Aqueous speciation is likely to control the stable isotopic fractionation of cerium at varying pH
Aqueous speciation is likely to control the stable isotopic fractionation of cerium at varying pH
复制标题
DOI：
10.1016/j.gca.2017.09.019
复制
发表时间：
2017-12
影响因子：
5
通讯作者：
R. Nakada;Masato Tanaka;M. Tanimizu;Y. Takahashi
中科院分区：
文献类型：
--
作者：
R. Nakada;Masato Tanaka;M. Tanimizu;Y. Takahashi
最近对Ce在吸附和沉淀过程中的稳定同位素分馏的研究表明，Ce(Ce)可以作为氧化还原的替代物。然而，该研究中的实验是在低于天然海水的pH条件下进行的。在本研究中，用2.25 mm溶解碳酸盐在pH 6.80、8.20和11.00的条件下进行了吸附和沉淀实验，以模拟自然环境中Ce的同位素分馏，并考察了液态Ce的同位素分馏与Ce形态的关系。吸附在水合铁上的Ce的液固相平均同位素分馏系数(α，Lq-So)不受pH条件和溶解Ce物种的影响。在Ce/δ-MnO2体系中，αLq-So值从Ce~(3+)到Ce(CO_3)_2−，随pH或碳酸盐离子数目的增加而从1.000411(±0.000079)降至1.000194(±0.000067)。在溶液中有Ce(CO3)2−存在的pH为8.20和11.00时的Ce/沉淀体系中，αLq-So值分别为0.999821(±0.000071)和0.999589(±0.000074)，这意味着在液相中观察到了较轻的同位素富集度，这与其他体系相反。即使在较高的pH下，Ce(CO3)+或Ce(CO3)2-−是主要的溶解物种，但在Ce/高铁水合物和Ce/沉淀体系中，Ce在固相中的第一配位球与在pH为5.00的溶液中以Ce(CO3+)为主的溶液中观察到的第一配位球相似。Ce/δ-MnO2体系固相中的Cesingle Bondo键长在pH为11.00时略有延长，这可能会导致Ce/SnO-MnO2体系中的同位素分馏减少。在吸附过程中，Ce的配位环境可能不会发生明显的变化，这是因为Ce在pH低于8.5~8.8(即水合水铁的零电荷点(PZC)的pH值)时，与其他阳离子在水合和吸附物种中的金属-O距离相似时，与水合和吸附物种中的中性表面羟基键合。当pH高于PZC时，Ce与表面带负电荷的羟基键合，而Ce还与溶解物种中的CO32−键合。具有相同趋势的溶解物种(lnβLq)和吸附物种(lnβSO)的还原分配函数(lnβ)相互抵消，这是因为结合阴离子降低了水合阳离子的lnβ，导致同位素分馏较小。因此，在本研究的整个pH条件下，Ce/亚铁水合物的同位素分馏可能很小。根据密度泛函理论(DFT)计算，估算了同位素分馏的方向，证实了Ce与碳酸盐离子形成络合物时，较轻的Ce在液相中富集。因此，本研究表明，在沉淀反应过程中，溶解物种可以控制稳定的Ce同位素分馏。
Cerium (Ce) can be used as a plaeoredox proxy as shown by a recent study of stable isotopic fractionation of Ce during adsorption and precipitation. However, the experiments in that study were performed at pH conditions lower than that of natural seawater. In the current study, adsorption and precipitation experiments were performed at pH 6.80, 8.20, and 11.00 with 2.25 mM dissolved carbonate to simulate Ce isotopic fractionation in the natural environment and examine the relationship between isotopic fractionation and Ce speciation in the liquid phase. Mean isotopic fractionation factors between liquid and solid phases (αLq-So) of Ce adsorbed on ferrihydrite did not depend on pH conditions or dissolved Ce species. In the Ce/δ-MnO2system,αLq-Sovalues decreased from 1.000411 (±0.000079) to 1.000194 (±0.000067) with increasing pH or number of carbonate ions, from Ce3+to Ce(CO3)2−. In the Ce/precipitation system at pH 8.20 and 11.00 where Ce(CO3)2−is present in solution, the αLq-Sovalues were 0.999821 (±0.000071) and 0.999589 (±0.000074), respectively, meaning that lighter isotope enrichment was observed in the liquid phase, which is the contrary to those of the other systems.Extended X-ray absorption fine structure (EXAFS) analyses were also performed to investigate the coordination structure of the adsorbed or precipitated Ce species that control the isotopic fractionation during adsorption. Even at higher pH, where Ce(CO3)+or Ce(CO3)2−are the dominant dissolved species, the first coordination sphere of Ce in the solid phase in the Ce/ferrihydrite and Ce/precipitation systems was similar to that observed at pH 5.00 where Ce3+was the main species in solution. A slight elongation in the Cesingle bondO bond length in the solid phase at pH 11.00, where negatively charged dissolved species are dominant in the liquid phase, may cause a decrease in isotopic fractionation in the Ce/δ-MnO2system. The coordination environment of Ce may not change significantly during the adsorption onto ferrihydrite, because Ce binds to the neutral surface OH group on ferrihydrite at pH below 8.5–8.8 (i.e. the pH of the point of zero charge (PZC) for ferrihydrite), similar to other cations when the metal–O distance was similar in hydrated and adsorbed species. At pH above PZC, Ce bonds to the negatively charged surface OH group, while Ce also bonds with CO32−in dissolved species. The reduced partition functions (lnβ) for dissolved species (lnβLq) and adsorbed species (lnβSo) with the same trends canceled each other, because lnβof hydrated cation was reduced by the binding anion, resulting in small isotope fractionations. Thus, isotope fractionations for Ce/ferrihydrite may be quite small at the entire pH conditions in this study. The direction of the isotopic fractionation was estimated based on density functional theory (DFT) calculations, which confirmed that lighter Ce is enriched in the liquid phase when Ce forms a complex with carbonate ions. Therefore, this study indicates that the dissolved species can control stable Ce isotopic fractionation during precipitation reactions.