21. Oxidant Production on BDD Anodes
and Advanced Oxidation Processes
N. Vatistas, Ch. Comninellis, R.M. Serikawa, and G. Prosperi
Effluents containing low concentrations of biorefractory organic
contaminants require specific treatments to transform selectively
the biorefractory organic species into biodegradable ones or into
fully inorganic species like CO2. The common characteristic of
these specific treatment methods, known as Advanced Oxidation
Processes (AOPs), is the production of the highly active hydroxyl
radical, which oxidizes efficiently these organic species [1]. Even
the process of electrochemical oxidation with boron-doped diamond
(BDD) anodes is due to the hydroxyl radicals produced on its
surface [2], and thus this method also has the characteristic that is
crucial for an AOP [3].
Industrial
wastewaters
with
low
concentrations
of
biorefractory organic species derive from the production of
pharmaceuticals, pesticides, pigments, dyes, wood preservatives and
rubber [4]. Wash effluents derive from the washing of multi-
purpose reactors. Scrubber effluents derive from solutions used to
eliminate
organic
species
from
gaseous
phase
streams.
Wastewaters that derive from two-phase reactions involve an
1
organic phase that contains the products of a reaction and an
aqueous phase that contains small concentrations of biorefractory
organic species.
AOPs include two consecutive steps. In the initial step,
chemical, photochemical or electrochemical energy is transformed
into a higher-level chemical energy by forming highly reactive
hydroxyl radicals [5]. In the subsequent step, these highly active
radicals oxidize efficiently the biorefractory organic species to
biodegradable ones or to fully inorganic species.
Active hydroxyl radicals have been detected on the surface of
BDD anodes, and their action explains the efficient elimination of
organic species [1]. The elimination of the organic species occurs
on the surface of the BDD anode, and thus it has the characteristic
typical of a heterogeneous AOP.
This chapter considers the effluent treatment with BDD
anodes
under
the
wider
point
of
view
of
an
advanced
electrochemical oxidation process in order to point out the
possibilities and limits of this anode in the wastewater treatment
field.
In fact, a new process is described in this work according to
which hydroxyl radicals produced on the BDD surface are trapped
by an oxidizable species like sulfate or carbonate to form the
corresponding peroxide. These peroxides are relatively stable and
can be produced at high concentration in the electrolyte without
any problem of mass transport limitations. The treatment of the
wastewater can take place in a separate chemical reactor; in this
reactor, the peroxide is activated thermally or with UV radiation
2
to produce hydroxyl radicals. These hydroxyl radicals oxidize the
organic pollutants in an AOP. A second possibility is to introduce
in the electrolyte an oxidizable species (like sulfate) during the
electrochemical treatment of the wastewater. In this case, the
peroxide formation avoids the side reaction of oxygen evolution
and can act as a mediator in the oxidation of the organic
pollutants.
21.1. Mass Transfer Limitation in the Direct
Electrochemical Wastewater Treatment Process
Boron-doped diamond has a high overpotential for oxygen
evolution,
in
contrast
to
traditional
anodes.
This
high
overpotential can allow the formation of the active hydroxyl
radical (OH°) by water discharge, according to the following
reaction (eq. 21.1):
H2O  HO° + H+ + e-
(21.1)
Fig. 21.1. Heterogeneous advanced oxidation process on the BDD
anode.
3
As Fig. 21.1 indicates, only organic species that reach the
anodic surface can be oxidized by electrogenerated hydroxyl
radicals. The degradation rate of organics by these hydroxyl
radicals is very fast, and the reaction take place in a thin film close
to the anode surface. This process is heterogeneous in nature, and
consequently it is subject to mass transfer limitations. As the
oxidation of the organic species on the BDD anode surface involves
hydroxyl radicals, the treatment can be considered as an
electrochemical AOP.
In previous work (see Chapter 20 in this book) a model has
been developed permitting prediction of the chemical oxygen
demand (COD) during the electrochemical oxidation of organic
pollutants on BDD under galvanostatic conditions, as shown in
Fig. 21.2.
Fig. 21.2. Schematic diagram of a direct batch electrochemical
wastewater treatment process.
The model assumes that the rate of organic oxidation at the
anode surface is fast and that the reaction is limited by mass
4
transfer. The proposed relation for COD estimation during anodic
oxidation under galvanostatic conditions (iappl>ilim) is given by eq.
21.2:
 Ak
1  
CODt   COD0 exp m t 

 
 VR
21.2
where COD0 is the initial COD value, COD(t) is that after the
treatment time t, A is the surface area of the anode, VR is the

volume of the solution, and  is the ratio of the applied current
density iappl, to the limiting current density ilim.
The limiting current density decreases during the treatment,
and it is related to the COD by eq. 21.5:
ilim t   4FkmCODt 
21.3
An efficient operation mode during the electrochemical
oxidation process is to modulate the applied current density in
order to operate always at the limiting current density. This can
avoid the side reaction of oxygen evolution and allow operation
with a current efficiency of 100%.
Under these conditions, the parameter  of the model
assumes a constant value (=1), and eq. 21.2 can be written as:
 Ak 
CODt   COD 0 exp   m t 
 VR 
21.4
From this relation, the required anodic surface area A, in
order to decrease the chemical oxygen demand from COD0 to
5
CODf, after an electrolysis time t, can be calculated using the
equation:
A
VR COD f
ln
k mt COD 0
21.5
The value of the required anodic surface area A vs. the final
COD value is shown in Fig. 21.3, when VR = 1 m3, t = 1 h, km = 2
10-5 m.s-1 and CODi = 3000 ppm and V = 3 V). The depicted
anodic surface area value vs. CODf, shows that, in order to reach
the required low COD values, high surface areas of BDD anode
must be used.
Fig. 21.3. Anodic surface area and electrical energy vs. final COD
concentration for the treatment of 1 m 3/h of a wastewater with an
initial CODi of 3000 mg dm-3.
6
The required electrical energy (E) for the treatment of 1 m3 of
the wastewater in order to decrease the chemical oxygen demand
from CODi to CODf, is:
E  4 F CODi  COD f V
(21.6)
where V, is the applied electrical potential.
The results depicted in Fig. 21.3 indicate that the direct
oxidation with BDD anodes allows one to use efficiently the
furnished electric energy, but the BDD anodes are not efficiently
utilized.
The mean value of the applied current density imean, indicates
the degree of utilization of BDD anode in this treatment, and its
value is related to the logarithmic mean COD concentration (eq.
21.7):
imean  4km F
CODi  COD f
CODi
ln
COD f
(21.7)
Figure 21.4 shows the values of mean current density vs. final
COD concentration for a given km (2 10-5 m.s-1) and initial chemical
oxygen demand CODi (3000 ppm):
7
Fig. 21.4. Mean current density vs. final COD concentration (km = 2 
10-5 m.s-1; CODi = 3000 ppm).
In conclusion, the low mean values of the applied current
density obtained indicate a low utilization of the rather expensive
BDD anodes during the direct electrochemical wastewater
treatment.
21.2. Peroxide Production on BDD Anodes
Followed by Advanced Oxidation Processes in a
Separate Chemical Reactor
As
has been shown previously, the low concentrations of the
organic species in the wastewater limit the efficient use of BDD
anodes in the direct electrochemical treatment. In this work, an
alternative method is proposed according to which hydroxyl
radicals produced on the BDD surface are trapped by an oxidizable
species like sulfate to form the corresponding peroxide (eq. 21.7):
2HO° + 2HSO4-  S2O82- + H2O
8
(21.7)
Fig. 21.5. Combination of oxidant production on a BDD anode and an
advanced oxidation process (AOP).
These peroxides are relatively stable and can be produced at
high concentration in the electrolyte without any problem of mass
transport limitations. The treatment of the wastewater occurs in a
separate chemical reactor, as shown in Fig. 21.5.
In the reactor, the peroxide is activated thermally or with UV
radiation to produce hydroxyl radicals that oxidize the organic
pollutants; in other words, an advanced oxidation process actually
occurs in the reactor. In the reactor, the peroxide is well mixed
with the wastewater before its activation in order to maximize the
contact between the oxidant and the organic species.
The above combined method of the local production of the
peroxide and the subsequent AOP step avoids the mass transfer
limitation of the direct electrochemical wastewater treatment. An
efficient
wastewater
treatment
of
low
concentrations
of
biorefractory organic species can be reached with this combined
method.
9
Peroxides like hydrogen peroxide, ozone, percarbonate and
peroxodisulfate can be produced efficiently with the use of the
BDD anode. The first two oxidants are normally used in AOPs,
while peroxodisulfate, despite its superior characteristics, has not
been sufficiently considered for this kind of process.
Experimental tests have indicated that, with the use of a nonelectroactive supporting electrolyte (HClO4), hydrogen peroxide
[6], ozone [7,8] and oxygen are easily produced on the BDD anode.
The hydrogen peroxide production is due to the recombination of
two hydroxyl radicals (eq. 21.8) that are just formed by water
discharge, according to eq. 21.1:
2HO°  H2O2
(21.8)
while the ozone production is due to the following reactions:
HO°  O° + H+ + e-
(21.9)
2O°  O 2
(21.10)
O° + O2  O3
(21.11)
The experimental results indicate that the concentrations of
both ozone and hydrogen peroxide in the electrolyte increase
linearly with the applied current density [9].
Recently, it has been reported that using concentrated
sulfuric acic solutions ([H2SO4] > 2 mol dm-3) and low temperature
(t < 21 °C) the peroxodisulfate is efficiently produced on BDD
anodes ( > 90% ):
2HO° + 2HSO4-  S2O82- + H2O
(21.12)
A small quantity of hydrogen peroxide and ozone are also produced
during this process [6,9].
10
These results show that the innovative BDD anode can be
used for the in situ production of strong oxidants, which can be
activated in a separated chemical reactor in order to produce
active hydroxyl radicals for the oxidation of organic pollutants.
The BBD anode facilitates the application of the advanced
oxidation process.
21.3. Homogeneous and Heterogeneous Advanced
Oxidation Processes
The efficiency of AOPs in wastewater treatment is due to the high
activity of hydroxyl radicals that are formed during the process.
On the BDD anode, hydroxyl radicals are formed during the
electrochemical wastewater treatment, and consequently this
treatment can be classified as an AOP.
Hydroxyl radicals are formed when UV radiation impinges
upon the surface of titanium dioxide, or when it impinges upon
solutions that contain hydrogen peroxide or ozone. Hydroxyl
radicals are formed in a solution when hydrogen peroxide is mixed
with ferrous ion (Fenton reactant), as well as when peroxodisulfate
is mixed with silver ion or when a peroxodisulfate solution is
heated.
Figure 21.6 depicts hydroxyl radical formation on surfaces, as
in the case of the BDD anode, TiO2/UV and O3(in air)/UV systems.
In this case, the AOPs are heterogeneous, and thus they are
subject to mass transfer limitations, especially when the
concentration of the organic species is low.
11
Fig. 21.6. Heterogeneous advanced oxidation processes: (a) BDD
anode, (b) TiO2/UV process and (c) O3/UV process.
The mass transfer limitation can be avoided by efficient
mixing of the peroxide with the wastewater before activation. A
typical example is the case of the H2O2/UV AOP: a more
homogeneous activation is obtained if the hydrogen peroxide is
well mixed with the wastewater before UV radiation, as indicated
in Fig. 21.7a.
Fig. 21.7. Homogeneous advanced oxidation processes: (a) H2O2/UV
process and (b) S2O82-/heat process.
When the peroxodisulfate/heat AOP is used, the scheme of
Fig. 21.7b is suggested. The pre-heating of the wastewater assures
12
a uniform temperature of the wastewater, and the subsequently
introduced peroxodisulfate is then more homogeneously activated.
21.4. Peroxodisulfate/Heat Advanced Oxidation
Processes
The standard redox potential E° of peroxodisulfate in aqueous
solution is 2.01 V, which is comparable to those of other AOP
oxidants: ozone (E° = 2.07 V) and hydrogen peroxide (E° = 1.78 V).
Peroxodisulfate is not an active oxidant at ambient temperature,
but it is activated with UV radiation or heating.
The effect of heating or UV radiation is formation of hydroxyl
radicals (eq. 21.12-21.13) that oxidize many organic species to
carbon dioxide:
S2O82- + heat/UV  2SO4-°
(21.12)
SO4-° + H2O  HO° + HSO4-
(21.13)
The efficient oxidation of many organic species with heat/
peroxodisulfate or UV/peroxodisulfate has led to the use of this
process as a standard for the determination of total organic carbon
(TOC), in wastewater [11]. The innovative BDD anodes reduce the
complexity of the actual peroxodisulfate production [12], and
consequently simplify the AOPs that use this oxidant.
Figures 21.8a and 21.8b depict two alternative methods for
wastewater treatment that use BDD anodes for the oxidation of
biorefractory organic species. In the first treatment (Fig. 21.8a),
peroxodisulfate is produced with a high current efficiency from a
concentrated sulfate solution. The oxidant (peroxodisulfate)
produced is subsequently mixed with the heated wastewater in
13
order to achieve its activation, i.e., the production of hydroxyl
radicals. The AOP occurs efficiently in the bulk of the wastewater,
and thus the mass transfer limitation is avoided.
Fig. 21.8. Alternative wastewater treatment using BDD anodes: (a)
homogeneous peroxodisulfate/heat process and (b) heterogeneous and
homogeneous peroxodisulfate/heat process.
In the second treatment (Fig. 21.8b), the wastewater is
initially heated, as before, but the electrochemical method is
applied in the wastewater using the BDD anode initially without
sulfate, and sulfate is added in the wastewater in order to produce
peroxodisulfate when the organic species reach low concentration
values and the applied current density is higher than the limiting
current.
In this electrochemical process, the organic species are
subjected
to
two
distinct
oxidation
mechanisms:
first,
a
heterogeneous AOP due to the hydroxyl radicals produced on the
surface of BDD anode, and second, a homogeneous AOP that is
due to the combination of the formed peroxodisulfate and the
heating of the solution. Experimental studies are in progress in
order to point out the characteristics of the two alternative
14
methods previously reported, and some results of these studies are
reported below.
21.5. The
Process
Peroxodisulfate/Heat
Homogeneous
The effect of temperature on peroxodisulfate solution reactivity is
due to the formation of active hydroxyl radicals. These radicals can
oxidize both organic species and water. The water oxidation is a
parasitic reaction, and it is usually described as a decomposition of
peroxodisulfate solutions. The efficiency  of the homogeneous
peroxodisulfate/heat process is given by the following:

Rate of organic species oxidation
Rate of water oxidation
21.14
This ratio needs to be optimized for the wastewater
investigated in order to obtain the maximum efficiency.
21.5.1 The peroxodisulfate decomposition reaction
in aqueous solutions
The temperature has a strong effect on the peroxodisulfate
aqueous solution stability.
15
Fig. 21.9. Peroxodisulfate concentration ratio vs. time of
peroxodisulfate decomposition (initial peroxodisulfate concentration,
12 g dm-3; pH = 1).
Figure
21.9
depicts
log(C/C0)
values
(peroxodisulfate
concentration at a given time t relative to the initial concentration)
vs. the reaction time at various temperatures. This figure indicates
that the reaction is first order with respect to peroxodisulfate. The
values of the first order rate constant kaq, for the decomposition of
peroxodisulfate have been estimated at various temperatures, and
Arrhenius behavior has been assumed:
  Ea ,aq 

kaq (T )  Ao,aq exp 
 RT 
The
frequency
factor
and
the
activation
(21.14)
energy
for
the
decomposition of peroxodisulfate have been estimated by fitting
the above equation (Ao,aq = 5.64x1015 min-1, Ea,aq = 118 kJ mol-1).
16
21.5.2. Formic acid oxidation
Formic acid has been oxidized with peroxodisulfate at various
temperatures. An initial series of experiments indicates that the
rate of the formic acid oxidation follows first order kinetics with
respect to peroxodisulfate and does not depend on formic acid
concentration
The
peroxodisulfate
concentration
was
measured,
and
log(C/C0) values vs. time are shown at various temperatures (Fig.
21.10). These results have been used in order to estimate the
pseudo-first-order rate constant kor for formic acid oxidation at
various temperatures.
Fig. 21.10. Peroxodisulfate concentration ratio vs. time of formic acid
oxidation (initial peroxodisulfate concentration, 8 g dm-3; pH = 1).
The increase of the rate constant with temperature follows
the Arrhenius equation, and its parameters have been estimated
17
for formic acid oxidation with peroxodisulfate (Ao,or = 1.85  1016
min-1, Ea,or = 116 kJ mol-1).
21.5.3. Efficiency of the peroxodisulfate/heat
homogeneous process
The efficiency of the peroxodisulfate/heat homogeneous AOP is
related to the following factors: (i) the oxidation rate of the organic
species; and (ii) the selectivity of the peroxodisulfate for the
oxididation of the organic versus the rate of peroxodisulfate
decomposition.The oxidation rate of the organic species is related
to the rate constant kor, while the selectivity is related to the ratio
kor/kaq. Figure 21.11 shows Arrhenius plots for both formic acid
oxidation (kor) and peroxodisulfate decomposition (kaq).
Fig. 21.11. The kinetic rate constants for peroxodisulfate degradation
kaq and formic acid oxidation kor vs. 1/T.
18
The strong effect of the temperature on the rate constant kor
indicates that a high oxidation rate can be reached by increasing
the reaction temperature. The observed large difference between
kor with respect to kaq assures a high selectivity for the oxidation of
the organic species versus the degradation of peroxodisulfate.
These results indicate that the peroxodisulfate/heat AOP oxidizes
efficiently formic acid at moderately high temperatures.
21.6. The Combined Heterogeneous-Homogeneous
Peroxodisulfate/Heat Process
One of the major problems in the direct electrochemical
wastewater treatment process with BDD electrodes is the large
anode surface needed, especially if low concentrations of organic
pollutants have to be treated (see §21.1). In fact, the maximum
operating current density (limiting current density) is dictated by
the COD value of the wastewater. Working above this limiting
current can result in efficiency losses due to the side reaction of
oxygen evolution.
The objective of the proposed combined process is to avoid the
side reaction of oxygen evolution. This can be achieved by
introducing in the wastewater a suitable amount of an inorganic
compound (for example, sulfate) which can be oxidized by trapping
the
electrogenerated
hydroxyl
radicals
to
produce
corresponding peroxo compound (for example, peroxodisulfate).
19
the
Fig. 21.12. Homogeneous and heterogeneous AOPs during the
electrochemical treatment of wastewater with the BDD anode.
As Fig. 21.12 shows, the oxidation of sulfate ions to
peroxodisulfate replaces the side reaction of oxygen evolution, and
at a sufficiently high temperature, the AOP is activated in the
bulk of the wastewater. The organic species are oxidized by the
hydroxyl radicals formed at first on the BBD anode and by those
formed subsequently in the solution. An increase of the total
efficiency of the process can be reached with the use of this
method. In order to observe if the combined effect of heterogeneous
and homogeneous AOP occurs, salicylic acid solutions with and
without sulfate have been treated with BDD anodes at various
temperatures.
20
Fig. 21.13. COD values vs. time of electrochemical treatment of
salicylic acid solution with and without sulfuric acid (current
density, 158 A m-2; temperature, 70°C).
Figure 21.13 depicts the electrochemical treatment results for
two salicylic acid solutions with and without sulfuric acid at 70°C
at the same value of current density (iappl = 158 A m-2). Under
these conditions, a more efficient elimination of COD was observed
when the sulfuric acid was present. The comparison of the results
obtained indicates that both homogeneous and heterogeneous
AOPs occur when the solution contains sulfate ions.
Figure 21.14 depicts the temporal evolution of COD during
formic acid treatments at 25 °C and 70 °C; sulfuric acid was added
in both cases (1 mol dm-3).
21
Fig. 21.14. COD values obtained during the electrochemical treatment
of salicylic acid at low and high temperatures (25 and 70 °C) with
sulfuric acid (1 mol dm-3).
The
lower
temperature
treatment
exhibited
a
lower
elimination rate compared to that obtained at 70°C. The results
obtained indicate that at low temperatures the peroxodisulfate
produced on the BDD surface is not activated and the organic
species are not oxidized by peroxodisulfate.
The concentration of peroxodisulfate has been analyzed
during the treatments. Figure 21.15 depicts the temporal evolution
of peroxodisulfate during treatments at 25 and 60°C using the
same current density (iappl = 158 A m-2) and sulfuric acid
concentration (1 mol dm-3).
22
Fig. 21.15. Peroxodisulfate concentration vs. time at two different
temperatures during formic acid oxidation (sulfuric acid
concentration, 1 mol dm-3; current density, 158 A m-2).
At
the
lower
temperature
(T
=
25°C),
relatively
high
concentrations of peroxodisulfate are formed; this is certainly due
to the fact that at this temperature peroxodisulfate is not
activated. The estimated current efficiency for peroxodisulfate
formation at the low temperature (T = 25°C), is about 30%. The
observed lower concentration of peroxodisulfate at the higher
temperature (T = 60°C in Fig. 21.15) indicates the activation of
peroxodisulfate to hydroxyl radicals, which further oxidize formic
acid.
The reported results show that the peroxodisulfate is
produced during the electrochemical treatment with BDD anodes
of wastewater containing sulfate. When the treatment occurs at
low temperature (T = 25°C) the peroxodisulfate produced is
23
inactive and is accumulated in the wastewater, while at high
temperature (T = 70°C), the peroxodisulfate produced is activated
to generate hydroxyl radicals, which oxidize the organic species.
21.7. Conclusions
The production of numerous active oxidants: hydroxyl radicals,
hydrogen peroxide, ozone, peroxodisulfate, etc., has been simplified
with the use of the BDD anodes. The AOPs use these oxidants to
destroy low concentrations of biorefractory organic species. Some
of these oxidants are unstable, and thus the innovative BDD
anodes allow an easier use of the AOPs in the field of wastewater
treatment.
The capacity of the BDD anode to oxidize organic species is
due to its ability to produce hydroxyl radicals on its surface. The
mass transfer of the organic species to the anode surface limits
their efficient use, particularly when the concentration of the
organic species reaches low values during the treatment. This
aspect, which is common for all of the heterogeneous AOPs, has
been considered, and alternative homogeneous AOPs have been
proposed.
In fact, a new combined two-step process is described in this
work, according to which hydroxyl radicals produced on the BDD
surface are trapped by an oxidizable species like sulfate to form
the
corresponding
peroxide,
e.g.,
peroxodisulfate.
The
peroxodisulfate is relatively stable and can be produced at high
concentration in the electrolyte without any problem of mass
24
transport limitations. The treatment of the wastewater takes place
in a separate chemical reactor. In the chemical reactor, the
peroxodisulfate is activated thermally to produce hydroxyl
radicals. These hydroxyl radicals oxidize the organic pollutants in
an AOP. An efficient wastewater treatment is obtained by use of
suitable operating conditions for the two successive steps.
The BDD anode easily produces active oxidants used in the
AOPs, and thus this innovative anode open up new possibilities in
the field of wastewater treatment, particularly for wastewater that
contains biorefractory organic species at low concentrations.
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26