Voltage-enhanced ion sieving and rejection of Pb²⁺ through a thermally cross-linked two-dimensional MXene membrane

Highlights

• Highly stable and anti-swelling 2D MXene membrane prepared via thermal cross-linking.

• Voltage improves the ion sieving of K⁺/Pb²⁺ pairs with the separation factor of 78.

• The rejection of heavy metal ion of Pb²⁺ is up to 99%.

Abstract

Highly stable and anti-swelling two-dimensional (2D) MXene (Ti₂C₃T_x) membranes can be applied for ion sieving and water purification under external voltage. This paper shows the effective ion sieving of mixed K⁺/Pb²⁺ pairs and the high rejection of the heavy metal ions (Pb²⁺) through a thermal cross-linked 2D MXene membrane by exerting external voltage. Due to the dehydration and cross-linking of hydroxyls during the drying process, the interlayer spacing of MXene membrane is tuned to be 6.7 ~ 6.92 Å, which lies in the selective ion sieving size of the mixed K⁺ and Pb²⁺. The rejection of hydrated Pb²⁺ is up to 99%, while the hydrated K⁺ can smoothly transport the channels of MXene membrane under external voltage. For 365-nm-thick MXene membrane, the separator factor of mixed K⁺ and Pb²⁺ reaches up to 78 under an applied voltage of 16.5 V. Furthermore, the application of external voltage doesn’t result in swelling of MXene membrane, showing stable separation of K⁺/Pb²⁺ pairs with effective rejection of Pb²⁺. The high rejection of Pb²⁺ and good anti-swelling demonstrate the stability of the resultant MXene membranes. Our work indicates that the thermal cross-linked MXene membrane is a promising alternative for ion sieving and the rejection of heavy metal ions.

Introduction

The recently emerging two-dimensional (2D) membranes, made from graphene, graphene oxide (GO), silica, zeolite or metal–organic frameworks (MOFs), have attracted extensive interests owing to their fascinating transport behavior similar to some biological membranes. These 2D membranes have potential applications in gas separation, water purification and desalination [1], [2], [3], [4], [5], [6], [7], [8], [9]. The presence of the nano- or sub-nanometer channels inside these membranes is responsible for the precise sieving properties with high selectivity for smaller molecules or ions to pass through but blocking the larger components. One typical example of these burgeoning 2D membranes is MXene, which was firstly reported by Gogotsi’s group in 2012 [10]. Due to its outstanding mechanical properties, excellent thermal stability, superior flexibility, high hydrophilic surfaces, and exceptional electrical conduction, MXene membrane has also been considered as a promising candidate in separation application in the last few years [11], [12], [13], [14]. For example, hydrogen separation with good selectivity up to 167 has been reported using an MXene membrane by several groups [15], [16]. By tuning their interlayer distance and pore size by ion intercalation or others, 2D MXene membranes have also demonstrated high ion rejection for water purification [11], [14], [17], [18]. However, due to notorious issue of MXene swelling occurring in water, it is still a challenge for hydrophilic MXene membrane to be applied for water treatment with ideal ion rejection.

To improve the rejection of ion/molecules, voltage-gated scheme has been introduced for electronic conductive MXene membrane with negative-charged surface, in which the electrostatic interaction between the membrane and ions can promote the selective rejection under positive/negative voltages [13], [19]. This method has also been demonstrated to improve other conducting membranes, such as metal-coated polymer, GO, MOF and MoS₂, which compels the gating effect toward molecules or ions permeation under an applied voltage [20], [21]. However, the swelling of these 2D membranes is still inevitable once immersing into the aqueous solution or directly subjected to the applied voltage, leading to a low ion rejection and sieving. Just subjected to 0.4 V, the interlayer-spacing of MXene membrane would increase from 3.7 to 6.5 Å [13]. Therefore despite improving smaller ion permeation rate, other undesirable larger metal cations like Pb²⁺ and Hg²⁺, could become more permeable under an electric field, which lowers the membrane performance due to the poor ion rejection or sieving. Meanwhile, due to the low applied voltage, it is time-consuming to reach the steady operation state [13]. To inhibit the swelling of hydrophilic 2D membrane, the cross-linking agent can be chosen to intercalate into the interlayer of these 2D nanosheets [22], [23]. In spite of the increased interlayer spacing, the ion rejection is enhanced with the occupied space of the molecular size itself. Recently, a strategy of self-crosslink has been proposed to effectively limit the swelling of MXene membranes [24]. In this method, self-crosslinking reactions between these hydroxyl terminal groups on the MXene would occur (−OH + −OH =  −O− + H₂O) under a facile thermal treatment, resulting in the formation of Ti−O−Ti bonds between the neighboring nanosheets and the improvement of the stability. Comparing with the pristine membranes, such thermally self-crosslinked MXene membranes show good anti-swelling and exhibit high permeation rates of the single monovalent metal ions, such as Li⁺, K⁺, and Na⁺. However, the separation of mixture with a monovalent like K⁺ and an undesirable bivalent like Pb²⁺ has not been evaluated via the thermally crosslinked MXene membranes. Moreover, it is feasible to further improve the separation performance of these electronic conducting 2D MXene membranes toward mixed metal ions with the application of voltage field.

To demonstrate this hypothesis, in this study, a novel device (Fig. 1) was developed to evaluate the separation of mixed K⁺/Pb²⁺ pairs and the rejection of heavy metal Pb²⁺ using the cross-linked 2D MXene membrane under the external voltage to improve the oriented transport of ions.