363
Journal of Environmental Protection, 2012, 3, 363-373
http://dx.doi.org/10.4236/jep.2012.35046 Published Online May 2012 (http://www.SciRP.org/journal/jep)
The Effect of Generated Chlorine Gas on
Electroremediation of Heavy Metals from Offshore Muds
Sanghee Shin1, George V. Chilingar1*, Muhammad Haroun2, Bisweswar Ghosh2, Najmedin Meshkati1,
Sibel Pamukcu3, J. Kenneth Wittle4, Manal Al Badawi2
1
Sonny Astani Civil and Environmental Engineering, University of Southern California, Los Angeles, USA; 2Petroleum Institute,
Abu Dhabi, UAE; 3Lehigh University, Bethlehem, USA; 4Electropetroleum Inc., Wayne, USA.
Email: *[email protected]
Received January 20th, 2012; revised February 19th, 2012; accepted March 21st, 2012
ABSTRACT
The removal efficiency of heavy metals from offshore muds is enhanced in the presence of generated chlorine gas (Cl2).
The tests showed a high removal efficiency of heavy metals at the anode end of cores after 24 hours of EK application.
In the initial tests, high electrokinetic flow potential was achieved; however, high levels of chlorine gas were produced
in the high-salinity environments. The process was improved by controlling and maintaining a certain fraction of the
chlorine gas (Cl2) in place. The pH was controlled by the chlorine gas maintained in-situ and transported from the anode
to cathode. The transports of four heavy metals were evaluated in this study. The chlorine gas can have two impacts on
the transport of metals in the system. One is to oxidize the metal ions to a higher oxidation state and the second is to
form chloride complexes, which have higher mobility in the system. Determination of oxidation state and the subsequent metal chloride complex are left for future research.
Keywords: Offshore Sediments; Chlorine Gas Removal; Electroremediation; Contaminated Muds; Electrokinetic
Efficiency
1. Introduction
Electroremediationof heavy metals from muds have been
studied by Haroun (2009) [1], Pamukcu (2009) [2] and
Wittle and Pamukcu (1993) [3]. The writers investigated the efficiency of electroremidiation in the presence
of chlorine gas produced in high-salinity environment.
The sample diameter of cores was 3.81 cm (1.5 inch); pH
and temperature were measured continuously. Various
voltage gradients were used. The electrokinetics proved
to be successful in enhancing oil recovery (EEOR) in
actual field tests (Wittle, J.K. et al., 2008; Wittle, J.K. et
al., 2011) [4,5].
2. The Algorithm
D.C current, which is determined by voltage gradient,
was employed using electrokinetic technology. H+ was
produced at the anode (1) and OH– at the cathode (2).
H 2 O  2e   1 2 O 2  2H 
2H 2 O  2e   H 2  2OH 
(1)
(2)
These two equations are fundamental reactions at the
*
Corresponding author.
Copyright © 2012 SciRes.
electrodes occurring in the electrokinetic cell.
The chlorine gas (Cl2) was produced at the anode, together with O2 in the presence of saline water:
H 2 O  2e   1 2 O 2  2H 
(1)
2Cl  2e   Cl2
(3)
The electrokinetic velocity of a fluid of certain viscosity and dielectric constant through a surface-charged porous medium with zeta or electrokinetic potential (ζ), under an electric potential gradient E, is given by the Helmholtz-Smoluchowski (H-S) equation as follows:
ξDE
and
4π
AξDE
Qe 
4π
ve 
(4)
where,
ve = electrokinetic velocity, Qe= electrokinetic flow
rate, A = cross-sectional area, D = dielectric constant,
ξ  zeta potential , E = potential difference, and  
viscosity of the fluid.
The ζ potential in Equation (4) has been shown to vary
with the pH and ionic concentrations of the pore fluid, as
well as the electric field. Therefore, it is not constant durJEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
364
ing electrokinetic transport in clay medium (Probstein
and Hicks, 1993) [6].
cent to refinery—industrial area and 2) offshore muds in
Port area.
Mud samples were sieved using a 1-mm sieve and then
compressed in a cylinder 3.81 cm in diameter and 30 cm
long for seven days under an applied pressure of 30 psi.
Two samples 30 cm in length were treated continuously with D.C current for 24 hours: sample (1) using
open system and (2) using closed system. Two other
samples were treated for 40 hours (sample No. 3, open
system, and sample No. 4, closed system) with two 8hour interruptions (total treatment time = 24 hours).
Concentrations of As, Cr, Cs, and Zn were determined
initially and after EK treatment (ICP-MS).
3. Apparatus and Procedure
The apparatus consists of two chambers with the anode
and cathode electrodes, D.C power supply, solution tanks,
volumetric cylinders with gas measurement devices, peristaltic pumps, pressure measurement devices, and digital
pH measurement device (Figure 1). The peristaltic pump
was employed to control the pH in the cathode chamber
(flow rate controller). Gas measurement devices were
mounted to determine the amount of gas generated.
The pH was continuously maintained at a value less
than 10 at the cathode end. By maintaining the pH at less
than 10, the process efficiency improved and scaling at
the cathode was minimized. The saline water was flushed
through the anode chamber by a peristaltic pump. The
pH in the anode chamber was measured continuously and
remained at less than about 2.5. Throughout the test the
chlorine gas was produced at the anode.
In a closed system, the Cl2 gas was not allowed to escape with resulting increase in pressure, whereas in the
open system the Cl2 gas was allowed to escape.
5. Data Analysis
The experimental results are presented in Figures 2 through
17. Figures 2 through 11 show the results of 24 hour
treatment, whereas Figures 12 through 17 show the results after EK treatment applied for 40 hours with two
8-hour interruptions (actual treatment time = 24 hours).
Application of potential gradient of 3.5 V/cm in the open
system resulted in an increased removal efficiency when
compared to application of 3.5 V/cm in the closed system.
Higher removal efficiency was obtained by taking advantage of Cl2 gas generated at the anode and transported
to the cathode in a closed system. Reduced power consumption with higher volumes of produced water was
achieved in the closed system.
4. Experimental Results
The samples were collected from two different contaminated areas in Abu Dhabi, UAE: 1) offshore muds adja-
H2
Cl2, O2
valve
Gas
measurement
Gas
measurement
Saline water
Pump
Temp, and pH meter
Pressure
Pressure
Used Saline
water
Pump
Saline water
& EDTA
Used Saline
water
Anode
Cathode
D.C
Figure 1. Apparatus used in electroremediation of heavy metals from offshore sediments with equipment for chlorine gas
removal.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
365
Al-30 cm, 3.5 V/cm, 24 hours, Closed system
1250
1150
Concentration (ppm)
1050
950
Initial Concentration
Final Concentration
850
750
650
1
2
3
4
5
Location in core sample
Figure 2. Concentration of Al upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Al-30 cm, 3.5 V/cm, 24 hours, Open system
1100.00
1050.00
1000.00
Concentration (ppm)
950.00
900.00
Initial Concentration
Final Concentration
850.00
800.00
750.00
700.00
1
2
3
4
5
Location in core sample
Figure 3. Concentration of Al upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
366
As-30 cm, 3.5 V/cm, 24 hours, Closed system
2.4
2.2
Concentration (ppm)
2
1.8
Initial Concentration
1.6
Final Concentration
1.4
1.2
1
1
2
3
4
5
Location in core sample
Figure 4. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
As-30 cm, 3.5 V/cm, 24 hours, Open system
1.85
1.75
Concentration (ppm)
1.65
1.55
Initial Concentration
Final Concentration
1.45
1.35
1.25
1
2
3
4
5
Location in core sample
Figure 5. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
367
Cr-30 cm, 3.5 V/cm, 24 hours, Closed system
13.5
12.5
11.5
10.5
Concentration (ppm)
9.5
8.5
Initial Concentration
7.5
Final Concentration
6.5
5.5
4.5
3.5
1
2
3
4
5
Location in core sample
Figure 6. Concentration of Cr upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Cr-30 cm, 3.5 V/cm, 24 hours, Open system
10.00
9.00
Concentration (ppm)
8.00
7.00
Initial Concentration
Final Concentration
6.00
5.00
4.00
1
2
3
4
5
Location in core sample
Figure 7. Concentration of Cr upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
368
Cs-30 cm, 3.5 V/cm, 24 hours, Closed system
0.1
0.09
Concentration (ppm)
0.08
0.07
Initial Concentration
Final Concentration
0.06
0.05
0.04
1
2
3
4
5
Location in core sample
Figure 8. Concentration of Cs upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Cs-30 cm, 3.5 V/cm, 24 hours, Closed system
0.08
0.07
Concentration (ppm)
0.07
Initial Concentration
0.06
Final Concentration
0.06
0.05
1
2
3
4
5
Location in core sample
Figure 9. Concentration of Cs upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
369
Zn-30 cm, 3.5 V/cm, 24 hours, Closed system
11
10
9
Concentration (ppm)
8
7
Initial Concentration
Final Concentration
6
5
4
3
1
2
3
4
5
Location in core sample
Figure 10. Concentration of Zn upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Zn-30 cm, 3.5 V/cm, 24 hours, Closed system
6.30
6.10
5.90
Concentration (ppm)
5.70
5.50
Initial Concentration
5.30
Final Concentration
5.10
4.90
4.70
4.50
1
2
3
4
5
Location in core sample
Figure 11. Concentration of Zn upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 24 hours.
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
370
Al-30 cm, 3.5 V/cm, 24 hours, Closed system
(with two 8-hour interruptions)
2350.00
2300.00
2250.00
Concentration (ppm)
2200.00
2150.00
Initial Concentration
2100.00
Final Concentration
2050.00
2000.00
1950.00
1900.00
1
2
3
4
5
Location in core sample
Figure 12. Concentration of Al upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
Al-30 cm, 3.5 V/cm, 24 hours, Open system
(with two 8-hour interruptions)
2200.00
2150.00
2100.00
Concentration (ppm)
2050.00
2000.00
Initial Concentration
1950.00
Final Concentration
1900.00
1850.00
1800.00
1750.00
1
2
3
4
5
Location in core sample
Figure 13. Concentration of Al upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
371
As-30 cm, 3.5 V/cm, 24 hours, Open system
(with two 8-hour interruptions)
1.70
1.60
Concentration (ppm)
1.50
1.40
Initial Concentration
1.30
Final Concentration
1.20
1.10
1.00
1
2
3
4
5
Location in core sample
Figure 14. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
As-30 cm, 3.5 V/cm, 24 hours, Closed system
(with two 8-hour interruptions)
1.90
1.80
Concentration (ppm)
1.70
1.60
Initial Concentration
1.50
Final Concentration
1.40
1.30
1.20
1
2
3
4
5
Location in core sample
Figure 15. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
372
Cr-30 cm, 3.5 V/cm, 24 hours, Open system
(with two 8-hour interruptions)
31.00
29.00
Concentration (ppm)
27.00
25.00
Initial Concentration
23.00
Final Concentration
21.00
19.00
17.00
1
2
3
4
5
Location in core sample
Figure 16. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
Cr-30 cm, 3.5 V/cm, 24 hours, Closed system
(with two 8-hour interruptions)
34.00
33.00
32.00
Concentration (ppm)
31.00
30.00
Initial Concentration
29.00
Final Concentration
28.00
27.00
26.00
25.00
1
2
3
4
5
Location in core sample
Figure 17. Concentration of As upon EK treatment along the core length of 30 cm. Potential gradient = 3.5 V/cm, treatment
time = 40 hours (with two 8-hour interruptions).
Copyright © 2012 SciRes.
JEP
The Effect of Generated Chlorine Gas on Electroremediation of Heavy Metals from Offshore Muds
Figures 12 through 17 displayed results of a second
set of tests to illustrate the effect of chlorine gas on removal efficiency.
The continuous application of D.C in closed system
gave better results than interrupted application of D.C.
Final pH at the cathode was reduced with the aid of
transported Cl2 gas.
6. Conclusions
In the case of tests involving Cl2 gas, in a closed system
with a potential gradient of 3.5 V/cm, removal efficiency
was higher than in the open system with 3.5 V/cm electric potential. Higher removal efficiency was obtained by
using the Cl2 gas generated at the anode and transported
to the cathode in a closed system. Reduced power consumption with higher volumes of produced water was
achieved in the closed system.
In this study, the continuous application of D.C in
closed system gave better results than interrupted application of D.C. Final pH at the cathode was reduced by
Cl2 gas. The chlorine gas possibly can have two impacts
on the transport of metals in the system. One is to oxidize
the metal ions to a higher oxidation state and the second
is to form chloride complexes which have higher mobility in the system. In conclusion, it appears that presence
of Cl2 gas improves the efficiency of remediation.
7. Acknowledgements
The authors gratefully acknowledge the support of The
Petroleum Institute, Abu Dhabi, U.A.E., University of
Southern California, CA, USA, Lehigh University, PA,
Copyright © 2012 SciRes.
373
USA, and Electropetroleum Inc., PA, USA.
REFERENCES
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Heavy Metals by Electroremediation of Offshore Muds,”
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