Sorption behaviour of radiocaesium in soils

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Sorption behaviour of radiocaesium in soils

 

Author: Cheung, Yin-ping Irene
Title: Sorption behaviour of radiocaesium in soils
Degree: M.Phil.
Year: 2002
Subject: Hong Kong Polytechnic University -- Dissertations
Calcium -- Isotopes
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1625984
URI: http://theses.lib.polyu.edu.hk/handle/200/1711
Abstract: The general purpose of this study is to investigate the sorption behaviour of 137Cs in soils in Hong Kong reservoir. This provides information about the long-term mobility of radiocaesium for remedial decisions and effective response to any accidental releases from the nearby nuclear power plants. Three sets of soils were collected from a local reservoir and characterized for their physical and chemical properties. The time variations of 137Cs concentration in water were monitored using five different isotherms until equilibrium had been established. The conductivity and the pH of the water-soil mixture were also monitored. The first isotherm was defined by varying the mass of soils added while maintaining a constant initial 137Cs activity concentration. The mass of soils ranged from 0.05 g to 5 g. The second isotherm was defined by varying the initial water activity concentration with constant mass of random sizes. The initial water activity concentrations varied from 29 kBq l-1 to 120 kBq l-1. The third isotherm was defined by varying the sizes of soil particles while keeping constant initial activity concentration and mass of soil. The diameters of soils ranged from 0.15 mm to 2 mm. The fourth isotherm was defined by varying the mass of soil added in one specific pH value while maintaining constant initial activity concentration. The mass varied from 2 g to 7 g and three different pH values of 2.6, 3.0 and 3.6 were used. The fifth isotherm was defined by varying the pH values of water but maintaining constant mass of soils. It was found that the high density polyethylene (HDPE) container also played a part in the sorption process and ways had to be devised to minimize its effect. The sorption behavior of 137Cs to soils has generally been described by the distribution coefficient Kd, a quantity commonly used by other researchers to the understanding and determination of the eventual fate of metal and radionuclides released in aquatic environments. It was found in this project that Kd depended on initial activity concentration, slurry ratio and sorption time. A logistic theory was devised to explain the experimental results. It was based on irreversible exchange of 137Cs to soil. However, the model could only explain certain trends but failed to describe some of the quantitative aspects of the findings. Agreement between theory and experiment was good only for small masses. The theory also failed to fit theoretical values with experimental data to consistently deduce the sorption coefficient, which should be unique for a particular type of soils. Experiments also showed that the fraction of activity concentration remaining in water was never zero, showing that there might be some reversible exchange mechanism taking place. A revised logistic theory was devised based on the simultaneous occurrence of the sorption and desorption processes, quantified by the sorption coefficients and desorption coefficients, respectively. By fitting theoretical with experimental data, reasonably constant sorption and desorption coefficients have been obtained for aparticular type of soils, independent of mass of soils added. The number of sorption sites available could also be estimated and was found to vary almost linearly with the mass of a particular type of soils. Our model provides coefficients that are time independent, unlike the conventional distribution coefficients Kd, with more information on the sorption sites and sorption-desorption process.

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