Proceedings of the 13th International Conference of Environmental Science and Technology
Athens, Greece, 5-7 September 2013
TREATMENT OF INDUSTRIAL WASTEWATERS CONTAINING MULTIVALENT
METALS BY MEANS OF ELECTRODIALYTIC TECHNIQUES
M.C. MARTI-CALATAYUD, M. GARCIA-GABALDÓN, E. ORTEGA and V. PEREZHERRANZ
1
IEC Group, Departamento de Ingeniería Química y Nuclear, Universitat Politècnica de
València, Camí de Vera s/n, 46022 València, Spain. P.O. Box 22012, E-46071
e-mail: [email protected]
EXTENDED ABSTRACT
Ion-exchange membranes are usually known for their use in fuel cells or in electrodialysis
systems applied to desalinate seawater. Apart from this, the potential use of ionexchange membranes to treat industrial waste waters, such as those generated in the
metal finishing industry or in leather tanneries, has not been completely exploited.
Electrodialysis does not involve the addition of chemicals and, in most cases, allows the
separation of the contaminants from the raw materials, which can be recycled to the
process. As a result, the quantity of toxic metals discharged to the environment is
reduced. However, the limited chemical resistance of the ion-exchange membranes
makes their use unfeasible to treat harsh effluents. In the present work, the transport of
Fe(III) and Cr(III) species through perfluorosulfonic Nafion membranes is studied. These
membranes are characterized by a high durability in oxidizing or very acidic
environments. Besides, in the case of electrolytes with multivalent cations, various
species of different charge and size can be present as a consequence of the chemical
equilibriums involved. This may imply a change in the electrolyte properties, and
therefore, may alter the nature of the mass transfer phenomenon taking place through the
membrane and the diffusion boundary layer.
In the present work, chronopotentiometric measurements are conducted to evaluate the
transport of sulphate salts of Cr(III) and Fe(III) through cationic membranes. This
technique consists of measuring the membrane voltage drop under the imposition of
different current values. The membrane voltage drop can be regarded as an indirect
measure of the concentration changes taking place in the membrane/solution interface,
and therefore can be used as a measure of the evolution of concentration polarization
phenomena. The chronopotentiometric results and the obtained current-voltage curves
prove the passage of different species as a function of the applied current density.
Moreover, the simultaneous transport of H+ ions provokes changes in the pH of the
diffusion boundary layer. As a consequence, the initial equilibrium is shifted and different
complex species are formed. The resistance of the membrane changes depending on
which species predominates in the electrolyte. Once surpassed the limiting current value
at which a drastic depletion of ions occurs near the membrane, the delivery of ions to the
membrane can be again enhanced if the membrane voltage drop exceeds a critical value.
This critical value has been proven to depend upon the size and concentration of the
main cation present in the electrolyte composition. The obtained results can serve as a
reference for the selection of the optimum operating conditions to achieve an effective
removal of contaminants.
KEYWORDS: metal recovery, waste water treatment, ion-exchange membranes,
electrodialysis
Please do not use page numbers
1. INTRODUCTION
The generation of industrial wastewaters with toxic and harmful substances constitutes a
great problem for the environment and, consequently, also for the health of the human
beings. Among different industrial waste streams, those which contain heavy metals are
of great concern because of their toxicity and their accumulation in the environment
(Hang et al., 2009). Besides this, these effluents are usually produced in large quantities
and represent important economic losses for the industries if an important amount of the
raw materials is not recovered. Typical examples of industries that produce these waste
streams are the metal finishing industry, the mining activities or the leather tanneries
(Fahim et al., 2006).
Conventional techniques applied to treat effluents containing heavy metals include
membrane filtration, the use of evaporative techniques or the precipitation of metals by
addition of caustic reagents. These procedures are usually directed to minimize the
undesired effects of the harmful substances on natural aquatic ecosystems. However,
usually they do not imply the recycling of raw materials, involve significant energy
consumptions and lead to the generation of contaminant sludge for disposal. Therefore,
the use of different techniques in which the recycling of resources in the industrial
processes is maximized could entail important advantages from both economic and
environmental points of view.
In this vein, the use of electrochemical reactors or electrodialysis (ED) cells to treat
industrial waste streams constitutes an interesting alternative to achieve the recycling of
raw materials, reduce the water and energy consumption and avoid the generation of
contaminant sludge. ED systems can be integrated in the industrial cycle to selectively
separate the ionic species that contaminate the industrial aqueous streams, reuse the
desalted water in auxiliary operations and obtain valuable by-products from the
concentrated ionic species. Moreover, ED operations are continuous processes which do
not need heat inputs to achieve changes of phase, and can be operated without the
addition of chemicals. On the other hand, one of the main drawbacks of ED is related to
the difficulty of finding ion-exchange membranes with sufficient chemical stability to treat
industrial solutions that usually are characterized by having extreme pH values and high
oxidizing strengths (Martí-Calatayud et al., 2013).
One of the main advances achieved in the field of ED processes is related to the
production of perfluorosulfonic cation-exchange membranes. These membranes stand
out, among other reasons, due to their increased chemical resistance and durability in
harsh environments. Therefore, they could be potentially used to remove metallic
contaminants from industrial wastewaters. Nonetheless, there is a lack of knowledge on
how the transport of multivalent metals through ion-exchange membranes occurs. In this
regard, it is known that heavy metal ions own specific characteristics different than those
of monovalent ions. Heavy metals such as iron or chromium are mostly present in their
multivalent forms and their bigger size and charge leads to differences in the diffusion
and migration rates of these ions through the membranes and in the diffusion boundary
layers formed at the membrane/electrolyte interfaces. Moreover, the anions present in the
electrolyte can associate with the multivalent cations, thus leading to the formation of
different complex species. In the present study, the effect of the concentration and the
chemical equilibria on the transport of trivalent metals through perfluorosulfonic Nafion
117 cation-exchange membranes is investigated. Chronopotentiometric features and
current-voltage characteristics are used in order to clarify some aspects of their transport
over a wide range of currents.
2. METHODOLOGY AND MATERIALS
Synthetic solutions of Fe2(SO4)3 and Cr2(SO4)3 (Panreac) were prepared with metal
concentrations ranging from 10-3 M to 2·10-2 M. The solutions were prepared with distilled
water. Cr(III) and Fe(III) were selected as the target metals to be investigated because
they are present in several industrial waste streams. Both metals are usually present as
contaminants in spent baths generated in the metal finishing industries (Martí-Calatayud
et al., 2012). Cr(III) is also present in tannery effluents (Sahu et al., 2009). In the case of
Fe(III), Fe3+ ions and sulfate compounds of Fe(III) are the most common species found in
acid mine drainage solutions generated in areas with an important coal mining activity
(Sobrón et al., 2007). The membranes to be used in the chronopotentiometric
experiments were equilibrated in the corresponding synthetic solutions during at least 24
h. The perfluorosulfonic cation-exchange membranes investigated were Nafion 117 (Du
Pont). Ionics ARZ-204 (Ionics Inc.) anion-exchange membranes were used as auxiliary
membranes.
The setup used in the experiments consists of an ED cell composed of three different
compartments, which is detailed in a previous study of the authors (Martí-Calatayud et
al., 2013). The cation-exchange membrane under study was clamped separating the
central and the cathodic compartment, whereas the anion-exchange membrane was
installed between the central and the anodic compartment in order to minimize the
influence of the H+ ions generated at the anode on the measurements conducted in the
vicinity of the cation-exchange membrane. The working and counter electrodes, made of
graphite, were placed at the side compartments and connected to a
potentiostat/galvanostat (Autolab, PGSTAT 20). Two Ag/AgCl reference electrodes
immersed in Luggin capillaries were installed at both sides of the cationic membrane. The
tips of the capillaries were placed facing each other at a distance of one mm from the
membrane surface to measure the membrane voltage drop (Um). The experiments were
conducted at room temperature (25°C). Chronopotentiometric experiments consisted on
the application of different current pulses between the working and the counter electrode
during 300 s of pulse duration. Then, the relaxation of the system was allowed for other
300 s without the application of current. The response of Um is registered during this
process and is used as an indirect measure of the concentration changes occurred in the
membrane/electrolyte system.
3.
RESULTS
3.1. Chronopotentiometric curves
Chronopotentiometry is used in the present study to investigate the dynamics of the
concentration polarization phenomena in the membrane/solution interface. An example of
the chronopotentiometric curves obtained with diluted chromic sulphate solutions is
shown in Figure 1. In these curves, the evolution of Um with time during the imposition of
a constant current is represented. The initial membrane voltage drop registered when the
current is switched on can be directly associated with the ohmic conductance of the
membrane, since the diffusion boundary layers are not developed at the beginning of the
pulse. During the course of the current pulse, the evolution of Um with time differs
depending on the value of applied current. At low current density values, the ohmic
overpotential
) remains as the main contribution to the final value of Um (Um,f). If higher
values of current density are applied, the concentration of ions at the dilute side of the
membrane decreases and this is reflected by an increase in Um. When a characteristic
current density known as the limiting value (ilim) is reached, the concentration of ions at
the membrane surface will approach zero and, consequently, a sharp increase in Um will
be registered. The instant at which this sharp increase in Um occurs can be clearly
detected from the maximum in the derivative of Um with time, and is known as the
transition time
. According to the concentration polarization theory, the supply of ions
to the membrane is suppressed when the ilim is reached and further increases in the
applied current density would only raise the membrane electrical resistance. However, a
further enhancement in the ionic transport through the membranes can be provoked if a
certain threshold in the membrane voltage drop is surpassed (Kim et al., 2012).
In the case of the solutions of trivalent metals and low electrolyte concentrations, more
than one transition times are detected in the chronopotentiograms (as denoted by various
maximums of
ure 1). This feature is rarely found in previous studies, which
m
are mostly focused on the study of binary salts of monovalent metals. In our case, the
obtained results suggest that the development of the diffusion boundary layers with
solutions containing trivalent metals occurs in various stages. The first transition time
would be associated with the depletion of a specific ion present in the electrolyte that
diffuses faster, whereas the second transition time would be related to the slower
transport of the remaining ions, which are depleted later from the diluted side of the
membrane.
0.6
0.008
1.13 mA/cm2
0.5
Um,f
0.006
0.92 mA/cm2
0.3
0.005
0.004
DUm/Dt
0.4
Um (V)
0.007
0.003
0.2
hW
0.50 mA/cm2
0.1
0.002
0.001
0.0
t1
0
t2
100
200
t (s)
300
0.000
400
Figure 1. Chronopotentiometric curves obtained for 2.5·10-3 M Cr2(SO4)3 solutions.
This behaviour was observed for all the studied concentrations of Cr2(SO4)3, except for
the most concentrated solution of 0.01 M Cr2(SO4)3, for which only one inflexion point was
detected. In order to interpret the phenomena associated with the different transition
times, the speciation diagrams of the studied solutions were obtained by considering the
chemical equilibria of Cr(III) ions with OH- and SO42- ions. The possible formation of
precipitates was also considered by taking into account the solubility product of Cr(OH)3.
The corresponding stability constants are taken from the literature (Kotrlý and Sucha,
1985). The speciation diagrams for Cr2(SO4)3 solutions at different concentrations are
shown in Fig. 2. The equilibrium conditions at the beginning of the experiments reveal the
coexistence of various positively charged Cr(III) species: Cr3+, CrSO4+ and CrOH2+ ions.
Moreover, the pH of these solutions corresponds to a H+ concentration of the same order
of magnitude as that of the Cr(III) species in the case of diluted solutions. Therefore,
taking into account the higher mobility of Cr3+ ions, the first transition time observed in the
chronopotentiograms presented in Figure 1 could be attributed to the depletion of Cr3+
ions. Moreover, the simultaneous transport of H+ ions would promote a shift from the
initial equilibrium conditions to higher pH values, at which the proportion of hydroxylated
compounds of Cr(III) increases. Accordingly, the less mobile complex species of Cr(III)
would be transported slower through the membrane and their depletion in the
membrane/electrolyte interface would be the phenomenon originating the latter transition
times. In the case of the most concentrated solutions of Cr2(SO4)3, the equilibrium
concentration of Cr3+ ions is significantly smaller than that of complex species of Cr(III).
Therefore, the preferential transport of Cr3+ ions is not so notorious in the development of
the diffusion boundary layers next to the membrane surface.
a) 1.0
b)
Cr(OH)2+
CrOH
Cr3+
0.8
2+
1
Cr(OH)2+
CrSO4+
0.8
CrOH2+
0.6
ai
ai
0.6
0.4
CrSO4+
0.4
Cr3+
0.2
0.2
0
0.0
0
1
2
3
4
5
6
0
7
1
2
3
pH
4
5
6
7
pH
Figure 2. Speciation diagrams calculated for chromic acid solutions: (a) 5·10-4 M
Cr2(SO4)3 solutions and (b) 10-2 M Cr2(SO4)3 solutions. The vertical dashed lines indicate
the initial equilibrium conditions and the pH value.
Some of the chronopotentiograms obtained with 10-2 M Fe2(SO4)3 solutions are shown in
Figure 3. It can be seen that the curves also present more than one transition time and
therefore have a similar shape to that of the curves obtained with Cr2(SO4)3 solutions.
However, for the most concentrated solutions and at high current densities (10.60
mA/cm2 in Fig. 3) the signal of Um does not reach a steady value and continues
increasing with time. This behavior corresponds to the formation of precipitates at the
anodic side of the membrane, as it was observed after the experiments.
1.8
0.009
Um (V)
10.60 mA/cm2
0.008
1.4
0.007
1.2
0.006
1.0
0.005
0.8
0.004
0.6
10.55 mA/cm
2
0.4
DUm/Dt
formation of Fe(OH)3
precipitates
1.6
0.003
0.002
10.48 mA/cm2
0.2
0.0
0
100
200
300
0.001
0
400
t (s)
Figure 3. Chronopotentiometric curves obtained for 10-2 M Fe2(SO4)3 solutions.
To explain the obtained results, the speciation diagram and the solubility (s) plot for
Fe2(SO4)3 solutions (shown in Fig. 4) were considered. Analogously as explained above
for Cr(III) solutions, in the case of Fe2(SO4)3 solutions, the transport of H+ ions at high
current densities would promote a shift in the speciation of Fe(III) species toward high pH
values. Consequently, FeSO4+, FeOH2+ and Fe(OH)2+ ions would be the species
responsible for the subsequent transition times observed in the chronopotentiograms of
Figure 3. Moreover, at high applied currents, the supply of the hydroxylated species of
Fe(III) to the membrane surface together with the basic pH originated by the H+ transfer
would originate the formation of precipitates at the anodic side of the membrane. In the
case of Fe(III) solutions, the formation of precipitates can take place at very acidic values,
as shown in Fig. 4(b), where the solubility of the Fe(III) and the Cr(III) species are
compared. On the contrary, in the case of Cr2(SO4)3 solutions the formation of
precipitates starts at significantly higher pH values, which would explain the absence of
the final increase in the Um values corresponding to the formation of precipitates in the
case of chromic sulfate solutions (see Figure 1).
a)
1
Fe(OH)2+
0.8
b)1.E+00
Fe(OH)3
1.E-01
FeSO4+
Cr2(SO4)3 10-2 M
Fe2(SO4)3 10-2 M
1.E-02
1.E-03
0.6
s
ai
1.E-04
1.E-05
0.4
FeOH2+
-
Fe(SO4)2
0.2
1.E-06
1.E-07
1.E-08
0
1.E-09
0
1
2
3
4
pH
5
6
7
0
2
4
6
8
10
12
14
pH
Figure 4. (a) Speciation diagram for 10-2 M Fe2(SO4)3 solutions and (b) solubility plot for
10-2 M Cr2(SO4) and 10-2 M Fe2(SO4)3 solutions. The vertical dashed line of Figure 4(a)
indicates the initial equilibrium conditions and the pH value.
3.2. Current-voltage curves
The data registered in the chronopotentiometric experiments also allows us to obtain the
current-voltage curves by means of plotting the values of current density against the Um,f
values. Typical current-voltage curves of monopolar ion-exchange membranes present
three characteristic parts: at low current densities a quasi-ohmic behavior describes the
direct relationship between the current density and Um; when the ilim value is achieved a
plateau region results from the increased membrane resistance; and finally a third region
appears, in which a rapid increase in the current with Um occurs due to the enhancement
of overlimiting mass transfer mechanisms.
Figure 5 shows different current-voltage curves obtained for varying concentrations of
Cr2(SO4)3 and Fe2(SO4)3. The curves obtained with Cr(III) solutions show an atypical
behavior in the case of diluted electrolytes. Two different limiting currents can be
identified and, consequently also two different plateau regions can be seen in the figure.
Taking into account the chronopotentiometric results, the first plateau (l1) would
correspond to the first transition time associated with the depletion of Cr3+ ions, whereas
the second (l2) would be related to the depletion of complex species of Cr(III) taking place
at higher current densities. In order to confirm this hypothesis, the resistance of each
characteristic region can be calculated from the inverse of the slope of the curve, as
indicated in the Figure, and is presented in Table 1. The higher conductance of Cr3+ ions
(multi-charged ions) contributes to the lower values obtained for R1 and this fact confirms
that Cr3+ ions are preferentially transported through the membrane at low current
densities. R2 values are significantly higher than those of R1, which is caused by the low
mobility of the positively charged complex Cr(III) species, which are much bigger in size
than Cr3+ ions and have a lower charge. On the contrary, the values of R3 corresponding
to the overlimiting region are intermediate between the values of R1 and R2. This is an
indicative that both Cr3+ ions and complex species of Cr(III) are transported through the
membrane when overlimiting currents are applied. In this case, it seems that the diffusion
and the mobility of the ions are not determining the supply of ions to the membrane
surface at overlimiting currents. On the contrary, convective phenomena would be
responsible for the supply of mixtures of ions towards the membrane surface in the same
proportion as they are in the bulk solution. The obtained results reinforce the idea that the
overlimiting mechanisms of ionic conductance are strongly related with hydrodynamic
instabilities under the application of strong electric fields.
a) 8
b)
1.5
5
1.5
7
ilim2
3
l2
ilim1
0.5
1/R2
1
0
0.0
0
0.5
1
ilim
2
1
Cr2(SO4)3 5·10-4 M
1/R1
1.0
3
0.5
Fe2(SO4)3 5·10-3 M
Cr2(SO4)3 10-2 M
l1
2
i (mA/cm )
4
2
1.0
1/R3
2
i (mA/cm )
2
i (mA/cm )
5
-4
Fe2(SO4)3 5·10 M
0
1.5
2
i (mA/cm )
4
6
0.0
0
0.5
1
1.5
Um (V)
Um (V)
Figure 5. Current-voltage curves obtained for (a) Cr2(SO4)3 solutions and (b) Fe2(SO4)3
solutions.
Table 1. Current-voltage characteristics obtained for Cr2(SO4)3 solutions.
C0 (mol/L) R1 ( ·cm2) R2 ( ·cm2) R3 ( ·cm2)
5·10-4
180.33
887.50
468.04
2.5·10-3
112.50
218.43
215.88
5·10-3
63.54
86.59
66.73
1·10-2
43.84
51.29
l1 (V)
0.500
0.077
0.045
-
l2 (V)
0.288
0.528
0.416
0.318
In the case of the most diluted Fe2(SO4)3 solutions the three characteristic regions are
clearly observed in the curves because the values of ilim1 and ilim2 are practically the same.
However, with concentrations of Fe2(SO4)3 of 5·10-3M and 10-2M the formation of
precipitates on the anodic side of the membrane suppresses the overlimiting region and
blocks the ionic transfer through the membranes. Consequently, Um increases toward
very high values with very small increments in the current density.
Formerly, the application of overlimiting currents in ED processes was not considered
because it was believed that it only increased the energy consumption. However, recent
studies have reported that operating in the overlimiting range of currents could lead to an
increase in the mass transfer of the ions of interest (Martí-Calatayud et al., 2012; Kim and
Lawler, 2012). In that case, the length of the plateau region of the current-voltage curves
can be regarded as a measure of the membrane voltage drop needed to be surpassed
for the overlimiting mass transfer mechanisms to appear. In order to ascertain which is
the effect of the concentration of trivalent metal cations on this parameter, in Fig. 6 the
plateau length is represented as a function of the concentration of Cr(III) (for which lplateau
= l1 + l2) and Fe(III). A clear decrease in the lplateau values are confirmed as the
concentration of multivalent metals increases. This would imply lower energy
consumptions when applying overlimiting currents with concentrated solutions of
multivalent metals.
0.9
0.8
0.7
Cr(III)
Fe(III)
lplateau (V)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.005
0.01
0.015
0.02
0.025
[M(III)] (mol/L)
Figure 6. lplateau values as a function of the metal concentration.
4. CONCLUSIONS
In the present study, various differences in the transport of free ions of multivalent metals
through cationic membranes with respect to that of charged complex species are
evidenced. There is a strong dependence of the ionic transport through the membranes
on the applied current. In the case of acidic solutions, Cr3+ ions are preferentially
transferred through the membranes. However, operating at current densities at which the
transport of H+ ions through the membranes is significant could lead to the transport of
other complex metallic species. The imposition of overlimiting currents is more
convenient in the case of concentrated solutions of multivalent metals, unless the
formation of precipitates occurs on the membrane surface. In the latter case, the
membrane surface is blocked; the transport of ions is suppressed, and the membrane
resistance would significantly increase the energy consumption of the process.
ACKNOWLEDGEMENTS
The authors of this work would like to express their gratitude to Ministerio de Ciencia e
Innovación (Spain) for their financial support (CTQ2008-06750-C02-01/PPQ) and to
Universitat Politècnica de València for a postgraduate grant (Ref. 2010-12).
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