Radium and strontium binding by a modified bauxite refinery residue - isotope exchange studies of pH-dependence, reversibility and ageing

The pH-dependence and reversibility of radium (Ra) and strontium (Sr) binding to a modified bauxite refinery residue (MBRR), and the effects of sample ageing, were studied in laboratory uptake/leaching experiments. Natural Ra-226 and stable strontium (Sr-nat) were placed in contact with the MBRR for an 8-day loading period (equilibrium pH c. 8.5). Following the addition of exchange isotopes Ra-228 and Sr-85, the samples were then leached in pH-dependent experiments, where the pH was decreased incrementally from 9 to 3 over 7 h. A further suite of samples was aged at 4 degrees, 23 degrees and 65 degrees C for 6 months after the initial addition of Ra-226 and Sr-nat and then studied in a similar set of pH-dependent exchange experiments. The relative concentrations of the Ra and Sr isotope pairs (Ra-226/Ra-228 and Sr-nat/Sr-85) provided insights into the adsorption strength, incorporation, reversibility and ageing effects. The Sr-nat data showed that the amount of bound Sr released from aged MBRR samples as the pH decreases is substantially lower than the unaged sample, showing that Sr has been incorporated in less accessible phases during the ageing period. It appears that the uptake of Ra-228 by the higher-temperature aged samples is somewhat stronger than the unaged samples; however, Ra isotope exchange plots for aged and unaged samples are similar. Consequently, the stronger binding of Ra-228 by the higher-temperature aged samples is probably driven by translocation to kinetically less-favourable surface sites rather than a significant incorporation of Ra within the mineral solids during ageing.