Calcium isotopic fractionation during adsorption onto and desorption from soil phyllosilicates (kaolinite, montmorillonite and muscovite)
Calcium isotopic fractionation during adsorption onto and desorption from soil phyllosilicates (kaolinite, montmorillonite and muscovite)
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DOI：
10.1016/j.gca.2019.02.017
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发表时间：
2019-04
影响因子：
5
通讯作者：
Jean-Michel Brazier;A. Schmitt;S. Gangloff;E. Pelt;F. Chabaux;E. Tertre
中科院分区：
文献类型：
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作者：
Jean-Michel Brazier;A. Schmitt;S. Gangloff;E. Pelt;F. Chabaux;E. Tertre
矿物土壤成分由于其吸附和解吸阳离子的能力而在养分的储存/释放中起着重要作用。层状硅酸盐矿物由于其小的尺寸和高的比表面积（高达800 m2·g−1）而具有特别的反应性。以前的工作强调了Ca同位素（δ44/40 Ca）在识别土壤中发生的次生过程中的潜力。然而，与钙的吸附和解吸从不同的层状硅酸盐矿物，通常在土壤中发现的同位素分馏（幅度和性质）的机制仍然知之甚少。这一步是根本的，以提高我们的理解在水-土壤-植物界面的钙地球化学循环。因此，可能的钙同位素分馏过程中的吸附/解吸现象的研究接近实验使用三个“模型”基板代表的页硅酸盐矿物内经常遇到的土壤（KGa-2高岭石，Swy-2蒙脱石和Tuftane白云母）。实验是在非生物条件下进行的“水-矿”批处理系统，通过精确控制的物理化学条件：pH值，固/液比，初始溶解钙浓度，粒度分布，反应时间和其他阳离子浓度。我们的研究结果表明，在KGa-2和粗粒级Tuftane白云母（50-200 µm）实验期间，无论使用何种物理化学参数，都没有与Ca吸附或解吸相关的显著同位素分馏。在Ca吸附到Swy-2和Tuftane白云母的细粒度部分（0.1-1 μm）上的过程中，轻同位素（例如，40 Ca）优先吸附在粘土矿物上，在吸附实验后回收的上清液中测得0.10‰ ~ 0.28‰的正表观同位素分馏。在Swy-2实验中观察到动力学和热力学同位素分馏，而在Tuftane白云母（0.1-1 µm）实验中仅发生热力学分馏。用氯化物六胺钴溶液进行的钙解吸实验得到的结果表明，钙吸附和相关的同位素分馏是完全可逆的。我们的研究结果表明，钙同位素分馏的强度在粘土矿物吸附现象的控制层电荷和比表面积所考虑的层状硅酸盐矿物，以及存在的层间空间开放的阳离子溶质。
Mineral soil constituents play an important role in the storage/release of nutrients due to their ability to adsorb and desorb cations. Phyllosilicate minerals are particularly reactive due to their small size and high specific surface area (up to ∼800 m2·g−1). Previous work has highlighted the potential of Ca isotopes (δ44/40Ca) to identify secondary processes occurring within soils. Nevertheless, the mechanisms of isotopic fractionation (amplitude and nature) associated with Ca adsorption onto and desorption from different phyllosilicate minerals that are commonly found in soil remain poorly understood. This step is fundamental to improve our understanding of the Ca biogeochemical cycle at the water-soil-plant interface. Consequently, the study of the possible Ca isotopic fractionation during adsorption/desorption phenomena was approached experimentally using three “model” substrates representative of the phyllosilicate minerals frequently encountered within soils (KGa-2 kaolinite, Swy-2 montmorillonite and Tuftane muscovite). The experiments were carried out under abiotic conditions in “water-mineral” batch systems by precisely controlling the physico-chemical conditions: pH, solid/solution ratio, initial dissolved Ca concentration, particle size distribution, reaction time and other cations concentrations. Our results show no significant isotopic fractionation associated with Ca adsorption or desorption during KGa-2 and coarse size fraction of Tuftane muscovite (50–200 µm) experiments regardless of the physico-chemical parameters used. During Ca adsorption onto Swy-2 and fine size fraction of Tuftane muscovite (0.1–1 µm), light isotopes (e.g.,40Ca) are preferentially adsorbed to the clay mineral, and a positive apparent isotopic fractionation between 0.10‰ and 0.28‰ is measured in the supernatant recovered after adsorption experiments. Kinetic and thermodynamic isotopic fractionation are observed during the Swy-2 experiments while only a thermodynamic fractionation took place during the Tuftane muscovite (0.1–1 µm) experiments. The results obtained for the Ca desorption experiments performed with a chloride hexaamine cobalt solution suggest that Ca adsorption and the associated isotopic fractionation are fully reversible. Our results show that the Ca isotopic fractionation intensity during adsorption phenomena onto clay minerals is controlled by the layer charge and specific surface area of the considered phyllosilicate mineral, as well as the presence of an interlayer space open to cationic solute.