Preparation of high-efficiency ceramic planar membrane and its application for water desalination
Journal of Advanced Ceramics
Preparation of high-efficiency ceramic planar membrane and its application for water desalination
Yan-Dong XU 1
a b b Jun-Wei WANG 0
Xin XU 0
0 CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China , Hefei 230026 , China
1 Sinopec Northwest Oilfield Branch, Research Institute of Petroleum Engineering , Ürümqi 830001 , China
Highly efficient Si3N4 ceramic planar membrane for water desalination process using membrane distillation was prepared by the dual-layer phase inversion tape casting and sintering method. In comparison with typical phase inversion tape casting method, the green tape was formed using Si3N4 slurry on the top and graphite slurry on the bottom. After consuming away the graphite structure, a ceramic membrane consisting of a two-layered structure (skin and finger-like layers) was obtained. The skin layer was relatively tight, and thus could act as a functional layer for separation, while the finger-like layer contained straight open pores with a diameter of 100 μm, acting as a support with low transport resistance. For comparison, typical Si3N4 ceramic membrane was fabricated by phase inversion technique without graphite substrate, resulting in a three-layered structure (skin, finger-like, and sponge layers). After membrane modification from hydrophilic to hydrophobic with polymer derived nanoparticle method, the water desalination performance of the membranes was tested using the sweeping gas membrane distillation (SGMD) with different NaCl feed solutions. With the increase of salt content from 4 to 12 wt%, the water flux decreased slightly while rejection rate maintained over 99.99%. Comparing with typical three-layered Si3N4 membrane, an excellent water flux enhancement of over 83% was resulted and the rejection rate remained over 99.99%.
Clean water demand has been one of the most serious
global problems during last decade [
distillation (MD) is regarded as an increasing water
supply method due to zero discharge and high water
recovery (~80% for seawater) . The concentrated
discharges from MD can be used in industry as raw
material (e.g., food industry).
MD is a thermally driven separation process, where
mass transfer is realized by evaporation of a volatile
solvent through the membrane, while non-volatile
substances such as ions, macromolecules, colloids, and
cell are rejected by the membrane structure [
Currently, the most common hydrophobic membranes
in MD are made of polymers including polypropylene
(PP), polytetrafluoroethylene (PTFE), and
polyvinylidenefluoride (PVDF) [
]. Compared to polymeric
membranes, ceramic membranes can withstand harsh
environments due to their excellent mechanical strength,
chemical stability, and thermal resistance [
researches have been reported to explore ceramic
membrane application in MD [
inorganic modifiers were used for ceramic membrane
modification to achieve hydrophobic structure. These
kinds of membranes retained the hydrophobicity under
harsh conditions and exhibited satisfactory long-term
stability, which are proposing the application of ceramic
membrane in membrane distillation [
]. In our last
few works, phase inversion tape casting method has
been investigated for preparation of porous ceramic
membrane to enhance the mass transfer. The typical
three-layered hydrophobic Si3N4 ceramic membrane
consists of skin layer, vertical finger-like voids, and
sponge-like region [
]. Sponge layer in conventional
three-layered membrane structure is considered as an
obstacle against gas transmission which is restricting
their application in MD process.
In this work, to overcome this demerit, the sponge
layer was removed by a dual-layer tape casting method
with a graphite sacrifice layer and carbonation reaction
at the bottom of the green tape. The Si3N4 membrane
showed satisfying water desalination performance,
wherein emerging its potential for practical applications.
Fabrication of ceramic membranes
Commercially available α-Si3N4 (SN-ESP, Ube Industries,
Ltd.; α-phase > 95%, average particle size: 0.55 μm),
graphite powders (Furunda Co., Ltd., Shandong, China;
average particle size: 1.05 μm), Al2O3 and Y2O3 (both
from Sinopharm Chemical Reagent Co., Ltd., Shanghai,
China) were used as starting powders. The dispersant
O-(2-aminopropyl)-O-(2-methoxyethyl)-polypropyleneglycol (AMPG) ( M w = 600, Sigma–Aldrich Co., Ltd.,
USA) and the binder polyethersulfone (PESf) (Radel
A-100, Solvay Advanced Polymers) were dissolved in
N-methyl-2-pyrrolidone (NMP) (CP, Sinopharm
Chemical Reagent Co., Ltd., Shanghai, China). Then
the starting powder mixtures were added into the
polymer solution, followed by planetary milling to form
a stable suspension. Si3N4 slurry and graphite slurry
Graphite slurry (wt%)
were milled separately for 24 h, with the compositions
which have been presented in Table 1. The as-prepared
slurries were degassed for 30 min using a vacuum
pump. Then the two slurries were co-tape cast on a
Mylar sheet with a doctor blade, as shown in Fig. 1.
The blade heights for the graphite and ceramic slurry
were 0.15 and 0.85 mm, respectively. The membrane
was coded as M2.
In order to compare the structure of conventional
three-layered ceramic membrane with the present
two-layered structure (M2), Si3N4 ceramic membrane
was prepared through phase inversion process. The
Si3N4 slurry was cast on a Mylar sheet of gap height
1.0 mm. The membrane was coded as M1.
The cast slurries were solidified by immersion in
water for 10 h at room temperature and then dried at
ambient conditions. After that, the green tape was cut
into round or rectangular pieces followed by sintering
in a graphite furnace at 1700 ℃ under flowing N2
atmosphere (500 mL/min) for 4 h.
Both the membranes were ultrasonic treated in ethanol
for 0.5 h. M2 was slightly polished to remove any
impurities from the surface of the membrane. Then the
membranes were immersed in boiling NaOH aqueous
solution (0.2 M) for 10 min to modify the membrane
surface for the grafting reaction. Dichloromethylsilane
and dichlorodimethylsilane (Adamas Reagent Co., Ltd.)
as the raw materials were first dissolved in n-heptane
(Sinopharm Chemical Reagent Co., Ltd.). Then the
prepared membranes were immersed into the mixture
solution and held in a boron nitride boat. The boat was
heated in a box-type resistance furnace under ammonia
atmosphere, with the temperature increase to 150 ℃ at
heating rate of 3 ℃/min, and held for 30 min to
evaporate solvent. Then, the temperature was raised to
600 ℃ at heating rate of 4 ℃/min. Before cooling it
down, the temperature was retained for 60 min. The
modified ceramic membranes were immersed into
ethanol solvent and then ultraphonic treated for 10 min
for further cleaning.
The crystalline phases were detected by X-ray
diffraction analysis (XRD, Philips PW 1700) using the
Cu Kα1 radiation at a scanning rate of 2 (°)/min. The
morphology of the membrane surface was studied
using a scanning electron microscope (SEM,
JEOLJSM-6390LA, Japan). The bending strength was
tested by a three-point bending method (INSTRON
Model 5567, UK) with a maximum load cell of 1 kN.
Water contact angle was measured using an apparatus
(SL200B, Solon Tech Co., Ltd., Shanghai, China). The
porosity of the membrane was determined by the
Archimedes method. The pore size distribution of the
membranes was determined by the bubble point
Gas permeation properties were measured using
nitrogen. The sample was fixed on the base of a male
connector, and then covered by a refined cylinder.
Nitrogen was fed into the cylinder at different
pressures, and the gas permeation through the fiber
sample was measured by a soap bubble flow meter.
Membrane distillation process was carried out with a
home-made device shown in our previous work [
Nine membranes with a total area of 25 cm2 were fixed in
a steel plate which was separating a steel-made chamber
into two compartments (the width of each compartment,
i.e., the span between the membrane and the outer wall,
was 2 mm). A feed solution was pumped into one
compartment at a flow rate of 100 L/h. The water vapor
of the feed solution passed through the membranes to
the other compartment. Then, the water vapor was
taken by nitrogen on the permeate side and condensed
in the chiller. Sweeping gas flow was 500 mL/min. The
condensed water was collected in a glass bottle and
weighed using an electronic balance. The conductivities
of the feed solution and the permeate were measured
by a conductivity meter (FE30, Mettler Toledo). The
NaCl rejection R was calculated by
R 1 100% (1)
where Cf and Cp are the conductivities of the feed
solution and permeate water, respectively [
Results and discussion
According to Figs. 2(a) and 2(b), with a sacrifice layer,
we successfully remove sponge region by dual-layer
phase inversion tape casting method, while the skin
layer and finger-like layer have remained constantly.
Figure 2(c) shows the microstructure of the skin layer.
Porous rod-like Si3N4 grains were disordered assembled
in three-dimensional space. As shown in Fig. 2(d),
finger-like pores penetrate through the membrane cross
section and expose at the bottom surface. The skin
layer has remained as the functional layer which
decreases the water vapor transfer resistance and
increases permeate flux in SGMD. The bending
strength of M2 is 25 MPa, which is comparable with
ceramic membranes reported in the literature [
The X-ray diffractograms of the M2 membranes
sintered at 1700 ℃ are shown in Fig. 3. The initial
membrane containing black graphite layer in the
bottom side, shows Si3N4, SiC, and C phases. Although
after ultrasound treatment, the black layer is removed
and green SiC is emerging at the surface of the
membrane, the impurity phase is still observed in the
XRD spectra. After slight polishing, pure Si3N4 phase
has been resulted. In the sintering procedure,
carbonation reaction occurs in the mixing layer of
sponge-like region, which helps to form a separate
layer. This layer has been completely removed by
slight polishing as reveled in the XRD result.
Surface modification was carried out with an
organosilane-derived inorganic particle product. It is an
efficient and long-term stable modifier to embellish the
membrane hydrophobicity, according to our previous
]. As we can see in Fig. 4, water contact angle
is about 142° for M1 and 140° for M2. Both the
membranes show hydrophobicity.
In Fig. 5, the pore size distributions of M1 and M2
membranes are depicted. It can be seen that both the
membranes exhibit a homogeneous pore size
distribution. The pore size distribution of M2 is
calculated in the range of 0.80–1.05 μm and the
average pore size is about 0.91 μm, while 0.82 µm has
resulted for M1 membrane. According to the principle
of the bubble-point method, the pore size corresponds
to the narrowest position of through pores, i.e., the
pore throat. So this pore diameter is associated with
the pores of the skin layer for M2 and skin
layer–sponge layer for M1. The porosity is increased
from 48% (M1) to 57% (M2), according to the
As shown in Fig. 6, gas permeation has been
dramatically increased by removing sponge layer of
membrane. The gas permeation of both membranes
decreases slightly after modification, which can be
explained by the reduction of pore size distribution or
clogging the membrane pores with deposition of the
modifier particles on the pore walls of the membrane.
More than 100% enhancement of gas permeation is
attained from 3.28×106 L/(m2·day) to 6.90× 106 L/(m2·day)
at 0.01 MPa for M1 and M2, respectively. M1
comprises a low-porosity skin layer at the top, a
fingerlike porous layer in the middle, and a sponge-like layer
at the bottom. The gas transmission process consists of
several steps in series. First, the gas is transferred
through the small pores of the top layer following by
transition through the finger-like large pores in the
middle layer. In the next, the gas phase is passed
through the small pores in the bottom layer. The
sponge-like layer acts as an extra diffusion barrier for
the gas transmission. Therefore, higher efficient separation
process is attained with a two-layered structure (M2).
Membrane distillation was carried out with a
homemade device (Section 2.4). The feed solutions contained
4–12 wt% NaCl. Sweeping gas flow was fixed at
500 mL/min. The temperature on the permeate side
was room temperature (25 ℃). Figure 7 shows the
permeate flux of M1 and M2 as a function of the feed
temperature with 4 wt% NaCl content. The permeate
flux enhances quickly when the feed temperature
increases from 45 to 75 ℃ for both membranes.
The MD flux is proportional to the transmembrane
water vapor pressure difference: P Pf Pp . The
water vapor pressure at the water side of the membrane,
Pf , is written as follows [
Pf exp C1 C2 C3Tf C4Tf2 C5Tf3 C6 lnTf
1 f 1 0.5f 10f2
where C1 – C6 are constants: C1 = –5.8002206×103,
C2 = –5.516256, C3 = –4.8640239×10–2, C4 =
4.1764768×10–5, C5 = –1.4452093×10–8, C6 =
6.5459673. Pf is expressed in kPa and Tf in K. In
the 4 wt% NaCl solution, the mole fraction is 0.0127.
Table 2 shows the Pf values at different temperatures.
The pressure at the permeate side, Pp , is calculated
as follows [
Although the humidity ratio along the membrane
module, , is unknown, but this parameter is
correlated with the N2 flux, ma , and the value of the
humidity at the module inlet, in , through following
in NA (4)
The humidity of the inlet N2 was 0. Membrane area
was 25 cm2. Sweeping gas flow was 500 mL/min.
According to the flux of the process, the pressure of
the permeate side and the driving force could be
calculated for both membranes, as shown in Table 2.
Therefore, higher temperature enhances the permeate
flux. It reaches 6.40 L/(m2·h) for M1 and 11.75 L/(m2·h)
for M2 at 75 ℃, while the rejection rate remains over
99.99%. Comparing M1 and M2, a great water flux
enhancement of over 83% occurs at 75 ℃ after removing
the sponge layer.
Figure 8 shows the water flux of M1 and M2 in
different NaCl solutions. 4–12 wt% NaCl concentrations
were used as feed solutions, and the sweeping gas
pressure was fixed at 0.15 bar and the temperature was
fixed at 75 ℃. With salt concentration increasing, the
water flux decreases slightly from 6.40 L/(m2·h) at 4
wt% to 5.00 L/(m2·h) at 12 wt% for M1 and from
11.75 L/(m2·h) at 4 wt% to 9.19 L/(m2·h) at 12 wt%
for M2. According to the equations of transmembrane
water vapor pressure difference, Eq. (2) and Eq. (3),
the driving force decreases with increasing salt
concentration. Thus, the flux is decreasing with
increasing of salt concentration. All the rejection rates
are over 99.99%. It has retained a good flux and high
rejection rate at high salt content. Over 83% enhancement
of water flux is gained at different concentration NaCl
The mean free path of the water molecules at 0–
100 ℃ is 0.2 μm. Its value is comparable with the
membrane pore size. The vapor transmission takes
place via a combined Knudsen–molecular diffusion
flow. The following relationship is applied for
calculation of mass transport coefficient B:
So the mean mass transport coefficient is
0.1805 L/(m2·h·kPa) for M1 and 0.3394 L/(m2·h·kPa)
High-efficiency two-layered ceramic planar membrane
was prepared by the double-layer phase inversion tape
casting and sintering method. The skin layer was
relatively tight which acts as a functional layer, while
the finger-like layer contained straight open pores,
acting as a support with low transport resistance. In
comparison with the membranes prepared by typical
phase inversion tape casting method, the average pore
size has increased slightly from 0.82 to 0.91 µm, while
more than 100% enhancement of gas permeation was
attained from 3.28×106 to 6.90×106 L/(m2·day) at
0.01 MPa. The surface of the membrane was modified
from hydrophilic to hydrophobic via grafting with
inorganic SiNCO nanoparticles. The membrane showed a
good performance of high salt concentration. With salt
content increasing, water flux decreased slightly from
11.75 L/(m2·h) at 4 wt% to 9.19 L/(m2·h) at 12 wt%,
while the rejection rate remained over 99.99%.
Comparing with typical phase inversion three-layered
structure Si3N4 membrane, a great enhancement over
83% of the water flux was achieved.
This research was supported by the National Natural
Science Foundation of China (Grant Nos. 51372238,
U1732115, and 11435012), the CNPC-CAS Strategic
Cooperation Research Program (Grant No. 2015A-4812),
and demonstration project of key technologies for EOR of
carbonate oil and gas fields in Tarim Basin (national
major project of China, Grant No. 2016ZX05053).
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