The rate of oxygen isotope exchange between nitrate and water

Abstract

The oxygen isotope exchange rate between nitrate and water was measured at a temperature of 50–80°C and pH −0.6 to 1.1. Oxygen isotope exchange is a first-order reaction, with the exchange rate being strongly affected by both reaction temperature and pH, with increased rates of isotope exchange at higher temperatures and lower pH values. The rate of oxygen isotope exchange under natural conditions is extremely slow, with an estimated half-life for isotope exchange of 5.5×109years at 25°C and pH 7. The extremely slow rate of oxygen isotope exchange between nitrate and water under typical environmental conditions illustrates that nitrate-δ18O signatures (and also nitrate δ17O and Δ17O signatures) associated with various nitrate sources, as well as isotope compositions produced by biogeochemical processes, will be preserved. Hence, it is valid to use the value of nitrate-δ18O to investigate the sources and biogeochemical behavior of nitrate, in a similar manner to the use of sulfate-δ18O signatures to study the sources and biogeochemical behavior of sulfate.Equilibrium oxygen isotope fractionation factors have been determined, although quantification of the nitrate–water equilibrium fractionation factor is not possible due to the presence of nitrate as both protonated (i.e. HNO3) and unprotonated forms (i.e. NO3-) under the experimental conditions, and the difficulty in accurately calculating nitrate speciation in low pH, high ionic strength solutions.