A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water

Although the stable oxygen isotope fractionation between dissolved sulfate ion ðSO 4 Þ and H2O (hereafter aðSO2 4 –H2OÞ) is of physico-chemical and biogeochemical significance, no experimental value has been established until present. The primary reason being that uncatalyzed oxygen exchange between SO 4 and H2O is extremely slow, taking 10 5 years at room temperature. For lack of a better approach, values of 16& and 31& at 25 C have been assumed in the past, based on theoretical ‘gas-phase’ calculations and extrapolation of laboratory results obtained at temperatures >75 C that actually pertain to the bisulfate ðHSO 4 Þ–H2O system. Here I use novel quantum-chemistry calculations, which take into account detailed solute– water interactions to establish a new value for aðSO2 4 –H2OÞ of 23& at 25 C. The results of the corresponding calculations for the bisulfate ion are in agreement with observations. The new theoretical values show that sediment dOSO2 4 -data, which reflect oxygen isotope equilibration between sulfate and ambient water during microbial sulfate reduction, are consistent with the abiotic equilibrium between SO 4 and water. 2009 Elsevier Ltd. All rights reserved.