Scale-up synthesis of zero-valent iron nanoparticles and their applications for synergistic degradation of pollutants with sodium borohydride

Article abstract of DOI:10.1016/j.molliq.2016.10.040

The scale-up synthesis of nZVI (zero-valent iron nanoparticles) was optimized by response surface modeling (RSM). The synthesized nZVI was employed for synergistic degradation of metronidazole (MNZ) or methyl orange (MO) in the presence of sodium borohydr

Full text of DOI:10.1016/j.molliq.2016.10.040

Scale-up synthesis of zero-valent iron nanoparticles and their applications for
synergistic degradation of pollutants with sodium borohydride
K. V. G. Ravikumar, Swati Dubey, Mrudula pulimi, Natarajan Chan-
drasekaran, Amitava Mukherjee
PII:
DOI:
Reference:
S0167-7322(16)32455-2
doi: 10.1016/j.molliq.2016.10.040
MOLLIQ 6444
To appear in:
Journal of Molecular Liquids
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Accepted date:
27 August 2016
8 October 2016
Please cite this article as: K.V.G. Ravikumar, Swati Dubey, Mrudula pulimi, Natarajan
Chandrasekaran, Amitava Mukherjee, Scale-up synthesis of zero-valent iron nanoparticles
and their applications for synergistic degradation of pollutants with sodium borohydride,
Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2016.10.040
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ACCEPTED MANUSCRIPT
Scale-up synthesis of zero-valent iron nanoparticles and their applications for synergistic
degradation of pollutants with sodium borohydride
Ravikumar K. V. G., Swati Dubey, Mrudula pulimi*, Natarajan Chandrasekaran, Amitava
Mukherjee**
Centre for Nanobiotechnology, VIT University, Vellore, Tamil Nadu, India
Corresponding Authors
*Dr. Mrudula pulimi
Associate Professor
Centre for Nanobiotechnology
VIT University
Vellore 632014, India
Email: pmrudula@vit.ac.in
**Dr. Amitava Mukherjee
Senior Professor & Deputy Director
Centre for Nanobiotechnology
VIT University,
Vellore 632014, India
Email: amit.mookerjea@gmail.com, amitav@vit.ac.in
Tel.: +91 416 220 2620
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Abstract
The scale-up synthesis of nZVI (zero-valent iron nanoparticles) was optimized by response
surface modeling (RSM). The synthesized nZVI was employed for synergistic degradation of
metronidazole (MNZ) or methyl orange (MO) in the presence of sodium borohydride (NaBH4).
Key parameters such as initial MNZ or MO concentration, nZVI concentration, and NaBH4
concentration for MNZ or MO removal process was optimized using central composite design
(CCD). Under the optimized conditions (nZVI concentration: 150 mg/L; NaBH4:0.1 M;
interaction time: 25 min for MNZ and 30 min for MO; and MNZ or MO initial concentration: 40
mg/L), the removal was synergistically increased to 100 % (MNZ) and 98.07±1.1 % (MO).
When nZVI was applied without NaBH4, the removal of MNZ and MO were 50.62±0.28 and
43.72±0.29 %, respectively. In the presence of NaBH4 alone, the removal of MNZ or MO was
6.5±1.1 % and 4.2±0.7 %, respectively. Addition of NaBH4 was found to enhance the
remediation ability of nZVI. The MNZ or MO degradation followed pseudo–first–order kinetics.
The degradation mechanism was studied by UV-visible spectroscopy, XRD, SEM-EDX, FT-IR,
and BET analyses. The applicability of the process was tested with MNZ-spiked or MO-spiked
lake water and ground water samples.
Keywords: Zero-valent iron nanoparticles; Scale-up synthesis; Metronidazole; Methyl orange;
synergistic removal; Degradation mechanism.
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1. Introduction
The global increase in the toxic materials burden in environmental media represents a
major ecological and human health threat [1, 2]. From this perspective, the utilization of
nanomaterials may allows provide an opportunity to reduce the environmental contamination [3].
Subsequently, the broad research attempt of the nanotech–community regarding functionalized
nanomaterials has evaluated key properties such as redox capability, sorption, also toxic organic
or inorganic entrapment of pollutants [3-5]. Thus far, variations of the nZVI technology are the
progressively more promising choice for the remediation of various hazardous and toxic wastes
as well as contaminated sites [5-8].
Scaling up of nanomaterials production is one of the important challenges in transitioning
from laboratory to the industrial scale [9]. Scale–up of biological and chemical synthesis of
nanoparticles has consistently been a challenge. Still there are no reports on a scale–up of
synthesis of nZVI; there are few prototypes have been designed in a batch reactor [5, 6]. A batch
processes are often imperfect as compared to synthesis in small–scale, because of the low
nanoparticle production yield and time intensiveness. A chemical reactor can generate more
suitable for a scale–up production of nanoparticles than the batch process. The nZVI technology
has turn into an essential research topic in modern years owing to their reducing (remediation)
property [5, 10, 11].
Recently, the peer-reviewed literature contains increasing reports regarding the presence
and need for treatment of pharmaceuticals and textile wastes in waste waters and surface waters
[12, 13] [14]. Pharmaceutical substances are used all over the world for improving the health of
humans and animals. Increased and continuous usage of drugs like antibiotics, analgesics and
anti–inflammatories have resulted in the increase in the release of these compounds And semi-
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degraded metabolites into the aquatic system [15]. Metronidazole is water soluble, non–
biodegradable drug with nitroimidazole ring structure and is used for its antibacterial and
antiprotozoal activities. Clearly, the presence of metronidazole in the aquatic environment can be
detrimental to the aquatic life and humans [16-18]. Therefore, its removal is necessary as well as
a challenge. Nanoscale zero-valent iron particles (nZVI) have been used for metronidazole
remediation.
Dyes released from synthesis plants and dye using industries as effluents are significant
environmental pollutant due to their non–biodegradable nature. Methyl Orange is a highly toxic,
water soluble azo dye that has a complex structure consisting of N=N group and is used in
printing, textile, pharmaceutical, paper manufacturing and food industries [19-21]. Several
methods like coagulation, chemical oxidation, photochemical degradation, flocculation,
membrane separation, floatation, reverse osmosis, and adsorption have been utilized for
remediation of dyes in water [22]. Nanoparticles like multiwalled carbon nanotubes, Zinc Oxide
nanoparticles, Titania nanoparticles, and nZVI for Methyl Orange removal from aqueous
solution in batch studies [23, 24]. Bentonite supported nZVI [20], and bimetallic nZVI/Pd
nanoparticles [21] were used by Chen et al. (2011) and Wang et al. (2013) for Methyl Orange
removal from aqueous solution in batch experiments.
The current study involves the optimization of scale–up synthesis of nZVI by response
surface modeling (RSM) and exploring their applications for degradation of dyes and antibiotics.
Metronidazole is a known essential antibiotic drug from the hospital effluents, and MO is an azo
dye commonly found in the textile industrial wastes, which cannot be degraded by
conventionally degraded by the biological method effectively [18, 25]. In the current study, the
synergistic degradation of MNZ or MO using nZVI in the presence of NaBH4, was attempted.
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Since NaBH4, is already being used for reductive synthesis of nZVI, it was worthwhile to study
its possible effects on degradation of pollutants by nZVI. To the best of our knowledge, this
heppens to be the first ever study on synergistic action NaBH4, and nZVI for degerdating the
dyes and antibiotics in aqueous solution. The effects of parameters like initial MNZ or MO
concentration, nZVI concentration and NaBH4 concentration on the degradation of MNZ or MO
by the nZVI was studied. The mechanism of MNZ or MO degradation using nZVI was also
explored.
2. Experimental
2.1 Materials
Ferrous sulfate (FeSO4·7H2O) and EDTA (C10H14N2Na2O8·2H2O) were obtained from
Sisco Research Laboratories Pvt. Ltd., India. Sodium borohydride (NaBH4) obtained from SD
Fine Chemicals, Mumbai, India. Metronidazole obtained from the Sigma–Aldrich, Methyl
orange (C14H14N3NaO3S) was acquired from Hi–Media Laboratories, Mumbai, India. The
chemicals used in this study were of analytical grade, and with deionized water the solutions
were prepared.
2.2 Scale-up synthesis of nZVI particles in chemical reactor
The synthesis of nZVI particles procedure was described in our previous report [6].
Briefly, FeSO4 and disodium salt of EDTA were prepared and mixed, and then, NaBH4 was
added at a speed of 1–2 drop sec−1 into the mixture in a nitrogen atmosphere at 60 °C under
vigorous and continuous stirring. The solution turned black due to precipitation of nano zero-
valent iron; the solution was filtered, and nZVI nanoparticles were rinsed with absolute ethanol
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three times and dried over night at 100°C. The experimental volume is 200 ml (1:1 ratio of
FeSO4 and NaBH4).
The experimental conditions for synthesis of nZVI particles were optimized by Box–
Behnken Design [6] (BBD) of Response surface modeling (RSM). The process was optimized by
varying the concentration of the variables which directly affects the yield percentage of the
synthesized nZVI. The parameters considered were concentrations of FeSO4 (0.05 to 0.15 M),
NaBH4 (0.1 to 0.75 M) and temperature (30 to 60 °C).
Here, yield percentage was considered as the response in conducted series of experiments. Yield
percentage was calculated by using the formula:
푌푖푒푙푑 % = 퐸푥푝푒푟ꢀ푚푒푛푡푎푙 푤푒ꢀ푔ℎ푡 표푓 푝푟표푑푢푐푡 푛푍푉퐼 × 100
(1)
( )
푇ℎ푒표푟푒푡ꢀ푐푎푙 푤푒ꢀ푔ℎ푡 표푓 푝푟표푑푢푐푡 ꢀ푟표푛
Where, experimental weight is the dry weight of synthesized nZVI; theoretical weight is the % of
iron content of the precursor material (FeSO4).
The optimized experimental condition of the synthesis of nZVI was carried out in a 2.5 L
chemical reactor.
2.3 Characterization of nZVI particles
The structure and crystallinity of the nZVI was analysed using powder X–ray diffraction
(XRD) (D8 Advance diffractometer, Bruker, Germany). The average crystalline size calculated
using Debey–Scherrer’s formula [26].
퐾휆
퐷 =
(2)
훽 퐶os(휃)
The nZVI size distribution was calculated by Dynamic Light Scattering (DLS) (Horiba,
SZ–100, Japan) in Milli–Q water. UV–visible spectral investigation of synthesized nZVI
particles was determined using UV–vis spectrophotometer (UV1750, Shimadzu Corporation,
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Kyoto, Japan). SEM image of the nZVI was analysed by scanning electron microscopy
(FESEM–SUPRA 55 – Carl Zeiss, Germany). The surface elemental analysis of synthesized
nZVI and reacted with MNZ or MO were conducted using Energy Dispersive X–ray
spectroscopy (FESEM–SUPRA 55 – Carl Zeiss, Germany). Surface area of the nZVI reacted and
un–reacted with MNZ or MO was analyzed by BET (Micromeritics, TriStar III, USA). The
surface chemical characteristics of the synthesized nZVI reacted and un–reacted with MNZ or
MO were analyzed by Fourier Transform Infra–Red Spectrometer (IR Affinity–1, Shimadzu,
Japan).
2.4. Batch experiments for removal of MNZ or MO using nZVI
2.4.1 Optimization of parameters for removal of MNZ or MO using nZVI using RSM
The degradation of MNZ (metronidazole) and Methyl orange (MO) using nZVI in the
presence of sodium borohydride experimental conditions was optimized using RSM modeling
[Central Composite Design (CCD)] [27]. Degradation of MNZ or MO experiments were
conducted in a conical flask (250 mL), in which the experiments the volume of the solution was
100 mL. The three major factors (initial MNZ or MO concentration, nZVI concentration, and
concentration of NABH4) were taken for the optimization of MNZ or MO degradation, where the
pH (pH–7) of MNZ or MO solution and experimental temperature (25 °C) was kept constant to
imitate the natural environment. At prescribed time intervals, 5 mL of samples were taken out
and immediately centrifuged. The MNZ absorbance was measured by using UV–vis
spectrophotometer at wavelength 314 nm. The MO absorbance was measured by using a UV–vis
spectrophotometer at wavelength 464 nm. The entire experiments were carried at constant pH–7
and reaction temperature 27 °C.
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2.4.2 Measurement of the removal percentage and removal capacity
The removal percentage of MNZ or MO has been calculated by the following equation
[20]:
(퐶 −퐶 )푋100
0
푒
( )
푅푒푚표푣푎푙 % =
(3)
퐶
0
The amounts of MO or MB removal capacity [qe (mg/g)] at equilibrium was calculated
by the expression [28]:
푞푒 =
(퐶 −퐶 )푋푉
0
푒
(4)
푊
where C0 and Ce (mg/L) are the MNZ or MO initial and equilibrium concentrations,
respectively; V (L) is the volume of solution; and W (g) is the weight of nZVI used.
2.4.3 Kinetic study
The similar equilibrium experiment method was used for kinetic tests, aqueous samples
were collected at selected time intervals and the MNZ or MO concentrations were measured. The
kinetics of MNZ or MO removal with nZVI was determined using pseudo first–order, pseudo
second–order, Elovich, and Intraparticle diffusion kinetic models [6]. The correlation
coefficients (r2) values were close or equal to 1, expressing the agreement between experimental
data and the model predicted values. A relatively high r2 value denotes that the model
successfully explains the kinetics of MNZ or MO removal [29].
The pseudo first order, pseudo second order, Elovich model, and Intraparticle diffusion model
equations are expressed in eq (5), (6), (7), and (8) respectively.
푞
푙푛 푞
= −퐾퐿푡
(5)
(6)
푒−
푡
푞
푒
푡
1
′
푡
=
+
푞
푒
2
푞
푡
2퐾 푞 푒
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1
1
(
)
푞푡 = 훽 푙푛 훼훽 + 훽 푙푛(푡)
(7)
(8)
푞푡 = 푘ꢀꢁ푡0.5
Where KL–removal rate constant of Lagergen (L/min); K′– rate constant adsorption of pseudo–
second order (mg/g), qe and qt are the amounts of MNZ or MO degrade (mg/g) at equilibrium
and time t respectively; α–initial adsorption rate (mg/g. min); β–desorption constant (g/mg); R–
percent MNZ or MO adsorbed; t–contact time (h); kid–Intraparticle diffusion rate constant (h–1);
The linear plots pseudo first order [log (qe–qt) vs t] and pseudo second order [linear plot t/qt vs
t], Elovich model [qt vs ln (t)], and intra–particle diffusion model [qt vs t0.5] yields a linear
relationship of and reaction from which KL, K′, α, β, and kid can be calculated from the slope
and intercept of the plot, respectively [29, 30].
2.5. MNZ or MO removal in environmental water matrices (lake and ground water)
Water samples were taken from different locations at Vellore. The ground water and lake
water samples were collected from Sipcot and VIT University Vellore respectively. The removal
of MNZ or MO spiked environmental water samples were carried out at the optimized
conditions.
2.6. Statistical analysis
All the experiments were carried out in triplicate, Using Design Expert 9 and one-way
ANOVA software was carried out to ensure the results statistical significance.
3. Results and discussion
3.1 Optimization of scaled up synthesis process of nZVI
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The BBD was used for optimizing the experimental conditions for synthesis of nZVI
particles. The successive experimental runs and ANOVA are shown in Table 1A, B. The F–value
of Model 53.84 implied that the model was significant. The values of "Prob > F" < 0.05 showed
that the terms of the model are significant. The values > 0.1 indicated that the model terms are
not significant. The standard deviation of the experimental design was found to be 1.41, R2–0.98.
The mean and C.V % of the experimental design were observed to be 74.00 and 1.90 (adjusted
R2 –0.96; predicted R2–0.87; adequate precision 23.87).
The predicted R2 (0.87) was in agreement with the adjusted R2 (0.96); i.e. < 0.2
difference. A ratio greater than 4 for the adequate precision (measures signal to noise ratio) is
desirable. An experimental design ratio of 23.87 denotes an adequate signal. This model may be
used to navigate the design space.
Fig. 1 showed the three–dimensional (3D) response of the counter image and also the
effect of varying factors like temperature, FeSO4, and NaBH4 concentration in the scale-up
synthesis of nZVI. Fig. 1A demonstrates that an increase in the concentration of FeSO4 and
NaBH4 causes an increase in the yield percentage at constant temperature (60 °C). When
constant NaBH4 concentration (0.75 M) was used by varying FeSO4 concentration and
temperature, the results demonstrated that an increase in the concentration of FeSO4 and
temperature resulted in an increase in the yield percentage (Fig. 1B). When FeSO4 concentration
(0.1 M) was maintained constant by varying NaBH4 concentration and temperature, the yield
percentage was increased with increase in the NaBH4 concentration and temperature as shown in
Fig. 1C.
The results proved that the FeSO4 and NaBH4 concentrations and temperature played an
important role in the synthesis of nZVI. With increase in concentration of FeSO4 (0.05 to 0.15
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M) at constant NaBH4 concentration (0.75 M) and temperature (60 °C), the yield percentage was
observed to be increased due to availability of sufficient amount of reducing agent in the
solution. Similar concentrations were used for small-scale synthesis of nZVI as mentioned in an
earlier report [6]. With a decrease in NaBH4 concentration at constant FeSO4 (0.1 M)
concentration and temperature (60 °C), the yield percentage was observed to be decreased,
probably owing to the insufficient amount of reducing agent available for synthesis. It is well
known that NaBH4 is a strong reducing agent, which plays an important role in nZVI synthesis
[6]. Therefore, NaBH4 concentration was considered as an important factor in nZVI synthesis.
With increase in temperature (30–60 °C), the yield percentage was found to be increased at
constant FeSO4 (0.1 M) and NaBH4 (0.75 M) concentration. Through previous studies, it can be
proved that with increase in temperature, there will be more formation of nanoparticles [31, 32].
Similar to the earlier reports that use a maximum temperature of 50 °C for nZVI synthesis, in our
experiments, we have used a maximum of 60 °C [6].
From the above results from BBD, the optimized experimental conditions were found to
be 0.13 M FeSO4, 0.05 M EDTA, 0.75M NaBH4, and a temperature of 60 °C. The maximum
nZVI yield was found to be 86.08±1.8 %. Small–scale synthesis in flask and large–scale
synthesis of nZVI in chemical reactor showed similar characterization data as shown in Table S1
(supplementary information). XRD analysis confirmed the formation of nZVI, and the 2θ peaks
(44.74 [6], 46.78) showed similar crystalline structures in both synthesis process. By using DLS
and BET analyses, the mean particle size and surface area of nZVI were found to be similar in
both synthesis processes. The SEM and EDX results showed comparable surface characteristics
and iron weight percentage. By analyzing these results, small scale and large scale synthesis of
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nZVI showed a similar result in size and nZVI formation. The scale–up synthesis of nZVI in a
chemical reactor can be suggested to produce large quantities of nZVI.
Due to reducing property of nZVI, it has been frequently used as a sorbent for
environmental remediation [7, 11]. To further study the applicability of synthesized nZVI for
environmental remediation in the present work, the removal of antibiotic (metronidazole) and
azo dye (methyl orange) from aqueous solution was performed.
3.2 Statistical optimization of MNZ or MO removal
The removal of MNZ and MO using nZVI was done in separate batch experiments. CCD
was performed to identify the best conditions for removal of MNZ or MO using nZVI in the
presence of NaBH4, enabling the statistical evaluation of the aspects that significantly influence
the removal of MNZ by nZVI. The experimental data for MNZ or MO removal by nZVI in the
presence of NaBH4 are shown in Table 2A, B. From the CCD model, the analysis of variance
(ANOVA) results for MNZ or MO removal are shown in Table 3A, B. The CCD model F–values
for removal of MNZ or MO using nZVI in the presence of NaBH4 was found to be 1621.31
(MNZ) and 544.85 (MO), which means that the CCD model is significant. The “Probability >F”
value was >0.05, which shows that present method is statistically significant for the removal of
MNZ by nZVI in the presence of NaBH4. The equations (9, 10) were produced by the analysis of
regression through the correlation coefficients (R2) of MNZ (0.99) and MO (0.99), and p<0.0001
for MNZ or MO removal using nZVI in the presence of NaBH4.
R1=66.63–13.40A+14.60B+13.33C–0.59AB+1.61AC–0.16BC+4.45A2–6.56B2–3.62C2 (9)
R1=63.84–8.16A+10.69B+20.10C–2.37AB+4.11AC–3.19BC+5.69A2–1.05B2–5.49C2 (10)
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where, R1−percentage of MNZ or MO removal using nZVI, A– initial concentration of
MNZ or MO, B– concentration of nZVI, and C– NaBH4 concentration. The result of factors on
the removal of MNZ or MO (R1) was correlated by the coefficient values of A, B, and C
independently.
For MNZ removal by nZVI in the presence of NaBH4, the coefficient of variation (CV)
was observed to be 1.13%. The "predicted R2" of 0.99 is in agreement with the "adjusted R2" of
0.99; i.e. the difference is < 0.2. Accordingly, the coefficient of variation (CV) was observed to
be 1.91%, and the “predicted R2" of 0.98 is in agreement with "adjusted R2" of 0.99 for MO
removal by the nZVI in presence of NaBH4. The CV for both nanoparticles was observed to be <
10%, which represents the reliability and precision of the experimental design. A ratio greater
than 4 for adequate precision (measures signal to noise ratio) is desirable. The ratio of MNZ
(31.70) and MO (89.92) denotes an adequate signal for MNZ or MO removal by nZVI. The
results recommended that the CCD model may be employed to pilot the design space.
The relationship between independent and dependent variables is illustrated in three-dimensional
representations of the response surfaces (Fig. 2). A decrease in the removal (%) of MNZ or MO
was observed with increase in the concentration of initial MNZ or MO concentration [100 to
18.43±1.2 % (MNZ) and 98.07±1.1 to 17.69±0.9 % (MO)]. The higher values of MNZ or MO
removal efficiencies were obtained by the increase in nZVI and NaBH4 concentration (Fig. 2). At
25-min reaction time for MNZ and 30 min for MO, the maximum removal of MNZ and MO was
100 % and 98 %, respectively. The consequence of surface interaction involving nZVI and MNZ
or MO was checked by varying the MNZ or MO initial concentrations (40–150 mg/L) and
NaBH4 (0.05–0.1 M), whereas the concentration of nZVI (150 mg/L) was set as constant (Fig.
2A, B).
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The increased NaBH4 concentration also leads to enhancement of MNZ or MO removal
percentage. The maximum removal of MNZ or MO >98 % was determined at constant NaBH4
concentration 0.1 M. Similarly, a decrease in the removal of MNZ or MO was observed upon
increasing the initial MNZ or MO concentrations above 40 mg/L in the solution. Fig. 2C, D
shows the surface interaction between nZVI and NaBH4, which was studied by varying the nZVI
concentrations (50–150 mg/L) and NaBH4 concentrations (0.05–0.1 M) at a constant initial MNZ
or MO concentration (40 mg/L). The removal percentage of MNZ or MO was gradually
enhanced with increase in nZVI dosage and NaBH4 concentration. The cause of surface
interaction between nZVI and MNZ or MO was observed by varying the initial concentrations of
MNZ or MO (40–150 mg/L) and nZVI (50–150 mg/L), whereas the NaBH4 concentration (~0.1
M) was kept constant (Fig. 2E, F). The removal percentage of MNZ or MO was observed to be
increased as the nZVI concentration increases; furthermore, maximum response was achieved at
150 mg/L for MNZ or MO removal. Similarly, a decrease in the MNZ or MO removal was noted
upon increasing the initial MNZ or MO concentration above 40 mg/L in the solution. Maximum
removal was attained at an initial concentration of MNZ or MO: 40 mg/L and 150 mg/L of nZVI
and 0.1 M of NaBH4.
Using nZVI alone, the MNZ or MO removal experiments were conducted according to
the design expert software, and their corresponding results are listed in Table S2A, B
(supplementary information). The quadratic equations for prediction of the optimum point were
obtained according to the CCD and input variables as follows:
For MNZ removal
R1=+10.47+7.06*A-13.86*B-5.58*AB+1.90*A2+11.80*B2
For MO removal
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R1=+16.59+5.77*A-14.52*B-5.60*AB+0.44*A2+3.72*B2
where R1 is the MNZ or MO removal efficiency (R %), and A and B represent the nZVI
concentration and MNZ or MO concentrations, respectively. ANOVA suggests that the model
equation derived by RSM could be used to describe the MNZ [Table S2C (supplementary
information)] or MO [Table S2D (supplementary information)] removal by nZVI. It is well-
known that a corresponding variable is more significant if its absolute p-value is smaller than
0.05. The three-dimensional representations of the response surface plots illustrated the
relationship between independent and dependent variables. The maximum removal of MNZ and
MO was obtained by the increase in nZVI concentration [Fig. S1A (supplementary
information)]. The decreased initial MNZ or MO concentrations also led to enhancement in the
removal percentage [Fig. S1B (supplementary information)].
Since nZVI is a strong reducing agent, [5, 6, 33] it was also reported as a well–known
sorbent for removing contaminants [5]. As the MNZ or MO concentration increases, there will
be more oxidation of Fe0, which can lead to loss of reactivity and results in the decrease of MNZ
or MO removal efficiency [18, 20]. Further, the increase in removal efficiency was observed at
fixed MNZ or MO (40 mg/L) and nZVI (150 mg/L) concentration by increasing the NaBH4
concentration This was due to the regeneration of nZVI particles, which led to the increase in
reaction sites on the surface of nZVI [18]. In addition, there was a decrease in the removal
efficiency observed at constant nZVI and NaBH4 concentration by increasing the initial MNZ or
MO concentration, presumably due to the limited availability of limited reaction sites on the
nZVI surface [6, 21, 33].
3.3 Optimized conditions for batch removal study
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The optimal conditions for maximum removal of MNZ or MO using nZVI in presence of
NaBH4 were found to be following; initial MNZ or MO concentration: 40 mg/L; nZVI
concentration: 150 mg/L; NaBH4 concentration: 0.1M; interaction time: 25 min for MNZ and 30
min for MO. Under these optimized conditions, the removal percentage of MNZ and MO was
found to be 100 and 98.07 %, respectively, and the removal capacity was observed to be 266±4.7
(for MNZ) and 261±3.1 (for MO) mg/g, respectively. Compared to previous studies, the
degradation of MNZ or MO using nZVI [20, 33, 34] in the present study showed high removal
(%) at low concentrations of nZVI as mentioned in Table 4.
The optimal experimental conditions for the removal of MNZ or MO using nZVI alone
were determined using the desirability function by the Design expert software. The optimum
conditions were found to be nZVI concentration: 150 mg/L and initial concentration of MNZ or
MO: 40 mg/L, which provide maximum MNZ and MO removal efficiencies of 50.62±0.28 and
43.72±0.29 %, respectively.
In the presence of only NaBH4, the removal percentages of MNZ or MO were only
6.5±1.1 and 4.2±0.7 %, respectively. These results indicated that nZVI in the presence of NaBH4
had a synergistic effect on the MNZ or MO removal in aqueous solutions.
3.4 Kinetic study
The effect of contact time on removal of MNZ or MO was investigated at constant nZVI
(150 mg/L) and NaBH4 (0.1 M) concentrations by varying the initial concentrations of MNZ or
MO solutions (40, 80, 120, 160 mg/L). The interaction time was checked for a period of 35 min.
As the reaction time increased to 25 min for MNZ and 30 min for MO, the removal percentage
of MNZ or MO was found to be maximum, and additionally, an equilibrium state was attained
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for MNZ and MO, respectively. No further increase in removal percentage (a maximum of 100
% for MNZ and 98 % for MO) was observed beyond the time points (Fig. 3A, B). In the case of
MO degradation, the color of solutions turned to colorless at 30 min after interaction with nZVI
in the presence of NaBH4 (Fig. 3C, D). The fast removal of MNZ or MO was observed at an
initial stage, which is due to the availability of a large number of reaction sites on the nZVI.
Until the point that equilibrium was attained, the MNZ or MO removal rate gradually decreased
due to the reduction in the MNZ or MO gradient concentration between nZVI and aqueous
phases. Pseudo–first–order, pseudo–second–order, elovich, and intraparticle diffusion were used
to study the mechanism and rate of reaction for the removal process. The calculated results fit the
kinetic models as presented in Table 5 and Fig. S2 (supplementary information). The correlation
coefficients (R2) values (>0.98) of the pseudo–first–order model was higher than that of the
pseudo–second–order model, indicating that the removal kinetic data of MO on nZVI was well
described by the pseudo–first–order model. Farzadkia et al. (2015) and Chen et al. (2011)
reported MNZ removal by nanoparticles and MO removal by nZVI particles, [20, 35] which
concluded that the kinetic model of pseudo–first–order may strongly rely on the nZVI
concentration. The experimental results confirmed that the high removal percentage, removal
capacity, and rapid kinetics reaction for MNZ or MO removal using nZVI make it a competent
tool for MNZ or MO removal in other environmental matrices and contaminated sites.
3.5 Characterization of nZVI before and after reaction with MNZ or MO
The XRD results of nZVI synthesized in the chemical reactor showed a sharp 2θ peak at
46.78°, which confirmed the presence of nZVI [JCPDS no. 00-006-0696] [8]. From the XRD
analysis, the crystalline size (D) was found to be 18nm. The XRD analyses of nZVI and MNZ or
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MO reacted with nZVI are shown in Fig. 4A, B, C. The evident 2θ peak at 44.94° also indicated
the presence of nZVI (Fig. 4A). The nZVI after reaction with MNZ or MO (Fig. 4B, C) indicated
the presence of Fe2O3 (2θ: 35.68° and 55.62°), Fe3O4 (2θ: 35.45° and 65.76°), MNZ peaks (**)
[36], and MO peaks (*). After interaction with MNZ or MO, the appearance of Fe (II) and Fe
(III) peaks confirmed the occurrence of redox reactions between Fe0 and MNZ or MO. This is in
concordance with other studies, where nZVI particles were used as a reductant [37-39]. The
average crystalline size of nZVI was found to be 18 nm, and the average crystalline size of nZVI
was increased after interaction with MNZ (38 nm) or MO (52 nm).
Table S3 (supplementary information) demonstrates that the average particle size of
nZVI before interaction to be 114 nm, and that after interaction, the average particle size was
found to be 394 nm with MNZ and 664 nm with MO. From BET analysis, the nZVI surface area
was calculated as 87 m2/g after interaction with MNZ (21 m2/g) or MO (15 m2/g). A decrease in
the surface area of nZVI was observed. The particle size was increased due to the binding of
degradation-related components of MNZ or MO on the surface of nZVI. This was confirmed by
the BET studies performed in the available literature, which showed that an increase in the size
of the particle will cause a decrease in the surface area [8].
When MNZ reacted with nZVI in the presence of NaBH4, the MNZ absorbance peak at
314 nm decreased with increase in time from 5 to 35 min as shown in Fig. 3A. In case of MO,
the absorbance peak was recorded at 464 nm, and this peak was also observed to decrease over
time. Also, an increase in the absorbance peak at 248 nm (Fig. 3B) showed that the degradation
of MO was occuring. This was also signified by the change from colored to colorless solution
over a reaction time of 30 min upon interaction with nZVI in the presence of NaBH4 (Fig. 3C,
D).
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The SEM results show that as-syntesized nZVI [Fig. S3A (supplementary information)]
was spherical in shape with an average particle size of 30 nm. After interaction with MNZ or
MO, the surface was observed to be rough and existed as aggregates as shown in Fig. S3B, C
(supplementary information). The EDX results of nZVI showed a 78 weight percentage of iron
content in the sample with trace elements shown in Fig. S3D (supplementary information),
which indicated that the synthesized nano–iron is in zero state i.e nZVI. Fig. S3E, F
(supplementary information) shows the elemental composition of nZVI after exposure to MNZ
or MO. After interaction with MNZ or MO, the atomic percentages of carbon, nitrogen, and
oxygen [Fig. S3E, F (supplementary information)] were increased due to the presence of
degraded components of MNZ or MO on the nZVI surface [19]. Thus, the EDX result
established the involvement of nZVI in the MNZ or MO removal process.
Fig. 5 shows the FT–IR spectra of nZVI particles before and after interaction with MNZ
or MO, from which a remarkable change in the absorption bands of the functional groups was
observed. The broad peaks at 3328 cm−1 suggested the presence of hydroxyl group (–OH) and
the peaks at 2365 cm−1 were due to C–H stretching band [40]. The peaks at 1418 cm−1 denoted
the presence of C–O (stretching vibration) group [41]. The FT–IR spectra were recorded for
nZVI before and after interaction with MNZ or MO (Fig. 5). An increase in absorbance at 3328
cm–1 (hydroxyl), 1418 cm–1 (carbonyl), 1039 cm−1 and 876 cm−1 were noted, which was due to
the complexation of MNZ or MO [34].
From the above results, a possible removal mechanism of MNZ can be proposed that
MNZ degrades into 1–(2–hydroxyethyl)–2–methyl–5–imidazole or 1–(2–hydroxyethyl)–2–
methyl–5–aminoimidazole, 2–methyl–5– nitroimidazole [34]. The nitro reduction and
hydroxyethyl cleavage or N–denitration are the pathways considered in the MNZ degradation
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process with nZVI surface that result in the generation of electrons and hydrogen species [42,
43].
In the case of MO, the degradation results in the formation of sulfanilic acid and aromatic
amines [20]. This is due to the cleavage of azo N = N bond by Fe0 to form hydrazine–like N–H
particular bonds, which consume H+ and lead to increase in pH. On the other hand, the instant
reaction of Fe0 with water to produce Fe2+, OH, and H2 can also cause pH increase in solutions
[19, 20]. In nZVI–MO system, the fluctuation of pH from 12 to 9.07 was observed within 20 min
of interaction. Consequently, the decrease in pH of the system was found to be 7.45 in 30 min.
This was attributed to the utilization of H+, resulting in an alkaline state for Fe2+ upon reacting
with OH to form Fe(OH)2 in the solutions [20]. Fig. 6 shows the schematic representation of the
nZVI scale-up synthesis and possible ways of metronidazole and methyl orange degradation
using nZVI.
3.6 MNZ or MO removal in environmental water matrices
The application of nZVI in the removal of MNZ or MO was studied by performing the
experiment in environmental water matrices. The removal percentage of MNZ was evaluated in
ground water (GW) and lake water (LW) taken from Sipcot and VIT Lake, Vellore, India, and
was spiked with 40 mg/L of MNZ or MO. In GW and LW, 88.75±0.6 and 79.74 ±1.2% of MNZ
removal, respectively, were achieved by nZVI (150 mg/L) in the presence of NaBH4 (0.1M). The
MO removal percentages of 81.54±1.3 and 71.01±1.2 % were observed by nZVI (150 mg/L) in
the presence of NaBH4 (0.1M) in MO-spiked GW and LW, respectively. The removal percentage
of MO or MNZ calculated in spiked environmental water samples was found to be decreased by
10 to 20 % in contrast with the experimental outcome quantified in distilled deionized water.
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However, a decline in the removal percentage calculated in natural samples was observed. This
decrease was due to the existence of natural colloids in the environmental water samples [6, 8].
Thus, the results suggested that the synthesized nZVI can be employed in MNZ or MO removal
in different aqueous samples, even though a lesser removal percentage was observed as
compared to the MNZ or MO removal percentage in distilled deionized water.
4. Conclusions
The scale–up synthesis of nZVI was successfully optimized by RSM. The synergistic
removal of MNZ or MO using nZVI in the presence of NaBH4 was investigated using CCD.
Results showed that the removal of MNZ or MO increased with increase in the nZVI
concentration and decreased with increase in the MNZ or MO initial concentration. The removal
percentage of MNZ or MO using nZVI in the presence of NaBH4 was found to be enhanced in
comparison to either nZVI or NaBH4 alone. The synthesized nZVI was successfully employed
for MNZ or MO removal in MNZ-spiked or MO-spiked environmental water samples. Further,
the scaling up of the synthesis process of nZVI can be utilized for the MNZ or MO removal
through in situ remediation process using permeable reactive barriers (PRB) technology and/or
by ex–situ “pump and treat” processes.
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Acknowledgements
We would like to acknowledge Department of Science and Technology–Science and
Engineering Research Board (DST–SERB) [SB/S3/ME/0019/13], Government of India for
providing the grant throughout this research work.
Compliance with ethical standards
In this study, neither human participants nor animals were involved.
Conflict of interest
The authors declare that they do not have any conflict of interest.
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Figure captions
Fig. 1. Scaling up the synthesis process of nZVI response surface plots (A) showing the effect of
FeSO4 and NaBH4 concentrations at constant temperature (60 °C) (B) showing the effect of
FeSO4 concentration and temperature at constant NaBH4 concentration (0.75 M) (C) showing the
effect of NaBH4 concentration and temperature at constant FeSO4 concentration.
Fig. 2. The response surface plots: (A) Effect of MNZ and NaBH4 concentration on MNZ
removal and (B) Effect of MO and NaBH4 concentration on MO removal at 150 mg/L of nZVI.
(C) Effect of nZVI and NaBH4 concentration on MNZ (40 mg/L) removal, (D) Effect of nZVI
and NaBH4 concentration on removal of MO (40 mg/L), (E) Effect of MNZ and nZVI
concentration on removal of MNZ at constant NaBH4 concentration (0.1 M), (F) Effect of MO
and nZVI concentration on removal of MO at constant NaBH4 concentration (0.1 M)
Fig. 3. UV–vis spectrophotometry analysis at different time intervals:
A) MNZ reacted with nZVI B) MO reacted with nZVI C) MO solution D) MO solution after
reaction with nZVI
Fig. 4. XRD image: (A) nZVI particles, (B) MNZ reacted nZVI, (C) MO reacted nZVI
Fig. 5. FT–IR spectra of nZVI, MNZ reacted nZVI, MO reacted nZVI
Fig. 6. Schematic representation of the nZVI scaling up the synthesis process of nZVI and
possible ways of MNZ and MO degradation using nZVI
29