Uranium uptake onto magnox sludge minerals studied using EXAFS

Around the world large quantities of sludge wastes derived from nuclear energy production are currently kept in storage facilities. In the UK, the British government has marked sludge removal as a top priority as these facilities are nearing the end of their operational lifetimes. Therefore chemical understanding of uranium uptake in Mg-rich sludge is critical for successful remediation strategies. Previous studies have explored uranium uptake by the calcium carbonate minerals, calcite and aragonite, under conditions applicable to both natural and anthropogenically perturbed systems. However, studies of the uptake by Mg-rich minerals such as brucite [Mg(OH)₂], nesquehonite [MgCO₃·3H₂O] and hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H ₂O], have not been previously conducted. Such experiments will improve our understanding of the mobility of uranium and other actinides in natural lithologies as well as provide key information applicable to nuclear waste repository strategies involving Mg-rich phases. Experiments with mineral powders were used to determine the partition coefficients (K_d) and coordination of UO₂²⁺ during adsorption and co-precipitation with brucite, nesquehonite and hydromagnesite. The K _d values for the selected Mg-rich minerals were comparable or greater than those published for calcium carbonates. Extended X-ray absorption fine structure analysis results showed that the structure of the uranyl-triscarbonato [UO₂(CO₃)₃] species was maintained after surface attachment and that uptake of uranyl ions took place mainly via mineral surface reactions.