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Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions
Journal Pre-proofs Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions Li-Ping Zhang, Zhuang Liu, Xing-Long Zhou, Chuan Zhang, Quan-Wei Cai, Rui Xie, Xiao-Jie Ju, Wei Wang, Yousef Faraj, Liang-Yin Chu PII: DOI: Reference:
S1383-5866(19)33682-2 https://doi.org/10.1016/j.seppur.2019.116364 SEPPUR 116364
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
17 August 2019 16 October 2019 28 November 2019
Please cite this article as: L-P. Zhang, Z. Liu, X-L. Zhou, C. Zhang, Q-W. Cai, R. Xie, X-J. Ju, W. Wang, Y. Faraj, L-Y. Chu, Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/ j.seppur.2019.116364
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Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions
Li-Ping Zhanga, Zhuang Liua,b,*, Xing-Long Zhoua, Chuan Zhanga, Quan-Wei Caia, Rui Xiea,b, Xiao-Jie Jua,b, Wei Wanga,b, Yousef Faraja,b, Liang-Yin Chua,*
a b
School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,
Sichuan 610065, China
*Corresponding
authors at: School of Chemical Engineering, Sichuan University, Chengdu,
Sichuan 610065, China. E-mail address: [email protected] (Z. Liu); [email protected] (L. Y. Chu). 1
Abstract Novel composite membranes are successfully developed for simultaneous adsorption of heavy metal ions and catalytic degradation of organic contaminants by blending polydopamine-coated ferroferric oxide (Fe3O4@PDA) nanoparticles in polyethersulfone (PES) matrix via liquidinduced phase separation (LIPS) method.
The permeabilities of the as-fabricated composite
membranes are significantly improved by introducing Fe3O4@PDA nanoparticles, because the hydrophilic coating on the surface of Fe3O4@PDA nanoparticles enables to generate more microporous structures between the PES matrix and nanoparticles during the LIPS process. The composite membrane containing 20 wt% Fe3O4@PDA nanoparticles shows water flux as high as 2640 L∙m-2∙h-1∙bar-1, which is six times larger than that of PES blank membrane.
The
composite membranes exhibit efficient and repeatable performances for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants based on the mechanism of Fenton-like reaction.
The proposed composite membranes with
multifunction of simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions provide a new pathway to deal with some hard-to-be-treated wastewaters from papermaking, leather, textile printing-dyeing industries, and so on.
Furthermore, the proposed
one-step strategy for fabricating composite membranes with both large permeability and high separation efficiency in this work is easy to be scaled up.
Keywords Composite membranes; Fe3O4@PDA nanoparticles; Fenton-like reaction; Degradation of organic contaminants; Removal of heavy metal ions
2
1. Introduction
An abundant of wastewater containing organic contaminants and heavy metal ions are produced in the papermaking, leather, and textile printing-dyeing industries [1-6].
Such
wastewaters are prohibited to discharge into the environment before they have been treated, because both organic dye contaminants and heavy metal ions are particularly hazardous to the ecological system [7-12].
Wastewater treatment processes, such as sedimentation, adsorption
and oxidation, enable to remove most of the organic pollutants and heavy metal ions; while trace contaminants and heavy metal ions are usually still remained in the pretreated water [13]. It is essential to develop advanced technologies to simultaneously remove such trace organic contaminants and heavy metal ions from wastewaters.
To further deal with the pretreated
wastewater, several methods have been developed, such as membrane filtration, biological and catalytic degradation [13-18].
The membrane technology is considered to be preferable owing
to its many advantages such as functional selectivity, high separation efficiency, simple process, low energy consumption, flexibility, and easy integration ability with other technologies [1922]. Up to now, three main categories of membrane technologies have been developed to simultaneously remove the organic contaminants and heavy metal ions from wastewaters, which are dual-rejection mode, rejection-adsorption mode, and dual-adsorption mode [13, 2331].
In principle, both organic contaminants and heavy metal ions could be excluded by
reverse osmosis and nanofiltration membrane technologies based on molecular size and charge. Such processes commonly suffer from high pressure to drive the solution permeating through the membrane [32].
Since the sizes of organic molecules are usually larger than those of ions, 3
some membranes are developed with enhanced permeability to reject organic contaminants but adsorb heavy metal ions via electrostatic attraction and/or complexation.
For example,
Moradi and co-workers [26] reported polyethersulfone (PES) nanofiltration membranes with magnetic graphene-based additives, which can directly reject Red 16 by electrostatic repulsion and remove copper ions (Cu2+) by adsorption. 1∙bar-1.
The flux of their membrane is only 8.3 L∙m-2∙h-
Hou and co-workers [33] developed an ultrafiltration membrane made of
polydopamine-(β-cyclodextrin)-modified graphene oxide sheets by drop-coating with vacuum filtration.
This membrane enables to reject the methylene blue by size sieving and achieve the
adsorptive removal of heavy metal lead(II) ions with a flux of 120 L∙m−2∙h−1∙bar-1.
To further
improve the processing efficiency, the dual-adsorption mode of membrane technology has been developed based on electrostatic attraction, chemical bonding, and/or physical bonding between the membranes and organic contaminants as well as heavy metal ions.
For instance, Xu and
co-workers [28] prepared a 3D polyurethane/polyacrylic acid membrane using solvent casting, salt leaching and thermal crosslinking.
The membrane can adsorb heavy metal lead(II) ions
and organic dye methylene blue with the removal ratio of 99.7% and 99.6% respectively, and possesses the water flux of 1085.1 L∙m-2∙h-1 under 0.06 MPa.
Yu and co-workers [13]
prepared a free-standing membrane made of carbon nanofibers to adsorb methylene blue and heavy metal ion pollutants effectively with a trans-membrane flux of 1580 L∙m-2∙h-1.
In
general, these previously reported membranes are developed to remove organic contaminants and heavy metal ion pollutants from wastewater.
However, during the membrane filtration
processes, the organic contaminants adsorbed on the membrane surfaces could cause the membrane fouling, resulting in the decrease of water flux [34, 35].
4
Even after the desorption
of organic contaminants and regeneration of membranes, the membranes could not be completely recovered [36].
Therefore, it is still highly desirable to develop novel efficient
membrane technology to remove organic contaminants and heavy metal ions simultaneously from wastewaters. In this work, we report on a novel polyethersulfone (PES) composite membrane containing polydopamine-coated ferroferric oxide (Fe3O4@PDA) nanoparticles for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants from water.
The composite membrane is easily prepared by blending Fe3O4@PDA nanoparticles
in PES membrane via liquid-induced phase separation (LIPS) approach (Fig. 1a, b).
PES is
chosen as the membrane substrate, because of its advantages of proper mechanical property, as well as outstanding oxidative, and chemical and thermal stability [37, 38].
The Fe3O4
nanoparticles are synthesized by hydrothermal method, followed by coating PDA layer via the self-polymerization property of dopamine in a weak alkaline aqueous solution to obtain Fe3O4@PDA nanoparticles [39-41].
The prepared Fe3O4@PDA composite nanoparticles are
blended into the PES membrane in the LIPS process.
The Fe3O4@PDA nanoparticles in PES
composite membrane could adsorb heavy metal ions by abundant functional groups in PDA coating layer on the surface of nanoparticles (Fig. 1c), and simultaneously produce active hydroxyl radicals (∙OH) to degrade organic contaminants based on Fenton-like reaction (Fig. 1d).
The hydrophilic PDA coating on the surface of nanoparticles enables to produce more
microporous structures between the PES matrix and nanoparticles during the LIPS to achieve large water flux (Fig. 1b).
The phenolic hydroxy and amino groups on the surface of the PDA
coating could chelate the cations to achieve the adsorption of heavy metal ion pollutants [425
44].
Moreover, electron transfer in the process of Fenton-like reaction can be promoted due
to phenoquinone structure on the surface of the PDA coating, which is beneficial to accelerate the catalytic reaction [45-47].
In principle, the fabrication of the proposed PES composite
membranes by LIPS approach can be easily and scalably acheived by using the existing membrane casting equipment.
The proposed PES composite membranes could be featured
with a large water flux and simultaneous removal of heavy metal ions by adsorption and organic contaminants by catalytic degradation from wastewater.
Fig. 1.
(a, b) Schematic illustrations of cross-sectional structure of the proposed PES
composite membrane by blending Fe3O4@PDA nanoparticles in PES via LIPS (a) and the enlarged view of the cross-section near surface of the PES composite membrane (b).
(c, d)
The mechanisms of the adsorption of heavy metal ions (c) and the catalytic degradation of organic contaminants by Fenton-like reaction (d) with the blended Fe3O4@PDA nanoparticles.
6
2. Materials and methods 2.1. Materials
Ethylene glycol (EG), ferric chloride hexahydrate (FeCl3∙6H2O), hydrogen peroxide (H2O2, 30 wt%), ammonia solution (NH3∙H2O, 30 wt%), sodium acetate (NaAc), N-methylpyrrolidone (NMP), methylene blue (MB), tri-hydroxymethyl-aminomethane (Tris), hydrochloric acid (HCl), dopamine hydrochloride (DA ∙ HCl), ethanol, lead nitrate, cadmium nitrate and silver nitrate were purchased from Chengdu Ke Long Chemicals without further purification. with a molecular weight of 66 kDa was purchased from BASF.
PES
Deionized water with
resistance of 18.2 MΩ∙cm used through the whole experiments came from Millipore Elix-10 water purification system.
2.2. Synthesis and characterization of Fe3O4@PDA nanoparticles
The Fe3O4 nanoparticles (Fe3O4 NPs) were synthesized according to the previous work [39].
The prepared Fe3O4 NPs were coated with a PDA layer via the oxidative self-
polymerization of dopamine in a weak alkaline aqueous solution [40].
Firstly, 0.1 g of Fe3O4
NPs was suspended in 50 mL of Tris-HCl solution (15 mM, pH 8.5) ultrasonically to obtain 2 mg/mL Fe3O4 NPs dispersion solution.
Secondly, 0.1 g of DA-HCl was put into the above
solution and stirred mechanically at ambient temperature for 6 h.
After the reaction was
completed, the obtained Fe3O4@PDA composite nanoparticles were isolated from the reaction solution using a strong magnet and cleaned alternately with ethanol and water for five times 7
and dried for several hours. The micromorphology and size of the Fe3O4@PDA composite nanoparticles in the dried state were observed by a transmission electron microscope (TEM, JEM-2100, JEOL).
The
Fe3O4@PDA samples were obtained by droplet casting NPs suspension on the carbon-coated copper grid and dried.
X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos) tests were
performed with Kratos Axis spectrometer attached with a monochromatic Al-Kα irradiation.
2.3. Preparation of composite membranes
The PES composite membranes were fabricated via liquid-induced phase separation. The PES polymer was dissolved in the NMP solvent with a mass ratio of 17.5 w/w% at room temperature.
First, the Fe3O4@PDA NPs were added with pre-determined amount and
dispersed homogeneously in NMP solvent by mechanical stirring and ultrasonication.
Next,
pre-dried PES polymer was added to the above dispersion with mechanical stirring until a homogenous casting solution was formed.
Then, after degassing in vacuum for 1 h, the
casting solution was casted on glass plate using a flat membrane casting equipment (FM-2, Ningbo Jiangbei) with 180 μm wet membrane thickness.
After casting operation, the wet
membranes were immerged in a water bath at room temperature so that the membranes can be sufficiently solidified and spontaneously detached off the glass plate in the coagulation water bath.
Finally, the PES membranes were washed with a large quantity of deionized water to
get rid of excess solvents.
The prepared PES composite membranes containing 0 wt%, 5
wt%, 10 wt% and 20 wt% Fe3O4@PDA nanoparticles were labeled as M1, M2, M3, and M4, 8
respectively.
2.5. Characterization of membranes
The surface and cross-sectional microstructures of the PES membranes were characterized by field emission scanning electron microscope (JSM-7500F, JEOL), and the surface element distributions of PES membranes were studied by EDS mapping.
In order to investigate the
cross-sectional structures of the as-prepared membranes, the dried PES membranes were immerged in liquid nitrogen and frozen for 30 s before mechanically fractured. membranes to be characterized by FESEM were spray-coated with gold.
All of the PES The mean pore
diameters of different PES membranes were measured using a mercury porosimeter (AutoPore IV 9500, Micromeritics). Contact angle measurement device equipped with a high-speed camera (DSA25, KRÜSS GmbH) was used to analyze the hydrophilicity of the membrane surface at room temperature. The hydraulic permeability of PES membranes was investigated by a weighing method with a dead-end filtration device at 0.1 MPa.
Briefly, the circular PES membranes were fixed on the
porous alloy gasket support of the dead-end filtration device, and then pure water was passed through the PES membranes by applying a pressure via compressed nitrogen.
The water
fluxes were measured by weighing the transmembrane water solution at certain time periods. The water flux (J) of the membrane under certain transmembrane pressure can be calculated by Equation (1):
9
J
m st
(1)
where, m refers to the mass of permeated solution, s is the effective permeable area of PES membrane, and t is the transmembrane time duration.
The permeable area in this study was
12.56 cm2, and the test temperature is controlled at room temperature.
Each water flux value
of the PES membrane was confirmed by taking average values acquired from five measurements to minimize the experimental errors, and three batches of PES membranes were measured to prove the repetitive reliability of the membrane preparation procedure.
2.6. Characterization of adsorptive performances of PES composite membranes to heavy metal ions
The adsorption properties of the PES composite membranes with different Fe3O4@PDA nanoparticle contents (M1, M2, M3, M4) for adsorptive removal of heavy metal ions were studied at 25 oC.
Firstly, the adsorptions of heavy metal Pb2+ ions on different PES
membranes were investigated.
The pH value of the feed solution was regulated to 4.0~5.4 by
0.1 M of hydrochloric acid to ensure the stability of Pb2+ in solution [48].
A peristaltic pump
was used to feed the Pb2+ solution into the membrane module continuously with a flow flux of 9.6 L∙m-2∙h-1.
The permeated solutions were collected every 5 mL at the outlet and were
measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, VG PQExCell, TJA).
The effective adsorption area of membrane was 12.56 cm2.
The Removal
efficiency of heavy metal ions by the membrane could be calculated by Equation (2):
10
Rem oval
efficiency (1
CJ ) 100% C0
(2)
where, CJ is the concentration of heavy metal ions in the permeated solution with flow flux of J, and C0 refers to the initial concentration of heavy metal ions in the feed solution. Next, in order to investigate the effect of transmembrane flow flux on the adsorptive removal characteristics of Pb2+ ions by the membrane M4, the adsorptive removal properties of Pb2+ by the membrane M4 under different transmembrane flow fluxes (9.6, 19.1 and 28.7 L∙m2∙h-1)
were tested.
The adsorptive removal performances of heavy metal ions Cd2+ and Ag+ by
the membrane M4 were also studied.
The initial concentrations of heavy metal ions Cd2+ and
Ag+ were respectively 100 ppb and 500 ppb, and the pH values of Cd2+ and Ag+ feed solutions were respectively adjusted to 5.2~6.8 and 5.5 [48, 49].
The transmembrane flow flux was
19.1 L∙m-2∙h-1, and the experimental procedures were the same as those for the above-mentioned Pb2+ adsorption experiments.
In the ICP-AES tests, the determination wavelengths of Pb, Cd,
and Ag were 220.4, 214.4, and 328.1 nm respectively.
2.7. Characterization of catalytic performances of PES composite membranes to organic contaminants
The catalytic performances of the PES composite membranes M1, M2, M3, and M4 with different contents of Fe3O4@PDA nanoparticles for catalytic degradation of organic contaminants were investigated at 25 oC.
Organic dye methylene blue (MB) was chosen as a
model organic contaminant, and with a low initial concentration of 10 ppm. reaction area of the membrane was 3.14 cm2. 11
The effective
5.5 mL of feed solution containing MB dyes
was pumped through the PES composite membrane with a flow flux of 38.2 L∙m-2∙h-1 by a peristaltic pump, and then the permeated solution was re-circulated to the feed solution to form a mixed solution.
During the reaction, the absorbance value (UV-vis) of MB dye at 664 nm
in the mixed solution was measured every 10 min to evaluate the removal ratio of MB dye by the PES composite membranes.
By contrast, the adsorption performances of the membranes
M1, M2, M3, and M4 for MB dyes were studied without adding any H2O2.
The Removal ratio
of MB dye by the membrane was calculated with Equation (3): Rem oval
ratio (1
Ct ) 100% C0
(3)
where, Ct refers to the concentration of MB dye in the mixed solution at time t, and C0 is the initial concentration of MB dye in the feed solution.
In addition, the adsorptive and catalytic
repeatability of the PES composite membrane M4 was tested.
2.8. Removal performances of heavy metal ions and organic contaminants by PES composite membranes
The performance of PES composite membrane M4 for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants was investigated.
The
mixed solution with Pb2+ as a model heavy metal ion contaminant and MB dye as a model organic contaminant was used in the experiments.
Briefly, a mixed solution with 1 ppm Pb2+
and 10 ppm MB dye was used as the feed solution, and then the pH value of the mixed feed solution was regulated to 4.0~5.4 by 0.1 M of hydrochloric acid [48]. hydrogen peroxide was added to the mixed feed solution. 12
Then, 8 v/v% of
A peristaltic pump was used to feed
the mixed feed solution into the membrane module continuously with the flow flux of 2.4 L∙m2∙h-1.
The permeated solutions were collected every 5 mL at the outlet to measure the
concentration of Pb2+ and MB dye using ICP-AES and UV-vis respectively. area of the membrane in the membrane module was 12.56 cm2.
The effective
The removal performances of
Pb2+ and MB dye were evaluated according to the above-mentioned methods.
3. Results and discussion 3.1. Characterization of Fe3O4@PDA nanoparticles
The transmission electron microscope (TEM) images of prepared Fe3O4 nanoparticles and Fe3O4@PDA nanoparticles in the dry state are shown in Fig. 2a and 2b, respectively. Compared with the Fe3O4 nanoparticles (Fig. 2a), the Fe3O4@PDA composite nanoparticles show an obvious core-shell structure with a coating thickness of about 13 nm (Fig. 2b).
The
polydopamine is formed in a weak alkaline aqueous solution mainly via the covalent bonds and non-covalent bonds [50].
Because the hydroxyl groups upon the surface of Fe3O4
nanoparticles can combine with catechol groups of dopamine molecule through dehydration [51], the PDA firmly adheres on the surface of Fe3O4 NPs, resulting in the core-shell structure of Fe3O4@PDA NPs.
The XPS spectra confirm the elements of Fe3O4 nanoparticles and
Fe3O4@PDA composite nanoparticles (Fig. 2c).
The characteristic peaks of Fe2p, O1s, and
Fe3p appear in the spectrum of Fe3O4 nanoparticles.
While, in the spectrum of Fe3O4@PDA
composite nanoparticles, the characteristic element N of the dopamine molecule appears but the peaks of Fe2p and Fe3p disappear.
The characteristic peak of Fe element is weakened due
13
to that the detection depth of XPS testing is 3-6 nm, while the PDA layer thickness of the Fe3O4@PDA composite nanoparticles is about 13 nm.
As shown in Table 1, the
concentrations of C element and N element have respectively risen from 17.87% to 73.36% and from 0% to 2.36%, and the proportion of O element decreases from 62.71% to 23.84%, because of the high content of C element in dopamine molecule and the low content of O element. Moreover, the new N1s peak after dopamine coating is not particularly strong because of the low content of N element.
From the above results, it can be confirmed that the dopamine can
self-polymerize on the surface of Fe3O4 nanoparticles in alkaline aqueous solution to form the Fe3O4@PDA composite nanoparticles.
Because of the PDA coating, the Fe3O4@PDA
nanoparticles show pH-depended zeta potential (Fig. 2d).
Under the neutral condition, the
zeta potential of Fe3O4@PDA nanoparticles is about -25 mV, which is beneficial for the adsorption of cations.
14
Fig. 2.
(a, b) TEM images of Fe3O4 nanoparticles (a) and Fe3O4@PDA nanoparticles (b).
The scale bars are 200 nm in (a1, b1) and 100 nm in (a2, b2). and Fe3O4@PDA (B) nanoparticles.
(c) XPS spectra of Fe3O4 (A)
(d) Zeta potential of Fe3O4@PDA nanoparticles at
different pH values. 15
Table 1.
XPS atomic concentrations of Fe3O4 and Fe3O4@PDA nanoparticles Atomic concentration (%)
Sample
C
O
Fe
N
Fe3O4 nanoparticles
17.87
62.71
19.42
0
Fe3O4@PDA nanoparticles
73.36
23.84
0.44
2.36
3.2. Morphology of PES composite membranes
The optical photographs of PES composite membranes with different contents of Fe3O4@PDA nanoparticles are shown in Fig. 3. Compared with the PES blank membrane M1, the PES composite membranes show brown color after adding Fe3O4@PDA nanoparticles. The color of the PES composite membrane gradually becomes darker with increasing the content of Fe3O4@PDA nanoparticles.
However, the addition of Fe3O4@PDA nanoparticles
makes no difference in the microstructures of cross-sections of the PES blank membrane (M1) and PES composite membranes (M2, M3, and M4) (Fig. 4).
The top surfaces of PES
composite membranes (M2, M3, and M4) are compact, and nearly no visible pores could be found at 2500 magnification (Fig. 4b1-d1), which is alike to that of PES blank membrane M1 (Fig. 4a1).
The cross-sections of all the membranes show typical LIPS-induced asymmetric
structures that constituted of near-surface functional layer and finger-like macro-void substrate layer [52] (Fig. 4a2-d2 and 4a3-d3).
On the membrane surfaces, Fe3O4@PDA nanoparticles
are clearly observed on the surfaces of PES composite membranes M2, M3, and M4.
With
increasing the content of blended Fe3O4@PDA nanoparticles, the number of nanoparticles on the surfaces of PES composite membranes also increases, which is confirmed by the EDS 16
mapping results (Fig. 5).
The oxygen element and iron element are found in the PES
composite membranes (M2, M3, and M4), which correspond to the distribution of Fe3O4@PDA nanoparticles in the PES composite membranes (Fig. 5b2-d2 and b3-d3).
The results confirm
that the Fe3O4@PDA nanoparticles have been embedded in PES polymer membranes successfully via physical blending and liquid-induced phase separation.
Fig. 3.
Optical photographs of the prepared membranes.
17
The scale bar is 1 cm.
Fig. 4.
FESEM images of top surfaces and cross-sectional views of PES membranes M1 with
0% NPs (a), M2 with 5% NPs (b), M3 with 10% NPs (c), and M4 with 20% NPs (d). Top surface; (a2-d2) Cross-section; (a3-d3) Magnified cross-section. (a1-d1) and (a3-d3), and 1 μm in (a2-d2).
18
(a1-d1)
Scale bars are 10 μm in
Fig. 5.
EDS mapping results of top surfaces of PES composite membranes.
(c) M3; (d) M4.
(a1-d1) The SEM images of membranes.
oxygen elements on the membrane surfaces. membrane surfaces.
(a) M1; (b) M2;
(a2-d2) The distribution of
(a3-d3) The distribution of iron elements on the
The scale bars are 2 μm.
3.3. Pore size, surface hydrophilicity and permeability of membranes
As shown in Fig. 6a, the mean pore diameters of M1, M2, M3 and M4 membranes are 96.2, 100.9, 115.3 and 126.4 nm respectively, which increase with increasing the added amount 19
of Fe3O4@PDA composite nanoparticles in the casting solution.
After incorporating with
Fe3O4@PDA nanoparticles, the water contact angles of PES composite membranes decrease (Fig. 6b).
The water contact angle reduces with increasing the content of Fe3O4@PDA
nanoparticles, showing that the PES composite membrane surface becomes more hydrophilic [53].
The pure water fluxes of the fabricated membranes are measured in a dead-end filtration
device at 0.1 MPa, and the results are shown in Fig. 6c.
The water fluxes of PES composite
membranes increase significantly with increasing the content of blended Fe3O4@PDA nanoparticles.
The water flux of PES composite membrane M4 increases to 2640 L∙m-2∙h-1,
which is six times larger than that of the PES blank membrane M1.
The hydrophilic PDA
layer on Fe3O4@PDA nanoparticles induces the non-solvent diffuses to the membrane surface and solvent could diffuse out from the PES matrix more easily in the LIPS process [39].
The
results confirm the water flux of PES membrane is enhanced by incorporating with Fe3O4@PDA nanoparticles, because additional porous microstructures between the PES membrane substrate and the Fe3O4@PDA nanoparticles are induced in the LIPS process due to the hydrophilic surfaces of Fe3O4@PDA nanoparticles (Fig. 1b).
20
Fig. 6.
(a,b) Mean pore diameters (a) and water contact angles on the surfaces (b) of different
membranes.
(c) Water fluxes across different membranes under constant transmembrane 21
pressure of 0.1 MPa.
3.4. Adsorptive performances of membranes to heavy metal ions
The adsorptive removal performances of heavy metal ions by the as-prepared PES composite membranes is displayed in Fig. 7.
In the transmembrane permeation process, the
breakthrough point is set as the permeate solution concentration reaches 10% of the feed solution concentration [54].
The breakthrough curves for removal of Pb2+ by different
membranes are shown in Fig. 7a and Fig. 7b.
The maximum permeating volumes of M1, M2,
M3 and M4 membranes are 5, 20, 35, 55 mL when the breakthrough points are reached (Fig. 7a).
Although the fresh PES blank membrane M1 shows certain ability to adsorb the Pb2+
ions, the Pb2+ removal efficiency decreases to nearly zero after treating 45 mL solution (Fig. 7c).
The Pb2+ removal efficiency of the PES composite membranes is significantly improved
because of the presence of Fe3O4@PDA nanoparticles, and the removal ability of Pb2+ increases with increasing the content of blended Fe3O4@PDA nanoparticles.
Because the Fe3O4@PDA
nanoparticles have negative charged surfaces, which are beneficial for adsorbing cations.
The
functional groups on the surfaces of Fe3O4@PDA nanoparticles can also chelate the heavy metal ions [42, 43].
The removal efficiency of Pb2+ by PES composite membrane M4 is still
about 90% after 5-hour treatment.
With increasing the flow flux, the removal efficiency of
Pb2+ by PES composite membrane M4 decreases to certain degree (Fig. 7d), which is due to the shortening of residence time of solution inside membrane.
In addition, the PES composite
membrane M4 can also effectively adsorb the heavy metal ions Cd2+ and Ag+, as shown in Fig. 7e and 7f.
The regeneration of PES composite membranes after adsorbing heavy metal ions 22
can generally use acid as desorbent to rinse the membranes with deionized water alternately [49].
Fig. 7.
Adsorptive removal performances of the membranes to heavy metal ions.
(a)
Breakthrough curves of M1, M2, M3 and M4 membranes for removal of Pb2+.
(b)
23
Breakthrough curves for the adsorptive removal of Pb2+ by the membrane M4 under different transmembrane flow fluxes.
(c) The Pb2+ removal efficiency of different membranes with a
transmembrane flow flux of 9.6 L∙m-2∙h-1.
(d) The Pb2+ removal efficiency of the PES
composite membrane M4 under different flow fluxes.
The Cd2+ removal efficiency (e) and
Ag+ removal efficiency (f) of PES composite membrane M4 with a transmembrane flow flux of 19.1 L∙m-2∙h-1.
3.5. Catalytic performances of membranes to organic contaminants
When the H2O2 is absent in the feed solution, PES composite membranes enable to take out the MB dye due to the adsorption (Fig. 8a).
Even though the removal ratio of MB dye
rises with increasing the content of blended Fe3O4@PDA nanoparticles, the highest removal ratio of MB dye by PES composite membrane M4 is about 87.6% (Fig. 8a). When adding the H2O2 in the solution, the Fe3O4@PDA nanoparticles embedded in PES composite membranes react with H2O2 to produce strongly oxidized hydroxyl radicals, which could be used to degrade MB dyes.
Therefore, combining with catalytic degradation, the PES composite membranes
can effectively remove the MB dye with removal ratio as high as 98.8% (Fig. 8b and 8c).
The
PDA coating on the surfaces of Fe3O4@PDA nanoparticles can accelerate the rate of catalytic reaction because the PDA coating has a phenoquinone structure, which can be used as redox medium to promote the electron transfer during the process of Fenton-like reaction [45-47]. The PDA layer of the composite nanoparticles blended in the membranes has abundant functional groups, thus the composite membranes prepared in this work show better catalytic performances than that reported in our previous work [39].
The results confirm that the PES
composite membranes blended with Fe3O4@PDA nanoparticles have excellent catalytic
24
property for degrading organic contaminants.
Fig. 8.
Adsorptive and catalytic performances of membranes.
ratios of different membranes without addition of H2O2. different membranes with addition of H2O2.
(a) The MB dye removal
(b) The MB dye removal ratios of
(c) The MB dye removal ratios of different 25
membranes at the adsorptive or catalytic time of 80 min.
The reproducibility of the PES composite membrane M4 was studied with a flow flux of 38.2 L∙m-2∙h-1.
The adsorptive removal ratio of MB dye by PES membrane M4 reduces
gradually after each circulation (Fig. 9a).
After repeated adsorption for five runs, the removal
ratio of MB dye by PES membrane M4 reduces from 87.6% to 54.6%.
MB dye are adsorbed
by Fe3O4@PDA nanoparticles in the PES composite membrane, and it is difficult to completely wash away the MB dye molecules from the membrane.
Thus, the adsorption capacity of the
membrane becomes weaker and weaker after each circulation of adsorption.
As expected,
upon adding H2O2, the removal ratio of MB dye by PES composite membrane M4 is still above 90% even after repeated catalytic experiments for five runs because of the catalytic degradation (Fig. 9b and 9c).
The results confirm that the catalytic performance of the PES composite
membranes is well repeatable, and the unique structures of the prepared membranes ensure long-term stability for degradation of organic contaminants [39].
26
Fig. 9.
The repeatability of catalytic performance of PES composite membrane M4.
(a, b)
Adsorption (a) and catalytic (b) performances of PES composite membrane M4 for MB dye without or with H2O2.
(c) Removal ratios of MB dye by PES composite membrane M4 in
repeated experiments at the adsorptive or catalytic time of 80 min in each run. 27
3.6. Simultaneous removal of heavy metal ions and organic contaminants by PES composite membranes
To demonstrate the simultaneous removal of heavy metal ions and organic contaminants by PES composite membranes, the treatment of a mixed solution containing Pb2+ and MB dye by the membrane M4 is investigated (Fig. 10).
The removal efficiency of Pb2+ by PES
composite membrane M4 is more than 80% after permeating 50 mL solution.
Compared with
the results in Fig. 7, the decrease in the removal efficiency of Pb2+ here is caused by the presence of MB dye, which can be competitively adsorbed to the Fe3O4@PDA nanoparticles. the removal efficiency of MB dye maintains above 90%.
While,
The results show that the PES
composite membrane effectively achieves adsorptive removal of Pb2+ and catalytic degradation of MB dye simultaneously.
Fig. 10.
The removal efficiency of heavy metal ion Pb2+ and MB dye in mixed solution by
PES composite membrane M4 with transmembrane flow flux of 2.4 L∙m-2∙h-1.
28
4. Conclusions
In summary, PES composite membranes have been successfully developed for simultaneous adsorption of heavy metal ions and catalytic degradation of organic contaminants by blending Fe3O4@PDA nanoparticles in PES matrix via LIPS.
The permeabilities of PES
composite membranes are significantly improved by introducing the blended Fe3O4@PDA nanoparticles, because the hydrophilic PDA coating on the surface of Fe3O4@PDA nanoparticles enables to produce more microporous structures between the PES matrix and nanoparticles during the LIPS process.
The PES composite membrane containing 20 wt%
Fe3O4@PDA nanoparticles shows water flux as high as 2640 L∙m-2∙h-1∙bar-1, which is six times larger than that of PES blank membrane.
The PES composite membranes show efficient and
repeatable performances for simultaneously removing heavy metal ions and organic contaminants from water.
The proposed PES composite membranes showing multifunction
of simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions provide a new pathway to deal with some hard-to-be-treated wastewaters containing both organic contaminants and heavy metal ions from papermaking, leather, textile printing-dyeing industries, and so on.
Importantly, the proposed one-step strategy for preparing composite
membranes with both high permeability and high separation efficiency in this work is easy to be scaled up.
Acknowledgements
29
The authors gratefully acknowledge support from the National Natural Science Foundation of China (21776182, 21490582), and the State Key Laboratory of Polymer Materials Engineering (sklpme2016-3-07).
The authors would like to thank Mr. Yi He and
Mr. Xi Wu at the Analytical & Testing Center of Sichuan University for the help of FESEM imaging and ICP-AES testing.
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Graphical abstract
38
Highlights
Membranes are developed for simultaneous removal of organic contaminants and heavy metal ions. The membranes are fabricated by blending Fe3O4@PDA nanoparticles in PES matrix via LIPS. The Fe3O4@PDA nanoparticles blended in the membranes act as both catalysts and adsorbents. The composite membranes exhibit both large permeability and high separation efficiency.
39
S1383-5866(19)33682-2 https://doi.org/10.1016/j.seppur.2019.116364 SEPPUR 116364
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
17 August 2019 16 October 2019 28 November 2019
Please cite this article as: L-P. Zhang, Z. Liu, X-L. Zhou, C. Zhang, Q-W. Cai, R. Xie, X-J. Ju, W. Wang, Y. Faraj, L-Y. Chu, Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/ j.seppur.2019.116364
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Novel composite membranes for simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions
Li-Ping Zhanga, Zhuang Liua,b,*, Xing-Long Zhoua, Chuan Zhanga, Quan-Wei Caia, Rui Xiea,b, Xiao-Jie Jua,b, Wei Wanga,b, Yousef Faraja,b, Liang-Yin Chua,*
a b
School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,
Sichuan 610065, China
*Corresponding
authors at: School of Chemical Engineering, Sichuan University, Chengdu,
Sichuan 610065, China. E-mail address: [email protected] (Z. Liu); [email protected] (L. Y. Chu). 1
Abstract Novel composite membranes are successfully developed for simultaneous adsorption of heavy metal ions and catalytic degradation of organic contaminants by blending polydopamine-coated ferroferric oxide (Fe3O4@PDA) nanoparticles in polyethersulfone (PES) matrix via liquidinduced phase separation (LIPS) method.
The permeabilities of the as-fabricated composite
membranes are significantly improved by introducing Fe3O4@PDA nanoparticles, because the hydrophilic coating on the surface of Fe3O4@PDA nanoparticles enables to generate more microporous structures between the PES matrix and nanoparticles during the LIPS process. The composite membrane containing 20 wt% Fe3O4@PDA nanoparticles shows water flux as high as 2640 L∙m-2∙h-1∙bar-1, which is six times larger than that of PES blank membrane.
The
composite membranes exhibit efficient and repeatable performances for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants based on the mechanism of Fenton-like reaction.
The proposed composite membranes with
multifunction of simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions provide a new pathway to deal with some hard-to-be-treated wastewaters from papermaking, leather, textile printing-dyeing industries, and so on.
Furthermore, the proposed
one-step strategy for fabricating composite membranes with both large permeability and high separation efficiency in this work is easy to be scaled up.
Keywords Composite membranes; Fe3O4@PDA nanoparticles; Fenton-like reaction; Degradation of organic contaminants; Removal of heavy metal ions
2
1. Introduction
An abundant of wastewater containing organic contaminants and heavy metal ions are produced in the papermaking, leather, and textile printing-dyeing industries [1-6].
Such
wastewaters are prohibited to discharge into the environment before they have been treated, because both organic dye contaminants and heavy metal ions are particularly hazardous to the ecological system [7-12].
Wastewater treatment processes, such as sedimentation, adsorption
and oxidation, enable to remove most of the organic pollutants and heavy metal ions; while trace contaminants and heavy metal ions are usually still remained in the pretreated water [13]. It is essential to develop advanced technologies to simultaneously remove such trace organic contaminants and heavy metal ions from wastewaters.
To further deal with the pretreated
wastewater, several methods have been developed, such as membrane filtration, biological and catalytic degradation [13-18].
The membrane technology is considered to be preferable owing
to its many advantages such as functional selectivity, high separation efficiency, simple process, low energy consumption, flexibility, and easy integration ability with other technologies [1922]. Up to now, three main categories of membrane technologies have been developed to simultaneously remove the organic contaminants and heavy metal ions from wastewaters, which are dual-rejection mode, rejection-adsorption mode, and dual-adsorption mode [13, 2331].
In principle, both organic contaminants and heavy metal ions could be excluded by
reverse osmosis and nanofiltration membrane technologies based on molecular size and charge. Such processes commonly suffer from high pressure to drive the solution permeating through the membrane [32].
Since the sizes of organic molecules are usually larger than those of ions, 3
some membranes are developed with enhanced permeability to reject organic contaminants but adsorb heavy metal ions via electrostatic attraction and/or complexation.
For example,
Moradi and co-workers [26] reported polyethersulfone (PES) nanofiltration membranes with magnetic graphene-based additives, which can directly reject Red 16 by electrostatic repulsion and remove copper ions (Cu2+) by adsorption. 1∙bar-1.
The flux of their membrane is only 8.3 L∙m-2∙h-
Hou and co-workers [33] developed an ultrafiltration membrane made of
polydopamine-(β-cyclodextrin)-modified graphene oxide sheets by drop-coating with vacuum filtration.
This membrane enables to reject the methylene blue by size sieving and achieve the
adsorptive removal of heavy metal lead(II) ions with a flux of 120 L∙m−2∙h−1∙bar-1.
To further
improve the processing efficiency, the dual-adsorption mode of membrane technology has been developed based on electrostatic attraction, chemical bonding, and/or physical bonding between the membranes and organic contaminants as well as heavy metal ions.
For instance, Xu and
co-workers [28] prepared a 3D polyurethane/polyacrylic acid membrane using solvent casting, salt leaching and thermal crosslinking.
The membrane can adsorb heavy metal lead(II) ions
and organic dye methylene blue with the removal ratio of 99.7% and 99.6% respectively, and possesses the water flux of 1085.1 L∙m-2∙h-1 under 0.06 MPa.
Yu and co-workers [13]
prepared a free-standing membrane made of carbon nanofibers to adsorb methylene blue and heavy metal ion pollutants effectively with a trans-membrane flux of 1580 L∙m-2∙h-1.
In
general, these previously reported membranes are developed to remove organic contaminants and heavy metal ion pollutants from wastewater.
However, during the membrane filtration
processes, the organic contaminants adsorbed on the membrane surfaces could cause the membrane fouling, resulting in the decrease of water flux [34, 35].
4
Even after the desorption
of organic contaminants and regeneration of membranes, the membranes could not be completely recovered [36].
Therefore, it is still highly desirable to develop novel efficient
membrane technology to remove organic contaminants and heavy metal ions simultaneously from wastewaters. In this work, we report on a novel polyethersulfone (PES) composite membrane containing polydopamine-coated ferroferric oxide (Fe3O4@PDA) nanoparticles for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants from water.
The composite membrane is easily prepared by blending Fe3O4@PDA nanoparticles
in PES membrane via liquid-induced phase separation (LIPS) approach (Fig. 1a, b).
PES is
chosen as the membrane substrate, because of its advantages of proper mechanical property, as well as outstanding oxidative, and chemical and thermal stability [37, 38].
The Fe3O4
nanoparticles are synthesized by hydrothermal method, followed by coating PDA layer via the self-polymerization property of dopamine in a weak alkaline aqueous solution to obtain Fe3O4@PDA nanoparticles [39-41].
The prepared Fe3O4@PDA composite nanoparticles are
blended into the PES membrane in the LIPS process.
The Fe3O4@PDA nanoparticles in PES
composite membrane could adsorb heavy metal ions by abundant functional groups in PDA coating layer on the surface of nanoparticles (Fig. 1c), and simultaneously produce active hydroxyl radicals (∙OH) to degrade organic contaminants based on Fenton-like reaction (Fig. 1d).
The hydrophilic PDA coating on the surface of nanoparticles enables to produce more
microporous structures between the PES matrix and nanoparticles during the LIPS to achieve large water flux (Fig. 1b).
The phenolic hydroxy and amino groups on the surface of the PDA
coating could chelate the cations to achieve the adsorption of heavy metal ion pollutants [425
44].
Moreover, electron transfer in the process of Fenton-like reaction can be promoted due
to phenoquinone structure on the surface of the PDA coating, which is beneficial to accelerate the catalytic reaction [45-47].
In principle, the fabrication of the proposed PES composite
membranes by LIPS approach can be easily and scalably acheived by using the existing membrane casting equipment.
The proposed PES composite membranes could be featured
with a large water flux and simultaneous removal of heavy metal ions by adsorption and organic contaminants by catalytic degradation from wastewater.
Fig. 1.
(a, b) Schematic illustrations of cross-sectional structure of the proposed PES
composite membrane by blending Fe3O4@PDA nanoparticles in PES via LIPS (a) and the enlarged view of the cross-section near surface of the PES composite membrane (b).
(c, d)
The mechanisms of the adsorption of heavy metal ions (c) and the catalytic degradation of organic contaminants by Fenton-like reaction (d) with the blended Fe3O4@PDA nanoparticles.
6
2. Materials and methods 2.1. Materials
Ethylene glycol (EG), ferric chloride hexahydrate (FeCl3∙6H2O), hydrogen peroxide (H2O2, 30 wt%), ammonia solution (NH3∙H2O, 30 wt%), sodium acetate (NaAc), N-methylpyrrolidone (NMP), methylene blue (MB), tri-hydroxymethyl-aminomethane (Tris), hydrochloric acid (HCl), dopamine hydrochloride (DA ∙ HCl), ethanol, lead nitrate, cadmium nitrate and silver nitrate were purchased from Chengdu Ke Long Chemicals without further purification. with a molecular weight of 66 kDa was purchased from BASF.
PES
Deionized water with
resistance of 18.2 MΩ∙cm used through the whole experiments came from Millipore Elix-10 water purification system.
2.2. Synthesis and characterization of Fe3O4@PDA nanoparticles
The Fe3O4 nanoparticles (Fe3O4 NPs) were synthesized according to the previous work [39].
The prepared Fe3O4 NPs were coated with a PDA layer via the oxidative self-
polymerization of dopamine in a weak alkaline aqueous solution [40].
Firstly, 0.1 g of Fe3O4
NPs was suspended in 50 mL of Tris-HCl solution (15 mM, pH 8.5) ultrasonically to obtain 2 mg/mL Fe3O4 NPs dispersion solution.
Secondly, 0.1 g of DA-HCl was put into the above
solution and stirred mechanically at ambient temperature for 6 h.
After the reaction was
completed, the obtained Fe3O4@PDA composite nanoparticles were isolated from the reaction solution using a strong magnet and cleaned alternately with ethanol and water for five times 7
and dried for several hours. The micromorphology and size of the Fe3O4@PDA composite nanoparticles in the dried state were observed by a transmission electron microscope (TEM, JEM-2100, JEOL).
The
Fe3O4@PDA samples were obtained by droplet casting NPs suspension on the carbon-coated copper grid and dried.
X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos) tests were
performed with Kratos Axis spectrometer attached with a monochromatic Al-Kα irradiation.
2.3. Preparation of composite membranes
The PES composite membranes were fabricated via liquid-induced phase separation. The PES polymer was dissolved in the NMP solvent with a mass ratio of 17.5 w/w% at room temperature.
First, the Fe3O4@PDA NPs were added with pre-determined amount and
dispersed homogeneously in NMP solvent by mechanical stirring and ultrasonication.
Next,
pre-dried PES polymer was added to the above dispersion with mechanical stirring until a homogenous casting solution was formed.
Then, after degassing in vacuum for 1 h, the
casting solution was casted on glass plate using a flat membrane casting equipment (FM-2, Ningbo Jiangbei) with 180 μm wet membrane thickness.
After casting operation, the wet
membranes were immerged in a water bath at room temperature so that the membranes can be sufficiently solidified and spontaneously detached off the glass plate in the coagulation water bath.
Finally, the PES membranes were washed with a large quantity of deionized water to
get rid of excess solvents.
The prepared PES composite membranes containing 0 wt%, 5
wt%, 10 wt% and 20 wt% Fe3O4@PDA nanoparticles were labeled as M1, M2, M3, and M4, 8
respectively.
2.5. Characterization of membranes
The surface and cross-sectional microstructures of the PES membranes were characterized by field emission scanning electron microscope (JSM-7500F, JEOL), and the surface element distributions of PES membranes were studied by EDS mapping.
In order to investigate the
cross-sectional structures of the as-prepared membranes, the dried PES membranes were immerged in liquid nitrogen and frozen for 30 s before mechanically fractured. membranes to be characterized by FESEM were spray-coated with gold.
All of the PES The mean pore
diameters of different PES membranes were measured using a mercury porosimeter (AutoPore IV 9500, Micromeritics). Contact angle measurement device equipped with a high-speed camera (DSA25, KRÜSS GmbH) was used to analyze the hydrophilicity of the membrane surface at room temperature. The hydraulic permeability of PES membranes was investigated by a weighing method with a dead-end filtration device at 0.1 MPa.
Briefly, the circular PES membranes were fixed on the
porous alloy gasket support of the dead-end filtration device, and then pure water was passed through the PES membranes by applying a pressure via compressed nitrogen.
The water
fluxes were measured by weighing the transmembrane water solution at certain time periods. The water flux (J) of the membrane under certain transmembrane pressure can be calculated by Equation (1):
9
J
m st
(1)
where, m refers to the mass of permeated solution, s is the effective permeable area of PES membrane, and t is the transmembrane time duration.
The permeable area in this study was
12.56 cm2, and the test temperature is controlled at room temperature.
Each water flux value
of the PES membrane was confirmed by taking average values acquired from five measurements to minimize the experimental errors, and three batches of PES membranes were measured to prove the repetitive reliability of the membrane preparation procedure.
2.6. Characterization of adsorptive performances of PES composite membranes to heavy metal ions
The adsorption properties of the PES composite membranes with different Fe3O4@PDA nanoparticle contents (M1, M2, M3, M4) for adsorptive removal of heavy metal ions were studied at 25 oC.
Firstly, the adsorptions of heavy metal Pb2+ ions on different PES
membranes were investigated.
The pH value of the feed solution was regulated to 4.0~5.4 by
0.1 M of hydrochloric acid to ensure the stability of Pb2+ in solution [48].
A peristaltic pump
was used to feed the Pb2+ solution into the membrane module continuously with a flow flux of 9.6 L∙m-2∙h-1.
The permeated solutions were collected every 5 mL at the outlet and were
measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, VG PQExCell, TJA).
The effective adsorption area of membrane was 12.56 cm2.
The Removal
efficiency of heavy metal ions by the membrane could be calculated by Equation (2):
10
Rem oval
efficiency (1
CJ ) 100% C0
(2)
where, CJ is the concentration of heavy metal ions in the permeated solution with flow flux of J, and C0 refers to the initial concentration of heavy metal ions in the feed solution. Next, in order to investigate the effect of transmembrane flow flux on the adsorptive removal characteristics of Pb2+ ions by the membrane M4, the adsorptive removal properties of Pb2+ by the membrane M4 under different transmembrane flow fluxes (9.6, 19.1 and 28.7 L∙m2∙h-1)
were tested.
The adsorptive removal performances of heavy metal ions Cd2+ and Ag+ by
the membrane M4 were also studied.
The initial concentrations of heavy metal ions Cd2+ and
Ag+ were respectively 100 ppb and 500 ppb, and the pH values of Cd2+ and Ag+ feed solutions were respectively adjusted to 5.2~6.8 and 5.5 [48, 49].
The transmembrane flow flux was
19.1 L∙m-2∙h-1, and the experimental procedures were the same as those for the above-mentioned Pb2+ adsorption experiments.
In the ICP-AES tests, the determination wavelengths of Pb, Cd,
and Ag were 220.4, 214.4, and 328.1 nm respectively.
2.7. Characterization of catalytic performances of PES composite membranes to organic contaminants
The catalytic performances of the PES composite membranes M1, M2, M3, and M4 with different contents of Fe3O4@PDA nanoparticles for catalytic degradation of organic contaminants were investigated at 25 oC.
Organic dye methylene blue (MB) was chosen as a
model organic contaminant, and with a low initial concentration of 10 ppm. reaction area of the membrane was 3.14 cm2. 11
The effective
5.5 mL of feed solution containing MB dyes
was pumped through the PES composite membrane with a flow flux of 38.2 L∙m-2∙h-1 by a peristaltic pump, and then the permeated solution was re-circulated to the feed solution to form a mixed solution.
During the reaction, the absorbance value (UV-vis) of MB dye at 664 nm
in the mixed solution was measured every 10 min to evaluate the removal ratio of MB dye by the PES composite membranes.
By contrast, the adsorption performances of the membranes
M1, M2, M3, and M4 for MB dyes were studied without adding any H2O2.
The Removal ratio
of MB dye by the membrane was calculated with Equation (3): Rem oval
ratio (1
Ct ) 100% C0
(3)
where, Ct refers to the concentration of MB dye in the mixed solution at time t, and C0 is the initial concentration of MB dye in the feed solution.
In addition, the adsorptive and catalytic
repeatability of the PES composite membrane M4 was tested.
2.8. Removal performances of heavy metal ions and organic contaminants by PES composite membranes
The performance of PES composite membrane M4 for simultaneous adsorptive removal of heavy metal ions and catalytic degradation of organic contaminants was investigated.
The
mixed solution with Pb2+ as a model heavy metal ion contaminant and MB dye as a model organic contaminant was used in the experiments.
Briefly, a mixed solution with 1 ppm Pb2+
and 10 ppm MB dye was used as the feed solution, and then the pH value of the mixed feed solution was regulated to 4.0~5.4 by 0.1 M of hydrochloric acid [48]. hydrogen peroxide was added to the mixed feed solution. 12
Then, 8 v/v% of
A peristaltic pump was used to feed
the mixed feed solution into the membrane module continuously with the flow flux of 2.4 L∙m2∙h-1.
The permeated solutions were collected every 5 mL at the outlet to measure the
concentration of Pb2+ and MB dye using ICP-AES and UV-vis respectively. area of the membrane in the membrane module was 12.56 cm2.
The effective
The removal performances of
Pb2+ and MB dye were evaluated according to the above-mentioned methods.
3. Results and discussion 3.1. Characterization of Fe3O4@PDA nanoparticles
The transmission electron microscope (TEM) images of prepared Fe3O4 nanoparticles and Fe3O4@PDA nanoparticles in the dry state are shown in Fig. 2a and 2b, respectively. Compared with the Fe3O4 nanoparticles (Fig. 2a), the Fe3O4@PDA composite nanoparticles show an obvious core-shell structure with a coating thickness of about 13 nm (Fig. 2b).
The
polydopamine is formed in a weak alkaline aqueous solution mainly via the covalent bonds and non-covalent bonds [50].
Because the hydroxyl groups upon the surface of Fe3O4
nanoparticles can combine with catechol groups of dopamine molecule through dehydration [51], the PDA firmly adheres on the surface of Fe3O4 NPs, resulting in the core-shell structure of Fe3O4@PDA NPs.
The XPS spectra confirm the elements of Fe3O4 nanoparticles and
Fe3O4@PDA composite nanoparticles (Fig. 2c).
The characteristic peaks of Fe2p, O1s, and
Fe3p appear in the spectrum of Fe3O4 nanoparticles.
While, in the spectrum of Fe3O4@PDA
composite nanoparticles, the characteristic element N of the dopamine molecule appears but the peaks of Fe2p and Fe3p disappear.
The characteristic peak of Fe element is weakened due
13
to that the detection depth of XPS testing is 3-6 nm, while the PDA layer thickness of the Fe3O4@PDA composite nanoparticles is about 13 nm.
As shown in Table 1, the
concentrations of C element and N element have respectively risen from 17.87% to 73.36% and from 0% to 2.36%, and the proportion of O element decreases from 62.71% to 23.84%, because of the high content of C element in dopamine molecule and the low content of O element. Moreover, the new N1s peak after dopamine coating is not particularly strong because of the low content of N element.
From the above results, it can be confirmed that the dopamine can
self-polymerize on the surface of Fe3O4 nanoparticles in alkaline aqueous solution to form the Fe3O4@PDA composite nanoparticles.
Because of the PDA coating, the Fe3O4@PDA
nanoparticles show pH-depended zeta potential (Fig. 2d).
Under the neutral condition, the
zeta potential of Fe3O4@PDA nanoparticles is about -25 mV, which is beneficial for the adsorption of cations.
14
Fig. 2.
(a, b) TEM images of Fe3O4 nanoparticles (a) and Fe3O4@PDA nanoparticles (b).
The scale bars are 200 nm in (a1, b1) and 100 nm in (a2, b2). and Fe3O4@PDA (B) nanoparticles.
(c) XPS spectra of Fe3O4 (A)
(d) Zeta potential of Fe3O4@PDA nanoparticles at
different pH values. 15
Table 1.
XPS atomic concentrations of Fe3O4 and Fe3O4@PDA nanoparticles Atomic concentration (%)
Sample
C
O
Fe
N
Fe3O4 nanoparticles
17.87
62.71
19.42
0
Fe3O4@PDA nanoparticles
73.36
23.84
0.44
2.36
3.2. Morphology of PES composite membranes
The optical photographs of PES composite membranes with different contents of Fe3O4@PDA nanoparticles are shown in Fig. 3. Compared with the PES blank membrane M1, the PES composite membranes show brown color after adding Fe3O4@PDA nanoparticles. The color of the PES composite membrane gradually becomes darker with increasing the content of Fe3O4@PDA nanoparticles.
However, the addition of Fe3O4@PDA nanoparticles
makes no difference in the microstructures of cross-sections of the PES blank membrane (M1) and PES composite membranes (M2, M3, and M4) (Fig. 4).
The top surfaces of PES
composite membranes (M2, M3, and M4) are compact, and nearly no visible pores could be found at 2500 magnification (Fig. 4b1-d1), which is alike to that of PES blank membrane M1 (Fig. 4a1).
The cross-sections of all the membranes show typical LIPS-induced asymmetric
structures that constituted of near-surface functional layer and finger-like macro-void substrate layer [52] (Fig. 4a2-d2 and 4a3-d3).
On the membrane surfaces, Fe3O4@PDA nanoparticles
are clearly observed on the surfaces of PES composite membranes M2, M3, and M4.
With
increasing the content of blended Fe3O4@PDA nanoparticles, the number of nanoparticles on the surfaces of PES composite membranes also increases, which is confirmed by the EDS 16
mapping results (Fig. 5).
The oxygen element and iron element are found in the PES
composite membranes (M2, M3, and M4), which correspond to the distribution of Fe3O4@PDA nanoparticles in the PES composite membranes (Fig. 5b2-d2 and b3-d3).
The results confirm
that the Fe3O4@PDA nanoparticles have been embedded in PES polymer membranes successfully via physical blending and liquid-induced phase separation.
Fig. 3.
Optical photographs of the prepared membranes.
17
The scale bar is 1 cm.
Fig. 4.
FESEM images of top surfaces and cross-sectional views of PES membranes M1 with
0% NPs (a), M2 with 5% NPs (b), M3 with 10% NPs (c), and M4 with 20% NPs (d). Top surface; (a2-d2) Cross-section; (a3-d3) Magnified cross-section. (a1-d1) and (a3-d3), and 1 μm in (a2-d2).
18
(a1-d1)
Scale bars are 10 μm in
Fig. 5.
EDS mapping results of top surfaces of PES composite membranes.
(c) M3; (d) M4.
(a1-d1) The SEM images of membranes.
oxygen elements on the membrane surfaces. membrane surfaces.
(a) M1; (b) M2;
(a2-d2) The distribution of
(a3-d3) The distribution of iron elements on the
The scale bars are 2 μm.
3.3. Pore size, surface hydrophilicity and permeability of membranes
As shown in Fig. 6a, the mean pore diameters of M1, M2, M3 and M4 membranes are 96.2, 100.9, 115.3 and 126.4 nm respectively, which increase with increasing the added amount 19
of Fe3O4@PDA composite nanoparticles in the casting solution.
After incorporating with
Fe3O4@PDA nanoparticles, the water contact angles of PES composite membranes decrease (Fig. 6b).
The water contact angle reduces with increasing the content of Fe3O4@PDA
nanoparticles, showing that the PES composite membrane surface becomes more hydrophilic [53].
The pure water fluxes of the fabricated membranes are measured in a dead-end filtration
device at 0.1 MPa, and the results are shown in Fig. 6c.
The water fluxes of PES composite
membranes increase significantly with increasing the content of blended Fe3O4@PDA nanoparticles.
The water flux of PES composite membrane M4 increases to 2640 L∙m-2∙h-1,
which is six times larger than that of the PES blank membrane M1.
The hydrophilic PDA
layer on Fe3O4@PDA nanoparticles induces the non-solvent diffuses to the membrane surface and solvent could diffuse out from the PES matrix more easily in the LIPS process [39].
The
results confirm the water flux of PES membrane is enhanced by incorporating with Fe3O4@PDA nanoparticles, because additional porous microstructures between the PES membrane substrate and the Fe3O4@PDA nanoparticles are induced in the LIPS process due to the hydrophilic surfaces of Fe3O4@PDA nanoparticles (Fig. 1b).
20
Fig. 6.
(a,b) Mean pore diameters (a) and water contact angles on the surfaces (b) of different
membranes.
(c) Water fluxes across different membranes under constant transmembrane 21
pressure of 0.1 MPa.
3.4. Adsorptive performances of membranes to heavy metal ions
The adsorptive removal performances of heavy metal ions by the as-prepared PES composite membranes is displayed in Fig. 7.
In the transmembrane permeation process, the
breakthrough point is set as the permeate solution concentration reaches 10% of the feed solution concentration [54].
The breakthrough curves for removal of Pb2+ by different
membranes are shown in Fig. 7a and Fig. 7b.
The maximum permeating volumes of M1, M2,
M3 and M4 membranes are 5, 20, 35, 55 mL when the breakthrough points are reached (Fig. 7a).
Although the fresh PES blank membrane M1 shows certain ability to adsorb the Pb2+
ions, the Pb2+ removal efficiency decreases to nearly zero after treating 45 mL solution (Fig. 7c).
The Pb2+ removal efficiency of the PES composite membranes is significantly improved
because of the presence of Fe3O4@PDA nanoparticles, and the removal ability of Pb2+ increases with increasing the content of blended Fe3O4@PDA nanoparticles.
Because the Fe3O4@PDA
nanoparticles have negative charged surfaces, which are beneficial for adsorbing cations.
The
functional groups on the surfaces of Fe3O4@PDA nanoparticles can also chelate the heavy metal ions [42, 43].
The removal efficiency of Pb2+ by PES composite membrane M4 is still
about 90% after 5-hour treatment.
With increasing the flow flux, the removal efficiency of
Pb2+ by PES composite membrane M4 decreases to certain degree (Fig. 7d), which is due to the shortening of residence time of solution inside membrane.
In addition, the PES composite
membrane M4 can also effectively adsorb the heavy metal ions Cd2+ and Ag+, as shown in Fig. 7e and 7f.
The regeneration of PES composite membranes after adsorbing heavy metal ions 22
can generally use acid as desorbent to rinse the membranes with deionized water alternately [49].
Fig. 7.
Adsorptive removal performances of the membranes to heavy metal ions.
(a)
Breakthrough curves of M1, M2, M3 and M4 membranes for removal of Pb2+.
(b)
23
Breakthrough curves for the adsorptive removal of Pb2+ by the membrane M4 under different transmembrane flow fluxes.
(c) The Pb2+ removal efficiency of different membranes with a
transmembrane flow flux of 9.6 L∙m-2∙h-1.
(d) The Pb2+ removal efficiency of the PES
composite membrane M4 under different flow fluxes.
The Cd2+ removal efficiency (e) and
Ag+ removal efficiency (f) of PES composite membrane M4 with a transmembrane flow flux of 19.1 L∙m-2∙h-1.
3.5. Catalytic performances of membranes to organic contaminants
When the H2O2 is absent in the feed solution, PES composite membranes enable to take out the MB dye due to the adsorption (Fig. 8a).
Even though the removal ratio of MB dye
rises with increasing the content of blended Fe3O4@PDA nanoparticles, the highest removal ratio of MB dye by PES composite membrane M4 is about 87.6% (Fig. 8a). When adding the H2O2 in the solution, the Fe3O4@PDA nanoparticles embedded in PES composite membranes react with H2O2 to produce strongly oxidized hydroxyl radicals, which could be used to degrade MB dyes.
Therefore, combining with catalytic degradation, the PES composite membranes
can effectively remove the MB dye with removal ratio as high as 98.8% (Fig. 8b and 8c).
The
PDA coating on the surfaces of Fe3O4@PDA nanoparticles can accelerate the rate of catalytic reaction because the PDA coating has a phenoquinone structure, which can be used as redox medium to promote the electron transfer during the process of Fenton-like reaction [45-47]. The PDA layer of the composite nanoparticles blended in the membranes has abundant functional groups, thus the composite membranes prepared in this work show better catalytic performances than that reported in our previous work [39].
The results confirm that the PES
composite membranes blended with Fe3O4@PDA nanoparticles have excellent catalytic
24
property for degrading organic contaminants.
Fig. 8.
Adsorptive and catalytic performances of membranes.
ratios of different membranes without addition of H2O2. different membranes with addition of H2O2.
(a) The MB dye removal
(b) The MB dye removal ratios of
(c) The MB dye removal ratios of different 25
membranes at the adsorptive or catalytic time of 80 min.
The reproducibility of the PES composite membrane M4 was studied with a flow flux of 38.2 L∙m-2∙h-1.
The adsorptive removal ratio of MB dye by PES membrane M4 reduces
gradually after each circulation (Fig. 9a).
After repeated adsorption for five runs, the removal
ratio of MB dye by PES membrane M4 reduces from 87.6% to 54.6%.
MB dye are adsorbed
by Fe3O4@PDA nanoparticles in the PES composite membrane, and it is difficult to completely wash away the MB dye molecules from the membrane.
Thus, the adsorption capacity of the
membrane becomes weaker and weaker after each circulation of adsorption.
As expected,
upon adding H2O2, the removal ratio of MB dye by PES composite membrane M4 is still above 90% even after repeated catalytic experiments for five runs because of the catalytic degradation (Fig. 9b and 9c).
The results confirm that the catalytic performance of the PES composite
membranes is well repeatable, and the unique structures of the prepared membranes ensure long-term stability for degradation of organic contaminants [39].
26
Fig. 9.
The repeatability of catalytic performance of PES composite membrane M4.
(a, b)
Adsorption (a) and catalytic (b) performances of PES composite membrane M4 for MB dye without or with H2O2.
(c) Removal ratios of MB dye by PES composite membrane M4 in
repeated experiments at the adsorptive or catalytic time of 80 min in each run. 27
3.6. Simultaneous removal of heavy metal ions and organic contaminants by PES composite membranes
To demonstrate the simultaneous removal of heavy metal ions and organic contaminants by PES composite membranes, the treatment of a mixed solution containing Pb2+ and MB dye by the membrane M4 is investigated (Fig. 10).
The removal efficiency of Pb2+ by PES
composite membrane M4 is more than 80% after permeating 50 mL solution.
Compared with
the results in Fig. 7, the decrease in the removal efficiency of Pb2+ here is caused by the presence of MB dye, which can be competitively adsorbed to the Fe3O4@PDA nanoparticles. the removal efficiency of MB dye maintains above 90%.
While,
The results show that the PES
composite membrane effectively achieves adsorptive removal of Pb2+ and catalytic degradation of MB dye simultaneously.
Fig. 10.
The removal efficiency of heavy metal ion Pb2+ and MB dye in mixed solution by
PES composite membrane M4 with transmembrane flow flux of 2.4 L∙m-2∙h-1.
28
4. Conclusions
In summary, PES composite membranes have been successfully developed for simultaneous adsorption of heavy metal ions and catalytic degradation of organic contaminants by blending Fe3O4@PDA nanoparticles in PES matrix via LIPS.
The permeabilities of PES
composite membranes are significantly improved by introducing the blended Fe3O4@PDA nanoparticles, because the hydrophilic PDA coating on the surface of Fe3O4@PDA nanoparticles enables to produce more microporous structures between the PES matrix and nanoparticles during the LIPS process.
The PES composite membrane containing 20 wt%
Fe3O4@PDA nanoparticles shows water flux as high as 2640 L∙m-2∙h-1∙bar-1, which is six times larger than that of PES blank membrane.
The PES composite membranes show efficient and
repeatable performances for simultaneously removing heavy metal ions and organic contaminants from water.
The proposed PES composite membranes showing multifunction
of simultaneous catalytic degradation of organic contaminants and adsorption of heavy metal ions provide a new pathway to deal with some hard-to-be-treated wastewaters containing both organic contaminants and heavy metal ions from papermaking, leather, textile printing-dyeing industries, and so on.
Importantly, the proposed one-step strategy for preparing composite
membranes with both high permeability and high separation efficiency in this work is easy to be scaled up.
Acknowledgements
29
The authors gratefully acknowledge support from the National Natural Science Foundation of China (21776182, 21490582), and the State Key Laboratory of Polymer Materials Engineering (sklpme2016-3-07).
The authors would like to thank Mr. Yi He and
Mr. Xi Wu at the Analytical & Testing Center of Sichuan University for the help of FESEM imaging and ICP-AES testing.
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Graphical abstract
38
Highlights
Membranes are developed for simultaneous removal of organic contaminants and heavy metal ions. The membranes are fabricated by blending Fe3O4@PDA nanoparticles in PES matrix via LIPS. The Fe3O4@PDA nanoparticles blended in the membranes act as both catalysts and adsorbents. The composite membranes exhibit both large permeability and high separation efficiency.
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