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Citation: Jiang, Q.; Zhang, K.
Optimization of Preparation
Conditions of Poly(m-phenylene
isophthalamide) PMIA Hollow Fiber
Nanofiltration Membranes for
Dye/Salt Wastewater Treatment.
Membranes 2022,12, 1258. https://
doi.org/10.3390/membranes12121258
Academic Editor: Davor Dolar
Received: 18 November 2022
Accepted: 9 December 2022
Published: 13 December 2022
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membranes
Article
Optimization of Preparation Conditions of Poly(m-phenylene
isophthalamide) PMIA Hollow Fiber Nanofiltration
Membranes for Dye/Salt Wastewater Treatment
Qinliang Jiang 1,2 and Kaisong Zhang 2, 3, *
1Institute of Energy Research, Jiangxi Academy of Sciences, Nanchang 330096, China
2Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment,
Chinese Academy of Sciences, Xiamen 361021, China
3Key Lab of Marine Environment and Ecology, Ministry of Education, Ocean University of China,
Qingdao 266100, China
*Correspondence: kszhang@iue.ac.cn or zhangkaisong@ouc.edu.cn
Abstract:
Externally selective thin film composite (TFC) hollow fiber (HF) nanofiltration membranes
(NFMs) hold great industrial application prospects because of their high surface area module. How-
ever, the complicated preparation process of the membrane has hindered its mass manufacture and
application. In this work, PMIA TFC HF NFMs were successfully prepared by the interfacial poly-
merization (IP) of piperazine (PIP) with 1,3,5-benzenetricarbonyl trichloride (TMC). The effect of the
membrane preparation conditions on their separation performance was systematically investigated.
The characterized results showed the successful formation of a polyamide (PA) separation layer on
PMIA HF substrates by the IP process. The as-prepared HF NFMs’ performance under optimized
conditions achieved the highest pure water permeability (18.20 L
·
m
−2·
h
−1
, 0.35 MPa) and superior
salt rejection in the order: R
Na2SO4
(98.30%) > R
MgSO4
(94.60%) > R
MgCl2
(61.48%) > R
NaCl
(19.24%). In
addition, the as-prepared PMIA HF TFC NFMs exhibited desirable pressure resistance at various
operating bars and Na
2
SO
4
feed concentrations. Excellent separation performance of chromotrope
2B dye was also achieved. The as-prepared PMIA HF NFMs thus show great promise for printing
and dyeing wastewater treatment.
Keywords: hollow fiber; nanofiltration; PMIA; preparation conditions; dye/salt wastewater
1. Introduction
In the textile printing industry, the production process inevitably generates and dis-
charges large amounts of wastewater with high salinity and high color [
1
,
2
]. Unfortunately,
a large number of synthetic dyes are present in this wastewater that are difficult to de-
grade because of their stable chemical structure and high molecular weight. Consequently,
excessive discharge of this wastewater causes serious pollution to the water ecological en-
vironment and wastes useful and economical resources [
3
,
4
]. In some fields of application,
dyes are of great economic importance, which has therefore necessitated their recycling
from dyeing wastewater. In addition, a large number of inorganic salts (such as ~6% NaCl
or ~5.6% Na
2
SO
4
) in the wastewater also give the printing and dyeing wastewater a mas-
sive potential recycling value [
5
]. At present, the processes of separating valuable dyes and
inorganic salts contained in printing and dyeing wastewater (including dye purification,
inorganic salt desalting, and treated wastewater reuse) have been of global concern [
6
].
Chiefly, this category of wastewater has been treated by flocculation, coagulation, oxidation,
and biological processes [
7
,
8
]. Unfortunately, most of these treatment methods suffer the
same defects, high energy consumption and cost, and the valuable resources contained in
these wastewaters cannot be fully recycled [9].
Compared with traditional wastewater treatment technology, the NF separation pro-
cess has the advantages of low investment, easy operation, high dye recovery rate, and no
Membranes 2022,12, 1258. https://doi.org/10.3390/membranes12121258 https://www.mdpi.com/journal/membranes
Membranes 2022,12, 1258 2 of 18
adverse impact on the environment, etc. [
10
–
12
]. At present, NF components are mainly
coil membrane components which have the merits of high filling density and low cost of
membrane components [
13
]. However, their disadvantages include increased pressure
drop caused by isolators in fluid channels, susceptibility to contamination, and failure
to backflush. As a result, they usually require a lot of pre-processing. On the contrary,
hollow fiber membranes have a higher packing density than flat sheet membranes and
have backwashing potential in fouling layer removal. At the same time, the usual upstream
pretreatment is omitted, thereby resulting in higher process efficiency and lower operation
costs [
14
]. Nevertheless, the commercial preparation technology of hollow fiber nanofil-
tration membranes is not completely mature, and there are not many mature and stable
hollow fiber nanofiltration membranes in the business. Therefore, adopting more simple
and mature membrane preparation parameters is pertinent to preparing economically
advanced nanofiltration membranes.
The most common commercial TFC NFMs currently directed at treating dye wastew-
ater treatment are usually prepared by the IP method [
15
,
16
]. Currently, the TFC NFM
available in the exchanges is mainly composed of three layers: the non-woven support layer,
a porous polymer substrate layer, and an ultra-thin barrier layer [
17
]. Meanwhile, HF TFC
NFM is a self-supporting structure, which is mainly composed of two parts: a porous poly-
mer substrate, and dense layers [
18
]. Although the NFM’s performance mainly depends
on the structure of the dense layer, the material properties of the polymer substrate layer
also have an obvious effect on the microstructure of the membrane separation layer [
19
,
20
].
Currently, most HF TFC NFMs use polysulfone (PSF), polyethersulfone (PES), or polyvinyl
chloride (PVC) ultrafiltration membranes as substrates because of their attractive physical
and chemical properties, which facilitate IP [
21
–
23
]. However, the low chemical resistance
of these substrates to ketones and alcohols limits NFMs’ structural stability. Given this,
researchers have attempted to develop different kinds of porous support layers, such as
polypropylene (PP), polyethylene (PE), and polyvinylidene fluoride (PVDF) microfiltration
membranes, which are used to produce commercial non-polar polymer membranes because
of their high solvent stability, good pressure resistance, low cost, and high porosity [
23
,
24
].
However, their poor surface wettability restricts the aqueous solution’s spread, making
it difficult to generate a polyamide separable layer on the substrate surface by the IP pro-
cess. Therefore, this justifies the need to find new substrate materials to overcome the
above limitations.
Poly (m-phenyleneisophthalamide) (PMIA) has been reported to be a very attractive
material for membrane preparation [
25
], because of its outstanding chemical stability,
lasting thermal stability, excellent flame retardance, relatively low cost, easy processing,
and excellent mechanical properties [
26
–
28
]. For instance, Chen et al. prepared a thermally
stable PMIA TFC NFM by the IP method on PMIA flat substrates [
27
]. However, the
preparation process of flat sheet TFC NF membranes is relatively simple and mature, and
the prepared membranes do not have significant separation advantages compared to other
reported related NF membranes. Additionally, Wang et al. prepared a PMIA HF substrate
and successfully synthesized a polyamide separation layer on the substrate by the IP
method. The prepared membrane displayed relatively good separation performance [
24
].
However, this study only reported the separation performance of the NF membrane under
one preparation condition. As a matter of importance, the IP parameters usually have
a great influence on the performance of HF TFC NFMs during the process of large-scale
industrial production. Therefore, effective optimization of the process parameters in
membrane preparation is of great importance.
On this backdrop, piperazine (PIP) (selected as the monomer of the water phase)
and 1,3,5-benzenetricarbonyl trichloride (TMC) (selected as the monomer of the organic
phase) were employed to develop a PA separation layer on the outer surface of a PMIA HF
substrate by the IP process. The preparation conditions of the HF NFMs were thoroughly
investigated by optimizing the membrane preparation process (including the effects of
monomer concentration, water contact time, and IP time) on the properties of the mem-
Membranes 2022,12, 1258 3 of 18
branes. The separation characteristics of prepared membranes under different operating
conditions were evaluated from two aspects of permeability and rejection rate using a
simulated saline dye solution, and the optimal preparation conditions were obtained.
2. Experimental
2.1. Materials
Hollow fiber substrates were prepared using PMIA (DuPont Company, Wilmington,
DE, USA). LiCl was acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
China). Polyethylene glycols (PEG, Mw = 400 g mol
−1
), PIP, TMC, and four inorganic salts
(Na
2
SO
4
, MgSO
4
, MgCl
2
, NaCl) were procured from Shanghai Aladdin Reagent Co. Ltd.
(Shanghai, China). N,N-dimethylacetamide (DMAC, >99%) was purchased from Shanghai
Jingwei Chemical Co., Ltd. (Shanghai, China). Chromotrope 2B (Mw = 513 g mol
−1
) and
Janus Green B (511 g mol
−1
) were acquired from Sigma Aldrich (Darmstadt, Germany) and
the characteristics of these two dyes are detailed in Table 1. All other chemicals involved in
experiments were acquired from Sinopharm Chemical Reagent Shanghai Co. (Shanghai,
China). The chemical structure of PMIA were shown in Figure 1.
Table 1. Characteristics of the Chromotrope 2B and Janus Green B.
Dye Name Molecular Structure Dye Types Relative Molecular
Weight (Da) Charge
Chromotrope 2B
Membranes 2022, 12, x FOR PEER REVIEW 3 of 20
On this backdrop, piperazine (PIP) (selected as the monomer of the water phase) and
1,3,5-benzenetricarbonyl trichloride (TMC) (selected as the monomer of the organic
phase) were employed to develop a PA separation layer on the outer surface of a PMIA
HF substrate by the IP process. The preparation conditions of the HF NFMs were thor-
oughly investigated by optimizing the membrane preparation process (including the ef-
fects of monomer concentration, water contact time, and IP time) on the properties of the
membranes. The separation characteristics of prepared membranes under different oper-
ating conditions were evaluated from two aspects of permeability and rejection rate using
a simulated saline dye solution, and the optimal preparation conditions were obtained.
2. Experimental
2.1. Materials
Hollow fiber substrates were prepared using PMIA (DuPont Company, USA). LiCl
was acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polyeth-
ylene glycols (PEG, Mw = 400 g mol−1), PIP, TMC, and four inorganic salts (Na2SO4,
MgSO4, MgCl2, NaCl) were procured from Shanghai Aladdin Reagent Co. Ltd. (Shanghai,
China). N,N-dimethylacetamide (DMAC, >99%) was purchased from Shanghai Jingwei
Chemical Co., Ltd. (Shanghai, China). Chromotrope 2B (Mw= 513 g mol−1) and Janus
Green B (511 g mol−1) were acquired from Sigma Aldrich (Darmstadt, Germany) and the
characteristics of these two dyes are detailed in Table 1. All other chemicals involved in
experiments were acquired from Sinopharm Chemical Reagent Shanghai Co. (China). The
chemical structure of PMIA were shown in Figure 1.
Figure 1. Chemical structure of PMIA.
Table 1. Characteristics of the Chromotrope 2B and Janus Green B.
Dye Name
Molecular Structure
Dye Types
Relative Molecular
Weight (Da)
Charge
Chromotrope 2B
Acid dyes
513.37
−2
Janus Green B
Basic dyes
511.06
+1
2.2. Preparation of PMIA Substrate and HF NFMs
2.2.1. Preparation of PMIA HF Substrate
Acid dyes 513.37 −2
Janus Green B
Membranes 2022, 12, x FOR PEER REVIEW 3 of 20
On this backdrop, piperazine (PIP) (selected as the monomer of the water phase) and
1,3,5-benzenetricarbonyl trichloride (TMC) (selected as the monomer of the organic
phase) were employed to develop a PA separation layer on the outer surface of a PMIA
HF substrate by the IP process. The preparation conditions of the HF NFMs were thor-
oughly investigated by optimizing the membrane preparation process (including the ef-
fects of monomer concentration, water contact time, and IP time) on the properties of the
membranes. The separation characteristics of prepared membranes under different oper-
ating conditions were evaluated from two aspects of permeability and rejection rate using
a simulated saline dye solution, and the optimal preparation conditions were obtained.
2. Experimental
2.1. Materials
Hollow fiber substrates were prepared using PMIA (DuPont Company, USA). LiCl
was acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polyeth-
ylene glycols (PEG, Mw = 400 g mol−1), PIP, TMC, and four inorganic salts (Na2SO4,
MgSO4, MgCl2, NaCl) were procured from Shanghai Aladdin Reagent Co. Ltd. (Shanghai,
China). N,N-dimethylacetamide (DMAC, >99%) was purchased from Shanghai Jingwei
Chemical Co., Ltd. (Shanghai, China). Chromotrope 2B (Mw= 513 g mol−1) and Janus
Green B (511 g mol−1) were acquired from Sigma Aldrich (Darmstadt, Germany) and the
characteristics of these two dyes are detailed in Table 1. All other chemicals involved in
experiments were acquired from Sinopharm Chemical Reagent Shanghai Co. (China). The
chemical structure of PMIA were shown in Figure 1.
Figure 1. Chemical structure of PMIA.
Table 1. Characteristics of the Chromotrope 2B and Janus Green B.
Dye Name
Molecular Structure
Dye Types
Relative Molecular
Weight (Da)
Charge
Chromotrope 2B
Acid dyes
513.37
−2
Janus Green B
Basic dyes
511.06
+1
2.2. Preparation of PMIA Substrate and HF NFMs
2.2.1. Preparation of PMIA HF Substrate
Basic dyes 511.06 +1
Membranes 2022, 12, x FOR PEER REVIEW 3 of 20
On this backdrop, piperazine (PIP) (selected as the monomer of the water phase) and
1,3,5-benzenetricarbonyl trichloride (TMC) (selected as the monomer of the organic
phase) were employed to develop a PA separation layer on the outer surface of a PMIA
HF substrate by the IP process. The preparation conditions of the HF NFMs were thor-
oughly investigated by optimizing the membrane preparation process (including the ef-
fects of monomer concentration, water contact time, and IP time) on the properties of the
membranes. The separation characteristics of prepared membranes under different oper-
ating conditions were evaluated from two aspects of permeability and rejection rate using
a simulated saline dye solution, and the optimal preparation conditions were obtained.
2. Experimental
2.1. Materials
Hollow fiber substrates were prepared using PMIA (DuPont Company, USA). LiCl
was acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polyeth-
ylene glycols (PEG, Mw = 400 g mol−1), PIP, TMC, and four inorganic salts (Na2SO4,
MgSO4, MgCl2, NaCl) were procured from Shanghai Aladdin Reagent Co. Ltd. (Shanghai,
China). N,N-dimethylacetamide (DMAC, >99%) was purchased from Shanghai Jingwei
Chemical Co., Ltd. (Shanghai, China). Chromotrope 2B (Mw= 513 g mol−1) and Janus
Green B (511 g mol−1) were acquired from Sigma Aldrich (Darmstadt, Germany) and the
characteristics of these two dyes are detailed in Table 1. All other chemicals involved in
experiments were acquired from Sinopharm Chemical Reagent Shanghai Co. (China). The
chemical structure of PMIA were shown in Figure 1.
Figure 1. Chemical structure of PMIA.
Table 1. Characteristics of the Chromotrope 2B and Janus Green B.
Dye Name
Molecular Structure
Dye Types
Relative Molecular
Weight (Da)
Charge
Chromotrope 2B
Acid dyes
513.37
−2
Janus Green B
Basic dyes
511.06
+1
2.2. Preparation of PMIA Substrate and HF NFMs
2.2.1. Preparation of PMIA HF Substrate
Figure 1. Chemical structure of PMIA.
2.2. Preparation of PMIA Substrate and HF NFMs
2.2.1. Preparation of PMIA HF Substrate
The PMIA HF substrate was fabricated using dry–wet phase inversion technology [
29
].
The casting solution formulation of the above substrate was 14 wt.% PMIA, 4 wt.% LiCl,
3 wt.% PEG, and 79 wt.% DMAC. The pre-determined PMIA, solvent, and additives were
dissolved in the flask in an appropriate order and pre-dissolved at 70
◦
C for a certain
time, followed by mechanical continuous stirring for at least 12 h. After the polymer was
completely dissolved, the dope solution was defoamed for use with continuous stirring
for at least 12 h. After the PMIA fibers were completely dissolved, the casting solution
was vacuum defoamed for use. The polymer solution and bore fluid are extruded together
under the action of a precision syringe pump equipped with a device to control the flow
rate of the viscous fluid. The nascent fibers were collected by the roller with a fixed take-up
speed through an air bath and a coagulant bath (tap water), and the related performance
Membranes 2022,12, 1258 4 of 18
parameters of the PMIA HF substrates are detailed shown in Tables S1 and S2. The
detailed spinning parameters are summarized in our previous study [
30
], and the casting
equipment was shown in Figure S1. The picture and SEM cross-section images of the PMIA
HF substrate was shown in Figure S2.
2.2.2. Preparation of PMIA HF TFC NF Membranes
The PMIA HF NFMs were obtained using IP technology. First, the homemade PMIA
HF porous substrates were soaked in deionized water to be used, drained in air, and then
put into the aqueous phase solution containing PIP to be submerged for a few minutes.
Then the PMIA substrate was removed from the aqueous phase, and the excess droplets on
the surface of the substrate were gently absorbed with paper towels and then dried with a
hair dryer for a few minutes. Next, the blown dry PMIA substrate was quickly transferred
to the oil phase solution of TMC for interfacial polymerization reaction for a few seconds.
Finally, the membrane was removed and dried in an oven at 60
◦
C for 2 min and stored in
deionized water until use before testing.
2.2.3. Characterizations of Membranes
SEM images of the prepared PMIA HF substrates and PMIA HF NFMs were collected
by a field emission scanning electron microscope (FE-SEM, HITACHI S4800, Hitachi,
Tokyo, Japan). A Hitachi E-1010 ion sputtering coating machine was used to sputter gold
nanoparticles in a vacuum before testing. Fourier transform infrared spectroscopy (FTIR,
Nicolet iS50, Nicolet, Madison, WI, USA) with a wavenumber range of 650–4000 cm
−1
was applied to analyze the IP reaction occurring on the PMIA HF substrates. The surface
elements of the relevant membrane were determined by X-ray photoelectron spectrometry
(XPS, Axis Supra, Kratos, Manchester, UK). The water contact angle (CA) of the prepared
membranes was evaluated by a CA analyzer (DSA100, KRUSS, Hamburg, Germany). The
surface roughness of the membranes was analyzed by atomic force microscopy (AFM,
Dimension Icon, Bruker, USA). The surface zeta potential of the membranes was tested
using streaming potential measurements with a SurPASS electrokinetic analyzer (SurPASS
3, Anton Parr GmbH, Glaz, Austria).
2.2.4. Permeation Experiments
The permeability of the prepared HF NFMs was investigated by a crossflow filtration
experiment after the pre-pressured process at 0.5 MPa with deionized water for 5 h and
collected permeate at 0.35 MPa. The effective membrane area was about 14.1 cm
2
. The
permeability of the membrane was then characterized by the rejection of the Na
2
SO
4
aqueous
solution (2000 ppm). The conductivity of inorganic salts was tested with a conductivity meter
(SensION + EC5, HACH, Loveland, CO, USA). To further evaluate the treatment potential
of the membranes in question for salt-containing dye solutions, they were used to separate
simulated textile wastewater. The concentration of dyes in the produced water is determined
by a UV-Vis spectrophotometer (UV3600, Shimadzu, Kyoto, Japan). Permeability flux (J)
and rejection (R) were expressed by Equations (1) and (2), respectively.
Jw=
Q
A·t(1)
R= 1−Cp
Cf!(2)
where J
w
is the permeation flux of the PMIA HF TFC NF membrane (L
·
m
−2·
h
−1
), Qis
water volume (L), Adenotes the membrane area (m
2
), and t is the filtration time (h). R is
solute rejection, Cfis feed concentration (mg/L), Cp is permeate concentration (mg/L).
Membranes 2022,12, 1258 5 of 18
3. Results and Discussion
3.1. The Effects of Preparation Conditions
To obtain optimal preparation conditions, the effects of IP reaction conditions (PIP and
TMC concentration, aqueous phase immersion time, IP reaction time) on the membrane
properties of the PMIA HF NFMs were investigated separately.
First, the effects of PIP in the aqueous phase solution and TMC concentration in the
oil phase solution on the membrane separation performance of PMIA HF NFMs were
investigated. The nanofiltration test conditions were 25
◦
C and 0.35 MPa, respectively. The
results were presented in Figure 2a,b. Figure 2a illustrated the curve of PMIA HF NFMs’
permeability with the change of water phase PIP concentration. As seen in Figure 2a, as
the PIP concentration in the aqueous phase was enhanced from 1.0 wt.% to 2.5 wt.%, the
pure water flux gradually decreased from 17.1 L
·
m
−1·
h
−1
to 13.5 L
·
m
−1·
h
−1
, whereas the
Na
2
SO
4
rejection increased from 82.6% to 96.5%. However, when the PIP concentration
was less than 2.0 wt.%, the PWF decreased rapidly and the Na
2
SO
4
rejection rate rose
quickly. With the PIP content exceeding 2.0 wt.%, both the PWF and the membrane
rejection performance changed slowly. This is mainly because when the PIP concentration
is less than 2.0 wt.%, with the rise of the PIP concentration, TMC and PIP undergo a
polymerization reaction at the interface to form a dense network macromolecular layer,
resulting in a rapid decreased in membrane flux and a rapid increased in salt rejection.
When the PIP concentration is greater than 2.0 wt.%, the amount of PIP diffusion into the
oil phase increases, resulting in a gradual thickening of the polyamide layer thickness and
only a slight increase in rejection rate.
The influence of TMC concentration in the oil phase solution on membrane permeabil-
ity is displayed in Figure 2b. When the concentration of TMC was lower than 0.35 w/v%,
the effects of TMC and PIP concentrations on the performance of PMIA HF NFMs showed
the same trend. When TMC concentration is further increased (w/v% > 0.35), it is mainly
due to the reaction rate being too fast, which resulted in an incomplete reaction of the PA
layer. As a result, the flux of the membrane increased and salt rejection declined.
Figure 2c showed the variation curve of PMIA HF TFC NFMs’ performance with the
water phase soak time. When the immersion time of the aqueous phase was less than 60 s,
with the prolonging of water phase soak time, the water flux of the membrane showed a
downward trend, while the salt rejection showed an upward trend. This is mainly because
the amount of PIP monomers in the permeating membrane hole will gradually increase
with soaking time in the aqueous phase. The density of the effective separation layer
thickness formed on the surface of the basement membrane rose continuously. Thus, the
NFMs flux decreased and the rejection rate increased.
Figure 2d shows that with the IP reaction time being enhanced from 10 to 30 s, the
PWF decreased while the Na
2
SO
4
rejection rate rose quickly. This is mainly because PIP
and TMC were highly reactive monomers, and the contact reaction rate constant between
them was very fast, leading to a rapid interfacial polymerization reaction at the interface
between the two phases, resulting in a dense separation layer, which leads to a decrease
in flux and a rise in rejection rate. While the IP reaction time exceeded 30 s, the Na
2
SO
4
rejection tended to stabilize and the permeation flux decreased.
In summary, the optimal preparation conditions by combining the above membrane
production processes parameters of the prepared PMIA HF TFC NF membrane were
achieved at an aqueous phase PIP concentration of 1.8 wt.%, aqueous phase soak time of
1 min, oil-phase TMC concentration of 0.35 w/v%, and IP time of 30 s.
Membranes 2022,12, 1258 6 of 18
Membranes 2022, 12, x FOR PEER REVIEW 6 of 20
Figure 2. The effects of preparation conditions on the separation properties of the PMIA HF TFC NF
membrane. (a) the effect of PIP concentration (TMC concentration was 0.35 w/v%, and the water
phase soak and IP reaction time were 1 min and 20 s, respectively). (b) the effect of TMC concentra-
tion (PIP concentration was 1.8 wt.%, and the water phase soak and IP reaction time were 1 min and
20 s, respectively). (c) the effect of water phase soak time (PIP and TMC concentrations were 1.8
wt.% and 0.35 w/v%, respectively, and the IP reaction time was 20 s). (d) the effect of IP reaction time
(PIP and TMC concentrations were 1.8 wt.% and 0.35 w/v%, respectively, and water phase soak time
was 1 min).
Figure 2d shows that with the IP reaction time being enhanced from 10 to 30 s, the
PWF decreased while the Na2SO4 rejection rate rose quickly. This is mainly because PIP
and TMC were highly reactive monomers, and the contact reaction rate constant between
them was very fast, leading to a rapid interfacial polymerization reaction at the interface
between the two phases, resulting in a dense separation layer, which leads to a decrease
in flux and a rise in rejection rate. While the IP reaction time exceeded 30 s, the Na2SO4
rejection tended to stabilize and the permeation flux decreased.
In summary, the optimal preparation conditions by combining the above membrane
production processes parameters of the prepared PMIA HF TFC NF membrane were
achieved at an aqueous phase PIP concentration of 1.8 wt.%, aqueous phase soak time of
1 min, oil-phase TMC concentration of 0.35 w/v%, and IP time of 30 s.
Figure 2.
The effects of preparation conditions on the separation properties of the PMIA HF TFC NF
membrane. (
a
) the effect of PIP concentration (TMC concentration was 0.35 w/v%, and the water
phase soak and IP reaction time were 1 min and 20 s, respectively). (
b
) the effect of TMC concentration
(PIP concentration was 1.8 wt.%, and the water phase soak and IP reaction time were 1 min and
20 s, respectively). (
c
) the effect of water phase soak time (PIP and TMC concentrations were 1.8 wt.%
and 0.35 w/v%, respectively, and the IP reaction time was 20 s). (
d
) the effect of IP reaction time
(PIP and TMC concentrations were 1.8 wt.% and 0.35 w/v%, respectively, and water phase soak time
was 1 min).
3.2. Membrane Characterization
3.2.1. Surface Properties
The SEM images of the PMIA substrate and the PMIA HF TFC NFMs are depicted
in Figure 3. Figure 3a shows that the surface of the PMIA substrate is smooth and porous,
while its cross-section in Figure 3c reveals a defect-free sub-structure and other multiples of
cross-section information are detailed in Figure S2. As can be seen from Figure 3b, there
are no visible pores on the surface of the PMIA HF NFMs. On the contrary, compared to
the morphologies of the PMIA substrate surface, there are some rounded bumps on the
surface of the PMIA NFM. Furthermore, the cross-section of the PMIA HF NFMs shows
that a dense layer with a thickness of about 176 nm was formed on the surface of the
PMIA substrate. In Figure 3d, there was an indication that the PA layer was resoundingly
developed on the surface of the PMIA HF substrate.
Membranes 2022,12, 1258 7 of 18
Membranes 2022, 12, x FOR PEER REVIEW 7 of 20
3.2. Membrane Characterization
3.2.1. Surface Properties
The SEM images of the PMIA substrate and the PMIA HF TFC NFMs are depicted in
Figure 3. Figure 3a shows that the surface of the PMIA substrate is smooth and porous,
while its cross-section in Figure 3c reveals a defect-free sub-structure and other multiples
of cross-section information are detailed in Figure S2. As can be seen from Figure 3b, there
are no visible pores on the surface of the PMIA HF NFMs. On the contrary, compared to
the morphologies of the PMIA substrate surface, there are some rounded bumps on the
surface of the PMIA NFM. Furthermore, the cross-section of the PMIA HF NFMs shows
that a dense layer with a thickness of about 176 nm was formed on the surface of the PMIA
substrate. In Figure 3d, there was an indication that the PA layer was resoundingly devel-
oped on the surface of the PMIA HF substrate.
The AFM photos of the PMIA substrates and PMIA HF TFC NFMs were shown in
Figure 4. It can be seen from Figure 4b that, compared with Figure 4a, the PMIA NF mem-
brane surface presents a ridge, and valley structure, which is consistent with SEM photos
of the PMIA HF NFM’s surface. This is a typical reactive film layer formed mainly due to
the polymerization of PIP in the aqueous phase and TMC in the oil phase when the aque-
ous and oil phases come into contact [27,31]. In addition, as can be seen from the data in
Table 2, as a result of the ridge and valley structure of the polyamide layer, the Ra value
of the PMIA NF membrane surface is greater than the Ra value of the substrate, which
means that the roughness of the PMIA NF film surface is higher than the roughness of the
substrate surface. Therefore, the results further reinforced the fact that a dense PA layer
was triumphantly formed on PMIA HF multi-aperture support layer.
Figure 3. Morphologies of surface (a,c) and cross-section (b,d) of PMIA hollow fiber substrate mem-
brane and NF membrane, respectively.
Figure 3.
Morphologies of surface (
a
,
c
) and cross-section (
b
,
d
) of PMIA hollow fiber substrate
membrane and NF membrane, respectively.
The AFM photos of the PMIA substrates and PMIA HF TFC NFMs were shown in
Figure 4. It can be seen from Figure 4b that, compared with Figure 4a, the PMIA NF
membrane surface presents a ridge, and valley structure, which is consistent with SEM
photos of the PMIA HF NFM’s surface. This is a typical reactive film layer formed mainly
due to the polymerization of PIP in the aqueous phase and TMC in the oil phase when the
aqueous and oil phases come into contact [
27
,
31
]. In addition, as can be seen from the data
in Table 2, as a result of the ridge and valley structure of the polyamide layer, the Ra value
of the PMIA NF membrane surface is greater than the Ra value of the substrate, which
means that the roughness of the PMIA NF film surface is higher than the roughness of the
substrate surface. Therefore, the results further reinforced the fact that a dense PA layer
was triumphantly formed on PMIA HF multi-aperture support layer.
Table 2. Surface properties of membranes.
Membrane
Roughness Values Water Contact Angle
(o)
Ra Rq
PMIA substrate 15 ±0.32 19 ±0.31 65.3 ±2.7
PMIA TFC NF 43 ±0.43 58 ±0.46 54.6 ±3.3
Membranes 2022,12, 1258 8 of 18
Membranes 2022, 12, x FOR PEER REVIEW 8 of 20
Figure 4. AFM photos of PMIA substrate (a) and PMIA HF TFC NF membrane (b).
Figure 4. AFM photos of PMIA substrate (a) and PMIA HF TFC NF membrane (b).
The Zeta potential of the NFM’s surface presents different surface charge properties at
different pH values of the test solution. Figure 5reveals the Zeta potential of the PMIA HF
NFMs at different pH values. Under low pH conditions, due to the adsorption of H
+
, the NF
membrane surface has a positive charge. At a high pH value, as a result of OH
−
adsorption,
the membrane surface possesses a negative charge [
32
]. The PMIA HF NFM’s surface
presented a negatively charged property when the pH values were between 3 and 9. This
is mainly related to the -COOH generated by the hydrolysis of TMC with an incomplete IP
reaction [
33
]. In addition, to further understand the variation of hydrophilicity between
the PMIA HF substrate and PMIA HF NFM’s surface, the water CA values of the PMIA
HF support layer and PMIA HF NFM’s surface were tested, and the results are shown in
Figure S4 and Table 2. The results show that PMIA HF NFMs have stronger hydrophilicity
compared to the PMIA substrate by reason that the existence of hydrophilic functional
groups, for instance, carboxylic acid and amine terminal groups on the surface of PMIA
NFMs [25].
Membranes 2022, 12, x FOR PEER REVIEW 9 of 20
Table 2. Surface properties of membranes.
Membrane
Roughness Values
Water Contact Angle
(o)
Ra
Rq
PMIA substrate
15 ± 0.32
19 ± 0.31
65.3 ± 2.7
PMIA TFC NF
43 ± 0.43
58 ± 0.46
54.6 ± 3.3
The Zeta potential of the NFM’s surface presents different surface charge properties
at different pH values of the test solution. Figure 5 reveals the Zeta potential of the PMIA
HF NFMs at different pH values. Under low pH conditions, due to the adsorption of H+,
the NF membrane surface has a positive charge. At a high pH value, as a result of OH−
adsorption, the membrane surface possesses a negative charge [32]. The PMIA HF NFM’s
surface presented a negatively charged property when the pH values were between 3 and
9. This is mainly related to the -COOH generated by the hydrolysis of TMC with an in-
complete IP reaction [33]. In addition, to further understand the variation of hydrophilic-
ity between the PMIA HF substrate and PMIA HF NFM’s surface, the water CA values of
the PMIA HF support layer and PMIA HF NFM’s surface were tested, and the results are
shown in Figure S4 and Table 2. The results show that PMIA HF NFMs have stronger
hydrophilicity compared to the PMIA substrate by reason that the existence of hydrophilic
functional groups, for instance, carboxylic acid and amine terminal groups on the surface
of PMIA NFMs [25].
Figure 5. Potential values of PMIA HF TFC NF membrane at various pH values.
3456789
-80
-60
-40
-20
0
20
Zeta Potential (mV)
pH values
Zeta potential
Figure 5. Potential values of PMIA HF TFC NF membrane at various pH values.
Membranes 2022,12, 1258 9 of 18
3.2.2. Chemical Properties of the Membrane
To identify the elemental composition of the membrane surface, elemental analysis
was implemented on the PMIA HF substrate and PMIA HF NF film surface. XPS was used
for the elemental analysis of polymer surfaces. In general, the XPS detection depth was
around 5–6 nm, which was much lower than the separation layer thickness [
32
,
34
]. Table 3
shows the composition of carbon, oxygen, and nitrogen elements on the PMIA substrates
and PMIA HF NFM’s surface.
Table 3. XPS analysis membrane samples.
Samples Atomic Composition from XPS (%)
C/(%) O/(%) N/(%) O/N
PMIA substrate 72.26 17.44 10.3 1.69
PMIA TFC NF membrane 73.41 15.33 11.26 1.36
Theoretically calculated date
Totally crosslinking structure 71.42 14.29 14.29 1.0
Fully linear structure
substituted with two carboxyl 65.00 25.00 10.00 2.50
Linear structure substituted
with one carboxyl 68.42 21.05 10.53 2.00
In general, PA layer structures obtained through the IP process can be classified into
three different types according to the O/N ratio. That is, when the calculated O/N ratio is
1.0, the PA structure is fully cross-linked. If the calculated O/N ratio is 2.0, the PA structure
is a linear structure substituted by one carboxyl group. Finally, when the calculated O/N
ratio is 2.5, it is considered a completely linear structure substituted by two carboxyl groups.
Based on the O/N ratio data on the surface of the PMIA NFMs in Table 3, the results in
Table 3show that PMIA NF membranes are more likely to form cross-linked structures,
which was similar to the previously reported results [24].
FTIR was applied to analyze the differences of chemical functional groups on the
surface of the PMIA substrate and PMIA HF NFMs, and the results are shown in Figure 6.
As can be seen from Figure 6, there is no major change in the spectral peaks between
650~2700 cm
−1
for the PMIA substrates and PMIA NFMs. This is because both PMIA and
PA layers are aromatic polyamide families and therefore have similar functional groups [
24
].
The peak at 1654 cm
−1
in the PMIA HF substrate layer and the PA layer is attributed to
the C=O asymmetric stretching vibration. Although the PA layer and PMIA HF substrate
layer revealed the same peaks at 3300 cm
−1
(-NH stretching) and peaks around 2920 cm
−1
(aliphatic C-H bond), while the FTIR intensity at 3300 cm
−1
and peaks around 2920 cm
−1
of
the PA layer was less than that of the PMIA support layer [
35
]. This was mainly because the
PMIA substrate’s surface was covered with a PA layer, which will affect the FTIR strength
of the PMIA NFM’s surface. This is consistent with other studies [
23
]. All these indicate
that the PA layer is triumphantly polymerized on the PMIA HF substrate layer.
3.3. Permeate and Salt Retention Performance of PMIA HF TFC NFMs
The ionic selectivity of the NF membrane is mainly determined by the combination of
the Donnan effect and the screening effect. To investigate the permeate and salt rejection
performance of the PMIA HF NFMs on inorganic salts, four typical inorganic salts were
selected to investigate the properties of the prepared membrane, and the results are depicted
in Figure 7.
Membranes 2022,12, 1258 10 of 18
Membranes 2022, 12, x FOR PEER REVIEW 11 of 20
Figure 6. FTIR spectra of PMIA substrate and PMIA TFC membrane.
3.3. Permeate and Salt Retention Performance of PMIA HF TFC NFMs
The ionic selectivity of the NF membrane is mainly determined by the combination
of the Donnan effect and the screening effect. To investigate the permeate and salt rejec-
tion performance of the PMIA HF NFMs on inorganic salts, four typical inorganic salts
were selected to investigate the properties of the prepared membrane, and the results are
depicted in Figure 7.
Figure 6. FTIR spectra of PMIA substrate and PMIA TFC membrane.
Membranes 2022, 12, x FOR PEER REVIEW 12 of 20
8
10
12
14
16
18
Flux (L·m−2·h −1)
F
R
Na2SO4MgSO4MgCl2NaCl 0
10
20
30
40
50
60
70
80
90
100
Rejection (%)
Figure 7. Separation performance of the PMIA HF TFC NF membrane on inorganic salts.
As seen in Figure 7, the order of salt retention in different valence states by PMIA
hollow fiber nanofiltration membrane is RNa2SO4 (98.30%) > RMgSO4 (94.60%) > RMgCl2 (61.48%)
> RNaCl (19.24%), which is consistent with the order of retention of salt in different valence
states by negative charge NF membrane in the literature [22,27]. This is mainly because
Na2SO4 and NaCl, and MgSO4 and MgCl2 have the same cation, respectively, and anions
with the same charge as the membrane surface were repelled by electrostatic repulsion.
The higher the valence state is, the more obvious the repulsion. Thus, the rejection rate of
Na2SO4 was greater than that of NaCl, and that of MgSO4 was higher than that of MgCl2,
which conformed to Donnan’s exclusion effect [24]. As for Na2SO4 and MgSO4 containing
the same anion, the hydrated radius of Mg2+ (0.428 nm) is higher than the hydrated radius
of Na+ (0.358 nm), and the divalent Mg2+ was easier to adsorbe on the surface of the neg-
atively charged PMIA HF NFMs, which have adsorption and shielding effects on the sur-
face charge of the membrane [36]. Therefore, when there was the same anion (SO42-), the
rejection rate of Na2SO4 was greater than that of MgSO4, which was consistent with the
results of the compound nanofiltration membrane with a negative charge [37].
The separation performance of the PMIA NFMs was different due to the different
solution salt concentrations and operation pressure. Therefore, it was of great significance
to study the relationship between salt solution concentration and membrane separation
performance to understand the membrane separation process. Salt solutions of 1000, 2000,
3000, and 4000 ppm Na2SO4 were prepared as the solution to be tested, and the influence
of solution concentration changes on membrane separation performance were investi-
gated at 25 °C and 0.35 MPa, and the results are displayed in Figure 8a. Figure 8a shows
that the rejection rate and flux of the membrane decreased to some extent with the increase
of salt concentration of the feed solution. When the operating pressure was constant, the
osmotic pressure of the solution increased with the increase of solution salt concentration,
and the driving force of the solution through the composite membrane decreased, thereby
Figure 7. Separation performance of the PMIA HF TFC NF membrane on inorganic salts.
Membranes 2022,12, 1258 11 of 18
As seen in Figure 7, the order of salt retention in different valence states by PMIA
hollow fiber nanofiltration membrane is R
Na2SO4
(98.30%) > R
MgSO4
(94.60%) > R
MgCl2
(61.48%) > R
NaCl
(19.24%), which is consistent with the order of retention of salt in different
valence states by negative charge NF membrane in the literature [
22
,
27
]. This is mainly
because Na
2
SO
4
and NaCl, and MgSO
4
and MgCl
2
have the same cation, respectively,
and anions with the same charge as the membrane surface were repelled by electrostatic
repulsion. The higher the valence state is, the more obvious the repulsion. Thus, the
rejection rate of Na
2
SO
4
was greater than that of NaCl, and that of MgSO
4
was higher
than that of MgCl
2
, which conformed to Donnan’s exclusion effect [
24
]. As for Na
2
SO
4
and MgSO
4
containing the same anion, the hydrated radius of Mg
2+
(0.428 nm) is higher
than the hydrated radius of Na+ (0.358 nm), and the divalent Mg
2+
was easier to adsorbe
on the surface of the negatively charged PMIA HF NFMs, which have adsorption and
shielding effects on the surface charge of the membrane [
36
]. Therefore, when there was the
same anion (SO
42−
), the rejection rate of Na
2
SO
4
was greater than that of MgSO
4
, which
was consistent with the results of the compound nanofiltration membrane with a negative
charge [37].
The separation performance of the PMIA NFMs was different due to the different
solution salt concentrations and operation pressure. Therefore, it was of great significance
to study the relationship between salt solution concentration and membrane separation
performance to understand the membrane separation process. Salt solutions of 1000, 2000,
3000, and 4000 ppm Na
2
SO
4
were prepared as the solution to be tested, and the influence
of solution concentration changes on membrane separation performance were investigated
at 25
◦
C and 0.35 MPa, and the results are displayed in Figure 8a. Figure 8a shows that the
rejection rate and flux of the membrane decreased to some extent with the increase of salt
concentration of the feed solution. When the operating pressure was constant, the osmotic
pressure of the solution increased with the increase of solution salt concentration, and the
driving force of the solution through the composite membrane decreased, thereby resulting
in a decrease in water flux. The decrease in Na
2
SO
4
rejection is mainly because PMIA HF
TFC NFMs carry a fixed charge, and according to Donnan balance, a higher content of
inorganic salts in the feed solution results in a higher concentration of ions of opposite
charge in the membrane pore [
38
]. That is to say, the higher the salt concentration in the
membrane pore, the higher the inorganic salt concentration in the osmotic solution, and
therefore, the membrane rejection rate is reduced.
Membranes 2022, 12, x FOR PEER REVIEW 13 of 20
resulting in a decrease in water flux. The decrease in Na2SO4 rejection is mainly because
PMIA HF TFC NFMs carry a fixed charge, and according to Donnan balance, a higher
content of inorganic salts in the feed solution results in a higher concentration of ions of
opposite charge in the membrane pore [38]. That is to say, the higher the salt concentration
in the membrane pore, the higher the inorganic salt concentration in the osmotic solution,
and therefore, the membrane rejection rate is reduced.
Figure 8. Effects of Na2SO4 concentration (a) and operation pressure (b) on the permeability of
PMIA.
Figure 8b shows that the retention rate of Na2SO4 by the PMIA HF NFMs increased
slightly with the increase in operating pressure, and the permeate flux of the NFMs in-
creased in a linear relationship during the pressure increase. The retention rate of the
membrane remained stable and the permeate flux of the solution increased from 6.2 to
27.6 L−1·m −2·h −1 when the pressure rose from 0.2 to 0.8 MPa. The effect of operation pres-
sure on the removal of Chromotrope 2B dyes by PMIA HF TFC NF membranes was de-
tailed in Figure S6. The NF test results show that PMIAHF NFMs have excellent pressure
resistance, which mainly depends on the excellent mechanical properties of the PMIA
substrates (Table S2).
HF NFMs, Test conditions: (a) 0.35 MPa (b) 2000 ppm Na2SO4, 25 ± 1 °C.
3.4. Simulation of Dye Wastewater Treatment
In the actual dye wastewater treatment process, the properties of the membrane vary
with the concentration and charge of dye, the pH value, and the inorganic salt concentra-
tion in the solution. Two dyes with different charges (chromotrope 2B and Janus Green B)
in this study were chosen for the test, the effects of pH and dye concentration on mem-
brane separation performance were investigated, and the results are depicted in Figure 9.
Figure 8.
Effects of Na
2
SO
4
concentration (
a
) and operation pressure (
b
) on the permeability of PMIA.
Figure 8b shows that the retention rate of Na
2
SO
4
by the PMIA HF NFMs increased
slightly with the increase in operating pressure, and the permeate flux of the NFMs in-
Membranes 2022,12, 1258 12 of 18
creased in a linear relationship during the pressure increase. The retention rate of the
membrane remained stable and the permeate flux of the solution increased from 6.2 to
27.6 L
−1·
m
−2·
h
−1
when the pressure rose from 0.2 to 0.8 MPa. The effect of operation
pressure on the removal of Chromotrope 2B dyes by PMIA HF TFC NF membranes was
detailed in Figure S6. The NF test results show that PMIAHF NFMs have excellent pressure
resistance, which mainly depends on the excellent mechanical properties of the PMIA
substrates (Table S2).
HF NFMs, Test conditions: (a) 0.35 MPa (b) 2000 ppm Na2SO4, 25 ±1◦C.
3.4. Simulation of Dye Wastewater Treatment
In the actual dye wastewater treatment process, the properties of the membrane vary
with the concentration and charge of dye, the pH value, and the inorganic salt concentration
in the solution. Two dyes with different charges (chromotrope 2B and Janus Green B) in
this study were chosen for the test, the effects of pH and dye concentration on membrane
separation performance were investigated, and the results are depicted in Figure 9.
Membranes 2022, 12, x FOR PEER REVIEW 14 of 20
Figure 9. The effect of the dye concentration (a,c), pH value (b,d), on separation performance of
PMIA HF TFC NF membrane, test conditions: 0.35 MPa, 25 ± 1 °C.
Figure 9a,c shows the effect of dye concentration on the properties of PMIA HF NFMs
treating simulated dye wastewater. The dye solutions containing 100, 200, 300, 500, and
700 ppm of chromotrope 2B (513Da) and Janus Green B (511Da) were configured to inves-
tigate the separation performance of the NFMs with a test pressure of 0.35 MPa. As seen
in Figure 9a,c, the membrane flux decreases with increasing concentrations of both chro-
motrope 2B (513 Da) and Janus Green B (511 Da), mainly because the concentration of the
dye in the solution rises, the osmotic pressure of the solution increases, and the concen-
tration polarization in the membrane separation process becomes severe, resulting in a
descent in the flux of the membrane as a result of the adsorption of dye molecules on the
membrane surface [39]. Under the same dye concentration conditions, the mechanism of
flux decrease can be analyzed by comparing the flux magnitude of the two longitudinally.
Chromotrope 2B has the same charge as the surface of the nanofiltration membrane, which
has a mutual repulsive effect and does not easily adsorb on the membrane surface, satu-
rating the membrane surface after 500 ppm, and slowly decreasing flux at 700 ppm. In
contrast, Janus Green B has the opposite charge to the membrane surface and the charges
attract each other, so the dye is easily adsorbed on the NMF’s surface and clogs the mem-
brane pores, resulting in a linear decrease in membrane flux as the dye concentration in-
creases. At the same time, Figure 9a,c shows that when the dye concentration increased
from 100 ppm to 700 ppm, the rejection rate of chromotrope 2B reduced from 97.5% to
89.9%, while the retention rate of the positively charged Janus Green B dye increased from
99.7% to 99.9%, reflecting that the dye molecules are easily adsorbed on the membrane
Figure 9.
The effect of the dye concentration (
a
,
c
), pH value (
b
,
d
), on separation performance of
PMIA HF TFC NF membrane, test conditions: 0.35 MPa, 25 ±1◦C.
Figure 9a,c shows the effect of dye concentration on the properties of PMIA HF NFMs
treating simulated dye wastewater. The dye solutions containing 100, 200, 300, 500, and
700 ppm of chromotrope 2B (513 Da) and Janus Green B (511 Da) were configured to
investigate the separation performance of the NFMs with a test pressure of 0.35 MPa. As
seen in Figure 9a,c, the membrane flux decreases with increasing concentrations of both
chromotrope 2B (513 Da) and Janus Green B (511 Da), mainly because the concentration
Membranes 2022,12, 1258 13 of 18
of the dye in the solution rises, the osmotic pressure of the solution increases, and the
concentration polarization in the membrane separation process becomes severe, resulting
in a descent in the flux of the membrane as a result of the adsorption of dye molecules on
the membrane surface [
39
]. Under the same dye concentration conditions, the mechanism
of flux decrease can be analyzed by comparing the flux magnitude of the two longitudinally.
Chromotrope 2B has the same charge as the surface of the nanofiltration membrane, which
has a mutual repulsive effect and does not easily adsorb on the membrane surface, saturat-
ing the membrane surface after 500 ppm, and slowly decreasing flux at 700 ppm. In contrast,
Janus Green B has the opposite charge to the membrane surface and the charges attract each
other, so the dye is easily adsorbed on the NMF’s surface and clogs the membrane pores,
resulting in a linear decrease in membrane flux as the dye concentration increases. At the
same time, Figure 9a,c shows that when the dye concentration increased from 100 ppm
to 700 ppm, the rejection rate of chromotrope 2B reduced from 97.5% to 89.9%, while the
retention rate of the positively charged Janus Green B dye increased from 99.7% to 99.9%,
reflecting that the dye molecules are easily adsorbed on the membrane surface when it is in
contact with the surface of the membrane. The retention rate increased because the pores
of the membrane were clogged and the pore size became smaller.
Figure 9b,d shows the effect of pH values on the properties of PMIA HF NFMs treating
simulated dye wastewater. Comparative experiments were conducted by configuring
a concentration of 100 ppm chromotrope 2B (negatively charged) and Janus Green B
(positively charged) dye solutions, respectively, with a test pressure of 0.35 MPa and a pH
range of 3 to 11, and the results are represented in Figure 9b,d.
Figure 9b shows that the change in the solution pH value had a great influence on the
fluxes of chromotrope 2B dye solutions. This is mainly because chromotrope 2B was an acid
dye. In industrial production, when used for acid dye, the optimal pH range for dyeing
is 2.5~4. Therefore, when pH = 3, the negative charge on the membrane surface becomes
weaker, which is more beneficial to the adsorption and deposition of negatively charged
chromotrope 2B molecules on the membrane surface to form a filter cake layer, thus leading
to the lowest achieved flux [
40
]. With the increase of the solution pH value, the negative
charge on the membrane surface increases. Under the action of electrostatic repulsion on
the membrane surface, the adsorption ability of negatively charged chromotrope 2B dye
molecules on the membrane surface decreased [
41
], so the flux of the PMIA NFMs increased
with the rise in pH value. On the contrary, for the positively charged Janus Green B dye,
the flux of the membrane decreases with increasing pH, which also verifies this statement.
Meanwhile, Figure 9b,d show that the change of pH value does not have much effect
on the retention rate of both dyes, and the rejection rate of the PMIA HF NFMs retains
more than 98% of both dyes in the pH value range from 3 to 11, which is mainly because of
the large molecular weight and molecular size of the dyes, and the sieving effect plays a
major role, so the retention rate of the membrane remains relatively stable for both dyes.
The retention rate of Janus Green B is above 99%, which is partly due to the sieving effect,
but may also be due to the positively charged Janus Green B dye being adsorbed on the
membrane surface to form a filter cake layer and the membrane pores becoming smaller,
resulting in a higher retention rate.
Figure 10a,b showed the effect of Na
2
SO
4
and NaCl concentrations on the dye removal
performance of PMIA HF NFMs. As seen in Figure 10a,b, the flux of the NFMs showed a
decreased trend as the concentration of salts rose from 1 g/L to 10 g/L. This was mainly
because, with the increase of the concentration of salt in the solution, the osmotic pressure
of the solution increases, leading to the reduction of the driving forces on both sides of the
membrane, thus reducing the flux [
42
]. Additionally, an increase in the salt concentration
of the solution makes the concentration of Na
+
in the solution increase. This positive action
has a shielding effect on the surface of the negatively charged membrane, thereby making
the dye molecules easy to be adsorbed on the surface of the membrane, which consequently
causes the blockage of the membrane holes, leading to a decline in the membrane flux.
Membranes 2022,12, 1258 14 of 18
Membranes 2022, 12, x FOR PEER REVIEW 16 of 20
Figure 10. The effect of the inorganic salt (Na2SO4 (a), NaCl (b)) concentration on separation perfor-
mance of PMIA HF TFC NF membrane, test conditions: 0.35 MPa, 25 ± 1 °C.
When the inorganic salt was Na2SO4, the rejection of the membrane to chromotrope
2B decreased from 97.5 to 74.7% with the rise in salt concentration (0 g/L to 10 g/L). When
the inorganic salt was NaCl, the dye rejection rate decreased from 97.5 to 96.0%. This is
because the salt ions in the mixing system will be coupled with charged chromotrope 2B,
which makes the dye disperse more evenly. Additionally, it was easier for it to pass
through the membrane holes, thus leading to the reduction in the rejection rate of chro-
motrope 2B. In addition, the rejection rate of Na2SO4 also descended with a rise in salt
concentration, especially when the salt concentration was more than 8 g/L, and the mem-
brane rejection rate decreased significantly from 88.3% to 74.5%. Because the increase in
salt concentration also causes an increase in the concentration gradient on the membrane
surface, and the increase in the cation Na+ in the solution system has a certain shielding
effect on the NF membrane surface (the electrostatic repulsion between the negatively
charged membrane surface and SO42− ions is weakened) [43], which increases the solute
flux to the membrane and leads to a falloff in the rejection rate of the membrane for salt,
while the concentration of NaCl in the solution has less effect on the dye rejection rate
than Na2SO4, mainly because both Na+ and Cl- ions have the same monovalence and both
have a small charge-shielding effect [40]. Therefore, the effect on the dye rejection rate is
also small.
3.5. Continuous Stability Operation of the Membrane
Continuous stable operation is one of the most important indexes to investigate the
comprehensive performance of PMIA HF TFC NFMs [44]. To investigate the continuous
stable operation of the membrane desalination, the mixed solution of 100 ppm chromo-
trope 2B and 4000 ppm NaCl was configured to conduct continuous stable operation ex-
periments for 72 h at 0.35 MPa and 25 °C. The experimental data were recorded every 3 h
and the experimental results are depicted in Figure 11, and the effect of operation time on
the removal of Chromotrope 2B dyes by PMIA HF NFMs was displayed in Figure S6.
Figure 11 shows that the membrane flux decreased significantly in the first 7 h, mainly
because some chromotrope 2B dye molecules were adsorbed on the surface of the mem-
brane to build a filter cake layer, and the membrane was compacted under pressure. When
the time exceeded 7 h, the membrane flux almost reached a stable state, and the membrane
retention rate of dye was more than 96.0%, while the rejection rate of NaCl remained at
about 15%. Comprehensive analysis showed that the membrane had a high rejection effect
on dye in the desalination test of chromotrope 2B dye, and a high pass rate on inorganic
salt NaCl, it shows that the membrane has good potential for desalination of dye
wastewater.
Figure 10.
The effect of the inorganic salt (Na
2
SO
4
(
a
), NaCl (
b
)) concentration on separation
performance of PMIA HF TFC NF membrane, test conditions: 0.35 MPa, 25 ±1◦C.
When the inorganic salt was Na
2
SO
4
, the rejection of the membrane to chromotrope
2B decreased from 97.5 to 74.7% with the rise in salt concentration (0 g/L to 10 g/L). When
the inorganic salt was NaCl, the dye rejection rate decreased from 97.5 to 96.0%. This is
because the salt ions in the mixing system will be coupled with charged chromotrope 2B,
which makes the dye disperse more evenly. Additionally, it was easier for it to pass through
the membrane holes, thus leading to the reduction in the rejection rate of chromotrope 2B.
In addition, the rejection rate of Na
2
SO
4
also descended with a rise in salt concentration,
especially when the salt concentration was more than 8 g/L, and the membrane rejection
rate decreased significantly from 88.3% to 74.5%. Because the increase in salt concentration
also causes an increase in the concentration gradient on the membrane surface, and the
increase in the cation Na
+
in the solution system has a certain shielding effect on the NF
membrane surface (the electrostatic repulsion between the negatively charged membrane
surface and SO
42−
ions is weakened) [
43
], which increases the solute flux to the membrane
and leads to a falloff in the rejection rate of the membrane for salt, while the concentration
of NaCl in the solution has less effect on the dye rejection rate than Na
2
SO
4
, mainly because
both Na
+
and Cl
−
ions have the same monovalence and both have a small charge-shielding
effect [40]. Therefore, the effect on the dye rejection rate is also small.
3.5. Continuous Stability Operation of the Membrane
Continuous stable operation is one of the most important indexes to investigate the
comprehensive performance of PMIA HF TFC NFMs [
44
]. To investigate the continuous
stable operation of the membrane desalination, the mixed solution of 100 ppm chromotrope
2B and 4000 ppm NaCl was configured to conduct continuous stable operation experiments
for 72 h at 0.35 MPa and 25
◦
C. The experimental data were recorded every 3 h and the
experimental results are depicted in Figure 11, and the effect of operation time on the
removal of Chromotrope 2B dyes by PMIA HF NFMs was displayed in Figure S6. Figure 11
shows that the membrane flux decreased significantly in the first 7 h, mainly because some
chromotrope 2B dye molecules were adsorbed on the surface of the membrane to build
a filter cake layer, and the membrane was compacted under pressure. When the time
exceeded 7 h, the membrane flux almost reached a stable state, and the membrane retention
rate of dye was more than 96.0%, while the rejection rate of NaCl remained at about 15%.
Comprehensive analysis showed that the membrane had a high rejection effect on dye in
the desalination test of chromotrope 2B dye, and a high pass rate on inorganic salt NaCl, it
shows that the membrane has good potential for desalination of dye wastewater.
Membranes 2022,12, 1258 15 of 18
Membranes 2022, 12, x FOR PEER REVIEW 17 of 20
Figure 11. Continuous stable operation test of the PMIA HF NF membrane. Test conditions: 100
ppm chromotrope 2B and 4000 ppm NaCl, 0.35 MPa, 25 ± 1 °C.
4. Conclusions
PMIA HF TFC NFMs were successfully fabricated by IP technology with a self-made
PMIA HF substrate. The effects of preparation conditions on the properties of the mem-
brane were systematically investigated. Under the optimum preparation conditions, the
PWF of the PMIA HF NFMs was 18.2 L·m−2·h −1 (0.35 MPa), and the rejection rate of Na2SO4
was 98.3%. The characteristics of the HF TFC NFMs’ surface were observed and analyzed
by SEM, AFM, XPS, and FTIR spectra. It was confirmed that a layer of PA separation layer
was developed on the surface of the PMIA substrates, which was composed of an ultra-
thin dense layer and spherical structure on the surface of the dense layer. PMIA HF TFC
NFMs were applied to the treatment of printing and dyeing wastewater and they showed
excellent pressure resistance, separation, and flux stability.
Supplementary Materials: The following supporting information can be downloaded at:
www.mdpi.com/xxx/s1, Figure S1: The casting equipment of the PMIA HF substrates; Figure S2:
Pictures of PMIA HF substrate and SEM images of its cross-section at different magnifications; Fig-
ure S3: The pore size and the pore size distribution of PMIA HF substrate; Figure S4: The contact
angle of PMIA HF substrate and PMIA HF NF membrane; Figure S5: The rejection of PEGs with
different molecular weights through PMIA HF TFC NF membrane; Figure S6: Effect of operation
pressure on the removal of Chromotrope 2B dyes by PMIA HF TFC NF membranes; Figure S7: Effect
of operation time on the removal of Chromotrope 2B dyes by PMIA HF TFC NF membranes; Table
S1 The properties of PMIA HF substrate; Table S2 Mechanical properties of PMIA HF substrates.
Author Contributions: Conceptualization, K.Z. and Q.J.; methodology, Q.J.; software, Q.J.; valida-
tion, K.Z. and Q.J.; formal analysis, Q.J.; investigation, Q.J.; resources, Q.J.; data curation, Q.J.; writ-
ing—original draft preparation, Q.J.; writing—review and editing, Q.J.; visualization, Q.J.; supervi-
sion, K.Z.; project administration, K.Z.; funding acquisition, K.Z. All authors have read and agreed
to the published version of the manuscript.
Figure 11.
Continuous stable operation test of the PMIA HF NF membrane. Test conditions: 100 ppm
chromotrope 2B and 4000 ppm NaCl, 0.35 MPa, 25 ±1◦C.
4. Conclusions
PMIA HF TFC NFMs were successfully fabricated by IP technology with a self-made
PMIA HF substrate. The effects of preparation conditions on the properties of the membrane
were systematically investigated. Under the optimum preparation conditions, the PWF
of the PMIA HF NFMs was 18.2 L
·
m
−2·
h
−1
(0.35 MPa), and the rejection rate of Na
2
SO
4
was 98.3%. The characteristics of the HF TFC NFMs’ surface were observed and analyzed
by SEM, AFM, XPS, and FTIR spectra. It was confirmed that a layer of PA separation
layer was developed on the surface of the PMIA substrates, which was composed of an
ultra-thin dense layer and spherical structure on the surface of the dense layer. PMIA HF
TFC NFMs were applied to the treatment of printing and dyeing wastewater and they
showed excellent pressure resistance, separation, and flux stability.
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/membranes12121258/s1, Figure S1: The casting equipment of the
PMIA HF substrates; Figure S2: Pictures of PMIA HF substrate and SEM images of its cross-section
at different magnifications; Figure S3: The pore size and the pore size distribution of PMIA HF
substrate; Figure S4: The contact angle of PMIA HF substrate and PMIA HF NF membrane; Figure
S5: The rejection of PEGs with different molecular weights through PMIA HF TFC NF membrane;
Figure S6: Effect of operation pressure on the removal of Chromotrope 2B dyes by PMIA HF TFC NF
membranes; Figure S7: Effect of operation time on the removal of Chromotrope 2B dyes by PMIA HF
TFC NF membranes; Table S1: The properties of PMIA HF substrate; Table S2: Mechanical properties
of PMIA HF substrates.
Author Contributions:
Conceptualization, K.Z. and Q.J.; methodology, Q.J.; software, Q.J.; validation,
K.Z. and Q.J.; formal analysis, Q.J.; investigation, Q.J.; resources, Q.J.; data curation, Q.J.; writing—
original draft preparation, Q.J.; writing—review and editing, Q.J.; visualization, Q.J.; supervision,
K.Z.; project administration, K.Z.; funding acquisition, K.Z. All authors have read and agreed to the
published version of the manuscript.
Membranes 2022,12, 1258 16 of 18
Funding:
This research was funded by This work was supported by the Bureau of Frontier Sciences and
Education (QYZDB-SSW-DQC044), the Bureau of International Cooperation (132C35KYSB20160018),
the Chinese Academy of Sciences and the Joint Project between CAS-CSIRO (132C35KYSB20170051),
the Postdoctoral Science Foundation of Jiangxi Academy of Sciences, the key R&D—key project of the
Jiangxi Province, China (20212BBG71001). Pilot Demonstration Project for the Contract Responsibility
System of the Provincial Science and Technology Plan Project of Jiangxi Academy of Sciences, China
(2022YSBG22015).
Institutional Review Board Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Acknowledgments:
This work was supported by the Bureau of Frontier Sciences and Education
(QYZDB-SSW-DQC044), the Bureau of International Cooperation (132C35KYSB20160018), the Chi-
nese Academy of Sciences and the Joint Project between CAS-CSIRO (132C35KYSB20170051), the
Postdoctoral Science Foundation of Jiangxi Academy of Sciences, the key R&D—key project of Jiangxi
Province, China (20212BBG71001). Pilot Demonstration Project for the Contract Responsibility Sys-
tem of the Provincial Science and Technology Plan Project of Jiangxi Academy of Sciences, China
(2022YSBG22015). The authors would like to acknowledge Lasisi Kayode Hassan for the valuable
comments and grammatical revision.
Conflicts of Interest: The authors declare no conflict of interest.
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