A Hydrofluoric Acid-Free Green Synthesis of Magnetic M.Ti₂CT_x Nanostructures for the Sequestration of Cesium and Strontium Radionuclide

An environmentally friendly MAX phase etching methodology was established Sodium hydroxide produced magnetic layered M.Ti₂CT_x nanostructure M.Ti₂C-A_III exhibited exceptional Sr²⁺ and Cs⁺ removal capacities of 376 and 142.88 mg/g Highly efficient magnetic nanostructures permitted selective radionuclide removal Highlights: MAX phases are the parent materials used for the formation of MXenes, and are generally obtained by etching using the highly corrosive acid HF. To develop a more environmentally friendly approach for the synthesis of MXenes, in this work, titanium aluminum carbide MAX phase (Ti₂AlC) was fabricated and etched using NaOH. Further, magnetic properties were induced during the etching process in a single-step etching process that led to the formation of a magnetic composite. By carefully controlling etching conditions such as etching agent concentration and time, different structures could be produced (denoted as M.Ti₂CT_x). Magnetic nanostructures with unique physico-chemical characteristics, including a large number of binding sites, were utilized to adsorb radionuclide Sr²⁺ and Cs⁺ cations from different matrices, including deionized, tap, and seawater. The produced adsorbents were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The synthesized materials were found to be very stable in the aqueous phase, compared with corrosive acid-etched MXenes, acquiring a distinctive structure with oxygen-containing functional moieties. Sr²⁺ and Cs⁺ removal efficiencies of M.Ti₂CT_x were assessed via conventional batch adsorption experiments. M.Ti₂CT_x-A_III showed the highest adsorption performance among other M.Ti₂CT_x phases, with maximum adsorption capacities of 376.05 and 142.88 mg/g for Sr²⁺ and Cs⁺, respectively, which are among the highest adsorption capacities reported for comparable adsorbents such as graphene oxide and MXenes. Moreover, in seawater, the removal efficiencies for Sr²⁺ and Cs⁺ were greater than 93% and 31%, respectively. Analysis of the removal mechanism validates the electrostatic interactions between M.Ti₂C-A_III and radionuclides.