Quaternary ammonium bearing hyper-crosslinked polymer encapsulation on Fe3O4 nanoparticles

Abstract

The Gibbs free energy involved in the transfer of ions from water to a hydrophobic medium with ion-exchange sites changes the selectivity pattern expected from the electrostatic interactions. Oxyanions are less hydrated and, therefore, their exchange with more hydrated anions at the ion-exchange sites of a hydrophobic matrix in contact with aqueous solution is energetically more favorable. This gives rise to a possibility of separating the highly toxic monovalent oxyanions such as HCrO₄⁻, MnO₄⁻, TcO₄⁻ and ClO₄⁻ etc. by the Gibbs free energy of the transfer controlled anion-exchange process. In the present work, hydrophobic anion-exchange polymer encapsulation was anchored on Fe₃O₄ particles by a simple and reproducible method for studying the selectivity pattern. The choice of Fe₃O₄ nanoparticles (NPs) as the host matrix was based on their higher dispersion, larger surface area, and easy retrieval using an external magnetic field that is best suited for treating a large volume of an aqueous stream and developing sustainable technology. The anion-exchange polymer encapsulation was formed by first coating Fe₃O₄ NPs with (3-aminopropyl)triethoxysilane, and subsequently reacting these precursor particles in a sequence with poly(vinylbenzyl chloride) (PVBCl), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 1,8-dibromooctane (DBO). Each step involved in the chemical treatments was monitored by C, H and N analyses. Energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were used to confirm the expected chemical structure of the encapsulation on the Fe₃O₄ NPs. This sequence of chemical treatments resulted in the formation of hyper-crosslinked, hydrophobic, and high fixed positive charge density polymer encapsulation on the Fe₃O₄ NPs. The analyses of the images obtained from field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) indicated that the sizes of the Fe₃O₄ NPs were increased from 13 ± 3 nm to 33 ± 3 nm due to the formation of polymer encapsulation and inter-particle crosslinking. Although a very dilute solution and ultrasonication were used, there was a possibility of crosslinking between the particles. However, the thus encapsulated Fe₃O₄ NPs retained their superparamagnetic properties having a reasonably good magnetic saturation (30 emu g⁻¹). The anion-exchange capacity was found to be 0.65 ± 0.03 meq g⁻¹. The Fe₃O₄@PVB–DABCO–DBO particles exhibited a selectivity pattern corresponding to the Hofmeister series, i.e. the least hydrated anions were adsorbed preferentially. For example, the least hydrated representative TcO₄⁻ anions adsorbed quantitatively in the Fe₃O₄@PVB–DABCO–DBO particles from aqueous solutions such as ground water, seawater, and 0.5 mol L⁻¹ HNO₃ with efficiencies of 98%, 80% and 75%, respectively.