chapter 2 review of literature -...
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CHAPTER 2
REVIEW OF LITERATURE
2.1 GENERAL
In 4000 BC, usage of iron began in Egypt. The utilization of bronze
tools vanished in the period from the 12th
to 10th
century, which was replaced
by iron, leading to ‘Iron Age’, a profound age for civilisation.
Today iron has become an integral part of our civilisation. With
respect to wastewater treatment, different oxidation states of iron especially
0,+2,+3 and +6 have been used widely. This chapter gives the background
information related to the use of different oxidation states of iron (0, +2 and
+6) in the degradation of xenobiotic organic compounds. The literature
pertaining to current research on these lines are reviewed and summarized
here.
2.2 FERRATE
Ferrate is an amethyst colored solid having the chemical formula
FeO42-
. The oxidation state of iron in ferrate is +6. Since, it is the highest
oxidation state of iron; it is called supervalent/hypervalent iron. Ferrate was
first discovered by Georg Ernst Stahl, Stahl found that when a mixture of
potassium nitrate (salt peter) and iron powder is ignited and dissolved in
water, a purple solution is obtained (Stahl 1715). Later, in the 19th
century,
Edmond Frèmy, again stumbled upon a water soluble purple colored solid
which was formed after the fusion of potassium hydroxide and iron oxide
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(Frèmy 1844). The dark purple colored solution is similar to that of potassium
permanganate, but is a better oxidizing agent than permanganate. The only
relatively recent contribution of that nature, by Russian authors, describes
preparation of potassium ferrate by calcination of a mixture of ferric oxide
and potassium peroxide at 350-370 °C under oxygen flow (Kiselev et al
1989).
Ferrate ion has tetrahedral co-ordination and is isostructural with
chromate and permanganate ions. The presence of four covalently bonded
oxygen atoms surrounding the iron atoms (Goff and Murmann 1971 and
Hoppe et al 1982) is responsible for the high oxidizing ability than other
common oxidants.
The redox potential of ferrate is compared with that of other
commonly used oxidants like ozone, hydrogen peroxide, permanganate,
hypochlorite, chlorine and oxygen in Figure 2.1. Some of the chemicals listed
in the figure like chlorine and hypochlorite are also used as disinfectant. From
the figure it is obvious that ferrate is a better oxidant (oxidation potential – 2.2
V) when compared with other chemicals. Ferrate has also been used for
disinfection purpose in many studies (Murmann and Robinson 1974, Gibert et
al 1976, Schink and Waite 1980, Jiang et al 2001).
The half cell reaction for ferrate reduction potential (Wood 1958) is
FeO42-
+ 8 H+ + 3 e
-Fe
3+ + 4H2O (2.1)
and the potential is 2.20 V ± 0.03 V at 25°C.
The stability of ferrate ion is highly dependent on pH. Wood in
1958, proposed the rapid exothermal decomposition of ferrate in highly acidic
medium. In alkaline medium (pH 10), the exchange of oxygen ligands with
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that of water is the slowest (Goff and Murmann 1971). Lee and Gai (1993)
found that the lowest rate of reduction of potassium ferrate was in the pH
range of 9.4 to 9.7.
Figure 2.1 Comparison of oxidation potential of ferrate with other
chemical oxidants
Till now, three protonated forms of ferrate has been reported (Rush
et al 1996) in 0.02 M phosphorous acetate buffers at 25°C
(Equations 2.2 to 2.4 )
H3FeO4+
H+ + H2FeO4 pK1 = 1.6 ± 0.2 (2.2)
H2FeO4 H+ + HFeO4
- pK2 = 3.5 (2.3)
HFeO4-
H+ + FeO4
2- pK3 = 7.3 ± 0.1 (2.4)
Including FeO42-
, there are four species of ferrate in the pH scale of
0 – 14. In highly acidic pH, dihydrogen ferrate as well as trihydrogen ferrate
ion exists. Whereas, in the case of basic pH, ferrate ion as well as hydrogen
15
ferrate ion (HFeO4-) predominates in the solution. The possibility of
formation of low concentration of diferrate was also reported by Carr et al
1985, Ernst et al 1979 and Rush et al 1996, in 0.2 M phosphate buffer.
2 FeO42-
+ 2 H+
Fe2O72-
+H2O (2.5)
Above pH 10, a decrease in the stability of ferrate ion is reported
(Graham et al 2004). This might be due to the formation of anionic species of
ferrate (Fe (OH)4- and Fe (OH)6
3-) instead of solid ferric hydroxide.
The kinetic constant for ferrate decomposition from neutral to
highly alkaline pH range was experimentally calculated by Li et al 2005. The
values of t½ suggests that as pH increases from 7.1 to 9.4, there is increase in
the stability of the ferrate ion. Above 9.4, up to 12 there is decrease in the
stability. This study reinforces the results obtained by Graham et al 2004.
Due to its less stability in neutral and acidic solutions, ferrate
decomposes spontaneously at a very rapid rate as per the Equation 2.6
(Williams and Riley 1974).
4FeO42-
+ 10 H2O 4 Fe(OH)3 + 8 OH- + 3O2 (2.6)
As the ferric hydroxide formed is innocuous in nature, ferrate is
also termed as environmental friendly oxidant (Sharma 2002).
There are three main applications of ferrate with respect to water
and wastewater treatment. They are:
i. Coagulation
ii. Disinfection
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iii. Degradation of xenobiotics
i Coagulation
According to Jiang et al 2006, the two important unit processes for
water and wastewater treatment are coagulation and oxidation/disinfection.
Coagulation destabilizes colloidal contaminants and transfers small particles
into large aggregates and adsorbs dissolved organic materials onto the
aggregates, which is finally removed by sedimentation and filtration.
Ferric hydroxide, which is the byproduct of ferrate reduction is
known to be a good coagulant. They are more preferred when compared to
aluminium salts, as they are able to produce more rapid floc growth, leading
to rapid settling characteristics. Ferrate has been successfully used as
coagulant for water and wastewater treatment (Jiang and Lloyd 2002, Jiang et
al 2001, Sharma 2002, Stanford et al 2010). For drinking water treatment,
ferrate removed 10-20% more UV254abs and DOC than ferrous sulphate for
the same dose compared in natural pH range 6 and 8 (Jiang et al 2006 a). In
sewage treatment also, ferrate removed 50% more Vis400abs and 30% more
COD when compared to ferrous sulphate and aluminium sulphate at the same
dosage (Jiang et al 2006 b).
ii Disinfection
Conventionally used disinfectant chlorine/hypochlorite was found
to be harmful as they form chlorinated by products which are toxic in nature.
Hence, studies are being carried out to find an alternative disinfectant.
Recently, ferrate has been widely experimented upon as an alternative
disinfectant for both water and wastewater (Schink and Waite 1980, Jiang et
al 2002, Sharma 2007, Sharma et al 2008, Bandala et al 2009, Murmann and
Robinson 1974, Gilbert et al 1976, Jiang et al 2001, Jiang et al 2006 (a) and
17
(b), Kazama 1995, Jiang et al 2003). Ferrate can inactivate 3-log10 more
bacteria when compared with aluminium sulphate or ferrous sulphate for
same dosage (Jiang et al 2006) in sewage treatment as well as drinking water
treatment (Jiang et al 2006).
iii Degradation of xenobiotics
In the past two decades, many researchers have used ferrate to
degrade wide range of xenobiotics. Owing to its innocuous nature of
reduction i.e., through electron transfer, ferrate is considered as green oxidant
which is not yet reported to have formed any harmful byproducts with
pollutants during degradation studies. Hence, the focus has completely shifted
onto ferrate as in near future it might be used as conventional oxidant for the
treatment of wastewater in most of the treatment plants. Altogether almost 69
compounds are found to be effectively degraded/removed by potassium
ferrate (Sharma et al 2002).
Ferrate is capable of degrading harmful endocrine disrupting
chemicals (EDCs) and drug related compounds also (Hu et al 2004, Jiang et al
2005, Lee et al 2005, Sharma et al 2006). The oxidation of estrone (E1), 17 -
estradiol (E2) and 17 -ethynylestradiol (EE2) was studied using ferrate as a
function of pH and dosages. The results suggest that pH 9 is the most
favorable condition to obtain the highest removal efficiency and complete
removal of organic pollutants can be obtained with ferrate:compound ratio
>3:1. The effectiveness of ferrate for the oxidation of phenolic EDCs was also
confirmed in both natural and wastewater (Lee et al 2005).
A list of chlorophenols which were reported to be oxidized by
ferrate is given in Table 2.1. Along with optimal pH, as pH plays a crucial
role in the efficiency of oxidation by ferrate.
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Table 2.1 List of chlorophenols degraded with potassium ferrate
Degradation
efficiency (%)
Chlorophenol Reference Optimal
ferrate:chlorophenol
ratiopH 5.8 pH 10.0
CP
(0.047mM CP)
5:1 52 89
DCP
(0.037mM DCP)
5:1 70 72
TCP
(0.031mM TCP)
Graham et
al 2004
5:1 86 50
From the table, it could be inferred that the degradation of
chlorophenols is highly dependent on pH and also requires high (5:1) ferrate:
compound ratio. Graham et al (2004) had prepared ferrate solution by
dissolving solid ferrate in distilled water prior to use, this might also lead to
loss of some ferrate activity.
Recently, ferrate coupled methods for degradation of recalcitrants
have been evolving. Already notable amount of research have been carried
out with ferrate and photocatalysis using TiO2 and UV (Sharma et al 2001,
Sharma and Chenay 2008, Winkelmann et al 2008). Ferrate helps in
photocatalysis by absorbing an electron from conducting band and this leads
to formation of Fe5+
which happens to be much more reactive than ferrate
Fe6+
. Coupling of ferrate with sonication has not yet been reported.
Similarly, Kinetic study and stable intermediates have also not yet
been reported in the case of chlorophenols degradation by ferrate.
2.2.1 Sonochemical degradation
Ultrasound (US) is the sound wave above the normal human
audible range. Its frequency is in the range of 20 – 500 kHz. US has been
found to enhance the chemical reactivity of the reactants (Lormier et al 1991).
19
Sonochemistry is the branch of chemistry which deals with the chemical
effects produced by subjecting a chemical reaction to sound waves especially
ultrasound waves (Bremner 1990).
In recent years, increasing attention has been focused on the
ultrasonic (US) process in wastewater treatment, owing to its greater
efficiency in decomposition of the refractory organic compound (Petrier et al
1992, Serpone et al 1992, Lin et al 1996). Chlorinated compounds such as
CCl4 (Francony and Petrier 1996), trichloroethylene (Drijvers et al 1999),
pentachlorophenate (Petrier et al 1992) and CFCs are the most studied class of
compounds.
Young et al (1997) decomposed 2CP (1.26 10-4
M) in aqueous
solution by ultrasonic waves. They reported that the decomposition is rapid in
acidic solutions where molecular 2CP dominates, part of the molecular
species may evaporate into the gaseous region (cavitation bubble), therefore
the overall decomposition of 2CP in acidic solution is considered to take place
in both the gaseous and film regions of the cavitation bubble.
Lin et al (1996) studied the decomposition of 2CP in aqueous
solution by US/H2O2 process. The decomposition rate of 100 mg/L of 2CP
was 99% with H2O2 (200 mg/L) at pH 3, after 360 min of reaction.
When an aqueous solution is exposed to ultrasound, large pressure
gradients occur within the liquid causing the transient expansion and
rarefaction of micro sized bubbles as shown in Figure 2.2. These bubbles are
called cavitation bubbles. This phenomenon is called acoustic cavitation
(Lormier and Mason 1988).
It was estimated that 4 × 108
bubbles / sec / m3
are produced. The
bubbles are of the order of 10 to 200 µm in diameter and they are short lived,
20
with a lifetime of around 10 µsecs. Therefore the bulk of the solution remains
unaffected (Suslick and Hammerton 1986). It has also been reported by them
that pressure of around 1000 atms and temperature of 5000 K are generated
on collapse of these bubbles.
Figure 2.2 Formation of a liquid microjet during bubble collapse near
an extended surface
The cavitation bubble generated contains the vapor from the
solvent and the solute (Makino et al 1983).
The efficiency of ferrate method is increased due to the following
reactions.
a) Direct pyrolytic degradation of solute molecules occurs due
to the enormous changes in both temperature and pressure
during the collapse of the cavitation bubble. The
fragmentation of the solute results in generation of reactive
free radicals, which further causes, the degradation of other
solute molecules.
b) Pyrolysis of water vapor yields hydroxyl and hydrogen
radicals (Equation 2.7). The hydrogen free radical reacts
with oxygen molecule to form hydroperoxyl free radical as
per Equation 2.8 (Ince et al 2001).
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H2O•OH + H
• (2.7)
H•+O2 HO2
• (2.8)
The hydroperoxyl and hydroxyl radical combines
individually to form hydrogen peroxide (Minero et al 2005)
as per equations 2.9 and 2.10.
2•OH H2O2 (2.9)
HO2•
H2O2 (2.10)
c) The atomic oxygen formed in Equation 2.11 leads to the
formation of hydroxyl radical in the presence of ultrasonic
waves (Equation 2.12).
O2 2 O• (2.11)
O• + H2O 2
•OH (2.12)
d) Super critical water oxidation: Super critical water is a
phase of water that exists above its critical temperature and
pressure, 647 K and 221 atm. This unique state of water has
different density, viscosity and ionic strength properties
than water under ambient conditions. Since the organic
contaminant has an increased solubility within super critical
water, these organic species are brought into close
proximity with the oxidant, usually oxygen from dissolved
air. Oxidation is therefore accelerated. During sonolysis, it
was proposed that super critical water is present in a small
thin shell around the bubble. According to the Hoffmann et
al (1996), this mode of destruction is expected to be
))))))
))))))
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secondary in importance because the fraction of water in the
super critical water state is estimated to be in the order of
0.0015 parts out of 100 parts of water. Alternatively, the
volume of the gaseous bubble was estimated to be 2 × 104
times greater than the volume of the thin super critical water
shell surrounding the bubble.
2.3 FERROUS IRON
Ferrous iron has divalent oxidation state. It is most commonly used
in wastewater treatment as Fenton’s method. The history of Fenton’s
chemistry dates back to 1894, when Henry J. Fenton reported that hydrogen
peroxide could be activated by ferrous salts to oxidise tartaric acid (Fenton
1894).
2.3.1 Fenton process
The Fenton’s reagent, mixture of hydrogen peroxide and ferrous
ion generates hydroxyl radical (•OH) at acidic condition according to
Equation 2.13 (Walling 1975).
Fe2+
+ H2O2 Fe3+
+ OH- +
•OH (2.13)
The hydroxyl radical formed reacts with the organic compound and
Fe2+
separately to produce active organic free radical and Fe3+
ion. The
reaction is shown in Equation 2.14.
•OH + RH H2O + R
• (2.14)
Equations 2.15 and 2.16 reveal the cyclic reduction and oxidation
reaction of iron, in which Fe3+
is converted to Fe2+
when it reacts with the
organic free radical. This results in the oxidized product formation. The same
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organic free radical converts Fe2+
back to Fe3+
and the free radical converts
back to its original form. This is a termination reaction for the active organic
free radical.
R• + Fe
3+ Fe
2+ + Oxidized Product (2.15)
R• + Fe
2 + Fe
3+ + RH (2.16)
To carry out the Fenton process efficiently, many authors illustrated
that the pH of the solution should be controlled between 2 and 4 (Sedlack and
Andrew 1991, Kochany and Bolton 1992).
The Fenton process, which is the ferrous – hydrogen peroxide
oxidation has been used for several decades to remove the hazardous organic
compounds from industrial wastewater (Kochany and Bolton 1992, Zhu et al
1996). The major advantage of Fenton’s method is that the reagent
components are easy to handle and are environmentally benign.
2.3.2 Heterogeneous Fenton
Kuznetsova et al (2004) studied the catalytic oxidation of organic
compound using H2O2 over heterogeneous Fenton type catalyst. A zeolite
named as FeZSM-5 was selected as the most active heterogeneous Fenton
type catalyst. The catalyst was active in oxidation of organic substances at pH
1.5-5, maximum activity was observed at pH 3. The FeZSM-5 effectively
oxidized a simulant warfare agent, diethylnitrophenil phosphate, which is
hardly detoxified by other methods. Most of the ferrous ions in zeolite are
heterogeneous and hence no complexation with phosphates occurred and was
stable during 30 catalytic runs and being active in a wide pH range.
Sabhi and Kiwi (2001) reported that the degradation kinetics of
DCP on Nafion-Fe membranes was more favorable than photo assisted
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Fenton process. At optimum pH 5.4, complete mineralisation of DCP
occurred within 1 h. The Nafion-Fe membrane was effective over many
cycles during the photo catalytic degradation of DCP without leaching out of
iron into solution.
The immobilization of iron species onto different types of ionic
exchange resin has been studied in order to expand their application for the
photodegradation of various dye pollutants with visible light and H2O2.
Whereas the cationic dyes are efficiently photodegraded on a cationic resin
exchanged Fe3+
catalyst (Fenton like method), the anionic dye is preferably
photodegraded on an anionic resin supported catalyst. In addition, a notable
dissolution of Fe species into the solution upon UV light irradiation is a
serious problem that needs to be faced in the future study about the
immobilized Fe3+
catalyst (Xuejun et al 2005).
Iron-containing SBA-15 catalyst consisting of crystalline hematite
particle oxides supported onto a mesostructured silica matrix has been shown
as a promising catalyst for the treatment of phenolic solutions through photo-
Fenton processes. The outstanding physico-chemical properties make this
material more attractive than unsupported commercial hematite iron oxide
catalyst, leading to a better overall photocatalytic performance. The
experimental design model carried out for different levels of catalyst and
hydrogen peroxide concentrations has demonstrated that the increase of
catalyst loadings from 0.5 to 1.5 g/L does not promote a significant
enhancement of TOC removal except for strong oxidant conditions. The
stability of the catalyst towards the leaching of iron species is strongly
dependent on the oxidant to catalyst ratio. For the highest catalyst and
hydrogen peroxide concentrations (1.5 g/L and 4100 ppm, respectively), the
amount of iron species detected into the aqueous solution does not exceed 8
ppm. This low leaching is accompanied with a complete removal of aromatic
25
compounds and a TOC conversion of ca. 80% after 240 min of reaction,
which is a remarkable result. The contribution of homogeneous photo-Fenton
reactions from the iron species leached out from the catalyst has been proven
to be poorly relevant (Martinez et al 2005).
To understand the photodegradation of azo dyes in natural aquatic
environment, a novel photo-Fenton-like system, the heterogeneous iron
oxide–oxalate complex system was set up with the existence of iron oxides
and oxalate (Jing et al 2006). Five iron oxides, including -FeOOH, IO-250,
IO-320, IO-420 and IO-520, were prepared and their adsorption capacity was
investigated in the dark. The results showed that the saturated adsorption
amount was ranked the order of IO-250 > IO-320 > -FeOOH > IO-420 > IO-
520 and the adsorption equilibrium constant (Ka) followed the order of IO-
250 > IO-520 > -FeOOH > IO-420 > IO-320. The results showed that the
photodegradation of orange I under UVA irradiation could be enhanced
greatly in the presence of oxalate. The optimal oxalate concentrations (C0 ox)
for -FeOOH, IO-250, IO-320, IO-420 and IO-520 were 1.8, 1.6, 3.5, 3.0 and
0.8 mM, respectively.
A novel supported iron oxide, prepared using a fluidized-bed
reactor (FBR), was utilized as a catalyst of the heterogeneous photoassisted
Fenton degradation of azo-dye Reactive Black 5 (RB5) (Hsueh et al 2006).
This catalyst is much cheaper than Nafion-based catalysts, and can markedly
accelerate the degradation of RB5 under irradiation by UVA ( = 365 nm).
The effects of the molar concentration of H2O2, the pH of the solution and the
catalyst loading on the degradation of RB5 are elucidated. A simplified
mechanism of RB5 decomposition that is consistent with the experimental
findings for a solution with a pH of up to 7.0 is proposed. About 70%
decolorization was reported and 45% of the total organic carbon was
eliminated on the surface of the iron oxide at pH 7.0 after 480 min in the
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presence of 0.055mM RB5, 5.0 g iron oxide/L, 29.4mM H2O2, under 15W
UVA
Pretreatment of wastewater of industrial origin has been carried out
on structured silica fabrics that have been exchanged with Fe-ions
(Bozzi et al 2002). Evidence is presented that the treatment under light
irradiation in the presence of added oxidant (H2O2) and the silica loaded
fabric involves fast recycling of the Fe-ions back on the fabric. The ratio
BOD5/TOC after treatment was enhanced more favorably by the EGF/Fe
(0.4%) fabric than by homogeneous Fenton reagent pointing out to higher
biodegradability attained by heterogeneous photocatalysis in spite of the fact
that TOC reduction was more marked in homogeneous media.
Through cation exchange reaction, hydroxyl-Fe pillared bentonite
(H-Fe- P-B) was successfully prepared as a solid catalyst for UV-Fenton
process (Chen and Zhu 2007). Compared with raw bentonite, the content of
iron, interlamellar distance and external surface area of H-Fe-P-B increased
remarkably. Heterogeneous UV-Fenton catalytic degradation of azo-dye Acid
Light Yellow G (ALYG) was investigated in aqueous using UVA (365 nm)
light as irradiation source It was reported that almost 100% discoloration and
more than 65% TOC removal of 50 mg/L ALYG could be achieved by
heterogeneous UVA-Fenton system in 120 min. The iron leaching rates of H-
Fe-P-B were all below 0.6% in multiple runs in the degradation of ALYG,
which indicated that the heterogeneous catalyst had long-term stability and
activity. Another advantage of this catalyst was its strong surface acidity,
which made the range of pH for heterogeneous UV-Fenton system extended
from 3.0 to 9.0. The results indicated that the H-Fe-P-B was a promising
catalyst for heterogeneous UV-Fenton system.
A review article on heterogeneous Fenton catalysts based on clay,
silica and zeolites (Navalon et al 2010) reports the vast amount of work
27
carried out on heterogeneous Fenton catalysts on the above mentioned support
materials. Clays such as montmorillonites, bentonite, saponite and synthetic
clay and hydro calcites are used as support material for ferrous by researchers
worldwide. Zeolites (alumino silicates) like ZSM5 and Zeolite y), also porous
silica can also be used for heterogeneous Fenton. From the review, it is
evident that all the three (clays, zeolites and mesorporous silica) are capable
of being efficient heterogeneous Fenton catalysts. Even though, their
operating cost less, the cost of montmorillonites, zeolites and mesoporous
silica is high. A very low cost and effective solid support is the need of the
hour.
2.3.3 Alginate
E.C.C. Stanford, British Pharmacist, discovered alginates in 1880s.
From 1929 onwards industrial production began in California. Alginic acid
and its salt occur mainly in marine brown algae (Pheophyta) and comprise
40% of its dry weight. Macrocystis porifera and Ascophyllum nososum are
major raw material for the major production of alginate in the world
(McNeely and Pettitt 1973). Alginate consists of polymeric chain made up of
polymannuronic and poly guluronic chains (blocks). It has been proved that
proportion of mannuronic and guluronic acid ranged from 0.34 to 1.79 (Haug
et al 1974). Thickening and gelling are the most important property of
alginates. It is hydrophilic, in water they swell, thickening the solution and
thus increasing its viscosity.
In alginate gels, hydrogen bridges between carboxyl groups are
organized in zones of fusion joining the adjacent polysaccharide chains; the
chains aggregate due to the formation of multiple bonds with ions, whose
arrangement is similar to ‘egg box’ structure. The main fields where alginates
are used are in the food, textile, cosmetic, paper and pharmaceutical
industries. Drug consisting of alginate as base even treats duodenal ulcer in
children aged 4-15 years (Miroshnichenko et al 1998). This report proves the
28
harmless nature of alginates to human body and further to the ecosystem.
Therefore alginate was chosen as a novel support material for Fenton catalyst.
The use of alginate for degradation / removal of organic pollutants is a very
new arena for research as only very few literatures are available on this.
Jodra and Mijangos 2003 reported about the use of calcium alginate
– activated carbon composition for removal of phenol from aqueous solution
by adsorption. The composite was prepared by mixing activated carbon with
3% (w/v) sodium alginate solution and followed by dripping of the mixture
into a calcium solution. Spherical beads of activated carbon immobilized in
calcium alginate matrix were obtained. The saturation of adsorbent material
was achieved in less than 30 min.
Alginate beads composed of activated carbon and iron (ferro fluid)
were prepared and used for adsorptive dye removal (Rocher et al 2010). The
Langmuir equation fitted well for the adsorption data with maximum
adsorption capacities of 0.02 mmol/g for methyl orange and 0.7 mmol/g for
methylene blue. It had equilibrium time of 60 min. The same authors reported
previously (Rocher et al 2008) about the removal of organic dyes by magnetic
nanopraticles and activated carbon encapsulated alginate beads. These beads
had equilibrium time of 3 h.
Another group of researchers (Lin et al 2005) had also reported
about the removal of organic compounds by alginate gel beads with entrapped
activated carbon. From the reported works, it is evident that alginate is a good
supporting material which can be used for water pollution abatement.
The synergistic effect of sonication in enhancing the degradation
efficiency has already been discussed in the previous section. Apart from
using a new support material, sonication has also been coupled with the
heterogeneous Fenton, to enhance its efficiency further. Factors, which affect
the sono Fenton process, are the ultrasonic wave frequency, Fenton’s reagent
ratio and pH (Lormier et al 1990).
29
The coupled ultrasonic process with the Fenton process increases
the decomposition efficiency and reduces the time required for the removal of
pollutants. The coupled method has been shown to be efficient for the
removal of hazardous organic compounds (Lin et al 1996).
During the last few years, several interesting studies concerning
heterogeneous Fenton and sonolytic degradation were reported and are listed
in Table 2.2 and 2.3. From the list, it is evident that degradation of
chlorophenols with ferrous alginate by heterogeneous and sono heterogeneous
Fenton has not yet been reported. Hence, alginate was used as support
material for this research work.
2.4 ZERO VALENT IRON
Zero-valence state metals (such as Fe, Zn, Sn and Al) are effective
agents for the remediation of contaminated groundwater for removal arsenate
and chromate (Powell et al 1995, Warren et al 1995). Zero-valent iron in
particular has been the subject of numerous studies over the last 10 years and
is becoming an increasingly popular choice for treatment of hazardous and
toxic wastes. The earliest report recording the use of zero valent state metal to
remove organic contaminant was in 1972.
In the patent literature, iron was first recognized as a chlorinated
pesticide degrader (Sweeny and Fischer 1972). In 1981, iron powders were
used to degrade various chlorinated hydrocarbons, such as trichloroethylene
(TCE) (Sweeny 1981a, 1981b). Additional suggestions for using zero valent
iron to degrade trichloroethylene and trichlorotethane were made in the late
1980’s (Senzaki and Kumagai 1988). It was reported that the TCE
degradation rate increased when increasing the ratio of iron mass to the
influent flux. However, little focused work on the application of zero valent
iron to remediation of polluted groundwater was reported until the early
1990’s (Reynolds et al 1990, Gillham et al 1992). These laboratory studies
30
indicated that ZVI could degrade a wide range of dissolved halogenated
compounds, with reaction rates of three to eight orders of magnitude higher
than the naturally-occurring abiotic degradation process. All the laboratory
studies indicated the great potential of using ZVI for contaminant removal
and led to the first field application of ZVI for in situ remediation of TCE-
contaminated groundwater at a Canadian Air Force Base, Ontario in 1991.
This trial involved use of ZVI in a permeable reactive barrier (PRB) and
achieved 95% TCE removal across the ZVI PRB (Gillham et al 1992).
In 1993 a patent was lodged by the University of Waterloo for
using zero valent iron for treating contaminated groundwater in-situ,
demonstrating the identification of zero valent iron as a remediation
constituent. Based on the success of the Canadian field demonstration, the
first commercial ZVI PRB was installed at an industrial site in California for
the removal of chlorinated hydrocarbons in groundwater in 1995 (Powell et al
1998).
31
Table 2.2 Heterogeneous Fenton process for wastewater treatment
Sl. No. Compound Conditions Major findings Reference
Chlorophenols
1 4-CP
4-cp=1.4 mM
H2O2=0.23 mM
pH 7.5
Hg UV lamp = 97 mW/cm2
Immobilized iron (Fe2+
) on Nafion as a
photocatalyst was demonstrated for the
degradation of 4-CP. The new photocatalyst
showed high efficiency at neutral range. It was
found that increased thicknesses of the
membrane did not improve the efficiency but
increasing the surface area improved the DOC
reduction. Reusing the membrane after the
treatment was possible and the photocatalyst
showed resistance to ageing
Maletzky et al
1999
2 2,4-DCP
DCP= 72mg/L
H2O2=10 mM
Nafion-Fe= 1.78%
pH= 2.8-11
suntest soloar simulator = 80
mW/cm2
The degradation kinetics of DCP on nafion-Fe3+
membranes was more facourable than photo-
assisted fenton process. At the optimum pH 5.4,
complete mineralization of DCP occurred within
1h. The nafion-Fe was effective over many
cycles during the photocatalytic degradation of
DCP without leaching out of Fe3+
ions onto the
solution. Results indicated that the degradation at
the surface of the nafion-iron membrane was
controlled by mass transfer and not by chemical
species of the solution
Sabhi and Kiwi
2001
32
Table 2.2 (Continued)
3 2-CP
2-CP=0.31 mM
H2O2= 1.96 mM
Geothite = 0.2 g/L
pH = 3
NaClO4 = 0.1 M
2-CP was effectively degraded by Fenton-like
oxidation using goethite as a catalyst at an acidic
pH. Degradation increased with increasing
concentration of goethite due to more surface
active sites for reductive dissolution. Dissolution o
f iron was enhanced in the presence of ligand like
oxalate and ascorbate, which act as a reductant
causing the conversion of ferric to ferrous, thereby
propagating the Fenton reaction. The oxidation
mechanism was found to be surface controlled for
2-CP
Lu et al
2002
4 2-CP
2-CP=15 mg/L
H2O2=9.8mM
Iron oxide = 1 g/L
Catalytic decomposition of hydrogen peroxide and
2-CP was studied in the presence of iron oxides
namely granular ferrihydrite, goethite and
hematite. The catalytic activity for hydrogen
peroxide decomposition follows the order:
granular ferrihydrite>goethite>hematite; whereas
for 2-CP degradation, the hematite exhibited
highest catalyzing power. Results showed that
ferrihydrite exhibited strong diffusion resistance
which was attributed to the microporous structure
or to the formation of oxygen in the pores of iron
oxide.
Huang et al
2001
5 2,4,6-TCP
TCP=62 mM
Iron tetrasulphophthalo cyanine
(FePCS)/TCP = 0.074
pH=7
H2O2/TCP=10 mol
TCP was effectively degraded to CO2 by hydrogen
peroxide catalysed Fe PCS. At 90 min, more than
69% of was recovered in the aqueous phase, 13%
was present in the hydrophobic phase and the
remaining was accounted for CO and CO2
Sorokin et al
1996
33
Table 2.2 (Continued)
Other organic pollutants
6 Phenol
Phenol 10-4
M
Fe(III)-HY = 0.25%
H2O2 = 10-2
M
0.25 wt.% Fe(III)-HY is active for the
degradation of phenol at pH = 6.
Heterogeneous Fe(III)-HY efficiency over
homogenous photo-Fenton system, that can
be applied at pH > 3. The enhanced activity
of heterogeneous Fe(III)-HY system due to
the synergistic effect of zeolite by adsorption
of pollutant molecules facilitating the rate of
degradation. No leaching of Fe from the solid
catalyst into the solution
Noorjahan et al
2005
7 Phenol
Fe resin catalysts
Temperature = 40- 80oC
Catalyst loading rate = 0.5-5
g/L
H2O2 conc. = .05-.3 mol/L
95% Phenol removed and 75% COD
removed by using Fe (III) resin catalyst
process.
Liou et al
2005
8 Phenol
a) Iron containing SBA-15
material
b) Amorphous SiO2-Fe2O3
mixed oxide
c) Iron containing zeolite
material (SiO2-FeO3)
Temperature maintained at
25oC
Phenol conc.= 0.5 g/L
H2O2 conc.= 2450 ppm
pH = 6
Reaction carried out in a pyrex batch
cylindrical magnetically stirred reactor
Irradiation performed with a mercury lamp
(UV). Stability of material in terms of metal
leachability was evaluated throughout the
photo-Fenton reaction. Activity and stability
depends on the environment of iron species
and features of silica support.
Martinez et al
2007
34
Table 2.2 (Continued)
9 Phenol
Fe-ZSM-5 = 0.4 g/L
H2O2 = 1.5 times Fe
Time = 360 min
pH = 5
Total elimination of phenol and significant
reduction of DOC at 90°C was achieved by
using Fe-ZSM-5 as a heterogeneous catalyst.
The reaction rate depends on the degree of
hydroxylation of aromatic compound in
contact with the catalyst. At the end of the
reaction period, no aromatic by product was
present whereas aliphatic compounds
accumulated. The concentration of ferric ion
leached was extremely low at the optimum
pH, while at low pH significant leaching
occurred which resulted in loss of catalyst
from the solid support.
Kuzretsova et al
2004
10 Benzoic acid
BA = 1.5-1.8 mM
H2O2= 11-12mM
Iron oxide
pH = 3.5
time = 50 min
Oxidation of BA by hydrogen peroxide was
performed with -FeOOH as a catalyst
supported on brick grains in a circulating
fluidized bed reactor- the oxidation rate of
BA was dependent on both hydrogen
peroxide and BA concentration. It was
affected strongly by Ph changes due to
ionization fractions of surface hydroxyl
groups. Results indicated that heterogeneous
oxidation of BA plays a dominant role at pH
4.4-7.0, while at acidic pH, homo generous
oxidation predominates due to reductive
dissolution of -FeOOH.
Chou et al
2001
35
Table 2.2 (Continued)
11
- (4- pyridyl-1-
oxide)-n-tert-butyl
nitrone
4-POBN= 2.6-8.8 mM
Crushed goethite +silica = 3.5 +
66.5 g
Peat = 0.14 g
H2O2 = 0.13 mM
The activity of Fenton driven hydroxyl
reactions was estimated in batch suspensions
comprised of silica sand, crushed goethite, peat
and hydrogen peroxide using 4-POBN as a
model compound. The hydroxyl radical
production was greater in peat amended
systems than in unamended control. The
presence of iron-rich organic phase in natural
sediments increased the rate of Fenton-
dependent contaminant oxidation
Huling et al
2001
12Trichloro ethylene
(TCE)
TCE = 0.1 mM
Iron perchlorate = 1-15mM
H2O2= 2-10mM
Goethite = 1-5 g/20mL
A standard Fenton’s system, a modified soluble
iron system with a input of peroxide and
goethite catalyzed systems at two different pH
was studied for contaminant degradation using
TCE as a model compound. In standard Fenton
system, 78% of TCE was degraded with not
more than two chlorine was released from TCE
degradation. In modified soluble iron system,
91% of TCE was degraded with two chlorine
per TCE molecule was released. In the goethite
system at pH 3 >99% of TCE was degraded
with near compete release of chloride ion. Only
22% TCE was degraded with minimal release
of chloride by goethite system at pH 7. It is
suggested that goethite catalyzed system at pH
3 can effectively degrade the parent compound
and mineralize the contaminants when used for
in situ and ex situ treatment methods
Teel et al
2001
36
Table 2.2 (Continued)
13Tetrachloro
ethylende (TeCE)
TeCE = 50 – 150 mg/L
Reticulated iron = 13.3 g
H2O2= 0.1%
pH = 5-9
circulating rate 2L/min
Results indicated that more than 99.8% of TeCE was
removed. In wastewater, the concentration of TCE did not
decrease below 1.5 mg/L even after 24 h of circulation. The
concentration was brought down to 0.1 mg/L in 6h by
aeration with Fenton’s reaction. Closed circulation with
aeration-agitation was reported to be effective for practical
wastewater treatment
Takemura et al
1994
14 Quinoline
Quinoline = 10 mg/L
H2O2= 500 mg/L
Iron oxide = 500 mg/L
pH = 7.7
The surface catalysed oxidation of quinoline in the presence
of iron oxide was examined and compared. Among the three
oxides, ferrihydrite, goethite and semi crystalline, the
catalytic activity of goethite was highest towards quinoline
degradation. Efficiency of quinoline oxidation could be
reduced to zero in the presence of high concentration of
humic acid, may be due to scavenging, inhibition of
catalytic site and radical site promoter. The stoichiometric
efficiency relating quinoline loss to peroxide decomposition
was unaffected by the presence of carbonates and
phosphates, although their rates were reduced.
Valentine and
Wang 1998
15 Dye wastewater
Dye = 0.6 g/200mL
H2O2 = 1000mg/L
Fe/MgO = 4-5 g/L
pH = 3
A novel technology was developed to decolorize the dye
wastewater. Both soluble and insoluble dyes were rapidly
decolorized at room conditions by using Fe/MgO catalyst
and H2O2. The DOC of soluble dye decreased from 75 to 0.1
mg/L and in insoluble dye 105 to 20 mg/L. the dye
wastewater from dye manufacturing unit was decolorized by
catalytic oxidation using Fe/MgO catalyst fluidizing in a
reactor. Both COD, BOD were removed from dye
wastewater by catalytic oxidation and their catalytic activity
lasts longer than 30 days in the reactor
Pak and Chang
1999
37
Table 2.2 (Continued)
16 Phenol
a) Fe-TiO2
b) Fe-C-TiO2
Sample powder = 0.1 g
Phenol conc.= 1.1x 10-4
M
Dark reaction for 3 h
3black light blue fluorescent lamps
with 20W power.
Wavelength range = 300–415 nm
H2O2 conc.= 0.03M
Modification of TiO2 photocatalyst by carbon and iron
enhanced the photoactivity for phenol decomposition
under the condition of UV + H2O2 via photo-Fenton
process. The formation of•OH radical photocatalyst was
reduced as compared to TiO2. Due to carbon coating. The
photocatalytic activity of Fe-C-TiO2 was high and
adsorption was low. Carbon-coated TiO2 particles were
beneficial for mounting of iron and its application for
phenol decomposition under UV + H2O2 in photo-Fenton
process.
Tryba et al
2006
17 Phenol
Black light blue fluorescent lamps
= 20 W power
Wavelength range = 300–415 nm
Catalyst = 0.1 g
Phenol conc. = 2.1 x 10-4
M
TiO2–PET was modified by FeC2O4 . Carbon-coated TiO2
catalyst prepared from PET showed high adsorption
capacity. The addition of H2O2 increased the
decomposition of phenol. Thus effective removal of
Phenol took place with the help of two processes
adsorption and decomposition by photo-Fenton reaction.
Tryba et al
2005
18 Phenol
a)TiO2
b)Fe–TiO2
c)Fe–C–TiO2
Photocatalysts loading = 0.2 g /L
Phenol conc. = 2.1 x 10-4
mol L-1.
Reaction in Dark for 3 h
3 black light blue fluorescent
lamps = 20W power
Wavelength range = 300–415 nm
H2O2 conc. = 0.03M
Decomposition of Phenol was faster with TiO2 and Fe–
TiO2. The highest rate of phenol decomposition was
observed with Fe–C–TiO2 (UV+H2O2).The photo-Fenton
process is very efficient in the decomposition of phenol
through the formation of•OH radicals, which have a high
potential of oxidation.
Tryba et al
2006
38
Table 2.2 (Continued)
19 Atrazine
Ferrihydrite
Atrazine conc. = 4.6µ M
Different pH and H2O2 conc.
Sharp increase in the oxidation rate observed from
pH 4-3 where ferrihydrite dissolution is strongly
increased.
Barreiro et al
2007
20
Reactive
brilliant red X-
3B
Fe-Ce oxide hydrate catalyst
X-3B = 100 mg/L
H2O2 = 34 mg/L
Fe-Ce = 1.0 g/L
UV lamp = 36 W
The UV-Fe-Ce-H2O2 system demonstrated better
decolourization as compared to other related
systems. Complete decolourization was seen within
30 min.
Zhang et al
2007
21 2,4,6 TNT
Iron minerals (ferrihydrite,
hematite,
goethite, lepidocrocite, magnetite and
pyrite)
TNT conc. = 2 g/kg
pH = 3
Fe (II) bearing minerals (Magnetite and pyrite)
were more effective than ferric oxides (hematite,
goethite, lepidocrocite and ferrihydrite) for TNT
transformation.
Matta et al
2007
22 Rhodamine B
iron(II) bipyridine complex–clay
hybrid
[Fe(bpy)3] 2+
= 4.4 x 10-5
M
FeBL = 5.0 mg
[H2O2] = 2.0 mM,
[RhB] = 2 x 10-5
M.
Characteristic band of RhB centered at 553 nm was
decreased promptly in the presence of FeBL and
H2O2 upon light irradiation. After 2 h of
photoreaction, the red solution faded completely.
FeBL was re-utilized for 12 times and the aged
catalyst almost retained the catalytic activity as the
fresh one through the process.
Cheng et al
2006
39
Table 2.2 (Continued)
23Textile
wastewater
a) Carbotrat AP
b) Carbotrat Premium
Both the carbons are produced by
Controlled deposition of iron oxides
Catalyst dosages = 100-300 g/L at
different pH
H2O2 conc. = 500-1000 mg/L
Composites of iron oxides/ carbon used in the
treatment of wastewater as heterogeneous catalysts
in the Fenton reaction. Color removal and aromatic
compound removal was achieved by adsorption
and reaction with the catalyst at different pH. 70%
COD removal was achieved at room temperature.
No iron leached to the aqueous phase
Dantas et al
2006
24 Organic
contaminants
Niobia / Iron oxide composite
H2O2 (30% w/v) = 2ml
Water = 5ml
Composite = 30 mg
5 ml sol. of 50mgL-1
methylene blue
and phenol
H2O2 (30% w/v) = 2ml
Composite = 30 mg
Methlyene blue = 10ml
Discoloration of an organic dye was observed.
Methylene blue oxidation showed hydroxylation
until CO2 is ultimately formed to reach
mineralization.
Oliveria et al
2007
40
Table 2.3 Sonolytic degradation studies
Sl.
No
Compound /
MethodConditions Major findings Reference
1
2 CP/ US/H2O2
US=20kHz
2CP=100mg/L
Time=360mins
Temp=25•C
pH=3
The decomposition of 2CP was 99% at
double amplitude (120µm) with H2O2 of
200mg/L. Lin et al
1996
2 2CP/US
US=20kHz
2CP=100mg/L
At pH 3 the decomposition of 2CP was
found to be high. Decomposition rate
decreases slightly at elevated temperature
(60•C).
Young et al 1997
3 DCP & DMP/AOP
US=23 KHz
Amplitude=20µm
pH=3
DCP&DMP= 0.4 mM
The degradation rate of DCP was found to
be in the order of H2O2/Fe2+
/UV > H2O2/Fe2+
> O3/US > O3 O3/UV > UV/H2O2 UV
and for DMP it is in the following order
H2O2/Fe2+
/UV > O3/US > O3 O3/UV >
H2O2/Fe2+
> US UV/H2O2 > UV.
Marina et al
1998
42CP/US/ Fenton’s
method
US=20kHz
2CP=100mg/L
Amplitude=120µm pH=3
Temp=25•C
More than 89% of 2CP was degraded at Fe2+
of 10mg/L and H2O2 of 500 mg/L.Lin et al 2000
5 1,4 - Dioxan/US
1,4 Dioxan=1mM
FeSO4=1mM
Time=2 h
In the presence and absence of iron the
fastest overall degradation of 1,4 Dioxan
took place at 358 kHz.
Michael et al
2003
41
Table 2.3 (Continued)
62,4-D/ Sonoelectro-
chemical
US = 20kKHz
2,4 D = 1.2 mM
50% of 2, 4 D were oxidized in just 60 secs.
Yasman et al 2004
7 Chlorophenols
Sonochemical degradation of dilute aqueous
solutions of 2-, 3- and 4-chlorophenol and
pentachlorophenol has been investigated under
air or argon atmosphere. The degradation
follows first-order kinetics in the initial state
with rates in the range 4.5–6.6 mM min 1
under air and 6.0–7.2 mMmin 1 under argon at
a concentration of 100 mM of chlorophenols.
The rate of degradation was faster in argon than
in air: 90–100% of starting CPs were degraded
by 40 min and virtually 100% by 1 h for
sonolysis under argon, whereas it was 70–80%
by 40 min and 80–90% by 1 h for sonolysis
under air.
Nagata et al 2000
8 2-Chlorophenol
2-CP = 50 mL;
temperature = 300
K; power
position = 20%;
stirrer rate = 200
rpm.
The removal of the pollutant by the catalyst
alone is related to the adsorption which is
increased by increasing the contact time. In
ultrasonic treatment,the pollutant degraded
mostly indirectly by the cavitation process
Entezari et al 2005
42
Table 2.3 (Continued)
9 4-Chlorophenol
100 mg/L of phenol
1 g/L iron powder
100 mg/L H2O2
Frequency = 28 to
600 kHz
The results showed that both the iron powder and
mill scale additions can accelerate the degradation
of 4-CP, although the effect is dependent on the
solution pH. All 4-CP could be decomposed for 2
min at pH = 3 and for 1 h at pH = 5.6. When
ultrasound was applied to 4-CP containing aqueous
solutions, variation in pH value within a range
between 2 and 5.6 gave little effect on its
degradation. The addition of H2O2 in the amount of
100 mg/L increased the degradation ratio from
40% to 65% during the ultrasonic irradiation for 60
min. The combination of ultrasonic irradiation
with the addition of iron powder or mill scale
resulted in a significant enhancement to the 4-CP
degradation rate depending on the amount of H2O2
addition to the solution. Initial pH also affected the
degradation rate. The remarkable results were
obtained when 1 g/L of iron or mill scale powder
was added to the 4-CP solution containing 100
mg/l of H2O2 at pH = 3. Under these conditions, 4-
CP was completely decomposed within 2 min of
ultrasonic irradiation when its initial concentration
in the solution was 100 mg/L.
Liang et al
2007
43
Table 2.3 (Continued)
10 2-Chlorophenol
2-chlorophenol = 0.1 mM
H2O2 = 1 mM
Power = 15.1 W
HRP = 0.165 unit/mL
This paper reports a three-step approach to remove 2-
chlorophenol from dilute aqueous solution and compares
each technique. The first step utilizes Horse Radish
Peroxidase (HRP) in presence of hydrogen peroxide to
oxidize this organic pollutant (enzyme treatment). For a
more efficient removal of 2-chlorophenol, it is necessary to
add the enzyme solution gradually to the contents of the
reactor instead of rapid addition. The second step, involving
ultrasonic waves eliminated 2-chlorophenol through
hydroxyl radical generated by the cavitation process (sono-
degradation). In the third step, a combination of ultrasonic
waves and enzyme was used (sono-enzyme degradation). In
enzyme treatment with a concentration 0.165 unit/ml,
approximately 70% of pollutant has been removed from
solution by precipitation in 60 min. Sonication can remove
90% of the pollutant at the same time. In the combined
method, the pollutant is virtually completely removed in
about 30 min.
Entezari et al
2006
11 4-Chlorophenol
4-CP = 500 µM
Ultrasonic power = 30 W
The sonochemical destruction rate of 4-CP is frequency
dependent. Of the range of frequencies studied here (20,
200, 500 and 800 kHz), the highest destruction rate occurs
at 200 kHz. At low frequency (20 kHz), the rate of
degradation almost doubles on decreasing the solution
temperature from 45 to 10 C while at high frequency (500
kHz), the rate of 4-CP degradation is minimally perturbed
over this temperature range but with a slight optimum at
around 40 C.
Jiang et al 2006
44
At present, besides the chlorinated hydrocarbons and hexavalent
chromium, ZVI has been used for the degradation of pentachlorophenol (Kim
and Carraway, 2000), nitroaromatic compounds (Klausen et al 2001), nitrate,
nitrite, bromate and chlorate (Alowitz and Scherer, 2002), degradation of
pesticides (Ghauch 2001), nitro aromatic compounds (Agrawal and Tratnyek
1996), nitrates (Huang et al 1998), chlorinated solvents (Gillham and
O’Hannesin 1994), azo dyes (Cao et al 1999) and chloro organic pollutants
(Ishai 2005) and removal of arsenic (Nikolaidis et al 2003), selenium (McRae
et al 1997) and uranium (Gu et al 1998, Morrison et al 2001) in groundwater
and surface water.
ZVI (Fe0) is a mild reducing agent with reduction potential of –
0.440 V. The ZVI particles exhibit a typical coreshell structure as illustrated
in Figure 2.3 (Li et al 2006). The core consists primarily of zero-valent or
metallic iron while the mixed valent (i.e.,Fe2+
and Fe3+
) oxide shell is formed
as a result of oxidation of the metallic iron. Iron typically exists in the
environment as ferrous and ferric-oxides, and as such, ZVI is a manufactured
material. Thus far, applications of ZVI have focused primarily on the
electron-donating properties of ZVI. Under ambient conditions, ZVI is fairly
reactive in water and can serve as an excellent electron donor, which makes it
a versatile reducing agent.
45
Figure 2.3 The core-shell model of zero-valent iron nanoparticles (Li et
al 2006).
2.4.1 Reductive transformation of inorganic contaminants
ZVI is effective for the reductive transformation of a diverse range
of contaminants including reduction of nitrate to gaseous N2 (Rahman and
Agrawal 1997, Chew and Zhang, 1999, Choe et al 2000), immobilization of
numerous inorganic cations and anions (Powell et al 1995, Pratt et al 1997,
Puls et al 1999, Su and Puls, 2001) and reduction of metallic elements
(Morrison et al 2002). ZVI can immobilize heavy metals in wastewater thus
minimising adverse impacts on receiving waters such as rivers, estuaries and
ocean. Immobilized metals were removed through settlement or filtration
prior to discharge. Industrial wastewater containing excessive amount of
heavy metals can be treated by application of this ZVI system under anoxic
conditions.
46
2.4.2 Reductive degradation of organic compounds
Many studies have demonstrated that ZVI is an effective
technology to completely remove a wide range of contaminated organics in
groundwater and industrial wastewater (Powell et al 1998). ZVI is effective
for the reduction of aromatic azo dye compounds (Nam and Tratnyek 2000,
Cao et al 1999) and other organics such as pentachlorophenol (Kim and
Carraway 2000) and haloacetic acid (Hozakski et al 2001). An extensive
number of laboratory studies and column tests on the degradation of
contaminant organics by zero-valent iron has been published in the last two
decades. There are three possible mechanisms for reductive removal of
halocarbon (RX) as indicated in Figure 2.4
Pathway A: Direct reduction at the ZVI surface Fe(0) provides
electrons to the adsorbed halocarbon(RX) at the metal-water interface, which
results in dehalogenation of halocarbon and production of Fe2+
.
Pathway B: Reduction by ferric iron Fe2+
resulting from corrosion
of Fe0 may dehalogenate RX, thereby producing Fe
3+.
Pathway C: Reduction by hydrogen with catalysis.
Hydrogen (H2) formed in the anaerobic corrosion of Fe2+
might
react with RX if an effective catalyst is present (e.g. Fe0).
47
Figure 2.4 Schematic of possible pathways for the reductive
degradation of halocarbon (RX) (Matheson and Tratnyek 1994)
Reductive dechlorination of trichloroethane (TCE) by ZVI was
observed by Orth and Gillham (1996). The experiment was conducted in a
column containing a mixture of silica sand and zero valent iron. The redox
half reactions considered to be involved are as follows:
Fe0
Fe2+
+ 2e (2.17)
RHCl + 3H+ + 6e RH + 3Cl ( 2.18)
48
The principle degradation product of TCE was ethane followed by
smaller amounts of other chlorinated and non-chlorinated hydrocarbons. The
chlorinated products of degradation include cis-1,2-DCE, trans-1,2-DCE, 1,1-
DCE and vinyl chloride which accounted for less than 4% of the transformed
TCE.
Dechlorination of pentachlorophenol (PCP) in aqueous solutions by
zero valent iron has been investigated by Kim and Carraway 2000. The results
indicated that PCP was transformed to tetrachlorophenol isomers with an
observed first order rate constant of 3.9 ( ± 0.7)×103 h
1 . However, it was
shown that PCP sorption to ZVI probably accounted for 50% or more of the
removal of PCP. As a result, it was apparent that aromatic compounds were
not easily degraded by the ZVI-mediated reductive approach. Oxidative
removal of aromatic organic compounds through a ZVI induced Fenton
process may be a more effective method for degradation of agrochemicals
(e.g. pesticides, insecticides and herbicides). Another constraint of reductive
degradation is the strict requirement for an anaerobic environment, which is
difficult to achieve in surface waters or shallow groundwater. Oxidative
degradation described in the following section provides an alternative
approach to overcoming these problems.
2.5 CONCLUSION
The oxidation states of iron, +6, +2 and 0 have found many
applications in water and wastewater treatment. The degradation takes place
at room temperature and pressure which is an advantage. Also, the reagents
are safe to handle and benign in nature. They are used not only for treatment
of simulated wastewater but also to treat real industrial wastewater and were
found to be satisfactory.
49
It is evident from the review of literature that considerable scope
exists in treating chlorophenols using +6, +2 and 0 oxidation states of iron.
Several aspects like use of liquid ferrate solution, use of acid/alkali instead of
buffer for pH adjustment, coupling of sonication with ferrate and ferrous
alginate, alginate as a support material for ferrous, impregnation of ZVI onto
silica were not focused earlier which paves way for this research work.