Chung-FO Technologies and Challenges for Clean Water and Clean Energy-COCHE 2012
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Transcript of Chung-FO Technologies and Challenges for Clean Water and Clean Energy-COCHE 2012
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FO membranes and cost-effectively recyclable draw
solutes. However, for direct fertigation [22] and osmotic
power generation, fertilizer and seawater are natural draw
solutes, respectively. Therefore, molecular design of FO
membraneswith high flux and power density has received
major attention in the R & D of these two applications.
For osmotic membrane bioreactor (MBR), the requiredmembrane performance may not be as stringent as those
for osmotic power and desalination [23,24], but finding
low cost and easy recyclable draw solutes for osmotic
MBR is still quite challenging unless RO retentate is
readily available to be used as the draw solute as RO
retentatemay provide adequate osmotic pressure and can
be obtained at low or no cost if available.
Although the fouling behavior of FO membranes is more
reversible than RO membranes [25,26,27], the removal
of foulants in the former is more complicated than thelatter because of the internal concentration polarization
when the feed stream faces the porous sublayer [24,28
32]. In addition, owing to the high hydraulic pressure in
the high-pressure compartment, it is believed that the
Current forward osmosis technology development Chung et al. 247
Figure 1
Diluted draw
solution
Draw solution
Fresh water
Feed
Concentrated
feed
FO
membrane
Draw solution
regeneration
Retentate of
recycled water
Clean
water
Seawater
River water
Seawater
Pressure
exchanger
TurbineDiluted
seawater
Membrane
Pressure
exchanger
Turbine
Diluted
seawater
Membrane
(a) (b)
RO
Current Opinion in Chemical Engineering
Schematic diagrams of(a) a typical forward osmosis (FO) process and (b) osmotic power generation from the mixing of seawater and freshwater (top)
and from the mixing of RO and recycled water retentates (bottom).
Table 1
Benefits and challenges of different applications of FO
Applications of FO Benefits Challenges
Desalination Low energy consumption for water transport
across the semi-permeable membrane
Ineffective membranes; lack of cost effective draw solutes
Direct fertigation Fertilizers are natural draw solutes; diluted
draw solution is useful for irrigation
Limited application sites
Osmotic power generation Seawater is a natural draw solute Pretreatments of seawater and river water;
complicated fouling phenomenon owing to thehigh pressure in the seawater compartment
Osmotic membrane bioreactor Low fouling and low energy consumption Need to find low cost and easy recyclable draw solutes
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fouling behavior during osmotic power generation may bequite different from that in low-pressureFO processes.So
far, apart from Statkraft patents [21], almost no academic
studies have touched this interesting subject. Table 1
summarizes benefits and challenges for each application
ofFO. Table 2 and Figure 2 show a comparison ofFO and
RO processes.
The state-of-the-art FO membranes for lowpressure processesThe desired FO membranes must have (i) high salt
retention and high water flux; (ii) low concentrationpolarization; and (iii) resistance to chlorine and wide
range of pH plus long-term stability in separation per-
formance and mechanical strength [6]. Up to the pre-
sent, four approaches have been adopted to prepare
polymeric FO membranes by using (i) the non-solvent
phase inversion method developed by Loeb and Sourir-
ajan [33]; (ii) the thin-film composition (TFC) method via
interfacial polymerization on porous substrates invented
by Cadotte [34]; (iii) the layer-by-layer (LbL) depositionof nanometer-thick polycations and polyanions on porous
charged substrates [35]; and (iv) aquaporin (Aqp) incorp-
orated biomimetic membranes [36]. Wholly integrated
asymmetric FO membranes made of cellulose triacetate
(CTA) [37,38], polybenzimidazole (PBI) [3942], cellu-
lose acetate [43,44,45,46] and polyethersulfone [47] are
typical examples of the 1st approach (as shown in
Figure 3(a)(d)), while FO membranes made of polya-
mide via interfacial polymerization on polysulfone basedsubstrates [48,49], sulfonated substrates [50,51], cel-
lulose acetate propionate (CAP) substrates [52] and nano-
fibers [53,54] belong to the second approach (as shown
in Figure 3(e), (f)). Examples of LbL FO membranes can
be found elsewhere [55,56].
Usually, membranes derived from the phase inversion
method have relatively low fluxes compared to those
membranes made from TFC approach. In addition totheir inherent differences in water permeability and salt
248 Energy and environmental engineering
Table 2
A comparison of FO and RO processes
Process Advantages Disadvantages Challenges
FO Less energy intensive for water transport acrossthe semi-membranes; more reversible fouling
Permeate water 6 product;requires a second separation step
Ineffective membranes; lack of costeffective draw solutes; limited studies
on foulingRO Permeate water = high quality product High energy consumption; some
irreversible fouling
How to improve energy recovery efficiency;
How to mitigate membrane fouling
Figure 2
Seawater(feed)
Osmotic pressure gradient (FO)No hydraulic pressure gradient
RO vs. FO
Seawater(feed)
Draw solution
NaCl
NaCl
The RO membrane is densified underhigh pressures
The FO membrane is loose underno or low pressures
Hydraulic pressure gradient(RO)
Water = Product Water Product
Reverse flux of
draw solutes
Thick sub-layer Thin sub-layer
Current Opinion in Chemical Engineering
A comparison of (a) RO and (b) FO processes.
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rejection [57,58,59], the phase inversion membrane also
tends to have a greater sublayer resistance and internal
concentration polarization (ICP) owing to the difficulties
in controlling the selective skin and sublayer morphology
simultaneously during the rapid phase inversion process,while the TFC membrane has more design flexibility by
separately tuning selective skins and sublayers with the
aid of using more porous or less tortuous membranes as
the TFC substrates. Furthermore, by applying the dual-
layer co-extrusion technology or electrospun nano-fibers,
one may
have
greater
capabilities
to
effectively
manip-ulate the sublayer morphology and significantly mitigate
the low flux issue [42,45,53,54].
To reduce the ICP effects, Wang et al. were the first in
inventing double-skinned FO membranes consisting of a
less selective nano-filtration (NF) skin layer, a fully porous
cross-section, and a highly selective RO skin layer [43].
Subsequent theoretical and experimental works have con-
firmed the unique characteristics and advantages of this
type ofmembranemorphology suchas low fouling and low
ICP [60,61]. In addition, Fang et al. [62] and Su et al. [63]
also extended the basic principle of double skins to fab-
ricate double-skinFO hollow fibers consisting of a NF and
a RO skins. On the contrary, Wang et al. [50] and Widjojo
et al. [51] adopt another scheme to circumvent the ICP.
They reported that the hydrophilicity of porous substratesplays an important role on TFC FO membranes. TFC
membranes that are interfacially polymerized on hydro-
philic porous substrates not only showreduced ICP effects
butalso have a very high water flux (as shown in Figure 4).
So far, 22 LMH (L m2 h1) is the highest ever reported
water
flux for
TFC
FO
membranes
in
seawater
desalina-tionusing2.0 MNaCl as thedraw solution (DS) byWidjojo
et al. [64].Consistent with Wang et al. [50] andWidjojo et al.
[51]observation, Arena et al. surface modified the support
layers ofcommercially availableROTFCmembranes with
polydopamine (PDA) to improve the membranes hydro-
philicity forpressure retarded osmosis (PRO) [65].Follow-
ing the similar principle, Han et al. [66] surface modified
hydrophobic polysulfone (PSf) substrates with polydopa-
minebefore conducting interfacial polymerization. Results
show effective enhancements in both water flux and salt
rejection of the resultant TFC membranes.
Current forward osmosis technology development Chung et al. 249
Figure 3
Single-layer PBImembrane
40
Dual Layer PBI/PESmembrane
CA flat sheet membrane
Thin-film interfacial polymerized flat-sheet FO membrane
HTI CTA membrane Thin-film interfacialpolymerized FO hollow fiber
(a) (b)OL
(c)
(d) (e) (f)
200 m
1.53m
10.61m
1 m
Current Opinion in Chemical Engineering
Some of typical FO membranes for water reuse and desalination. (a) Single-layer polybenzimidazole (PBI) membrane1; (b) Dual Layer PBI/polyethersulfone (PES) membrane2; (c) CA flat sheet membrane3; (d) Hydration Technology Innovations (HTI) CTAmembrane; (e) Thin-film interfacial
polymerized flat-sheet FO membrane4; and (f) Thin-film interfacial polymerized FO hollow fiber5.1 Reprinted from ref. [40] with permission from Elsevier.2 Adapted from ref. [42] with permission from Elsevier.3 Adapted with permission from ref. [43]. Copyright (2010) American Chemical Society.4 Reprinted from ref. [50] with permission from John Wiley and Sons.5 Reprinted with permission from ref. [111]. Copyright (2012) American Chemical Society.
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So far, FO membranes made from LbL have shown a
trade-off between the water fluxes and the reverse salt
leakages. LbL membranes reported either have low
reverse salt leakages but low water fluxes (for the
cross-linkedones) [56] or high water fluxesbutwith high
reverse salt leakages (for the un-cross-linked ones) [55].Since most data were obtained by using MgCl2 as the
draw solute, no experimental data have proven that FO
membranes prepared from LbL have good rejections to
NaCl. A better design of LbL morphology and appro-
priate choices of electrolytes and cross-linkers are essen-
tial to advance LbL FOmembranes for real applications
in water reuse and desalination. FO membranes madefrom TFC/nano-fibers [53,54] also show significant
differences in performance; observed water fluxes of
66 vs. 26 LMH have been reported using 1.5 M NaCl
as the draw solution.Abetter understandingof the causes
of the differences is essential for the advancement of this
technology.
Novel
Aquaporin
(Aqp)
incorporated biomi-metic FOmembranes have recently been developed by
Wang et al. [36]. The membranes were prepared by
rupturing the AqpZ-embedded triblock copolymer
vesicles on the acrylate-functionalized polycarbonate
tracked-etched (PCTE) substrates. The planar pore-
spanning biomimetic membrane displays the highest
water flux of 142 LMH ever reported with very low
reverse salt leakage using 2.0 M NaCl as the draw
solution. However, the Aqp embedded membranes are
not mechanically strong because the selective layer is
only 10 nm in thickness.
FOmembranes for osmotic energy under PROTheoretically, the hydraulic pressure difference in the
seawater compartment during the mixing of river water
and seawater across a semi-permeable membrane under
PRO is preferred to operate at about 13.5 bars for sea-
water consisting of 3.5 wt% NaCl in order to generate themaximal energy output [19,20,21]. Since most conven-
tional FO membranes are designed for no-pressure or
low-pressure operation environments, currently available
FO membranes are likely to be damaged under this high
pressure condition.For example, based on a recent visit to
Statkraft, the latest membranes used in Statkraft are only
operated at about 6 bar because of membrane limitations[67]. Han et al. have recently developed flat asymmetric
membranes with osmotic power density in the range of 6
10W/m2 that can withstand up to 15 bar using model
seawater (0.59 M NaCl) and DI water [68,69,70]. To the
best of our knowledge, among the available membranes
for
osmotic
power
generation [19
,20,21,68
,7174], this
isthe first FO membrane that can withstand a hydraulic
pressure difference over 13.5 bar and also produce a high
energy output. It is worth mentioning that the exper-
iments to estimate membranes power density must be
conducted in actual PRO setup in which the hydraulic
pressure varies in the high pressure compartment. As the
real power density usually deviates a lot from the power
density calculated from an extrapolation of water flux vs.
pressure from the initial water flux under no hydraulic
pressure difference. As a result, any conclusion derived
from ideal theoretical predictions could be misleading.
250 Energy and environmental engineering
Figure 4
20 mCross section
Porousbottomsurface
Sponge-likecross-section
Macrovoid free
Hydrophilic substrate by usingBASF materials
Draw solution concentration, NaCl (M)
Waterflux(LMH)
(a) (b)
0 1 210
20
30
40
50
60
3 4 5
Pressure retardedosmosis (PRO) mode
Forward osmosis(FO) mode
Feed (DI water): flux 33 LMH (DS: 2 M NaCl), salt reverse flux 3.6 gMHSeawater (3.5 wt% NaCl): 15 LMH (DS: 2 M NaCl)
Cross section 40k
Current Opinion in Chemical Engineering
thin film
TFCFOmembrane with macrovoid-free substrate. (a)Water flux as a function of draw solution concentration, and (b) SEM images of TFCmembrane.
Adapted from ref. [51] with permission from Elsevier.
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Although an increase in membrane thickness and poly-mer concentration during casting or spinning may
improve membranes mechanical strengths, it also results
in a much lower water flux and power density. Therefore,
the FO membrane design for power generation under
high salinity
gradients
will
be
significantly different fromthose used in conventional low-pressure environments.
One must consider membranes physicochemical proper-
ties in the wet state as well as their changes under tensile,
elongation, compression, and bending stresses [75,76].
If RO retentate is to be used as the draw solution while
retentate of recycled water (i.e. retentate fromwastewater
reclamation processes) as the feed solution as proposed
elsewhere [6] and shown inFigure 1(b), as the salinity of
RO retentate is much greater than that of seawater (about
7.98.5 vs. 3.5 wt%), it will then increase the salinity
gradient and generate higher power output. The former
can result in a much higher osmotic pressure (about 7077
vs. 28 bar at 22.5 8C) and osmotic energy than the latter,
but also create tremendous challenges for membrane
scientists to design high flux FO membranes with superhigh mechanical strengths. However, if osmotic power
generation and RO plants can be successfully integrated,
not only can it make seawater desalination less energy
dependent and more sustainable, but also significantly
alleviate the disposal and environmental issues of wasteRO retentate. In addition, since the RO retentate has
been well pre-treated in its previous processes, it can
significantly reduce the membrane fouling in the high
pressure compartment. As a result, the integration may
save some
of
expensive pre-treatment
costs
originallyrequired for seawater before PRO. In addition, the integ-
ration of RO and osmotic power generation will signifi-
cantly alleviate the disposal of highly concentrated brine
back to ocean. Therefore, from the environmental stand-
point, the integration may provide a better ecosystem for
habitats and species, water composition, and landscape.
The development of draw solutesCompared to FO membranes, the progress in draw
solutes is much slower. This is owing to the fact that it
is not trivial to design draw solutes with characteristics of
(i) good water solubility; (ii) high osmotic pressures; (iii)
low leakages or reverse fluxes; (iv) easy recovery; and (v)
membrane compatibility and (vi) zero toxicity.
Since the 1960s, many efforts have been devoted to
discover suitable draw solutes such as sulfur dioxide[77], aluminum sulfate [78], glucose [79,80], fructose
[80,81], sucrose [63], fertilizers [22], and inorganic salts
[3856,6062,82]. Prof. Elimelech and his colleagues at
Current forward osmosis technology development Chung et al. 251
Figure 5
Fe(acac)3+2-pyrrolidine245C
reflux
Fe(acac)3+triethylene glycol280C
reflux
Fe(acac)3+triethylene glycol+ polyacrylic acid
280C
reflux
2-Pyrol-MNP:
TREG-MNP:
PAA-MNP:
Structure of Tris(acetylacetonato) Iron: Fe(acac)3
OO
O
O
O
O
Fe
Current Opinion in Chemical Engineering
Schematic diagram of syntheses of water soluble magnetic nano-particles (MNP): 2-Pyrol-MNP, TREG-MNP, and PAA-MNP.Reprinted with permission from ref. [86]. Copyright (2010) American Chemical Society.
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Yale University developed the first generation of drawsolution from water and ammonium bicarbonate
(NH4HCO3) mixtures for desalination in the early
2000s. The draw solute of NH4HCO3 decomposes to
ammonia and carbon dioxide upon heating at about
65 8C and
it
can
be
regenerated
by
re-dissolution[2,83]. However, using compounds with small mol-
ecules as draw solutes may not be economic and practical
because of high energy consumption in their recycles and
significant reverse fluxes in FO processes. Small molecu-
lar salts may also induce clogging in the supporting layer
and lead to severe fouling and internal concentration
polarization [30,31,84,85].
By taking advantages of the characteristics of high surface
area and high osmotic pressure, hydrophilic magnetic
nanoparticles were developed by Ling et al. [86] and
Ge et al. [87] as draw solutes. The original idea was to
produce pure water as well as to recapture nanoparticles
by using a magnetic separator. Figure 5 shows syntheses
of water soluble magnetic nano-particles [86] and
Figure 6 illustrates the draw solution regeneration ofwater soluble magnetic nano-particles in FO processes.
However, the nanoparticles gradually clumped together
owing to the strongmagnetic field.As a result, the osmotic
pressure of draw solutions reduced after regeneration and
so did the yield of fresh water. Ling and Chung demon-
strated that the use of an ultrafiltration (UF) process can
eliminate the magnetic field induced agglomeration [88].
To enhance the separation efficiency of nanoparticles
from water and minimize the loss of nanoparticles during
the UF recycle process, Ling et al. designed the nano-
particles comprising an outer layer of a temperaturesensitive amphiphilic polymer [89]. Below 34 8C, the
nanoparticles performed as draw solutes because of stronghydrogen bonding interactions with water, while above
37 8C, the nanoparticles clumped together as hydro-
phobic globules, making them easier to be captured by
means of UF.
Recently, a series of novel draw solutes based on poly-
electrolytes of PAA-Na salts were developed by Ge et al.
[90]. The characteristics of high solubility in water and
flexibility in structural configuration enable this type of
draw solutes to generate high water fluxes yet with
insignificant reverse salt fluxes in the FO process. These
unique properties not only ensure high efficiency in water
reclamation and high quality in water product, but also
lower the replenishment cost of draw solutes. In addition,
PAA-Na salts have good stability and show repeatable
performance after many recycles. Figure 7 shows some
common draw solutes and preparation of poly(acrylic acid
sodium) (PAA-Na).
Integrated systems for clean water productionand draw solute regenerationSustainable integrated systems for water production and
draw solute recycle must be developed in order to suc-
cessfully market FO technologies. For seawater desalina-
tion, researchers have proposed the integration of FO and
RO/NF processes for draw solute recovery and clean
water production [9193]. They are technically feasiblebut economically and industrially unpractical because of
high energy costs to operate RO and NF for draw solute
recycles. If waste heat or cold energy is available, anintegratedFOMD (forward osmosismembrane distilla-
tion) system (as shown in Figure 8(a)) is a promisingprocess for seawater desalination [94]. The cold energy
252 Energy and environmental engineering
Figure 6
Diluted drawsolution
Concentrated drawsolution
Feed(seawater)
Concentrated brine
FO membrane
Draw solution regeneration
N SProduct water
Magnetic field
Magnetic nano-particlesrecycled back to FO
Current Opinion in Chemical Engineering
Schematic diagram of water soluble magnetic nano-particles draw solutes for FO processes.
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refers to the heat absorption effect from the ambient
surrounding when liquefied natural gas (LNG) is re-
gasified at the LNG terminals [95]. However, the pro-
vision of low-cost waste heat for MD is the pre-condition
for the success of the integrated FOMDprocess [96,97].
By using highly hydrophilic nano-particles as draw
solutes, one may minimize the fouling issues including
scaling and crystallization in MD. Several attempts have
been made. Yen et al. [98] and Wang et al. [99] were one of
the firsts demonstrating the FOMD process for water
reuse and protein enrichment applications, respectively.
Su et al. extended their works by using a novel CAP
polymer as the FO membrane material and 0.5 M MgCl2as the draw solution for wastewater reclamation [100],
while Ge et al. developed a polyelectrolyte-promoted
FOMD hybrid system for the recycle of wastewater
Current forward osmosis technology development Chung et al. 253
Figure 7
C CC
O OH
n
NaOH
H
H
H
C CC
O O-Na+
n
H
H
H
PAA PAA-Na
Mg2+Cl2
Na+Cl
NH4HCO3or NH3/CO2
O
OH
OH
OHO CH2OH
CH2OH
CH2OH
OH
OHO
Sucrose
(b)
(a)
Magnetic nanoparticlesSalts
Current Opinion in Chemical Engineering
(a) Some common draw solutes and (b) preparation of poly(acrylic acid sodium) (PAA-Na).
Figure 8
Drawsolution
feedsolution
(a) (b)
Drawsolution
Drawsolution MD
FeedClean water
FO MD
feedsolution
Drawsolution
Drawsolution
Feed
Clean water
FO RO
Current Opinion in Chemical Engineering
Integrated (a) FOMD and (b) FORO systems to regenerate the draw solution and produce water.
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and an acid dye [101]. High performance home-made
polyvinylidene fluoride (PVDF) membranes [102104]
were used in their studies to recycle draw solute and
produce product water. Many useful design principles
and membrane modifications for MD can be found else-
where
[105
107].
However,
to
the
best
of
our
knowledge,no demonstration of FOMD process is available for
seawater desalination. This is owing to the fact that we
still lack (1) high performance FO membranes with high
fluxes and high salt rejections; and (2) cost effective draw
solutes with high osmotic pressures and minimal reverse
fluxes.
For water reuse, FOUF, FONF, FOMD and FORO
integrated systemsmay have great potential depending on
thequalityof feed solutions, physicochemical properties of
draw solutions, and applications [63,99,100,101,108,109].
UF is the most preferred because it is a well-established
low energy filtration process compared to RO and NF
[110]. In addition, various types of UF membranes and
modules are commercially available.As a result, theoverall
system development and operation cost for FOUF areeasier and more affordable compared to other integrated
systems.On the contrary, ifwastebutcleanRO retentate is
employed as the draw solute, a FORO integrated system
(as shown in Figure 8(b)) is recommended to remove
mono-valent ions. A combined system comprising FO,
UF and magnetic separators may be also a good choice
for water reuse. However, most existingmagnetic separa-
tors possesshighmagneticfieldsbecause they aredesigned
for other purposes. To avoid particle aggregation, tailored
magnetic separators with tunable magnetic strengths are
needed to recycle different magnetic nanoparticles in theFO process.
ConclusionIt took about 40 years (from about 1960 to about 2000) for
RO to surpass thermal multi-effect evaporation technol-
ogies as the dominant technology in seawater desalina-
tion. Technology evolutions on both RO membranes andprocess design have been continuously taking place to
increase membrane performance and achieve better
energy efficiency and mitigate fouling. Similarly, FO
technologies may appear promising but are still in the
infancy stage. Time andmoreR &D efforts are needed in
order to
have
significant breakthroughs on
FO
mem-branes, draw solutes and their regeneration methods so
that the FO technologies can compete effectively with
the well-established RO technologies for seawater desa-
lination. Commercialization of FO for fertigation appears
promising,while cost effective and easily recyclable draw
solutes must be found for water reuse. The use of RO
retentate as the draw solute for water reuse may lower the
operation cost and bring FO closer to commercialization.
A successful integration of osmotic power generation
and RO desalination plants will entirely revolutionizethe future power and desalination industries. However,
membrane scientists must overcome the challenges todesign high flux FO membranes with extremely robust
mechanical properties to withstand the operating pressure
in the high pressure PRO process. Encouragingly, a few
breakthroughs on high flux and high strength FO mem-
branes, draw
solutes with
high
osmotic
pressures, andadvanced integrated systems for water production and
draw solute recycle have been recently demonstrated.
AcknowledgementsThis research was funded by the Singapore National Research Foundationunder its Competitive Research Program for the project entitled,Advanced FO Membranes and Membrane Systems for WastewaterTreatment, Water Reuse and Seawater Desalination (grant numbers: R-279-000-336-281 and R-279-000-339-281). The authors also thank Miss SuiZhang, Dr. Jincai Su, Dr. NataliaWidjojo, Miss Sicong Chen, Miss Yue Cuifor their help and suggestions. Special thanks are due to BASF, EastmanChemicals and Mitsui Chemicals for their financial supports as well as Prof.Donald R. Paul, University of Texas at Austin, Dr. J.J. Qin, Public UtilitiesBoard (PUB, Singapore), Prof. D. Bhattacharyya, University of Kentucky,Dr. Subhas Sikdar, National Risk Management Research Laboratory, USEPA, Prof. Gary Amy, KAUST as well as the editorial team ofCOCHE
(Prof. Sirkar and Prof. Agrawal) for their valuable suggestions.
References and recommended readingPapers of particular interest, published within the period of review,have been highlighted as:
of special interest
of outstanding interest
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49.
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