Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)
Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)
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DOI：
10.1016/s0016-7037(03)00266-7
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发表时间：
2002-09
影响因子：
5
通讯作者：
S. Welch;B. Beard;C. Johnson;P. S. Braterman
中科院分区：
文献类型：
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作者：
S. Welch;B. Beard;C. Johnson;P. S. Braterman
在一系列氯化物浓度（0、11、110 mM Cl−）和两个温度（0和22°C）下测量的含水亚铁和三价铁物种之间的平衡和动力学Fe同位素分馏表明，Fe同位素分馏是温度的函数，但在研究的范围内与氯化物含量无关。利用~（57）Fe富集示踪实验，二价铁和三价铁的同位素交换动力学可用二级速率方程或一级方程拟合。在22°C下，交换是快速的，大约60-80%在5秒内完成，而在0°C下，交换速率大约慢一个数量级。同位素交换率随氯化物含量而变化，其中亚铁-铁同位素交换率在11 mM HCl溶液中比0 mM Cl−（约10 mM HNO 3）溶液慢25%至40%;同位素交换率在0和110 mM Cl−溶液中相当。在22°C下，0、11和111 mM Cl−溶液中的平均平衡同位素分馏ΔFe（III）-Fe（II）在实验误差范围内相同，分别为+2.76 <$0.09、+2.87 <$0.22和+2.76 <$0.06 ‰。这与约翰逊等人（2002年a）在稀盐酸溶液中测得的值非常相似。在0°C下，对于0、11和111 mM Cl−溶液，平均测量的ΔFe（III）-Fe（II）分馏分别为+3.25 <$0.38、+3.51 <$0.14和+3.56 <$0.16 ‰。在物种分离过程中，部分再平衡对同位素分馏的影响的评估表明，对于22°C和0°C的实验，测得的同位素分馏平均分别低了0.20 ‰和0.13 ‰。使用校正的分馏因子，我们可以将同位素分馏的温度依赖性定义为：103 ln αFe（III）-Fe（II）= [0.334 ± 0.032]* 106 T2 − 0.88 ± 0.38其中同位素分馏在这些实验中使用的范围内与Cl−含量无关。这些结果证实了Fe（III）-Fe（II）分馏大约是光谱数据预测的一半，并表明至少在中等Cl−含量下，同位素分馏对Fe-Cl物种形成相对不敏感。
Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl−) and at two temperatures (0 and 22°C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using57Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first-order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22°C, ∼60–80% complete within 5 seconds, whereas at 0°C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were ∼25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl−(∼10 mM HNO3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl−solutions. The average measured equilibrium isotope fractionations, ΔFe(III)-Fe(II), in 0, 11, and 111 mM Cl−solutions at 22°C are identical within experimental error at +2.76±0.09, +2.87±0.22, and +2.76±0.06 ‰, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0°C, the average measured ΔFe(III)-Fe(II)fractionations are +3.25±0.38, +3.51±0.14 and +3.56±0.16 ‰ for 0, 11, and 111 mM Cl−solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by ∼0.20 ‰ and ∼0.13 ‰ for the 22°C and 0°C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0°C to 22°C as: 103ln αFe(III)-Fe(II)= [0.334 ± 0.032]*106T2− 0.88 ± 0.38 where the isotopic fractionation is independent of Cl−contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl−contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation.