Carbon-nanotube-based sandwich-like hollow fiber membranes for expanded microcystin-LR removal applications

Accepted Manuscript Carbon-Nanotube-Based Sandwich-Like Hollow Fiber Membranes for Expanded Microcystin-LR Removal Applications Gaoliang Wei, Xie Quan, Xinfei Fan, Shuo Chen, Yaobin Zhang PII: DOI: Reference:

S1385-8947(17)30290-5 http://dx.doi.org/10.1016/j.cej.2017.02.125 CEJ 16564

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

22 December 2016 21 February 2017 23 February 2017

Please cite this article as: G. Wei, X. Quan, X. Fan, S. Chen, Y. Zhang, Carbon-Nanotube-Based Sandwich-Like Hollow Fiber Membranes for Expanded Microcystin-LR Removal Applications, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.02.125

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Carbon-Nanotube-Based Sandwich-Like Hollow Fiber Membranes for Expanded Microcystin-LR Removal Applications Gaoliang Wei, Xie Quan*, Xinfei Fan, Shuo Chen, Yaobin Zhang

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Corresponding author: E-mail: [email protected]; Phone: +86-411-84706140; Fax: +86-411-84706263;

1

Abstract The worldwide presence of harmful micropollutants in water resources drives the development of innovative and energy-efficient water treatment technologies. Herein, a novel carbon-nanotube-(CNT)-based hollow fiber membrane, with a sandwich-like structure in its cross section, is designed and prepared for expanded micropollutant removal under electrochemical assistance. The CNT membranes consist of (1) outer CNT layer as separation layer, (2) middle porous polyvinylidene fluoride layer and (3) inner CNT layer as support. Apart from their intrinsic functions as separation membranes, they can construct a complete electrochemical system, in which two CNT layers are also designed as electrodes and the PVDF

layer

as

insulating

separator.

Low-concentration microcystin-LR

can be

cost-efficiently and continuously removed (>99.8%) by these CNT ultrafiltration membranes through facile switches between adsorption and desorption/electrochemical oxidation. Such switches can be achieved at a high flux of 500 L m−2 h−1 without terminating filtration process. Degradation product analysis has evidenced the breaking of Mdda chains that are largely responsible for the toxicity of microcystins. This work synergistically combines adsorption and electrochemistry with membrane separation, and highlights their potentials for advanced wastewater treatment and drinking water purification. KEYWORDS: membrane, electrochemical oxidation, carbon nanotube, adsorption, microcystin-LR

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1. Introduction Thousands of industrial and natural chemical compounds resulting in the increasing worldwide contamination of freshwater systems have raised considerable concerns. [1,2] Although some of these organic compounds in water are usually present at low concentrations (µg/L or ng/L), many of them show notable toxicological effects, high persistence as well as long-range transport, commonly known as organic micropollutants. As one of typical micropollutants, microcystins have revealed the acute lethal toxicity through inhibition of protein phosphatase-1 and -2A, leading to liver damage and tumor promotion. [3,4] Among more than 150 types of microcystins identified, [5] microcystin-LR (MC-LR, Fig. S1 of Supporting Information) is one of the most frequently reported and most toxic microcystins, [6] with a worldwide distribution from rivers to lakes. The World Health Organization has consequently proposed a provisional guideline value of 1 µg/L of MC-LR in drinking water. [7] Due to their cyclic and stable structure, [8] conventional biological treatments cannot provide efficient elimination. [9,10] High costs associated with chemical processes generally make them unaffordable in practical treatments. Adsorption-based processes, regarded as a simple, effective and relatively cost-saving technology for the removal of MCs, have been widely investigated using a variety of adsorbents such as activated carbon, [11] peats, [12] mesoporous silica, [10] mesoporous carbon materials. [13,14] However, it is still a challenge for the energy-efficient regeneration of these materials after adsorption saturation. As a potentially viable alternative, nanofiltration-based treatment technology has demonstrated a sustained high removal efficiency for MC-LR. [15] Combined with their relatively low-cost 3

and environmentally sustainable advantages, nanofiltration membranes have been thus practically applied for wastewater advanced treatment and domestic drinking water purification. The retention of micropollutant molecules (including MC-LR) by nanofiltration membranes can be generally achieved through size exclusion, adsorption onto membrane, and charge repulsion. [16] Steric exclusion is a first mechanism, in which pollutant molecules larger than the membrane pore size (about 1 nm) are normally retained because of a sieving effect. Some micropollutant molecules with similar or smaller size than the membrane pores can be also retained due to charge repulsion between membranes and the micropollutants, or their interfacial adsorption onto membranes. [17-20] However, their small pore size generally results in extremely low water permeability, greatly reducing treatment efficiency and increasing cost. Additionally, MC-LR molecules retained on membranes should be further decomposed in principle to avoid secondary pollution. To overcome these technical limitations, we herein report a novel sandwich-like heterogeneous carbon nanotube (CNT) hollow fiber ultrafiltration membrane composed of three layers in their cross section: (1) outer CNT layer as separation layer and cathode, (2) porous PVDF insulating separator and (3) inner CNT layer as support and anode. Apart from its intrinsic sieving capability, the CNT cathode is also designed in this membrane structure to capture low-concentration MC-LR with a high dynamic adsorption capacity, and can also readily desorb these molecules in presence of a low voltage, achieving its regeneration for sequential adsorption toward MC-LR. The MC-LR desorbed from cathode will penetrate the porous PVDF separator and can be subsequently electrochemically decomposed by CNT 4

anode. Thus, through facile switches between adsorption and electrochemical oxidation, micropollutants can be continuously removed by these bipolar CNT ultrafiltration membranes at high flux. We design and construct this membrane structure aiming at reserving the intrinsic sieving capability of ultrafiltration membranes for removal of viruses, bacteria and colloids, as well as coupling membrane adsorption and chemical oxidation for removal of pollutant molecules. 2. Experimental sections 2.1. Chemicals and materials. The pristine multiwalled CNTs with 40–60 and 60–100 nm in outer diameters were commercially obtained from Shenzhen Nanotech Port Co., Ltd., China. Unless otherwise stated, “60–100 nm CNTs” and “40–60 nm CNTs” denote the CNTs with outer diameters of 60–100

nm

and

40–60

nm,

respectively.

N,N-Dimethylacetamide

(DMAc)

and

N,N-Dimethylformamide (DMF) were provided by Tianjin Fuyu Fine Chemical Co., Ltd., China. Polyvinylidene fluoride (PVDF) powders with average molecular weight of 534 kDa were commercially available from Sigma-Aldrich Co. LLC. Polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP, K30) and polyvinyl alcohol (PVA) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Microcystin-LR (purity ≥95%) samples were purchased from Dalian Bioscizone Biotechnology Co., Ltd., China. Ultrapure water -1

(resistivity >18.0 MΩ cm ) obtained from a purification system (Laikie Instrument Co., Ltd., Shanghai, China) was used in all experiments. 2.2. Surface functionalization of CNTs. About 3 g CNTs were added into a mixture of 40 mL concentrated nitric acid and 120 mL 5

concentrated sulfuric acid, and the mixture was subsequently kept at 60 °C for 30 min under stirring. Then the CNTs were recovered by vacuum filtration after diluting the mixture with about 4 L water. After being washed for several times until pH of the filtrate was about 7.0, the CNTs were dried at 60 °C for 6 h. 2.3. Preparation of common CNT hollow fiber membranes. The common CNT hollow fiber membranes were prepared according to the method reported in our previous work. [21] In a typical experiment, 3 g of surface-functionalized 60–100 nm CNTs and 1.5 g PVB were added in 36 g DMF to form homogeneous spinning solution under assistance of sonication and stirring. The spinning solution was subsequently squeezed into water through the outer stainless-steel capillary of spinneret by a constant-flow pump, accompanied with a water flow within the core capillary of spinneret. The obtained CNT/PVB hollow fibers were dried at room temperature (20 °C) after being washed for several times, and then calcined at 600 °C for 2 h in Ar flow. 2.4. Preparation of sandwich-like CNT hollow fiber membranes. Typically, 4 g PVDF powders, 1 g water and 3 g PVP powders were dissolved in 16 g DMAc at 60 oC under stirring to form a homogeneous casting solution. Then, common CNT hollow fiber membranes were dipped into the PVDF casting solution (60 oC) for 30 s and then pulled out at a speed of 0.5 mm s-1 with a high-precision dip coater. The CNT membranes coated with PVDF layers were immediately immersed into water for 12 h to remove PVP and DMAc. Subsequently, they were vertically immersed into the 40 –60 nm CNT ethanolic dispersion (5 g L-1) with one end sealed. When the other end was connected to a vacuum pump, the 40–60 nm CNTs would be filtered on the surface of CNT/PVDF fibers 6

to form a compact layer, whose thickness was controlled by the volume of CNT dispersion filtered. At last, the sandwich-like CNT hollow fiber membranes were immersed into 0.5 wt% PVA aqueous solution for 10 min, followed by being dried at 80 oC for 6 h to reinforce the membranes. 2.5. MC-LR removal experiments. All performance tests of the membranes were performed using a dead-end membrane filtration system, in which voltages were supplied by a DC power (Fig. S2). Two filtration modules with different structures were designed for common and sandwich-like CNT hollow fiber membranes, respectively. In the module for common CNT hollow fiber membranes, a stainless steel network was bent into a cylinder as cathode, and the membranes as anode were positioned in its axis with a distance from it of 0.5 cm (schematically shown in Fig. S2). The initial concentration of MC-LR is 0.5 mg L-1 in feed containing 0.05 M Na2SO4 as electrolyte. The concentrations of MC-LR in filtrate and their degradation products were analyzed by an Agilent 1100 HPLC-tandem an Agilent 6410 Triple Quadruple mass spectrometer (MS/MS) with full scan from mass-to-charge ratio (m/z) 100 to 1500 in positive electrospray ionization (ESI) mode. The mobile phase was 0.1% formic acid (v/v) and acetonitrile (70:30, v/v) at a flow rate of 0.3 mL min-1. 3. Results and discussion 3.1. Preparation and characterization of sandwich-like CNT hollow fiber membranes. The sandwich-like CNT membranes are prepared with a sequential layer-by-layer coating method. After a facile dip-coating procedure, the common CNT hollow fiber membrane is wrapped with a 100-µm-thick PVDF layer, forming a heterogeneous two-layered 7

configuration, as shown in scanning electron microscope (SEM) images (Fig. 1(a,b)). Magnified outer surface reveals a porous structure (Fig. 1(c)). Further deposition of 40–60 nm CNTs on this CNT/PVDF hollow fiber by vacuum filtration will creates three-layered CNT membranes (Fig. 1(d,e)) composed of (1) 40–60 nm CNT outer layer, (2) porous PVDF middle layer and (3) 60–100 nm CNT inner layer. These three layers with a concentric circle configuration reveal a sandwich-like structure. The magnified membrane surface reveals a random pore structure, characterized by an interwoven network of CNTs (Fig. 1(f)). As demonstrated in our previous work, the thickness of 60–100 nm CNT layer (common CNT hollow fiber membrane) can be facilely controlled. [21] And the 40–60 nm CNT layer, involved in this work, could be also easily tunable in their thickness by controlling the volume of CNT dispersion filtered. Thus, various desirable sandwich-like CNT membranes can be accordingly constructed. Taking the one presented in Fig. 1(d) for an example, we will investigate its performance and elaborate its unique function.

8

Fig. 1. SEM images of CNT/PVDF hollow fibers (a-c) and sandwich-like CNT membranes (d-f): (a,d) cross senctions; (b,e) magnified cross sections; (c,f) magnified membrane surfaces.

The micrometer-sized pores (Fig. 1(c)) in PVDF layer will greatly decrease hydrodynamic transfer resistance from outer to inner CNT layer. Additionally, the embedded PVDF layer as skeleton in this membrane structure will significantly increase their mechanical strength, which has been confirmed by stress-strain curves (Fig. 2(a)). As revealed, the tensile strength of common CNT hollow fiber membrane is measured to be 6.3 MPa, and it increases to 10.0 MPa for sandwich-like hollow fiber membranes. As shown in Fig. 2(b), fluxes of the sandwich-like membranes linearly increase with an increase in pressure, indicating that their pore channels are not compressed under a pressure below 1 bar. Typically, they can afford a flux of 1050 ± 210 L m-2 h-1 bar-1 with a pore size of 67 nm (Fig. 2(b), inset). Based on the relation between nanofiber diameter and resultant pore size, [21] the 40–60 nm CNT layer will possess smaller pore size than both PVDF and 60–100 nm CNT layer. It thus can function as the actual separation layer of the heterogeneous membranes, which will effectively decrease the hydrodynamic resistance. Consequently, despite the large thickness, their flux is still higher than that of many commonly-used ultrafiltration membranes with 2–100 nm pore sizes. [22]

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Fig. 2. (a) Typical stress-strain curves of prepared common and sandwich-like CNT hollow fiber membranes. (b) Water flux of sandwich-like CNT membranes as a function of pressure. Inset shows their pore size distribution measured by a porometer (poroluxTM 1000, Germany). 3.2. Switch between adsorption and desorption/electrochemical oxidation for MC-LR removal. 3.2.1. Electrochemical oxidation of MC-LR on common CNT hollow fiber membranes. Common CNT hollow fiber membranes (details are shown in Fig. S3) are firstly used to investigate adsorption and electrochemical oxidation of MC-LR molecules on CNTs. CNT-based membranes can provide abundant active pore channel interfaces that can non-covalently interact with a great many molecules through π-π coupling (between bulk π systems on CNT surfaces and organic molecules with C=C double bonds or benzene rings), hydrogen bonds (because of the functional groups on CNT surfaces), electrostatic interactions (because of the charged CNT surface), and surface condensation (multilayer adsorption). [23-25] MC-LR molecules structurally have benzene ring, conjugated C=C double bonds, hydroxyls, and ionizable amides and carboxyls. As a consequence, the CNT-based membranes will possess an ultrahigh adsorption capacity towards MC-LR and can thus efficiently remove these small molecules from water with 120 nm pores through adsorption. 10

Based on the breakthrough curves shown in Fig.3(a), the prepared common CNT hollow fiber membranes can afford a removal of >99.8% for MC-LR before 70 min when the influent (0.5 mg L−1) is performed with a single pass through the membranes, demonstrating a dynamic adsorption capacity of about 9.8 mg g-1. While for commercial PVDF membranes and Al2O3 ceramic membranes with the similar effective mass, complete removal of MC-LR is observed only before 0.5 min and 0.3 min, respectively, which suggests the dynamic adsorption capacity of CNT membranes is about 150-200 times higher than that of these two membranes. Additionally, the >99.8% removal could be achieved at a very high flux of 860 ± 50 L m−2 h−1 which is about 10−100 times higher than that of most currently commercially available

MC-LR concentration (mg L-1)

MC-LR concentration (mg L-1)

nanofiltration membranes capable of rejecting MC-LR. (a)

0.5 0.4 0.3

CNT membrane PVDF membrane Al2O3 membrane

0.2 0.1 0.0 0

1

2

3

4 60 Time (min)

80

100

(b)

0.5 2.0 V 2.5 V 3.0 V

0.4 0.3 0.2 0.1 0.0 0

10

20 30 Time (min)

40

Fig. 3. (a) Breakthrough curves for adsorption of MC-LR by CNT membranes, PVDF membranes and Al2O3 membranes. (b) Concentrations of MC-LR as a function of duration at various voltages. Concentration of MC-LR in feed is 0.5 mg L-1; Na2SO4 concentration is 0.05 M; pH is ~6.4.

After adsorption saturation, the CNT membrane is given a positive potential without stopping the filtration process. As shown in Fig. 3(b), at a voltage of 2.0 V, the concentration of MC-LR in filtrate decreases to about 0 mg L-1 in 38 min. This is because MC-LR can be electrochemically degraded on CNT membranes. It is also shown that, if the voltages are 11

increased to 2.5 and 3.0 V, the time can be further decreased to 30 min and 18 min, respectively. The improved efficiency may be attributed to the increased electron transfer kinetics with increasing potential. [26] It is should be emphasized that, in real water, the concentration of MC-LR is much lower than 0.5 mg L-1 used in this work, which will greatly inhibit the conventional batch bipolar electrochemical reactions because of limited mass transfer. [26] Contrastingly, convective mass transport in membrane filtration process and active surfaces of CNTs will greatly benefit the capture and enrichment of MC-LR on CNTs, thus avoiding the mass transfer limitation. Unfortunately, continuous application of a potential will significantly increase the treatment cost, making the electrochemical membrane filtration technology less affordable for practical applications. 3.2.2. Intermittent removal of MC-LR by sandwich-like CNT hollow fiber membranes. It will be an effective strategy to decrease cost by reducing the voltage-applied time on the CNT membranes under conditions of complete and continuous removal of the MC-LR. We consequently design the sandwich-like hollow fiber membrane containing both anode and cathode. After adsorption saturation on 40-60 nm CNT cathode, negatively charged MC-LR molecules (pH 6.4) can be readily desorbed due to the electrostatic repulsion at a low voltage, and subsequently penetrate the porous PVDF layer to arrive at CNT anode where they can be directly or indirectly electrochemically decomposed. This process will be finished in several minutes. By switching off the electricity, the cathode component can be recovered to trap MC-LR molecules again. To evaluate the design, we firstly filter the MC-LR solution (0.5 mg L-1) containing 10 mM Na2SO4 at a permeation flux of 500 L m-2 h-1 for 60 min, then a voltage of 2.0 V is applied on the membrane. As shown in Fig. 4(a), the bipolar CNT 12

membranes are recovered in 6 min and can remove MC-LR again. In addition, this process can be continuously repeated and no obvious deterioration in their removal ability towards

40

-1

Concentration of MC-LR (mg L )

-1

Concentration of MC-LR (mg L )

MC-LR is observed after three “adsorption-desorption/oxidation” cycles.

(a)

Electricity on

30 20 10

Electricity off

0 0

40

80

120 160 200 240 Time (min)

40

(b)

2.0 V

30 20 10 0 0

1

2

3 4 Time (min)

5

6

7

-1

20 (a)

Concentration of MC-LR (mg L )

-1

Concentration of MC-LR (mg L )

Fig. 4. (a) Three “adsorption-desorption/oxidation” cycles on bipolar CNT membranes. (b) The concentration of MC-LR as a function of time at a votage of 2.0 V.

2.5 V

15 10 5 0 0

1

2 3 Time (min)

4

5

6 (b)

3.0 V

5 4 3 2 1 0 0

1

2 Time (min)

3

4

Fig. 5. Concentration of MC-LR as a function of time at a votage of 2.5 V (a) and 3.0 V (b). 3.2.3. Continuous removal of MC-LR by sandwich-like CNT hollow fiber membranes. It is shown in Fig. 4(b) that, at a voltage of 2.0 V, the concentration of MC-LR demonstrates an obvious increase in 3.0 min, followed by a significant decrease from 38.0 to 0 mg L-1, resulting in a maximum concentration Cmax of 38.0 mg L-1 in filtrate. This is because that MC-LR molecules are rapidly desorbed from CNT cathode and the high-concentration MC-LR cannot be completely and timely oxidized. Higher Cmax generally 13

represents higher desorption efficiency than that of electrochemical oxidation. To a very great extent, voltages determine both desorption rate and electrochemically oxidative efficiency. As shown in Fig. 5, Cmax will decrease to 18.2 mg L-1 at 2.5 V, and further decrease to 5.8 mg L-1 if the voltage increases to 3.0 V. This negative correlation suggests that increasing voltages will result in higher oxidation efficiency than that of desorption. On the basis of these findings, we have shortened the adsorption duration to achieve the complete and continuous removal of MC-LR. As shown in Fig. 6(a), if the adsorption duration is reduced to 30 min, MC-LR can be completely oxidized at 2.0 V when they pass through anode layer. After a 5-min presence of the voltage, the bipolar CNT membranes are recovered and can subsequently adsorb MC-LR again, achieving their continuous removal in whole process. If the voltage is increased to 3.0 V, adsorption duration can be correspondingly increased to 45 min and the membranes can be recovered in 3 min on condition of continuous removal of MC-LR (Fig. 6(b)). As designed, through facile switches between adsorption on cathode and electrochemical oxidation on anode, MC-LR can be energy-efficiently and continuously removed.

(b) 100 Electricity Off Electricity On (2 V)

MC-LR removal (%)

MC-LR removal (%)

(a) 100

95

90 0

20

40 60 Time (min)

80

100

Electricity Off Electricity On (3 V)

95

90 0

20

40

60 80 100 120 140 Time (min)

Fig. 6. Continuous removal of MC-LR by bipolar CNT hollow fiber membranes through switches between adsorption and electrochemical oxidation at 2.0 V (a) and 3.0 V (b). 14

3.2.4. Identification of the degradation intermediates. The intermediate products are identified by LC-MS/MS to investigate the electrochemical oxidation pathways of MC-LR and evaluate the toxicity of water after treatment. As shown in the total ion chromatogram (TIC) of original MC-LR solution (Fig. S4(a)), an intense peak at a retention time of 5.185 min is observed. The peak, corresponding to m/z 995.7 in the mass spectrometric analysis (Fig. S5(a)), is consistent with MC-LR ([M+H]+/z). In the TIC of filtrate (Fig. S4(b)) collected at 2 V (red curve in Fig. 6(a)), the peak at 5.185 min disappears, and intermediates with m/z 835.5, 967.6, 983.6, 1001.5, 1011.6 and 1029.6 are observed (Fig. S5(b-f)), suggesting that all MC-LR molecules have been electrochemically decomposed by the CNT bipolar membranes. Previous works have demonstrated that the aromatic ring undergoes hydroxyl substitution of an aromatic hydrogen to form the m/z 1011.5 intermediate, [27] whose mechanistic steps consist of the addition of a hydroxyl radical (HO•) in one of the aromatic double bonds to form a carbon-centered radical and its rapid reaction with oxygen to form a peroxy radical. [28] The intermediates with m/z 983.6 could be the reaction products of the m/z 1011.5 intermediate at the free carboxyl group of the Glu residue after a series of free-radical-involved reactions [29]. Different aromatic hydrogen of Adda chain (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid) being substituted by hydroxyls would explain why m/z 983.6 exhibits multiple peaks (Fig. S4(b)).

15

Fig. 7. Possible electrochemical degradation pathway of MC-LR on bipolar CNT hollow fiber membranes. It is generally accepted that conjugated double bonds of Adda chain is most prone to oxidation mainly because of its position in the toxin molecule. [28] The m/z 1029.6 is intermediate product related to double hydroxyl addition. The double hydroxylation (1,2 addition) can occur on any of the double bonds pairs, resulting in two kinds of intermediates with same m/z 1029.5. Their further hydroxyl addition can generate intermediate (m/z 1063.6), of which catalytic hydrolysis in peptide bonds is rapid and loss of a MeAsp fragment ultimately produces intermediate product (m/z 967.6). Another conversion route in which m/z 1029.5 intermediates will lose the carbonyl group (−C=O) at the free carboxyl group of the Glu residue produces m/z 1001.5 intermediates, similar with the source of m/z 983.6 product. Additionally, a series of conversions inducing bond cleavage of Mdda chain produce a ketone-derivative (m/z 835.5). [28] On the basis of above analysis, we have proposed a possible electrochemical degradation pathway of MC-LR on the bipolar CNT membranes 16

(Fig. 7). It deserves mentioning that, apart from MC-LR molecules, other charged contaminant molecules can also be efficiently removed from water with same strategy by the bipolar CNT hollow membranes. 4. Conclusions In conclusion, a novel hollow fiber membrane containing both CNT cathode and CNT anode in an electrochemical filtration system is prepared with a sequential layer-by-layer coating method for the efficient removal of organic micropollutant molecules. As revealed, the CNT-based bipolar membranes with unique heterogeneous structure possess a high water flux, especially the capacity for synergistic integration of adsorption and electrochemical oxidation into a simple membrane filtration system besides the intrinsic sieving function. They are consequently competent in the low-cost and continuous purification of water contaminated by MC-LR at high permeation flux through facile switches between adsorption and electrochemical oxidation on this bipolar membrane. This study implies a strong possibility for their practical applications for energy-efficient drinking water purification, and may serve as an inspiration for the design of new and advanced membranes for a variety of other important applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (21437001) and the Programme of Introducing Talents of Discipline to Universities (B13012).

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Carbon-Nanotube-Based Sandwich-Like Hollow Fiber Membranes for Expanded Microcystin-LR Removal Applications Gaoliang Wei, Xie Quan*, Xinfei Fan, Shuo Chen, Yaobin Zhang

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Graphical Abstract

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Carbon-Nanotube-Based Sandwich-Like Hollow Fiber Membranes for Expanded Microcystin-LR Removal Applications Gaoliang Wei, Xie Quan*, Xinfei Fan, Shuo Chen, Yaobin Zhang

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Highlights: (1) A novel carbon-nanotube-based sandwich-like hollow fiber membrane was prepared. (2) Low-concentration microcystin-LR was cost-efficiently and continuously removed. (3) Facile switches between adsorption and desorption/electrochemical oxidation for MC-LR removal.

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