Redistribution of Pb during transformation of monohydrocalcite to aragonite
Redistribution of Pb during transformation of monohydrocalcite to aragonite
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DOI：
10.1016/j.chemgeo.2014.08.024
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发表时间：
2014-11
期刊：
影响因子：
3.9
通讯作者：
Takashi Munemoto;K. Fukushi;Y. Kanzaki;T. Murakami
中科院分区：
文献类型：
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作者：
Takashi Munemoto;K. Fukushi;Y. Kanzaki;T. Murakami
在溶液中铅的存在下，一水方解石转化为文石，以了解铅的再分配机制和动力学过程中的转变。单水方解石合成了（Ca 0.98 Mg 0.02 CO 3·1.00 H 2 O），并沿着Pb原液，加入到缓冲溶液中;批量吸附实验进行了（i）在不同的初始铅浓度范围从1到100 μM（1-30和50-100 μM，称为低和高初始铅浓度，（ii）在1和100 μM的初始Pb浓度下，在不同的反应时间下反应1-24 h。在整个实验过程中，CO 32 −浓度（24.4 meq/L）、pH值（9.50）和温度（25 °C）在实验误差范围内保持恒定。在低初始铅浓度下，一水方解石的转化在24 h内完成，可能在15 h内完成，但在高初始铅浓度下，一水方解石的转化甚至在24 h内完成，表明水溶液中铅浓度影响一水方解石的溶解速率和/或文石的成核和晶体生长速率，从而影响转化速率。文石是在一水方解石溶解后不久形成的，几乎所有从一水方解石中溶解的Ca都形成文石。~ 90-96%的初始Pb在转化开始前被吸附在一水方解石上，并且在任何反应时间和任何初始Pb浓度下，> 95%的初始Pb分布在固相中，表明Pb的再分布速率与一水方解石的转化速率一样快。碳酸铅（可能是水白铅矿）形成在高初始铅浓度，但不是在低初始铅浓度。碳酸铅的沉淀速率比一水方解石的转化速率慢。文石对铅的吸附主要以掺入方式进行。转化后，相应地，几乎所有的铅被纳入文石在低初始铅浓度，并在文石和碳酸铅在高初始铅浓度。Pb在文石中的掺入是通过（Ca，Pb）CO 3固溶体的形成来解释的。Pb在单水方解石向文石转化过程中的再分配现象将有助于我们更深入地了解微量元素在地表的迁移过程。
Monohydrocalcite was transformed to aragonite in the presence of Pb in solution to understand the Pb redistribution mechanisms and kinetics during the transformation. Monohydrocalcite (Ca0.98Mg0.02CO3·1.00H2O) was synthesized, and along with Pb stock solution, added to a buffer solution; batch sorption experiments were carried out (i) at different initial-Pb concentrations ranging from 1 to 100 μM (1–30 and 50–100 μM, referred to as low and high initial-Pb concentrations, respectively) for 24 h and (ii) at different reaction time for 1–24 h at 1 and 100 μM of the initial Pb concentrations. The CO32 −concentration (24.4 meq/L), pH (9.50) and temperature (25 °C) were kept constant within experimental errors throughout the experiments. The transformation of monohydrocalcite was completed in 24 h, probably in 15 h, at the low initial-Pb concentrations but not even in 24 h at the high initial-Pb concentrations, suggesting that the aqueous Pb concentration affects the rates of dissolution of monohydrocalcite and/or nucleation and crystal growth of aragonite, and thus, the transformation rate. Aragonite was shortly formed as monohydrocalcite was dissolved and almost all amount of Ca dissolved from monohydrocalcite formed aragonite. ~ 90–96% of the initial Pb was adsorbed onto monohydrocalcite before the transformation started, and > 95% of the initial Pb was distributed in solid phases at any elapsed time of the reaction and at any initial-Pb concentrations, indicating that the Pb redistribution rate was as fast as the transformation rate of monohydrocalcite. Pb carbonate (possibly hydrocerussite) was formed at the high initial-Pb concentrations but not at the low initial-Pb concentrations. The precipitation rate of the Pb carbonate is slower than the transformation rate of monohydrocalcite. The Pb sorption on aragonite occurred dominantly by incorporation. After the transformation, accordingly, almost all Pb was incorporated in aragonite at the low initial-Pb concentrations, and in aragonite and Pb carbonate at the high initial-Pb concentrations. The incorporation of Pb in aragonite is well explained by the formation of (Ca,Pb)CO3solid-solution. The Pb redistribution during transformation of monohydrocalcite to aragonite revealed by the present study will give a deeper understanding of trace element transport at the Earth's surface when the transport is accompanied by mineral transformation.