Dong Wenming

Sorption and desorption of Eu and Yb on alumina: mechanisms and effect of fulvic acid.

The effects of pH, ionic strength and FA (fulvic acid) on the sorption and desorption of Eu(III) and Yb(III) on alumina were respectively investigated by using batch technique and radiotracers 152 + 154Eu and 169Yb. The distribution coefficients for sorption and desorption of Eu on alumina at pH 4.4, 4.6 and 5.7 in 1 mol/l NaCl solutions as a function of solid phase concentration were determined in the presence or absence of FA. The effects of pH, FA and ionic strength on the distribution coefficients for sorption and desorption of Yb on alumina were determined in 0.01-2.0 mol/l NaNO3. It was found that pH and FA influenced the sorption of Eu(III) and Yb(III) on alumina greatly. A surface hydrolysis model can satisfactorily and qualitatively explain the observations on bare alumina. The competition among the complexations of surface free hydroxyl groups, soluble and sorbed fulvic acids can satisfactorily and qualitatively explain the observations on the coated alumina.

Comparative study on sorption/desorption of radioeuropium on alumina, bentonite and red earth: effects of pH, ionic strength, fulvic acid, and iron oxides in red earth.

The sorption and desorption of Eu(III) as a representative of trivalent lanthanides and actinides on bentonite, alumina, red earth and red earth treated to remove free iron oxides were comparatively investigated by using batch technique and radiotracer 152 + 154Eu. The effects of pH, ionic strength, fulvic acid, iron oxides in red earth and the sorption mechanism were also discussed. As compared to alumina and red earth, Eu(III) presents a considerable distribution coefficient (Kd) onto bentonite. It was found that the pH and the presence of clay minerals are the main factors dominating the sorption/desorption characteristic of Eu3+ in the soil, and that a sorption-desorption hysteresis on bentonite and red earth actually occurs. Furthermore, the main sorption mechanism of lanthanides onto bentonite, alumina and red earth is the formation of bridged hydroxo complexes with the surface, and there are negative effects of fulvic acid and free iron oxides in red earth on the sorption of Eu(III). The results of this paper indicate that the additivity rule on the sorption characteristic of a soil from the individual components characteristics is not general.

Sorption characteristics of zinc(II) by calcareous soil–radiotracer study

65Zn was used as a radiotracer to study the sorption characteristics of zinc by two calcareous soils from Gansu Province (China). The sorption and desorption isotherms of Zn on two untreated calcareous soils and on two soils treated to remove CaCO3 were determined at 20 +/- 2 degrees C, pH 7.8 +/- 0.2 in the presence of 0.001 mol/l CaCl2. The contribution of CaCO3 to the Zn sorption by the two calcareous soils was approximately 70% of the total amount sorbed and the sorption-desorption hysteresis was definitely demonstrated, thus the CaCO3 in calcareous soil is undoubtedly the most significant sink for zinc at high pH range.

Use of the ion exchange method for the determination of stability constants of trivalent metal complexes with humic and fulvic acids II. Tb3+, Yb3+ and Gd3+ complexes in weakly alkaline conditions

The conditional stability constants for tracer concentrations of Tb(III), Yb(II), and Gd(III) with three soil humic acids, three soil fulvic acids and a fulvic acid from weathered coal were determined at pH 9.0-9.1 (these values are similar to those in calcareous soils) in the presence of NaHCO3 by using the anion exchange method. It was found that 1:1 and 1:2 complexes were simultaneously formed in the weakly alkaline conditions. The conditional stability constants of these 1:1 and 1:2 complexes were calculated from the distribution coefficients of rare earth elements at various concentrations of humate or fulvate. The stability constants indicate the very high stability of trivalent Tb3+, Yb3+ and Gd3+ complexes with humic substances in weakly alkaline conditions. The key parameters necessary for the experimental determination of the conditional stability constants of metal ions with humic substances in the presence of NaHCO3 by using an anion exchange method were discussed. The conditional stability constants of these 1:1 and 1:2 complexes were compared in this paper. It was found that stabilities of Tb3+ 1:1 and 1:2 complexes with humic acid are greater than the corresponding ones with fulvic acid from the same soil. In addition, the effect of the presence of Ca2+ as a competitor on the stabilities of 1:1 and 1:2 complexes of Yb was examined and no pronounced change of stabilities of 1:1 complex was found, even though Ca2+ is in a 10(3) excess to Yb3+.

Sorption of Radiocobalt on Bentonite and Kaolinite

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.

Sorption and desorption of radiocesium on calcareous soil: Results from batch and column investigations

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.

Sorption characteristics of radioeuropium on bentonite and kaolinite

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.

SORPTION AND DESORPTION OF RADIOSELENIUM ON RED EARTH AND ITS SOLID COMPONENTS

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.

Additivity rule and its application to the sorption of radionuclides on soils

Summary Previous authors reported that for some sorption systems the overall distribution coefficient (Kd) for a radionuclide or trace element on a mixture of minerals could be predicted from the additivity rule, the distribution coefficients on individual minerals and their mass fractions and that for some sorption systems, deviations from the additivity rule were found. In this paper, the additivity rule and its applicability to soils were analyzed from the theoretical considerations and the nature of soils. It was found that under the simplifying assumptions: 1) reversible and independent sorption reactions, 2) at equilibria of sorption reactions, 3) homogeneous surfaces, 4) insolubility of ion exchangers or sorbents, 5) no new phase formation, and 6) no interactions between individual minerals of the mixture, the additivity rule is formally valid. In practice, because of these crucial assumptions, the nature of soils and the difference in the experimental conditions for the sorption on mixtures and for the sorption on individual minerals, in general, the additivity rule is not applicable to the sorption systems of soils.

Use of the Anion Exchange Method for Determination of Stability Constants of Metal-Humic Complexes

The stability constants of complexes of rare earth elements with humic and fulvic acids were measured at the in situ conditions involving trace metal concentration, relatively high pH and presence of carbonate and hydroxyl groups, by using the anion exchange equilibrium method. A new correction technique for the sorption of humic substance and its complexes on anion exchange resin was developed and examined.