Interaction between Uranyl Cations and Layered Double Hydroxide Nanoparticles: Implications for Nuclear Wastewater Management

Effective uranium (U) capture is required for the remediation of contaminated solutes associated with the nuclear fuel cycle, including fuel reprocessing effluents, decommissioning, or nuclear accident cleanup. Here, interactions between uranyl cations (UO2 2+) and a Mg-Al layered double hydroxide (LDH) were investigated using two types of uranyl-bearing LDH colloids. The first (ULDH) was synthesized by coprecipitation with 10% of Mg2+ substituted by UO2 2+. Alternatively, UO2 2+ was added to a neoformed LDH to obtain the second uranyl-bearing LDH colloid (LDHU). In both the LDHU and ULDH colloid systems, schoepite (UO2)8O2(OH)12·12H2O, was formed. The presence of U significantly reduced the size of both LDHU and ULDH compared to a reference LDH colloid. Surface charge and aggregation of the ULDH and LDHU colloids were compared in NaCl, Na2CO3, Na2SiO3, and Na3PO4 solutions that are often present in nuclear wastewaters. Aggregation of ULDH and LDHU in the presence of Na2SiO3 or Na3PO4 promotes colloid restabilization. While the uranyl cation was not incorporated into the LDH structure, it influences nanoparticle growth in addition to imparting modified surface properties that affect aggregation. This has implications for radioactive waste disposals, where LDH, which can also incorporate a variety of other radionuclides, is used for remediation.