Biological nano-mineralization of Ce phosphate by Saccharomyces cerevisiae

In order to understand the nanoscale mineralization process of REE phosphate on microorganism surfaces, we have investigated the mechanism underlying Ce sequestration by yeast Saccharomyces cerevisiae after exposure to Ce(III) solution for 4–120 h at pH 3, 4, or 5. A variety of analytical techniques have been employed, including field-emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (FESEM-EDS), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), and synchrotron-based X-ray absorption fine structure (XAFS). concentration in solutions decreases as a function of exposure time and more rapidly at pH 5 than at pH 3 or 4. Analyses of the yeast cells by FESEM, TEM, and XAFS show that needle-shaped Ce(III) phosphate nanocrystallites with a monazite structure formed on the yeast cells by exposure to Ce(III) for 42 h, even though the initial solutions did not contain any P species. The Ce(III) phosphate nanocrystals grew from about 50 nm to hundreds of nanometers when pH increased from 3 to 5. Lower pH resulted in higher P concentration in the solution after the yeast cells were inoculated, indicating the release of P from the yeast cells. These results suggest that the sorbed Ce on the cell surfaces reacted with P released from inside the yeast cell, resulting in the formation of Ce(III) phosphate nanocrystallites. This post-sorption nanocrystallization on the microbial cell surface should play a key role in constraining the long-term migration of REEs and trivalent actinides in geological repositories.