A rapid deposition of polydopamine coatings induced by iron (III) chloride/hydrogen peroxide for loose nanofiltration

Mussel-inspired polydopamine (PDA) coatings have received widespread concern due to the advantages of eco-friendliness, adhesion nature, and film-forming feasibility. However, self-polymerization of dopamine assisted by air-oxidation under alkaline condition is time-consuming, and the ensuing uneven PDA coatings restrict their applications. In this study, we proposed a rapid PDA deposition triggered by a facile system of iron (III) chloride/hydrogen peroxide (FeCl₃/H₂O₂) under acidic condition. The oxygen-radical species generated by FeCl₃/H₂O₂ largely promote covalent polymerization and deposition rate of dopamine. This not only considerably shortens the deposition time of PDA, but also improves the stability of PDA coatings, combined with the chelation of Fe ions in PDA matrices. SEM, AFM, XPS, zeta potential and water contact angle analyses confirmed the formation of a hydrophilic, smooth, and negatively charged PDA layer onto several membrane substrates. Herein, PDA-coated hydrolyzed polyacrylonitrile membranes yield a remarkable NF performance with superior dye retentions (direct red 23: 98.6%, Congo red: 99.0%, reactive blue 2: 98.2%) and a high water permeability (17.5 L m^-2 h^-1 bar^-1). Furthermore, a low salt rejection (NaCl: 5.6%) of PDA-modified membranes demonstrates their great potential in fractionation of dye/salt mixtures. Meanwhile, the PDA-modified membranes show an excellent organic fouling resistance and a long-term stability. This facile, environmental-friendly method provides a rapid PDA deposition onto various substrates for a wide range of applications, including filtration membranes.