Micro Reactor for Chlorophenols

Electrocatalytic Oxidation

O. Radovicia, A. Banua, C.Pirvua

a
Politehnica University from Bucharest, ROMANIA, Splaiul Independentei 313

Abstract

Chlorophenols are universal priority pollutants listed by all

Environmental Agencies. The removal of chlorophenols from
contaminated waters is carried out by microbiological and
chemical methods. In this work is presented a combination of three
methods that are electrochemical oxidation, electrochemical
reduction and photo-Fenton-oxidation. Results performed in a
micropilot reactor showed that it is feasible to degrade
chlorophenols by these combined methods, with a removal rate of
82%, at a current density of 6.66 Adm-2. The current efficiency
was calculated and the results indicated that this combined method
is more efficient that separated ones (electrochemical oxidation or
reduction).

Introduction

Chlorophenols represent one of the most important water pollutants. These

compounds are present in waters from different industries, also they could be present in
domestic water since they are widely used as pesticides, disinfectants and antiseptics.
Chlorophenols can be oxidized by chemical, electrochemical, photochemical and
microbiological processes. However biological treatment of waste waters containing
chlorophenols requires longer retention time and in the case of higher concentration, the
activated sludge formation can be intoxicated and destroyed.
There has been numerous investigation using alternative processes to oxidize organic
compounds including chlorophenols called Advanced Oxidation Processes (AOP).
Among these we mention electro-Fenton and electro-photo-Fenton reaction which are
based on the use of a very strong oxidizing agents such as hydroxyl radical OH· which
is generated in situ in the reaction medium. Hydroxyl radicals are able to non selectively
destroy most organic pollutants until their complete mineralization.
Electrochemical oxidation and reduction are most effective technique for the direct
production of OH· via anodic oxidation at Pt, PbO2, diamond electrodes (1,2,3), or
indirect generation utilizing Fenton’s reagent (4,5,6,7) in which the O2 electro generated
at the cathode reacts with Fe+2 ions dissolved in the aqueous solution leading to the
formation of hydroxyl radicals. The effectiveness of this treatment is enhanced if is
utilized a O2- diffusion cathode (8, 9).
The recent developments of anodic oxidation and electro-Fenton reaction take into
consideration the effect of coupling with other processes such as photo-electro- Fenton

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(10, 11) which present great capacity of oxidation or mineralization of aqueous
toxicants.
The Fenton reaction is (among AOP) one of the most investigated. The Fenton
reagent is a mixture of H2O2 and ferrous ions (Fe+2) which produces OH· radicals
according to the equation:

Fe +2 + H2O2 → Fe+3 +OH- +OH· ↔ Fe(OH)+2 + OH· [1]

In addition, Fenton reaction efficiency can be enhanced in two ways:

a. In electro-Fenton processes the electrolysis generates the hydrogen peroxide, at
the PTFE oxygen electrode (cathode):

O2 + 2H+2 +2e- → H2O2 + Fe +2 → Fe(OH)+2 + OH· [2]

This is electro- Fenton reaction.

b.In the presence of UV irradiation. The iron product in the reaction [1] can
be reduced regenerating Fe+2 and producing more OH· radicals:
hν
Fe (OH) +2
⎯⎯→ Fe +2 + OH· [3]

This is photo-Fenton reaction.

Hydroxyl radical with the second highest oxidation potential 2.33V, (which lead to
the unselective oxidation property) permit complete degradation of phenolic compounds
to organic acids and CO2.
The major advantage of Fenton’s reagent as a hazardous treatment technology is that
both, H2O2 and Fe +2 are cheap and nontoxic, and also no mass transfer limitation due to
its homogeneous catalysis nature.
The electro oxidation of chlorophenols at different electrodes (Pt, Au, C etc) was
studied by different authors by cyclic voltametry (12-17). The results obtained indicate
that the oxidation of chlorophenols depends on the pH, the chlorophenol concentration,
the number of chloride atoms in the aromatic ring and of the position of chloride atom
with respect to the phenolic OH. At low scan rate and/or high chlorophenols
concentration phenoxy radicals condense forming polymers or at least oligomers
(dimmers) that precipitate at the anode surface, the resulting film behaving as an
insulator “passivating” the surface, or at high potential scan rate or low chlorophenols
concentration the film is porous which allow the charge transfer to continue.
The aim of our work is to provide information about the efficiency of anodic
oxidation and photo-electro-Fenton method for decontamination of some important
chlorinated contaminants (2-chlorophenols) contained in waste or underground waters
utilizing an electrochemical micropilot reactor coupled with reservoir provided with an
UV lamp. To test the efficiency of proposed method, waters spiked with pollutants in
laboratory were tested under laboratory and micro pilot reactor condition.

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 Materials and Methods

Materials

Platinum anode 99,99%-15 cm2 surface cathode oxygen diffusion electrode

carbon/Pt/PTFE made of carbon felt ETEK 12.5x12.5 cm impregnate with carbon
powder Vulcan XC-72 and N-PTFE dispersion (Du Pont).
Chemical used
Chemicals were obtained from Aldrich Chemical and Fluka purity: sodium sulphate
99%Fluka, 2-chlorophenol Sigma Aldrich 98%.
Analytical procedure

The Total organic carbon (TOC) of treated contaminated water was obtained by a
Shimazu 5050 TOC Analyzer.

Methods

Photo- electro- Fenton

The method is based on an oxidation reaction with hydroxyl radicals (Fenton
reaction). The treatment device consist of a continuously operated electrochemical micro
reactor (electrochemical cell) coupled with a reservoir provided with a UV radiation
emitting fluorescent lamp (1). The schema of the micro pilot plant unit is presented in
figure 1.

Figure 1. The schema of the electrochemical micro pilot unit

The electrolyte solution was recirculated in the reactor system with 60, 120 and 180
Lh-1 respectively. Working temperature was 35ºC. The solution was irradiated in UV

3
reactor (reservoir) with a fluorescence lamp (Philips 10W) with radiation emission of
300-400 nm. The electrochemical cell reactor is presented in the figure 2.

Figure 2. The electrochemical cell reactor

As is shown in the figure 2 the electrochemical cell consists of two compartments.

First compartment represents electrochemical cell itself, with platinum anode 15cm2 and
C-PTFE cathode 15cm2. The second compartment represents the oxygen compartment
which fuels the oxygen diffusion electrode. For decontamination were utilized aqueous
solutions of 2-chlorophenol in 0.05M Na2SO4. The concentration of 2-chlorophenol was
as follows: 100ppm; 200ppm and 300ppm. The electrolysis was performed
galvanostatically at: 0.3A, 0.7A and respectively1.0A.

Results and discussions

Due to catalytic activity of many metals oxides (PtO/PtO2) the anodic oxidation is the
most utilized electrochemical method for decontamination of organic pollutant including
chlorophenols. This method utilizes electrodes as Pt, PbO2, BOD (diamond) to favor
generation of hydroxyl radicals adsorbed at its surface from water oxidation by reaction
[4] see (18,19).

M + H2O →M─OH· + H+ + e [4]

M─OH·→MO- + H+ [5]
MO- ↔MO + e [6]
2MO↔ 2M + O2 [7]

The hydroxyl radical is a powerful oxidizing agent that reacts with chlorophenols
via hydroxylation until their transformation in CO2 and organic acids. In the same time

4
the Pt anode covered with PtO/PtO2 act as a catalyst and favors the formation of a
polymeric film on the electrode surface.
The electro-Fenton method as an advanced oxidation process is based on
continuous supply to a contaminated aqueous solution of hydrogen peroxide (H2O2)
formed from the reduction of the O2 gas at an O2 diffusion cathode (see eq.3).
A small quantity of Fe+2 added to the solution strongly increase the oxidation power of
electrogenerated H2O2.
Electrochemical reaction [2] can takes place using a two electrodes undivided
recirculating cell. In this case the generation rate of H2O2 is controlled by a (PTFE)/O2 -
diffusion cathode [4, 8, 20, 21]. Chlorophenols are then oxidized by the combined action
of OH· produced at the anode by means of reaction [4] and catalytic activity of PtO/PtO2
(formation of polymers) and in homogeneous medium by means of Fenton reaction.
Photo-electro-Fenton is a distinct method where contaminated solution is irradiated with
UV light of λ=300-400nm that causes the photo-Fenton reaction of Fe(OH)+2 the
predominant species of Fe+3 in acid medium, as is shown by equation [3] ( 22, 23).
Hydroxyl radicals can thus be produced at high rate by simultaneous reactions [1]
and [3].

Degradation of 2-chlorophenols

The degradation experiments of 2-chlorophenols performed with our micro

reactor are illustrated by table 1 and figures 3-5.
For different values of flow rate of working solution i.e. 60, 120 180 Lh-1 were found
similar results which show that electro-Fenton process is not affected by diffusion of
reactants. The figure 3 shows the significant role played by the applied current on the
degradation rate of 2-cholorophenols. A faster decay rate of pollutant destruction with
the increasing current can be observed (from 62% for i=0.3A to 81.4% for i=1A after 5 h
of electrolysis).

Figure 3. The influence of applied current on degradation rate of 2-chlorophenols (a-1A;

b-0.7A; c-0.3A)

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Table 1 Electrochemical results of 2-chlorophenol degradation
Current Voltage Flow rate Electrolysis TOC Degradation
density V L/h Time Efficiency
2
A/dm h ppm %
6.66 21 120 0 302
6.66 21 120 1 144 52.2
6.66 21 120 2 122 59.5
6.66 21 120 3 82 72.8
6.66 21 120 4 65 78.4
6.66 21 120 5 57 81.1
6.66 21 60 0 298
6.66 21 60 1 143 52.5
6.66 21 60 2 120 60.1
6.66 21 60 3 81 73.1
6.66 21 60 4 66 78.1
6.66 21 60 5 55 81.7
6.66 21 180 0 301
6.66 21 180 1 145 51.8
6.66 21 180 2 110 63.5
6.66 21 180 3 82 72.8
6.66 21 180 4 68 77.4
6.66 21 180 5 56 81.4
4.66 15 120 0 303
4.66 15 120 1 175 42.2
4.66 15 120 2 138 54.2.
4.66 15 120 3 114 62.5
4.66 15 120 4 96 68.5.
4.66 15 120 5 85 71.1
2.0 6 10 120 0 301
2.0 6 10 120 1 203 32.5
2.0 6 10 120 2 163 45.8
2.0 6 10 120 3 138 54.1
2.0 6 10 120 4 125 58.4
2.0 6 10 120 5 114 62.1
6.66 21 120 0 200
6.66 21 120 1 115 42.5
6.66 21 120 2 74 63
6.66 21 120 3 52 74
6.66 21 120 4 42 79
6.66 21 120 5 36 82
6.66 21 120 0 102
6.66 21 120 1 72 29.4
6.66 21 120 2 53 48.0
6.66 21 120 3 39 61.7
6.66 21 120 4 21 79.4
6.66 21 120 5 16 84.3

Accordingly to the results presented in the figure 4 which represent combined

processes of reduction and oxidation which were used to degrade 2-chlorophenols, is
shown that 2-chlorophenols can be degraded more than 82% in 5 hours.

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Figure 4. Efficiency of degradation reaction photo-electro-Fenton of 300 ppm of
2-chlorophenols at 1A electrolysis current: A- for each hour of electrolysis; B- total
efficiency

Our results compared with others (24-27) indicated that the combination between
reduction (electro-Fenton) and oxidation was better that single reduction or single
oxidation for degradation of 2-chlorophenols. However we may point out that cathodic
electro-Fenton reduction has much better effect on the degradation of 2-chlorophenols
than anodic oxidation. The cathodic reduction has much higher current efficiency than
anodic oxidation.
It was observed that during electrolysis at the anode 2-chlorophenols is not degraded,
but rather appears to undergo a coupling reaction to form water insoluble high molecular
weight coupling products, which cover the anode surface. The polymer product may be
collected as a deposit film from the anode surface. This type of collection as well as
removing the organic compound from the waste stream suggest that far higher loading
of 2-chlorophenols toxicants can be removed, than are possible by other processes.

Current efficiency
The power consumed during degradation of 2-chlorophenols could be calculated
using the following equation:

(
P kWhm −3 = ⎢)
⎡ EIh ⎤ TOC in
⎥ [8]
⎣ V ⎦ ∆TOC

where E =cell voltage (volts), I= cell current (amps) V=volume (m-3);

TOC in total organic carbon at t=0; ∆TOC=TOCin – TOC at t. The results are presented in
figure 5.

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Figure 5 Power consumption for total degrading of 300ppm 2-chlorophenols solution at:
A-0.3A; B-0.7A and C-1A

From this figure can be seen that lowest power consumed was for o.3Amps for the
first hour of degradation process, 8KWhm-3 and 30KWh m-3for the 5th hour of
degradation. Comparing our results with the power consumption during anodic
degradation of 2-chlorophenols (502 KWh m-3) founded by E. Brillas et al. (25) it is
obvious the great efficiency of the combined photo-electro-Fenton process for
chlorophenols degradation.

Conclusions

Photo-electro-Fenton combined reduction and oxidation processes were used to

degrade the waste water of 2-chlorophenols. Results indicated that the electro photo
Fenton process was much better that single reduction oxidation processes for the
degradation of chlorophenols or intermediates.
Results showed that the energy cost of anodic oxidation was greater that than of
electro photo Fenton process and so the electro-photo-Fenton process is an effective and
feasible method for degradation of chlorophenolic contaminants.

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