García-Rosales G, Drot R, Mercier-Bion F, Lagarde G, Simoni E
Université Paris-Sud 11, Institut de Physique Nucleaire d'Orsay, 15 Rue G. Clemenceau, Bat 100, Orsay Cedex, France.
J Colloid Interface Sci. 2009 May 1;333(1):104-13. doi: 10.1016/j.jcis.2009.01.049. Epub 2009 Jan 29.
The purpose of this work is the study of the interaction mechanisms between U(VI) ions and SrTiO(3) surfaces as a function of pH and temperature (25, 50, 75 and 90 degrees C) by coupling thermodynamic and spectroscopic approaches. First, the reactivity towards U(VI) for both surface sites of the strontium titanate ([triple bond]Ti-O and [triple bond]Sr-O) has been investigated as a function of the temperature. The N(2)-BET specific area was measured: 2.4+/-0.2 m(2)g(-1). The surface site density has been determined from potentiometric titrations (6 sites/nm(2) for each site [triple bond]Ti-O and [triple bond]Sr-O). The potentiometric titration data have been simulated, for each temperature, using the FITEQL 4.0 software and the constant capacitance model, taking into account both protonation of the [triple bond]Sr-OH surface sites and deprotonation of the [triple bond]Ti-OH ones (one pK model). The intrinsic strontium protonation constant increases with an increasing temperature, while the titanate deprotonation one decreases. Moreover, both enthalpy and entropy changes corresponding to the surface acid-base reactions have been evaluated using the van't Hoff relation. The uranium(VI) ions are sorbed onto SrTiO(3) surfaces in the 0.5-5.0 pH range with an initial cation concentration equal to 10(-4) M. The U(VI) surface complexes were identified by using time-resolved laser-induced fluorescence spectroscopy (TRLFS). For all the studied samples, the fluorescence spectra and the corresponding lifetime values do not change with the pH and the temperature. Two U(VI) complexes sorbed onto SrTiO(3) were detected and the corresponding lifetimes are 60+/-5 and 12+/-2 micros whatever the temperature (25, 50, 75 and 90 degrees C). The sorption edges were simulated with the FITEQL 4.0 code. The sorption equilibrium constants of the U(VI)/SrTiO(3) system between 25 and 90 degrees C were obtained with the constant capacitance model (CCM), considering two reactive surface sites. According to the spectroscopic characterization, two types of surface complexes, namely ([triple bond]SrOH)([triple bond]TiOH)UO(2) and ([triple bond]TiOH)([triple bond]TiO)UO(2), were considered. Finally, enthalpy (Delta(r)H(o)) and entropy (Delta(r)S(o)) changes were calculated from the temperature-dependent sorption constants, by the application of the van't Hoff formalism. The formation of the ([triple bond]SrOH)([triple bond]TiOH)UO(2) surface complex was found to present an endothermic character associated to an increase in the disorder of the system. On the contrary, the formation of the ([triple bond]TiOH)([triple bond]TiO)UO(2) surface complex led to an exothermic process with only a slight increase in the disorder of the system.
本工作的目的是通过结合热力学和光谱学方法，研究U(VI)离子与SrTiO₃表面之间的相互作用机制，该机制是pH值和温度（25、50、75和90摄氏度）的函数。首先，研究了钛酸锶的两个表面位点（[三键]Ti - O和[三键]Sr - O）对U(VI)的反应活性与温度的关系。测量了N₂ - BET比表面积：2.4 ± 0.2 m²g⁻¹。通过电位滴定法确定了表面位点密度（每个[三键]Ti - O和[三键]Sr - O位点为6个位点/nm²）。使用FITEQL 4.0软件和恒电容模型，针对每个温度模拟了电位滴定数据，同时考虑了[三键]Sr - OH表面位点的质子化和[三键]Ti - OH表面位点的去质子化（单pK模型）。本征锶质子化常数随温度升高而增加，而钛酸酯去质子化常数则降低。此外，利用范特霍夫关系式评估了与表面酸碱反应对应的焓变和熵变。在初始阳离子浓度等于10⁻⁴ M的情况下，U(VI)离子在0.5 - 5.0的pH范围内吸附到SrTiO₃表面。通过时间分辨激光诱导荧光光谱（TRLFS）鉴定了U(VI)表面络合物。对于所有研究的样品，荧光光谱和相应的寿命值不随pH值和温度变化。检测到吸附在SrTiO₃上的两种U(VI)络合物，无论温度（25、50、75和90摄氏度）如何，其相应的寿命分别为60 ± 5和12 ± 2微秒。用FITEQL 4.0代码模拟了吸附边界。考虑两个反应性表面位点，用恒电容模型（CCM）获得了25至90摄氏度之间U(VI)/SrTiO₃体系的吸附平衡常数。根据光谱表征，考虑了两种类型的表面络合物，即[([三键]SrOH)([三键]TiOH)UO₂]²⁺和[([三键]TiOH)([三键]TiO)UO₂]²⁺。最后，通过应用范特霍夫形式，根据温度依赖性吸附常数计算了焓变（ΔᵣHₒ）和熵变（ΔᵣSₒ）。发现[([三键]SrOH)([三键]TiOH)UO₂]²⁺表面络合物的形成具有吸热特征，与体系无序度增加相关。相反，[([三键]TiOH)([三键]TiO)UO₂]²⁺表面络合物的形成导致放热过程，体系无序度仅略有增加。
