Development of a reactive zone technology for simultaneous in situ immobilisation of radium and uranium

Simultaneous in situ immobilisation of uranium (U) and radium (Ra-226) by injectible amounts of grey cast iron (gcFe), nano-scale iron (naFe) and a gcFe/MnO2 mixture (1: 1) was studied in batch and column tests. Both 0.5 g/ L naFe and gcFe are effective in Ra-226 and U removal from mine water, whereas MnO2 addition clearly increased the efficiency of gcFe for Ra-226 and U immobilisation. In a column test with 0.6 wt% gcFe/MnO2 mixture (1: 1), neither Ra-226 nor U was detected in the effluent after replacement of 45 pore volumes. A sequential extraction under flow condition revealed Ra-226 to be mostly occluded in manganese oxides. Uranium was mostly sorbed onto poorly crystalline iron hydroxides, but a significant part was found to be occluded in manganese oxides also. The results of this study suggest that MnO2 promotes iron hydroxide formation under slightly reducing environmental conditions resulting in an increased pollutant retention capacity.