731 Korean J. Chem. Eng., 32(4), 731-742 (2015) DOI: 10.1007/s11814-014-0290-1 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: sadikuo@tut.ac.za, funmi2406@gmail.com Copyright by The Korean Institute of Chemical Engineers. Microscopical characterizations of nanofiltration membranes for the removal of nickel ions from aqueous solution Oluranti Agboola * ,† , Jannie Maree ** , Richard Mbaya * , Andrei Kolesnikov * , Rotimi Sadiku * , Arne Verliefde *** , and Arnout D’Haese *** *Department of Chemical, Metallurgical and Material Engineering, Faculty of Engineering and the Built Environment, Tshwane University of Technology, Pretoria 0001, South Africa **Department of Environmental Science and Water Care, Faculty of Science, Tshwane University of Technology, Pretoria 0001, South Africa ***Particle and Interfacial Technology Group, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Gent, Belgium (Received 23 May 2014 • accepted 27 September 2014) Abstract-The nanofiltration (NF) process is electrostatically governed and the surface free energy plays a key role in the separation of particulates, macromolecules, and dissolved ionic species. Streaming potential measurement and the surface charge mapping by Kelvin probe atomic force mircoscopy (AFM) have been carried out. Forces of interaction near the surface of nanofiltration membranes were further studied by a force spectroscopy using atomic force micros- copy. The two membranes used are more negatively charged at high pH values; hence the higher the solution chemis- try, the higher and faster will be adhesion of ions on the surface of the nanofiltration membranes. It was observed that the three acquired signals from non-contact AFM (contact potential difference, amplitude and phase) were rigorously connected to the surface structure of the nanofiltration membranes. In addition to the surface structure (roughness), electrostatic interactions can also enhance initial particle adhesion to surfaces of nanofiltration membranes. The perfor- mance of the NF membranes was further investigated for the removal of nickel ions from aqueous solution, and the results were correlated to the mechanical responses of the nanofiltration membranes obtained from AFM and the stream- ing potential measurement. Keywords: Nanofiltration Membranes, Streaming Potential, Surface Charge, Atomic Force Microscopy, Forces of Inter- action, Amplitude Mode INTRODUCTION The physical and chemical properties of nanofiltration mem- branes are very important in understanding nanofiltration mem- brane functions. For optimum operation, the membrane has to pos- sess the physical attributes that gives appropriate interactions with solutes in the process stream [1]. The important physical properties are fouling, surface morphology, pore size distribution and electri- cal double layer interaction. Nanofiltration fouling involves the ac- cumulation and deposition of consistuent in the feed stream on the membrane surface. Electrostatic interaction between the charged surfaces and colloidal particles has been calculated traditionally within the framework of a mean-field pseudo-one-component for- mulation, known as Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [2]. However, this method only allows for the calculation of some average values of an electrical potential at some unspecified shear plane for materials with surface chemical inhomogeneties or significant roughness [1]. Many studies have been done to investigate the surface proper- ties of membranes by using electrokinetic techniques such as stream- ing potential measurements [3-7]. Some studies have also dealt with the quantification of membrane surface potential through electro- phoretic mobility of nanofiltration membranes from experimental point of view [8-10]. Molecular dynamic simulations have recently clarified the underlying processes from atomistic point of view, and this has allowed them to study the role that cations play on mem- brane structure and stability [11-13]. Atomic force microscopy has proved to be a very useful technique employed for surface images with sub-nanometric resolution. By imaging membranes with atomic force microscopy, molecular structure and morphological aspect were demonstrated [14-16]. Although streaming potential measurements are the most fre- quently used for the evaluation of surface charge properties, they have been criticized [17]. According to Brat et al. [17], the results from prior investigations revealed some uncertainty in individual measurement and data scatter because the differences in instru- ment design and the lack of calibration standard for streaming potential analyzer makes comparison of data among laboratory challenging. An advantage AFM has over streaming potential is that AFM is an advanced physical instrument with high resolution imaging of any surface, which has the novelty of directly measur- ing the interfacial interaction between a probe and a membrane surface. Several studies have been conducted in the past two decades to