Uranium Retardation Capacity of Lithologies from the Negev Desert, Israel—Rock Characterization and Sorption Experiments

Abstract

A series of batch experiments were performed to assess the uranium sorption capacity of four mineralogically distinct lithologies from the Negev Desert, Israel, to evaluate the suitability of a potential site for subsurface radioactive waste disposal. The rock specimens consisted of an organic-rich phosphorite, a bituminous marl, a chalk, and a sandstone. The sorption data for each lithology were fitted using a general composite surface complexation model (GC SCM) implemented in PHREEQC. Sorption data were also fitted by a non-mechanistic Langmuir sorption isotherm, which can be used as an alternative to the GC SCM to provide a more computationally efficient method for uranium sorption. This is because all the rocks tested have high pH/alkalinity/calcium buffering capacities that restrict groundwater chemistry variations, so that the use of a GC SCM is not advantageous. The mineralogy of the rocks points to several dominant sorption phases for uranyl (UO22+), including apatite, organic carbon, clays, and iron-bearing phases. The surface complexation parameters based on literature values for the minerals identified overestimate the uranium sorption capacities, so that for our application, an empirical approach that makes direct use of the experimental data to estimate mineral-specific sorption parameters appears to be more practical for predicting uranium sorption.

Desorption and Co‐Dissolution of Uranium‐Bearing Solids During Alkalinity‐Enhanced Flushing of Contaminated Sediments

“…K d values are notorious for introducing high uncertainty into transport estimates (Zhu 2003; Dangelmayr et al 2018; Dangelmayr et al 2022) since they lump complex and variable chemical processes into a single parameter. As a consequence, it is not surprising that the K d values derived from the column experiments were highly variable changing not only during each influent phase but also during the beginning of the column experiment (with the exception of P2C1).…”

“…Alkalinity-enhanced flushing (AEF) of uranium presents a possible alternative to MNA as a method to restore a contaminated aquifer to background concentrations. High carbonate concentrations have been shown to increase uranium desorption from mineral phases such as ferric oxides (Waite et al 1994;Davis et al 2004, Dangelmayr et al 2017, clays (Bachmaf et al 2008), as well as natural organic matter (Lenhart et al 2000;Dangelmayr et al 2022) due to the formation of negatively charged uranyl-carbonate species (Geipel et al 1998) which are unlikely to sorb to the negatively charged surface sites of mineral phases. In the presence of divalent cations (such as Mg and Ca) uranyl-carbonates form even stronger ternarycomplexes that are highly soluble (Bernhard et al 2001;Dong and Brooks 2006;Dong and Brooks 2008).…”

Radionuclide Transport Simulations Supporting Proposed Borehole Waste Disposal in Israel

“…The uranium transport model incorporates sorption coefficients informed by uranium sorption and desorption experiments that were performed on representative Negev Desert rock samples [14]. Due to the strong pH and alkalinity buffering capacity of Negev Desert carbonates studied, a non-mechanistic, linear partitioning coefficient (Kd) for uranium sorption is an adequate approximation and greatly reduces computational complexity.…”

“…The generally low permeability, high uranium-sorption capacity, and high pH and alkalinity buffering capacity of the shallow marine lithologies constituting the YP subsurface indicate that a proposed site in the Negev Desert may be favorable as a natural barrier to HLW migration. In particular, a ~28 m thick phosphorite layer has been identified to have high uranium-sorption potential through laboratory tests [14][15][16][17] and may represent an important geochemical barrier in the unlikely case of waste canister failure and migration of HLW into the rock surrounding the borehole [8,[13][14][15][16][17]. Below the water table, a primary concern is lateral transport of radionuclides due to regional hydraulic gradients [18,19].…”

“…Because the uranium-sorption capacity of the YP lithologies is an important factor for establishing the feasibility of the proposed disposal borehole, Dangelmayr et al (2021) performed uranium batch sorption laboratory tests on lithologies in the horizon of the proposed IBD site [14]. The results indicate that non-linear sorption isotherms best predict the sorption behavior of the experiments and mechanistic surface complexation models for sorption capture changes of sorption behavior under evolving chemical conditions.…”