Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
www.acsami.org
Direct Three-Dimensional Visualization of Membrane Fouling by Confocal Laser Scanning Microscopy Yi-Min Lin,† Chen Song,‡ and Gregory C. Rutledge*,‡ †
Department of Materials Science and Engineering and ‡Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/29/19. For personal use only.
S Supporting Information *
ABSTRACT: Membrane-based separation is an important technique for removing emulsified oil from water. However, the mechanisms of fouling are complex because of the deformability and potential for coalescence and break-up of the oil droplets. Here, we report for the first time direct, threedimensional (3D) visualization of oil droplets on electrospun fiber microfiltration membranes after a period of membranebased separation of oil-in-water emulsions. High-resolution 3D images were acquired by a dual-channel confocal laser scanning microscopy (CLSM) technique in which both the fibers and the oil (dodecane) were fluorescently labeled. The morphology of dodecane as the foulant was observed for two different types of fibers, an oleophobic nylon (PA6(3)T), and oleophilic polyvinylidene fluoride (PVDF). Through direct visualization, the rejected oil was found to form droplets of clam-shell shape on the PA6(3)T fibers, whereas the oil tended to wet the PVDF fibers and spread across the membrane. The morphology was also analyzed as a function of separation time (i.e., “4D” imaging), as the oil accumulated within and upon the membranes. The observations are qualitatively consistent with a transition from blocking of individual pores in the membrane to coalescence of oil droplets into coherent liquid films with increasing filtration time. Analysis of permeate flux using blocking filtration models corroborate the transition of fouling modes indicated by the 3D images. This direct, 3D visualization CLSM technique is a powerful tool for characterizing the mechanisms of fouling in membranes used for liquid emulsion separations. KEYWORDS: 3D imaging, membrane fouling, oil emulsion, microfiltration, direct visualization
1. INTRODUCTION Oily wastewater is one of the major byproducts of industries such as petroleum refining, food processing, and metal finishing, the inadvertent release of which can cause concerns for the aquatic environment.1 The method to remove oil from contaminated water typically depends on the size and stability of the oil droplets.2 Free-floating and unstable suspended oil can be removed inexpensively by gravity-based techniques such as flotation and settling. However, these techniques are not efficient for removing emulsified oil stabilized by surfactants because of the small diameters of the droplets (
