On the growth of anhydrous Mg-bearing carbonates – Implications from norsethite growth kinetics
On the growth of anhydrous Mg-bearing carbonates – Implications from norsethite growth kinetics
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DOI：
10.1016/j.gca.2018.07.013
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发表时间：
2018-10
影响因子：
5
通讯作者：
M. Lindner;G. Saldi;Salvatore Carrocci;P. Bénézeth;J. Schott;Guntram Jordan
中科院分区：
文献类型：
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作者：
M. Lindner;G. Saldi;Salvatore Carrocci;P. Bénézeth;J. Schott;Guntram Jordan
在环境条件下，白云石[CaMg（CO 3）2]和菱镁矿[MgCO 3]都不容易从水溶液中生长.对此的一个常见解释是Mg 2+的高度水合特性。然而，诺氏石[BaMg（CO 3）2]异常容易的生长清楚地表明，原则上，Mg 2+离子即使在环境条件下也能迅速溶解并结合到无水碳酸盐矿物中。然而，由于缺乏可靠的定量数据，无法对这些含镁矿物的反应性进行必要的比较。为了阐明无水Mg 2+离子可能的掺入率的跨度，我们提出了第一个系统的定量研究的诺赛特生长动力学以及诺赛特溶解度积的测定在很宽的温度范围内，以取代目前存在的扩散定性知识。使用氢电极浓差电池从30 ° C至150° C在0.1 M NaCl水溶液中测定诺赛特溶解度，其提供氢离子摩尔浓度的连续原位测量。诺耳石的溶度积可以用log 10 K sp°-nrs= a+ B/T+ cT来描述，其中a= 31.007，B=− 7321.122，c=− 0.0811。诺赛特生成的吉布斯自由能（Δ f G 298.15 0）和焓（Δ f H 298.15 0）分别为− 2167±2 kJ/mol和− 2351±2 kJ/mol。生长实验是在混合流反应器中进行的，涵盖了溶液组成（pH值：7.0-8.5，[Ba]：3× 10− 6-5× 10− 3 M，[Mg]：1× 10− 4-9× 10− 2 M，离子强度：0.1 M，Ω norsethite= 1-290）和温度（40、65和100° C）的显著跨度。由实验数据确定了诺氏石生长速率常数的表观活化能为Ea = 80±7 kJ/mol。外推到25° C时，反应速率常数k nrs 25 <$C= 1.8× 10− 2 nmol m− 2 s− 1，反应级数为1.2±0.1。这些结果允许不同的无水含镁碳酸盐矿物的生长速率的直接，定量的比较。这一比较表明，在100° C下，诺赛特的生长速率常数比菱镁矿高约三个数量级，比白云石高约五个数量级。对于诺氏石，显然必须存在某种有效的方法来促进Mg离子的脱水，并使脱水的Mg 2+快速地结合到生长的矿物中，这种促进作用必须发生在诺氏石的表面，在那里Mg 2+的水化能可以明显地不同于在本体溶液中的已知值。因此，不仅含水金属络合物本身的稳定性是控制无水含镁碳酸盐矿物生长速率的重要因素，而且给定表面削弱该络合物稳定性的手段。
At ambient conditions, both dolomite [CaMg (CO 3) 2] and magnesite [MgCO 3] do not readily grow from aqueous solution. A common explanation for this is the highly hydrated character of Mg 2+. The exceptionally easy growth of norsethite [BaMg (CO 3) 2], however, clearly shows that Mg 2+ ions, in principle, can rapidly dehydrate and incorporate into anhydrous carbonate minerals even at ambient conditions. Still, the lack of reliable quantitative data prevents the necessary comparison of the reactivities of these magnesium-bearing minerals. In order to shed light on the span of possible incorporation rates of anhydrous Mg 2+ ions, we present the first systematic quantitative study of norsethite growth kinetics as well as the determination of norsethite solubility product over a wide range of temperatures to replace the diffuse qualitative knowledge existing so far. Norsethite solubility was determined in 0.1 M NaCl aqueous solutions from 30 to 150° C using a hydrogen-electrode concentration cell, which provides a continuous in-situ measurement of hydrogen ion molality. The solubility product of norsethite can be described by log 10 K sp°-nrs= a+ b/T+ cT, where a= 31.007, b=− 7321.122, and c=− 0.0811. Gibbs free energy (Δ f G 298.15 0) and enthalpy (Δ f H 298.15 0) of norsethite formation were determined to be− 2167±2 kJ/mol and− 2351±2 kJ/mol, respectively. Growth experiments were conducted in mixed-flow reactors covering a significant span of solution compositions (pH: 7.0–8.5,[Ba]: 3× 10− 6–5× 10− 3 M,[Mg]: 1× 10− 4–9× 10− 2 M, ionic strength: 0.1 M, Ω norsethite= 1–290) and temperatures (40, 65, and 100° C). From the experimental data, the apparent activation energy of norsethite growth rate constant was determined to be E a= 80±7 kJ/mol. An extrapolation to 25° C resulted in a rate constant of k nrs 25∘ C= 1.8× 10− 2 nmol m− 2 s− 1 with a reaction order of 1.2±0.1. These results allowed for a direct, quantitative comparison of the growth rates of different anhydrous Mg-bearing carbonate minerals. This comparison revealed that the growth rate constant of norsethite at 100° C is approximately three orders of magnitude higher than that of magnesite and five orders of magnitude higher than that of dolomite. In the case of norsethite, obviously some effective means must exist which promotes the dehydration of the Mg ion and allows for the rapid incorporation of dehydrated Mg 2+ into the growing mineral. This promotion has to take place at the norsethite surface where the hydration energy of Mg 2+ can significantly differ from the well-known value in bulk solution. Consequently, not only the stability of the aqueous metal complex per se is an important factor controlling the growth rate of anhydrous magnesium bearing carbonate minerals but also the means of a given surface to weaken the stability of this complex.