Synthesis and Applications of Carboxylic Terminated Ti3C2Tx MXenes for Enhanced Adsorption of Cr(VI) and Pb(II)

Abstract

This research focused on the synthesis of carboxylic terminated Ti3C2Tx MXenes nanosheets and their application to achieve an enhanced adsorption of toxic Cr(VI) and Pb(II) contaminants in an aqueous media. An original, facile, and one-pot protocol for the synthesis of carboxylic terminated Ti3C2Tx nanosheets incorporating non-hazardous citric acid and NH4HF2 was articulated. To conduct a comparative study of the protocol, three different etchants: (a) NH4HF2, (ii) NH4HF2 and citric acid, and (iii) NH4F and citric acid, were used to etch the Ti3AlC2 MAX phase. Thus, three samples of Ti3C2Tx nanosheets were synthesized and comparatively analysed using XRD, SEM, TEM, EDX, AFM, FTIR, XPS, BET, TGA and AAS techniques. Firstly, it was established that among the Ti3C2Tx samples, the best-exfoliated and few-layered carboxylic terminated nanosheets of 1.36 nm thickness were successfully produced using NH4HF2 and citric acid. Secondly, the nanosheets exhibited extraordinary features owing to their superior morphology, microporous structure, improved surface chemistry, and enhanced adsorption performance. For instance, the Ti3C2Tx nanosheets showed a d-spacing of 1.25 nm, and a SSA of 42.63 m2 /g with numerous active sites in terms of (-COOH) terminations alongside (=O), (-F), and (-OH) terminal groups. The adsorption efficiency was evaluated under varying sonication, metal ions dosage and solution temperatures, and optimum adsorption capacities of 1090 mg/g and 1135 mg/g for Cr(VI) and Pb(II) were attained within 7 and 4 minutes, respectively. The adsorption capacities achieved for Cr(VI) and Pb(II) were superior to the capacities of titanium carbide and activated carbon-based adsorbents, reported to date. Moreover, adsorption kinetics and isotherms studies confirmed that the experimental data fitted well with pseudo-second-order reaction and Freundlich models. It was also established that the main interactions to drive the adsorption reactions were the electrostatic forces between the metal ions and Ti3C2Tx nanosheets. Furthermore, (-COOH) and (-OH) terminal groups were the main contributors to the adsorption of Cr(VI) and Pb(II) pollutants through an ion exchange mechanism. Besides the ion exchange mechanism, chemical coordination, entrapment of the contaminants, and van der Wall forces led to a physiochemical interaction between the metal ions and Ti3C2Tx nanosheets. In addition, these nanosheets showed better selectivity towards Pb(II) removal than Cr(IV) xx in an aqueous solution as well in the presence of other competing cations i.e., Ca(II), Na(I) and K(I). The nanosheets also exhibited more than 80 % removal efficiency even after six cycles of regeneration and reusability. Furthermore, the nanosheets showed better dispersity and stability in water, up to 40 days as compared to the conventional Ti3C2Tx MXenes. Hence, these high-quality carboxylic terminated Ti3C2Tx nanosheets are a promising candidate for eradication of the other hazardous metal ions contaminants from the aquatic environment.