Post-decontamination treatment of MXene after adsorbing Cs from contaminated water with the enhanced thermal stability to form a stable radioactive waste matrix

Highlights

• Cs adsorbed MXene was thermally stable when mixed with hydroxyapatite and cold sintered at 200 °C.

• Hydroxyapatite significantly enhanced the oxidation resistance of MXene by reacting with its terminal group.

• MXene-ceramic composite exhibited a > 65 °C increase in oxidation temperature compared to pure MXene in an air environment.

• Dried MXene-ceramic composite was cold sintered (200 °C, 10 min) without using any transient phase.

• The sintered matrix was highly dense with good micro hardness of 2.2 ± 0.2 GPa and low leaching rates of 10⁻⁴ g•m⁻²•d⁻¹.

Abstract

Safe and stable immobilization of spent adsorbents is a crucial step in nuclear and radioactive waste management. This study provides the first example of the green immobilization of simulated radioactive cesium-adsorbing MXene (Ti₃C₂T_x) by using hydroxyapatite (HAp) ceramic as a host matrix. The MXene-HAp (MX-HAp) composite exhibited a significantly enhanced thermal stability than that of the pure Cs-adsorbing MXene. The immobilization process was carried out by cold sintering at a low temperature (200 °C), which prevented the possible oxidation and chemical degradation of the spent adsorbent. The developed composite ceramic matrix was highly dense (3 ± 0.01 g·cm⁻³), mechanically strong (microhardness 2.2 ± 0.2 GPa), and chemically stable. The normalized leaching rate of simulated radioactive cesium was calculated to be 2.02 (±0.09) × 10⁻⁴ g·m⁻²·d⁻¹. Thus, the thermal stability of MXene was enhanced using the HAp dried ceramic powder and non-volatile immobilization of the dried MX-HAp composite was achieved by cold sintering. The reported immobilization process showed no adverse effects on the stoichiometry of the composite matrix materials and no volatilization loss of the adsorbed simulated radionuclide was observed. This study is set to take the green immobilization process for spent adsorbents to the next level by combining MXenes and HAp to develop different types of composite materials.

Introduction

The storage and handling of nuclear waste is a major issue that needs to be addressed urgently to avoid environmental and health hazards. Therefore, several kinds of studies have been conducted to elucidate the strategies for achieving the storage and handling of nuclear waste [1]. Water that is contaminated by fission product radionuclides and their ionic species, can be purified by the use of adsorbents that selectively capture unwanted ions. However, the spent adsorbents pose another risk due to the handling and storage problems. Conventionally, the spent adsorbents are stored in concrete and cement, which have the ability to hold the material for several years [2,3]. Thus, contaminants may leach out of these concrete blocks when subjected to harsh environments [4].

High-temperature processing for the immobilization of spent adsorbents, using different types of bonding materials such as glass, has serious disadvantages; these include the volatilization of adsorbed contaminants, low-waste loading capacity, high-energy consumption, and comparatively long processing times. Recently, a very low-temperature (200 °C) sintering process of ceramic-based spent adsorbents was reported [5], [6], [7], wherein sodalite containing simulated radioiodine, hydroxyapatite-magnetite composite with adsorbed corrosion metal products, and hydroxyapatite with adsorbed simulated radioactive cobalt were sintered without using any bonding material such as glass or cement. The spent adsorbent itself was consolidated to become the waste matrix. There was no volatilization of the loaded simulated radionuclides, and the stoichiometry of the starting materials remained unaltered [5], [6], [7]. The reported results could be used for the sintering of materials with low degradation (oxidation) temperatures.

MXenes have been recently discovered as a large family of two-dimensional materials, and have emerged as new adsorbents for the removal of heavy metal ions from water [8]. Due to their hydrophilic nature, they can be easily dispersed in water and provide stable aqueous dispersions [9]. Their rich surface chemistry, due to the presence of functional groups (-F, -O, -OH) and inherently large surface area, contributes to the good adsorption of ions on their surface [10,11]. Khan et al. recently showed the effectiveness of MXenes in removing Cs from water, where the adsorbent reached a steady state within 1 min, with a maximum Cs adsorption capacity of 25.4 mg·g⁻¹ at room temperature [12]. Jun et al. achieved 47 times higher adsorption capcity of Cs with MXenes compared to porous activated carbon and attributed this result to the higher negative surface charge of MXenes and electrostatic interactions between the Cs and MXenes [13]. MXenes are reported to be radiation damage resistant, which qualifies its use as an adsorbent for radionuclides [14], [15], [16]. MXenes have also been explored for the removal of heavy metal ion pollutants such as Pb²⁺, Hg²⁺, U⁶⁺, Cu²⁺, and Ba²⁺ from the environment [17], [18], [19], [20], [21], [22]. Fard et al. reported a high removal efficiency of Ba (90% removal, 9.3 mg·g⁻¹) from water within 10 min [22]. In another study, Ying et al. reported Cr⁶⁺ ion adsorption in the anionic form with MXenes, where a high removal of Cr⁶⁺ ions was obtained in excess of 250 mg·g⁻¹ [23]. Similarly, Mu et al. utilized MXenes for the efficient removal of radioactive palladium from HNO₃ aqueous solution, and reported a maximum adsorption capacity of 184.56 mg·g⁻¹ at 45 °C [24]. In contrast to these excellent properties, MXenes are prone to thermal degradation and can be fully oxidized under high-temperature processing and applications [25,26]. Therefore, different immobilization processes are required, in order to avoid the oxidation of MXene-based spent adsorbents. However, to the best of our knowledge, there are no published results available in this regard.

Herein, for the first time, we report the immobilization of an MXene-based spent adsorbent and its chemical durability in an aqueous environment. Hydroxyapatite (HAp) dried ceramic powder was selected as a host matrix, based on its low solubility in an aqueous environment and its cold sintering ability reported in the previous papers [5,27].