Selective radionuclide co-sorption onto natural minerals in environmental and anthropogenic conditions

Abstract

Anthropogenic activities can redistribute the constituents of naturally occurring radioactive materials (NORM), posing potential hazards to populations and ecosystems. In the present study, the co-sorption of several RN from the U decay chain– ²³⁸U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, onto common minerals associated with mining activities (chalcopyrite, bornite, pyrite and barite) was investigated, in order to identify the various factors that control long-term NORM mobility and retentivity in environmental acid-mine drainage systems and hydrometallurgical processing. The results show selective RN co-sorption to the various natural minerals, suggesting that mineral-specific mechanisms govern the variability in NORM mobility and retentivity. Both ²²⁶Ra and ²¹⁰Po underwent significant sorption onto the natural minerals investigated in this study. The order of co-sorption in sulfate media for chalcopyrite and bornite was ²¹⁰Po>²²⁶Ra>²⁰⁶Pb>²¹⁰Pb>²³⁸U/²³⁰Th. Conversely, both pyrite and barite showed increased affinity for ²²⁶Ra; the order of co-sorption in sulfate media was ²²⁶Ra>²¹⁰Po>²⁰⁶Pb/²¹⁰Pb>²³⁸U/²³⁰Th for pyrite and ²²⁶Ra>²⁰⁶Pb/²¹⁰Pb>²³⁰Th/²³⁸U/²¹⁰Po for barite. Similar orders of co-sorption were observed in the nitrate media: for chalcopyrite and bornite the order was ²¹⁰Po>²²⁶Ra/²⁰⁶Pb/²¹⁰Pb/²³⁸U/²³⁰Th compared to ²²⁶Ra>²¹⁰Po/²⁰⁶Pb/²¹⁰Pb/²³⁸U/²³⁰Th for pyrite and barite. The behavior of ²¹⁰Po was found to the anomalous: in both sulfate and nitrate solutions, ²¹⁰Po had little affinity for barite compared to the sulfides. Thermodynamic modeling indicated the formation of a reduced PoS(s) phase at the surface of sulfide minerals, leading to the suggestion that ²¹⁰Po likely undergoes reductive precipitation on the surface of sulfide minerals. The high sorption of both ²⁰⁶Pb and ²¹⁰Pb observed in the sulfate systems were likely as a result of co-precipitation as insoluble anglesite compared to nitrate where they mainly remained in solution. Overall, barite showed the highest affinity for ²²⁶Ra, given its propensity to sorb ²²⁶Ra (similar ionic size). Both ²³⁸U and ²³⁰Th were highly mobile in acidic sulfate and nitrate solutions. The results highlighted here identify the various constraints on the natural variability and fractionation of NORM in the environment, as well as the mineral-specific mechanisms that control co-sorption of RN. This information provides a framework for predicting RN transport within soils and ground waters with variable geochemical conditions and in metallurgical extraction processes, in order to develop effective strategies towards NORM mitigation.