Investigation of the Effect of Anthropogenic Organic Ligands on the Immobilisation of Anions by Cementitious Materials

The disposal concept for radioactive waste in England, Wales and Northern Ireland depends on the classification of the waste. One disposal route for waste classified as intermediate level (ILW) or low level waste (LLW) is based on a multibarrier system in which the waste is immobilised with cement in waste containers and surrounded by cementitious backfill materials in a geological disposal facility (GDF). The immobilisation cement and backfill materials are intended to provide both physical and chemical retention of radionuclides. Along with radionuclides, the ILW and LLW also contain quantities of chelating agents and cellulose. Under the repository conditions, the latter will form cellulose degradation products (CDP). Previous studies have suggested that the presence of these organic ligands decrease the retention of anions by the cement in the GDF. This research used solution analysis of samples taken from batch sorption experiments to investigate the effect of various organic ligands on the retention of selenium and iodine (in the form of SeO32- and I-) by cementitious materials. Four types of cementitious material were studied: CEM I, Nirex reference vault backfill (NRVB), ground granulated blast-furnace slag (GGBS): ordinary Portland cement (OPC) and pulverised fuel ash (PFA):OPC. The organic ligands assessed were picolinic acid (Pic), ethylenediaminetetraacetic acid (EDTA), gluconic acid (Glu), isosaccharinic acid (ISA), and nitrilotriacetic acid (NTA). Retention of selenite in the absence of organic ligands was proposed to be dependent on the calcium concentration in the cement for all cements, except GGBS:OPC. This was suggested to be due to the abundance of monosulphate (AFm) and ettringite (AFt) cement phases in GGBS:OPC or the change in species of anion causing a change in immobilisation. The study found that the retention of selenite by CEM I, NRVB and GGBS:OPC was reduced by 26.3, 39.1 and 18.4%, respectively, in the presence of Glu. Additionally, the in presence of ISA a decrease in the retention by NRVB (36.6%) and CEM I (61.3%) was seen. Moreover, a reduction in selenite immobilisation was also observed in NRVB (48.4%) and CEM I (29.3%) in the presence of EDTA. However, Pic reduced the retention (41.2%) in NRVB only. A relationship was observed between the magnitude of the decrease in selenite sorption in NRVB due to the presence of an organic ligand and the equilibrium constants for the formation of organic-calcium complexes. The isotherm study completed by changing the concentration of selenite was effective in showing the retention of the anion in NRVB and this experiment should be completed for other organic and cement combinations. Additional optimisation of the analytical method should be completed to obtain information on the retention of iodide on cement