Polydopamine interface encapsulating graphene and immobilizing ultra-small, active Fe3O4 nanoparticles for organic dye adsorption

Abstract In this study, a combined process of bio-inspired modification and magnetic treatment is presented for the preparation of a polydopamine (Pdop)-modified graphene (Pdop-G)-based adsorbent which incorporates ultra-small, active Fe3O4 nanoparticles (with an average size of 6.5 nm). Not only can the nanoparticles impart superparamagnetism to the modified graphene adsorbent but also enhance the adsorption performance. The ultra-small size of Fe3O4 nanoparticles allows the exposure of a high proportion of low-coordinated sites such as corners and edges. Additional active sites can thus be provided to bind methylene blue molecules, in addition to the active Pdop-G surface with catechol and amine groups which induce hydrogen bonding, electrostatic attraction, and π-π stacking interactions. The Pdop interface wraps graphene and immobilizes Fe3O4, endowing the magnetic Pdop-G (MPG) with high adsorption capacity, easy recyclability, and excellent reusability for the organic pollutant removal. In stark contrast, the counterpart without the interfacial Pdop layer suffers from severe Fe3O4 aggregation, causing its adsorption performance inferior to that of MPG. The MPG-based adsorption obeys the pseudo-second-order kinetics, and the intraparticle diffusion model also indicates the complex adsorption pathway, including the external and intraparticle mass transfer. The Langmuir isotherm can better fit the experimental data than the Freundlich isotherm, with the theoretical maximum adsorption capacities estimated to be 131.6, 140.3, and 152.0 mg/g at 30, 40, and 50 °C, respectively. The adsorption is endothermic and spontaneous, along with an increase in the randomness at the solid-solution interface. The separation factor (RL) reveals the favorable adsorption process with MPG. The superparamagnetism imparted via the Fe3O4 composition makes MPG easily recyclable. Furthermore, the removal rate can be maintained at about 90% after 5 runs of repeated usage of MPG. This study opens up a new avenue to the magnetization of adsorbents for enhancing adsorption performance in addition to imparting magnetism.