Nanofiltration membranes via co-deposition of polydopamine/polyethylenimine followed by cross-linking Yan Lv, Hao-Cheng Yang, Hong-Qing Liang, Ling-Shu Wan, Zhi-Kang Xu n MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China article info Article history: Received 14 August 2014 Received in revised form 10 November 2014 Accepted 15 November 2014 Available online 24 November 2014 Keywords: Nanofiltration membrane Composite membrane Polydopamine Polyethylenimine Co-deposition abstract Novel composite nanofiltration membranes (NFMs) were simply fabricated via co-deposition of mussel- inspired polydopamine (PDA) and polyetheylenimine (PEI) followed by glutaraldehyde (GA) crosslinking. A uniform, robust and defect-free selective layer was generated on the hydrolyzed polyacrylonitrile (HPAN) ultrafiltration membrane substrate, endowing the composite NFMs with high separation performance for multivalent ions. Zeta potential measurements indicate these NFMs are slightly positively charged, resulting in a salts rejection sequence of MgCl 2 4CaCl 2 4MgSO 4 4Na 2 SO 4 4NaCl at pH 5.5. The nanofiltration performance can be tuned by changing the co-deposition time and the mass ratio of dopamine/PEI. A mass ratio of 2/2 with 4 h co-deposition is the optimum protocol for the membrane performances including surface hydrophilicity, water flux and salt rejection. Moreover, the composite NFMs show a good structural stability for immersing in ethanol or for long-term nanofiltra- tion process. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, nanofiltration has become a rapidly developing and expanding area with tremendous potential for desalination, water softening and wastewater recycling [1]. In general, nanofiltration is defined as a pressure-driven process with properties between reverse osmosis and ultrafiltration [2]. Due to its special separation properties, nanofiltration owns the advantages of both aforemen- tioned processes including low operation pressure, high flux, high retention of multivalent ions and organic molecular (200–1000 Da) along with low investment, operation and maintenance costs [1,3]. Nanofiltration membranes (NFMs) are mainly fabricated via inter- facial polymerization technique [4–8]. It is a facile and fast met- hod to prepare composite NFMs. The resulted active layer can be attached onto various substrates. Nevertheless, the compatibility between the support layer and the skin layer is usually so poor that the skin layer can be easily detached from the substrate in harsh environments containing organic solvents such as ethanol. Great effort has been made to enhance the adhesive strength between the skin layer and the substrate surface, including creating covalent linkage and constructing adhesive transition layer [9–12]. For exa- mple, grafting method can strongly binding the active layer onto the substrate by covalent bonds and significantly improve the adhesive strength [13,14]. However, these methods are complicated or far from precisely control. As a consequence, a facile fabrication method is in great demand for composite NFMs with excellent structural stability. Inspired by the universal adhesion of the adhesive proteins in mytilus edulis foot, Messersmith and his co-workers demonstrated that dopamine could be oxidized in alkaline environment and formed a polymer-like coating on various substrates with great adhesive strength [15–20]. After that, numerous of studies have applied mussel-inspired coating technique into membrane science such as surface modification, functionalization and fabrication of thin film composite membranes [21–30]. Zhu et al. fabricated a novel hydrophilic NFM by simply immersing polysulfone ultrafil- tration substrate in dopamine solution [31]. The work of Li et al. further chemically modified the membrane with a fluorinated polyamine after dopamine deposition to obtain a stable composite membrane with good antifouling performance [32]. However, the pure polydopamine (PDA) coating is not dense enough for excel- lent salts rejection. Scientists then grafted polyethyleneimine (PEI) onto PDA deposited PES membrane with or without further crosslinking to obtain a much denser and more compact compo- site membrane with good salts rejection performance [33,34]. Nevertheless, it is well known that the deposition process of pure dopamine is generally time-consuming and heterogeneous, the resultant PDA particles are not compactly or uniformly coated on Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2014.11.024 0376-7388/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ86 571 8795 1773. E-mail address: xuzk@zju.edu.cn (Z.-K. Xu). Journal of Membrane Science 476 (2015) 50–58