(I)     
Electrochemical Oxidation (direct/indirect)

Investigated parameters: current
density and reaction temperature in electrochemical reactor (Körbahti and Artut, 2010),
initial pH and cell voltage (Yan et al., 2011).

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(II)    Electro-Fenton

In the
Electro-Fenton process, Fe(II) is oxidized by H2O2 to form Fe(III).
This lead to forming a hydroxyl radical (HO•) and a hydroxide ion (OH?) in the process as
well. In the next step Fe(III) is then reduced back to Fe(II) by another
molecule of H2O2, forming a radical of (HOO•)
and a proton (H+). The main effect of adding H2O2 is
to create two different oxygen-radical species, with water (H+ + OH?)
as a byproduct (Ishak
and Malakahmad, 2013).

 

Fe2+ + H2O2
? Fe3+ + HO• + OH?

(1)

Fe3+ + H2O2
? Fe2+ + HOO• + H+

(2)

In the
second reaction free radicals of HOO are produced. Hydroxyl radical (HO•)
is an authoritative, strong, and non-selective oxidant which can start the new
reactions rapidly.  Oxidation of an
organic compound by Fenton’s reagent can be done very quickly but it involved
with exothermic reactions that results in increasing the temperature of the
solutions. The main purpose of this process is to oxidation of pollutants to
primarily carbon dioxide and water (Kavitha, V., & Palanivelu, K., 2005). Generally, Fe(II) sulfate (FeSO4)
is used as catalyst in the reactions. In case of electro-Fenton process,
hydrogen peroxide is produced in situ from the electrochemical reduction of
oxygen. Also, Fenton’s reagent during the radical substitution reaction is used
in organic synthesis for the hydroxylation of aromatic hydrocarbon (Casado et al., 2005). For instance, classical conversion
of benzene (C6H6) into phenol (C6H5OH)
can be expressed as: (Casado
et al., 2005)

C6H6 + FeSO4
+ H2O2 ? C6H5OH

(3)

Meinero and Zerbinati
(2006) investigated
the oxidative and energetic efficiency of various electrochemical oxidation
processes. The electro-Fenton process was verified to have the best degradation
efficiency in terms of energy consumption: for that case the specific energy
consumption was 0.3 kWh/g of COD, corresponding to 41.8 kWh/m3.

Many
works classified electro-Fenton or the very Fenton process as advanced
oxidation process (AOP). Some of AOPs are, electro-Fenton process, TiO2/H2O2,
photocatalysis reactions, etc., that are chemical oxidation processes mainly
used as an attractive pretreatment method to improve the biodegradability of
various industrial discharges, that is able to generate and use hydroxyl free
radicals (•OH) as strong oxidant (Klamertha
et al., 2010; Sin et al., 2011).

The application of AOPs not only
reduces the COD load and contaminants levels in wastewater, but also generates
fewer toxic effluents. Besides, AOPs augment the biodegradability of wastewater
through forming intermediates similar to the metabolic pathway substances (Ollis, 2000). Advanced oxidation process (AOP)
which employ strong oxidant agents (ozone, hydrogen peroxide and UV, Fenton,
etc.), are able to remove organic and phenolic pollutants of the Olive Mill Wastewater
(OMW) (Madani et al. 2015).

The
Fenton process could be enumerated as one of the promising alternative
oxidation methods because of its cost efficiency, operation simplicity, lack of
residue, and ability to treat a spectrum of substances. Fenton process, which
is in fact a synthesis of oxidation and coagulation reaction, reduces toxicity
and COD concentration using hydrogen peroxide and ferrous sulfate (Madani et al. 2015). To be specific, the oxidation
mechanism by the Fenton process is due to the generation of hydroxyl radical in
an acidic solution by the catalytic decomposition of hydrogen peroxide and in
presence of ferrous (II) ions (Ledakowicz
et al., 2001).

Fenton’s
reagent (a solution of hydrogen peroxide (H2O2) and an iron catalyst (like FeSO4,
iron electrode, FeSO4.7H2O (ferrous sulfate
heptahydrate), etc.)) is used to oxidize contaminants or organic compounds in
wastewaters such as trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PCE), and
refinery wastewater to augment biodegradability. The Fenton reaction is shown
in Eqs. (4) to (13). At acidic pH it leads to the production of ferric ion and
hydroxyl radical (Ishak
and Malakahmad, 2013):

H2O2 + Fe2+ ? Fe3++ •OH + OH-

(4)

Fe3+ + H2O2 ? Fe-OOH2+ + H+ ? •H2O
+ H+

(5)

Hydroxyl radicals may be scavenged
by reaction with another Fe2+ or with H2O2:

•OH + Fe2+ ? OH? + Fe3+ 

(6)

•OH
+ H2O2 ? H­O2 • + H2O

(7)

Hydroxyl
radicals may react with organic and starting a chain reaction:

•OH
+ RH ? H2O
+ R• (RH=organic substrate)

(8)

R•
+ O2 ? ROO• ? products of
degradation

(9)

Ferrous ion and radicals are produced during the reactions:

H2O2 + Fe3+ ? H+ + FeOOH2+

(10)

FeOOH2+ ? HO2• + Fe2+

(11)

HO2• + Fe2+ ? HO2? + Fe3+

(12)

HO2• + Fe3+ ? O2 + Fe2++
H+

(13)

Ishak and Malakahmad
(2013) showed
that Fenton process is able to augment the biodegradability of refinery
wastewater as a pretreatment for recalcitrant contaminants. Studied operational
parameters were reaction time (20 – 120 min), H2O2/COD (2 – 12) and
H2O2/Fe2+ (5 – 30) molar ratios. They
determined that BOD5/COD as an index of biodegradability of
wastewater increased from 0.27 to 0.43 under optimum conditions of operational
parameters, including reaction time (71 min), H2O2/COD (2.8) and H2O2/Fe2+
(4) molar ratios: the process was optimized using response surface methodology
based on a five-level central composite design.

In addition to low biodegradability of petroleum refinery
wastewater, the higher concentration of COD in characterized refinery
wastewater is because of presence of some compounds such as phenols and
sulfide. So, such wastewater with low BOD and high COD is consider as low
biodegradability wastewater (Metcalf and Eddy,
2003). Moreover, considering high concentration of some contaminants
including oil and grease; Benzene and Toluene as PHCs; Ethylbenzene and Xylene
as aromatic hydrocarbons, it could be implied that the petroleum wastewater or
other oily wastewaters containing biorecalcitrant contamination or heavy metals
requires pretreatment before application of any biological decontamination (Ishak and Malakahmad, 2013).

According to Ishak and Malakahmad
(2013), although the range of time factor in Fenton process was from 20
to 120 min, the results revealed that in the first 20 minutes of the Fenton
reaction, more than 90% of COD and BOD removal was achieved. Also, BOD5/COD
ratio of 0.40 was attained within 20 minutes. This finding, shows very short
period of time required for a significant biodegradability improvement and
pollution reduction in a Fenton process which is of special interest in the
industrial application of Fenton’s reagent: hydroxyl free radicals
bear a the short half-life, so the extension of reaction time
does not improve degradation.

Even though by increasing of H2O2
concentration better organic degradation will be attained due to more
generation of more hydroxyl radicals (Kang and
Hwang, 2000), at a certain limit, the complete organic removal could not
be obtained even with higher than stoichiometric quantity of H2O2/COD
and this eventually led to reducing the removal efficiency. Generally, it means
biodegradability declined after increasing H2O2/COD molar
ratio to more than 2 (Ishak and Malakahmad, 2013).

Regarding the third studied influential factor, i.e. H2O2/Fe2+,
it has been verified that both peroxide dose and iron concentration (Fe2+)
are influential factors in the Fenton reaction for better degradation efficiency  and reaction kinetics, respectively (Kavitha and Palanivelu, 2005; Siedlecka and Stepnowski,
2005). In that experiment, decrease of H2O2/Fe2+
molar ratio (i.e. higher concentration of Fe2+) caused more
biodegradability and higher removal of the target compound and formation of
early intermediates, i.e. generating more hydroxyl radicals for the degradation
process (Catalkaya and Kargi, 2007; Ishak and
Malakahmad, 2013). Excessive amount of Fe2+ competes with the
organic carbon for hydroxyl radicals when high Fe3+ concentration is
used.