Molten salt synthesis of MXene-derived hierarchical titanate for effective strontium removal.

The removal and recovery of radioactive Sr(II) from wastewater and seawater has been of great concern due to the negative environmental impacts of nuclear energy development and the potential risk of nuclear accidents. Herein, a facile molten salt synthesis strategy was developed to systematically investigated the reaction of different types of MXenes with nitrates. Among the products, K+ intercalated hierarchical titanate nanostructures (K-HTNs) obtained from the direct chemical transformation of multilayered Ti3C2Tx exhibited unique layered structures, good physicochemical properties, and outstanding adsorption performance for Sr(II). The maximum adsorption capacity of Sr(II) by K-HTNs reached 204 mg·g-1 at ambient temperature, and the good regeneration and reusability of the titanate was also demonstrated. K-HTNs showed preferential selectivity for Sr(II) in different environmental media containing competing ions, and the removal efficiency of Sr(II) in real seawater was as high as 93.3 %. The removal mechanism was elaborated to be the exchange of Sr2+ with K+/H+ in the interlayers of K-HTNs, and the adsorbed Sr(II) had a strong interaction with Ti-O- termination on the titanate surface. Benefiting from the merits of rapid and scalable synthesis and excellent adsorption performance, MXene-derived K-HTNs have broad application prospects for the purification of 90Sr-contaminated wastewater and seawater.

Thorium(IV) adsorption onto multilayered Ti3C2Tx MXene: a batch, X-ray diffraction and EXAFS combined study.

The interlayer regulation of layered environmental adsorption materials such as two-dimensional early transition metal carbides and carbonitrides (MXenes) plays an important role in their purification performance for specific pollutants. Here the enhanced uptake of ThIV by multilayered titanium carbides (Ti3C2Tx) through a hydrated intercalation strategy is reported. ThIV adsorption behaviors of three Ti3C2Tx samples with different c lattice parameters were studied as a function of contact time, pH, initial concentration, temperature and ion strength in batch experiments. The results indicated that the ThIV uptake was pH and ionic strength dependent, and the adsorption process followed the pseudo-second-order kinetics and the heterogeneous isotherm (Freundlich) model. Thermodynamic data suggested that the adsorption process of all MXene samples was a spontaneous endothermic reaction. The dimethyl sulfoxide intercalated hydrated Ti3C2Tx featured the largest interlayer space and exhibited the highest ThIV adsorption capacity (162 mg g-1 at pH 3.4 or 112 mg g-1 at pH 3.0), reflecting the significant increase in available adsorption sites from Ti3C2Tx interlayers. The adsorption mechanism has been clarified based on adsorption experiments and spectroscopic characterizations. An ion exchange process was proposed for the interaction between hydrated MXenes and ThIV, where H+ from surface [Ti-O]-H+ groups were the primary active sites on Ti3C2Tx. Extended X-ray absorption fine structure (EXAFS) fitting results, in combination with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses, clearly indicated that ThIV mainly formed the outer-sphere complexes on Ti3C2Tx surface through electrostatic interaction under strong acid conditions, while at pH > 3.0 the adsorption mechanism was determined by inner-sphere coordination and electrostatic interaction together.