Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
In Situ Visualization of Concentration Polarization during Membrane Ultrafiltration Using Microscopic Laser-Induced Fluorescence Bo-yang Meng† and Xiao-yan Li*,†,‡,§
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†
Environmental Engineering Research Centre, Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China ‡ Shenzhen Environmental Science and New Energy Laboratory, Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen 518055, China § Shenzhen Engineering Research Laboratory for Sludge and Food Waste Treatment and Resource Recovery, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China S Supporting Information *
ABSTRACT: A novel noninvasive techniquemicroscopic laser-induced fluorescence (micro-LIF)has been applied to achieve in situ visualization of concentration polarization (CP) of nanoparticles during cross-flow ultrafiltration at high resolutions. The reversible, highly dynamic nature of CP and its sensitive response to the filtration conditions were investigated and validated by direct visualization of the CP layer and the well depicted concentration profile near the membrane surface. Using micro-LIF, the formation of a CP layer during filtration and its back-diffusion after the filtration ceased can be directly observed. The dynamic variation of the CP layer with the cross-flow velocity and transmembrane pressure (TMP) change has also been demonstrated. The results showed that CP reached the steady state approximately 1 min after the filtration condition change. A higher cross-flow velocity and/or a lower TMP decrease the CP concentration and thickness. Further quantitative analysis of the filtration test results using the film theory model helps to obtain the particle concentration at the membrane surface and the thickness of the CP layer (30−50 μm). Accordingly, the nature of CP dynamics was characterized and the deficiency of the traditional CP model was explored.
1. INTRODUCTION Membrane filtration has been increasingly used as an efficient separation technology in water purification, wastewater treatment, and reclamation. However, membrane fouling remains a major obstacle for membrane applications, greatly reducing membrane filtration capacity and hence increasing treatment cost.1−4 Membrane fouling is often connected with the phenomenon of concentration polarization (CP), the initial stage of membrane fouling.5,6 It is therefore of fundamental importance to gain an accurate description and sound understanding of CP for developing effective fouling mitigation strategies. CP occurs when rejection of solutes by the membrane results in the accumulation of solutes in the boundary layer near the membrane surface, forming a CP layer.6,7 CP is a complex and dynamic process affected by multiple factors. Efforts have been made to model and simulate CP formation and characteristics in connection with membrane fouling.8−12 However, experimental studies of CP and its characteristics have been rather limited. For membrane fouling, various techniques have been used to realize real-time investigation © XXXX American Chemical Society
and characterization, such as direct observation through the membrane,13−15 electrical impedance microscopy,16−18 ultrasonic time domain reflectometry,19,20 and optical coherence tomography.21−26 However, unlike the membrane fouling layer, which remains on the membrane surface after filtration, the CP layer is highly dynamic, unstable, and reversible, and it disappears rapidly by diffusion after filtration ceases.27 Owing to the reversible and dynamic nature of CP, it remains difficult to achieve real-time CP detection and monitoring during the filtration process. Chen et al.28,29 reviewed some noninvasive techniques that can be used for in situ CP investigation. Shadowgraphy30,31 and refractometry,32,33 which are based on the light deflection that results from the refractive index gradient within the CP layer, have been used to obtain a concentration profile in deadend ultrafiltration. McDonogh et al.34 applied the electron Received: Revised: Accepted: Published: A
October 12, 2018 January 24, 2019 January 29, 2019 January 29, 2019 DOI: 10.1021/acs.est.8b05741 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 1. (a) Schematic diagram of the experimental setup, (b) detailed schematic diagram, and (c) photograph of the membrane chamber, the LIF laser emitter and laser sheet, and the camera with a microscope lens.
diode array microscope (EDAM) technique to observe the growth of the CP layer during cross-flow ultra- and microfiltration. The EDAM measurement could be made as close as 20 μm from the membrane surface with a resolution of 2 μm, whereas the refractometry technique had a measurable distance of 200 μm above the membrane surface with a resolution of 5 μm.32,33 In recent years, digital holographic interferometry,35,36 an interferometric technique, has been
used to monitor the CP layer via interference fringes during cross-flow reverse osmosis. The dynamics of the CP layer were verified by the appearance and disappearance of the interference fringes. However, the resolution was rather low due to the discontinuity of the interference fringes, and as the concentration gradient was steep near the membrane surface, the interference fringes became too dense to distinguish. Smallangle X-ray scattering (SAXS) was reported to achieve in situ B
DOI: 10.1021/acs.est.8b05741 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology CP characterization during cross-flow ultrafiltration.37 The substance concentration was determined from analysis of the SAXS patterns. However, the resolution of the detection was 50 μm starting at 100 μm from the membrane surface, and the build-up and diffusion of the CP layer could not be directly visualized. Generally, limitations in resolution and accuracy continue to undermine the investigation and characterization of detailed CP dynamics. Novel noninvasive detection techniques are therefore much needed to achieve real-time, high-resolution CP observation and detection of the concentration profile near the membrane surface during the filtration process. Laser-induced fluorescence (LIF) is an advanced spectroscopic imaging technique for nonintrusive flow visualization and concentration determination.38−41 By applying a microscopic lens, a very high resolution (approximately 1 μm/pixel) can be achieved for observation and visualization. Using the LIF technique, a high-speed charge coupled device (CCD) camera can capture images of real-time flow within the target area, and the concentration field can be determined by the correlation between the fluorescent light intensity and fluorescent dye concentration. However, the LIF technique has not been used to investigate CP during membrane filtration. In this study, the micro-LIF technique was applied for the first time for in situ visualization of the CP layer during the cross-flow ultrafiltration process. The formation and disappearance of the CP layer were directly observed at a high resolution. The concentration profile of the CP layer was also determined by image processing and analysis. The dynamic and reversible features of the CP layer was verified, and the influence of cross-flow velocity and transmembrane pressure (TMP) on the CP characteristics was also studied. In addition, the experimental results were compared with theoretical predictions, and the deficiency of the conventional CP model was analyzed.
The micro-LIF system (Dantec Dynamics, Denmark) consisted of a laser source (high-power Nd:YAG lasers) emitting 532 nm laser light, a HiSense Zyla sCMOS Camera with a long-pass cutoff filter and a 5× microscopic objective lens, and a computer equipped with the Dynamic Studio software package for image acquisition. A laser generator was used to form a 1 mm thick laser light sheet to illuminate the membrane chamber. The laser sheet plane cut perpendicularly along the center of the membrane surface in the flow chamber. The camera was positioned facing the observatory window and focused at the center of the arch-shaped membrane surface, which was illuminated by the laser sheet (Figure 1b). In this way, the flow field near the membrane surface could be captured from the side view. The cutoff filter in front of the camera filtered off light with a wavelength less than 570 nm, whereas fluorescent light could pass through. Thirty images were acquired at a frequency of 20 ms/image to obtain the mean image for every acquisition. The images had a resolution of 2560 × 2160 pixels for a size of 2.45 × 2.07 mm, corresponding to a high resolution of
