Kinetics of ikaite precipitation and dissolution in seawater-derived brines at sub-zero temperatures to 265 K
Kinetics of ikaite precipitation and dissolution in seawater-derived brines at sub-zero temperatures to 265 K
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DOI：
10.1016/j.gca.2014.05.031
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发表时间：
2014-09
影响因子：
5
通讯作者：
S. Papadimitriou;H. Kennedy;P. Kennedy;Davis Thomas
中科院分区：
文献类型：
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作者：
S. Papadimitriou;H. Kennedy;P. Kennedy;Davis Thomas
采用恒加实验技术研究了六水碳酸钙（ikaite）在海水和海水卤水中的沉淀和溶解动力学。这两个过程的稳态速率被发现是一个函数的偏差的平衡相对于ikaite的溶液，并符合相同的经验速率定律的无水碳酸钙多晶型物，方解石和文石。除了盐水相对于伊凯特的饱和状态外，盐水的盐度和反应温度显然对伊凯特的沉淀动力学有一定的控制作用，而多晶型物的溶解动力学则不受这两个参数的明显影响。实验的盐度和温度条件相当于极地海洋海冰覆盖中盐水和冰之间的热平衡条件。通过将海洋等效物外推到海冰盐水而对CO2系统进行的简单建模表明，海水离子的物理浓度以及伊凯特石溶解度随盐度和温度的变化（两者都是海冰系统固有的）不足以使新生盐水达到伊凯特石过饱和，并使海冰中的降水降至零下8摄氏度。溶解无机碳向海冰气相和共生自养生物的损失是两种独立的机制，在自然界中，这两种机制可以促使盐水CO2系统向ikaite过饱和和沉淀。在这些条件下，稳定状态的沉淀率的ikaite被认为是足够快的快速形成在短时间尺度内（天到几周）在海冰。观察到的ikaite溶解动力学也被发现有利于在腐蚀性溶液，如表面海水中的几个小时到几天的短周转时间尺度。
The kinetics of calcium carbonate hexahydrate (ikaite) precipitation and dissolution were investigated in seawater and seawater-derived brines at sub-zero temperatures using the constant addition experimental technique. The steady state rate of these two processes was found to be a function of the deviation of the solution from equilibrium with respect to ikaite and conformed to the same empirical rate law as the anhydrous CaCO3polymorphs, calcite and aragonite. In addition to the saturation state of the brine with respect to ikaite, the salinity of the brine and the temperature of the reaction evidently exerted some control on the ikaite precipitation kinetics, while the dissolution kinetics of the polymorph were not noticeably influenced by these two parameters. The experimental salinity and temperature conditions were equivalent to those at thermal equilibrium between brine and ice in the sea ice cover of polar seas. Simple modelling of the CO2system by extrapolation of the oceanic equivalent to sea ice brines showed that the physical concentration of seawater ions and the changes in ikaite solubility as a function of salinity and temperature, both inherent in the sea ice system, would be insufficient to drive the emergent brines to ikaite supersaturation and precipitation in sea ice down to −8 °C. The loss of dissolved inorganic carbon to the gas phase of sea ice and to sympagic autotrophs are two independent mechanisms which, in nature, could prompt the brine CO2system towards ikaite supersaturation and precipitation. Under these conditions, the steady state precipitation rate of ikaite was found to be fast enough for rapid formation within short time scales (days to weeks) in sea ice. The observed ikaite dissolution kinetics were also found conducive to short turn-over time scales of a few hours to a few days in corrosive solutions, such as surface seawater.