vailable online at www.sciencedirect.com
Journal of Hazardous Materials 152 (2008) 423–430
Electrocoagulation of cutting oil emulsions using
aluminium plate electrodes
K. Bensadok a,∗, S. Benammar a, F. Lapicque b, G. Nezzal a
aLaboratoire de G´enie des Proc´ed´es et de l’Environnement, F.G.M.G.P., USTHB, B.P. 32,
El Alia, 35111 Algiers, Algeria
bLaboratoire des Sciences du G´enie Chimique, CNRS-ENSIC, BP 20451, F-54001 Nancy, France
Received 4 January 2007; received in revised form 29 June 2007; accepted 29 June 2007
Available online 7 July 2007
The treatment of very concentrated oil–water emulsions by electrocoagulation (EC) was experimentally investigated as a pre-treatment step prior
to a membrane process. The oil–water emulsion was prepared from a cutting mineral oil B22 currently used for drilling and machining operations.
The electrocoagulation progress was followed by the measurement of COD, turbidity and pH in a batch process with recirculation of the liquid.
This study is mainly focused on the effects of operating parameters such as initial pH, current density, oil concentration and recirculation rate, on
the de-emulsiﬁcation efﬁciency. Kinetic curves showed that the EC process exhibits two phases: a “reactive phase” during which the COD and the
turbidity removals increase with electrolysis, and a stationary phase for which further aluminium dissolution is useless in the pollution abatement.
The results showed that the treatment efﬁciency increases with increasing current density, but decreases with oil concentration. It appears that
treatment of the considered cutting oil is completed through dissolution of around 10 mg Al/g oil, with a slight positive effect of the liquid ﬂow
rate. Best results are also obtained with initial pH near 7.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Electrocoagulation; Cutting oil emulsions; Aluminium plate electrodes; Very high COD
Cutting oils also called “soluble oils”, are used in particular
during mechanical operations of cutting and machining metals.
They combine the properties of cooling and lubrication. A cut-
ting ﬂuid concentrate usually contains a mineral oil, a surfactant
mixture, in some cases water and various additives, which are
included to meet the speciﬁcations of commercial concentrates
such as resistance to bacterial growth and low corrosion capac-
ity . During their use, cutting ﬂuids loose their properties and
effectiveness because of their thermal degradation and the pro-
duction of suspended metal particles. The oils have therefore
to be replaced periodically and the organic wastes generated
have to be taken away and treated. This constitutes a danger
∗Corresponding author at: University of Sciences and Technology H. Boume-
diene, F.G.M.G.P., Laboratory of Process and Environmental Engineering, B.P.
32, El Alia, 35111 Algiers, Algeria. Tel.: +213 76 14 60 02;
fax: +213 21 24 79 19.
E-mail address: email@example.com (K. Bensadok).
to the environment because these efﬂuents are highly charged
in surface-active agents and other organic matters. Moreover,
because of their great capacity of penetration in the ground,
they constitute a very serious threat for groundwater. Sokovic
and mijanovic  give an analysis of the ecological parameters
of the cutting ﬂuids. The signiﬁcant developments of the formu-
lations of cutting oils as well as the preparation of the synthetic
or semi-synthetic emulsions complicate some more the issue of
the puriﬁcation of these efﬂuents. Because of the stability of
these emulsions, there is no universal solution for their treat-
ment, and it can be necessary to combine one or two treatment
processes for highly effective puriﬁcation. Several techniques
have been applied to treat these types of oily wastewater, e.g.
chemical  and biological  destabilization, ultraﬁltration 
and nanoﬁltration . Adsorption on mixed Ca and Mg oxides
obtained by thermal decomposition of dolomite was studied to
remove exhausted oils .
Coagulation makes it possible to destabilize the suspended
particles which cannot settle naturally because of their submicro-
metric size. The destabilization of negatively charged particles
0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
424 K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430
takes place upon addition of positively charges species, which
can be supplied either by addition of chemicals, e.g. iron chloride
or aluminium sulphate, for conventional chemical treatment, or
by the dissolution of anodes in the case of electrocoagulation
(EC). Electrocoagulation is initiated by the oxidation of sac-
riﬁcial anodes out of aluminium or iron yielding, respectively
Al3+ and Fe2+ ions. The latter ions are rapidly oxidized to Fe3+
by air oxidation. The metal ions combine to the hydroxyl ions
produced by the water electrolysis at the cathode, to form the
corresponding metal hydroxides, which favor the formation of
the ﬂocs by destabilisation of the contaminants or particulate
suspensions. The ﬂocs formed can be recovered from the liquid
surface by scraping – when the bubbles of hydrogen produced at
the cathode allow ﬂotation – or settle depending on their density.
The development of EC process has been hindered for
years by the high investment costs and severe competition with
the chemical processes. It started to regain importance with
improvement of electrochemical processes and promulgation of
more stringent environmental legislations on wastewater. This
new rise of electrocoagulation has also been due to the rela-
tive reduction in the operation and investment costs. EC has
the potential to be competitive with respect to both economical
and environmental criteria for treatment of wastewater and other
related water management issues . This technique has been
applied for treatment of waters containing suspended solids ,
oils and greases [10–13], dyes and textile wastewaters [14,15],
or industrial wastes containing heavy metals  and phosphate
. EC was also applied for deﬂuoridation of water  and
urban wastewaters .
The electrochemical reaction occurring at the anode and
involving metal M-aluminium in the present case, is written as
Hydrogen evolution occurs at the cathode depending on pH.
2H2O(l) +2e−→H2(g) +2OH(aq)−(2)
The generated Al(aq)3+ ions combine with water and hydroxyl
ions to form corresponding hydroxydes and/or polyhydroxides
[20–22] as follow:
•monomeric species such as Al(OH)2+, Al(OH)2+, and
Al(OH)4−by Eqs. (4), (5) and (7),
•polymeric species such as Al2(OH)24+ and Al2(OH)5+,
•amorphous and less soluble species such as Al (OH)3by Eq.
(6) and Al2O3.
Considering only mononuclear speciation, the concentration
of the various Al forms present in solution (α) was calculated
Fig. 1. Solubility diagram of aluminium hydroxide Al(OH)3(s) considering only
mononuclear aluminium species. Data from .
by Holt et al.  depending on pH. Fig. 1 provides the speci-
ation diagram obtained by the authors. Al complexes acting as
coagulants are adsorbed on the particles and thus neutralise the
colloidal charges, resulting in destabilization of the emulsion.
This phenomenon is similar to the action of chemical coagu-
lants in the conventional chemical treatment. Hydrogen bubbles
formed at the cathode can adsorb on the ﬂocculated species and
induce their ﬂotation. The bubbles formed also reduce fouling
of the cathode surface which could occur by the formation of
NaCl is usually employed to increase the conductivity of the
water or the wastes to be treated. The presence of the chloride ion
in solution has been reported to decrease passivation of the Al
surface and thereby increase the efﬁciency of electrocoagulation
processes [23,24]. All authors attributed the effect of chloride
ion to its role in the “pitting” corrosion of the metal surface.
Mameri et al. postulated a mechanism for the reduction
in Al passivation by the oxide layer formed, and the overall
equations can be written as
2Al +6HCl →2AlCl3+3H2(8)
Because of EC complexity, previous researchers have
adopted a largely empirical approach to understand the various
processes involved[8,19,26]: the capability of removal the pollu-
tant by their technology was clearly demonstrated and analysed.
The published works have often resulted in various EC systems
whose design and operations were developed to meet the speci-
ﬁcation of the speciﬁc pollutants investigated. However most of
these contributions have failed to extract and to quantify the key
underlying mechanisms of pollutant removal.
The efﬁciency of EC processes is to be controlled by several
parameters, e.g. current density, pH, ﬂow rate, electrode mate-
rials, and the amount of matter to be removed. In this study,
treatment of cutting oil emulsions was considered as a pre-
treatment step prior to a membrane process. The progress of
the treatment carried out in a batch system provided with recir-
culation of the ﬂow, was followed by time variations of COD
K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430 425
Characteristics of the emulsions used
Oil concentration 2% 4% 6%
COD (mg O2/L) 60 282 108 208 116 128
Turbidity (NTU) 26 400 51 712 64 125
pH before NaCl addition 8.63 9.09 9.69
pH after NaCl addition 8.43 9 9.3
Conductivity before NaCl
257 322 419
Conductivity after NaCl
3 150 3 350 3 730
2. Materials and methods
2.1. Chemical and analytical techniques
The metal used for the cell electrodes was Al alloy AU4G
(2017-Al). It is mainly produced by recovery of aluminium
waste, which explains its very accessible cost. The alloy con-
tains Cu at 4%, Fe, Mg and Mn each at 0.7%, Si at 0.5% and
lower percentages of Zn and Cr.
Oil–water emulsions were prepared from a cutting mineral
oil B22 supplied by Naphtal (Sonatrach, Algeria) and currently
used for drilling and machining operations. The emulsions were
diluted in deionized water to form very stable emulsion with
mean diameter droplet and the zeta potential equal, respectively
to 112 nm and −30 mV. The zeta potential was measured using
a Malvern Zetasizer 3000HS. The diameter of the oil droplet
was measured using Malvern Mastersizer nano S. In all cases,
sodium chloride at 1.5gL
−1concentration was added for suf-
ﬁcient electrical conductivity of the emulsion to be treated, as
suggested by S´
anchez-Calvo et al.  and Chen et al. . This
addition has negligible effect on the initial pH of the emulsion
(Table 1). Carmona et al.  observed that the characteristics
of these types of emulsions were not affected by the addition
of supporting electrolyte such as NaCl at low/moderate con-
centrations. COD and turbidity levels of the dilute emulsions
are very high (Table 1) and nearly proportional to their concen-
tration, as expected. The progress of electrocoagulation in the
batch process was followed by measurement of COD, turbid-
ity, conductivity and pH. The COD levels of the samples were
determined using the standardized colorimetric technique with
an excess of hexavalent chromium and subsequent measurement
of the optical density. Turbidity was measured by a Hanna Instru-
ment LP 2000 turbidimeter which measures the quantity of light
absorbed by the suspended particles in comparison with a stan-
dard solution. Accuracy of the two analytical techniques was
better than 5%. The pH was measured using a Hanna Instrument
pH 211 Microprocessor pH Meter. Conductivity was measured
using a Hanna Instrument Conductivity Meter EC 214. The main
features of the emulsions used are given in Table 1.
2.2. Set-up and protocol
The batch experimental set up is shown in Fig. 2. The
system consisted of an EC cell, a cylindrical reactor, a peri-
staltic pump for the recirculation of the efﬂuent and a sampling
valve to collect the samples. The EC cell consisted of two
polymeric halves which were bolted to form the rectangu-
lar channel cell. An aluminium electrode with dimensions of
100 mm ×50 mm ×12 mm, was embedded in each of the two
valves: only one face was exposed to the solution and the
effective electrode area was 5.0×10−3m2. The electrode gap
was maintained constant at 20 mm. The electrodes were con-
nected to a digital dc power supply (P. Fontaine, MC4020C,
40 V, 2 A). Two digital multimeters (Multimetrix X1000) were
used to measure the current passing through the circuit and the
applied potential, respectively. The tank reactor with a capacity
of 1.5 ×10−3m3was made in PlexiglassTM and was provided
with inlet and outlet ﬁttings for recirculation of the liquid.
Homogenisation of the efﬂuent was ensured by magnetic stir-
ring at approximately 250 rpm. The gentle agitation allowed the
gases to be separated from the liquid, thus avoiding the formation
of foam which could affect the occurrence of electrocoagulation.
Fig. 2. Schematic diagram of experimental set-up. (1) dc power supply; (2) magnetic stirrer; (3) plexiglass reactor; (4) peristaltic pump; (5) electrochemical cell; (6)
aluminium electrodes; (7) conductimeter; (8) pH meter.
426 K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430
To avoid passivation of the electrodes, the electrochemical cell
was entirely cleaned after each experiment with detergent and
acetone, as described by Kobya et al. . All experiments were
carried out at room temperature near 25 ◦C.
One litre of fresh emulsion was introduced into the tank,
and after the NaCl was added, the selected current was applied
to the cell. At regular intervals, samples were collected and
pH was adjusted to 7 by the addition of aliquots of concen-
trated hydrochloric acid for optimal precipitation of aluminium
hydroxide as reminded in Fig. 1 . The collected samples were
allowed to settle for 24 h, COD and turbidity were then deter-
mined from the clear fractions recovered. Contrary to previous
observations with others suspensions  the only adjustment
of pH did not result in visible COD abatement in the B22 oil sus-
pensions. It has to be mentioned that accidental entrainment of a
small sludge particle could occur drawing the clear liquid from
the settling tube, causing an unexpected additional error in the
analytical procedure. Replicates of most experiments were made
to overcome this experimental issue. Analysis of the results of
replicate experiments led to estimate that the global uncertainty
in determination of COD and turbidity was near 5%.
3. Results and discussion
3.1. General aspects
This study is mainly focused on the electrocoagulation treat-
ment of cutting oil emulsions with very high concentration for
investigation of the effects of the main operating parameters,
e.g. initial pH, current density, oil concentration and recircu-
lation rate. Turbidity and COD values were used to evaluate
the EC progress and the removal efﬁciency. For all cases, two
phases were observed during the discontinuous treatment: (i) a
“reactive” phase for which the abatement of COD and turbidity
increases regularly with time and (ii) a steady phase for which
further Al dissolution has no effect on the treatment efﬁciency.
The intercept between the two periods can be considered as the
minimum time required for the treatment, corresponding to the
minimum required concentration of dissolved Al.
The progress of electrocoagulation is to be linked to the
amount of dissolved aluminium. The theoretical concentration
of dissolved Al, CAl in g/L, can be expressed by Faraday’s law
where M,I,t,n,Fand V, are, respectively the molecular weight
of aluminium (g/mol), the current (A), the electrolysis time (s),
metal valence (3 for Al), Faraday constant (96 500 C/mol) and
the emulsion volume in the overall circuit. Qis the electrical
charge passed during the galvanostatic runs and Φeis the current
yield. For appreciable change in the volume of the liquid in the
ﬂow rig because of the repeated samplings, Eq. (10) had to be
modiﬁed accordingly. The current efﬁciency for Al dissolution
was previously found to be larger than unity [10,29], and the
actual concentration of Al can attain 150% of the theoretical
level, due to the chloride-induced dissolution mentioned above.
3.2. Effect of oil concentration
The experiences were followed at constant current density
(i= 100 A/m2), the initial pH of emulsion was adjusted to 7, and
the liquid velocity in the cell was 5.33 10−3m/s, corresponding
to 320 cm3min−1through the 10 cm2cross-sectional area. We
observe in Fig. 3 that removal of COD and turbidity expressed
in %, are lower with higher oil concentration. This is can be
explained by the fact that higher amounts of Al ions are required
for the treatment of more concentrated oil suspensions. For a
2 vol.% emulsion, the treatment does not progress any further
after 35 min, the residual COD was equal to 1475mg O2/L, cor-
responding to 98% COD abatement. Final turbidity was near
507 NTU, corresponding to the same abatement. For higher
oil concentrations, the stationary plateau of COD and turbidity
was reached after more than 1 h. This result is not in agreement
with previous work  also conducted with suspensions of
machining oil, for which the abatement yield was observed to
be nearly independent of the amount of suspended matter. How-
ever, the dependence of the concentration was shown to largely
depend on the nature of the pollutant to be treated and latex
suspensions had been shown to behave like the present cutting
The above results were plotted on the basis of the speciﬁc
electrical charge per gram oil, Q/moil (Fig. 4). Such a plot shows
that the electrical charge required for the treatment of the inves-
tigated cutting oil – and the amount of dissolved aluminium –
is directly proportional to the concentration of oil in the emul-
Fig. 3. Time variations of COD (a) and turbidity (b) removal depending on the
oil concentration at i= 100 A/m2, initial pH 7, liquid ﬂow rate = 320 cm3/min.
Oil concentrations: (䊉) 2%; (×) 4%; () 6%.
K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430 427
Fig. 4. Variations of COD (a) and turbidity (b) removal with the speciﬁc charge
(Q/moil) depending on the oil concentration at i= 100A /m2, initial pH 7, liquid
ﬂow rate = 320 cm3/min. Oil concentrations: (䊉) 2%; (×) 4%; () 6%.
sion to be treated. Treatment conducted in the above conditions
appears to be completed upon application of 70 As/g oil.
3.3. Inﬂuence of the current density
Three current densities 100, 150 and 200 A m−2were tested
for the treatment of 4 vol.% emulsions. The initial pH was the
natural pH of the diluted emulsion, around 9, and the ﬂow rate
was 320 cm3min−1. The results, expressed in the form of COD
and turbidity reduction versus time, are shown in Fig. 5. Dur-
ing the reactive phase, the reduction rate of COD and turbidity
were observed to increase with the current density. This can be
explained by the fact that the amount of Al3+ species formed
by dissolution of the anode, increases with the current den-
sity according to Faraday’s law (Eq. (10)) and previous results
[12,15,29]. Higher amounts of dissolved aluminium allowed
higher coagulation efﬁciency and more signiﬁcant destabilisa-
tion of the emulsion. Moreover as shown by Khemis et al. ,
higher production rates of hydrogen allowed by higher currents,
favors the ﬂotation of the ﬂocculated matter.
This is not true in the second phase of the treatment: during
this period the separation process is not controlled by coagula-
tion but by other phenomena, e.g. formation of solid, liquid/solid
aggregation and adsorption of destabilized particles onto solid
Al(OH)3formed. As done in Section 3.1, the data could be suc-
cessfully plotted versus the speciﬁc charge (Q/moil), as shown in
Fig. 6. For the initial pH considered, around 80 As/g are required
for the treatment of the emulsion whatever the current density.
Fig. 7 represents the variation of the emulsion pH during
the electrolytic run. The pH was observed to increase rapidly
during the reactive phase, then to stabilise at pH close to 11.
This progressive increase in pH is explained by the occurrence
of water electrolysis resulting in hydrogen evolution and pro-
duction of OH−ions which are partly buffered by the various
forms of Al-hydroxides. As can be expected from [20,21], and
demonstrated in Fig. 1, at pH equal or higher than 11, aluminium
hydroxide is mainly in the form of soluble forms, which is to
limit the efﬁciency of the ﬂocculation process. The relative sta-
bilisation of the pH, for 80 As/g, corresponds to the end of the
3.4. Effect of liquid ﬂow rate
The effect of the ﬂow conditions on the electrocoagulation
efﬁciency of the liquid waste has been scarcely investigated. We
studied here the inﬂuence of this parameter for the treatment of
4 vol.% emulsions. As shown in Fig. 8, the EC progress observed
by the removal of COD and turbidity, was slightly better at high
ﬂow rates. This result could be explained by the fact that more
steady convection allowed by higher ﬂow rates are to improve the
rates of transport and transfer phenomena of the various species
in the electrochemical cell. In addition, higher ﬂow velocity is
to induce a greater number of collisions between the particles
of Al(OH)3and the destabilized oil droplets, thus improving the
Fig. 5. Time variations of COD (a) and turbidity (b) removal depend-
ing on the current density; initial pH 9; oil concentration = 4%, liquid
ﬂow rate = 320 cm3/min. Current density: () 100A/m2;(×) 150 A/m2;(䊉)
428 K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430
Fig. 6. Variations of COD (a) and turbidity (b) removal with the speciﬁc charge
(Q/moil) depending on the current density; initial pH 9, oil concentration = 4%,
liquid ﬂow rate = 320 cm3/min. Current density: () 100 A/m2;(×) 150 A/m2;
(䊉) 200 A/m2.
However, we observed that the ﬂow rate effect on the tur-
bidity abatement was more signiﬁcant for the highest values of
current density (Fig. 8). This reveals the synergetic effect of: (i)
the anode dissolution, forming Al3+ and allowing coagulation,
(ii) the transfer phenomena by convective diffusion, and (iii) the
ﬂocculation phenomena. However, the inﬂuence of the ﬂow rate
on the COD abatement was more signiﬁcant at low current den-
sities, and was hardly visible at 200 A m−2. These observations
Fig. 7. Variation of the liquid pH with the speciﬁc charge (Q/moil) during the
electrocoagulation according to current density; initial pH 9, oil concentra-
tion = 4%, liquid ﬂow rate = 320 cm3/min. Current density: () 100 A/m2;(×)
150 A/m2;(䊉) 200 A/m2.
Fig. 8. Time variation of COD (a) and turbidity (b) removal depending on the
ﬂow recirculation, initial pH 9, i= 100 A/m2, oil concentration = 4%. Liquid ﬂow
rate: (䊉) 130 cm3/min; () 320 cm3/min; (×) 520 cm3/min; () 750 cm3/min.
are illustrated in Fig. 9 which gives the inﬂuence of the ﬂow
rate and the current density on the COD and turbidity abate-
ment, after 90 min of electrolysis, corresponding to maximum
treatment in all cases.
Fig. 9. Effect of the recirculation ﬂow rate on removal efﬁciency, initial pH 9,
oil concentration = 4% and electrolysis time = 90 min.
K. Bensadok et al. / Journal of Hazardous Materials 152 (2008) 423–430 429
Effect of the initial pH on efﬁciency removal after 60 min of electrolysis
Initial pH Final COD (mg O2/L) Final turbidity (NTU) COD removal (%) Turbidity removal (%) Final pH
3 10 036 24 587 91 52 6.8
5 9 856 20 083 91 61 7.8
6 8 652 630 92 99 8.0
7 10 058 610 91 99 8.4
9 28 516 19 024 74 63 10.5
11 43 251 23 581 60 54 10.6
Current density = 100 A/m2; oil concentration=4vol.%, i.e. (Q/moil) = 56.25 As/g; liquid ﬂow rate = 750 cm3/min.
3.5. Effect of initial pH of emulsions
It has been established that the initial pH is an important
operating factor inﬂuencing the performance of EC process
[10,12,27]. To examine its effect, the pH of the emulsion to be
treated was adjusted to the desired value by aliquots of sodium
hydroxide or hydrochloric acid. The current density was ﬁxed at
−2and the oil concentration was 4 vol.%. Table 2 shows
the removal yields of COD and turbidity after 60 min of elec-
trolysis, depending on the initial pH. The maximum removals
of COD, and turbidity were observed at neutral pH 6–7, and this
is in agreement with many previous works related to EC using
aluminium electrodes [11,13,15]. For neutral or acidic pH, the
effect is less signiﬁcant on the COD removal whose value is
higher than 90%. The drop in turbidity removal, in acid or basic
media, is in accordance with the amphoteric character of alu-
minium hydroxide Al(OH)3whose solubility increases as the
solution becomes either more acidic or alkaline (Fig. 1). Solid
precipitate of aluminium hydroxide formed at pH 6–7 is a pre-
cursor for oil removal by coagulation. Furthermore, as shown
in Table 2, the more acid the initial liquid is, the higher is the
increase in pH during the run. This phenomenon was ascribed
by Vik et al.  to hydrogen evolution at cathode and according
to reaction (2).
In alkaline medium (pH >7), the ﬁnal pH does not vary very
much and a slight drop was recorded. However for initial pH over
10, ionic forms of Al-hydroxides predominate, which reduces
the efﬁciency of the treatment by electrocoagulation.
Electrocoagulation using aluminium electrodes is a feasible
process for treatment of very stable cutting oil emulsions, char-
acterized by high COD and turbidity. Whatever the operating
conditions, EC proceeds in two phases: a reactive phase in which
the removal efﬁciency increases regularly with time – and the
amount of dissolved Al – and a stationary phase for which fur-
ther energy consumption for solubilisation of aluminium anode
is useless. The reactive phase is as longer as the oil concentra-
tion is high, and can be reduced by using higher current density
and, to a lower extent by higher liquid recirculation rate. The
optimal initial pH was found equal to 6–7 and remain for which
the COD and turbidity removal correspond, respectively to 92
In spite of very important removal efﬁciency, the ﬁnal val-
ues of COD (8652 mg O2/L) and turbidity (610 NTU) remain
higher than standard rejections (120 mg O2/L and 5 NTU). In
this fact, the EC process would be appropriate perfectly as pre-
treatment step prior to a membrane process. Besides, treatment
of the investigated cutting oil, although incomplete, consumes
from 60 to 80 As/g oil, corresponding to 5.6–7.4 mg Al/g oil
assuming a current efﬁciency at 100%. In reality, higher dissolu-
tion yields are to be expected, and the Al concentration required
would be of the order of 10 mg/g oil. This value is a positive
aspect of the technique, since the sludge to be produced by the
treatment is to have an Al content of 1 wt.%, which is satisfactory
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