In order to assess radionuclide diffusion and transport properties in compacted bentonite, the “in-diffusion” method based
on bentonite filled capillaries is used. The effect of 99TcO4- concentration and pH value of the solution, the contact time and the dry density of compacted bentonite on the apparent diffusion
coefficient (Da) and on the distribution coefficient (Kd) values obtained from the capillary test was studied. The Da and Kd values decrease with increasing of the bulk dry density of compacted bentonite. Ion exclusion influences the diffusion of
99TcO4- in the same substance. As compared to literature data, the Kd values obtained from capillary tests are in most cases lower than those from batch tests, the difference between the two
Kd values is a strong function of dry density of the compacted bentonite.
The effect of pH and fulvic acid on the sorption of Sr on bentonite was investigated by using batch experiments. The sorption
and desorption isotherms of Sr on bentonite were determined at room temperature, at pH 6.0±0.2 and in presence of 0.1M NaCl.
It was found that the sorption of Sr is independent at pH<8, and then increases slightly with increasing pH. Fulvic acid increases
the sorption of Sr significantly on bentonite at low pH, but decreases the sorption of Sr at pH>8. The sorption of Sr on bentonite
can be described by a reversible sorption process and the sorption mechanism consists mainly of ion exchange.
The sorption of cesium onto g-Al2O3 was studied by using batch technique and ultra-filtration method at room temperature, at pH 4.0, 6.0 and 8.0 and ionic strength ranged from 0.001M to 0.1M NaClO4. The pore size of the filter 1000K was used to separate the liquid from the solid phase. UV vis spectrophotometer was used to determine the concentration of humic acid in the solution at wavelength of 254 nm. The sorption of humic acid and Cs on the g-Al2O3 was studied in the pH range from 2 to 12. The sorption of humic acid on the g-Al2O3 was strong (»98% HA was sorbed on the surface of g-Al2O3 below pH 9) and strongly depended on the pH values (above pHzpc the sorption of HA decreased markedly with the increasing of pH), humic acid has a little negative effect on the sorption of cesium. This may be attributed to the formation of HA-Cs complexation in the solution. The sorption of Cs onto g-Al2O3 was weak depending on the pH and independent on the ionic strength. The sorption of Cs onto g-Al2O3 may be contributed to cation exchange and surface complexation mechanism.
Authors:Wang Xiangke, Dong Wenming, Gong Yingchun, Wang Changhui, and Tao Zuyi
The factors affecting the sorption of radioeuropium(III) by bentonite and kaolinite were studied with the aim to assess the important factors which should be included in modeling of radioeuropium(III) migration in soils and sediments. Europium(III) is an analogue of trivalent actinides. The distribution coefficients of radioeuropium for sorption on bentonite and kaolinite from aqueous solutions were determined by using the batch method, and it was found that they were sensitive to the loading, the pH, the humic substance and the sorption direction. Thus, these sorption characteristics of radioeuropium on bentonite and kaolinite were found to be different from those of radiocobalt1, and the mathematical modeling of trivalent lanthanides and actinides migration will be more complicated than that of radiocobalt. It is improbable that the migration modeling with a constant distribution coefficient will be successful in the case of trivalent lanthanides and actinides.
Authors:Dong Wenming, Wang Xiangke, Du Jinzhou, Wang Dongqi, and Tao Zuyi
The effect of organic matter and iron oxides as solid components of the red earth on the retention of SeO3 has been investigated by a batch technique and selective extraction method. The sorption and desorption isotherms of SeO3 on the untreated red earth and the three treated soils were determined at 20°C, pH 6.8 or 7.2 and in the presence of 0.01M
CaCl2. It was found that the sorption-desorption hysteresis for untreated an treated soils is obvious and the clays play an important
role in the sorption-desorption hysteresis, and that the retention of SeO3 on red earth is attributed to the iron oxides to a great extent.
Authors:Dong Wenming, Wang Xiangke, Shen Yi, Zhao Xudong, and Tao Zuyi
The factors affecting the sorption of radiocobalt by bentonite and kaolinite were studied with the aim to assess important factors which should be included in modelling radiocobalt migration in soils and waters. The distribution coefficients of radiocobalt sorption on bentonite and kaolinite from aqueous solutions were determined by using the batch method, and it was found that they were insensitive to the loading, the initial pH, the ionic strength, the humic substance and the sorption direction. Thus, the sorption characteristics of radiocobalt on bentonite and kaolinite facilitated the mathematical modelling of radiocobalt migration. In addition, the sorption kinetics and the sorption mechanism of radiocobalt on bentonite and kaolinite were also examined.
Authors:Xiangke Wang, Jinzhou Du, Zuyi Tao, and Zhongxiu Fan
The migration of radionuclides 85+89Sr2+, 134Cs+, 125I- and 75SeO32- in calcareous soil from Yuzhong county of Gansu Province (China) at pH 7.8±0.2 and 152+154Eu(III) in red earth from Yingtan county of Jiangxi Province (China) at pH 4.6±0.2, in presence of CaCl2 was studied using column experiments. Results indicate that the negative anions, iodide and selenide are mobile nuclides while the migration of positive cations is related to the sorption capacity for the element. Iodide can be assumed to be non-reactive in calcareous soil. The breakthrough curves (BTCs) were fitted to the analytical solution or numerical solution of one-dimensional convection-dispersion transport models. Good agreements were obtained between the measured and predicted concentration profiles.
Authors:Guodong Sheng, Jiang Sheng, Shitong Yang, Ju Hu, and Xiangke Wang
The effects of pH, ionic strength, competing ions and initial metal concentrations on the uptake behavior and mechanism of
radioactive Ni(II) onto MnO2 was investigated using a combination of classical macroscopic methods and the extended X-ray absorption fine structure (EXAFS)
spectroscopy technique. The results indicated that the uptake of Ni(II) on MnO2 is obviously dependent on pH but independent of ionic strength, which suggested that the uptake of Ni(II) onto MnO2 is attributed to an inner-sphere surface complex rather than an outer-sphere surface complex. EXAFS analysis shows that the
hydrated Ni(II) is adsorbed through six-fold coordination with an average Ni–O interatomic distance of 2.04 ± 0.01 Å. It can
be inferred from the EXAFS analysis that the inner-sphere surface complex of Ni(II) onto MnO2 is involved in both edge-sharing and corner-sharing linkages. Both the macroscopic uptake data and the molecular level evidence
of Ni(II) surface speciation at the MnO2-water interfaces should be factored into better prediction of the bioavailability and mobility of Ni(II) in soil and water
MX-80 bentonite is considered as one of the best backfill materials for high-level radioactive nuclear waste. Herein, the
bentonite is characterized by using XRD and FTIR techniques. Sorption of radionickel to MX-80 bentonite in the presence/absence
of humic acid (HA) or fulvic acid (FA) as a function of pH is investigated. The results indicate that the presence of HA or
FA decreases the sorption of Ni2+ obviously. The different experimental processes do not affect the sorption of nickel to FA/HA bound bentonite. The sorption
of Ni2+ on FA/HA-bound bentonite decreases with the increasing FA/HA content in the systems. The mechanism of nickel sorption is
also discussed in detail.
Authors:Wang Xiangke, Dong Wenming, Du Jinzhou, and Tao Zuyi
The sorption and desorption of radiocesium on a calcareous soil from Jiuqian County of Gansu Province (China) were studied
by using batch and column experiments. The sorption-desorption isotherms and the breakthrough curves, displacement curves
on the whole soil and two treated soils were determined. Based on these results, it was found that the sorption and retention
of cesium are mainly determined by the clay minerals, that the sorption-desorption hysteresis of cesium on the calcareous
soil is obvious and that the organic matter has a little positive contribution and the calcium carbonate has a little negative
contribution to the sorption of cesium on the whole soil. The results from batch experiments were consistent with the results
from column experiments.