Nanoporous networks as effective stabilisation matrices for nanoscale zero-valent iron and groundwater pollutant removal

Article abstract of DOI:10.1039/c5ta05025a

Nanoscale zero-valent iron (nZVI), with its reductive potentials and wide availability, offers degradative remediation of environmental contaminants. Rapid aggregation and deactivation hinder its application in real-life conditions. Here, we show that by caging nZVI into the micropores of porous networks, in particular Covalent Organic Polymers (COPs), we dramatically improved its stability and adsorption capacity, while still maintaining its reactivity. We probed the nZVI activity by monitoring azo bond reduction and Fenton type degradation of the naphthol blue black azo dye. We found that depending on the wettability of the host COP, the adsorption kinetics and dye degradation capacities changed. The hierarchical porous network of the COP structures enhanced the transport by temporarily holding azo dyes giving enough time and contact for the nZVI to act to break them. nZVI was also found to be more protected from the oxidative conditions since access is gated by the pore openings of COPs.

Full text of DOI:10.1039/c5ta05025a

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DOI: 10.1039/C5TA05025A
Journal of Materials Chemistry A
PAPER
Nanoporous networks as effective stabilisation matrixes for
nanoscale zero valent iron and groundwater pollutant removal†
P. D. Mines,a,b J. Byun,b Y. Hwang,a,c H. A. Patel,b H. R. Andersen,a and C. T. Yavuz*b
Received 00th January 20xx,
Accepted 00th January 20xx
Nanoscale zero valent iron (nZVI), with its reductive potentials and wide availability, offers degradative remediation of
environmental contaminants. Rapid aggregation and deactivation is hindering its application in real life conditions. Here,
we show that by caging nZVI into the micropores of porous networks, in particular the Covalent Organic Polymers (COPs),
we dramatically improved its stability and adsorption capacity, while still maintaining its reactivity. We probed the nZVI
activity by monitoring azo bond reduction and Fenton type degradation of the naphthol blue black azo dye. We found that
depending on the wettability of the host COP, the adsorption kinetics and dye degradation capacities changed. The
hierarchical porous network of the COP structures enhanced the transport by temporarily holding azo dyes giving enough
time and contact for the nZVI to act to break them. nZVI was also found to be more protected from the oxidative
conditions since access is gated by the pore openings of COPs.
DOI: 10.1039/x0xx00000x
www.rsc.org/MaterialsA
dyes with a stabilized nZVI makes for a simple indicator test of the
particles’ effectiveness for the abiotic reduction of both azo bonds
Introduction
and various other chemical bonds that ZVI is capable of reducing.5
Despite the superior decontamination properties, nZVI by itself
is not a suitable material for practical water contaminant treatment
in real-world applications. Bare nZVI particles have very limited
transportability and flow in a porous medium, such as a subsurface
contaminant plume, and exhibit a higher sticking coefficient,
resulting in aggregated larger particles with substantially reduced
reactivity.6,7 The driving force behind the aggregation of nZVI
particles comes from van der Waals and magnetic attraction
forces.6,8 In order to combat these inherent attractive forces of the
nZVI particles and allow nZVI to be commercially used on a much
larger scale, the idea of adding a stabilizing agent has been
identified in order to alter the surface characteristics of nZVI.
Previously utilized stabilizing agents include a variety of organic and
inorganic species, including anionic surfactants, carbon particles,
silica, clay, poly-acrylic acid, guar gum, xanthan gum, starch,
carboxymethyl cellulose, various polyelectrolytes, biopolymers,9
and most recently a synthetic organoclay.10 Despite a high volume
of research dedicated to nZVI and its further applications, the
Current treatment practices for groundwater contamination, in
particular hard to degrade contaminants like halogenated organics
and azo dyes, have seen the application of zero valent iron (ZVI)
emerge as one of the most discussed and researched reactive
materials. These materials are often in the form of in-situ
permeable reactive barriers, while applied extensively in ex-situ (i.e.
pump-and-treat) methods as well.1 Globally, the use of ZVI has
quickly gained traction because iron is ubiquitous in the earth’s
crust, cheap to manufacture, environmentally safe, and an effective
reductant (E0 = -0.44 V). Synthesizing ZVI on the nanoscale has
been proven to dramatically increase the reactive capability of the
iron particles due to the increase in reactive surface area, increasing
from 0.9 m2/g in commercially available micron-scale Fe-powder to
33.5 m2/g in nano-sized zero valent iron (nZVI).2 Over the years,
nZVI has demonstrated the ability to be an effective degrading
agent of a wide variety of pollutants, including antibiotics, azo dyes,
chlorinated solvents, chlorinated pesticides, organophosphates,
nitroamines, nitroaromatics, p-chlorophenol, polychlorinated
biphenyls, inorganic ions, alkaline earth, transition, and post-
transition metals, metalloids, and actinides.3 In this study, our focus
was dedicated towards azo dyes, which are the most widely used
dyes in textile industries; and, because the destruction of the azo
(N=N) bond by nZVI has been well documented over a range of
challenge remains to develop
a new synthetic method for
generating stable and dispersive nZVI particles, without
compromising their reactivity and the surface area in the presence
of the stabilizing agents.
various azo dyes.4
decolourization, making the reaction easy to identify and monitor in
a standard laboratory. In this light, targeting the treatment of azo
Azo bond destruction allows for dye
The past decade has seen the development of various porous
organic materials targeting a plethora of applications, most often in
capture of carbon dioxide gas, such as those termed Covalent
Organic Frameworks (COFs),11 with certain studies reporting very
promising results in this field.12 However, these COFs have failed to
reach the theoretical predictions for carbon dioxide capture
capabilities13 and have also proven, in many cases, to be instable in
water;14,15 unless, for instance, the pores can be alkylated and
hydrolysis can be prevented.16 This lack of stability in water creates
a major concern when dealing with environmental applications of
organic polymers, in particular, when treating contaminated water
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23
sources. A particular type of porous network polymer, namely performed following previously developed methods. Synthesis of
View Article Online
Covalent Organic Polymers (COPs), have received significant COPs 60 and 61 were performed similarly toDOthI:a1t0.o10f3C9O/CP5-1TA, 0in50t2h5aAt
interest, particularly for environmental applications such as gas 2,2’,4,4’-tetrahydroxybenzophenone (COP-60) and 1,1,1-tris(4-
capture,17–19 and water purification.20 The main advantage of hydroxyphenyl)-ethane (COP-61) monomers were added to
nanoporous COPs is their synthetic diversity in which various benzene tricarbonyl trichloride before brought to reflux for 24
building blocks are employed to construct a rigid and stable porous hours (COP-60) and 14 hours (COP-61), followed by
polymer network with high surface area. Recently, it has been
reported that COPs could be utilized as a porous support to
generate palladium nanoparticles,21 these composites of COPs and
palladium nanoparticles exhibited excellent and stable performance
as a heterogeneous catalyst for the carbon monoxide oxidation
reaction. High surface area and functionality of the COPs enhance
mass transfer for adsorption of carbon monoxide molecules, and
further stabilize the palladium nanoparticles without hindering the
overall reactivity and surface area.
Table 1: Chemical structure of each network used in this study.
Network
Structure
COP-1
COP-6
Herein, we introduce COPs as a new type of support material
to stabilize nZVI particles. Rather than typical methods employed
to stabilize nZVI that utilize a chemical coating technique, COPs
present a highly porous polymeric network capable of physically
caging nZVI within its matrix. This ability to trap nZVI stems from
the amorphous web of interwoven polymer chains, anywhere from
10 – 50 nm in thickness, inherent in the polymer construction. Five
different COPs having different surface area and functionalities
(amine, imine, thiol and ester) are employed to immobilize nZVI,
where Fe3+ ions in aqueous solution were adsorbed into the pore of
COPs and simply converted into Fe0 by in-situ chemical reduction
using sodium borohydride (NaBH4). Hybrid composites exhibit
relatively high surface area compared to other solid supports
reported in literature,8,22 showing good distribution of nZVI with
good stability. Furthermore, the large granule of the composites is
easily separated from aqueous solution by simple filtration, which
both prevents secondary water contamination and improves
recyclability of the composites. The composites with nZVI and COP
vary in their morphology, surface area, and wettability, and we
compare the effect of the difference in physicochemical properties
of the composites by analysing both the stability of composites in
water, as well as azo dye removal efficiency. We believe that the
proposed composites with different characteristics would offer
ideas to understand key parameters for producing a stable and
active nZVI for real field water treatment in both in-situ and ex-situ
operations.
COP-19
COP-60
COP-61
purification by repetitive washing with selected solvents.
Supporting information, section S2, has in-depth details on the COP
chemical compositions and synthesis methods of COPs 60 and 61.
COP/nZVI composite synthesis
The procedure for the synthesis of the various COP/nZVI composite
materials was derived from a method developed for immobilization
of nZVI with alginate beads.24 A solution containing 2.0% (w/v)
COP/solvent, the solvent being water or N,N-dimethylformamide
(DMF), and 0.05 M of FeCl3 was mixed for 24 hours, in order to
ensure full diffusion of Fe3+ into the porous polymer matrix (e.g. 0.4
g of COP, 0.163 g FeCl3, and 20 mL deionized water) . This solution
was then filtered under vacuum through a 0.45 µm glass-fiber filter
until dry. The dried product was then re-solubilized with solvent
(water or DMF) and sonicated, to ensure complete dissolution.
Conversion of the Fe3+ to Fe0 was performed by adding a 0.3 M
solution of NaBH4 in deionized water drop-wise to an equal volume
of the filtered and re-solubilized COP/Fe3+ product, and mixed for
30 minutes under N2-atmospheric conditions (e.g. 0.113 g NaBH4 in
10 mL deionized water added to recovered COP/Fe3+ mixed in 10
mL deionized water); this amount of NaBH4 ensured that there was
at least three times the amount of NaBH4 for every Fe3+. Following
reduction, the solution was centrifuged and washed with ethanol
three times, then dried for 12 hours in a vacuum environment at
120°C. As for the control experiments, pure nZVI was prepared in
the same manner, except the absence of COP for immobilization.
Materials and methods
Chemicals
All chemicals were used, as obtained from the supplier; except for
the organic solvents used in COP synthesis, of which the syntheses
required that the solvents be made anhydrous via a solvent
purification system. Complete details can be found in the
supporting information, section S1.
Synthesis of covalent organic polymer networks
In order to maintain nomenclature uniformity among various
publications, COPs are numbered using an internal ordering system
(visit PorousPolymers.com for a complete list). And, to better
understand the polymeric structure of each COP, a table (Table 1)
has been provided to outline the polymer structure of each COP
used. Synthesis of COP-1 was performed following previously
developed methods.17 Synthesis of COP-6 was performed following
previously developed methods.18 Synthesis of COP-19 was
Physical characterisation
Fourier transform infrared (FTIR) spectroscopy was used to
determine the functional groups of the individual COPs, spectra
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were recorded as KBr pellet using
PAPER
a
Perkin-Elmer FT-IR and pH=7 had already been pre-determined in a previous study27 of
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spectrometer (Waltham, MA, USA). Morphological observation of Fe0 in water to degrade contaminants as DtOheI: 1o0p.1t0im39a/Cl 5cToAn0d5i0ti2o5nA.
the COP/nZVI composites was performed using a field emission Composite material was added to the dye solution at
a
transmission electron microscope (FE-TEM, Tecnai G2 F30 S-Twin concentration of 1.5 g/L (Fe content of ~0.15 g/L), sonicated, and
model, FEI, The Netherlands). Preparation of the samples was done placed on a rotating mixer, then samples were measured over a 30
by solubilizing 5 mg of dried composite powder with 2 mL of minute period, in a wavelength range of 300 – 800 nm, in order to
ethanol, sonicating to ensure total dissolution, and placing three obtain a peak diminishing/decolourization curve from the spectra;
droplets of said solution onto a carbon-coated copper mesh grid similar to azo dye decolourization by Fe0 methods reported
(300-mesh, FCF300-Cu Formvar Carbon Film, Electron Microscopy previously.28,29 All decolourization experiments were done using
Sciences, Hatfield, PA, USA). The solution was allowed to dry on the deionized water under oxic conditions, to not only simulate real
Cu-grid for 30 minutes, and then subsequently analysed with FE- application, but also, as it has been previously determined that azo
TEM. Total iron contained in each composite, including both dyes are more effectively degraded by ZVI in oxic conditions.30
impregnated iron within polymer matrix and bound iron attached to Degradation products, specifically aniline, from the dye
the outside of the polymer matrix, was measured using inductively decolourization was measured using high performance liquid
coupled plasma – mass spectrometry (ICP-MS, 7700 Series, Agilent chromatography (HPLC, Agilent Technologies, 1100 Series, 1260
Technologies, Santa Clara, CA, USA). Samples were prepared by Infinity Degasser, Waldbronn, Germany; Sigma-Aldrich, Supelco
digesting a solution of 2 mg composite and 1 mL of HNO3 (60%) for Analytical Discovery® C18 column, 5 µm, 15cm x 4.6mm, Bellefonte,
approximately one hour (or until sample was completely dissolved) Pennsylvania, USA); using an eluent of methanol and 5 mM
at 80°C, then filtering the digested solution through a 0.45 μm potassium dihydrogen phosphate.31
polytetrafluoroethylene (PTFE) and diluted with deionized water.25
Crystalline structures of the COP/nZVI composites were analysed
using X-Ray diffraction (D/Max-2500 model, Rigaku, Japan).
Samples were scanned over a 2θ range of 5 – 80° at scan speed of
Results
Properties of covalent organic polymers
2° per minute. Analysis of the samples was performed in a vacuum-
controlled environment. Total Brunauer-Emmett-Teller (BET)
surface area and pore volume of each polymer and composite was
measured using Physisorption Analyzer (ASAP 2020,
Following synthesis of each COP and before utilizing each COP, FTIR
analysis was run on the polymers to confirm that the synthesis was
a success and that the desired bonds had formed and certain
functional groups were present (see supporting information,
section S3 for full analysis and FTIR scans). BET surface areas of
each of the polymers varied substantially, from as low as 8.8 m2/g in
COP-61, to as high as 600 m2/g in COP-19; BET results for all the
polymers and composites used are detailed in Figure 1 (A).
a
Micromeritics, Norcross, Georgia, USA). Prior to analysis, samples
were degassed for 5 hours at 150°C.
Particle stability – sedimentation analysis
Stability of COP/nZVI composites was evaluated via sedimentation
kinetics, using a Hach UV-Vis Spectrophotometer (DR 5000,
Loveland, CO, USA). It was determined that the absorbance of
metal nanoparticles in suspension is directly related to the particle
size.26 Being that all samples were prepared in the same manner, it
was determined that the sedimentation rate was independent of
solution viscosity. Freshly prepared samples were mixed with
deionized water at a concentration of 550±20 mg/L, sonicated, and
transferred to a 1 cm plastic cuvette. Absorbance spectra of the
samples were measured over a 60 minute period, at 508 nm, in
order to obtain sedimentation curves, which were then interpreted
using the following equation.6
Properties of COP/nZVI composites
Morphologies varied between the various COP/nZVI composites,
exhibiting globular cluster-like formations to thin sheath-like
shapes. Although the exact shape and morphology is not
necessarily important, being that the COPs are, by nature,
amorphous; TEM images confirm that the particles are nano-sized,
Figure 2. Furthermore, when compared to similar TEM images of
the COP by itself, like those already published of COP-1,17 the
composites display darker inner cores, evidence that there is iron
embedded within the COP matrix. In all cases, the cores of the
matrix appear to have dark iron formations, where the iron has
been trapped. Additionally, the reduction in BET surface area of
each composite confirms that there is nZVI being trapped and
loaded into the pores of the polymer; which correlates similarly
with a previous study that entrapped nZVI into the pores of
activated carbon.32 Also, it was found that the average pore size
increased slightly with nZVI impregnation. This is mainly due to the
presence of free-standing ZVI nanoparticles formed near and
around the COPs, making additional pores by self-stacking of
nanoparticles themselves.33
!ꢀ
ꢀ
ꢀꢀ = ꢀꢀ ∗ ꢀ
Where ꢀꢀ is the absorbance of the sedimenting solution at time ꢀ,
ꢀꢀ is the initial absorbance, and ꢀ is the characteristic time of
sedimentation for the particles in solution.
Particle reactivity – dye decolourization
The reactivity of the composite particles was determined by their
ability to decolorize a solution of the azo dye, naphthol blue black.
Previous work has proved the ability for granular Fe0 to decolorize a
variety of azo dyes,5 and naphthol blue black was chosen in
particular, due to its higher peak absorbance wavelength (λmax
=
618 nm), in order to minimize any absorbance interference from
Fe2+ or Fe3+ ions that may form in solution during the test. All dye
decolourization experiments used a solution of 10 mM HEPES
buffer and 60 µM naphthol blue black in deionized water. This
particular condition set with an organic buffer (HEPES) at 10 mM
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DOI: 10.1039/C5TA05025A
Fig. 1: BET surface area comparison of the bare polymer network vs. the COP/nZVI
composite (A); and, total iron content contained in each composite (B).
Fig. 3: XRD spectra patterns of composites of COP-1 (A) and COP-19 (B). Clearly visible
is the Fe0 peak at 44.9° 2θ, indicating the presence of ZVI contained within the
composite matrices.
A
B
The bernalite, can be explained by the elemental iron reacting
with water and dissolved oxygen once the NaBH4 is consumed, see
below reaction scheme.
ꢀꢁꢂ + ꢀꢀꢀ + ꢀꢀꢀꢀ → ꢀꢀꢁꢀ! + ꢀꢁꢀꢁ! → ꢀꢁꢂ(ꢀꢁ)ꢀ
D
C
The lawrencite can be explained from the FeCl3 precursor by
the reaction of elemental iron with excess ferric chloride still in
solution during the NaBH4 reduction process, see below reaction
scheme.
ꢀꢁ + ꢀꢁꢂꢀꢁꢀ → ꢀꢁꢂꢀꢁꢀ
Fig 2: TEM images demonstrating composite morphology of COP-1 (A); COP-6 (B); COP-
19 (C); and COP-60 (D).
And also as the product of anaerobic corrosion of iron in water
followed by the precipitation of FeCl2, see below reaction scheme.
ꢀꢁ + ꢀꢀꢀꢀ → ꢀꢁꢀ! + ꢀꢀ + ꢀꢁ!
ꢀꢁꢀ! + ꢀꢀꢁ! → ꢀꢁꢀꢁꢀ
ICP-MS analysis of the composites revealed that iron had
indeed been impregnated into the COP matrix. For the most part,
the composites contained roughly 10% iron, by mass. Composites
of COPs 1, 6, 19, 60, and 61 contained 9.1, 8.3, 9.2, 16.3, and 9.9%
iron, respectively; outlined in Figure 1 (B). This is slightly higher
than the observed values of 7.4% iron and 8.2% iron, when
immobilizing nZVI onto alginate beads24 and activated carbon,32
respectively, indicating that the porous nature of COPs are better
suited than both alginate and activated carbon for the impregnation
of iron. In every case, the BET surface area of each of the
composite dropped, with respect to the bare polymer used for that
synthesis, due to both the filling and blocking of the micropores by
nZVI. The comparison of which is outlined in Figure 1 (A).
When COPs are analysed with XRD, their amorphous non-
crystalline structure yields no peak pattern, and therefore that data
is not presented here. However, this means that any peak found in
the composite scans is directly related to the iron impregnation;
subsequently, the XRD analysis revealed that iron contained in the
composites did reveal a Fe0 peak. The characteristic peak of Fe0 at
44.9° 2θ was clearly visible in the scans of each composite, except
for the COP-6 composite, which did not exhibit any discernible peak
pattern. Other than the composite of COP-6, the remaining XRD
scans all produced a group of similar peak patterns. Although a
definitive peak pattern could not be isolated for any one particular
compound, various iron-based peaks do occur; in particular,
bernalite (Fe3+(OH)3), maghemite (Fe2O3), lawrencite (FeCl2), and
magnetite (Fe2+Fe3+2O4), as is plotted in Figure 3 for composites of
COP-1 (A) and COP-19 (B). Although the Fe0 peak appears small
compared to the other peaks in the XRD patterns, it should be
noted that the peak’s intensity is still quite high compared to pure
nZVI. For example, the Fe0 peak in COP-1 is still 52% intensity of the
pure nZVI peak and the Fe0 peak in COP-19 is 79% intensity of the
pure nZVI peak. As for the other peaks, Fe2O3 and Fe3O4 are easily
produced during the air oxidation of iron.
Therefore, the appearance of the various other peaks in the
XRD spectra can be attributed to the inherently imperfect reduction
process of Fe3+ and potential oxidation during the sample
preparation for XRD. XRD spectra patterns for the remaining
composites, as well as for pure nZVI, are presented in the
supporting information, section S4.
Effect of COP on nZVI stability
The first and foremost requirement for making nZVI an applicable
treatment technology is to make the nZVI itself stable in an aqueous
environment. Therefore, for COPs to be an effective support
material, it must be established that these COPs are both immune
to the phenomenon of hydrolysis, as well as to establish that the
composites are capable of maintaining suspension while in aqueous
solution. The first parameter has already been proven, COP-1 for
example, has been shown to remain stable in boiling water for
weeks at a time.17 Measuring the rate at which the composites
aggregate and sediment out of solution can determine the second
parameter, how effective the COPs are as a stabilizing agent for
nZVI, preventing iron aggregation and loss of reactivity. To
illustrate this, Figure 4 (A) plots the relative absorbance of each
composite suspended in deionized water, as well as pure nZVI,
relative to the initial absorbance over a period of one hour;
included in the plot are the calculated sedimentation curves, as
conducted per the aforementioned method for particle stability.6
Pure nZVI particles show very poor stability in deionized water, as
expected, sedimenting out over 80% of its total mass, with nearly
all of it settling out very quickly, within the first 15 minutes. COPs 1,
6, and 61 show improved stability compared to nZVI, sedimenting
out 64, 64, and 59% respectively; while also showing a much slower
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A
B
Fig. 4: Sedimentation profiles of nZVI and COP/nZVI composites, measured by
absorbance at 508 nm, in terms of absorbance relative to initial absorbance, monitored
over time (A); naphthol blue black decolorization over time by the composites, bare
nZVI, and activated carbon (B).
and linear sedimentation profile over the course of 60 minutes.
COP-61 had even greater stability, sedimenting out only 39%. And,
COP-19 exhibited highly stable sedimentation properties, after 60
minutes, only 3.9% of the initial absorbance had decreased.
However, every one of these composites vastly outperforms certain
previous attempts at a stabilized nZVI. For example, a study
Fig. 5: UV-Vis absorbances for COP-1 (A); COP-19 (B); activated carbon (C); and bare
nZVI (D).
analyzed
a surfactant modified nZVI from the NANOIRON®
spectra for the composites not detailed in Figure 5 can be found in
the supporting information, section S5. In order to compare how
the composites fared against the standard adsorbent for the water
treatment industry, activated carbon was also tested for
decolorization of naphthol blue black. It was observed that
activated carbon is a very poor agent for decolorization of naphthol
blue black; the spectra, Figure 5 (C), displayed a slow and minimal
shift downwards and absolutely no change in shape, indicating no
degradation, only slow adsorption to the active binding sites of the
carbon. COPs 60 and 61 proved to be very ineffective, much less so
than activated carbon; however, it should be noted that COPs 1, 6,
and 19 all fared better than that of activated carbon.
Company (Rajhrad, Czech Republic) at a concentration of 300 mg/L,
compared to 550 mg/L in this study, and found that over 80% of the
particles settled out after a period of only 30 minutes.34 At the
equivalent point in time for this study, at nearly double the
concentration, composites of COPs 1, 6, 19, 60, and 61 settled out
47, 49, 2.5, 32, and 50%, respectively. These percentage numbers
can easily be put into perspective using the equation from Phenrat
et al. (2006) to calculate the characteristic time, ꢀ , which can also
be interpreted as a sedimentation “half-life” that determines how
long it will take for particles to fall out of solution.6 Characteristic
times for bare nZVI and the composites of COPs 1, 6, 19, 60, and 61
are 14, 52, 49, 1340, 93, and 52 minutes, respectively; meaning that
there is an increase in stability for each of the composites with
respect to nZVI, and a very substantial increase for COP-19, proving
the effectiveness of COPs as stabilizing agents for nZVI. Although,
the composites of COPs 1, 6, 60, and 61 have only similar or mildly
better characteristic times as various other studies stabilizing
nZVI,35–37 the composite with COP-19 has proven to be drastically
better at stabilizing nZVI by over a whole order of magnitude
greater.
It was observed, over time, in the spectra for COPs 1 and 19,
that the peak absorbance (λmax = 618 nm), indicative of azo bonds,
gradually shifted to 588 nm and that the spectra peak at 310 nm,
indicative of naphthalene groups, was also reduced. This pattern in
the spectra observed in COP-1 and, to a lesser extent, in COP-19,
Figure 5 (A,B), correlated to previous observations when degrading
naphthol blue black with Fe0, where it was concluded that these
two phenomena in the peaks at 618 nm and 310 nm, degraded the
azo bonds via reduction and oxidized the naphthalene groups via
the Fenton reaction, respectively.38 In order to confirm the results
of Chang et al. (2009), when bare nZVI in this study was tested, the
peaks at 618 and 310 nm behaved in the same manner, Figure 5 (D).
Furthermore, this spectral pattern in naphthol blue black, and
primarily the peak absorbance shift to lower wavelengths, was
previously observed when reducing naphthol blue black by
hydrogenation in the presence of platinum black,39 and again when
performing visible light induced degradation of naphthol blue black
on TiO2 nanoparticles.40 Due to the pattern similarity of the
observed spectra and experimental conditions, it can be assumed
that the composite of COP-1 is also degrading naphthol blue black
in the same manner. It is also believed that the composite of COP-
19 is behaving similarly; however, the powerful adsorptive
properties of COP-19 make determination of naphthol blue black
degradation harder to distinguish. If in fact degradation of the dye
is taking place, certain degradation products would be likely to
appear in the dye solution; two primary coloured degradation
products have been identified, Chromotrope 2B and Chromotrope
2R, with peak absorbances of 514 nm and 512 nm, respectively.40
However, the coloured nature of these compounds could lead to
analytical uncertainty when using UV-Vis spectroscopy. So, it was
decided to focus on the smaller degradation products, in particular,
Effect of COP/nZVI composites on azo dye removal
There was a wide range of results in the ability of the composites to
effectively decolorize the azo dye solution, which has been
summarized in Figure 4 (B), plotting the absorbances (at the λmax
=
618 nm) of the naphthol blue black solution when mixed with each
composite, as well as activated carbon and bare nZVI, over time.
After a 30-minute reaction period, the spectra of COPs 60 and 61
displayed almost no change, only producing a minimal reduction in
total absorbance, 8.0 and 7.0%, respectively. Fairing slightly better,
COP-6 was capable of decolorizing the solution by 36%; however
the shape of the curve remained exactly the same, indicating only
adsorption and no degradation. Ultimately, COPs 1 and 19 achieved
excellent dye removal rates, producing 95 and 97% absorbance
decreases, respectively, Figure 5 (A,B). The remaining UV-Vis
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Journal of Materials Chemistry A
aniline, which can be quantitatively identified via HPLC. Another
study focused on the degradation of naphthol blue black, utilizing
photocatalysis, has proposed the mechanism (Scheme 1) for how
this aniline production would be expected to take place.31
View Article Online
DOI: 10.1039/C5TA05025A
Fig. 6: Relationship of composite surface area with decolourization ability towards
naphthol blue black, in terms of removal percentage, organized by water affinity.
The relationship of both the surface areas and the polymer
wettability, with respect to decolorize naphthol blue black, is
illustrated in Figure 6.
Scheme 1: Mechanism of the reductive degradation of naphthol blue black in the
presence of nZVI.
In order to confirm what the spectra indicated, that there was
degradation taking place, samples from the dye reaction with the
most promising candidate, the COP-1 composite, were tested for
aniline production. Analysis with HPLC produced positive results,
and did confirm the presence of aniline after the reaction began,
however only a minimal amount of 0.9 µM could be detected, 2.5%
of the total amount if every dye molecule produced an aniline
molecule during the reaction. Initial thoughts were that resultant
aniline was subsequently adsorbed into the COP matrix, but further
testing with dedicated sorption tests of aniline in aqueous solution
with COP-1 and COP-19 could not prove that either are capable of
adsorbing aniline. The combination of the observed patterns in the
spectra and the detection of aniline conclude that, in addition to
the dye decolourization due to adsorption, it is possible to degrade
naphthol blue black; however, the primary benefit of these COPs
appear to be first, stabilization of nZVI in solution, and second, the
adsorption of azo dyes and potentially an entire range of organic
contaminants in the environment.
Ultimately, the main determining factors in the ability for the
composite to remove the azo dye from solution were the BET
surface area and the wettability of the base polymer. In general, the
polymers with higher surface areas showed higher azo dye removal,
the increased surface area and pore volume allowed for first
adsorption and subsequent degradation once the dye had
penetrated the polymer matrix. Even more significant is the water
wettability, or ease of mixing into the dye containing water
solution, of the root polymer. Although exact solubility parameters
have not yet been determined, COPs 1 and 19 exhibit strong
hydrophilic properties due to the presence of polar groups, while
COPs 6, 60, and 61 have exhibited quite hydrophobic properties.
Possible reasons for the hydrophilicity most likely have to do with
presence of tertiary amine groups for COP-1 and the presence of
unreacted aldehyde, secondary amine, and tertiary amine groups
for COP-19; all of which can form hydrogen bonds and increase
water solubility. Hydrophobicity for COP-6 is due mainly to the
presence of thioether groups and the aromaticity; COPs 60 and 61
also exhibit a high level of aromaticity; both of which make these
compounds very insoluble in water. Wettability is a key factor, in
that it allows for the contaminant to migrate into the composite
material, where it can then be adsorbed, and further degraded by
the impregnated nZVI.
Conclusion
All five of the COPs utilized in this study, regardless of BET surface
area, pore volume, or wettability, were capable of successfully
being impregnated with iron, at approximately equal loading rates.
Although the morphologies varied widely from one COP to the
other, ranging from thin sheath-like structures to globular clusters,
iron was still observed to be contained within the various polymer
matrices. The BET surface areas decreased in each polymer once
the iron was impregnated, indicating that the pores were being
filled.
Each of the composites, especially COP-19, showed
remarkable stability with respect to particle aggregation and
sedimentation; vastly out-performing and remaining in suspension
much longer, compared to commercially available products, which
is a key parameter in the effectiveness of Fe0 as a degrading agent.
However, the composites of COPs 1 and 19 proved to be the most
effective materials for decolorizing naphthol blue black, exhibiting
high amounts of adsorption, and subsequent azo bond reduction as
well as naphthalene group oxidation. This can be explained by both
the high surface areas and the increased wettability of these two
composites. Although, this phenomenon is more pronounced in the
COP-1 composite, the COP-19 composite maintains too much
adsorption to accurately determine degradation capacity.
However, this combination adsorption/degradation provides a
synergistic effect where large amounts of the contaminant can be
drawn into and adsorbed within the composite matrix; then,
subsequently providing an environment where the impregnated Fe0
can degrade the contaminant. Constructing hydrophilic and high
surface area COPs into larger aggregates with larger pore volumes
would allow for not only more iron to be impregnated, but also
allow for more contaminant diffusion into the composite matrix,
providing
a
more suitable environment for contaminant
degradation. These larger particles would then be ideal for
immersion into a flow-through filter column designed for water
treatment.
Acknowledgements
P.D.M. and H.R.A. acknowledge funding for this study from the
Technical University of Denmark (DTU) through the KAIST-DTU
Signature Project on Integrated Water Technology.
Y.H.
6 | J. Mater. Chem. A, 2014, 00, 1-3
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PAPER
acknowledges funding for this study through a DFF-Individual 25.
postdoctoral grant from the Danish Council for Independent
Research – Technology and Production Science (4005-00393B). J.B.
thanks the National Research Foundation of Korea (NRF) for a 26.
global Ph.D. fellowship (2013H1A2A1033423). J.B., H.A.P., and
C.T.Y. acknowledge the financial support by Basic Science Research 27.
Program through the National Research Foundation of Korea (NRF),
ICT & Future Planning (2013R1A1A1012998), and IWT (NRF-2012- 28.
C1AAA001-M1A2A2026588).
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Journal of Materials Chemistry A
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NBB
NBB
NBB
NBB
NBB
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=
R-N=N-R
e-
H2O
R-NH-NH-R
H+
OH-
2+
Fe
H+
e-
0
Fe
NBB
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R-NH2 H2N-R
Fe-oxide
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aniline
p-nitro-aniline