Selective Removal of ²²⁶Ra²⁺ from Gas-Field-Produced Waters

Abstract

The 226Ra2+ selectivity of both the self-assembled (iso)guanosine-based systems and ionizable thiacalix[4]crown dicarboxylic acids was determined in gas-field-produced water and a metal ion-containing model solution (simulant). Seven gas-field-produced water samples have been analyzed. From a sample (K5D) with average metal ion concentrations ([metaltot] = 0.14 M), thiacalix[4]crown-5 dicarboxylic acid (10-4 M) extracts 60% of the 226Ra2+ content. Extractions performed with the model solution (MK5D) indicate that in K5D there is significant competition in 226Ra2+ extraction due to the organic constituents of K5D, in particular with self-assembled extractants guanosine and isoguanosine. Nevertheless, all four extractants extract 226Ra2+ both from the produced water K5D and the model solution MK5D, even with a 100-fold excess of [metaltot] to [extractant]. The extracted 226Ra2+ cations could effectively be stripped from the extractants by washing with pH 2 water. The results obtained with the extractants used, especially thiacalix[4]crown-5 dicarboxylic acid 3, clearly demonstrate the way to selectively remove Ra2+ from gas-field-produced waters.