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

Abstract

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 °C and pressures of 172–241 bar. δ 13C composition of residual HCOOH (aq) was also monitored to examine kinetic isotope effects associated with oxidation processes. Our results reveal that decomposition of H 2O 2(aq) in presence of magnetite follows pseudo first order kinetics, highly enhanced relative to the homogeneous H 2O 2(aq)–HCOOOH (aq)–H 2O system, which possibly reflect synthesis of hydroxyl radicals ( OH) through Fenton processes. The kinetic rate constants of HCOOH (aq) decarboxylation to CO 2(aq) are also elevated relative to those previously measured in H 2O 2(aq) free experiments. However, reaction kinetics are slightly slower in the case of H 2O 2(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. δ 13C values of residual formic acid in the homogeneous H 2O 2(aq)–HCOOH (aq)–H 2O system are consistent with previous studies. However, magnetite-bearing experiments produce a negative shift in δ 13C of residual formic acid, perhaps specific to OH-imposed oxidation of organic compounds. This would indicate that isotopic fractionations by this oxidation pathway are opposite to kinetic fractionation effects expected in biologically driven oxidation processes. This could have important implications for putative H 2O 2(aq)-bearing Martian subsurface environments and the evolution of organics at low-temperature hydrothermal conditions.