Removal Behavior and Mechanism of Uranium by Bacillus Siamensis Based on Spectroscopic Analysis : the Role of Biological Phosphorus

In this experiment, Bacillus Siamensis (B. siamensis) was taken as research object, and B. siamensis had a high ratio of surface area to volume, indicating that B. siamensis has a great performance on adsorbing heavy metals. However, the studies of previous experts and scholars on B. siamensis mostly focused on degradation of starch or cellulose and antifungal activity. And the mechanism of the interaction between B. siamensis and heavy metals or radionuclides was to tally not researched. Therefore, the purposes of this experiment were to use ICP-OES and ICP-MS to study the effects of pH value of solution, initial uranium concentration and biomass on removal of uranium by B. siamensis and the relationship between biological phosphorus released by cells and removal of uranium during the process, FTIR and SEM to characterize the morphology and group changes of B. siamensis before and after interaction with uranium, XPS and EDS to analyze the distribution and valence of elements on the surface of B. .siamensis, and then the removal mechanism of uranium by B. siamensis was discussed. The results showed that the removal of uranium by B. siamensis under different pH varied greatly due to the difference of growth activities of B. siamensis, the existence forms of uranium and the amount of phosphorus element released by cells in the process under different pH values conditions. The removal effect was the best at pH=5. 0. Increasing the biomass was beneficial to the removal of uranium by B. siamensis. The Langmuir and Freundlich adsorption isotherm models were used to fit the experimental data, and the fitting results showed that the removal behavior of uranium by B. siamensis conformed to the Langmuir isotherm adsorption model. And the maximum adsorption capacity obtained from the initial uranium concentration experiment was higher than the theoretical maximum adsorption capacity calculated from the Langmuir isotherm adsorption model, indicating that the removal of uranium by B. siamensis could be a combination of physical and chemical behavior. B. siamensis could effectively remove uranium from water. The maximum removal rate obtained in this experiment was 96. 5% and the maximum adsorption capacity was 450. 3 mg . g(-1), which were higher than those of most of the Bacillus strains used to adsorb uranium. SEM test of B. siamensis before and after the reaction showed that scale-like precipitate appeared on the surface of the cells after the reaction. XPS and EDS showed that the precipitate was a phosphorus-containing uranium substance. Combined with FTIR analysis, it was presumed that the removal mechanism of uranium by B. siamensis is as follows: Firstly, uranium was rapidly attracted to the surface of B. siamensis through electrostatic action, then adsorbed by phosphate groups, amino groups, hydroxyl groups and carboxyl groups on the bacteria in coordination form, and at the same time interacting with phosphate-containing substances released by the bacteria to form phosphorus-containing precipitation of uranium and then immobilized on the surface of the bacteria. During this process, a part of the hexavalent uranium was reduced to tetravalent uranium by intracellular substances released by the B. siamensis and then settled. It was speculated that the precipitation on the surface of the cell might be the mixture of the phosphate precipitation of uranium and the complexes of phosphorus-containing compounds and uranium complexes.