Quantification of common aminopolycarboxylic acids in trench leachate from the Low Level Waste Repository

Abstract

The aminopolycarboxylic acids (APCAs), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and nitrilotriacetic acid (NTA), are used as decontamination agents throughout the nuclear industry; therefore, APCAs are often found in radioactive waste. Limits of acceptance on APCAs are imposed on wastes consigned to the Low Level Waste Repository (LLWR) because, when present in the waste, the ligands have the potential to mobilise otherwise surface-bound or solid radionuclides, making them available for transport to groundwater and ultimately to the bio-sphere. A selective and sensitive methodology to detect and quantify these ligands in a range of complex matrices is advantageous in supporting waste acceptance processes. A reversed-phase high-performance liquid chromatography (HPLC) procedure has been applied for quantification of EDTA, DTPA and NTA in their Fe(III)-complex form. Method validation results show linearity (r2 > 0.999), precision (intra/inter-day %RSD ≤ 10%), accuracy (recovery = 100 ± 3%), sensitivity (minimum limits of detection = 0.31, 0.38 and 4.3 μM for EDTA, DTPA and NTA, respectively) and selectivity (simultaneous determination of the three APCA complexes achieved with baseline resolution) for Fe(III)-APCAs in aqueous solution. Chromatographic peak overlap is observed for samples containing Fe(III)- and Co(III)-EDTA; two deconvolution methods (2D least-squares fitting vs. PARAFAC) were applied to resolve the peaks and the performances compared. The optimised HPLC method was applied to trench leachate samples from the LLWR site. EDTA was detected with 0.4 μM < concentrations < 1 μM in samples from four of the six sampling locations tested. The levels are not considered sufficient to increase the risk of radionuclide mobilisation. The technique is considered to be robust and will be considered further in informing limits of acceptance on APCAs.