redox reactions with more positive potentials. NADP+ + 2H+ + 2e- NADPH + H+ O2 + 4H+ + 4e- 2H2O -0.32 +0.81 NADPH + H+ NADP+ + 2H+ + 2eO2 + 4H+ + 4e- 2H2O 2 NADPH + O2 + 2H+ 2 NADP+ + 2 H2O +0.32 +0.81 +1.13 ELECTRON TOWER more negative more positive oxidized/reduced forms potential acceptor/donor BOM – Figure 5.9 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 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 glucose • transport of electrons coupled to pumping protons CH2O CO2 + 4 e- + H+ 0.5 O2 + 4e- + 4H+ H2O e- Proton Motive Force (PMF) • Enzymatic reactions pump H+ outside the cell, there are a number of membranebound 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… HOW IS THE PMF USED TO SYNTHESIZE ATP? • catalyzed by ATP synthase BOM – Figure 5.21 ATP generation II • Alternative methods to form ATP: • Phosphorylation coupled to fermentation, low yield of ATP 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! Minimum Free Energy for growth • Minimun free energy for growth = energy to make ATP? • What factors go into the energy budget of an organism?? 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 (> ~0.1 mg L-1). Reducing waters (anoxic) - waters that contain both dissolved iron and sulfide. The Redox ladder O2 Oxic Aerobes H2O NO3- Denitrifiers Sub-oxic anaerobic N2 MnO2 Mn2+ Manganese reducers Fe(OH)3 Fe2+ Sulfidic Iron reducers SO42H2S Methanic Sulfate reducers CO2 CH4 Methanogens H2O H2 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. (Gibb’s free energy becomes more positive going down the steps)
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