(371g) Ionophore-Decorated Magnetic Graphene Oxide As a Composite Adsorbent for Precious Metal Recovery

Recovery of lithium from aqueous streams has gained significant attention as it can contribute in resolving the anticipated challenges on shortage of lithium supply in the future. Given the variety and complexity in the composition of aqueous Li⁺ sources, more advanced materials must be developed for selective Li⁺ capture. It is also desirable to synthesize multi-functional materials which can be readily regenerated and reused for long term application.

Herein, a multifunctional adsorbent (CE-rGO-Fe₃O₄) was successfully synthesized and used as a selective lithium ion (Li⁺) adsorbent. The adsorbent was decorated with crown ethers (CE) as Li⁺-specific ionophores, magnetite (Fe₃O₄) for its easy separation and recyclability, and graphene oxide (GO) as a two-dimensional, high-aspect ratio support of the CE and Fe₃O₄.

The GO was prepared by oxidizing graphite powder through a modified Hummerâ??s method. The Fe₃O₄ nanoparticles were then immobilized on the surface of the GO through a solvothermal technique which also partially reduced the GO nanosheets (rGO-Fe₃O₄). Alkyne moieties were grafted on the rGO-Fe₃O₄ surface using 4-ethynylaniline The alkyne terminals in 4-ethynylaniline served as â??clickableâ? CE-specific attachment sites. Meanwhile, Li⁺-selective CE (i.e. 2-hydroxymethyl-12-Crown-4 Ether) was prepared via a two-step mesylation-azidation reaction to obtain 2-(azidomethyl)-12-Crown-4 Ether. The azidated CE was then â??clickedâ? on the alkyne- rGO-Fe₃O₄ at 60 °C via the 1,3 cycloaddition click chemistry reaction.

The synthesized composite adsorbent (12CE4-rGO-Fe₃O₄) was characterized using Transmission Electron Microscopy (TEM), Boehm Titration, X-ray Diffraction Spectrometry (XRD), Fourier-Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). TEM was used to observe the morphology of the adsorbent, which revealed the presence of Fe₃O₄ NPs on the surface of the GO nanosheets. The XRD spectrum and FTIR conservatively confirmed the presence of different functional groups on the GO support, which was also consistent with the TGA results. Raman Spectroscopy also reflected the degree of functionalization of the GO as indicated by the changes in the D- and G-bands and their corresponding I_D/I_G ratios. The changes in the chemical states of the elements present in the adsorbent upon subsequent modification and functionalization was also confirmed via XPS. XPS results revealed the presence of the Fe₃O₄ in the adsorbent (12CE4-rGO-Fe₃O₄) as indicated by the Fe2p signals whereas the successful attachment of the CEs via click reaction was confirmed with the presence of the peak attributed to the triazole functional group found in the N1s spectra.

The adsorption results show the Langmuir-type Li⁺ uptake of the 12CE4-rGO-Fe₃O₄, with the maximum Li⁺ uptake at 7.33 mg g^-1. The results also show that the material has high selectivity towards Li⁺ as compared to the other cations. The integration of magnetite allowed the easy separation and recyclability of the adsorbent. Overall results demonstrate the suitability of the 12CE4-rGO-Fe₃O₄ for long-term Li⁺ adsorption application.

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (2015R1A2A1A15055407) and by the Ministry of Education (No. 2009-0093816).