Performance and Mechanisms of Ultrafiltration Membrane Fouling

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Performance and Mechanisms of Ultrafiltration Membrane Fouling Mitigation by Coupling Coagulation and Applied Electric Field in a Novel Electrocoagulation Membrane Reactor Jingqiu Sun, Chengzhi Hu, Tiezheng Tong, Kai Zhao, Jiuhui Qu, Huijuan Liu, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01189 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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Environmental Science & Technology

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Performance and Mechanisms of Ultrafiltration

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Membrane Fouling Mitigation by Coupling

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Coagulation and Applied Electric Field in a Novel

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Electrocoagulation Membrane Reactor

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Jingqiu Sun1, 2, Chengzhi Hu1, 3*, Tiezheng Tong3, Kai Zhao1, Jiuhui Qu1, Huijuan

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Liu1, Menachem Elimelech3

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Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2

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Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-

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University of Chinese Academy of Sciences, Beijing 100049, China

Department of Chemical and Environmental Engineering, Yale University, New Haven,

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Connecticut 06520-8286, United States

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*Corresponding

author: e-mail address: [email protected] (C.H.); Tel: +86-10-62849160; Fax:

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+86-10-62849160; Add: 18 Shuangqing Road, Haidian District, Beijing, 10085, P.R. China.

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ACS Paragon Plus Environment


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ABSTRACT

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A novel electrocoagulation membrane reactor (ECMR) was developed, in which ultrafiltration

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(UF) membrane modules are placed between electrodes to improve effluent water quality and

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reduce membrane fouling. Experiments with feed water containing clays (kaolinite) and natural

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organic matter (humic acid) revealed that the combined effect of coagulation and electric field

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mitigated membrane fouling in the ECMR, resulting in higher water flux than the conventional

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combination of electrocoagulation and UF in separate units (EC-UF). Higher current densities

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and weakly acidic pH in the EMCR favored faster generation of large flocs and effectively

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reduced membrane pore blocking. The hydraulic resistance of the formed cake layers on the

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membrane surface in ECMR was reduced due to an increase in cake layer porosity and polarity,

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induced by both coagulation and the applied electric field. The formation of a polarized cake

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layer was controlled by the applied current density and voltage, with cake layers formed under

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higher electric field strengths showing higher porosity and hydrophilicity. Compared to EC-UF,

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ECMR has a smaller footprint and could achieve significant energy savings due to improved

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fouling resistance and a more compact reactor design.

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TOC Art

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INTRODUCTION

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Pressure-driven membrane filtration technologies are widely used in water treatment.1

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Ultrafiltration (UF) in particular is commonly employed in drinking water treatment because it

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removes multiple contaminants in one step and is economically feasible.2,

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inherently constrained by membrane fouling, with dissolved organic substances (e.g., humic acid

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and proteins) acting as major foulants.4-6 Membrane fouling compromises the performance and

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lifetime of UF membranes, increasing the cost and energy demand of UF systems. Therefore,

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effective strategies for fouling mitigation are highly desirable to improve the efficiency and

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applicability of UF systems.7

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However, UF is

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Electrocoagulation (EC) is an attractive process in water treatment, and its performance and

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mechanisms have been widely investigated.8 Since EC could effectively remove particles and

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humic acid from raw water, applying EC as a pretreatment step is one of the promising solutions

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to control UF membrane fouling.9-14 The properties of flocs generated by EC have a significant

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influence on the structure of membrane cake layers and the extent of membrane fouling.9, 11 For

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example, it has been reported that the steady-state normalized specific water flux increased from

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51% for microfiltration to 72% for electrocoagulation-microfiltration.15 If the porosity and

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structure of the formed cake layer can be effectively controlled in the EC process, a highly

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porous protective layer could be formed to further alleviate membrane fouling.

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Applied electric field has also been considered as an efficient method to reduce membrane 16-18

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fouling.

It has been demonstrated that deposition of charged foulants, such as humic acid

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and inorganic particles, was prevented by electrostatic repulsion induced by an electric field.19, 20

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In this case, charged foulants migrate away from the membrane surface by electrostatic force.21,

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22

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fouling.23 Although electric field can alter the dipole moment of organic foulants, its effect on

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cake layer structure has not been investigated. Furthermore, previous studies typically combined

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EC with membrane filtration as separate units8, 24, 25, in which membranes were located outside

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of the electric field zone. We hypothesize that when EC and UF are integrated in one unit,

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membrane fouling would be mitigated by the combined effect of both coagulation and electric

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field, leading to further improvement of membrane fouling control.

It was further suggested that the dipole moment of foulants can also influence membrane



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In this study, we designed an integrated electrocoagulation membrane reactor (ECMR), with

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membrane modules placed between a pair of electrodes. We evaluated the efficiency of ECMR

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in mitigating membrane fouling, and analyzed the effect of electrochemical parameters on the

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ECMR performance. To better understand the antifouling mechanisms of ECMR, we focused on

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the evolution of particle size and cake layer polarity in ECMR, which are influenced by

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coagulation and electric field, respectively. Our study is the first to propose the concept of

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ECMR and systematically delineate its design and working principles.

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MATERIALS AND METHODS

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Chemicals and Synthetic Feed Water. A stock solution of humic acid (Sigma-Aldrich,

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U.S.A) was prepared in deionized water (DI, Millipore Milli-Q) after filtration through a 0.45

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µm mixed cellulose ester membrane. The molecular weight of humic acid was mainly distributed

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around ~270 kDa and ~10 kDa (Figure S1).The prepared stock solution had a dissolved organic

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carbon (DOC) concentration of 777±20 mg/L and was stored at 4 °C until use. Sodium chloride

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(50 mM) was added in the synthetic feed water as the background electrolyte, and sodium

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bicarbonate (0.5 mM) was used to provide a buffer capacity. DOC of the synthetic feed water

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was adjusted to 5 mg/L and clay (kaolinite) concentration to 50 mg/L. Sodium hydroxide and

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hydrochloric acid were used to adjust the solution pH for experiments investigating the effect of

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pH on ECMR performance. Sodium chloride, sodium bicarbonate, sodium hydroxide, and

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hydrochloric acid were received at analytical pure grade; kaolinite was at chemical pure grade

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with particle size ranging from ~400 to ~1200 nm (Figure S2). The ATR-FTIR spectra of humic

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acid and kaolinite are presented in Figure S3. All reagents were purchased from Sinopharm

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Chemical Regent Co., Ltd, China.

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ECMR System and Membrane Fouling Experiments. Figure 1 describes the design

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of the ECMR system and membrane modules. The UF membrane module was placed between

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two plate electrodes, so that both the EC process and applied electric field could work in concert

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to mitigate membrane fouling in one unit. When the membrane module was placed outside the

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electrodes, the process was considered as an EC-UF system instead of ECMR. Unlike other

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systems that combined (electro) coagulation with ultrafiltration or microfiltration10, 15, 26, EC-UF

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was performed in one single reactor in our study. The effective volume of the reactor was 735

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mL (14 cm ×5 cm ×10.5 cm). As shown in Figure 1b, hollow fiber membranes (PVDF, Tianjin


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MOTIMO Membrane Technology Co., Ltd., China) were used in the UF system. A membrane

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module composed of ten fibers with a length of 3 cm was employed to provide an effective

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membrane area of 24 cm2. A small tube was placed on top of the membrane module for permeate

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collection. According to the manufacturer, the membrane average molecular weight cutoff was

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100 kDa, which is corresponding to an average pore size of ~30 nm.

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FIGURE 1

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In each experiment, 1.4 L of synthetic feed water was recycled by two peristaltic pumps (36

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mL/min) in order to generate cross-flow and keep a constant water level in the ECMR system

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(Figure 1 and Figure S4). The membrane module was placed between two aluminum electrodes

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parallel to the water flow, with the distance between the two electrodes fixed at 2.5 cm. The feed

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water was well mixed by a magnetic stirrer at 300 rpm. A digital balance (ME2002E, Mettler

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Toledo, Switzerland) connected to a personal computer was used to record the mass of permeate

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during system operation. A pressure sensor was employed to monitor the transmembrane

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pressure, which was maintained at 30 kPa by a peristaltic pump. The initial water flux of each

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membrane module was 96.25 L·m-1·h-1. Unless otherwise specified, pH was adjusted to 7.0 and

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the current density was 10 A/m2. When investigating the effects of electric field strength on

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membrane fouling, different electrolyte concentrations were used to keep the current density

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constant. The electrolyte concentration (measured as conductivity) exhibited little effect on water

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flux in our experiments (Figure S5).

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Analytical Methods. UV absorbance at 254 nm (UV254) was measured by an ultraviolet-

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visible spectrophotometer (U-3010, Hitachi High Technologies Co., Japan) as an indicator of

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humic acid concentration. Dissolved organic carbon (DOC, after filtration through 0.45 µm

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mixed cellulose ester membranes) was determined with a total organic carbon (TOC) analyzer

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(TOC-VCPH, Shimadzu, Japan).

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In order to investigate cake layer properties, fouled membrane fibers were cut from the

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membrane modules and dried at room temperature for 24 h. In the preparation of cross-section

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samples of membranes, the hollow fiber membranes were frozen in liquid nitrogen, and then cut

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instantly to preserve the cake layer attached to the membrane surface. The pristine and fouled

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membrane samples were platinum-coated by a sputter and observed under a scanning electron 4 ACS Paragon Plus Environment


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microscope (SEM, JSM7401F, JEDL, Japan). Attenuated total reflectance Fourier transform

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infrared (ATR-FTIR) spectra were collected by a Fourier Transform Infrared Spectrometer

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(Nicolet 8700, Thermo Fisher Scientific, USA). Furthermore, the hydrophilicity of the fouled

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membranes formed under different electric fields was measured by a dynamic contact angle

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tensiometer (Data Physics DCAT, LPD Lab Services Ltd., UK). Floc size as a function of time

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was directly monitored by a laser particle size analyzer (Mastersizer 2000, Malvern, UK).27 The

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flocs were monitored by pumping water through the optical unit of the laser particle size

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analyzer and back into the reactor by a peristaltic pump on the return tube (5 mm internal

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diameter). Particle size was recorded every 20 seconds during the experiments by a computer.

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RESULTS AND DISCUSSION

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Performance of ECMR. The removal rate of humic acid (as measured by UV254) and

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membrane water flux were measured to evaluate the performance of ECMR compared to EC-UF,

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EC, and UF (Figure 2). As shown in Figure 2a, the removal rate of humic acid by ECMR

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attained 93.7±0.2% within 10 minutes and maintained the same level afterwards. This removal

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rate was comparable to that of EC-UF (94.0±2.5%), but significantly higher than those of EC

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(61.1±2.9%) and UF (51.3±3.0%). For UF, size exclusion is the main mechanism for humic acid

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removal. The low removal efficiency of humic acid by UF indicates that humic acid

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macromolecules smaller than the membrane pore size were able to pass through the UF

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membrane. Although higher removal efficiency (~80%) could be achieved by EC than UF,

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longer time was needed for floc sedimentation. The ECMR and EC-UF processes combined the

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advantages of the EC and UF processes to achieve a much higher removal efficiency of humic

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acid within a short time.

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FIGURE 2

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As shown in Figure 2b, water flux decline of ECMR caused by membrane fouling was less

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than that observed with EC-UF and UF. The flux decline curves for ECMR and EC-UF exhibited

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two stages that corresponded to different fouling mechanisms, consistent with the pore

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blockage–cake filtration model developed by Ho and Zydney.28 This model accounts for a

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smooth transition of fouling mechanism from pore blockage to cake formation during filtration,

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and has been successfully used in investigating humic acid fouling in microfiltration.29 Our

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observation indicates that initial fouling was due to deposition of humic acid aggregates on the 5 ACS Paragon Plus Environment

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membrane surface blocking membrane pores, followed by formation of a cake layer on the

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regions of membrane that had been covered by the foulant aggregates.

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The water flux for UF alone decreased continuously within 15 min, suggesting that pore

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blocking played a primary role. In contrast, a rapid flux decline was only observed in the

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beginning period (i.e., the first 4-7 min) for ECMR and EC-UF. After this initial period, the

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water flux was maintained at a relatively steady level, indicating that a cake layer had been

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formed on the membrane surface. This two-stage mode of flux decline was consistent with the

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results reported in other studies (Table S1).30-32 The final relative water flux, J/J0, of the ECMR

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(87.2%) was higher than that of EC-UF (77.8%). Therefore, ECMR achieved comparable humic

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acid rejection but provided a higher water flux than EC-UF. This finding indicates that

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coagulation and electric field likely influence the structure and physicochemical properties of the

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formed cake layer, which will be discussed in more detail in the following subsections.

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Key Operational Parameters Governing EMCR Fouling Behavior. Water flux

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decline by pore blockage mechanism depends on foulant size.33 Severe membrane fouling can be

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caused by pore blocking when the particle size is comparable to the membrane pore size. As the

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EC process proceeds, particle size increases in the coagulation process.34, 35 Furthermore, current

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density and pH could control aluminum coagulant dosage and hydrolysis rate, respectively,

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thereby leading to different sizes of generated flocs.36

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FIGURE 3

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As shown in Figure 3a, an increase of current density promoted fouling resistance in ECMR.

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Under a current density of 2 A/m2, the water flux of ECMR decreased to 70.5% of the initial

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value and kept steady afterwards. When the current density increased to 10 A/m2 and 20 A/m2,

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the water fluxes were maintained at 83.1% and 97.3% of the original flux, respectively.

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Furthermore, the applied current density influenced the growth of particles formed in ECMR. As

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shown in Figure 3b, particles kept stable at first and then started to grow in size rapidly at 2 min,

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4 min, and 11 min under current densities of 20 A/m2, 10 A/m2, and 2 A/m2, respectively.

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Notably, this critical time for particle growth roughly matched the inflection point of the flux

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decline curve when water flux became stable.



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The electric field strengths varied with the different current densities applied in ECMR,

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thereby creating two variables (electric field strength and current density) that potentially

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influenced membrane fouling. This dependence may interfere with the interpretation of the

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observed flux decline curves under different current densities (Figure 3). Therefore, the effect of

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current density on water flux was investigated in EC-UF (i.e. UF membrane outside the reactor

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and under no electric field) to exclude the concomitant influence of electric field. As shown in

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Figure S6, the trend of flux decline under various current densities in EC-UF was consistent with

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that in ECMR. This observation suggests that higher current densities produced more aluminum

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coagulant species per unit time, thus enhancing aggregation of foulants (humic acid and kaolinite)

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and reducing membrane fouling.

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FIGURE 4 Solution pH also influenced EMCR performance because pH governs the coagulation

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mechanisms and the growth of particles by metal salt coagulants.35,

37, 38

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previous work38, lower solution pH resulted in a more rapid growth of flocs with a more porous

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structure, which might contribute to the mitigation of membrane fouling. As shown in Figure 4a,

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J/J0 reached a steady state of 94.7% at pH 4.0, whereas it decreased to 87.6% and 77.4% at pH

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7.0 and 9.0, respectively. Figure 4b shows that the particle size kept small initially and then

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started to grow rapidly at 2 min, 4 min, and 6 min for pH at 4.0, 7.0, and 9.0, respectively.

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Consistent with the phenomenon observed in Figure 3, the time when particle size started to

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increase roughly matched the inflection point of the flux decline curves for every pH, which

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underscores the importance of particle size in reducing membrane fouling in ECMR.

According to our

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Our results demonstrate that current density and solution pH are two critical parameters for

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membrane fouling control in ECMR. Hence, proper control of these operational parameters

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could be used to optimize membrane resistance to pore blocking.

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Cake Layer Properties under Applied Electric Field. Water flux decline curves

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under various initial electric field strengths are presented in Figure 5. The current density was

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kept constant, whereas the electric field strength was varied by altering the electric conductivity

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of the feed solutions. Electrolyte concentration (measured as conductivity) exhibited negligible

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effect on membrane fouling and humic acid removal in our experiments (Figure S5 and S7). 7 ACS Paragon Plus Environment

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Feed solutions with lower conductivities resulted in stronger electric field strengths. Results

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show that the final normalized water flux (J/J0) was positively correlated to the initial strength of

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the electric field. Specifically, J/J0 values were 85.4%, 91.0%, and 94.9% under electric field

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strengths of 0.5 V/cm, 1 V/cm, and 2 V/cm, respectively. Polarization effects induced by the

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electric field likely played an important role in membrane fouling, especially during the process

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of cake layer formation, as also recently observed by Fan et al.20. To further explore this

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observation, we analyzed the morphology and hydrophilicity of the fouled membranes in order to

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understand the alteration of cake layer properties under different electric fields.

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FIGURE 5

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The surface morphologies of the fouled membranes subjected to different electric field

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strengths were observed by SEM (Figure 6a-c). The cake layers formed under different electric

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field showed similar compositions (Figure S8). But when the strength of electric field increased,

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the cake layer showed a more irregular and rougher surface with enhanced porosity. The

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observed average pore sizes of the cake/fouling layers were 7.9 nm, 12.6 nm, and 12.8 nm under

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electric field strengths of 0.5 V/cm, 1 V/cm, and 2 V/cm, respectively (Figure S9). Also, the

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thickness of cake layers was larger at higher electric field strengths (Figure 6d-f).

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FIGURE 6

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As the electric field increases, the structure of the formed cake layers is increasingly

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influenced by orientation polarization.40 Negatively charged functional groups would orient

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toward the anode and positively charged groups would orient toward the cathode, which results

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in more functional groups oriented parallel to the direction of electric field. In addition, the

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surface charge of the foulants could be enhanced by displacement polarization.41 The foulant

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molecules would be stretched along the direction of electric field, and the end functional groups

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on foulants could concentrate more charges. Therefore, the interaction between foulants is

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strengthened in the direction parallel to electric field. In contrast, this interaction was weakened

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perpendicular to electric field, resulting in an increased pore size of cake layers. According to

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this mechanism, charged foulants under higher electric field strengths were more prone to be

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connected to attach to the membrane surface, contributing to an increase of cake layer thickness.

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FIGURE 7 8 ACS Paragon Plus Environment


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Figure 7 indicates that cake layers formed under higher electric field strengths were more

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hydrophilic than those formed at a lower electric field strength. As the electric field strength

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increased from 0.5 V/cm to 2 V/cm, the water contact angle of the fouled membrane surface

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decreased from 114.7±5.3 ° to 99.4±6.3 ° (Figure 7a). It has been reported that a more

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hydrophilic cake layer resulted in a greater membrane fouling resistance in UF.42 Furthermore,

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ATR-FTIR was used to characterize the surface functionality of the fouled membranes. The

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peaks at 2850~2880 cm-1, 2930 cm-1 (aliphatic C-H stretching), and 3300 cm-1 (O-H stretching

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vibration), which are characteristic for humic substances,36, 43-45 showed higher signals as the

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electric fields increased from 0 V/cm to 2 V/cm (Figure 7b). This result indicates an increase of

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humic acid amount in the cake layers, which was in agreement with the thicker cake layer

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observed at higher electric field strengths. Many functional groups of humic acid, such as

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carboxyl, hydroxyl and amine groups, are known to be influenced by electric field,40 in particular

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hydroxyl and amide groups that have higher electric dipole moments and are more readily

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polarized. In addition, charge distribution with foulant (humic acid) molecules was induced by

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the applied electric field, resulting in more charges being concentrated at both ends of the

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molecules along electric field direction. These polarized functional groups rendered humic acid

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more inclined to attach to the membrane surface at higher electric fields due to the stronger

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electrostatic force. As a result, cake layers with higher polarity were formed, exhibiting high

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affinity to water molecules and subsequently improving water permeability. An illustration of the

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proposed mechanism for the impact of electric field on fouling is summarized in Figure 8.

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FIGURE 8

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ECMR Fouling Mitigation Mechanisms and Implications for System Operation

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and Design. Particle size, which performed as a key factor in reducing membrane fouling in

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ECMR, was controlled by the current density and solution pH. Particles with sizes larger than the

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membrane pore size reduced membrane pore blocking and facilitated the formation of a cake

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layer that served as a protective layer. Meanwhile, a loose and hydrophilic cake layer was

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formed in ECMR, due to oriented polarization of foulants under the electric field. Charged

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foulants on the membrane surface were aligned in accordance with the direction of the applied

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electric field. The increased cake layer polarity induced by both coagulation and electric field

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further improved the affinity of the membranes to water. Therefore, the combined effect of 9 ACS Paragon Plus Environment

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coagulation and electric field in ECMR improved the fouling resistance of UF the membrane and

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resulted in a higher water flux.

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ECMR not only improves removal efficiency of organic matter but also greatly mitigates

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membrane fouling. ECMR performs better at high current densities and under weakly acidic pH

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conditions. It is also feasible to further minimize membrane pore blocking by starting the

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membrane filtration process after the floc size becomes larger than the membrane pore size.

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Furthermore, increasing the applied voltage can result in a more hydrophilic cake layer. The

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above operation conditions favor effective mitigation of membrane fouling in practice.

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Water production per unit volume of reactor can be increased with the design of multiple

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electrodes and membrane modules in the ECMR. The energy consumption per unit water

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production for ECMR at 2 A/m2 was comparable to that of EC-UF. When applying current

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densities of 10 A/m2 and 20 A/m2, ECMR reduces energy consumption by 6.4% and 1.3%,

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respectively, compared to EC-UF (Supporting information Table S2 and Equation S8). Therefore,

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energy savings could be achieved by changing the water treatment system configuration from

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EC-UF to ECMR. In addition, the land use of ECMR will be reduced due to a more compact

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reactor design.

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ASSOCIATED CONTENT

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Supporting information available: molecular weight distribution of humic acid (Figure S1);

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particle size distribution of kaolinite (Figure S2); ATR-FTIR spectra of humic acid and kaolinite

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(Figure S3); supplementary scheme for the design of ECMR (Figure S4); normalized water flux

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decline curves at different electric conductivity (Figure S5); calculation of particle size for light

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scattering measurements; fitted parameters for pore blocking resistance-limited model (Table

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S1); normalized water flux decline curves at different current densities in EC-UF (Figure S6);

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influence of ionic strength on EC (Figure S7); energy dispersion spectrum analysis (Figure S8);

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pore size analysis (Figure S9); parameters for energy consumption calculation (Table S2). This

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material is available free of charge via the internet at http://pubs.acs.org.

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AUTHOR INFORMATION 10 ACS Paragon Plus Environment


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Corresponding Author

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*Chengzhi Hu, Phone: +86-10-62849160; fax: +86-10-62849160; email: [email protected].

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript.

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ACKNOWLEDGMENT

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The authors are grateful for financial support from the National Natural Science Foundation of

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China (No. 51678556 and 51438011) and the Major Science and Technology Program for Water

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Pollution Control and Treatment (2015ZX07402003-3).

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17. Liu, L.; Liu, J.; Gao, B.; Yang, F.; Chellam, S., Fouling reductions in a membrane bioreactor using an intermittent electric field and cathodic membrane modified by vapor phase polymerized pyrrole. J. Membr. Sci. 2012, 394, 202-208. 18. Ronen, A.; Duan, W.; Wheeldon, I.; Walker, S.; Jassby, D., Microbial Attachment Inhibition through Low-Voltage Electrochemical Reactions on Electrically Conducting Membranes. Environ. Sci. Technol. 2015, 49, (21), 12741-12750. 19. Chen, J.-P.; Yang, C.-Z.; Zhou, J.-H.; Wang, X.-Y., Study of the influence of the electric field on membrane flux of a new type of membrane bioreactor. Chem. Eng. J. 2007, 128, (2-3), 177-180. 20. Fan, X.; Zhao, H.; Quan, X.; Liu, Y.; Chen, S., Nanocarbon-based membrane filtration integrated with electric field driving for effective membrane fouling mitigation. Water Res. 2016, 88, 285-292. 21. Tsai, Y.-T.; Weng, Y.-H.; Lin, A. Y.-C.; Li, K.-C., Electro-microfiltration treatment of water containing natural organic matter and inorganic particles. Desalination 2011, 267, (2-3), 133-138. 22. Chen, X.; Deng, H., Removal of humic acids from water by hybrid titanium-based electrocoagulation with ultrafiltration membrane processes. Desalination 2012, 300, 51-57. 23. Nghiem, L. D.; Schafer, A. I.; Elimelech, M., Pharmaceutical retention mechanisms by nanofiltration membranes. Environ. Sci. Technol. 2005, 39, (19), 7698-7705. 24. Zhu, B. T.; Clifford, D. A.; Chellam, S., Comparison of electrocoagulation and chemical coagulation pretreatment for enhanced virus removal using microfiltration membranes. Water Res. 2005, 39, (13), 3098-3108. 25. Tanneru, C. T.; Rimer, J. D.; Chellam, S., Sweep Flocculation and Adsorption of Viruses on Aluminum Flocs during Electrochemical Treatment Prior to Surface Water Microfiltration. Environ. Sci. Technol. 2013, 47, (9), 4612-4618. 26. Chen, Y.; Dong, B. Z.; Gao, N. Y.; Fan, J. C., Effect of coagulation pretreatment on fouling of an ultrafiltration membrane. Desalination 2007, 204, (1-3), 181-188. 27. Yu, W.; Hu, C.; Liu, H.; Qu, J., Effect of dosage strategy on Al-humic flocs growth and re-growth. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2012, 404, 106111. 28. Ho, C. C.; Zydney, A. L., A combined pore blockage and cake filtration model for protein fouling during microfiltration. J. Colloid Interface Sci. 2000, 232, (2), 389-399. 29. Yuan, W.; Kocic, A.; Zydney, A. L., Analysis of humic acid fouling during microfiltration using a pore blockage-cake filtration model. J. Membr. Sci. 2002, 198, (1), 51-62. 30. Lee, S.; Cho, J. W.; Elimelech, M., Combined influence of natural organic matter (NOM) and colloidal particles on nanofiltration membrane fouling. J. Membr. Sci. 2005, 262, (1-2), 2741. 31. Xiao, F.; Xiao, P.; Zhang, W. J.; Wang, D. S., Identification of key factors affecting the organic fouling on low-pressure ultrafiltration membranes. J. Membr. Sci. 2013, 447, 144-152. 32. Yuan, W.; Zydney, A. L., Humic acid fouling during microfiltration. J. Membr. Sci. 1999, 157, (1), 1-12. 33. Howe, K. J.; Marwah, A.; Chiu, K.-P.; Adham, S. S., Effect of coagulation on the size of MF and UF membrane foulants. Environ. Sci. Technol. 2006, 40, (24), 7908-7913. 34. Xu, Y.; Chen, T.; Cui, F.; Shi, W., Effect of reused alum-humic-flocs on coagulation performance and floc characteristics formed by aluminum salt coagulants in humic-acid water. Chem. Eng. J. 2016, 287, 225-232. 13 ACS Paragon Plus Environment

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35. Ching, H. W.; Elimelech, M.; Hering, J. G., Dynamics of Coagulation of Clay Particles with Aluminum Sulfate. J. Environ. Eng. 1994, 120, (1), 169-189. 36. Tremblay, L.; Alaoui, G.; Leger, M. N., Characterization of Aquatic Particles by Direct FTIR Analysis of Filters and Quantification of Elemental and Molecular Compositions. Environ. Sci. Technol. 2011, 45, (22), 9671-9679. 37. Cao, B.; Gao, B.; Liu, X.; Wang, M.; Yang, Z.; Yue, Q., The impact of pH on floc structure characteristic of polyferric chloride in a low DOC and high alkalinity surface water treatment. Water Res. 2011, 45, (18), 6181-6188. 38. Hu, C.; Sun, J.; Wang, S.; Liu, R.; Liu, H.; Qu, J., Enhanced efficiency in HA removal by electrocoagulation through optimizing flocs properties: Role of current density and pH. Sep. Purif. Technol. 2017, 175, 248-254. 39. Chen, M.-Y.; Lee, D.-J.; Yang, Z.; Peng, X. F.; Lai, J. Y., Fluorecent staining for study of extracellular polymeric substances in membrane biofouling layers. Environ. Sci. Technol. 2006, 40, (21), 6642-6646. 40. de Menezes, V. M.; Mota, R.; Zanella, I.; Fagan, S. B., Pristine and functionalized capped carbon nanotubes under electric fields. Phys. Status Solidi B 2014, 251, (3), 649-654. 41. Li, H.; Hu, T.; Zhang, R.; Liu, J.; Hou, W., Preparation of solid-state Z-scheme Bi2MoO6/MO (M=Cu, Co3/4, or Ni) heterojunctions with internal electric field-improved performance in photocatalysis. Appl. Catal. B: Environ. 2016, 188, 313-323. 42. Peter-Varbanets, M.; Margot, J.; Traber, J.; Pronk, W., Mechanisms of membrane fouling during ultra-low pressure ultrafiltration. J. Membr. Sci. 2011, 377, (1-2), 42-53. 43. Litvin, V. A.; Minaev, B. F., Spectroscopy study of silver nanoparticles fabrication using synthetic humic substances and their antimicrobial activity. Spectrochim. Acta A 2013, 108, 115-122. 44. Hatch, C. D.; Gierlus, K. M.; Zahardis, J.; Schuttlefield, J.; Grassian, V. H., Water uptake of humic and fulvic acid: measurements and modelling using single parameter Kohler theory. Environ. Chem. 2009, 6, (5), 380-388. 45. Jarusutthirak, C.; Amy, G., Role of soluble microbial products (SMP) in membrane fouling and flux decline. Environ. Sci. Technol. 2006, 40, (3), 969-974.

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Figure 1. Schemes for (a) the design of ECMR system: 1.4 L of synthetic feed water was recycled by two

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peristaltic pumps (36 mL/min) in order to generate cross-flow and keep a constant water level in the

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ECMR system. The membrane module was placed between two aluminum electrodes parallel to the water

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flow, with the distance between the two electrodes fixed at 2.5 cm. A pressure sensor was employed to

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monitor the transmembrane pressure, which was maintained at 30 kPa by a peristaltic pump. Permeate

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water was generated via peristaltic pump from UF membrane module and weighted by a digital balance,

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which connected to a computer. (b) Hollow fiber membrane module (one eighth was cut off in order to

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exhibit the inside structure of the membrane module). Water passed through membranes from the fiber

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outside (brown arrow), flowed inside the membranes (purple arrow) and was collected by a small tube on

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top of the membrane module (blue arrow).

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(a)

UF EC EC-UF ECMR

1.0 0.8

C/C0

0.6 0.4 0.2 0.0 0

Normalized Water Flux

1.1

10

20

30

40

50

(b)

60

ECMR EC-UF UF

1.0 0.9 0.8 0.7 0.6 0.5 0

447

2

4

6

8

10

12

14

Time (min)

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Figure 2. (a) Removal rate of humic acid (as measured by UV254) by UF, EC, EC-UF, and ECMR.

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Removal rates were obtained from three independent fouling experiments. (b) Final relative water flux

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(J/J0) of ECMR and EC-UF at 10 A/m2 and UF alone. Feed water was composed of sodium chloride (50

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mM), sodium bicarbonate (0.5 mM), humic acid (5 mg/L DOC), and kaolinite (50 mg/L). Initial solution

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pH was 7.0 and temperature was kept at 25 °C.

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Figure 3. (a) Normalized water flux decline curves at different current densities in ECMR; (b) particle

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size evolution with time at different current densities at initial pH 7. Feed solution composition is

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described in the caption of Figure 2.

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Figure 4. (a) Normalized water flux decline curves under different initial solution pH; (b) particle size

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evolution with time under different initial solution pH at 10 A/m2. Feed solution composition is described

462

in the caption of Figure 2.

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Figure 5. Normalized water flux decline curves under different initial electric field strengths. The

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corresponding initial electric conductivities were 500, 1000, and 2000 µS/cm for 2.00 V/cm, 1.00 V/cm,

467

and 0.50 V/cm, respectively; current density was 10 A/m at pH 7.0. Feed solution composition is

468

described in the caption of Figure 2.

2

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Figure 6. Scanning electron microscopy (SEM) images of fouled hollow fiber membrane surface under

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electric fields of (a) 0.5 V/cm, (b) 1 V/cm, and (c) 2 V/cm and cross-sections under electric fields of (d)

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0.5 V/cm, (e) 1 V/cm, and (f) 2 V/cm, respectively. The corresponding initial conductivities were 500,

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1000, and 2000 µS/cm, for 2 V/cm, 1 V/cm, and 0.5 V/cm, respectively; current density was 10 A/m2 at

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pH 7.0 and experiment time was 30 min. Feed solution composition is described in the caption of Figure

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2.

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Figure 7. (a) Contact angle and (b) attenuated total reflectance Fourier transform infrared spectroscopy

480

(ATR-FTIR) spectra of fouled membranes at different initial electric fields. The corresponding initial

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conductivities were 500, 500, 1000, and 2000 µS/cm, for 0 V/cm, 2 V/cm, 1 V/cm, and 0.5 V/cm,

482

respectively; the current density was 10 A/m2 at pH 7.0. Contact angle measurements were repeated three

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times.

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Figure 8. Schematic description of the proposed mechanisms for membrane fouling mitigation in ECMR.


Performance and Mechanisms of Ultrafiltration Membrane Fouling

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