Spectroscopic and microscopic approach of U(VI) sorption on Acidovorax facilis for remediation purpose

The Gram-negative betaproteobacterium Acidovorax facilis is a suitable candidate for in situ bioremediation of contaminated waste waters and environments [1]. For spectroscopic and microscopic studies kinetic U(VI) sorption experiments were performed under aerobic conditions at 30°C by adjusting an initial U(VI) concentration to 0.1 mM at a neutral pH range by adding UO2(NO3)2 to the batch culture.
A high-resolution image of the cellular localization of U by A. facilis was achieved by using electron microscopy (STEM/HAADF). The elemental distribution analysis of phosphorus and uranium clearly indicates that U is entirely present in the cell membrane. By cryo-Time-resolved laser-induced fluorescence spectroscopy (cryo-TRLFS) studies the spectra deconvolution indicates a fast binding of U(VI) on phosphorylic functionality groups during the first hour with a subsequent formation of Uranyl-carboxylic species in addition to the Uranyl-phosphorylic species. Compared to emission spectra of the Uranyl-lipopolysaccharide [2] we suggest an interaction of UO22+ with cell membrane components of the outer membrane of A. facilis cells, whereas lipopolysaccharide will form the most stable complex. These results support those obtained by Extended X-ray absorption fine structure spectroscopy (EXAFS), where a relative short average U-Oeq bond length of 2.35 Å were observed for the U(VI) interaction with lipopolysaccharide indicating a binding of the U(VI) via organic phosphate groups in a monodentate fashion. The strong interaction of U(VI) with phosphorylic and carboxylic groups was reinforced by in-situ attenuated total reflection Fourier-transform (ATR FT-IR) spectroscopic studies due to the presence of characteristic phosphoryl vibrations. Most of the bound U(VI) presumably remained on the cells, more precisely on the phosphorylic functionalities at the cell membrane.
[1] Gerber et al. (2016). J. Hazard. Mater. 317, 127–134.
[2] Barkleit et al. (2008). Appl. Spectrosc. 62(7), 798-802.