It is well known that uranium (U) is a kind of radionuclide with chemical toxicity. Once entering the soil system, it will inevitably undergo physical, chemical and biological effects, which is extremely important to the geochemical behaviors of U.  

In this study, the researchers found that under different pH conditions, the sorption mechanism of U(VI) on kaolinite and illite is mainly attributed to ion exchange, outer-sphere complexes (OSCs), and inner-sphere complexes (ISCs).  

Aspergillus niger could obviously change the charge density, sorption sites and other physical and chemical properties of illite and kaolinite to a certain extent, thereby promoting the sorption of U(VI).  

At the same time, the sorption mechanism of U(VI) on the surface of the clay- Aspergillus niger composites could be only attributed to the ISCs. It showed that the sorption process of U(VI) on the bio-composites is significantly different from clay minerals.  

The study also showed that U(VI) mainly complexed with the phosphate group, amide group and carboxyl group derived from the biomass of Aspergillus niger in the bio-composites, which further weakened the effect of ion exchange and complexation with Si-OH and Al-OH on illite and kaolinite.  

Besides, according to the X-ray photoelectron spectroscopy, U(VI) was partly reduced to non-crystalline U(IV) by Aspergillus niger in the bio-composites.  

The findings further clarify the geochemical process of U(VI) in a complicated environmental system, and provid an important theoretical basis for accurately assessing and predicting the environmental behavior of uranium. 

This research has been published in Chemosphere in an article entitled "New insights into the sorption of U(VI) on kaolinite and illite in the presence ofAspergillus niger."  

The sorption process and mechanism of U(VI) on kaolinite- and illite-A. niger composites surface. (A)-(D): SEM images of kaolinite- and illite-A. niger composites after U(VI) sorption. (E): The contents of U(VI) at the different areas of clay, A. niger, and clay-A. niger composites (Green and Blue points are the hyphae surface and clay, respectively. Red square is the area of clay-A. niger composites). (F)-(H): The relationship of element U with S, P and N elements. (I): The sorption mechanism of U(VI) on clay-A. niger. (Image by FAN Qiaohui)