Oxygen isotope (18O/16O) fractionation between water and hydroxide ion

In any reaction involving water, the educt oxygen is either derived from H₂O or OH^- (i.e., the hydroxide ion). For example, during carbonate precipitation the relative proportion of (de)hydration (CO₂ + H₂O ⇔ H⁺ + HCO₃^-) and (de)hydroxylation (CO₂ + OH^- ⇔ HCO₃^-) reactions is pH-dependent. When modelling this system, the oxygen isotopic composition of water can be measured directly, but the oxygen isotopic composition of hydroxide must be calculated from the respective fractionation factor (1000lnα_H2O–OH-). Experimental studies from the 1960s determined 1000lnα_H2O–OH- to be 39.22(±2.88)‰ at 25 °C and estimated its temperature dependence at ‑0.5‰ °C^-1 (1-3). These empirical observations were recently questioned by a theoretical study that implied a much lower fractionation factor of 23.18–18.91‰ at 25 °C as well as a lower temperature dependence of ‑0.05‰ °C^‑1 (4).

To provide new experimental data to solve this controversy, we performed quantitative witherite (BaCO₃) precipitation experiments. Tank CO₂ gas of known oxygen and carbon isotopic composition was injected into saturated Ba(OH)₂ solution of known oxygen isotopic composition. Following the hydroxylation of the CO₂, BaCO₃ instantly precipitated from the high pH (>12) solution. Since the precipitate directly inherited 1/3 of its oxygen from the hydroxide ion and 2/3 from the tank CO₂, the δ¹⁸O value of the OH^- can be calculated via mass balance. Subsequently, the 1000lnα_H2O–OH- value can be derived. Altogether 18 experiments were performed at a range of temperatures (1–80 °C) and using solutions of different oxygen isotopic composition (range of ca. 20‰). Minor variations between the δ¹³C values (< 1‰) of the BaCO₃ and the tank CO₂ attests the quantitative precipitation of the reference gas.

Our 1000lnα_H2O–OH- values show a similar temperature dependence as the recent theoretical study of Zeebe (4), but our fractionation factor at 25 °C is much closer to the values reported in the 1960s. Reasons for the discrepancies between our study and previous publications in terms of 1000lnα_H2O–OH- and its temperature dependency will be discussed. One of our hypotheses is that the H₂O ⇒ OH^- + H⁺ reaction introduces a large kinetic effect as isotopically light H₂O is pyrolysed more frequently. In contrast, the back reaction proceeds rapidly without an isotopic preference. Hence, the self-ionisation of water cannot be described as a classic equilibrium. Alternative explanations such as unidentified kinetic isotope effects in our precipitation experiments (i.e., on the crystal surface) cannot be ruled out.

(1) H. R. Hunt, H. Taube, The relative acidity of H₂O¹⁸ and H₂O¹⁶ coördinated to a tripositive ion. J. Phys. Chem. 63, 124-125 (1959).
(2) E. R. Thornton, Solvent isotope effects in H₂O¹⁶ and H₂O¹⁸. J. Am. Chem. Soc. 84, 2474-2475 (1962).
(3) M. Green, H. Taube, Isotopic fractionation in the OH^-–H₂O exchange reaction. J. Phys. Chem. 67, 1565-1566 (1963).
(4) R. E. Zeebe, Oxygen isotope fractionation between water and the aqueous hydroxide ion. Geochim. Cosmochim. Acta 289, 182-195 (2020).