Sorption behaviour of cesium, strontium, cobalt, ruthenium, cerium, europium and uranium has been studied on soil selected for the disposal of low level radioactive waste solutions.
Physical, chemical and mineralogical characteristics of the soil used for these studies were determined. The soil was a silty clay and alkaline in nature. The cation exchange capacity of the soil was found to be 35 meq/100g. The principal clay mineral identified by X-ray diffraction analysis was illite.
The sorption experiments were performed by batch technique using radioactive nuclides of the elements as tracers. Distribution coefficients were determined for soil-element solution systems as a function of all pertinent variables, Le., time of reaction, pH, adsorbent and adsorbate concentrations. The interfering effects of complementary cations and organic complexing ligands were also investigated. Desorption experiments were also carried out.
The sorption equilibrium for Cs and Sr was achieved almost instantaneously. The sorption of Eu was rapid and that of Ce was initially fast but later on became slow. The uptake of Co was rapid in the beginning and was slow afterwards. The sorption of U was also rapid and did not change much with time. The sorption of Ru was a slow process right from the beginning.
The sorption of Cs, Sr, Co, Ru, Ce and Eu increased with increasing pH, while of U showed variable behaviour with pH.
The distribution coefficients (Kd) values of Cs, Ce and U increased with increasing adsorbent concentrations. Kd values for Co and Eu first increased and then decreased with adsorbent concentrations. Kd values of Sr and Ru decreased with increasing adsorbent concentrations.
Kd values of Cs, Sr, Co, Ce, Eu and U increased with decreasing adsorbate concentrations. This effect was greater for Cs and Sr and smaller for Co, Ce, Eu and U. Kd values for Ru were unaffected with changes in adsorbate concentrations. Tile experimental data filled well into the Freundlich and Langmuir adsorption isotherms.
The effects of interfering cations such as Ca, Mg, K and Na in the concentration range of 0.005 eqt.dm-3 to 3.0 eqt.dm-3, on the sorption of elements under consideration were studied. The sorption of Cs was found to decrease by the interfering cations and the reducing effect of cations followed the order K>Ca> Mg > Na, where as the effect of cations in reducing the sorption of Sr, Co, Ce and Eu was found to follow the order, Ca > Mg > K > Na. The effect of interfering cations on the sorption of U was found in the order Ca > Mg > Na ,>K. The sorption of Ru on soil was found to remain unaffected by the presence of these cations.
The sorption of Cs, Sr, Co, Ce and Eu was studied in the presence of 1.0x10-5 to 1.0x10-3 mol.dm-3 of synthetic organic chelating agents like EDTA, DTPA and 1.0x10-5 to1.0x10-4 mol.dm-3 of CDTA. The uptake was also, investigated in the presence of 1.0x10-4 to 1.0x10-3 mol.dm-3 of natural organic ligands like oxalic acid and citric acid. The sorption of Co, Ce and Eu was , appreciably reduced by higher concentrations of EDTA, DTPA and CDTA. The sorption of Co was also affected by citric acid, while oxalic acid had little effect. The sorption of Cs and Sr was also reduced by higher concentrations of both synthetic and natural ligands but the effect was smaller as compared to Co, Ce and Eu.
The desorption behaviour with respect to time and desorbent of all the elements except uranium was also studied. Groundwater and 0.01 mol.dm-3 CaCI2 were used as desorbents for these studies. It was found that Sr was partially desorbed (about 12-14%) with groundwater and completely des orbed with 0.01 mol.dm-3 CaCI2 solution. About 7-8% of Cs was desorbed with groundwater and 18-21% by CaCI2 solution. Practically no desorption of Co, Ce and Eu was observed with either groundwater or CaCI2 solution. The desorption pf all the elements studied. showed an insignificant dependence upon time.
The sorption of Cs, Sr and Co on < 2 μm fractions of clay minerals such as montmorillonite, illite and kaolinite was also studied under controlled experimental conditions, i.e., at pH 5,7 and 9 in the presence of 0.01 eqt.dm-3 CaCI2 solution as background electrolyte at 0.001 eqt.dm-3 elemental concentration. It was found that Cs sorbed to a greater extent under these conditions followed by Co and Sr. The sorption mechanism of these elements has been explained by using Hard-Soft Acid-Base Principle, according to which, hard acids tend to associate with hard bases and soft acids with soft bases.
The sorption of Cs, Co and Sr was also investigated under the above mentioned experimental conditions in the presence of other background electrolytes such as Ca(NO3)2 and CaSO4. Montmorillonite was found to sorb softer Cs and Co ions to a greater extent than Sr. It was found on the basis of experimental data that montmorillonite was a soft base than illite and kaolinite.
The effects in the presence of Ca(NO3)2 and CaSO4 on the sorption of these elements on clay minerals followed the same order as was found for CaCI2 solution. The distribution coefficients of the elements under study were practically unaffected in the presence of anions such as CI-, NO3- and SO42-.
The experimental data shows that illitic soil can be effectively used for the disposal of low level liquid radioactive waste under controlled conditions, i.e., pH of the disposable solution should be alkaline and elemental concentration should be low. The interfering cations must be present in minimum quantities. Higher concentrations of organic complexing agents must be absent in the radioactive waste solutions.
It is also concluded that Lewis Hard-Soft Acid-Base Principle can be used in understanding sorption sequence of the elements on mineral surfaces.