Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST
Article
Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid Discharge Casey Finnerty, Lei Zhang, David L. Sedlak, Kara L. Nelson, and Baoxia Mi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03040 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29
Environmental Science & Technology
1 2
Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid Discharge
3 4
Revision submitted to
5 6
Environmental Science & Technology
7 8
August 29, 2017
9 10 11
Casey Finnerty, Lei Zhang, David L. Sedlak, Kara L. Nelson, Baoxia Mi*
12 13 14
Department of Civil and Environmental Engineering
15
University of California, Berkeley, California 94720, United States
16 17 18 19
*
The author to whom correspondence should be addressed. fax: +1-510-643-5264
e-mail:
[email protected]; tel.: +1-510-664-7446,
ACS Paragon Plus Environment
Environmental Science & Technology
20
ABSTRACT
21
Water vapor generation through sunlight harvesting and heat localization by plasmonic and
22
carbon-based porous thin film materials holds great promise for sustainable, energy-efficient
23
desalination and water treatment. However, the applicability of such materials in high-salinity
24
environment emphasizing zero liquid discharge brine disposal has not been studied. This paper
25
reports the characterization and performance of a nature-inspired synthetic leaf made of graphene
26
oxide (GO) thin film material, which exhibited broadband light absorption and excellent stability
27
in high-salinity water. Under 0.82-sun illumination (825 W/m2), a GO leaf floating on water
28
generated steam at a rate of 1.1 liters per m2 per hour (LMH) with a light-to-vapor energy
29
conversion efficiency of 54%, while a GO leaf lifted above water in a tree-like configuration
30
generated steam at a rate of 2.0 LMH with an energy efficiency of 78%. The evaporation rate
31
increased with increasing light intensity and decreased with increasing salinity. During a long-
32
term evaporation experiment with a 15 wt.% NaCl solution, the GO leaf demonstrated stable
33
performance despite gradual and eventually severe accumulation of salt crystals on the leaf
34
surface. Furthermore, the GO leaf can be easily restored to its pristine condition by simply
35
scraping off salt crystals from its surface and rinsing with water. Therefore, the robust high
36
performance and relatively low fabrication cost of the synthetic GO leaf potentially unlock a new
37
generation of desalination technology that can be entirely solar-powered and achieve zero liquid
38
discharge.
1
ACS Paragon Plus Environment
Page 2 of 29
Page 3 of 29
Environmental Science & Technology
39
INTRODUCTION
40
As water resources dwindle while demands soar, desalination is becoming a viable water
41
treatment option to augment water supplies. Major advances have been made in recent years to
42
reduce the energy consumption of desalination technologies. For example, state-of-the-art
43
reverse osmosis (RO) membrane systems desalinate seawater with a recovery rate of 50% while
44
consuming as little as 2 kWh/m3, which is only 25% higher than the practical minimum energy
45
requirement.1 However, as feed water salinity increases, the energy consumption, operating
46
pressure, and fouling/scaling potential of the membrane system increase. This issue makes it very
47
difficult and costly to further enhance the recovery of an RO-based desalination system or
48
operate it under high salinity conditions.2 In the meantime, existing brine treatment technologies
49
such as zero-liquid discharge (ZLD) systems are particularly energy-intensive. For example, the
50
most widely used technology, which relies on mechanical vapor compression (MVC)-based brine
51
concentrators and crystallizers, consumes 20-25 kWh/m3 and 52-66 kWh/m3, respectively.2
52
Recently, brine concentrators based on membrane processes such as forward osmosis3 and
53
membrane distillation
54
low-grade heat as an energy source, but it is unclear if they offer substantial advantages over
55
existing MVC-based concentrators. Because high energy consumption remains a major barrier to
56
ZLD, innovative approaches that are more sustainable and energy-efficient than current
57
technologies are desirable.
58
4
have been the subject of increasing interest due to their ability to use
Recent developments in photothermal steam generation have greatly enhanced the solar 5
59
light-to-heat conversion efficiency
and hold immense promise for increasing the overall
60
efficiency of desalination and water treatment. In this process, materials exhibiting mesoscopic
2
ACS Paragon Plus Environment
Environmental Science & Technology
61
porosity and efficient light absorption6-12 are used for localized heat transfer from the light-
62
absorbing material to water in pores. However, the absorption bandwidth of the plasmonic
63
nanoparticles frequently used for absorbing light and converting it to heat under surface plasmon
64
resonance depends on their size and shape,13 which increase the complexity and cost of the
65
materials.14, 15 Additionally, plasmonic nanoparticle films tend to be fragile and thus limit their
66
use under realistic operation conditions.
67
In contrast, the robust and inexpensive carbon-based materials such as graphene,5, 16
68
graphene oxide (GO),17-19 and graphite20 are natural broadband light absorbers that are capable of
69
absorbing about 97% of incident sunlight, making them an ideal material for sustainable solar-
70
powered desalination.
71
prior to their effective use for photothermal steam generation. This is because hydrophilic
72
materials like GO readily swell or dissolve in water21 and are thus unsuitable for direct water
73
treatment without necessary modification,22-24 while hydrophobic materials like graphene and
74
black carbon cannot maintain good contact with water and thus have limited effectiveness in
75
transferring heat to water.
76
However, further development of carbon-based materials is required
Looking to nature, mangrove trees rely on a complex salt filtration process involving 25
to thrive in waters with a salinity higher than that of seawater.26
77
passive water transport
78
Evaporation of water from leaf stomata under solar irradiation, together with capillary action,
79
creates a negative pressure that transports water from the growing medium into the roots,
80
through the xylem, and eventually to the leaves. Coupled with such water transpiration,
81
mangrove trees have developed two salt-regulating mechanisms. One is to partially exclude salt
82
from water at their roots to minimize the accumulation of salt in their xylem and leaves, and the
3
ACS Paragon Plus Environment
Page 4 of 29
Page 5 of 29
Environmental Science & Technology
83
other is to excrete salt from glands, which are ~30 µm in diameter and mostly distributed over
84
the upper surface of mangrove leaves.27 By mimicking the water transpiration and salt-excreting
85
capability of mangroves, we may innovate an artificial tree system that simultaneously acts as a
86
water purifier and a salt crystallizer, leading to a solar-powered desalination technology with
87
ZLD capability and eliminating the two energy-intensive steps — brine concentration and
88
crystallization — required by the existing technologies.
89
This paper reports the first-phase study of a nature-inspired technology for solar-
90
powered desalination with ZLD by using synthetic leaves to enable photothermal water
91
evaporation. We used GO to fabricate a synthetic leaf that is stable in water and capable of
92
generating steam with a high energy conversion efficiency. The evaporation performance of the
93
synthetic GO leaf was evaluated in two configurations of the solar desalination system: (1) a
94
floating configuration, in which the GO leaf floated on and was in direct contact with bulk water,
95
and (2) a tree configuration, in which the GO leaf was lifted above and connected via a water-
96
absorbing sheet to bulk water. Infrared thermal imaging was used to visualize the evaporation
97
mechanism of each configuration. The effects of light intensity and water salinity on the
98
performance of the GO leaf in the tree configuration were systematically investigated. Lastly, the
99
durability and reusability of the GO leaf operated in high-salinity conditions were studied to
100
assess its potential for long-term effective treatment of highly saline water.
101
MATERIALS AND METHODS
102
Chemicals. Chemicals needed for GO synthesis included graphite flakes with 99% purity
103
(Alfa Aesar, Ward Hill, MA), sodium nitrate (Sigma-Aldrich, St. Louis, MO), sulfuric acid
104
(Sigma-Aldrich, St. Louis, MO), potassium permanganate (Fisher Scientific, Hampton, NH), and
4
ACS Paragon Plus Environment
Environmental Science & Technology
105
hydrogen peroxide (30% solution, Fisher Scientific, Hampton, NH). Additionally, hydrochloric
106
acid (37% solution, Fisher Scientific, Hampton, NH) and ethanol (Koptec, King of Prussia, PA)
107
were used as washing solutions. Fabrication of the synthetic GO leaf required
108
triethylenetetramine (TETA) and 1,4-butanediol diglycidyl ether (BDGE) (Sigma-Aldrich, St.
109
Louis, MO) as crosslinking agents to stabilize GO nanosheets. Cellulosic filter paper (Whatman,
110
Maidstone, United Kingdom) was used as a substrate. Deionized water was produced by a
111
Barnstead Smart2Pure water purification system (Thermo Fisher Scientific, Waltham, MA).
112
GO Nanosheet Preparation.
113
procedure adapted from a previous study,24 which was based on the modified Hummers’
114
method.28, 29 Briefly, 5 g of powdered graphite flakes was oxidized in a mixture of 2.5 g of
115
sodium nitrate, 115 mL of sulfuric acid, and 15 g of potassium permanganate in a 35°C water
116
bath for 30 min, then transferred to a 98°C water bath and diluted by 230 mL of deionized water,
117
where it remained for 15 min. After cooling for 10 min, the mixture was diluted to 700 mL and
118
then added with 5 mL of hydrogen peroxide. The suspension was filtered using a polyethylene
119
terephthalate fiber, and the filtrate was centrifuged at 8,000 x g for 2 h with an Avanti J-30I
120
centrifuge (Beckman Coulter, Indianapolis, IN). The supernatant was decanted and the
121
precipitate was washed in successive cycles of deionized water, hydrochloric acid, and ethanol
122
using the centrifuge until the supernatant reached pH 7. Next, the GO precipitate was freeze-
123
dried at -50°C and under a pressure of less than 0.2 mbar in a freeze dryer (FreeZone 1,
124
Labconco, Kansas City, MO) to obtain GO powder. The GO was then resuspended in a 20 g/L
125
solution by sonication with a Q500 Sonicator (Qsonica, Newton, CT) in an ice bath at an
126
amplitude of 80% for 2 h. The resulting GO suspension was purified by placing it in a dialysis
127
bag and soaking the bag in 2000 mL deionized water, which was replaced daily for 6 days or
GO nanosheets were synthesized from graphite flakes using a
5
ACS Paragon Plus Environment
Page 6 of 29
Page 7 of 29
Environmental Science & Technology
128
until the water conductivity stabilized.
129
GO Leaf Fabrication.
130
nanosheets with TETA and BDGE.30 As schematically illustrated in Figure 1, 0.5 mL of 20 g/L
131
GO suspension was first mixed with 0.1 mL of 0.01 M sodium hydroxide in a 2 mL tube on ice.
132
The solution was then mixed with an Analog Mini Vortexer (Fisher Scientific, Hampton, NH) for
133
1 min and placed back on ice. Next, 24 µL of TETA was added and the solution was vortex-
134
mixed for 1 min and placed back on ice. Then, 65 µL of BDGE was added and the solution was
135
vortex-mixed before placing the tube in an ice bath that was sonicated at 40% amplitude for 4
136
min. Two circular coupons, each with a diameter of approximately 4.5 cm, were cut from the
137
filter paper with a circle cutter (Fiskars, Finland). The coupons were dip-coated in the GO-
138
TETA-BDGE solution and left undisturbed at room temperature for 1 min, leading to a GO mass
139
loading density of about 0.29 mg/cm2 on the coupon. The coupons were then frozen by placing
140
them on a plastic petri dish wrapped in aluminum foil and suspending it in liquid nitrogen for 2
141
min. The petri dish with the coupons was placed in a freeze dryer (Fisher Scientific, Hampton,
142
NH) overnight at -50°C and under a pressure of less than 0.2 mbar, and then the coupons were
143
transferred to a 100°C oven and stored for 24 h to facilitate the crosslinking reaction. After that,
144
the coupons were soaked in deionized water for 24 h, during which the water was changed at
145
least 5 times. Finally, the resulting GO-TETA-BDGE coupons were stored at 60°C in an Isotemp
146
General-Purpose Heating and Drying Oven (Fisher Scientific, Hampton, NH) to remove any
147
residual water.
The synthetic GO leaf was prepared by crosslinking the GO
148
FIGURE 1
149
GO Leaf Characterization. To investigate the stability of the synthetic GO leaf, its elemental
6
ACS Paragon Plus Environment
Environmental Science & Technology
150
compositions before and after solar irradiation were analyzed by X-ray photoelectron
151
spectroscopy (XPS). To verify the light absorption capability of the GO leaf, its absorption
152
spectrum was measured using an ASD QualitySpec Pro UV-Vis spectrophotometer with a
153
Muglight attachment (ASD Inc., Boulder, CO). The absorption spectrum of the filter paper
154
substrate was also collected as a control to determine light absorption enhancement by GO. The
155
deposition of GO-TETA-BDGE onto the filter paper substrate was verified by collecting the
156
absorption spectra with an iS50 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher
157
Scientific, Waltham, MA). Lastly, the pore size and thickness of the GO leaf were determined by
158
visualizing it with a Gemini Ultra-55 analytical field emission scanning electron microscope
159
(SEM) (Zeiss, Jena, Germany).
160
Solar Evaporation Experiments and Evaluation. Inspired by the water transpiration
161
process of a natural tree, a bench-scale setup was built to evaluate the solar evaporation
162
performance of the synthetic GO leaf. As shown in Figure 2, the synthetic GO leaf was placed on
163
top of a water reservoir (100 mL beaker with an inner diameter of 4.5 cm) by positioning it on a
164
piece of polystyrene sheet. Water was transported from the bulk water to the GO leaf by capillary
165
action through a water-absorbing sheet (Nalgene Versi-Dry Surface Protectors, Thermo Fisher
166
Scientific, Waltham, MA). To promote good contact between the GO leaf and the water absorber,
167
the two materials were sandwiched together using two sheets of polystyrene plastic. As a control,
168
a floating configuration was evaluated, in which the beaker was filled with deionized water and
169
the GO leaf was placed directly on the water surface. Furthermore, baseline tests on water
170
evaporation in the absence of a GO leaf were conducted for the tree and floating configurations,
171
respectively.
7
ACS Paragon Plus Environment
Page 8 of 29
Page 9 of 29
172
Environmental Science & Technology
FIGURE 2
173
The evaporation experiment was started by irradiating the water in the reservoir (with or
174
without a GO leaf installed) with artificial sunlight, which was produced by a 1,000-watt large
175
area light source solar simulator (Spectra-Physics, Stratford, CT) and had a solar spectrum
176
comparable to that of natural sunlight over the visible light region (Figure S1). A P-8001 mass
177
balance (Denver Instrument, Bohemia, NY) was placed beneath the light source of the solar
178
simulator to record the mass of water remaining in the reservoir at 1-min intervals during
179
irradiation cycles. The evaporation rate was then calculated as the change in the recorded mass of
180
water normalized by the leaf area and the corresponding time interval, that is, the slope of the
181
curve that depicted the relationship between the mass of water evaporated per unit leaf area and
182
the irradiation time. The same analysis was also performed for all control experiments (where
183
light irradiation was not applied) to obtain baseline evaporation rates in dark conditions. The
184
light intensity and solar spectrum were obtained by an LX1330B lux meter (Precision Vision,
185
Woodstock, IL) and an EPP 2000 radiometer (StellarNet Inc., Tampa, FL), respectively. Unless
186
otherwise specified, each solar experiment was performed with a light intensity of 825 W/m2,
187
which corresponds to 0.82-sun illumination since 1,000 W/m2 is often considered as the default
188
value for one sun condition in related literature.17
189
taking infrared images with a Ti100 infrared camera (Fluke Corporation, Everett, WA) before
190
irradiation and at regular time intervals during irradiation.
191
Solar Desalination Performance Evaluation. The stabilized evaporation rates for the tree
192
configuration were determined by running the evaporation experiments for at least 5 h using bulk
193
NaCl solutions of different initial salinities (1.7, 3.5, 5.25, and 7 wt.%), representing 0.5, 1.0, 1.5,
8
The temperature evolution was monitored by
ACS Paragon Plus Environment
Environmental Science & Technology
Page 10 of 29
194
and 2.0 times the concentration (3.5 wt.%) of NaCl in seawater. The crystal formation on the GO
195
leaf surface was examined by SEM when the salinity of the solution remaining in the reservoir
196
reached a certain level (~8.8 wt.% NaCl). The long-term evaporation performance of the GO leaf
197
was examined using 15 wt.% NaCl solution (~4 times seawater salinity), and the effects of salt
198
crystals formed over the leaf surface on temperature distribution and evaporation performance
199
were investigated. The reusability of the material was assessed by scraping off the accumulated
200
salt crystals from the GO leaf using a metal scoopula and briefly soaking it in deionized water
201
for 5 min before reusing the cleaned GO leaf in another evaporation cycle.
202
RESULTS AND DISCUSSION
203
Properties of GO Leaf.
204
photothermal capability, hydrophilicity, and low cost. The TETA-BDGE-crosslinked GO
205
nanosheets were deposited across the whole depth of the filter paper, as indicated by the change
206
of color from white to black on both top and bottom surfaces of the filter paper after GO
207
deposition (Figure S2).
208
dissolving in water) and durability (to resist mechanical damages), as the GO leaf did not exhibit
209
signs of cracking or tearing after repeated folding and unfolding in either dry or wet/humid
210
condition (Figure S3).
211
elemental composition of the GO leaf due to sunlight irradiation were small.
GO was selected as the solar absorber in this study because of its
Such a fully interwoven structure led to excellent stability (to avoid
Furthermore, XPS analysis (Figure S4) revealed that the changes in
212
A major advantage of the GO leaf is its broadband light absorption capability, which is
213
critical for enhancing the overall energy efficiency of solar evaporation. As shown in Figure 3A,
214
the light absorption of the GO leaf was consistently higher than that of the filter paper substrate.
215
Absorption throughout the visible and near infrared regions of the electromagnetic spectrum is
9
ACS Paragon Plus Environment
Page 11 of 29
Environmental Science & Technology
216
attributable to both the broad absorption spectrum and surface roughness (which enhances
217
internal light scattering) of GO.17 Such mechanisms for photothermal conversion are
218
advantageous over plasmonic heating, which requires noble metals (e.g., gold and silver) and
219
complex material processing to convert narrowband to broadband light absorption.
220
The composition of the GO leaf was evaluated by FTIR. As shown in Figure 3B, the
221
FTIR spectrum of the GO leaf exhibits peaks at 1204, 1566, 1645, and 1724 cm-1 that are not
222
present in the spectrum of the filter paper, indicating the existence of epoxy, carboxylate, amide,
223
and ester groups, respectively, in the GO leaf. It is believed that the carboxylate and, to a lesser
224
extent, epoxy and ester peaks were likely contributed by GO, the amide peak was solely due to
225
TETA, and the epoxy and ester peaks were primarily associated with BDGE. The surface (Figure
226
3C-D) and cross-section (Figure 3E-F) SEM images reveal that the fabricated GO leaf was
227
around 170-µm-thick and had an extremely porous structure, both external and internal, created
228
by the crosslinked GO nanosheets on the cellulosic fibers of the filter paper substrate.
229
The pores of the 3D microporous network of the GO leaf had a planar diameter in the
230
range of 10 to 30 µm, similar to the size of stomatal openings in natural tree leaves. However,
231
unlike stomata, which mainly serve as gates to regulate the escape of water vapor,31, 32 the fixed-
232
sized openings of the GO leaf contributed to water evaporation in three different ways. First, the
233
presence of these pores increased the effective surface area and thus enhanced the overall rate of
234
evaporation. Second, because water diffuses much faster near the edge of a pore than at its center,
235
a phenomenon known as the perimeter effect,33 the rate of pore evaporation was higher than that
236
of bulk water evaporation. Third, the porous GO structure enabled effective heat localization by
237
directly transferring the heat generated by absorbed light to water within the pores.
10
ACS Paragon Plus Environment
Environmental Science & Technology
238
Page 12 of 29
FIGURE 3
239
Heat Localization Enabled by Tree Configuration.
240
GO leaf in the floating configuration (where the GO leaf was in direct contact with bulk water)
241
and the tree configuration (where the GO leaf was lifted above and insulated from bulk water, as
242
assembled in Figure S5) were compared to understand how the GO leaf enhanced the water
243
evaporation through heat localization. The GO leaf heated up much faster than its control in the
244
floating configuration (Figure 4A-B) as well as in the tree configuration (Figure 4C-D). This is
245
because the photothermal properties of the GO leaf facilitated more light to be converted into
246
heat, increasing the temperature at the evaporation interface. The heat generated in the floating
247
configuration was partly transferred into the bulk water, as evidenced by an increase in the
248
temperature of bulk water, thereby reducing the amount of heat available for evaporation in the
249
GO leaf. In contrast, the bulk water did not significantly heat up in the tree configuration,
250
indicating that heat was mainly retained in the pores of the GO leaf. Note that compared with the
251
floating configuration, the tree configuration only led to a small increase of temperature at the
252
evaporation surface. This is because, although the tree configuration improved heat localization,
253
the corresponding higher rate of evaporation required more energy to vaporize additional water,
254
thus limiting the amount of heat available for increasing the temperature of the GO leaf.
255
The temperature evolutions of the
FIGURE 4
256
Solar Evaporation Performance.
257
configurations were examined. As shown in Figure 5A, the water evaporation rate (calculated as
258
the slope of the curve) of the GO leaf in the floating configuration was 1.1 LMH, marking a 59%
259
increase from 0.70 LMH of its control experiment without GO leaf. Similarly, the evaporation
The evaporation performances of both floating and tree
11
ACS Paragon Plus Environment
Page 13 of 29
Environmental Science & Technology
260
rate of the GO leaf in the tree configuration was 2.0 LMH, which more than doubled the 0.88
261
LMH of its control without GO leaf.
262
The evaporation efficiency (i.e., light-to-vapor energy conversion efficiency), as
263
calculated in the Supporting Information with baseline dark-condition corrections (Figure S6),
264
increased from 20% for bulk water to 54% for the GO leaf in the floating configuration and 78%
265
for the GO leaf in the tree configuration. While both configurations relied on the photothermal
266
properties of the GO leaf to convert sunlight into usable heat, the evaporation rate for the tree
267
configuration was higher due to better heat localization at the evaporation surface. Figure 5B
268
illustrates the variation in the relative proportions of the total solar energy having different
269
ultimate fates — loss by light reflection and transmission, loss to the surrounding environment
270
through heat transport (i.e., conduction, radiation, and convection), and latent heat for phase
271
change (evaporation) — among different configurations under 0.82-sun illumination. Detailed
272
calculation of the energy distribution is explained in the Supporting Information. For both bulk
273
water and tree configurations without a GO leaf, most (72-83%) of the energy from the incident
274
light was lost due to light reflection and transmission. In contrast, adding the GO leaf in both
275
floating and tree configurations made it possible to capture significantly more solar energy while
276
reducing the amount of energy lost as reflected/transmitted light to 10%. Besides, the tree
277
configuration with a GO leaf was very energy-efficient viewing that only 12% of the total energy
278
was lost as heat into the environment. In contrast, the floating GO leaf configuration lost 36% of
279
the total energy to the bulk water that had a direct contact with the GO leaf. Therefore, a key
280
advantage of the tree configuration over the floating configuration is that more heat can be
281
locally trapped by the lifted GO leaf due to the air insulation between the GO leaf and bulk water,
282
thus significantly minimizing the amount of heat energy lost into the aqueous environment.
12
ACS Paragon Plus Environment
Environmental Science & Technology
Page 14 of 29
283
Our data agreed well with literature data in revealing the importance of having an
284
insulation layer between the bulk water and evaporation surface to enhance the overall light-to-
285
vapor energy conversion efficiency.
286
78% under 0.82 sun, which is comparable to the efficiency (80-85% under one sun) of other
287
recently reported carbon-based evaporators with good insulation.5, 16-20 Some other biological
288
and engineered materials (e.g., carbonized mushroom, two-dimensional MXene) with good
289
photothermal properties and proper insulation also led to a relatively high efficiency (~80%).34, 35
290
However, when an effective insulation layer was absent, a much lower efficiency (50-64%) was
291
reported for GO,17 plasmonic material,12,
292
conversion efficiency in general increases with increasing light intensity. For example,
293
efficiencies as high as 90-94% were achieved by GO 16 and plasmonic material-based systems 12,
294
15
Our tree configuration with a GO leaf had an efficiency of
15
and other materials.36
Note that the energy
when subjected to a much higher light intensity (> 4 suns).
295
Additional characterization of the GO leaf in a tree configuration showed a positive
296
correlation between the evaporation rate and light intensity. As plotted in Figure 5C, the
297
evaporation rate increased approximately linearly with light intensity and nearly doubled as the
298
light intensity increased from 450 to 900 W/m2. Note that other environmental factors such as
299
humidity, temperature, and wind speed also likely affect the evaporation rate.37 Research on the
300
effects of these factors is warranted in the future but is beyond the scope of the present study.
301
FIGURE 5
302
Solar Desalination Performance. The GO leaf was tested in a tree configuration
303
under the 0.82-sun condition (825 W/m2) and at different salinity levels. It is observed in Figure
304
6A that the stabilized evaporation rate of the GO leaf was inversely proportional to water salinity.
13
ACS Paragon Plus Environment
Page 15 of 29
Environmental Science & Technology
305
Specifically, the evaporation rate decreased from 2.0 LMH for fresh (i.e., zero-salinity) water to
306
0.71 LMH for water with an initial salinity of 7.0% NaCl, twice that of seawater. As salinity
307
further increased, the evaporation rate appeared to decrease much more slowly and reached ~0.5
308
LMH at a salinity level of 15 wt.% NaCl. The evaporation rate at each salinity typically
309
decreased initially and stabilized within the first hour, as demonstrated in Figure 6B using the
310
salinity level of 15% as an example. Therefore, compared with the thick accumulation of salt
311
crystals on the GO leaf, the initial formation of salt crystals, which took place within the pores
312
and grow heterogeneously at random locations on the surface, likely played an important role in
313
reducing the evaporation rate. In order to study the structure of salt crystals initially formed on
314
GO surface before it was fully covered by a thick crystal layer, we used SEM to observe the
315
crystal-free regions of the GO leaf with naked eyes after crystals started to accumulate in other
316
regions, using NaCl solutions with initial salinity levels of 1.75% (Figure 6C), 3.5% (Figure S7),
317
5.25% (Figure S7), and 7.0% (Figure 6D), respectively. As expected, an increase in salinity led
318
to more severe crystal formation and thus narrower open pores of the GO leaf, decreasing the
319
rates of water transport and evaporation (Figure 6A).
320
It is observed in Figure 6D that open pores still existed in the GO leaf even after it had
321
experienced significant crystal build-up at high salinity. According to Kelvin equation,38
322
evaporation would occur more rapidly on the convex/flat surface of salt crystals formed over the
323
GO leaf surface than on the concave water surface formed within the pores. We hypothesize that
324
such preferential evaporation helped the pores of the GO leaf to remain open as crystals kept
325
growing on the already-formed ones, thereby enabling water evaporation to continue despite salt
326
crystal accumulation on the leaf surface. Similar preferential sites for salt accumulation was also
327
observed in the process of water evaporation from porous media.39
14
ACS Paragon Plus Environment
Environmental Science & Technology
Page 16 of 29
328
It is interesting to note that after the initial pore blockage the additional salt
329
accumulation on the GO leaf surface did not appear to further reduce the evaporation rate. As
330
shown in Figure 6B, the water evaporation vs. time curve has an almost constant slope during a
331
5-day test (except the first hour) at a very high salinity of 15 wt.% NaCl. The time-lapse
332
snapshots in Figure 6E demonstrate that crystals gradually grew on the GO leaf surface during
333
the 5-day test, with the overall thickness reaching more than 1 cm by day 5. A time-lapse video
334
documenting the 5-day test is available in the Supporting Information. In addition, the formation
335
of a thick crystal layer did not significantly decrease the light absorption efficiency of the GO
336
leaf; on the contrary, thermal images (Figure 6F) show that the surface temperature of the GO
337
leaf during the evaporation experiment using high-salinity (15%) water was actually higher than
338
that using deionized water (Figure 4D), indicating that a large portion of light was still able to
339
transmit through the salt crystals accumulated on the GO leaf. Note that the evaporation rate was
340
likely affected by a number of factors, including the surface temperature, pore size, effective
341
evaporation area, and capillary action within the leaf structure, some of which may have
342
conflicting effects on evaporation on the GO leaf.
343
GO leaf might quickly (say, within hours) reach a steady state and become the most limiting
344
factor during subsequent evaporation.
It is suspected that capillary action within the
Future work to fully understand such effects is needed.
345
Although the salt accumulation did not noticeably affect the water evaporation rate in the
346
present study, the salt build-up on the GO leaf surface could eventually damage the overall solar
347
desalination system due to, for example, the added weight. To resolve this issue, the accumulated
348
salt should be cleaned regularly during the evaporation operation — salt crystals were scraped
349
off from the GO leaf using a metal scoopula and then the GO leaf was soaked in deionized water
350
for 5 min (Figure S8A). In this way, the pristine state of the GO leaf was restored without
15
ACS Paragon Plus Environment
Page 17 of 29
Environmental Science & Technology
351
apparent signs of physical damage or performance deterioration (Figure S8B). Note that the
352
accumulation of inorganic (e.g., gypsum) and organic (e.g., natural organic matter, protein and
353
other macromolecules) species on the GO leaf could also affect the evaporation performance.
354
Evaluation of such effects is necessary in the future toward further development of the present
355
solar desalination technology.
356
FIGURE 6
357
Outlook of Artificial Tree for Solar Desalination and Wastewater Treatment with
358
ZLD. An artificial tree system consisting of multiple synthetic GO leaves will be the next-
359
phase experimental configuration toward scaling up the present single-leaf device. We expect
360
that the multi-leaf configuration will make the solar desalination system more compact so as to
361
better withstand real-world environmental stressors. Besides, the multi-leaf configuration will
362
increase the so-called leaf area index (LAI), which is defined as the ratio of total one-sided leaf
363
area to canopy footprint area.40
364
efficient use of solar light by absorbing scattered/transmitted light and secondary irradiation, but
365
also enhance convective evaporation by increasing the available leaf surface area. In the
366
meantime, future research is warranted to optimize the structure and properties of the GO leaf for
367
maximum utilization of convective evaporation (in addition to direct solar evaporation) along
368
with the capillary force to take up and transport water. Eventually, water vapor generated by
369
synthetic leaves will be collected and condensed to produce clean water. Note that the latent heat
370
released from vapor condensation could be recovered and reused for evaporation, further
371
increasing the overall energy efficiency.
372
An artificial tree with a larger LAI will not only enable more
The direct crystallization of salt on synthetic leaves offers an unprecedented opportunity
16
ACS Paragon Plus Environment
Environmental Science & Technology
Page 18 of 29
373
for brine disposal with ZLD. The unique capability of natural mangrove trees to predominantly
374
excrete NaCl crystals on leaf surface, which results in a buildup of magnesium and potassium
375
ions in leaves,27 suggests the possibility of salt separation and purification during crystallization.
376
In addition to salt excretion on synthetic leaves via water evaporation, salt exclusion at synthetic
377
roots made of semipermeable membranes can be implemented to enhance the overall
378
desalination performance, leading to a complete tree configuration. We expect that the artificial
379
tree system will be capable of treating high-salinity waters and recovering salts using sunlight as
380
the sole energy source. Considering the scale-up feasibility and low fabrication cost of the
381
synthetic GO leaf, the artificial tree system offers a great potential for important applications
382
such as off-grid, robust, and economical wastewater treatment and seawater desalination with
383
minimum environmental impacts.
384
385
ASSOCIATED CONTENT
386
Supporting Information.
387
(Figure S1); Comparison of the color and morphology of the filter paper substrate before and
388
after GO leaf synthesis (Figure S2); Folding and unfolding of the GO leaf to demonstrate its
389
material durability (Figure S3); XPS analysis of the GO leaf before and after sunlight irradiation
390
(Figure S4); Step-by-step assembly of the experimental setup for the tree configuration with a
391
GO leaf (Figure S5); Methods for calculating the evaporation efficiencies of the GO leaf in
392
floating and tree configurations, respectively; Evaporation performance of the GO leaf and the
393
control bulk water under dark conditions in floating and tree configurations, respectively (Figure
394
S6); Method for calculating the fate of solar energy irradiated on the evaporation surface; SEM
Comparison of the solar spectra of natural and simulated sunlights
17
ACS Paragon Plus Environment
Page 19 of 29
Environmental Science & Technology
395
images of early-stage formation of salt crystals on the GO leaf surface at salinities of 3.5 and
396
5.25 wt.% NaCl, respectively (Figure S7); Reusability of the GO leaf after facile physical
397
cleaning (Figure S8). (PDF).
398
A time-lapse video documenting the salt crystal formation on the surface of the GO leaf in a tree
399
configuration, which was tested at a high salinity of 15 wt.% NaCl during a 5-day evaporation
400
experiment. (.mpg)
401
402
ACKNOWLEDGEMENTS
403
The material is based upon work supported by the U.S. National Science Foundation under
404
award no. CBET-1565452 and the U.S. Department of Energy under award no. DE-IA0000018.
405
Besides, CF acknowledges the support of the U.S. National Science Foundation Graduate
406
Research Fellowship (DGE 1106400), and BM is grateful for a Hellman Fellows fund. Work at
407
the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences,
408
of the U.S. Department of Energy under award no. DE-AC02-05CH11231. However, the
409
opinions expressed herein are those of the authors and do not necessarily reflect those of the
410
sponsors. We also thank Dr. Zhongying Wang at the University of California, Berkeley for the
411
assistance in the GO leaf characterization.
412
REFERENCES
413 414 415 416
1. Elimelech, M.; Phillip, W. A., The future of seawater desalination: Energy, technology, and the environment. Science 2011, 333, (6043), 712-717. 2. Tong, T.; Elimelech, M., The global rise of zero liquid discharge for wastewater management: Drivers, technologies, and future directions. Environmental Science & Technology
18
ACS Paragon Plus Environment
Environmental Science & Technology
417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462
Page 20 of 29
2016, 50, (13), 6846-6855. 3. McGinnis, R. L.; Hancock, N. T.; Nowosielski-Slepowron, M. S.; McGurgan, G. D., Pilot demonstration of the NH3/CO2 forward osmosis desalination process on high salinity brines. Desalination 2013, 312, 67-74. 4. Al-Obaidani, S.; Curcio, E.; Macedonio, F.; Di Profio, G.; Al-Hinai, H.; Drioli, E., Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation. Journal of Membrane Science 2008, 323, (1), 85-98. 5. Ito, Y.; Tanabe, Y.; Han, J.; Fujita, T.; Tanigaki, K.; Chen, M., Multifunctional porous graphene for high-efficiency steam generation by heat localization. Advanced Materials 2015, 27, (29), 4302-4307. 6. Jiang, Q.; Tian, L.; Liu, K.-K.; Tadepalli, S.; Raliya, R.; Biswas, P.; Naik, R. R.; Singamaneni, S., Bilayered biofoam for highly efficient solar steam generation. Advanced Materials 2016, 28, (42), 9400-9407. 7. Wang, Z. H.; Liu, Y. M.; Tao, P.; Shen, Q. C.; Yi, N.; Zhang, F. Y.; Liu, Q. L.; Song, C. Y.; Zhang, D.; Shang, W.; Deng, T., Bio-inspired evaporation through plasmonic film of nanoparticles at the air-water interface. Small 2014, 10, (16), 3234-3239. 8. Liu, Y.; Chen, J.; Guo, D.; Cao, M.; Jiang, L., Floatable, self-cleaning, and carbon-blackbased superhydrophobic gauze for the solar evaporation enhancement at the air–water interface. ACS Applied Materials & Interfaces 2015, 7, (24), 13645-13652. 9. Chang, C.; Yang, C.; Liu, Y. M.; Tao, P.; Song, C. Y.; Shang, W.; Wu, J. B.; Deng, T., Efficient solar-thermal energy harvest driven by interfacial plasmonic heating-assisted evaporation. ACS Applied Materials & Interfaces 2016, 8, (35), 23412-23418. 10. Liu, Y.; Lou, J. W.; Ni, M. T.; Song, C. Y.; Wu, J. B.; Dasgupta, N. P.; Tao, P.; Shang, W.; Deng, T., Bioinspired bifunctional membrane for efficient clean water generation. ACS Applied Materials & Interfaces 2016, 8, (1), 772-779. 11. Liu, Y. M.; Chen, J. W.; Guo, D. W.; Cao, M. Y.; Jiang, L., Floatable, self-cleaning, and carbon-black-based superhydrophobic gauze for the solar evaporation enhancement at the airwater interface. ACS Applied Materials & Interfaces 2015, 7, (24), 13645-13652. 12. Zhou, L.; Tan, Y.; Wang, J.; Xu, W.; Yuan, Y.; Cai, W.; Zhu, S.; Zhu, J., 3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination. Nat Photon 2016, 10, (6), 393-398. 13. Yuan, N.; Caixia, K.; Qi, G.; Jingjing, W.; Haiying, X.; Changshun, W., Heat generation and stability of a plasmonic nanogold system. Journal of Physics D: Applied Physics 2016, 49, (5), 055302. 14. Bae, K.; Kang, G.; Cho, S. K.; Park, W.; Kim, K.; Padilla, W. J., Flexible thin-film black gold membranes with ultrabroadband plasmonic nanofocusing for efficient solar vapour generation. Nat Commun 2015, 6, 10103. 15. Zhou, L.; Tan, Y.; Ji, D.; Zhu, B.; Zhang, P.; Xu, J.; Gan, Q.; Yu, Z.; Zhu, J., Selfassembly of highly efficient, broadband plasmonic absorbers for solar steam generation. Sci Adv 2016, 2, (4), e1501227. 16. Zhang, P.; Li, J.; Lv, L.; Zhao, Y.; Qu, L., Vertically aligned graphene sheets membrane for highly efficient solar thermal generation of clean water. ACS Nano 2017. 17. Li, X.; Xu, W.; Tang, M.; Zhou, L.; Zhu, B.; Zhu, S.; Zhu, J., Graphene oxide-based efficient and scalable solar desalination under one sun with a confined 2D water path. Proc Natl Acad Sci U S A 2016, 113, (49), 13953-13958. 18. Hu, X.; Xu, W.; Zhou, L.; Tan, Y.; Wang, Y.; Zhu, S.; Zhu, J., Tailoring graphene oxide-
19
ACS Paragon Plus Environment
Page 21 of 29
463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508
Environmental Science & Technology
based aerogels for efficient solar steam generation under one sun. Advanced Materials 2017, 29, (5), 1604031. 19. Li, X.; Lin, R.; Ni, G.; Xu, N.; Hu, X.; Zhu, B.; Lv, G.; Li, J.; Zhu, S.; Zhu, J., Threedimensional artificial transpiration for efficient solar waste-water treatment. National Science Review 2017, https://doi.org/10.1093/nsr/nwx051. 20. Ghasemi, H.; Ni, G.; Marconnet, A. M.; Loomis, J.; Yerci, S.; Miljkovic, N.; Chen, G., Solar steam generation by heat localization. Nat Commun 2014, 5, 4449. 21. Zheng, S.; Tu, Q.; Urban, J. J.; Li, S.; Mi, B., Swelling of graphene oxide membranes in aqueous solution: Characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11, (6), 6440-6450. 22. Mi, B., Graphene oxide membranes for ionic and molecular sieving. Science 2014, 343, (6172), 740–742. 23. Huang, H.; Song, Z.; Wei, N.; Shi, L.; Mao, Y.; Ying, Y.; Sun, L.; Xu, Z.; Peng, X., Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes. Nat Commun 2013, 4, 2979. 24. Hu, M.; Mi, B., Layer-by-layer assembly of graphene oxide membranes via electrostatic interaction. Journal of Membrane Science 2014, 469, 80-87. 25. Kim, K.; Seo, E.; Chang, S. K.; Park, T. J.; Lee, S. J., Novel water filtration of saline water in the outermost layer of mangrove roots. Scientific Reports 2016, 6. 26. Lovelock, C. E.; Feller, I. C., Photosynthetic performance and resource utilization of two mangrove species coexisting in a hypersaline scrub forest. Oecologia 2003, 134, (4), 455-462. 27. Atkinson, M.; Findlay, G.; Hope, A.; Pitman, M.; DW Saddler, H.; West, K., Salt regulation in the mangroves rhizophora mucronata lam and aegialitis annulata rbr. Australian Journal of Biological Sciences 1967, 20, (3), 589-600. 28. Hummers, W. S.; Offeman, R. E., Preparation of graphitic oxide. Journal of the American Chemical Society 1958, 80, (6), 1339-1339. 29. Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z. Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M., Improved synthesis of graphene oxide. ACS Nano 2010, 4, (8), 4806-4814. 30. Ye, S. B.; Feng, J. C.; Wu, P. Y., Highly elastic graphene oxide-epoxy composite aerogels via simple freeze-drying and subsequent routine curing. Journal of Materials Chemistry A 2013, 1, (10), 3495-3502. 31. Lamb, M.; Koch, G. W.; Morgan, E. R.; Shafer, M. W., A synthetic leaf: The biomimetic potential of graphene oxide. Proc. of SPIE 2015, 9429, 942915-1-10. 32. Sillett, S. C.; Van Pelt, R.; Carroll, A. L.; Kramer, R. D.; Ambrose, A. R.; Trask, D. A., How do tree structure and old age affect growth potential of california redwoods? Ecological Monographs 2015, 85, (2), 181-212. 33. Li, J.-m.; Liu, C.; Xu, Z.; Zhang, K.-p.; Ke, X.; Li, C.-y.; Wang, L.-d., A bio-inspired micropump based on stomatal transpiration in plants. Lab on a Chip 2011, 11, (16), 2785-2789. 34. Xu, N.; Hu, X.; Xu, W.; Li, X.; Zhou, L.; Zhu, S.; Zhu, J., Mushrooms as efficient solar steam-generation devices. Advanced Materials 2017, 29, (28), 1606762-n/a. 35. Li, R.; Zhang, L.; Shi, L.; Wang, P., Mxene Ti3C2: An effective 2D light-to-heat conversion material. ACS Nano 2017, 11, (4), 3752-3759. 36. Zhang, L.; Tang, B.; Wu, J.; Li, R.; Wang, P., Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating. Advanced Materials 2015, 27, (33), 4889-4894.
20
ACS Paragon Plus Environment
Environmental Science & Technology
509 510 511 512 513 514 515 516 517 518
37. Kuiper, P. J. C., The effects of environmental factors on the transpiration of leaves, with special reference to stomatal light response. H. Veenman & Zonen: 1961. 38. Thomson, W., On the equilibrium of vapour at a curved surface of liquid. Philosophical Magazine 1871, 4(42), (282), 448-452. 39. Rad, M. N.; Shokri, N.; Keshmiri, A.; Withers, P. J., Effects of grain and pore size on salt precipitation during evaporation from porous media. Transport in Porous Media 2015, 110, (2), 281-294. 40. Breda, N., Ground-based measurements of leaf area index: A review of methods, instruments and current controversies. Journal of Experimental Botany 2003, 54, 2403-2417.
519
520
21
ACS Paragon Plus Environment
Page 22 of 29
Page 23 of 29
521 522
Environmental Science & Technology
T.O.C.
523
22
ACS Paragon Plus Environment
Environmental Science & Technology
524
525 526 527 528
Figure 1. Schematic illustration for the fabrication of synthetic GO leaf.
23
ACS Paragon Plus Environment
Page 24 of 29
Page 25 of 29
Environmental Science & Technology
529
530 531 532 533
Figure 2. Solar evaporation through the synthetic GO leaf to mimic water transpiration in natural trees.
24
ACS Paragon Plus Environment
Environmental Science & Technology
Page 26 of 29
534 A
B
100 GO Leaf
1204 1566
Absorption
Absorption (%)
80 60 40
1645
Filter Paper
20
GO Leaf
1724
Filter Paper
0 500
535
1000 1500 2000 Wavelength (nm)
2500
500
1000
1500 2000 2500 Wavenumber (cm-1)
3000
536 537 538 539 540
Figure 3. Characterization of the synthetic GO leaf. (A) Absorption spectra and (B) FTIR spectra of the filter paper substrate and GO leaf; SEM images with different magnifications of (C and D) the surface morphology and (E and F) cross section of the GO leaf.
541 542 543
25
ACS Paragon Plus Environment
Page 27 of 29
544 545 546 547
Environmental Science & Technology
Figure 4. Temperature evolution during water evaporation in different configurations: (A) bulk water as a control, (B) GO leaf floating on water surface, (C) tree configuration without GO leaf as a control, and (D) tree configuration with GO leaf.
548
26
ACS Paragon Plus Environment
Environmental Science & Technology
Page 28 of 29
549
A
Light Intensity = 825 W/m2 (0.82 sun)
B
4
3
Tree with GO leaf 2.0 LMH
3
Floating GO 1.1 LMH
2
No GO
0.88 LMH
1 Bulk evaporation 0.70 LMH
0 0
20
40
550
B
60 80 Time (min)
Floating GO
72%
1
500
600 700 800 900 Light Intensity (W/m2)
Tree without GO
36%
1000
Tree with GO Leaf
83% 12%
Evaporation Loss as heat Loss as light
551 552 553 554 555 556
2
0 400
100
Bulk Evaporation
Evaporation Rate (L/m2/h)
Water Evaporation (L/m2)
GO Leaf in Tree Configuration
78%
Light reflection 10% 8%
20%
54%
6%
10%
11%
Figure 5. Solar evaporation enhanced by the GO leaf: (A) the water evaporation, with the slope denoting the evaporation rate in L/m2/h (LMH), for each configuration; (B) the fate of solar energy for each configuration; (C) the effect of light intensity on the evaporation rate of the GO leaf in the tree configuration.
27
ACS Paragon Plus Environment
Environmental Science & Technology
Evaporation Rate (L/m2/h)
A
3 Light Intensity = 825 W/m2 (0.82 sun)
2
Seawater
1
B
70
0.8
Water Evaporation (L/m2)
Page 29 of 29
60
0.6
50
0.4 0.2
40 0.0
30
0
10
20 30 40 50 Time (Minutes)
60
0.5 L/m2/h
20 10 Salinity = 15 wt.% NaCl 0
0 0
557
558 559 560 561 562 563 564 565
2
4 6 8 10 12 14 Water Salinity (wt.% NaCl)
0
16
1
2 3 Time (Days)
4
5
Figure 6. Solar desalination by the GO leaf in a tree configuration. (A) Effect of water salinity on the stabilized evaporation rate; (B) 5-day evaporation performance of the GO leaf using 15 wt.% NaCl solution; (C and D) SEM images of the GO leaf surface after salt crystallization occurred with 1.75 and 7 wt.% NaCl solutions respectively ; (E) Time-lapse snapshots of salt crystal formation on the GO leaf surface during a 5-day evaporation experiment using 15 wt.% NaCl solution; and (F) thermal images revealing the effect of salt accumulation over the GO leaf surface on temperature evolution, using 15 wt.% NaCl solution.
28
ACS Paragon Plus Environment