Evaluating ion transport and selectivity of 2D titanium carbide membranes in presence of different driving forces

Transition metal carbides, carbonitrides, and nitrides or MXenes represent a large family of 2D materials derived from a class of ternary ceramic materials. It has a formula of Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and n ranges from 1 to 3 and T can be any functional moieties, usually -OH, -O or –F. The termination of these layers with these moieties gives it an inherent negative surface charge. More recently, they have attracted interest for both gas and aqueous phase separations due to their high stability in aqueous environment, flexibility, high surface area and conductivity. This includes organic contaminant removal from water, energy storage and gas separation. The layered structure of MXene allows the inter-layered spacing to be tuned to allow rejection of selective ions based on their size and charge. When compared to graphene oxide membranes, MXene membranes have higher water flux due to 2-3 layers of water molecules in the spacing between the layers. The high strength of 2D layers, high surface area, and hydrophilicity are perfect properties for a material to be used in water treatment. Like graphene and GO, MXenes’ treatment mechanism is via size exclusion through the stacked layers and via electrostatic interaction between the ions and the MXene surface. Here, we aim to evaluate ion transport through MXene membranes, by investigating single-salt and multicomponent mixtures. We also investigate ion transport due to varying different driving forces – diffusion and pH-driven migration. Diffusion occurs due to a concentration gradient across the membrane whereas migration is achieved through the generation of a pH gradient across the membrane. Experiments were carried out using a H-Cell apparatus and the ion transport flux and selectivity was measured with time. The selectivity of the ions in mixtures follows the hydrated ion size and the diffusivity values of these ions. So, smaller the size of hydrated ion, higher is the diffusivity of the ion and thus, faster the transport and better selectivity. Thus, the transport and selectivity have the following order – – K+>Na+>Li+>Ca2+ The selectivity of K+ ranges from 1.4 to 1.2 while for Ca2+ it ranges from 0.6 – 0.8 when compared with sodium for diffusion tests. While for pH-driven tests, due to the initial faster ion transport to compensate for the charge gradient across the membrane, the K+ selectivity is initially highest at a maximum value of 1.7 while the Ca2+ selectivity is lowest initially at 0.4. These selectivity values ultimately reach similar values observed for diffusion tests after around 8 hours when the system equilibrates.

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