Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti3C2Tx carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti3C2Tx by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti3C2Tx stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti3C2Tx and COOH-Ti3C2Tx. Compared to Ti3C2Tx, COOH-Ti3C2Tx has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti3C2Tx is lower. For both Ti3C2Tx and COOH-Ti3C2Tx, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg2+ concentration of 50 ppm and pH of 6, COOH-Ti3C2Tx has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg2+ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti3C2Tx (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.