Determination of radium and uranium isotopes in natural waters by sorption on hydrous manganese dioxide followed by alpha-spectrometry

Summary

Water samples, spiked with ¹³³Ba and ²³²U radiotracers, are scavenged for radium and uranium isotopes using hydrous manganese dioxide which is produced in-situ, by reacting manganese (+2) and permanganate ions at pH 8-9. The precipitate is solubilized with ascorbic and acetic acids and the resulting solution filtered through a glass fibre filter GF/F to remove particulate matter. The radium is co-precipitated with barium ions by the addition of a saturated Na₂SO₄ solution where a small amount of BaSO₄ suspension is introduced to initiate crystallization. The micro precipitate containing the radium is collected on a 0.1 µm membrane filter and the filtrate saved for follow-up uranium analysis. The ²²⁶Ra on the filter is determined by alpha-spectrometry and its recovery is assessed by measuring the ¹³³Ba on the same filter using gamma-spectrometry. The filtrate containing uranium is passed through a Dowex AG 1x4 ion-exchange resin in the SO₄^2- form which retains uranium while other ions are eluted by dilute (0.25M) sulphuric acid. Uranium is eluted from the column by distilled water, electrodeposited on a silver disc and the uranium isotopes and their recovery are determined by alpha-spectrometry. The method was tested on a variety of natural and spiked water samples with known concentrations of ²²⁶Ra and ²³⁸U and was found to yield accurate results within ±10% RSD of the target values.