DES-09969; No of Pages 6
Desalination xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Desalination
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Characterization of iron oxide nanocatalyst in mineralization processes
Altai Bach ⁎, Adva Zach-Maor, Raphael Semiat
Grand Water Research Institute – Rabin Desalination Laboratory, Wolfson Faculty of Chemical Engineering, Technion – Israel Institute of Technology, Technion City, Haifa 32000, Israel
a r t i c l e
i n f o
Article history:
Received 17 April 2010
Accepted 10 May 2010
Available online xxxx
Keywords:
AOP
Fenton process
Iron oxide
Total organic carbon (TOC)
a b s t r a c t
The reuse of iron oxide nanoparticles as a catalyst of organic matter in the H2O2/iron oxide mineralization
process was investigated. The particle size and morphology of iron oxide particles obtained from TEM
(Transmission Electron Microscopy) images and DLS (Dynamic Light Scattering) measurements, indicate the
formation of a rod-like morphology with an average length of 50 ± 10 nm. An electron diffraction pattern
identified the particles as either α-FeOOH or β-FeOOH. Stability of iron oxide nanoparticles in organic model
compound solutions was studied as a function of pH solution and was correlated with average size. The
optimal pH for maximum mineralization in the H2O2/iron oxide system was found to be 2.8. Finally, results
indicated that at least seven stages of catalytic mineralization-recovery cycles can take place without a
reduction in the catalytic properties of the iron oxide nanocatalyst.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
+ 2
+ 1
þ
+ H2 O→ FeðH2 OÞ4 ðOHÞ2
+ H3 O
FeðH2 OÞ5 ðOHÞ
Iron oxide plays an important role in wastewater treatment. The
high versatility and structural chemistry variability of iron oxide is a
result of the existence of two oxidation stable states for iron, ferrous
ions (Fe+ 2) and ferric ions (Fe+ 3) in a wide range of pH and the
condensation phenomenon [9–11,23].
Ferric iron introduces hexacoordinated complexes within water.
The polarization of the water molecules depends upon the oxidation
states of the cations (ferrous or ferric ions) and their size. It was
reported [10], that hydroxylated complexes (an addition of hydroxyl
group into the iron oxide molecule) are generated from bivalent and
trivalent iron in different pH spectrums. In acidic pH between 0 and 5,
3
is formed with hydroxylated comhydrated ferric ion, Fe(OH2)+
6
+2
1
plexes such as FeOH(OH2)5 and Fe(OH)2(OH2)+
4 . It was demonstrated that in this acidic pH hydroxylated complexes of ferrous iron
do not exist.
Hydrated ferric ions can be generated via hydrolysis of ferric
chloride within water [3]. The chloride performs a displacement as
can be seen from the following equation.
The acidic solution is formed due to the proton that migrates from
the coordinated water molecules (Eq. (3)) to the Fe+ 3 ion and then to
the free water molecule [7].
The generation of complexes' clusters and particles in the solution
is due to the condensation phenomena of the iron species. The
condensation process of any hydroxylated complex can be performed
through two main mechanisms.
The first mechanism is olation, which releases a coordinated water
molecule and forms hydroxyl group bridges between the ferric ions.
The second mechanism is oxilation, which represents the condensation of oxohydroxo species — complexes which do not contain
coordinated water molecules.
The condensation mechanism depends on the nature of the
cations' coordination sphere. Therefore, ferric hydroxo complexes
have a pronounced tendency to condense via the olation mechanism.
Complete hydrolysis occurs in time when all six water ligands are
deprotonated in order to form iron oxides, i.e.
þ 3
−
½FeCl3 ⋅6ðH2 OÞ→ FeðH2 OÞ6
+ 3Cl
+ 3
þ
→FeOðOHÞ + 3H + 4H2 O
FeðH2 OÞ6
ð1Þ
While hydrolysis continues, the hydroxide ion replaces the water
molecules with tight bonds [12] and forms the following hydroxylated
complexes:
+ 2
+ 3
þ
+ H2 O→ FeðH2 OÞ5 ðOHÞ
+ H3 O
FeðH2 OÞ6
⁎ Corresponding author. Tel.: +972 50 5576599.
E-mail addresses: [email protected], [email protected] (A. Bach).
ð2Þ
ð3Þ
ð4Þ
Advanced oxidation processes (AOPs) have received considerable
attention as wastewater treatment processes due to their ability to
degrade and, in many cases, mineralize various organic compounds
that are otherwise resistant to conventional biological and chemical
treatments. AOPs are based on strong oxidants, such as hydrogen
peroxide (H2O2) and/or a combination of strong oxidants with
catalysts, including the Fenton process that is based on transition
metal ions (ferrous ions) combined with H2O2.
0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2010.05.016
Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016
2
A. Bach et al. / Desalination xxx (2010) xxx–xxx
1.1. Advanced oxidation process (AOP)
AOP is an oxidation technique by which a large quantity of
hydroxyl radicals (OH•) are generated and various organic pollutants
are oxidized [27]. These radicals enable very rapid reactions with most
organic contaminants mainly due to their high oxidation potential
(2.8 V) and non-selective nature. Since hydroxyl radical half-life is
only microseconds, they must be generated incessantly in situ through
chemical reactions. Environmental applications of the Fenton process
include the use of hydrogen peroxide, with the substitution of
different catalysts such as ferric iron and naturally occurring iron
oxides [2].
The mechanism of the Fenton reagent has been reported
extensively [15,17,18,26].
1.2. H2O2/iron oxide system
Dissolution of iron oxide, such as FeOOH, occurs via protonation
[7]. The general reaction between iron oxide and the protons is
presented in Eq. (5):
h
in +
+ ðn−1ÞH2 O
FeOðOHÞ + nH → FeðOHÞð3−nÞ
þ
aq
þ
3+
+ 2H2 O
2.2. Experimental Setup
Batch experiments were carried out in a 250 mL beaker stirred by
magnetic stirrers, with 100 mL of organic model compounds in deionized (DI) water at the desired concentration and room temperature. Hydrogen peroxide and iron oxide were added simultaneously at
the beginning of each experiment. pH was adjusted using HCl (1 N) or
NaOH (5 N). At pre-determined time intervals, samples were taken
from the beaker for analysis.
2.3. Analysis
ð5Þ
For n = 3, the reaction will be:
FeOðOHÞ + 3H →Fe
Ethylene glycol is used in large quantities as a car cooling fluid or
as an airplane and runway deicer. Deicer large quantities of ethylene
glycol have created environmental hazards leading to the serious
pollution of drinking water [1,16,21,22].
Several types of industrial waste contain phenols. They are very
harmful and highly toxic towards microorganisms [6,8]. Many phenol
compounds are used as solvents or reagents in industrial processes
and are therefore very common contaminants in industrial wastewater and contaminated drinking water sources.
ð6Þ
The nano-iron oxide particle (such as FeOOH) oxidation mechanism includes dissolution, a step that requires proton, which binds to
the iron oxide surface and releases ferric ion into the bulk solution.
The dissolution of iron oxide increases with a decrease in pH solution.
The high affinity of protons to structural O2− helps release the iron
atom from the surface to the bulk solution. The detachment of ferric
ion to the solution initiates a chain of reactions, called Fenton-like
reagents, as shown in Fig. 1.
The objective of this work was to investigate the ability to reuse
iron oxide nanoparticles in the H2O2/iron oxide system for the
mineralization process based on Fenton-like reagents (Fig. 1). The
stability of iron oxide nanoparticles in de-ionized water with
dissolved organic solution was studied as a function of pH solution
and was correlated with the particles' average size. The optimal pH
value for the mineralization process was determined. The organic
model compounds selected for the current study were phenol and
ethylene glycol.
2. Experimental
2.1. Materials
Iron (3)-chloride hexa-hydrate, FeCl3·6H2O and hydrogen peroxide, 30%, were purchased from Merck, Germany; phenol, analytical
grade (Fluka) and chemically pure ethylene glycol were purchased
from Bio Lab Ltd., Israel. All chemicals were analytical grade and were
used as received.
Multi N/C 2100/2100 analyzer (Analytik Jena, Germany) was used
to determine total organic carbon (TOC).
pH solution was measured with a Consort C931 pH meter.
Imaging was done with Transmission Electron Microscopy (TEM)
using a FEI Tecnai T12 G2 TEM, with low electron dose at an
acceleration voltage of 120 kV. Characterization of the dispersion was
made using Cryogenic Transmission Electron Microscopy (cryo-TEM).
Samples were prepared in a controlled-environment vitrification
system (CEVS) [12,25].
Total iron concentration was measured by the HACH DR2010
Portable Data logging spectrophotometer using method 8008 with
FerroVer Iron Reagent Powder Pillows.
NanoZS-3600 (Malvern Instruments Ltd., UK) was used to
determine average particle size, size distribution and zeta potential
of the FeOOH particles.
The preparation procedure of FeOOH nanoparticles is explained in
detail elsewhere [4].
3. Results and discussion
3.1. Iron nanoparticle characterization
In order to characterize the particles in their natural environment,
a cryogenic microscope was used. Cryo-TEM sample preparation
involves the fast-freezing of thin liquid films of solution and therefore
does not produce drying artifacts. Fig. 2 illustrates cryo-TEM images
and the electron diffraction pattern of the synthesized iron
nanoparticle.
From these microphotographs, mostly the particles having a rodlike morphology with average lengths of 50 ± 10 nm are observed.
The rod-like crystals were identified as either α-FeOOH or β-FeOOH
from the electron diffraction patterns.
As expected, beside the immediate hydrolysis that takes place to a
strong acid and a weak base, slow hydrolysis also occurs that could
Fig. 1. Reaction scheme for mineralization organic pollutants using FeOOH as a catalyst (based on [14]). R represents organic matter.
Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016
A. Bach et al. / Desalination xxx (2010) xxx–xxx
3
Fig. 2. TEM images of FeOOH samples synthesized with distilled water and the corresponding electron diffraction pattern. The iron oxide nanoparticles were at a concentration of
200 mg/L as Fe.
take days and even months for its termination. These results are in
good agreement with Wagner et al. [23] and Jean-Pierre et al. [11].
The slow hydrolysis was monitored not only with an increase in
conductivity and a decrease in pH, but also with color changes in the
nanoparticle solution, from yellowish to red. Particle size analysis
indicates differences in particle size only during the first few hours,
after which it remained stable and did not change over a lengthy
period of time. Fig. 3 illustrates the continuous size distribution of
FeOOH nanoparticles. The horizontal axis shows size distribution in
nm, while the vertical axis shows the partial volume fraction as
calculated from light intensity distribution.
All of the above samples were taken from the same freshly made
200 mg/l as ferric iron solution and were monitored with time. The
first measurements were taken each hour between the fourth and
sixth hour and the last measurement was taken after one month.
According to Fig. 3, after 4 h, the sample contains mainly small
particles (around 15 nm) and a much less amount of larger particles
(around 50 nm). An hour later the distribution changes and displays
an almost even distribution between the small and large particles.
However, after 6 h it seems like the small particles disappears and a
mono modal peak appears, indicating the existence of one population
of particles around 50 nm. That size distribution seems to be constant
as for it didn't change over long periods of time (even after one
month). The same behavior repeated itself with different Iron oxide
nanoparticles samples after different aging periods. The high
solubility of smaller particles may lead to their transformation into
larger ones via solution, a process called Ostwald Ripening [20].This
thermodynamically-driven spontaneous process occurs because
larger particles are more energetically favored than smaller particles.Large particles, which posses greater volume to surface area ratio,
represent a lower energy state (and have a lower surface energy). As
the system tries to lower its overall energy, molecules on the surface
of a small (energetically unfavorable) particle will tend to diffuse
through solution and adhere to the surface of larger particles.
Therefore, the smaller particles continue to shrink, while larger
particles continue to grow until equilibrium is reached.
3.2. Solution stability
The correlation between pH solution, and the stability and average
size of iron oxide particles was studied. It is important to find out the
conditions where the particles are stable as nano-particles and where
they aggregate to form larger particles.
Fig. 4a and b shows the zeta potential versus the pH of samples of
de-ionized water and water containing an organic model compound
(ethylene glycol or phenol), respectively. The two compounds were
chosen to represent common industrial water pollutants. The
concentration of the iron oxide particles was 200 mg/L as iron in
both solutions and the organic compounds concentration was
200 ppm as TOC. Solutions with zeta potential higher than 30mV or
Fig. 3. Size distribution over time of freshly prepared iron nanoparticle solution generated by dynamic light scattering. Red- After 4 Hours, Green- After 5 Hours, Blue- After 6 Hours,
Black- After 1 Month. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016
4
A. Bach et al. / Desalination xxx (2010) xxx–xxx
Fig. 4. a. Zeta potential as a function of pH in de-ionized water solution.
[Fe]Total = 50 mg/L. b. Zeta potential as a function of pH in organic model compounds
solution. [Fe]Total = 50 mg/L, [TOC]0 = 200 mg/L.
Fig. 5. a. Average size versus pH in de-ionized water solution. [Fe]Total = 50 mg/L.
b. Average size versus pH in organic model compounds solution. [Fe]Total = 50 mg/L,
[TOC]0 = 200 mg/L.
lower than −30 mV are normally considered stable. Between these
two values the solution is considered to be unstable. The stability and
instability of the solutions were examined by raising the pH from 2 to
8.7 and reducing it back to 2.
The de-ionized water zeta potential results (Fig. 4a) indicate that
between a pH of 2 and 4.7, the solution could be regarded as stable.
The pH range at which the solution is stable decreases when
organic model compounds are introduced into the solution (Fig. 4b).
Results are identical for the two organic compounds that were
checked. Here, the stability ranges between pH 2 and 3.2. Both
solutions become unstable at the point the particles would most likely
aggregate. Both samples display almost complete overlap when
decreasing or increasing solution pH.
When comparing average particle size shifts with increasing pH
(Fig. 5a and b), it is clear that the result of average size in both
solutions corresponds to the results in Fig. 4a and b. Above a pH of 4.5
(Fig. 5a) and 3.2 (Fig. 5b), the solutions become unstable and the
average particle size increases dramatically from around 50 nm and
100 nm to a few tens of microns respectively.
Both solutions show clearly the reversible effect by displaying that
there is no hysteresis when raising or lowering the solution pH.
The iron oxide particles' surface is positively charged when pH is
kept in the stability range. When base is added to either solution, it is
attracted to the surface of the suspended particles, therefore, the
particles do not repel each other and have the tendency to aggregate.
These aggregates are growing with pH increase (Fig. 5a, b) and finally
sediment due to gravity. Under these conditions, both solutions
become unstable (Fig. 4a, b). However, the formation of those
aggregates could be overturned; hence, adding an acid causes a
reverse affect. First the aggregates become separated and thus the
suspended particles have a greater positive charge which induces a
rejection between them.
The average size of iron oxide nanoparticles in the organic model
solution was greater compared to the size in de-ionized water (for the
Fig. 6. Ethylene glycol and phenol mineralization versus pH. [H2O2]/[Fe]Total = 16,
[TOC]0 = 200 mg/L, t = 120 min, [H2O2]0 = 8250 mg/L.
Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016
A. Bach et al. / Desalination xxx (2010) xxx–xxx
same pH) (Fig. 5a, b). It has been proven previously [5] that iron oxide
has the ability to adsorb organic matter, therefore it is suggested that
the difference between average particle sizes for both solutions is
probably due to the adsorption of dissolved organic matter onto the
iron oxide nanoparticles.
3.3. Iron oxide nanoparticles as a catalyst
Some soluble iron ions exist in aqueous solution due to the
dissolution of iron oxide, as explained earlier. The complexes
mentioned, along with the detachment of ferric ion (due to the
dissolution process) from the solution, initiate a chain reaction, called
the Fenton-like process.
The catalytic behavior of the H2O2/iron oxide system was
investigated versus pH in two organic model solutions: ethylene
glycol and phenol.
The experiments were performed at pH values ranging from 1 to 7.
The weight ratio between hydrogen peroxide and total iron was 16.
The percentage removal of total organic compound (TOC) as a
function of the solution pH for both organic compounds is shown in
Fig. 6.
Generally, the results show that the Fenton-like reaction in the
presence of the H2O2/iron oxide system efficiently mineralizes the
organic model solutions. The behavior of both organic models as a
function of pH is similar.
5
The highest mineralization was achieved for both organic models
between pH 2.5 and 3. In the optimal pH, nearly 95% and 85% of the
ethylene glycol and phenol solutions respectively were mineralized.
Phenol and ethylene glycol mineralization was not observed at pH
above 7 and below 1 within the reaction time applied (120 min).
These results is suitable with previous studies that have shown that
Fenton's process is most effective at an acidic pH between 2 and 4,
with an optimum pH of approximately 3 [19,24]. Lucking et al. (1998)
[13] in his study was demonstrated that the oxidation properties of
H2O2 and hydroxyl radicals depend on the solution's pH. At a solution
pH of 0 the redox potential for H2O2 and hydroxyl radicals are 1.77 V
and 2.8 V respectively. At a solution pH of 14 the redox potential for
H2O2 and hydroxyl radicals is 0.88 V and 2 V respectively. As a result
of this fact, acidic conditions are applied for oxidative treatment of
wastewaters with H2O2.
Regarding nano-catalyst aging, it was observed but not shown
here that even after two years, the iron oxide nano-catalyst in solution
can mineralize organic pollutants with the same efficiency as after
they are produced.
3.4. Iron oxide nanocatalyst recovery
A series of experiments was conducted in order to investigate the
catalytic behavior of the virgin and recovered iron oxide nanoparticles
in successive catalytic runs. The results are summarized in Fig. 7a and
b. The weight ratio between the hydrogen peroxide and the total iron
was 16 and the pH values ranged from 2.5 to 3. Seven mineralization
cycles are demonstrated both of ethylene glycol (Fig. 7a) and phenol
(Fig. 7b).
In both samples, aqua solution was separated from iron oxide
particles after the mineralization process. Removal of the Iron oxide
nanocatalyst was performed by means of precipitation via adjusting
solution pH level to 4.0–4.5 (according to previous results, see Fig. 4b).
After its recovery, the spent iron oxide nanocatalyst was used once
again for a second catalytic run. No iron nanocatalyst residues were
found in the treated solutions after their recovery. The glass with the
spent iron particles was filled with fresh solution of the organic model.
The fresh solution pH was then adjusted to 2.75–3.0 (according to
previous results, see Fig. 6) by adding HCl, after which solutions were
mixed for 3 h at room temperature in order to obtain optimal acidity
required for catalytic mineralization of the organic matters. This
procedure was repeatedly performed for an additional six catalytic
runs. Results are showing that at least seven stages of catalytic
mineralization-recovery cycles can be made without a reduction in
the catalytic properties of the Iron oxide nanocatalyst. All seven cycles
showed that nearly 95% and 85% for ethylene glycol and phenol
solutions were mineralized respectively. Higher mineralization may
be achieved at modified conditions.
The use of the reported technique for cleaning water from organic
matter requires catalyst recovery which is possible through sedimentation or filtration. The consumption cost of hydrogen peroxide, acid
and base needed for the process should be compared with other water
cleaning techniques in order to provide the best economical solution
to a problem. Obviously, the reported technique may be used in order
to clean relatively small streams of industrial or maybe secondary
treated wastewater.
4. Conclusions
Fig. 7. a. Ethylene glycol mineralization with time as a function of recovery rate of
FeOOH particles. [H2O2]/[Fe]Total = 16, [TOC]0 = 200 mg/L, [H2O 2]0 = 8250 mg/L.
b. Phenol mineralization with time as a function of recovery rate of FeOOH particles.
[H2O2]/[Fe]Total = 16, [TOC]0 = 200 mg/L, [H2O2]0 = 8250 mg/L.
The disadvantage of a classic Fenton process derives from the fact
that when a homogeneous catalyst is added as an iron salt, it cannot
be retained in the process, thus causing additional water pollution. In
our study, the use of a nanocatalyst was investigated. Based on
characterization and batch experiments with the organic model
solutions, it was suggested that the iron oxide nanoparticle could
enable us to both separate it from the final solution and reuse it in at
Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016
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A. Bach et al. / Desalination xxx (2010) xxx–xxx
least six more successive catalytic runs. Further investigations are
needed to study these results with real water treatment, which
consist of various inorganic and organic constituents that may affect
the efficiency of reused nanoparticles.
The optimal pH value for mineralization of nearly 95% and 85% of
the ethylene glycol and phenol solutions, respectively, was found to
be 2.8.
Acknowledgements
The authors wish to acknowledge with thanks the partial support
of this research project by the Ministry of Science, the Peres
Foundation for Peace and the Hurovitz Foundation.
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Please cite this article as: A. Bach, et al., Characterization of iron oxide nanocatalyst in mineralization processes, Desalination (2010),
doi:10.1016/j.desal.2010.05.016