Rossberg André, Ulrich Kai-Uwe, Weiss Stephan, Tsushima Satoru, Hiemstra Tjisse, Scheinostt Andreas C
Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf P.O. Box 51 01 19, 01314 Dresden, Germany.
Environ Sci Technol. 2009 Mar 1;43(5):1400-6. doi: 10.1021/es801727w.
Previous spectroscopic research suggested that uranium(VI) adsorption to iron oxides is dominated by ternary uranyl-carbonato surface complexes across an unexpectedly wide pH range. Formation of such complexes would have a significant impact on the sorption behavior and mobility of uranium in aqueous environments. We therefore reinvestigated the identity and structural coordination of uranyl sorption complexes using a combination of U LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and iterative transformation factor analysis, which enhances the resolution in comparison to conventional EXAFS analysis. A range of conditions (pH, CO2 partial pressure, ionic strength) made it possible to quantify the variations in surface speciation. In the resulting set of spectral data (N=11) the variance is explained by only two components, which represent two structurally different types of surface complexes: (1) a binary uranyl surface complexwith a bidentate coordination to edges of Fe(O,OH)6 octahedra and (2) a uranyl triscarbonato surface complex where one carbonate ion bridges uranyl to the surface. This ternary type B complex differs from a type A complex where uranyl is directly attached to surface atoms and carbonate is bridged by uranyl to the surface. Both surface complexes agree qualitatively and quantitatively with predictions by a charge distribution (CD) model. According to this model the edge-sharing uranyl complex has equatorial ligands (-OH2, -OH, or one -CO3 group) that point away from the surface. The monodentate uranyl triscarbonato surface complex (type B) is relevant only at high pH and elevated pC0O. At these conditions, however, it is responsible for significant uranyl sorption, whereas standard models would predict only weak sorption. This paper presents the first spectroscopic evidence of this ternary surface complex, which has significant implications for immobilization of uranyl in carbonate-rich aqueous environments.
以往的光谱研究表明，在一个出人意料的宽pH范围内，铀（VI）对铁氧化物的吸附主要由三元铀酰 - 碳酸根表面络合物主导。此类络合物的形成将对铀在水环境中的吸附行为和迁移性产生重大影响。因此，我们结合使用U LIII边扩展X射线吸收精细结构（EXAFS）光谱和迭代变换因子分析，重新研究了铀酰吸附络合物的特性和结构配位情况，与传统EXAFS分析相比，迭代变换因子分析提高了解析度。一系列条件（pH值、二氧化碳分压、离子强度）使得量化表面形态的变化成为可能。在所得的一组光谱数据（N = 11）中，方差仅由两个成分解释，这两个成分代表两种结构不同类型的表面络合物：（1）一种二元铀酰表面络合物，与Fe（O，OH）6八面体的边缘形成双齿配位；（2）一种铀酰三碳酸根表面络合物，其中一个碳酸根离子将铀酰与表面桥连。这种三元B型络合物不同于A型络合物，在A型络合物中铀酰直接附着于表面原子，而碳酸根则由铀酰与表面桥连。两种表面络合物在定性和定量上均与电荷分布（CD）模型的预测相符。根据该模型，边缘共享的铀酰络合物具有指向远离表面的赤道配体（-OH2、-OH或一个-CO3基团）。单齿铀酰三碳酸根表面络合物（B型）仅在高pH值和高pC0O时才相关。然而，在这些条件下，它是铀酰大量吸附的原因，而标准模型预测的吸附作用较弱。本文首次给出了这种三元表面络合物的光谱证据，这对富含碳酸盐的水环境中铀酰的固定具有重要意义。
