Myrttinen A, Becker V, Mayer B, Barth J A C
GeoZentrum Nordbayern, Friedrich-Alexander Universität Erlangen-Nürnberg, Schlossgarten 5, 91054, Erlangen, Germany.
Rapid Commun Mass Spectrom. 2014 Aug 15;28(15):1691-6. doi: 10.1002/rcm.6950.
Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10(3) lnα(13) C) between H2 CO3 (*) (H2 CO3 + CO2(aq) ) and gaseous CO2 (CO2(g) ) are so far only available up to 60 °C and were typically determined at or near atmospheric pressures. Here we experimentally expand this dataset to temperature and pressure conditions close to the supercritical state for CO2 . The objective is to improve the applicability of stable carbon isotopes as a tracer in environments where such conditions prevail.
Eighteen stable carbon isotope laboratory experiments were conducted in a steel vessel. Deionised water that was acidified with hydrochloric acid (HCl, 1 N) to a pH of 2.4 was equilibrated with CO2(g) at pressures (pCO2 ) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 °C. H2 CO3 (*) and CO2(g) were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry.
At 20 °C, average 10(3) lnα(13) CH2CO3 * -CO2(g) values of -1.0 ± 0.1 ‰ were observed with a preference for (12) C in H2 CO3 () consistent with previous research. At elevated temperatures of 120 °C, 10(3) lnα(13) CH2CO3 * -CO2(g) values decreased to an average value of -0.7 ± 0.1 ‰. The resulting temperature dependence for carbon isotope fractionation between H2 CO3 () and CO2(g) was 10(3) lnα(13) CH2CO3 * -CO2(g) = (0.0025 ± 0.0004) T(°C) - (1.0 ± 0.03) ‰. Carbon isotope equilibrium between H2 CO3 (*) and CO2(g) was reached within reaction times of 18 h and mostly within 5 h or less.
10(3) lnα(13) CH2CO3 * -CO2(g) data are now available for temperatures up to 120 °C and for pressures of up to 55 bar. The results suggest that higher pCO2 levels possibly shorten carbon isotope equilibration times. These data are critically important for using δ(13) C values as tracers, for instance at geological CO2 sequestration sites and corresponding natural analogues.
目前关于实验得出的碳酸（H₂CO₃，即H₂CO₃ ＋ CO₂(aq)）与气态CO₂（CO₂(g)）之间平衡稳定碳同位素分馏（10³lnα¹³C）的文献数据仅涵盖至60°C，且通常是在大气压或接近大气压的条件下测定的。在此，我们通过实验将该数据集扩展至接近CO₂超临界状态的温度和压力条件。目的是提高稳定碳同位素在存在此类条件的环境中作为示踪剂的适用性。
在一个钢制容器中进行了18次稳定碳同位素实验室实验。用盐酸（HCl，1N）酸化至pH为2.4的去离子水在55巴的压力（pCO₂）下与CO₂(g)平衡2至188小时。实验在20、60、80、100和120°C下进行。分别对H₂CO₃和CO₂(g)进行采样，并通过同位素比质谱法测定它们的碳同位素比。
在20°C时，观察到平均10³lnα¹³CH₂CO₃ * -CO₂(g)值为 -1.0 ± 0.1‰，H₂CO₃中优先富集¹²C，这与先前的研究一致。在120°C的高温下，10³lnα¹³CH₂CO₃ * -CO₂(g)值降至平均值 -0.7 ± 0.1‰。由此得出的H₂CO₃与CO₂(g)之间碳同位素分馏的温度依赖性为10³lnα¹³CH₂CO₃ * -CO₂(g) ＝ (0.0025 ± 0.0004)T(°C) - (1.0 ± 0.03)‰。H₂CO₃与CO₂(g)之间的碳同位素平衡在18小时内达到，且大多在5小时或更短时间内达到。
现在已有10³lnα¹³CH₂CO₃ * -CO₂(g)数据，涵盖温度高达120°C和压力高达55巴的情况。结果表明，较高的pCO₂水平可能会缩短碳同位素平衡时间。这些数据对于将δ¹³C值用作示踪剂至关重要，例如在地质CO₂封存地点及相应的天然类似物中。
