(381o) Co(II) Recovery from Electronic Waste Based on Ion-Imprinting of Reduced Schiff Base Ligand on Magnetite

The huge amount of generated electronic wastes (e-waste) worldwide is an attractive secondary source for the recovery of valuable metals. Cobalt in spent lithium-ion batteries (LIBs) is a critical metal that can be recycled into LIBs and other materials such as magnets, alloys, and printed circuit boards (PCBs). Thus, selective Co(II) extraction and recovery from secondary sources is of great importance. Schiff base ligands are known for their affinity towards Co(II) and have been used as extractants in liquid-liquid extraction (LLE). Schiff base-anchored support materials have also been reported, but none so far with magnetic particles as support. Schiff bases are typically synthesized via condensation of diamines or triamines with substituted benzaldehydes; their use as Co(II) ligands is relatively cheap and convenient. However, Schiff bases are susceptible to acid-catalyzed hydrolysis, especially during desorption of Co(II) prior to adsorbent recycling. The imine group of Schiff bases must be reduced to amine to render it acid-resistant while it retains its complexing ability with Co(II). In this study, Schiff bases as Co(II) ligand were prepared from the reaction of 4-allyloxy-2-hydroxybenzaldehyde (AHB) with o-phenylenediamine (PDA) or tris(2-aminoethyl)amine (TAEA) to form AHB-PDA and AHB-TAEA, respectively, which were further reduced by NaBH₄ (r-AHB-PDA and r-AHB-TAEA). All synthetic compounds were confirmed via infrared spectroscopy (FTIR), proton and carbon nuclear magnetic resonance (¹H-NMR and ¹³C-NMR).

Preliminary results from LLE of Co(II) reveal q = 105.3 mg Co(II) g^-1 for r-AHB-PDA and 12.2 mg Co(II) g^-1 for r-AHB-TAEA. The ligand r-AHB-PDA was chosen for the synthesis of ion-imprinted polymers (IIPs) on a magnetic support material, a practical approach that would minimize reagent use and simplify the Co(II) recovery process. Ion-imprinting technique imparts selectivity towards Co(II) as the r-AHB-PDA binding sites are pre-arranged or templated specifically for the target Co(II). Meanwhile, the magnetic core renders the adsorbent useful for cycled adsorption and desorption of Co(II). For the synthesis of Co(II) selective magnetic-IIP using Schiff base as ligand, magnetite (Fe₃O₄) was initially prepared by co-precipitation of FeCl₃/FeCl₂ under basic conditions. Its surface was then coated with silica by tetraethylorthosilicate (Fe₃O₄@SiO₂), then functionalized with alkene by 3-methacryloyloxypropyltrimethoxysilane (Fe₃O₄@SiO₂-MATES) as co-monomers of r-AHB-PDA ligand. Radical polymerization between the Fe₃O₄@SiO₂-MATES and r-AHB-PDA was then carried out using azobisisobutyronitrile (AIBN) as radical initiator and ethylene glycol dimethacrylate (EGDMA) as cross-linker. The adsorbent was thoroughly characterized via FTIR, SEM, XRD, TGA and elemental analysis. Its adsorption properties (i.e. isotherms, kinetics and selectivity) and recyclability are currently being investigated. This work was supported by National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017R1D1A1B03028102 and 2017R1A2B2002109) and Ministry of Education (2009-0093816 and 22A20130012051 (BK21Plus)).