Preliminary investigation on biosorption mechanism of 241Am by Rhizopus arrhizus

As an important radioisotope in nuclear industry and other fields, (241)Am is one of the most serious contamination concerns due to its high radiation toxicity and long half-life. Encouraging biosorption of (241)Am from aqueous solutions by free or immobilized Rhizopus arrhizus (R. arrhizus) has been observed in our experiments. In this study, the preliminary evaluation on the mechanism was further explored via chemical or biological modification of R. arrhizus using europium as a substitute for americium. The results indicated that in approximately 48 hours R. arrhizus was able for efficient adsorption of (241)Am. The pH value of solutions decreased gradually with the uptake of (241)Am by R. arrhizus, implying that H(+) was released from R. arrhizus via ion-exchange. The biosorption of (241)Am by the decomposed cell wall of R. arrhizus was as efficient as by the intact fungus. The adsorption ratio for (241)Am by deacylated R. arrhizus dropped, implying that carboxyl functional groups of R. arrhizus play an important role in the biosorption of (241)Am. Most of the investigated acidic ions have no significant influence on the adsorption of (241)Am, while saturated EDTA can strongly inhibit the biosorption of (241)Am by R. arrhizus. When the concentrations of coexistent Eu(3+), Nd(3+) were 300 times more than that of (241)Am, the adsorption ratios would decrease to about 86% from more than 99%. It could be noted by transmission electron microscope (TEM) analysis that the adsorbed Eu is scattered almost in the whole fungus, while Rutherford backscattering spectrometry (RBS) indicated that Ca in R. arrhizus have been replaced by Eu via ion-exchange. The change of the absorption peak structure in the IR spectra implied that there was complexation between metals and microorganism. The results implied that the adsorption mechanism of (241)Am by R. arrhizus is very complicated involved ion-exchange, complexation process as well as nonspecific adsorption in the cell wall by static electricity.