(186g) Thermo-Responsive Ionophore Polymer Brushes Grafted on Magnetic Graphene Oxide As a Dual-Functional Composite Adsorbent for Selective Lithium Ion Recovery from Seawater

Lithium (Li) is one of the most important components for energy storage due to its high energy density. Sufficient amount of lithium is currently produced from ores and brines, but a supply shortage is anticipated due to its soaring demand in mobile electronics and electric vehicle industries. Furthermore, the uneven global distribution of lithium reserves necessitates its recovery from alternative lithium sources. Seawater is identified as a potential alternative lithium resource as it contains 2x10⁶ Mt Li⁺. However, the complex composition of seawater makes it difficult to recover Li⁺ due to its significantly lower concentration than Na⁺, K⁺, Mg²⁺, and Ca²⁺.

In this work, a multi-functional thermo-responsive, Li⁺-selective adsorbent with magnetic property was successfully synthesized. The adsorbent used magnetic graphene oxide (rGO-Fe₃O₄) as a support, on which the polymer brushes of co-polymerized Crown Ether (CE) and N-Isopropylacrylamide (NIPAAm) were grafted via surface-initiated atom transfer radical polymerization (SI-ATRP). Magnetite (Fe₃O₄) is used for material separation and recyclability, and graphene oxide (GO) as a two-dimensional, high-aspect ratio support material for the polymer brushes and magnetite. The CE component is responsible for the selective Li⁺ capture whereas the NIPAAm imparts the thermo-responsive character.

The GO support decorated with magnetite (rGO-Fe₃O₄) was synthesized through a modified Hummers’ method and solvothermal process. This was followed by grafting hydroxyl moieties (OH-rGO-Fe₃O₄), via diazonium surface modification technique and further functionalized to immobilize the SI-ATRP initiator. On the other hand, Li⁺-selective CE (i.e. 2-hydroxymethyl-12-Crown-4 Ether) was modified to introduce an allyl-functional group. The allyl-CE was mixed with NIPAAm and reacted with the ATRP initiator to produce the polymer brushes.

The synthesized composite adsorbent material (12CE4/NIPAAm@HCTP-rGO-Fe₃O₄) was characterized using Boehm Titration, Transmission Electron Microscopy (TEM), X-ray Diffraction Spectrometry (XRD), Thermogravimetric Analysis (TGA), Fourier-Transform Infrared Spectroscopy (FT-IR), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). UV-Vis was utilized to determine the Lower Critical Solution Temperature (LCST) of the polymer brushes which is critical for the control of the adsorption/desorption process of Li⁺. The adsorbent material was systematically tested to determine the Li⁺ adsorption capacity and adsorption kinetics. The selectivity of 12CE4/NIPAAm@HCTP-rGO-Fe₃O₄ was tested using seawater as the feed solution. Repeated material adsorption-desorption experiments were done by utilizing the thermo-response of the material for the capture and release of the captured Li⁺ without the utilization of acid. Thus, the integration of both magnetite and the polymer brush on the GO support material resulted to the Li⁺-selective, magnetic and thermal-responsive 12CE4/NIPAAm@HCTP-rGO-Fe₃O₄ which is suitable for long-term Li⁺ adsorption application.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. 2017R1A2B2002109) and by the Ministry of Education (No. 2018R1D1A1B07048007 and No. 22A20130012051(BK21Plus)).