Removal of heavy metal cations and co-existing anions in simulated wastewater by two separated hydroxylated MXene membranes under an external voltage

Abstract

The removal of heavy metal cations and co-existing anions represents one of the promising pathways to produce purified water for industry. Laminar MXene membrane has attracted tremendous research interest in the separation field in the past decade. The thermal self cross-linking MXene membrane has been proven to exhibit good anti-swelling performance. However, the decrease of negative charge during self-crosslinking leads to decreasing ion separation performance due to fading adsorption and/or weakened surface-charged controlling effect. To regenerate the surface charge, the hydroxylation of MXene membrane is adopted by substituting –F with –OH using the alkali of KOH. The resultant hydroxylated MXene membrane demonstrates overall better performance than the pristine MXene membrane in wettability and Zeta potential, promoting the rejection and separation of ions. When two selective membranes are applied to remove the heavy metal cations (Pb2+, Cu2+, and Cd2+) and co-existing anions (Cl− and/or NO3−) under the electronic field with appropriate voltage, the rejection of 99.5% can be obtained at 12 V for a 383 nm-thick hydroxylated MXene membrane, higher than that of pristine MXene membrane. As a result, the co-existing anions were also excluded at ~97% via increasing surface-charged controlling effect. Moreover, the hydroxylated MXene membrane also shows selective separation permeance on Na+/Pb2+, Na+/Cu2+, and Na+/Cd2+ pairs. By controlling the interlayer spacing through thermal self-linking route, good operational durability and reproducibility are also achieved for the hydroxylated MXene membrane during the 70-min test. These highlight the potential of developing and applying 2D laminar membrane for separation and treatment of wastewater.