Modification of Ti₃C₂T_x nanostructure with KH₂PO₄ and chitosan for effective removal of strontium from nuclear waste

Nanostructure titanium carbide MXene (Ti₃C₂Tx) was modified with KH₂PO₄ and chitosan to effectively remove strontium from nuclear wastewater. Nuclear waste includes radionuclides of uranium, thorium, strontium, and cesium, which are classified depending on the concentration of radionuclides. Nuclear waste with a high strontium concentration is the production waste of radiopharmaceutical production centers. Ti₃C₂Tx was synthesized from Ti₃AlC₂ using HF40% and HF in situ (MILD-Ti₃C₂T_x) in 24 h at 313.15 and 333.15 K. Morphology, structure, and functional groups were investigated using the XRD, SEM, EDS, FTIR, and BET analyses. The Sr(II)'s adsorption capacity on Ti₃C₂T_x-HF and Ti₃C₂T_x-HF in situ was obtained as 61.9 and 253.5 mg g^-1, respectively (temperature, 298.15 K; pH, 7.00; contact time, 180 min; and Sr(II) concentration, 150 mg l^-1). Ti₃C₂T_x-HF in situ showed fourfold adsorption due to more hydroxyl functional groups and larger interlayer spacing. Ti₃C₂T_x was modified with KH₂PO₄ and chitosan to investigate the mechanism of change of Sr(II)'s adsorption capacity, which increased to 370 and 284 mg g^-1, respectively. The structural results of modified Ti₃C₂T_x showed that the surface functional groups increased when modified with chitosan. In addition, modification with KH₂PO₄, through encapsulating large amounts of KH₂PO₄ between Ti₃C₂T_x layers, increased the possibility of Sr(II) diffusion between layers and electrochemical interactions with hydroxyl groups, and thus, increased its adsorption. Some experiments were designed to investigate the effect of parameters like initial concentration of Sr(II), contact time, temperature, and pH solution, as well as modified- and unmodified-Ti₃C₂T_x on adsorbent. The results revealed that the adsorption process of Sr(II) with pristine and modified-Ti₃C₂T_x follows pseudo-second-order kinetics and Freundlich heterogeneous isotherm model. Freundlich model isotherm indicates the presence of various functional groups on the surface and between the pristine and modified Ti₃C₂T_x layers. Electrostatic reactions and intra-sphere complexation were the two dominant mechanisms of the adsorption process.