Physicochemical Characterization and Antimicrobial Properties of Lanthanide Nitrates in Dilute Aqueous Solutions

This work presents the results of studying dilute aqueous solutions of commercial <i>Ln</i>(NO₃)₃ · <i>x</i>H₂O salts with <i>Ln</i> = Ce-Lu using X-ray diffraction (XRD), IR spectroscopy, X-ray absorption spectroscopy (XAS: EXAFS/XANES), and pH measurements. As a reference point, XRD and XAS measurements for characterized <i>Ln</i>(NO₃)₃ · <i>x</i>H₂O microcrystalline powder samples were performed. The local structure of <i>Ln</i>-nitrate complexes in 20 mM <i>Ln</i>(NO₃)₃ · <i>x</i>H₂O aqueous solution was studied under total external reflection conditions and EXAFS geometry was applied to obtain high-quality EXAFS data for solutions with low concentrations of <i>Ln</i>³⁺ ions. Results obtained by EXAFS spectroscopy showed significant contraction of the first coordination sphere during the dissolution process for metal ions located in the middle of the lanthanide series. It was established that in <i>Ln</i>(NO₃)₃ · <i>x</i>H₂O solutions with <i>Ln</i> = Ce, Sm, Gd, Yb (c = 134, 100, 50 and 20 mM) there are coordinated and, to a greater extent, non-coordinated nitrate groups with bidentate and predominantly monodentate bonds with <i>Ln</i> ions, the number of which increases upon transition from cerium to ytterbium. For the first time, the antibacterial and antifungal activity of <i>Ln</i>(NO₃)₃ · <i>x</i>H₂O <i>Ln</i> = Ce, Sm, Gd, Tb, Yb solutions with different concentrations and pH was presented. Cross-relationships between the concentration of solutions and antimicrobial activity with the type of <i>Ln</i> = Ce, Sm, Gd, Tb, Yb were established, as well as the absence of biocidal properties of solutions with a concentration of 20 mM, except for <i>Ln</i> = Yb. The important role of experimental conditions in obtaining and interpreting the results was noted.