Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Surface charge characterization of nanofiltration membranes by potentiometric titrations and electrophoresis: Functionality vs. zeta potential Hojung Rho a , Kangmin Chon b, ⁎ , Jaeweon Cho a, ⁎ a School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea b Department of Environmental Engineering, College of Engineering, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon-si, Gangwon-do 24341, Republic of Korea ARTICLE INFO Keywords: Functionality Point of zero charge Surface charge Titration Zeta potential ABSTRACT The surface charge properties (i.e., functionality and zeta (ζ) potential) of two nanofiltration (NF) membranes were characterized by potentiometric titrations and electrophoresis to predict the electrostatic transport at the membrane surfaces affecting their salt rejection and fouling propensities. The ζ potential was not suitable for evaluating the rejection of Na + (NE20 membrane = 21–25%; NE70 membrane = 65–70%) and Cl - ions (NE20 membrane = 19–22%; NE70 membrane = 60–63%), and the fouling propensities of organic materials in the NF membranes due to its inherent measurement inaccuracies (Δ ζ potential = -1.8 × (Δ amount of desorbed organic foulants) + 45.9, R 2 = 0.07). The functionality accurately predicted both the rejection of NaCl and the fouling propensities of the organic materials, as the charge densities of the membranes determined by the functionality measurements (only TFC membranes are applicable) truly reflected the acid dissociation constants of the carboxylic and amine functional groups and the points of zero charge values. These results indicate that potentiometric titrations may provide valuable insights into the electrostatic transport at the membrane surface influencing the salt rejection and fouling mechanisms of the NF membranes. 1. Introduction During recent decades, great progress has been made in the field of membrane technologies, which has enabled their use in practical ap- plications, including drinking water production, wastewater treatment, seawater desalination, and energy recovery [1–4]. However, the effi- cient operation of membrane processes is still hindered by membrane fouling, which significantly influences their performance in terms of salt rejection and permeate flux decline [5]. Indeed, the surface charge properties of the membranes are considered as key factors affecting their salt rejection and permeate flux decline. For example, electrostatic interactions between charged solutes and membrane surfaces (known as Donnan exclusion) can have a significant effect on the salt rejection [6–8]. Furthermore, membrane electrostatically repel charged organic materials, colloids, and particles due to their charged surfaces at neutral pH values, which ultimately plays a critical role in the accumulation of foulants on the membrane surfaces which is associated with the decline of permeate flux [9–11]. To address these issues, many researchers have investigated the surface charge properties of membranes and their relationship to the electrostatic transport at the surfaces, which in turn determines their salt rejection and fouling mechanisms [12–15]. The surface charge properties of membranes in contact with aqu- eous media are induced by the ionization of surface functional groups (e.g., –COOH, –NH 2 , and –SO 3 H) as indicated below [16–20]: ↔ + + R–COOH R–COO H – (1) ↔ + + + R–NH R–NH H 2 (2) ↔ + − + R–SO H R–SO H 3 3 (3) Therefore, the ζ (zeta) potential of the membrane surface, as de- termined by electrokinetic measurements, has been widely used as a surrogate parameter to represent the surface charge properties of membranes, as it theoretically corresponds to the electric potential at the shear plane between the Stern and diffuse layers, according to the electrical double layer theory [12]. However, the ζ potential cannot accurately represent the surface charge properties of the membranes due to the distance between the surface and the shear plane of the electrical double layer. Hence, the ζ potential is typically lower than the https://doi.org/10.1016/j.desal.2017.11.003 Received 30 June 2017; Received in revised form 26 October 2017; Accepted 2 November 2017 ⁎ Corresponding authors. E-mail addresses: kmchon@kangwon.ac.kr (K. Chon), jaeweoncho@unist.ac.kr (J. Cho). Desalination 427 (2018) 19–26 0011-9164/ © 2017 Elsevier B.V. All rights reserved. MARK