TABLE 2

Free energy changes (ΔG′) associated with different N and Fe cycling processes at pH 7.0 and 30°C

EquationReactionFree energy changes (kJ/reaction)aFree energy boundaries for total reactions (kJ/reaction)b
InitialFinalMaximumMinimum
1NO3 + 0.25 CH3COO → NO2 + 0.25 H+ + 0.5 HCO3−152.5 (AC)−137.1 (BD)
22 Fe3+(aq) + H2O + 0.25 CH3COO → 2.25 H+ + 2 Fe2+(aq) + 0.5 HCO3−247.8 (GC)−206.2 (HD)
3NO2 + 2 Fe2+(aq) + 4.5 H2O → 0.5 N2O(g) + 3 H+ + 2 Fe(OH)3(amorph)−169.6 (BHE)−108.4 (AGF)
42 Fe(OH)3(amorph) + 3.75 H+ + 0.25 CH3COO → 5 H2O + 2 Fe2+(aq) + 0.5 HCO3−52.3 (GC)−52.3 (HD)
50.5 N2O(g) + 0.125 CH3COO → 0.5 N2(g) + 0.125 H+ + 0.25 HCO3−136.8 (ED)−154.2 (FD)
6NO2 + 0.75 CH3COO + 1.25 H+ + H2O → NH4+ + 1.5 HCO3−386.1 (BC)−363.2 (AD)
7NO3 + 0.875 CH3COO + 2 Fe3+(aq) + 0.5 H2O → 0.5 N2(g) + 2 Fe2+ + 1.75 HCO3 + 1.875 H+−606.8−532.4
8NO3 + 0.875 CH3COO + 2 Fe(OH)3(amorph) + 4.125 H+ → 0.5 N2(g) + 2 Fe2+ + 1.75 HCO3 + 5.5 H2O−411.3−378.5
9NO3 + CH3COO + 1.0 H+ + H2O → NH4+ + 2 HCO3−538.6−500.3
  • a Initial and final ΔG′ values were calculated in Geochemists Workbench (Aqueous Solutions, LLC). Initial free energy change values were calculated using the concentrations of reactants/products present at the beginning of the reactions. Final free energy change values were calculated using the concentrations of reactants/products measured or estimated at the end of the reactions. Bold values indicate an abiotic reaction not directly coupled to microbial energy metabolism. Concentrations used for calculations reflecting the experimental conditions were as follows: NO3, 1 mM; NO2, 0.01 mM (A) or 1.0 mM (B); Fe3+ (ferric citrate), 8 mM; CH3COO, 10 mM; HCO3, 0.1 mM (C) or 2.0 mM (D); N2O(g), 0.01 ppmv (E) or 9,600 ppmv (F); Fe2+, 0.01 mM (G) or 6.0 mM (H); NH4+, 3 mM. Letters in parentheses refer to the concentrations noted for the reactant or the product noted. amorph, amorphous; aq, aqueous.

  • b The total free energy changes associated with complete NO3 conversion to N2 (equations 7 and 8) or NH4+ (equation 9) were calculated by adding the ΔG′ values for the individual reactions (equations 1 to 6). The values reported as maximums represent the highest possible free energy change (i.e., most-negative ΔG′ values) associated with the respective reactions, while the values reported as minimums represent the lower threshold for free energy changes (i.e., the most-positive ΔG′ values). To calculate the maximum free energy values, the largest (i.e., most-negative) free energy change values from the individual reactions (equations 1 to 6) were added together. To calculate the minimum free energy values, the smallest (i.e., most-positive) free energy change values were added. Note that the ΔG′ value for Fe3+ was added twice in equation 8 to account for amorphous Fe3+ produced during chemodenitrification. Equation 7 was calculated by adding equations 1, 2, 4, and 5. Equation 8 was calculated by adding equations 1, 4 (twice), and 5. Equation 9 was calculated by adding equations 1 and 6.