Insights into High Li⁺/Mg²⁺ Separation Performance Using a PEI-Grafted Graphene Oxide Membrane

Abstract

Lithium extraction from brine or seawater using membrane technology has attracted extensive attention in recent years. Graphene oxide (GO), as one of the two-dimensional materials, has been proven as a competitive candidate for membranes. However, the GO membranes still suffer challenges for ion sieving due to the swelling in the aqueous solution. In this work, a GO–PEI membrane with positively charged channels was constructed by polyelectrolyte polyethyleneimine (PEI) molecular chain-grafted GO nanosheets. The GO–PEI membrane showed a high selectivity of 22.2 for Li+/Mg2+, together with a competitive Li+ permeation rate of 0.09 mol m–2 h–1 in a binary permeation test. In addition, the membrane showed excellent stability during the separation process. The enhanced Li+/Mg2+ separation performance of the GO–PEI membrane could be mainly attributed to the synergistic effect of size sieving and electrostatic repulsion. We further systematically studied the influence of various variables, such as PEI molecular weight, PEI content, membrane thickness, and ion concentration, on separation performance. This work deepens the understanding of membrane ion selectivity from multiple perspectives and puts forward a strategy to design a two-dimensional membrane structure from microscopic materials.

Graphene Oxide/Polyethyleneimine-Modified Cation Exchange Membrane for Efficient Selective Recovery of Ammonia Nitrogen from Wastewater

“…A schematic drawing of the GO-PEI membrane NH 4 + /Mg 2+ DD separation is presented in Figure 8. The -NH 2 in the positively charged PEI can be covalently cross-linked with the -COOH at the edge of the negatively charged GO during the impregnation process, forming a GO-PEI layer with controlled surface charge by layer self-assembly at the CEM [39]. The GO-PEI-modified layer changes the charge on the membrane surface from negative to positive (see Figure 3), granting stronger electrostatic membrane surface repulsion of highly valent cations (e.g., Ca 2+ , Mg 2+ ) [40].…”

“…In addition, GO can form π-metal cation conjugation with Mg 2+ and adsorb part of Mg 2+ [28], thus achieving selective retention of Mg 2+ . Wang et al [39] prepared ion-selective separation membranes using GO-PEI deposited on the surface of microfiltration membranes, performing monovalent/multivalent ion-selective separation through two-dimensional nanochannels with electrostatic repulsion and size-screening capabilities. Different from the report in the literature, the TEM observation of the sample sections (see Figure S3) revealed that the GO nanosheets were not uniformly stacked on the CEM surface to form two-dimensional nanochannels.…”

An Antisolvent Extraction Strategy for Extrusion Granulation Enhancement of Aluminum-Based Lithium Adsorbent Used in Ultrahigh Mg2+/Li+ Salt Lake Brines

“…Lithium has seen a constant increase in demand with the rapid promotion of electric vehicles and the vigorous development of energy storage technology worldwide. − Lithium recovery from brines is relatively feasible from economic and environmental considerations, which has attracted enormous interest from researchers in recent decades. − The adsorption method of extracting lithium is particularly suitable for low-lithium-grade brines with an ultrahigh Mg 2+ /Li + mass ratio that is far more than 20 because it is highly selective, simple, low cost, and environmentally friendly. − Up to date, the aluminum-based lithium adsorbents, lithium/aluminum-layered double hydroxides (Li/Al-LDHs), are the first successfully industrialized adsorbents used in brines for lithium recovery, attributing to their desorption properties without solubility loss. − Li/Al-LDHs are typical two-dimensional structures and consist of positively charged host layers formed by Al–O octahedra and linked by hydrogen bonds with anions and water molecules sandwiched. Lithium cations exist in the holes of the hydroxide layers composed of the Al–O octahedral without forming chemical bonds with other elements. , …”