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Article
Nylon-6−Mordenite Composite Membranes for Adsorption of
Ethylene Gas Released from Chiquita Bananas
Phuong Thanh Ton Nu and Takaomi Kobayashi*
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ABSTRACT: Nylon-6−mordenite composite membranes were
fabricated for adsorbing ethylene released from bananas. In the
membrane fabrication, the wet-phase inversion method was
employed when a nylon-6/mordenite/methanol solution was
immersed in water. The composite membranes had a porous
structure with a cavelike structure or a spongelike one in the top
layer, depending on the mordenite content from 0 to 30 wt %. For
ethylene adsorption ability, as the mordenite loading increased in
the membrane, high adsorptibility appeared. Then the removal of
ethylene released from Chiquita bananas was practically examined.
The ethylene production from bananas in 10 days storage was
monitored in the closed container with the presence and absence
of membranes. The results showed that 30 wt % mordenite loaded membranes adsorbed ethylene with less than 0.7 μmol/gbanana for
10 days at 20 °C in the presence of bananas and kept the bananas fresh. This meant the membranes could extend the lifetime of
fruits.
and microporous solid, it can adsorb ethylene and ethane13
and present the gas adsorption capacity for ethylene and
hydrogen.14 Although zeolite powders have high adsorption
ability, in the adsorption processes, the powder form is difficult
to handle in practical operation, leading to the limitation of the
utilization of the adsorbents. To overcome this feature, the
zeolite powder was composited or mixed within a polymeric
matrix and then used as potential approaches as followed in
several reports on zeolite/polymer composites in their forms of
membranes,15 fibers,16 and beads.17 As mentioned above,
mostly the adsorbents had forms of porous powder; but
relative to such powder adsorbents, some disadvantages are in
handing off and settle in the operation process. Thus, those
composite membranes have advantageous properties in the
process for practical uses with lower volume and easily move
the compact forms in the preferred positions.16,18,19 In
addition, porous composite materials have high adsorption
capacity and potential for recovery property. In the case of
gaseous separation, there were studies on membrane formed
adsorbers.15,20,21 For instance, Norwahyu et al. fabricated
1. INTRODUCTION
Ethylene, the first identified gaseous hormone, affects various
aspects of plant processes throughout the life cycle. Ethylene
acts on seed germination, plant growth, opening flower, fruit
ripening, deciduousness, organ destruction, and then, the age
progression.1,2 In the ripening fruits, for example, ethylene is
responsible for the changes in texture, softening, color, and
flavor since some fruits release ethylene while ripening such as
bananas, apples, kiwi fruits, etc.3,4
However, the effect of ethylene on the fruit life is cumulative
as is higher self-release ethylene which is allowable for ripening
and rotting of fruits and vegetables. Therefore, indeed, during
the store and transportation processes of the fruit of their
fruits, it is necessary to exclude the accumulation of ethylene in
the packaging in order to maintain the quality for extension of
the shelf life. At present, there are many materials which
display high potential in ethylene removal such as potassium
permanganate (KMnO4),5 activated carbon,6 calcium chloride
(CaCl2),7 and zeolites.8 When using KMnO4, ethylene from
the storage environment of fruit is converted into carbon
dioxide (CO2) and water by absorbing and oxidizing it on
KMnO4.5,9 Also, KMnO4 is water-soluble, resulting in it being
contaminated with the food. A similar problem can happen in
CaCl2.10 For activated carbon, ethylene gas is adsorbed on the
surface area11 as well as zeolite. However, it was known that
the ethylene was released at the small volume from the
activated carbon when the environmental temperature was
changed.12 In the case of zeolite, which is an aluminosilicate
© XXXX American Chemical Society
Received:
Revised:
Accepted:
Published:
A
November 7, 2019
March 28, 2020
March 30, 2020
March 30, 2020
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composite membranes containing different loadings of zeolite
T into a 6FDA−durene polymer and used them for CO2/CH4
separation.15 Also, Murali et al. reported the effect of zeolite 4A
loading in the Pebax-1657 membranes used as a base polymer
matrix for the separation of CO2, CH4, O2, and N2.21 In the
adsorption process, porous nylon-6 membranes were fabricated by the wet-phase inversion process and applied for
desalination at low pressure.22 Furthermore, Patcharin et al.
studied modified nylon fibers with amino chelating groups for
adsorption of Pb(II) and Cr(IV).23 Also, Hong et al. prepared
polyaniline−nylon-6 composite fabric by polymerizing aniline
on the surface of nylon-6 fabrics for an ammonia gas sensor.24
In those studies of the nylon fabrication processes, generally,
formic acid which is toxic and harmful was used. So, an
exception study without such formic acid solvent is expected.
Unfortunately, there is a lack in the study of nylon membranes
using a common solvent like alcohol. Also, little is known
about the application of the zeolite/polymer composite
membrane for ethylene adsorption from fruits. Especially,
polyamides such as nylon were quite interesting polymers from
the point of view of conventionally used plastic, film, fabric,
nonwovens, etc. However, the heating molding makes it
difficult to prepare a porous matrix even though zeolite
powders are imbedded in a dense plastic layer. For this reason,
it is very interesting to use an alcohol solvent to fabricate
nylon-6−zeolite composite membranes. In the present work,
the nylon-6−mordenite composite membranes were fabricated
in the methanol/CaCl2 solvent by wet-phase inversion as in the
first report. The characterized properties and application for
removing ethylene gas from Chiquita bananas were investigated.
Article
Scheme 1. Fabrication Procedure of Nylon-6−Mordenite
Composite Membranes
and NMC 30, respectively. Scheme 1 shows the fabrication
process of the nylon-6−mordenite membranes by wet-phase
inversion by using the methanol/CaCl2 solvent system.
For the membrane formation of nylon-6−mordenite, these
homogeneous mixture solutions were independently casted in
Petri plates and immersed in distilled water at 25 °C for 24 h.
Because water penetrated into the polymer solution, nylon-6
and mordenite were deposited in the water medium by the
solvent exchange process. Finally, the resultant membranes
were obtained by well washing in distilled water to remove
residual methanol and CaCl2 additive.26,27
2.3. Characteristics of Nylon-6−Mordenite Composite Membranes. For characterization, several tests were
conducted on the composite membranes. First, the density of
membranes was determined by the Archimedes method. A dry
specimen with known weight was immersed in excess
methanol until the specimen in methanol reached equilibrium.
The density of prepared specimens (ρ) was calculated as
W
ρ = W − sW × (ρo − d) + d where ρ was density (g/cm3), Ws
2. EXPERIMENTAL SECTION
2.1. Materials. Nylon-6 was obtained from Toray
Industries, Inc. (Japan) as a product of CM 1017. Mordenite
powder was a product of Nitto Funka Trading Co. Ltd.
(Japan). All chemicals used for the fabrication of nylon-6−
mordenite composite membranes were analytical grade.
Methanol (MeOH), calcium chloride, and other chemicals
were obtained from Nacalai Tesque, Inc. (Japan). Pure
ethylene (99.9%) and CO2 (99.5%) were supplied by Taiyo
Nichiyu Co., Ltd. (Japan). Ethylene standard of 997 ppm in
helium was purchased by GL Sciences (Japan). For the
molecular weight of nylon-6, the viscosity of the polymerformic acid was measured at 25 °C using an Ubbelohde
viscometer. The molecular weight of nylon-6 was calculated as
M = 16826 g/mol, according to the literature by using the
following equation: η = K × Ma (K = 22.6 × 10−3 mL/g, a =
0.8225).
2.2. Preparation of Nylon-6−Mordenite Composite
Membranes. In the wet-phase inversion process of nylon-6
solution to membranes, methanol/CaCl2 solution and
mordenite powder (size <0.1 mm) were employed for the
fabrication of nylon-6−mordenite composite membranes, as
shown in Scheme 1. Briefly, CaCl2 was completely dissolved
into methanol at 15 wt %. After 6 h of vigorous stirring at 60
°C, nylon-6 gradually dissolved. The polymer membrane
matrix was finally obtained by wet-phase inversion when the
water coagulation medium was used. After the coagulation of
the nylon-6 was done in the water medium, the mordenite
powders were embedded and folded in the polymer matrix
with loadings of 0, 10, 20, and 30 wt % and stirred well for
homogeneous mixtures named nylon-6, NMC 10, NMC 20,
s
w
was denoted specimen weight in air (g), Ww represented
specimen mass in methanol (g), ρo was density of methanol
(g/cm3), and d stood for density of air (g/cm3). Tensile
strength and elongation of membranes were examined by a
loading measure instrument (LTTU-500N, Minebea Co. Ltd.,
Japan) with an operating head load of 500 N at 23 °C with
50% RH. The gauge length and the cross-head speed were 20
mm and 1.5 mm/min, respectively. Five specimens were tested
for each sample. The thickness of each sample was measured
by using a micrometer (Mitutoyo 103-177, Japan). The values
of tensile strength and elongation were calculated using the
following equations: tensile strength (MPa) = maximum load/
cross-sectional area and elongation (%) = 100 × (elongation at
rupture/initial gauge length). Water content (WC) of
membranes was measured at 25 °C by immersing the dry
membrane (40 mm × 20 mm) in distilled water. After the
immersion period was 24 h to get equilibrium sorption, the
membrane was quickly wiped with soft paper and weighted.
The value of WC was calculated for each sample by the
m−m
following equation: WC (%) = m o × 100, where mo was
0
the dry weight, and m was the weight of the membrane after 24
h immersion in distilled water. The surface area of mordenite
and the composite membranes was identified by the BET
(Brunauer−Emmett−Teller) method at the temperature of
B
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Table 1. Properties of Nylon-6−Mordenite Composite Membranes
sample
mordenite
content (wt %)
thickness
(μm)
density
(cm3/g)
tensile
strength
(MPa)
Mordenite
Nylon-6
NMC 10
NMC 20
NMC 30
100
0
10
20
30
84
110
169
190
1.18
1.22
1.26
1.39
0.65
0.41
0.36
0.33
elongation
BET surface
area (m2/g)
water
content
(%)
porosity
(%)
BJH pore
volume (cm3/g)
BJH pore
size (nm)
32.05
19.13
12.34
11.61
56.33
11.78
13.56
15.33
17.09
276.36
212.56
130.46
118.29
12.78
24.00
29.35
45.34
0.047
0.021
0.022
0.027
0.034
65.37
78.89
73.7
64.36
59.88
preclimacteric stage. Each banana sample was weighed by
analytical balance (Mettler Toledo-Me 3002) to record the
initial weight and stored in the absence or the presence of the
composite membranes or mordenite for 10 days in a closed
container (the volume of 3 L). The produced gas in the
presence of the banana sample in the closed container was
sampled by the Autogas sampler (GS 5100, GL Science) (b) to
gas chromatography (c). Figure 2 illustrates the experimental
liquid nitrogen (77.35 K) by using Tristar II 3020 (Micromeritics TriStar II, Shimadzu-Japan). Before measurement,
samples were degassed by vacuum overnight at 140 °C. These
values of ρ, tensile strength, elongation, WC, and BET surface
areas are listed in Table 1. To investigate changes in the
material structure, Fourier transform infrared spectroscopy
(FTIR) spectra of mordenite and the composite membranes
were characterized on the IR Prestige-21 spectrometer
(Shimazu, Japan) in absorbance mode in the range 4000−
500 cm−1. The morphology of the composite membranes was
characterized by scanning electron microscopy (SEM, JSM5300LVB, JEOL Ltd., Japan) on the surface and cross section
of the membranes.
2.4. Measurements for Adsorption Isotherms of
Ethylene. First, mordenite powders and the membranes cut
in the small size were vacuumed overnight at 130 °C. Then,
the gas adsorption test for ethylene was conducted by using the
Tristar II 3020 instrument at 301 K. The ethylene gas (99.9%,
Taiyo Nichiyu Co., Ltd. (Japan)) was provided to the
instrument for the ethylene adsorption process to the
membrane. The equipment could monitor the equilibrium
pressure of the ethylene gas at each absolute pressure. Before
dosing, first of all, the ultrahigh vacuum at 1.33 × 10−5 kPa in
the sample folder was made of a vacuum system with two
rotary pumps (Edwards Leroy somer) and a turbo molecular
pump. Then, the absolute pressure of ethylene was conducted
in the range of 0−100 kPa. All the tests were repeated at least 3
times in order to confirm the precision of the obtained results.
Then, from the experimental adsorption data, a plot of
absolute pressure/quantity adsorbed (P/Qe) versus absolute
pressure (P) and a plot of ln(Qe) against ln(P) were drawn to
investigate the adsorption mechanism for ethylene gas on
mordenite and composite membranes. It was known that the
Freundlich model simulated adsorption on a heterogeneous
surface with the interaction between adsorbed molecules.28
The Freundlich equation was given as qe = KF × P1/n, where qe
was the ethylene adsorbed amount per unit mass of adsorbed
at equilibrium (mmol/g), P was the equilibrium pressure
(kPa), KF was the Freundlich adsorption constant related to
the adsorption capacity of the adsorbent (mmol/g), and n was
a dimensionless constant, that could be utilized to elucidate the
extent of adsorption and the adsorption intensity. Typically,
the value of n was below or equal to unity, the process was
chemisorption, but if greater than unity the process is favorable
in physical adsorption.29
2.5. Adsorption Experiments of Ethylene Gas
Released from Chiquita Bananas. It is well-known that
the climacteric fruits such as papayas, avocadoes, apples, pears,
and bananas release ethylene at an initial phase of ripening.30
When ethylene gas was released and accumulated in the
surrounding environment, the ripening process was in progress
for fruits. In the present work, Chiquita bananas were chosen
as an endogenous source of ethylene production at the
Figure 1. Appearance of nylon-6−mordenite composite membranes
with a different content of mordenite.
setup for the measurement of emitted gas in the closed
container having the banana and composite membranes, the
gas sampler, gas chromatography, and representative chromatogram.
To achieve the maximum adsorption of endogenous
ethylene, the mass of the composite membrane was changed
at 5, 10, 20, 30, 50, and 70% relative to the banana mass.
Besides, the temperature effect on the ability of ethylene
adsorption of the composite membranes and the ripening rate
of fruits was investigated by storing the NMC 30 treated
bananas at 5, 20, and 30 °C for 10 days. For determination of
the concentration of mixture gases in the container, the gas
composition inside was measured periodically in a 24 h period
during the storage time of 10 days for every treatment. First, 1
mL of headspace air from the container was withdrawn using
the autogas sampler (GS 5100, GL Sciences, Japan) and
injected into gas chromatography (GC-4000 plus, GL
Sciences, Japan). The gas chromatography was equipped
with a flame ionization detector (FID). The SUS (2 m × 3 mm
I.D.) column had filler material, Porapak N (GL Sciences,
Japan), and was installed into the GC equipment operated with
the following conditions: helium as the carrier gas at 10 mL/
min, the column temperature of 40 °C, the injection
temperature of 120 °C, and the detector temperature of 200
°C. To determine each gas peak in the chromatography, the
standard gas of O2, N2, CO2, and ethylene was used for their
calibration. The retention time of each gas was confirmed as
0.37, 0.65, 1.70, and 2.65 min for O2, N2, CO2, and ethylene,
respectively, as seen in Figure 2. The quantitative analyses were
conducted by using the calibration curves of each gas analyte
prepared from a known standard gas in the range of 0.1−1.0
μmol/L for ethylene, 0.05−0.5 μmol/L for CO2, and 1.5−9.0
μmol/L for O2. The correlation coefficient of each calibration
curve was greater than 0.997. The repetition on analysis of the
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Figure 2. Experimental setup for measurement of mixture gases released from the banana sample by gas chromatography. Part (a) was for the
closed container, and part (b) was for the autogas sampling device to gas chromatography (c).
standard samples had the standard deviation as 1.8 × 10−3, 9.3
× 10−3, and 6.7 × 10−3μmol/L for ethylene, CO2, and O2. The
analytical accuracy for all analytes was determined to be less
than 7%. The detection limit of measurement was defined as 3
times the standard deviation of the determination divided by
the slope factor of the calibration curve. The detection limit of
ethylene, CO2, and O2 was 2.9 × 10−5, 4.0 × 10−6, and 1.5 ×
10−4 μmol/L, respectively. The C2H4, CO2, and O2 contents
inside the container were expressed as μmol/gbanana·day. At the
end of the experiments, the banana mass and weight of water
emanating from the respiratory process of bananas (released
water) were recorded.
3. RESULTS AND DISCUSSION
3.1. Adsorption Isotherms of Ethylene on Nylon-6−
Mordenite Composite Membranes. Figure 1 contained a
picture view of the composite membranes. It was apparent that
the successful fabrication of the composite membranes was
recognized in the coagulation process of nylon-6 dissolved in
methanol and CaCl2 solution. As seen in Figure 3, SEM
pictures of resultant membranes for the surface (left side) and
the cross section (right side) were determined.
The left-side images displayed that the surface of the
composite membranes had many tiny pores. The resultant
nylon-6 membrane had large void pores. When containing 30
wt % mordenite loading, membranes had the least pores on the
cross section, and it was noted that visible mordenite powders
were present in the membranes. In the case of the nylon-6
membrane, the cross section had an asymmetric structure,
consisting of two layers. The thin dense skin at the top layer
and the porous sublayer were formed with a cavelike structure,
implying that spontaneous coagulation of the nylon-6 solution
proceeded in the water medium.31,32 Also, the rate of water−
methanol separation in the nylon-6 solution was fast, resulting
in creating many pores on the surface of the nylon-6
membrane. When the mordenite loading in the membrane
was high, the cavelike structure in the cross section was
replaced with the spongelike structure. For example, in NMC
20 and NMC 30, the cave structural views were obscured in
the cross section. Table 1 lists several values of thickness,
density, porosity, water content, BJH parameters, and
mechanical properties for their membranes.
With increasing the mordenite loading in the membranes,
the membrane thickness and density had a tendency to be
increased. Also, the porosity and surface area were increased.
In contrast, both the tensile strength and elongation of nylon6−mordenite composites (NMCs) declined steadily with the
continuous addition of mordenite to the nylon matrix. This
Figure 3. SEM images of the surface area (a) and the cross section
(b) of nylon-6 membrane and the composite membranes at the
magnification of 5000 and 500, respectively.
might be due to the increment in the porous void space in
composite membranes leading to the decrease of the
membrane strength and increase of the pore volume as
displayed in Table 1. Figure 4 displays adsorption isotherms
for ethylene gas of mordenite and the composite membranes
containing different mordenite loadings from 0 to 30 wt %.
The pressure of ethylene gas was varied from 0 to 100 kPa at
301 K.
The behavior on ethylene gas absorption in NMC 20, NMC
30, and mordenite increased sharply in the low range of
relative pressure until 20 kPa, while in the high-pressure area
the isotherms became approximately linear. Obviously, the
mordenite had the highest adsorption ability for ethylene gas,
while the nylon-6 membrane having no mordenite was quite
low. This meant that the ethylene adsorption depended on the
presence of mordenite in the membrane. Thus, in the
composite membranes, the increased mordenite loading
resulted in efficient ethylene adsorption. Truly, as shown in
Table 1, the introduction of the mordenite powders increased
the values of the BET surface area of the membrane, meaning
D
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Table 2. Parameters on Freundlich Isotherm
Mordenite
Nylon-6
NMC 10
NMC 20
NMC 30
n
KF (mmol/g)
R2
0.69
0.42
0.58
0.71
0.72
10.7
4.1
4.2
4.3
5.7
0.993
0.998
0.997
0.997
0.996
different loadings of mordenite powders, when the membrane
and the banana were installed inside the closed container (a) in
Figure 2. Then, each produced gas was injected into the gas
chromatography (c) by using the autosampling equipment (GS
5100, GL Science) and monitored at different times. Here, the
amount of one banana was introduced in the container, and
the weight was about 120 g and weighted before dosing it. The
weights of the membrane relative to the banana sample were
used to calculate the mass ratio % defined by the following
Figure 4. Ethylene adsorption isotherms of mordenite, nylon-6, and
composite membranes.
that the composite membranes became more porous. This was
due to the introduction of the mordenite having a mesoporous
structure. To clarify the adsorption mechanism, a plot of
absolute pressure/quantity adsorbed (P/Qe) versus absolute
pressure (P) and a plot of ln(Qe) against ln(P) from the
experimental adsorption data were constructed.
As seen in Figure 5(b), a plot of ln(Qe) versus ln(P) of all
samplers yielded a straight line. This suggested that the
adsorption process of ethylene on adsorbents was obeyed to
the Freundlich isotherm model.33 It meant that in the gas
adsorption process of composite membranes, ethylene
molecules were adsorbed onto mordenite and nylon-6 matrix
heterogeneously. From their linear relation, the parameters of
the Freundlich model were summarized in Table 2 for the
mordenite, nylon-6, and composite membranes.
So again, the correlation coefficient R2 which was near equal
to 1 indicated that the Freundlich model was fitted to the
present data. Furthermore, the Freundlich constant n of all
composite membranes was lower than 1 that suggested
favorable chemical adsorption of the ethylene onto the
adsorbent surface.33
3.2. Adsorption Capacity of Composite Membranes
for Ethylene Gas Released from a Chiquita Banana. In
order to investigate releasing gas in the fruit storage process,
the ethylene gas released from a Chiquita banana was
examined in the presence of the composite membranes with
equation:
(
mmembrane
mmembrane + mbanana
) × 100.
Figure 6 shows gas
production in the closed container at different storage times
at 20 °C. Here, the mass ratio % of the membrane and banana
in the container was fixed at 20% for each membrane and
mordenite. As seen, the production of ethylene (a), CO2 (b),
O2 (c), and released water (d) was measured for 10 days. The
ethylene production in the container was apparently decreased
significantly when the mordenite loading in membranes was
increased for the membrane system. Besides, the ethylene
accumulation in the control container became higher in the
membrane and mordenite system, when the amount was
compared with that of the control without the NMC
membrane or mordenite. After 4 days of storage at 20 °C,
the ethylene production in the container was 3.08, 1.30, 0.74,
0.69, 0.55, and 0.50 μmol/gbanana·day for the control, nylon-6,
NMC 10, NMC 20, NMC 30, and mordenite, respectively.
Then, gas production was decreased for 10 days in the case of
the control. It meant that the high value of ethylene production
became lower when the NMC membrane was present. In the
case of CO2, in 5 days the peak value of CO2 production in the
container was 2.74, 1.62, 0.74, 0.63, 0.57, and 0.29 μmol/
gbanana·day for the control, nylon-6, NMC 10, NMC 20, NMC
30, and mordenite, respectively; but the NMC 10, 20, and 30
and mordenite were somewhat faster in the CO2 production in
5 days, as shown in Figure 6(b). This might be due to that the
Figure 5. Plot of P/Qe vs P (a) and plot of ln(Qe) vs ln(P) (b) for C2H4 of mordenite, nylon-6, and the composite membranes.
E
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Figure 6. Effect of mordenite or the NMC membranes on the production of ethylene (a), CO2 (b), and O2 (c) in the presence of the banana in the
container stored at 20 °C. The released water (d) was water production in the container after the 10 day experiment was finished. Here, the mass
ratio of the adsorbent and banana was fixed 20%.
measurements were finished in 10 days. Herein, the values of
the released water indicated for the rotting level of the fruit for
nylon-6, NMC 10, NMC 20, NMC 30, and mordenite were
decreased to 4.6, 3.7, 3.2, 2.6, and 1.6 g relative to that of the
control, respectively. In the NMC 30 and mordenite, the lower
production of water was due to the low ripening process of the
banana.
In order to investigate the effect of the amounts of the NMC
30 on the gas production at different storage times, the
amounts of the NMC 30 relative to the banana weight was
changed. Here, the mass ratio % of the NMC 30 and banana
was in the range of 0−70%. During the storage period, the gas
production inside the container is shown in Figure 7. As seen
in Figure 7(a, b), the ethylene and CO2 detected in the
presence of the banana behaved as well as the results of Figure
6. Figure 7(d) was for water production in the container after
the 10 days had passed.
The control data showed that both production amounts of
ethylene and CO2 were increased until 4 days and 5 days,
respectively. In addition, it was noted that both gases produced
in the container were significantly restricted when the amount
of the NMC 30 membrane in the container was increased. In
contrast, as seen in the control, the banana emitted ethylene in
larger amounts for 5 days, and then there was a tendency to
decrease the amounts until 10 days due to the climacteric
process.36 During the period of the 5−10 days of storage, the
nylon-6 matrix adsorbed ethylene and CO2 generated from the
banana sample. Also, the higher loading of mordenite in the
NMC membrane presented a tendency of higher capacity of
the adsorptivities of ethylene and CO2 emitted from the
banana sample. In the case of the O2 production (Figure 6c), it
was apparent that the control was lower than the NMC
systems and mordenite. The reason for the higher oxygen
residence in the NMC systems and mordenite was concerned
with the lower concentration of ethylene emitted in the
containers. As known,34 banana is a typical climacteric fruit
that exhibits a characteristic rise in ethylene production and
respiration rate during ripening. This meant during the
ripening process that the banana could produce ethylene,
CO2, and water by consuming O2. Also, the banana ripening
process is very sensitive to the presence of ethylene, and so
atmospheric ethylene concentration of 0.0041 μmol/L caused
a trigger of ethylene production internally initiating the
ripening process.35 In contrast, in the NMC systems and
mordenite, the amounts of ethylene released from bananas
were significantly adsorbed by the NMC and mordenite in the
closed container. As a result, the rate of ripening and
respiration became lower by the O2 consumption, and there
was a decrease in the control for the O2 production, leading the
higher oxygen residence to be due to the lower ripening
process of the banana in the NMC systems and mordenite.
Figure 6(d) was related to water production after whole
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Figure 7. Effect of NMC 30 mass on the production of ethylene (a), CO2 (b), O2 (c) and released water (d) in the presence of the banana stored
at 20 °C.
Figure 8. Pictures of banana samples treated with the different masses of NMC30 after the 10 days had passed at 20 °C.
that the O2 produced in the container decreased gradually in
the storage period, meaning that the respiration process took
place and consumed O2 even though there were the composite
membranes. In this case of the presence of large membrane
amounts, the ripening was on the process delayed for the
ripening and respiration at 20 °C. It was known that the
storage condition of the low ethylene circumstance restricted
the ripening and respiration.37 Therefore, ethylene production
influenced causing the quality change of the banana sample
during the storage time. Apparently, this can lead to firmness,
color evolution, and aroma development3,38. Figure 8 shows
pictures of the banana stored in the container with the
presence of different amounts of NMC 30 at 20 °C for 10 days.
case without the NMC 30, the control system produced much
water (Figure 7d). This might be the reason for the respiratory
process for the control, indicating that the fruits arrived at the
postclimacteric stage. From those data, it was generally
accepted to consider that the banana synthesized ethylene
and CO2 and consuming O2. On the other hand, in the
presence of the NMC 30, ethylene, CO2, and water could be
still produced by consuming O2. The results indicated that the
case of 70% of the NMC 30 relative to the banana behaved as
influenced in the lower process of the ripening and respiration.
Because of the efficient adsorption of ethylene and CO2 by the
NMC 30, it implied that the ripening was delayed, leading to
higher O2 residence, as seen in Figure 6(c). It was also seen
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Figure 9. Effect of the mass ratio of NMC 30 and the banana on the weight loss of the banana stored at 20 °C after the 10 days had passed.
Figure 10. Effect of NMC 30 mass of gas production of ethylene (a), CO2 (b), O2 (c), and released water (d) in the presence of the banana stored
at the different storage temperatures. Here, the mass ratio of the adsorbent and banana was fixed 30%.
Apparently, the presence of NMC 30 significantly decreased
the ethylene concentration inside the container and led to a
delay in the banana ripening process. This meant the extension
of the storability period at 20 °C. As seen, for the control and
5% of the NMC 30, the brown spots emerged dominantly on
the banana skin after 10 days of storage. Contrarily, the change
in skin color of bananas was postponed with the increase in the
amounts of NMC 30, meaning an extension of shelf life for
banana storage.
It was noted that the measurement of the banana quality
variables indicated NMC 30 treatment had an effect on the
quality deterioration of the banana. As seen in Figure 7(c), the
released water which indicated a rotting level of the banana
stored at 5, 10, 20, 30, 50, and 70% of the NMC 30 mass ratio
decreased to 5.4, 4.4, 2.6, 1.8, 1.7, and 1.0 g in comparison to
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Figure 11. FTIR spectra of mordenite (MOR), nylon-6, and NMC 30 before and after the adsorption of ethylene released from the banana.
the control banana at 20 °C, respectively. Also, it was
considered that the weight loss was one of the most important
factors associated with postharvest handling. Therefore, the
weight loss of bananas after the storage period was recorded
and shown in Figure 9. As compared to the control banana, the
weight loss of the bananas stored in adding the NMC 30 with
the mass ratio at 5 and 30% was reduced to 6.6 and 2.1 g,
respectively. Those results strongly suggested the efficacy of
NMC 30 as ethylene adsorbent in delaying a quality
deterioration of the bananas, hence prolonging the freshness
of the banana.
It was known that the storage temperature affects the
ripening changes in fruits.39 Thus, the temperature in the
storage of the banana was changed to investigate the influence
of NMC 30 on the gas production rate of the banana in the
cases of 5, 20, and 30 °C for 10 days. Figure 10 shows the
production of ethylene (a), CO2 (b), O2 (c), and released
water (d) in the container.
As expected, in the presence of the NMC 30, the bananas
released less ethylene at all storage temperatures. At 20 °C, the
control had almost 3.08 μmol/g in the ethylene production in
4 days; while in the presence of NMC 30, the 20 °C storage
emitted about 0.36 μmol/g ethylene. Generally, the released
ethylene was increased when the temperature was enhanced.
This is because that higher temperature prefers the ripening
process. In other words, the banana was retarded in the
ripening process, specifically in the cases of the lower
temperature and the presence of the NMC 30. The storage
at 5 °C with the lower ethylene from the banana was more
efficient in the presence of the ethylene adsorber, NMC 30.
This suggested that the presence of NMC 30 might cause a
decrease rate of metabolism and respiration. Also, the
production of CO2 and O2 (Figure 10(b, c)) was similarly
seen as such the tendency in Figures 6 and 7. Also, the
decrease in the released water from the banana was also
observed similarly as shown in Figure 10(d). As compared to
the control, the released water from the banana with the NMC
30 system decreased to 11.3, 1.8, and 0.9 g at 30, 20, and 5 °C,
while the control was 40.5, 5.5, and 3.5 g for 30, 20, and 5 °C,
respectively. These results strongly suggested that the lower
temperature was effective in the banana storage with the NMC
30.
To elucidate the adsorption mechanism of ethylene, the
interaction among ethylene, mordenite, and nylon-6 was
considered. Thus, FTIR spectra were measured as shown in
Figure 11.
It was seen that the peak shifted in the FTIR spectra of
mordenite, nylon-6, and the composite membranes after the
ethylene adsorption. Before adsorbing ethylene, for the nylon
spectra, the typical peaks of the amide group were seen at 1535
cm−1 (amide II, C−N stretch, and CO−N−H bend) and 1637
cm−1 (amide I, CO stretch), those of −NH bending were
seen at 3062 cm−1, and those of −NH stretching were seen at
3286 cm−1.40,41 The FTIR bands of mordenite exhibited that
the intense broad peaks of the Si−O stretching vibration were
observed at 1035 cm−1, and the bands at 1647 cm−1 were
attributed to the bending vibration of H−OH. Isolated OH
stretching at 3628 cm−1 was assigned to the interaction
between the water hydroxyl and cations (Al−OH stretching).16,42 It is noted that the characteristic band for the amide
group at 1535 and 1637 cm−1 as well as the −NH vibration
band at 3062 and 3286 cm−2 in the FTIR band of nylon-6 was
clearly seen in the NMC 30. Furthermore, compared to the
mordenite spectra, the stretching vibration peak of Si−O in
FTIR bands of the NMC 30 was broadened and shifted to a
lower wavenumber from 1035 to 1029 cm−1. Also, the
incorporation of mordenite with nylon scaffold resulted in
the broadness and intensity reduction of the Al−OH stretching
band at 3630 cm−1. After the uptake of ethylene, some shifted
peaks in the FTIR bands of nylon-6, mordenite, and NMC 30
were confirmed. For instance, the peak shift at 1035 cm−1
attributed to the Si−O at 1041 cm−1 in the MOR after
ethylene adsorption. This meant the presence of the
interaction between ethylene and the Si−O sites in the
MOR.43 Also, the band for the vibration of Al−OH at 3628
cm−1 slightly shifted to 3616 cm−1 in the spectra for
mordenite. In the case of nylon-6 after ethylene adsorption,
the −NH vibration band at 3286 cm−1 was shifted toward
3302 cm−1. In addition, the amide I band at 1637 cm−1 shifted
toward 1641 cm−1, meaning that interaction with ethylene
occurred. It might be due to the CH···O interaction between a
C−H bond of ethylene and oxygen on the amide group of the
−N(−H)−C(O) of nylon-6 having been created. A similar
discussion was approved in the case of the NMC 30. In the
I
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FTIR of NMC 30 after the uptake of ethylene, the peak shift
was observed at the amide group, the group of Si−O, and Al−
OH. This suggested that the adsorption of ethylene released
from bananas cooperatively interacted with ethylene on the
mordenite powder embedded in the nylon-6 scaffold
membrane.
4. CONCLUSION
A methanol/CaCl2/nylon-6 solution was reported by the wet−
phase inversion process as the first paper in the fabrication of
nylon membranes loaded mordenite for ethylene adsorption.
The mordenite content varied from 10 to 30 wt % of the
polymer was homogeneously embedded within the polymer
solution. The SEM images of the composite membranes were
shown evidence for porous membrane adsorbents. The
experiments for ethylene gas adsorption showed that
composite membranes had better adsorption ability than that
of nylon-6 membrane under the same conditions. With the
increment of mordenite loading in the membrane, the
adsorption capacity for ethylene gas of the composite
membranes also increased. In the equilibrium study, ethylene
gas adsorption behavior was better fitted to the Freundlich
model. This meant that ethylene molecules were adsorbed by
multilayer and heterogeneous adsorption onto the adsorbent in
the sites of mordenite. The composite membranes were also
examined for ethylene removal in the presence of Chiquita
bananas at different mordenite loadings, different adsorbent
masses, and different storage temperatures. The results showed
that the composite membranes adsorbed significantly the
endogenous ethylene emanated from bananas, hence prolonging the storage life of fruits.
■
AUTHOR INFORMATION
Corresponding Author
Takaomi Kobayashi − Department of Materials Science and
Technology, Nagaoka University of Technology, Nagaoka,
Niigata 940-2188, Japan; Email: takaomi@nagaokaut.ac.jp
Author
Phuong Thanh Ton Nu − Department of Materials Science and
Technology, Nagaoka University of Technology, Nagaoka,
Niigata 940-2188, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.iecr.9b06149
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of all members
in the Kobayashi Lab, Nagaoka University of Technology,
Japan. Besides, the authors confirm that this research did not
receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
■
■
Article
ABBREVIATIONS
MOR, mordenite
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