Radionuclide sorption studies of Co, Cs and Sr onto soils from an Australian legacy radioactive waste site

This paper discusses results of radionuclide sorption studies on a soil profile taken from a low level radioactive waste site in South Eastern Australia, known as the Little Forest Burial Ground. Low level radioactive waste was buried at this site in a series of shallow trenches in the 1960s, and the site has an on-going environmental monitoring program. This site has been the focus of a field study in recent years and an extensive core-drilling program was undertaken in 2009. Measurable amounts of Co-60, Sr-90, Cs-137 and traces of actinides have been observed at this legacy waste site in some soils, groundwater and vegetation samples taken in close proximity to the disposal area.

The sorption of Co, Cs and Sr has been studied at four depth intervals from one corehole located near the trenches using radioactive tracers and a batch sorption method to assess the key site characteristics governing contaminant release and transport. The studies were conducted on bulk samples (< 1 mm) with a mass loading of 10 g/L, ionic strength of 0.01 M (NaCl) in the presence of air. Strong sorption was observed for Cs over the entire pH range studied, whereas the sorption of Co and Sr on the soils was pH dependent with sorption edges between pH 3 and pH 6. Distribution coefficients (Kd values) for Cs sorption were similar for each soil over the entire pH range, with variations of less than one order of magnitude between samples. However, the Kd values for Sr and Co sorption varied over two and three orders of magnitude, respectively, over the pH range studied.

The four soil samples were mineralogically characterised and the BET surface areas and cation exchange capacities (CEC) determined. The bulk mineralogy of the soils was found to be similar with quartz, kaolinite and interstratified illite/smectite to be the main mineralogical phases. Most soils also contained iron oxides and anatase as minor minerals. The BET surface areas of the bulk samples varied from 27 m2/g to 47 m2/g and no strong correlation of surface area with sorption was observed. For the bulk samples, those with the higher clay fraction had the highest CEC, with CEC ranging between 10 and 24 cmol/kg. The CEC of the clay fractions were significantly higher, ranging from 21 to 34 cmol/kg and 34 to 55 cmol/kg for the < 2 µm and < 0.2 µm fractions, respectively. Further studies to elucidate the role of the various minerals with respect to sorption are in progress.