Oxidation pathways for formic acid under low temperature hydrothermal conditions: Implications for the chemical and isotopic evolution of organics on Mars

In order to evaluate the oxidation effect of dissolved hydrogen peroxide and the catalytic role of iron oxides on the kinetics of formic acid decarboxylation, a series of flow-through hydrothermal experiments was conducted at temperatures ranging from 80 to 150 degrees C and pressures of 172-241 bar. delta C-13 composition of residual HCOOH(aq) was also monitored to examine kinetic isotope effects associated with oxidation processes. Our results reveal that decomposition of H2O2(aq) in presence of magnetite follows pseudo first order kinetics, highly enhanced relative to the homogeneous H2O2(aq)-HCOOOH(aq)-H2O system, which possibly reflect synthesis of hydroxyl radicals ((OH)-O-center dot) through Fenton processes. The kinetic rate constants of HCOOH(aq) decarboxylation to CO2(aq) are also elevated relative to those previously measured in H2O2(aq) free experiments. However, reaction kinetics are slightly slower in the case of H2O2(aq) aqueous solutions coexisting with magnetite than in the absence of mineral phases. This behavior is attributed to the possible formation of Fe-bearing hydroxyl formate aqueous species that could serve as stable transition states leading to a decrease in the activation entropy of formic acid decomposition.