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 G0R H0R T S0R Gibbs Free Energy G is a state variable measured in KJ mol or Cal mol 0 0 0 GR ni Gi products i nG i i reactants i Tabulated values of G0R available Equilibrium Constant aCc a Dd RT ln a b RT ln Q a A aB for aA bB cC dD Restate the equation as GR G0R RT ln Q GR 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 i 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 imi The activity coefficient i is found via some empirical foundations Activity Coefficients Extended Debye Huckel approximation valid for I up to 0 5 M log 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 2 mCa L 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 2 n x Geochemical models Hundreds of equations solved iteratively for speciation solve for GR All programs work on same concept for speciation thermodynamics and calculations of mineral equilibrium lots of variation in output specific info Oxidation Reduction Reactions R E 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 G s y a s 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 L O E 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 H 2O 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 a Mn 2 a H4 ae2 Solving for the electron activity aMn 2 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 Gr Gr n E 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 rxn EH is relative to SHE Std Hydrogen Electrode At non standard conditions a b RT a 0 A aB E H E H ln c d nF aC aD At 25 C a b 0 0592V a A aB 0 E H E H 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 Fe 2 and Fe3 Because Gr 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 Gr 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 transport of electrons coupled to pumping protons CH2O CO2 4 e H 0 5 O2 4e 4H H2O e glucose 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 …