Separation and Purification Technology 79 (2011) 139–143

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Separation and Purification Technology

journal homepage: www.elsevier.com/locate/seppur

Electrokinetic remediation with high frequency sinusoidal electric fields

Adrian Rojo ∗ , Henrik K. Hansen, Milagros Agramonte
Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa Maria, P.O. Box 110 V, Valparaíso, Chile

a r t i c l e i n f o a b s t r a c t

Keywords: In this work an electrokinetic remediation cell for copper mine tailings using sinusoidal electric field was
Electroremediation analyzed. The sinusoidal electric field was obtained by applying simultaneously continuous-alternating
Alternating current voltages; in this work an alternating voltage of high frequency was applied. The system was tested con-
Direct current
sidering the effect of: (1) the effective voltage applied to the cell and (2) the polarity reversal of the cell.
Copper mine tailings
According to the conditions studied in this investigation, the laboratory results showed that a high fre-
quency sinusoidal electric field improves the EKR process, and increasing the effective voltage improves
the remediation action, especially when a polarity inversion is present, which reduces polarization during
the process.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction materials if it could be possible to extract the metals during the

remediation processes.
In recent years, the copper mining industry in Chile has reached Due to the magnitude of the mining activity in Chile, it becomes
annual production levels higher than 5.0 million metric tons (MMT) necessary to find solutions to mitigate the impact of mining waste
of fine copper [1]. This production, according to the efficiencies of on the environment. One aspect of the solution to the problem is
the current mining processes and the copper ore grades exploited, to give stability to the mining waste by heavy metal’s remediation
generates a variety of wastes that represent a considerable envi- processes. One method that could be suitable for remediation of
ronmental liability. In the case of copper mine tailings, which mining wastes is the use of electric fields for the removal of metals,
can be characterized as low-grade ores, without any treatment the so-called electrochemical remediation: electrokinetic or elec-
are deposited in safe places such as dumps and ponds. These trodialytic remediation (EKR or EDR). This principle has been used
deposits have been accumulating large quantities of waste, and rep- successfully during around 20 years for the treatment of heavy
resent a serious environmental risk due to their content of heavy metal polluted soil [4–10]. Especially metals such as copper, zinc,
metals, for example the copper content in typical residues from lead and arsenic have been removed or concentrated when apply-
copper mining is in the interval 100–2000 ppm [2], other metals ing electric fields—and these metals are typically also found in
found in elevated concentrations are Zn, As, Cd, Pb, and Fe. Conse- mining waste.
quently, since wastes are not treated before disposal, they typically The objective of this work is the application of high frequency
become unstable over time due to natural processes of oxidation sinusoidal electric field obtained by applying simultaneously
[3]. continuous-alternating (DC–AC) voltages, to enhance the behavior
Unfortunately wastes not only have a damaging effect on hydro of an EKR cell for copper mine tailings. The application of DC–AC
resources by natural leaching, but also generate effects on flora and electric field as power source for electroremediation of organic con-
fauna, and have serious effect on air quality by the generation of taminants in soils has been investigated lately [11]. In the case
fugitive emissions of fine particles. The more complex case occurs of EKR with copper mine tailings; according to the magnitude of
when heavy metals leach into groundwater and rivers, because the DC–AC voltages a periodic reversal of polarity of the system can
distribution of these pollutants in the environment becomes fast be produced. This periodic phenomenon improves the efficiency of
and uncontrollable. Therefore action has to be taken to deal with EKR technique with copper mine tailings [12,13], mainly because
the growing problem of accumulation of mining waste, and one it reduces the polarization in the cell. In this work an alternating
possible way to treat this problem is to remediate in situ these voltage of high frequency (50 Hz) was applied.
wastes. This could both be of direct importance for the environment
by excluding leaching of metals, but also a minimisation of raw 2. Experimental

2.1. Experimental tailings

∗ Corresponding author. Tel.: +56 32 2654463; fax: +56 32 2654478. The mine tailing used for remediation experiments was sam-
E-mail address: adrian.rojo@usm.cl (A. Rojo). pled from the Caren impoundment at Codelco-El Teniente copper

1383-5866/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2011.03.029
140 A. Rojo et al. / Separation and Purification Technology 79 (2011) 139–143

Table 1 solid particles by vacuum through a 0.45 ␮m filter and diluted to

Characteristics of the mine tailing sample.
100.0 mL. The metal content was determined by Atomic Absorption
Quartz SiO2 Spectrometry (AAS) in flame.
Muscovite KAl2 (Si3 Al)O10 (OH,F)2 The soluble copper content of the tailings was determined by
Ferric clinochlore (Mg,Fe)6 (Si,Al)4 O10 (OH)8 adding 50 mL H2 SO4 5% (v/v) to 5.0 g of dry material, and stirring
Calcic albite (Na,Ca)Al(Si,Al)3 O8 the sample in a 250 mL Erlenmeyer flask for 30 min. The liquid was
Anorthite CaAl2 Si2 O8 separated from the solid particles by vacuum through a 0.45 ␮m
Hydrated calcium sulfate CaSO4 ·0.6H2 O
filter and diluted to 100.0 mL by adding 10 mL concentrated HCl
Chalcocite Cu2 S
Brochantite Cu4 SO4 (OH)6 and distilled water. The metal content was determined by AAS in
Chalcopyrite CuFeS2 flame.
Ramsbeckite Cu15 (SO4 )4 (OH)22 ·6H2 O pH was measured by mixing 5.0 g dry matter and 25.0 mL dis-
Wroewolfeite Cu4 (SO4 )(OH)6 ·2H2 O
tilled water. After 1 h of contact time, pH was measured using a pH
Guildite CuFe(SO4 )2 (OH)·4H2 O
Total copper concentration [mg/kg] 920 electrode.
Soluble copper concentration [mg/kg] 413
pH 6.5 2.3. Tailings pre-treatment

Before remediation experiments, the tailings were stove-dried

mine in VI Region of Chile. Table 1 gives characteristics of the mine for two days at 70 ◦ C. Once dried, the material was pulverized in a
tailings used, determined by X-ray diffraction analysis and by ana- mortar and sieved with meshes #4 and #20, until a homogeneous
lytical methods. sample was obtained. For all experiments, 1 M sulfuric acid was
added to the tailings until an average humidity of 20% was reached.
2.2. Analytical methods
2.4. Experimental cell design
The total and soluble copper was determined according to the
following methods. In both cases the analysis was done in triplicate, A schematic description of the remediation cell is given in Fig. 1.
and an average was used. Experiments were carried out in an opened acrylic straight paral-
The total copper content of the tailings was determined by lelepipeds cell with a rectangular base of 30 cm × 10 cm and 10 cm
adding 20 mL 1:1 HNO3 to 1.0 g of dry material and treating the height, divided in three compartments, the length of the central
sample in autoclave, according to the Danish Standard DS 259:2003 compartment was 20 cm, and two electrode compartments 5 cm
(30 min at 200 kPa (120 ◦ C)). The liquid was separated from the each. The power supply, which combines AC–DC voltages, was con-

Fig. 1. Schematic description of EKR cell.

 A. Rojo et al. / Separation and Purification Technology 79 (2011) 139–143 141

Table 2 Table 3
Remediation conditions. Final total copper concentration, CF [mg/kg] and pH.

Exp Applied voltage Veffective [V] Vmax [V] Vmin a [V] Exp Anode Center Cathode Anode Center Cathode

DC [V] AC [V] CF pH

1 7.2 23.0 17.8 30.2 −15.9 1 740 880 610 3.2 3.1 3.2
2 15.3 23.0 22.3 38.3 −7.8 2 420 630 570 3.3 3.4 3.5
3 23.3 23.0 28.4 46.3 0.3 3 842 860 1075 3.1 3.2 3.2
4 29.0 23.0 33.2 52.0 6.0 4 480 880 1150 3.6 2.5 3.5
5 23.3 31.0 31.8 54.3 −7.7 5 460 950 840 2.9 2.9 3.0
6 29.0 31.0 36.4 60.0 −2.0 6 490 920 610 3.0 3.1 6.7
7 23.0 – – – – 7 450 620 1000 2.9 3.1 3.5
a
Negative values, experiment with periodic polarity reversal.

Table 4
General remediation results in terms of removal and/or accumulation [%].
nected to the cell by two titanium rods immersed in the electrode Exp Anode Center Cathode Cell
compartment. In order to separate the central compartment from  CF

the lateral ones, nylon mesh (149 ␮m) and filter paper (grade 131) 1− Co
× 100

were used. The pre-treated mine tailings were placed in central 1 19.6 4.3 33.7 19.2
compartment. Initially, in the anode compartment, the electrolyte 2 54.3 31.5 38.0 42.2
was distilled water, and in the cathode compartment was dilute sul- 3 8.5 6.5 −16.8 1.6
4 47.8 4.3 −25.0 8.4
furic acid solution, later a continuous drop addition of concentrated
5 50.0 −3.3 8.7 18.7
acid to maintain pH below 4 was supplied. On the other hand, to 6 46.7 0.0 33.7 28.2
control pH in the cathode compartment, a sample was taken each 7 51.1 32.6 −8.7 24.6
day for pH monitoring.
After the experiments were carried out, mine tailing sample was
segmented into three slices of equal size, where copper concentra- 3.1. Copper removal
tion was measured. In this work anode zone is defined as the slice
closest to the anode, center zone the slice in the middle, and cathode EKR with sinusoidal electric field obtained by applying DC and
zone the slice closest to the cathode. AC voltages simultaneously can achieve a copper removal from the
cell better than EKR with continuous electric field. In this context,
comparing experiments 2 and 7, which were made at a simi-
2.5. Experimental plan lar voltage, the use of a sinusoidal electric field represents a 70%
improvement in the copper removal from the cell. In all experi-
Seven EKR experiments with a remediation time of 7 days were ments, no significant electroosmotic flow was observed, meaning
carried out with the conditions given in Table 2. In all experiments, that the main mechanism for copper removal is electromigration.
a sample of approximately 1.6 kg solid dry weight of mine tail- Moreover, experiments with sinusoidal electric field in which
ings was adjusted to an initial humidity of 20%, using sulfuric acid a polarity reversal occurs during the cycle, indicated by the nega-
solution. tive minimum voltage shown in Table 2 (experiments 1, 2, 5 and
The objective of the experiments was to evaluate the effect of: (i) 6), copper removal was also observed in the cathode zone. In addi-
the effective voltage applied to the cell and (ii) the polarity reversal tion, experiment 2 with sinusoidal electric field reached a copper
of the cell, in the copper removal and the electrical energy con- removal from the cell which involved virtually all soluble copper
sumption of the process. A reference was conducted to compare in the mine tailing sample.
these experiments with conventional EKR. In the analysis of copper removal and/or accumulation, the effect
With a sinusoidal electric field obtained by applying simul- of the effective voltage (Veffective ) was evaluated by considering the
taneously continuous-alternating (DC–AC) voltages, the effective following combinations: (i) VAC = 23.0 [V] fixed and VDC varying
voltage applied to the cell is determined by: between 7.2 and 29.0 V and (ii) VDC = 23.3 and 29.0 [V] fixed and
VAC varying between 23.0 and 31.0 [V] in both cases.
V = VDC + VAC · sin(2ft) Tables 5 and 6 show the copper removal and/or accumulation as
the effective voltage and the combinations defined in the preceding
 paragraph.
 T Comparing the experiments that remained fixed VAC and VDC
1
Veffective = · V 2 dt was variable; Table 5 shows that copper removal from the cell
T 0 increases proportionally with the effective voltage in the range
17.8–22.3 [V] and 28.4–33.2 [V]. In the first, 17.8–22.3 [V], the
where VDC : continuous voltage [V]; VAC : alternating voltage [V]; f: removals are greater because the polarity of the cell during the
frequency [s−1 ]; t: time [s]; T: period [s]; Veffective : effective voltage
[V].
Table 5
Removal and/or accumulation [%] vs Veffective . VAC = 23.0 V and VDC variable.
3. Results and discussion
Exp Veffective [V] Anode Center Cathode Cell
 CF

Table 3 shows copper concentrations in the anode, center, and 1− Co
× 100
cathode zone of the remediation sample after EKR experiments. 1 17.8 19.6 4.3 33.7 19.2
Table 4 includes a summary with general EKR results in terms of 2 22.3 54.3 31.5 38.0 42.2
removal (positive values) and/or accumulation (negative values) of 3 28.4 8.5 6.5 −16.8 1.6
total copper from the already mentioned zones and the whole cell. 4 33.2 47.8 4.3 −25.0 8.4
142 A. Rojo et al. / Separation and Purification Technology 79 (2011) 139–143

Table 6
Removal and/or accumulation [%] vs Veffective . VDC = 23.3 and 29 V and VAC variable.

Exp Veffective [V] Anode Center Cathode Cell

 CF

1− Co
× 100

3 28.4 8.5 6.5 −16.8 1.6

5 31.8 50.0 −3.3 8.7 18.7
4 33.2 47.8 4.3 −25.0 8.4
6 36.4 46.7 0.0 33.7 28.2

Fig. 3. Copper removal from the cell.

According to the highest copper removal efficiency of 42.2%

obtained in experiment 2, it should be noted that in fresh mine
tailings, copper could be expected to be found as residual insolu-
ble copper sulfide, which was not liberated in the grinding process
prior to flotation. The apparent low copper removals obtained here
were due to the soluble copper content in the tailings of 44.9%, so
the highest removal efficiency of 42.2% of the total copper contents
seems to be promising.
Fig. 2. Schematic definition of the time ratio  for a sinusoidal voltage.
On the other hand, the content of soluble copper, is variable
due to the heterogeneous origin of the mine tailings in the ponds,
cycle is reversed, this phenomenon does not occur in the second
among other reasons: copper grades depend on the original char-
range of 28.4–33.2 [V].
acteristics of the tails disposed, aging of the tailing in the ponds
On the other hand, comparing the experiments in which VDC
as consequence of physical–chemical changes due to weathering
remained fixed in 23.3 (experiments 3 and 5) and 29.0 [V] (experi-
and bacterial actions in time. The use of this remediation technol-
ments 4 and 6) and VAC was variable; Table 6 also shows that copper
ogy will imply the periodic application of the method in order to
removal from the cell follows the same trend. In this case, for both
remove the additional soluble copper that will be generated with
fixed DC voltage the removal increase is due to the reversal polarity
time. Therefore, the remediation action for this heterogeneous solid
of the cell during the cycle.
waste is to remove the soluble copper in the tailings and in this way
Consequently, according to the results in Tables 5 and 6, the
making the final residue more stable.
reversal of the polarity of the cell is the decisive phenomenon in
improving the copper removal from the whole cell.
To compare the results of experiments with a reversal polarity 3.2. Electrical energy consumption
of the cell, the time ratio  between the time of normal polarity
(t+) and the time of polarity reversal (t−) during a cycle (tcycle). Table 8 shows the net electric charge flowing through the
Fig. 2 shows schematically how the different times are defined experimental cell, in coulombs, and the necessary parameters for
during a cycle with a sinusoidal electric field obtained by applying estimating the electrical energy consumption of the EKR experi-
simultaneously AC–DC voltages. ments. For all experiments the net electric charge was calculated
Table 7 shows the copper removal and/or accumulation of the from current-time recording and confirmed with copper coulomb-
4 experiments with periodical reversal polarity of the cell, charac- meter measurements.
terized by the time ratio . The electrical energy consumption for experiments with sinu-
The results in Table 7 show that the copper removal from the soidal electric field was estimated from the active power obtained
cell increases proportionally to the ratio of time, . This is consistent with the effective voltage and current, and in the experiment with
with the experiments, since the time ratio is increased by reducing continuous electric field, energy was estimated with the net elec-
the time of reversal polarity, and accordingly to the frequency of tric charge and voltage. For its part, the mass considered in the
applied AC voltage of 50 Hz two positive effects are obtained: (i) calculations corresponds to the copper removed from the whole
times greater of normal polarity for the copper removal and (ii) cell.
reduction of polarization by the periodical polarity reversal of the The results in Table 8 show that EKR with sinusoidal electric
cell. field can achieve low levels of electrical energy consumption of that
Moreover, experiments 1 and 5 reached a similar copper obtained with a continuous electric field. In this context, comparing
removal from the cell, despite the effective voltage applied was experiments 2 and 7, which were made at a similar electric field,
different, 17.8 and 31.8 [V] respectively, demonstrating the impor-
tance of the effect of time ratio as shown in Fig. 3. Table 8
Electrical energy consumption.
Table 7
Removal and/or accumulation [%] vs time ratio . Exp Veffective [V] Net charge [C] IEffective [mA] Energy [KWh/kg]

1 17.8 11,537 46 31
Exp  Anode Center Cathode Cell
 CF
 2
3
22.3
28.4
33,670
7313
75
12
167
1426
1− Co
× 100
4 33.2 37,521 69 2181
1 1.50 19.6 4.3 33.7 19.2 5 31.8 37,910 82 1607
5 1.50 50.0 −3.3 8.7 18.7 6 36.4 105,685 210 3048
6 1.86 46.7 0.0 33.7 28.2 7 23.0a 86,324 – 1439
2 2.33 54.3 31.5 38.0 42.2 a
Continuous electric field.
 A. Rojo et al. / Separation and Purification Technology 79 (2011) 139–143 143

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This work was supported by the FONDECYT Project 1110057. (2010) 1095–1100.