Study Of Adsorption Behavior Of²³⁹Pu On Bentonite

In order to prevent radioactive nuclide migrationto the surface, and ensureabsolute security environment, buffer/backfill material should meet the long-termthermal stability, mechanical properties, low permeability, swelling, radionuclidemigration hysteresis, radiation resistance, thermal conductivity and economiccharacteristics as filled with the last artificial barrier between waste objects. A largenumber of domestic and international research shows, montmorillonite as the maincomponent of bentonite is the ideal buffer/backfill material in high-level radioactivewaste disposal.239Pu is an important key nuclide of the high-level radioactive wastegeological disposal. It has long life, high specific activity and heat release rate, highbiological toxicity and so on, it is always the important research objects of t animportant key nuclide and environmental safety assessment. At present, our countrysome related scientific research units have already studied on the adsorptioncharacteristics of a variety of radionuclide migration on bentonite. But for239Pu, therelated theory and technology research is not sufficient. Therefore, to carry outresearch on bentonite adsorption characteristics of239Pu on bentonite, that has certainsignificance for China’s high-level radioactive waste geological disposal work.In this paper, GMZ-bentonite, Ca-bentonite, Na-bentonite and Bleaching clay asadsorption medium, by static adsorption experiments, were studied in differentenvironments to the adsorption behavior of plutonium. The studies contents andresearch results are as follows:(1)Four kinds of bentonite could reach adsorption equilibrium after6days, theKdvalues were GMZ-bentonite Kd=23746mL/g, Ca-bentonite Kd=24311mL/g,Na-bentonite Kd=20499mL/g, Bleaching clay Kd=18312mL/g, respectively. Andobtained four kinds of bentonite adsorption capacity size:Ca-bentonite>GMZ-bentonite>Na-bentonite>Bleachingclay.(2)To explore cationic (Na+, K+, Mg2+, Ca2+, Ba2+and Fe3+), anionic (CO32-,HCO3-, SO42-, Cl-and NO3-), ion concentration(CO32-, Fe3+and Ca2+), pH,temperature and plutonium initial concentration and other factors affecting theadsorption behavior of239Pu on bentonite. To calculate the distribution coefficientKdvalues, then to conclude the influenceregulation.The results showed: The adsorption capacity of bentonite increases with theincrease of pH value; The existence of some external ions(Mg2+, Ca2+, Ba2+, Fe3+,CO32-, HCO3-and SO42-) affect adsorption behavior; The adsorption capacitydecreases with the increase ofions (CO32-, Fe3+and Ca2+) concentration; Thedistribution coefficient value Kddecreases with the concentration of239Pu increases;The higher the temperature, the adsorption capacity of bentonite is higher.(3)By adsorption affect experiments, the mechanism of adsorption behavior aswell as the factors of influence mechanism were analyzed. Concluded bentonitesurface functional groups which were deprotonation can react with plutonium and itshydrolysis products, so as to achieve the purpose of plutonium adsorption. pHcanaffect the surface functional groups of bentonite to the protonation of239Pu.andhydrolysis reaction, therefore the pH effect on the adsorption behavior largely. Theaqueous phase anions can cooperate with239Pu in reaction, impact the adsorptionbehavior of239Pu on bentonite. The presence of cation which affects the ion exchangeof bentonite. The existence of Fe3+which mainly affect the pH value of water phase,and Ca2+which can react with anion of bentonite, so thathave larger effect on theadsorption of239Pu.(4)To draw adsorption isotherms, determine Freundlich isotherm model ofadsorption behavior of239Pu on bentonite. And calculate the thermodynamicparameters, enthalpy, entropy, Gibbs energy change. The thermodynamic parametersconfirm the adsorption behavior of239Pu on bentonite is spontaneous endothermicprocess.