Biosorption Behavior And Mechanism Of ～(241)Am By Microorganisms

Biosorption behavior of metal or radioactive elements by microorganism and theirapplication in treatment of industrial solution or radioactive waste is a very activeresearch area. For more than 50 years, much effort has been invested in this area.However, it is still in a laboratory stage and need more effort to apply the biosorptiontechnology to industrial solution or radioactive waste. Meanwhile, until now there is nosystematic theory for the biosorption mechanism, due to the extensiveness and varietyof microorganism and complicate chemical behavior of metal or radioactive elementsin aqueous solutions. Unfortunately, while no or few reports were involved in thebiosorption behavior or mechanism of actinium and fission elements, the chemical ormigration behavior of these elements in the conditions with microorganism and inunderground conditions, especially in our country. Obviously, it is very interesting andimportant to do research related to this area.As a transuranium element without stable isotopes, americium has about 20radioisotopes or isomers. Among them, ²⁴¹Am (T_1/2=433 years, E_α=5.468 MeV, 86.6%;5.443 MeV, 12.3%; F_γ=0.0596 MeV, 35%) is the most important one in that it isgenerally used as target material in nuclear technology or excitation resource in somescientific instruments. Also, it has widespread use in other fields. For example, most ofthe fire alarms in hotels, hospitals and service centers contain a ²⁴¹Am source.Unfortunately, ²⁴¹Am is one of the most serious concerns due to its long half-life andα-particle emission, especially, the tendency to deposit on several key tissues or organs,such as skeleton and liver, if it enters haman body. The maximum permissible quantityfor ²⁴¹Am in human body is 11.1KBq and the maximum permissible concentration inwater is 1.0Bq/L. For those reasons, all countries over the world have paid considerableattention to disposal or treatment of wastewater containing ²⁴¹Am. However, the most studies were focused on the solvent extraction of ²⁴¹Am from highly radioactive liquids,while no or few reports were involved in the treatment for radioactive solutionscontaining ²⁴¹Am using biosorption technology. In this paper, the first systematicattempt was made to investigate the biosorption behavior and mechanism of ²⁴¹Am bymicroorganisms.Several microorganism were chosen as biosorbents for accumulation of ²⁴¹Am fromsolution, in order to find a feasible method for the disposal of the radioactivewastewater produced in the process of preparing ²⁴¹Am in fire alarms. In particular, theeffects of various experimental conditions on the biosorption were investigated. Amongthe investigated microorganism, Saccharomyces cerevisiae (S. cerevisiae), Rhizopusarrihizus (R. arrihizus), Aspergillus niger (A. niger) and Candida albicams (Candidasp.) could efficiently removed ²⁴¹Am from solutions of 5.6-111 MBq/L(44.3-877.2μg/L)(C_o), with maximum adsorption rate (R) of 99% and maximumadsorption capacities (W) of 237.9MBq/g biomass (dry weight) (1880.0μg/g). It couldbe noted that ²⁴¹Am uptake on the microorganism is pH-dependent process and thebiosorption equilibrium is achieved within 1-2 hour except on Candida sp. for 4 hour.No significant differences on ²⁴¹Am adsorption were observed at 10℃-45℃, or insolutions containing Au³⁺ or Ag⁺, even 1500 times above the ²⁴¹Am concentration. Thebiosorption process of ²⁴¹Am by S. cerevisiae, R. arrihizus and A. niger could bedescribed by Langmuir adsorption isotherm as well as Freundlich adsorption isotherm,while the biosorption process on Candida sp corresponded to Langmuir adsorptionisotherm better.S. cerevisiae and Rhizopus Arrihizus (R. arrihizus), which had shown encouragingability in removing ²⁴¹Am from solutions in experiments above, were immobilized bycalcium alginate. The various factors that affect the ²⁴¹Am biosorption by immobilizedS. cerevisiae or R. arrihizus were investigated. The experimental results showed thatimmobilized S. cerevisiae and R. arrihizus not only can adsorb ²⁴¹Am as efficiently asfree fungus, but also be used repeatedly or continuously. The biosorption equilibriumwas achieved within 2 hour, and more than 95% of ²⁴¹Am was removed by immobilizedS. cerevisiae or R. arrihizus from ²⁴¹Am solutions of 1.08MBq/L in the range of pH1-4 or pH1-7, respectively. However, no significant differences in ²⁴¹Am biosorption wereobserved at 15℃-45℃. Even after repeated adsorption for 6-8 times, the immobilized S.cerevisiae or R. arrihizus still adsorbed more than 90% of ²⁴¹Am from the solutions.Compared with free microorganisms, the immobilized S. cerevisiae or R. arrihizus hadwider pH range and could be used repeatedly or continuously. All these implied that theimmobilized microorganism is more suitable for further practical use. In fact, theimmobilized S. cerevisiae and R. arrihizus had been used to treat the real ²⁴¹Amwastewater from a ²⁴¹Am fire alarm plant run by CAEP (Mianyang, P. R. China) and thepreliminary result is very encouraging. After repeated adsorption for 4 times, theradioactivity could be reduced to about 1Bq/L from more than 800 Bq/L in the real²⁴¹Am wastewater, lower or near to the maximum permissible concentration in water.The preliminary evaluation on biosorption mechanism of ²⁴¹Am by S. cerevisiae orR. arrihizus was fitrther explored via chemical or biological treatment ofmicroorganism. Especially, europium or neodymium was used as substitutes foramericium, and the Eu or Nd adsorbed microorganism instead of the ²⁴¹Am-adsorbedmicroorganism was analyzed by Transmission Electron Microscope (TEM) andRutherford Backscattering Spectrometry (RBS). The results indicated that the culturetimes of more than 16 hours for S. cerevisiae or 40 hours for R. arrihizus were suitableand the efficient adsorption of ²⁴¹Am by the S. cerevisiae and R. arrihizus were able toachieve. The biosorption of ²⁴¹Am by the decomposed cell wall, protoplasm or cellmembrane of S. cerevisiae or R. arrihizus was same efficient as by the intact fungus.However, the adsorption ratio for ²⁴¹Am by the chemically pretreated S. cerevisiae or R.arrihizus dropped obviously, implying that protein or carboxyl functional groups havean important role in the biosorption of ²⁴¹Am by S. cerevisiaece or R. arrihizus. Mostof the investigated acidic ions have no significant influence on the ²⁴¹Am adsorptionbut minute changes of pH value, while the saturated EDTA can strong inhibit thebiosorption of ²⁴¹Am on S. cerevisiae or R. arrihizus. When the concentrations ofcoexistent Eu³⁺, Nd³⁺ were 100 times more than that of ²⁴¹Am, the adsorption ratioswould drop. It could be observed by TEM micrographs that the adsorbed Eu or Nd isalmost scattered in whole fungus, while RBS analysis results indicated that Ca in S. cerevisiae can be replaced by Eu or Nd via ion exchange. All the results implied thatthe mechanism of ²⁴¹Am absorption is very complicate and at least involved in ionexchange, complexation process as well as adsorption in cell wall because of staticelectricity.In summary, this paper made the first systematic attempt to investigate thebiosorption behavior and mechanism of ²⁴¹Am by microorganisms. The research resultsnot only would help us to clearly understand the adsorption behavior and mechanism of²⁴¹Am by microorganisms, but also provide a new way for the treatment of radioactivesolutions containing ²⁴¹Am, especially for low-medium radioactivity waste.Additionally, the results would be useful in the further investigation on chemical andmigration behavior of ²⁴¹Am in the conditions with microorganisms, especially inunder ground conditions.