Coordination Chemistry of Uranyl Ion with Citric Acid

Citric acid (H3Cit), 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 with H3Cit 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 in form of (UO2)nHrCit (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 H3Cit the complexes are demonstrated in respect to the significant polarization of the hydroxyl group upon the complexation with uranyl. The hydroxyl group of H3Cit undergoes deprotonation at low pH region down to 1.5 when bonding with uranyl, and H3Cit easily bridges 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−1Cit4−, which facilitates the formation of the mononuclear complex species of 1∶−2∶2 and 1∶−3∶3.