Impact of dissolved organic carbon on the determination of trace concentration of Pu-239+240 in the groundwater

Influence of dissolved organic carbon (DOC) on the quantitative estimation of Pu-239+240 in the groundwater was investigated. Impact of DOC in the range of 5–30 ppm was studied in solutions which were directly taken from different wells, whereas to conduct the experiment at the elevated level of DOC, i.e. in the range of 27–60 ppm, groundwater was spiked with stock solution of humic acid (HA), a high molecular weight organic compound. Samples with varying concentrations of DOC were artificially contaminated with a pre-calibrated solution of Pu-239+240 in the laboratory in the range of 200 μBq–17 Bq l–1, and the pH was controlled at 7.4. Quantitative estimation of plutonium was carried out by pre-concentrating it with Fe(OH)3. This purified plutonium from Ca2+, Mg2+ and amphoteric elements. The percentage recovery of Pu-239+240 was 90–92% in the solution having DOC ≅ 10 ppm, whereas in samples with DOC in the range of 10–60 ppm, the recovery was 42–43%. Therefore, to improve the recovery of plutonium in the case of the elevated level of DOC, a modified procedure for its quantitative estimation was adopted. In this procedure, after filtration the samples were refluxed with conc. HNO3 and H2O2 to destroy the DOC before pre-concentrating Pu with Fe(OH)3. By this modified procedure, the average recovery of plutonium at high concentrations of DOC was improved to 90–95%.