Removal of Cobalt from Drinking Water by Alternating Current Electrocoagulation Technique
Saber E. Mansour,
Ibrahim H. Hasieb
Hussein A. Khalaf
This research presents a preliminary study for the removal of cobalt from drinking water using Alternating Current Electrocoagulation (ACE) technology. The experiments were carried out using batch apparatus. Batch experiments with two monopolar aluminum plate anodes and cathodes were employed as electrodes. The effect of operational parameters such as initial pH, current density, reaction time, initial concentrations, solution conductivity and inter-electrode distance were studied in an attempt to reach higher Co (II) ion removal efficiency. Important operating parameters were optimized to attain higher (98.60%) Co(II) removal efficiency as follows: inter-electrode distance: 0.005 m, current density: 0.04 A m-2, operating time: 35 min, pH: 7.5, conductivity: 13 S m-1, frequency: 50 Hz and voltage: 50 V. The adsorption process followed first- order kinetics and the temperature studies showed that the adsorption was exothermic and spontaneous in nature.
Received: October 11, 2011;
Accepted: April 14, 2012;
Published: June 20, 2012
Suspended and dissolved materials in water are produced by the natural weathering
of minerals. Inorganic particles may consist of iron oxides, salts, sulfur,
silts and clays. Depending on the concentration of these particles in raw water
sources, human health effects can vary from beneficial to toxic. Dissolved cobalt
compounds are naturally occurring substances that can impact human health. Cobalt
is an essential oligoelement necessary for the formation of vitamin B12 (hydroxocobalamin).
The total daily intake of cobalt is variable and may be as much as 1 mg. Investigations
of health risk caused by the exposure to cobalt in drinking water at very low
levels have revealed the unlikely potential for adverse health effects to humans
(Karim, 2011). However, excessive administration of
this trace element (>5 mg day-1) produces abnormal thyroid functions,
polycythemia and overproduction of red blood cells (erythropoiesis) with increased
production of the hormone erythropoietin from the kidneys (Lauwerys
and Lison, 1994; Barceloux, 1999). Therefore, a
main goal in supplying quality drinking water is the maximum removal of such
Direct Current Electrocoagulation (DCE) technology has proven to be effective
in the removal of ionic species from wastewater, particularly heavy metals (Mollah
et al., 2004; Parga et al., 2007).
This process can eliminate over 99% of some heavy metal cations (Duffey,
1983). It is also able to remove significant amounts of the destabilized
suspended, emulsified, or dissolved contaminants from an aqueous medium (Benefield
et al., 1982). The fundamental operating principle is that cations
produced electrolytically from the sacrificial iron and/or aluminum anodes provide
continuous supply of polyvalent metal cations (coagulants) near the anode. These
cations react with the OH¯ ions generated at the cathode during the evolution
of hydrogen, to produce various forms of gelatinous monomeric and polymeric
hydroxo cationic species (Babu et al., 2007).
These activated intermediates can interact with the destabilized contaminants
via several routes (Mollah et al., 2001; Mollah
et al., 2004), creating metal oxides and hydroxides which agglomerate
and settle out of suspension. The proposed EC mechanisms for the electrode reactions
and the production of H2 (g) and OH- (at the cathode)
and H+ and O2 (g) (at the anode) are discussed elsewhere
(Moreno et al., 2009).
However, The DCE technology is associated with the formation of the passivation
layers on the cathode as well as corrosion of the anode due to oxidation. This
prevents the effective current transfer between the electrodes and therefore,
leads to the loss of efficiency of DCE processes. These drawbacks have been
controlled by applying Alternating Current Electrocoagulation (ACE) in the removal
processes (Vasudevan et al., 2011; Mansour
and Hasieb, 2012). Alternating current was used to prevent he passivity
or polarization of electrodes. It is also believed that the ac cyclic energization
can retard electrophoretic transport of the charged particles and may induce
dipoledipole interactions in water containing non spherical charged species
and thus disrupt the stability of balanced dipolar structures existing in such
a system. The main objective of this study was to investigate the effects of
alternating current on the removal of cobalt from drinking water using aluminum
electrodes and to determine the effects of several parameters, namely initial
pH, current density, initial Co(II) ion concentrations, inter-electrode distance,
conductivity and electrolysis time, on the removal efficiency.
MATERIALS AND METHODS
ACE processes were conducted in a lab-scale batch system, which was composed of an electrolysis cell, ac power supply, a magnetic stirrer and pH meter. An electrolysis cell was made of rectangular glass tank of 2 L capacity in which a pair of commercially obtained aluminum plates of size 0.15x0.15x0.004 m were used as electrodes. The electrodes were immersed vertically in the bottom of the reactor to a 15 cm depth with an effective area of 225 cm2 each. Currents (ranging 0.01-0.04 A m-2) were applied between the electrodes. One liter experimental solution of initial Co(II) concentration (ranging 5-25 mg L-1) was placed in ACE reactor and slowly stirred with a magnetic bar at 200 rpm. The inter-electrode distances were varied from 0.005-0.02 m. During electrolysis, samples of 5 mL were taken every 5 min and filtered using Whatman filter paper (Grade 40) and analyzed for cobalt. The concentration changes of Co(II) ions were determined by UV-Spectrophotometer (Beckman, DU 800). The electrical conductivity and viscosity were measured using conductivity meter and suspended Ubbelohde type viscometer, respectively.
The calculation of % removal efficiency of Co(II) after ACE treatment was performed
using the equation (Daneshvar et al., 2006):
%RE = [ Co-C/Co]x100
where, Co and C concentration of dissolved Co(II) ions before and after ACE process in mg L-1, respectively.
RESULTS AND DISCUSSION
Applied current density plays significant role in electrolytic treatment as
it is the only operational parameter that can be controlled directly. It determines
the rate of electrochemical metal dosing and electrolytic bubble production
and size and the floc growth resulting in a faster removal of pollutants (Khosla
et al., 1991; Holt et al., 2002).
Measurements were carried out at different current densities 0.01, 0.02 ,0.03
and 0.04 A m-2 at fixed electrode spacing of 0.005 m, with the same
concentration of 25 mg L-1 of Co(II) solution of pH = 7.5. According
to Fig. 1 the removal rates of the studied metal increased
with increasing current density. Also with decrease of electrical current, the
required time for achieving similar efficiencies increases. This was attributed
due to the fact that at high current densities, the extent of anodic dissolution
increased and in turn the amount of hydroxo-cationic complexes resulted in increase
of cobalt removal. These findings are in line with the results of Cr6+
removal investigations (Kumar et al., 2004; Bazrafshan
et al., 2007).
It is well known that pH is an important parameter influencing the efficiency
of the EC process (Mollah et al., 2004). As can
be seen in Fig. 2 the ACE treatment using aluminum electrodes
induces an increase in the pH when the initial pH value of the medium was 7.5
||Percentage removal efficiency of Co(II ) ions vs. time at
different current densities in solutions containing 25 mg L-1
of the metal
||Variation of pH of Co(II) solutions with time at different
current densities. Inter-electrode distance: 0.005 m, Initial Co(II) concentration:
25 mg L-1, Conductivity: 13 S m-1, Current frequency:
50 Hz and Potential: 50 V
||Residual concentration of Co(II) vs. time. Inter- electrode
distance: 0.005 m, Current density: 0.04 A m-2, AC of frequency:
50 Hz and Potential: 50 V, pH: 7.5, Conductivity 13 S m-1
This solution pH stabilizes at nearly constant value around 8.7. It is also
noticed (Fig. 3), that the removal efficiency of the studied
metal after 35 min of electrolysis time at the constant current density of 0.04
A m-2 reached very high values, 98.60% in the pH range 7.5-8. These
results could be explained by the excess of hydroxyl ions produced at the cathode
and by the intensification in Al (OH)3 generated in solution during
electrolysis time in neutral and slightly alkali conditions which produce more
of aluminum hydroxides with a consequent removal of Co(II) (Daida,
2005; Ghernaout et al., 2008). The stabilization
of pH at nearly constant value around 8.7 may be ascribed to the buffering capacity
of complex nature of aqua Al3+/Al(OH)3 system (Kobya
et al., 2006).
To explain the effect of initial Co(II) concentration and the time required
for its quantitative removal, a set of experiments were performed with five
different solutions containing initial concentrations of 25, 20, 15, 10 and
5 mg L-1 of the Co(II) ion. The solutions were treated at a constant
current density, frequency, and potential of 0.04 A m-2, 50 Hz and
50 V, respectively and different times of electrolysis. It is evident from Fig.
3, that ACE enables lowering [Co2+] to Ca, 0.2 of the initial
concentration within a reasonable time scale (in the first 5 min of operation),
indicating that ACE with Al electrodes is a promising method for reducing the
contamination level of cobalt without adding mediators. These observations are
in line with results of previous studies of the Arsenic removal efficiency (Kumar
et al., 2004) and Cr(VI) removal (Chaudhary et
al., 2003; Bazrafshan et al., 2008) by
EC processes. A complete removal is observed at the end of 35 min o f process.
The higher initial concentrations needed longer operating time to be quantitatively
||Effects of inter-electrode distance on Co(II) removal. Current
density: 0.04 A m-2, AC of frequency: 50 Hz and Potential: 50
V, pH: 7.5
Certainly, when the initial cobalt concentrations were higher, more aluminum
hydroxides were required to decrease the dissolved cobalt concentrations.
To explore the effect of inter-electrode distances on removal efficiency ,
the distance between electrodes was varied at the same energy input. As can
be seen from Fig. 4, When the inter-electrode distances changed
from 0.005-0.025 m under 50 Hz and 50 V, the removal (%) of the Co(II) was decreased
from 98.60-85.90%. As expected, An increase of local concentration of the Co(II)
ions with monomeric and polymeric hydroxo cationic species generated in a smaller
space will increase electrostatic interactions, leading to an increase of %
removal of dissolved ions (Mansour and Hasieb, 2012).
It appears that conductivity also had some effect on the removal efficiency
of Co(II) in the investigated range shown in Fig. 5. It is
observed, that the % RE of Co(II) ion increased with increasing conductivity.
An increase in conductivity decreases the internal resistance (or the ohmic)
drop between electrodes and therefore more aluminum ions could be produced at
the same energy input. Also, the formation of (AlCl2+)
ions is expected to enhance chemical dissolution of Al electrodes (Szynkarczuk
et al.,1994). All the results obtained were consistent with the previous
studies (Daida, 2005; Kim et al.,
2002). Concentration: 25 mg L-1, Current frequency: 50 Hz and
Potential: 50 V.
In the ACE process, the removal rate of Co(II) ions is proportional to the
amount of hydroxyl cationic complexes (Al(OH)3) which can effectively
remove Co(II) ions. Further, the applied current density determines the rate
of aluminum hydroxide production.
|| Pseudo-kinetic rate constants with first-order and second-order
models for cobalt ion removal at various initial cobalt concentrations
||Equivalent conductance (cm2 ohm-1 eq)
as a function of residual cobalt ion concentrations (mg L-1).
Inter- electrode distance: 0.005 m, initial Co(II) concentration: 25 mg
L-1, Conductivity: 13 S m-1 current frequency: 50
Hz and potential: 50 V
||Pseudo-first-order kinetics plot for the adsorption of various
initial cobalt ion concentrations on hydroxyl cationic complexes
In order to process, both pseudo first-order and pseudo second-order kinetic
models were used to fit the experimental data. For a pseudofirstorder
kinetic model, the integrated rate law is:
ln Ct = - k1t+ln C0
Here, C0 (mg L-1) is the initial concentration and Ct
(mg L-1) is the concentration at time t.
||Pseudo-second-order kinetics plot for the adsorption of various
initial cobalt ion concentrations on hydroxyl cationic complexes
As shown in Fig. 6, The slopes of the plots of ln Ct
versus t give the values of the rate constants k1 (min-1).
For a pseudo-second-order kinetic model, the integrated rate law is:
1/Ct = k2t+1/C0
The plots of 1/Ct versus t should give the value of the rate constant k2 (L mg-1 min). The values of the rate constants for first order and second-order models for Co(II) ion removal at various initial Co(II) ion concentrations are listed in Table 1. The conformity between experimental data and the model values was evaluated using the correlation coefficient values R2. As shown in Table 1, regardless of the initial Co(II) ion concentration, R2 values for the first- order model were dramatically higher than that for the second-order model. It may also be ascertained from the experimental results that the reaction rate constant k1 is independent of the initial cobalt concentration and other system parameters.
Therefore, the adsorption of Co(II) ion on hydroxyl cationic complexes is more
appropriately followed by the pseudo-first-order kinetic model. Further, the
nonlinearity plots at various initial concentrations of second-order model shown
in Fig. 7 suggested that the experimental data was best fitted
pseudo-first order kinetics.
||Thermodynamic parameters of the adsorption of Co(II) ions
in ACE process
Similar modeling results are also found in the kinetic studies on removal of
Mn2+ ions from synthetic wastewater by EC process (Shafaei
et al., 2010) and the removal of arsenate (Vasudevan
et al., 2010). It was also reported (Emamjomeh
and Sivakumar, 2006) that the defluoridation rate of the EC follows first
order kinetics with respect to fluoride concentration.
The Gibbs free energy change (ΔG) is the fundamental criterion of the spontaneity of a process. The thermodynamic parameters of the adsorption process are summarized in Table 2. The negative values of free energy ΔG indicate the feasibility of the process and its spontaneous nature. The ΔG values at different temperatures approximately remain constant for adsorption of Co(II) ions indicating that there is no effect of temperature on free energy of adsorption. The ΔH and ΔS were calculated from the plot of ln K versus 1/T and their values are shown in Table 2.
The negative values of ΔH for the present system confirmed the exothermic
nature of adsorption. The positive value of ΔS observed for the adsorption
of Co(II) ions suggested the increased randomness at the solid-solution interface
during the adsorption process. The Co(II) ions in the aqueous media are hydrated.
When the Co(II) ions get adsorbed on the adsorbent surface, the water molecule
bonded to the Co(II) ions by hydrogen bonds get released and dispersed in the
solution, this results in an increased in the entropy. The positive values of
entropy change also reflect good affinity, either physical or chemical, of the
Co(II) ions toward the adsorbent. The entropy changes results in the present
study are in excellent agreement with the literature (Vasudevan
et al., 2011).
The results of this study on cobalt ions removal from aqueous solution using Alternating Current Electrocoagulation (ACE) system can be summarized as follows:
||The gelatinous charged aluminum hydroxides generated in ACE
process can efficiently remove cobalt ions by adsorption. Considering the
removal efficiency at a specific energy input, a current density: 0. 04
A m-2, an inter-electrode distance: 0.005 m, operating time:
35 min, pH: 7.5, conductivity: 13 S m-1 , frequency: 50 Hz and
voltage: 50 V were found to be the optimum values for the present electrocoagulation
||It was found that increasing the initial cobalt ion concentration from
5 to 25 mg L-1 decreased the removal efficiency of cobalt ions
||Values of kinetic rate constants for cobalt ion removal at various initial
concentrations were calculated. The kinetic results showed that a pseudo-first-order
kinetic model matched satisfactorily with the experimental observations
||Values of thermodynamic parameters for cobalt ion removal as a function
of temperature were calculated. The temperature studies showed that the
adsorption was exothermic and spontaneous in nature
The authors are grateful for support of this project provided by Omar Al-Mukhtar University, Al-Bayda, Libya.
1: Barceloux, D.G., 1999. Cobalt chloride administration in athletes: A new perspective in blood doping? J. Toxicol. Clin. Toxicol., 37: 201-206.
2: Bazrafshan, E., A.H. Mahvi, S. Nasseri and M. Shaieghi, 2007. Performance evaluation of electrocoagulation process for diazinon removal from aqueous environments by using iron electrodes. Iran. J. Environ. Health Sci. Eng., 4: 127-132.
Direct Link |
3: Bazrafshan, E., A.H. Mahvi, S. Naseri and A.R. Mesdaghinia, 2008. Performance evaluation of electrocoagulation process for removal of chromium (VI) from synthetic chromium solutions using iron and aluminum electrodes. Turk. J. Eng. Environ. Sci., 32: 59-66.
Direct Link |
4: Benefield, L.D., J.K. Judkins and B.L. Weand, 1982. Process Chemistry for Water and Wastewater Treatment. Prentice-Hall, NJ, USA.
5: Chaudhary, A., N. Goswami and S.M. Grimes, 2003. Electrolytic removal of hexavalent chromium from aqueous solution. J. Chem. Technol. Biotechnol., 78: 877-883.
6: Daida, P., 2005. Removal of arsenic from water by electro coagulation using Al-Al Fe-Fe electrode pair systems and characterization of by product. UMI Microform, pp: 1-68.
7: Daneshvar, N., A. Oladegaragoze and N. Djafarzadeh, 2006. Decolorization of basic Dye solutions by electrocoagulation: An investigation of the effect of operational parameters. J. Hazard Mater., 129: 116-122.
8: Duffey, J.G., 1983. Electrochemical Removal of Heavy Metals from Wastewater. Product Finishing Inc., Hertfordshire, UK., pp: 72..
9: Emamjomeh, M.M. and M. Sivakumar, 2006. An empirical model for defluoridation by batch monopolar Electrocoagulation/Flotation (ECF) process. J. Hazard. Mater., 131: 118-125.
10: Ghernaout, D., A. Badis, A. Kellil and B. Ghernaout, 2008. Application of electrocoagulation in Escherichia Coli culture and two surface waters. Desalination, 219: 118-125.
11: Holt, P.H., G.W. Barton, M. Wark and C.A. Mitchell, 2002. A quantitative comparison between chemical dosing and electrocoagulation. Colloids Surf. A: Physicochem. Eng. Aspects, 211: 233-248.
Direct Link |
12: Karim, Z., 2011. Risk assessment of dissolved trace metals in drinking water of Karachi, Pakistan. Bull. Environ. Contam. Toxicol., 86: 676-678.
CrossRef | PubMed |
13: Khosla, N.K., S. Venkatachalam and P. Somasundaran, 1991. Pulsed electrogeneration of bubbles for electroflotation. J. Appl. Electrochem., 21: 986-990.
Direct Link |
14: Kim , T.H., C. Park, E.B. Shin and S. Kim, 2002. Decholorization of disperse and reactive dyes by continuous electro coagulation process. Desalination, 150: 165-175.
15: Kobya, M., E. Demirbas, O.T. Can and M. Bayramoglu, 2006. Treatment of levafix orange textile dye solution by electrocoagulation. J. Haz. Mater., 132: 183-188.
16: Lauwerys, R. and D. Lison, 1994. Health risks associated with cobalt exposure-an overview. Sci. Total Environ., 150: 1-6.
17: Mollah, M.Y.A., R. Schennach, J.R. Parga and D.L. Cocke, 2001. Electrocoagulation (EC)-science and applications. J. Hazard. Mater., 84: 29-41.
CrossRef | Direct Link |
18: Mollah, M.Y.A., P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga and D.L. Cocke, 2004. Fundamentals, present and future perspectives of electrocoagulation. J. Hazard. Mater., 114: 199-210.
19: Moreno, C.H.A., D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson and C. Garcia, 2009. Electrochemical reactions for electrocoagulation using iron electrodes. Ind. Eng. Chem. Res., 48: 2275-2282.
20: Parga, J.R., J.L. Valenzuela and C.T. Francisco, 2007. Pressure cyanide leaching for precious metals recovery. JOM J. Miner. Metals Mater. Soc., 59: 43-47.
21: Babu, R.R., N.S. Bhadrinarayana, K.M.M.S. Begum and N. Anantharaman, 2007. Treatment of tannery waste water by electro coagulation. J. Univ. Chem. Technol. Metall., 42: 201-206.
Direct Link |
22: Kumar, P.R., S. Chaudhari, K.C. Khilar and S.P. Mahajan, 2004. Removal of arsenic from water by electro coagulation. Chemosphere, 55: 1245-1252.
CrossRef | PubMed |
23: Shafaei, A., M. Rezayee, M. Arami and M. Nikazar, 2010. Removal of Mn2+ ions from synthetic wastewater by electrocoagulation process. Desalination, 260: 23-28.
24: Szynkarczuk, J., J. Kan, T.A.T. Hassan and J.C. Donini, 1994. Electrochemical coagulation of clay suspensions. Clay Clay Miner., 42: 667-673.
Direct Link |
25: Vasudevan, S., J. Lakshmi and G. Sozhan, 2010. Studies on the removal of arsenate by electrochemical coagulation using aluminum alloy anode. CLEAN-Soil Air Water, 38: 506-515.
26: Vasudevan, S., J. Lakshmi and S. Ganapathy, 2011. Effects of alternating and direct current in electrocoagulation process on the removal of cadmium from water. J. Hazard. Mater., 192: 26-34.
27: Mansour, S.E. and I. H. Hasieb, 2012. Removal of nickel from drinking water by electrocoagulation technique using alternating current. Curr. Res. Chem., (In Press).