Eu³⁺ Luminescence in High Charge Mica: An In Situ Probe for the Encapsulation of Radioactive Waste in Geological Repositories

Abstract

Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the 5D1 and 5D0 excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the 5D1 excited state, (2) the energy shift of the 5D0 → 7F0 transition, (3) the crystal-field splitting of the 7F1 Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network.

Mechanistic Insight into Energy‐Transfer Dynamics and Color Tunability of Na4CaSi3O9:Tb3+,Eu3+ for Warm White LEDs

“…7 F 2 emissions. [3] However,t he parity-forbidden 4f!4f transitions of Eu 3 + generally show low absorption in the near-UVr egion, severelyh ampering the utilization of Eu 3 + for practical applications. Enhancing the absorption and luminescence intensity of Eu 3 + in the near-UVorb lue range is therefore of crucial importance.…”

Exploring the Local Environment of the Engineered Nanoclay Mica-4 Under Hydrothermal Conditions Using Eu3+ as a Luminescent Probe

“…Isomorphous substitutions of Si 4+ by Al 3+ in the tetrahedral sheet originate a low negative charge density compensated by hydrated Na + in the interlayer space. Two interaction mechanisms between Eu 3+ cations and the clay can be univocally distinguished during the hydrothermal treatments by analyzing the luminescence features of the sample, as demonstrated by our previous work in Mica-2 [25]. The first mechanism consists on both the adsorption of Eu 3+ cations in non-specific sites on the surface of the clay and the cation exchange reaction in which the interlayer Na + is substituted by Eu 3+ .…”

“…Das et al [24] use Eu 3+ to explore Na 0.1 Sr 9.8 Eu 0.1 (PO 4 ) 6 F 2 both as a potential phosphor material and host for radioactive waste immobilization. Moreover, the incorporation of the luminescent cation Eu 3+ into the structure of the high charge mica with 2 charges per unit cell, Mica-2, has been previously used to monitor the long-term physicalchemical changes under mild hydrothermal conditions [25]. Eu 3+ emission is a sensitive spectroscopic probe since the luminescence from both 5 D 0 and 5 D 1 excited states to all low-lying states as well as their emission lifetimes can be used to monitor the Eu 3+ local environment [26].…”

Adsorptive Capture of Ionic and Non-Ionic Pollutants Using a Versatile Hybrid Amphiphilic-Nanomica

“…The optical properties of the amphiphilic clay upon contact with a solution of europium nitrate have been studied in detail. Excitation and luminescence measurements of Eu 3+ were previously used, considering Eu 3+ as a probe, to explore the adsorption mechanism of cationic species in high charge micas [30].…”