铀酰离子与柠檬酸的配位化学研究

Coordination Chemistry of Uranyl Ion with Citric Acid

柠檬酸(2-羟基-1,2,3-丙三羧酸,H_(3)Cit)常用于铀污染的去污和铀污染土壤的修复。然而以往对水溶液中铀酰离子与柠檬酸配位化学的研究大多是在低pH区域和柠檬酸与铀酰离子比例较低的条件下进行的,在更宽的pH范围内进行的实验和去污实践中,所得到的配位模型和热力学常数不能很好地解释大多数观察结果。为探究UO_(2)^(2+)与H_(3)Cit在宽pH范围和高铀酰柠檬酸比例下的配位反应,本文通过采用电位滴定和吸收光谱滴定方法测定不同条件下生成的一系列UO_(2)^(2+)与H_(3)Cit配合物的稳定常数,并通过解析滴定过程中采集的光谱获得各物种的标准摩尔吸收谱。结果表明,在pH=2~12区间,随着pH的升高,UO_(2)^(2+)与H_(3)Cit可逐渐形成11种不同配比和组成的(UO_(2))_(n)H_(r)Cit_(m)^(2n+r-3m)配合物,分别为n∶r∶m=1∶1∶1、1∶0∶1、2∶0∶2、2∶−1∶2、2∶−2∶2、1∶0∶2、1∶0∶3、3∶−5∶2、3∶−5∶3、1∶−2∶2、1∶−3∶3,其中1∶0∶3、1∶−2∶2和1∶−3∶3三个配合物尚未见文献报道。培养并获得了3∶−5∶3物种晶体,结合其拉曼光谱,通过UO_(2)^(2+)-H_(3)Cit拉曼光谱滴定归属了各物种的拉曼谱带。通过热力学和光谱信息,明确了H_(3)Cit的羟基去质子化并参与配位的机理。H_(3)Cit羟基在参与和UO_(2)^(2+)的配位时,UO_(2)^(2+)对羟基O—H键的强极化作用,使得H_(3)Cit羟基能在pH=1.5时发生去质子化,并通过羟基氧和羧基氧与UO_(2)^(2+)桥接形成多核配合物;在pH>9和更碱性的溶液中,Cit^(3−)的羟基将更易发生去质子化形成H_(−1)Cit^(3−),这有利于形成1∶−2∶2和1∶−3∶3的单核配合物物种。

Citric acid(H_(3)Cit),which is an important intermediate chemical in the tricarboxylic acid cycle metabolism of organisms,widely presents in nature and also is added in food and other daily necessities.Due to its strong chelating ability towards high valent metal ions and biodegradable property,it is commonly used in nuclear industry for the decontamination of uranium pollution and the remediation of uranium contaminated soil.However,because of the versatile coordination modes of citrate and the property of uranyl ion prone to hydrolysis in aqueous solutions,previous studies on the coordination chemistry of uranyl ion with citric acid in aqueous solutions was mostly conducted in low pH region under conditions of low ratios of citric acid to uranyl ion.Indeed,the obtained coordination models and thermodynamic constants cannot well explain most observation in experiments performed in wider pH range and in practice of decontamination.Therefore,the main purpose of this work is to study the complexation of UO_(2)^(2+)with H_(3)Cit under conditions of a wide pH range and high uranyl citrate ratios.With pH potentiometric titration and spectral titration methods,in the solutions of pH=2-12 eleven complex species of UO_(2)^(2+)in form of(UO_(2))_(n)H_(r)Cit_(m)^(2n+r-3m)(n∶r∶m=1∶1∶1,1∶0∶1,2∶0∶2,1∶0∶2,1∶0∶3,3∶−5∶2,3∶−5∶3,1∶−2∶2,and 1∶−3∶3)were identified,and among these three complexes(n∶r∶m=1∶0∶3,1∶−2∶2,1∶−3∶3)are reported for the first time.Along with the thermodynamic stability constants,the molar absorption spectra of each species are obtained by the deconvolution of the spectra collected during the titrations.In combination with speciation distribution based on the complexation model and the corresponding stability constants,the Raman shift of uranyl in each species are also assigned with regard the spectral and structural information of the known 3∶−5∶3 complex in single crystals.With the thermodynamic and spectroscopic information,the varying coordination modes of the hydroxyl group of H_(3)Cit the complexes are demonstrated in respect to the significant polarization of the hydroxyl group upon the complexation with uranyl.The hydroxyl group of H_(3)Cit undergoes deprotonation at low pH region down to 1.5 when bonding with uranyl,and H_(3)Cit easily bridges UO_(2)^(2+)ions through hydroxyl and carboxyl oxygen to form multinuclear complexes.In solutions of pH>9 and more alkaline,the hydroxyl group of citrate are more prone to deprotonation to form H_(−1)Cit^(4−),which facilitates the formation of the mononuclear complex species of 1∶−2∶2 and 1∶−3∶3.