Kinetics of radiocesium sorption in soils

Abstract

The general purpose of this study is to investigate the kinetics and sorption behaviour of radiocesium (137Cs) in soils in Hong Kong reservoirs. This is important as the sorption of radiocesium to soil constituents is one of the major factors which determine the availability of 137Cs to biological systems. Such information provides the long-term mobility of radiocesium in soils which is a useful tool in emergency planning and in countermeasure implementation during any accidental release from the nearby Guangdong and Lingao nuclear power stations. It is well known that soils can adsorb radiocesium rapidly and strongly. In general, the sorption of radiocesium in soils can be described in terms of three kinds of sorption sites which have different ion-exchange selectivities of cesium. One type of sites is the planar sites from which 137Cs is generally exchangeable and are called the 'regular exchange sites' (RES). The second type of sites is called the 'wedge edge sites' (WES) where 137Cs exchange is sterically limited to cations of similar size and charge. The other sites are interlayer sites from which 137Cs is not readily exchanged and are called the 'frayed edge sites' (FES). Significant methodological development has been proposed to allow a quantitative assessment of both the number and the selectivity pattern of such specific sites in soils (Cremers et al, 1998, 1990). The method is based on the use of the silver thiourea (AgTU) complex to mask the bulk of the RES and the WES. By dispersing soils in a solution containing suitable background level of AgTU, cesium ions are excluded from the bulk of the ion-exchange complex and their action becomes restricted to the FES. Soil samples in this study were collected from different reservoirs in Hong Kong and characterized for their physical and chemical properties such as clay content, organic matter content, cation exchange capacity, potassium ions availability and soil pH. Two types of experiments were conducted. The first type of experiments was to observe the sorption behaviour of 137Cs on different types of soils as a function of time. 1.0-g soil samples were dispersed in a 200-ml deionized water of initial 137Cs water activity concentration of 240 kBq l-1 in a 1.25-l high density Polyethylene (HDPE) container. The Cs137 water activity was monitored with time until equilibrium reached. The experimental data were fitted well with a simple box model by using a simulation software called ModelMaker. Rate constants for different chemical processes of 137Cs sorption in soils were then estimated. Attempts were also made to correlate the rate constants with the characteristic properties of soil samples such as clay content, organic matter content, cation exchange capacity, potassium ions availability and soil pH. The second type of experiments was to observe the sorption behaviour of 137Cs on the frayed edge sites (FES) in soils as a function of time. Soil samples were dispersed with 0.015M AgTU solution in a 1.25-l HDPE container. The 137Cs water activity was monitored with time until equilibrium reached. Experiments with different masses of soil samples at two different initial 137Cs water concentrations of 160 kBq l-1 and 240 kBq l-1 were performed. The capacity of the FES was also determined. 16-g of soil samples were dispersed in 0.015M AgTU solution containing varying amounts of cesium and a 137Cs label for 24 h. Sorption isotherm was obtained by plotting the amount of cesium sorbed against various amounts of cesium until a plateau was reached. The capacity of the FES is then the plateau value of the isotherm. To describe the irreversible 137Cs sorption behaviour on the FES in soils, a two-compartment conceptual model was used. The sorption rate was assumed to depend not only on a rate constant, but also on some other factors, e.g. the difference between the 137Cs water activity concentration in aqueous phase and that in particulate phase, the fractional 137Cs water activity concentration and the number of available sorption sites. Good fitting was obtained between experimental data and theory. The number of the FES for different masses of soils was also postulated and verified by experiments. Finally, a four-compartment modified model based on the two-compartment conceptual model was proposed using the simulation software of ModelMaker to describe the 137Cs sorption behaviour on the FES in soils. The FES were assumed to be composed of sub-sites where ion exchange and fixation of 137Cs could simultaneously occur. A very good fitting was obtained between experimental data and the model. This model provided a good and reliable tool to investigate the retention of 137Cs in soils.