D/H fractionation in the H2–H2O system at supercritical water conditions: Compositional and hydrogen bonding effects

Abstract

A series of experiments has been conducted in the H2–D2–D2O–H2O–Ti–TiO2 system at temperatures ranging from 300 to 800°C and pressures between ∼0.3 and 1.3GPa in a hydrothermal diamond anvil cell, utilizing Raman spectroscopy as a quantitative tool to explore the relative distribution of hydrogen and deuterium isotopologues of the H2 and H2O in supercritical fluids. In detail, H2O–D2O solutions (1:1) were reacted with Ti metal (3–9h) in the diamond cell, leading to formation of H2, D2, HD, and HDO species through Ti oxidation and H–D isotope exchange reactions. Experimental results obtained in situ and at ambient conditions on quenched samples indicate significant differences from the theoretical estimates of the equilibrium thermodynamic properties of the H–D exchange reactions. In fact, the estimated enthalpy for the H2(aq)–D2(aq) disproportionation reaction (ΔHrxn) is about −3.4kcal/mol, which differs greatly from the +0.16kcal/mol predicted for the exchange reaction in the gas phase by statistical mechanics models.The exothermic behavior of the exchange reaction implies enhanced stability of H2 and D2 relative to HD. Accordingly, the significant energy difference of the internal H2(aq)–D2(aq)–HD(aq) equilibrium translates to strong differences of the fractionation effects between the H2O–H2 and D2O–D2 isotope exchange relationships. The D/H fractionation factors between H2O–H2(aq) and D2O–D2(aq) differ by 365‰ in the 600–800°C temperature range, and are indicative of the greater effect of D2O contribution to the δD isotopic composition of supercritical fluids.The negative ΔHrxn values for the H2(aq)–D2(aq)–HD(aq) equilibrium and the apparent decrease of the equilibrium constant with increasing temperature might be because of differences of the Henry’s law constant between the H- and D-bearing species dissolved in supercritical aqueous solutions. Such effects may be attributed to the stronger hydrogen bonding in the O–H⋯O relative to the O–D⋯O environment. This difference allows enhanced gas solubility in the denser and more polar H2O clusters, and thus, affects the D/H exchange between the H2–D2 volatiles and the coexisting H2O–D2O mixtures. The proposed role of temperature in promoting differences in the density and polarity of hydrogen-bonded OHO and ODO molecules may be explained with isotope-specific molar volume effects similar to those suggested to account for the hydrogen isotope fractionation between H2O and hydroxide mineral phases (e.g. brucite) across large pressure intervals.