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The electrodeposition of trace metallic impurities: dependence on the supporting electrolyte concentration—a comparison between bipolar and monopolar porous electrodes
SHORT
COMMUNICATION
THE ELECTRODEPOSITION OF TRACE METALLIC IMPURITIES: DEPENDENCE ON THE SUPPORTING ELECTROLYTE CONCENTRATION-A COMPARISON BETWEEN BIPOLAR AND MONOPOLAR POROUS ELECTRODES M. ABDA, Y. OREN and A. SOFFER Chemistry
Division,
Nuclear
(Received
Research
20 May
Center-Negev,
Beer-Sheva,
P.O. Box 9001,
1986, in revised form 26 Nooember
Israel 84190
1986)
Abstract-Heavy metal impurities occur in contaminated water resources and industrial streams in trace amounts and very often in the presence of much higher concentrations of indifferent electrolytes. It is shown that under diffusion controlled conditions, the removal efficiency of metallic contaminants by electrodeposition using monopolar porous electrodes is high as long as the electrolyte concentration is high enough so that the ir drop of the solution in the pores is small. In the bipolar mode, the efficiency decreases considerably in both high and low concentrations.
INTRODUCTION The growing awareness for environment protection and the frequent classification of new materials as potential threats for human’s health, brings about new regulations which enforce very low concentrations of poisonous contaminants in the various types of water resources and industrial streams. Ionic dissolved mercury is considered highly hazardous and as such, its maximum concentration in drinking water is limited to l-2 ppb. Since in most cases the impurities occur in trace amounts in presence of much greater concentrations of inert electrolyte, the study of such systems is worthwhile. In the present communication, the removal of trace concentrations of mercuric ions by a graphite felt electrode from solutions of various supporting elcctrolyte concentrations is described. A comparison of results obtained from monopolar[l] and bipolar[l-3] electrode arrangements is presented. This type of electrode material was shown to be more efficient than others[4].
EXPERIMENTAL Two electrochemical flow cells are shown in Fig. 1 a and b. Each porous electrode is made by stacking layers of graphite felt 1 mm thick each. This arrangement made it easy to analyze sections of the electrodes for deposited mercury after disassembling the cell. In the monopolar mode the anode and cathode are separated by a 1 mm thick porous polypropylene disc through which the solution flows freely. In the bipolar mode, two polypropylene discs are used to separate the two platinum wire current collectors from the bipolar
graphite felt. For thorough comparison, the cathodic felt of the monopolar arrangement had the same length (20 mm) and diameter (7 mm) as the single bipolar electrode. The flow rate was 7 mlmin-‘. The cathode was at the solution inlet side and a constant current of 4 mA was passed. This current is about 3.4 times greater than required for complete removal of mercury from the flowing solution. Solutions containing HgZ + [as Hg(NOs)J and NaN03 as a supporting electrolyte in the concentration ranges 10 ppm and 2-0.001 M, respectively were used.
RESULTS
AND
DISCUSSION
Figure 2 shows the profiles of the mercury electrodeposited along the bipolar electrode bed at several electrolyte concentrations. It is apparent that mercury removal changes from negligible at high electrolyte concentrations, increases at intermediate and decreases again at the lowest concentrations. It is also clear that the depth of penetration varies as the electrolyte concentration changes. The same analysis was done for the monopolar case where it was found that the depth of penetration of mercury is the least for the lowest supporting electrolyte concentration. These results may be interpreted as follows. In the monopolar mode, at high electrolyte concentration, the resistivity of the solution in the pores is low, so that the interfacial potential will be constant throughout the electrode length. In this case the deposition of mercury will be diffusion controlled and the depth of deposited mercury will extend until the removal is complete. At lower concentrations the ir drop will be significant so that the interfacial potential 1113
1114
M. ABDA~~
al. Solution 0llt1er
Solution inlet
I
P;
Pt
POt-0US polyprpylene
separator
separator
I Solution inlet
Fig. 1. Experimental setup. A, monopolar;
B,
bipolar.
_~~~___ Bipolar,
NJNf13
4 m7
(M) \ h
0.5 0.1 0.01
-\ 1
ELECTRODE
Fig. 2. Normalized
LENGTH
@ n
h
(sun)
concentration of deposited mercury us electrode length.
The electrodeposition of trace metallic impurities
0
I 0.001
I 0.01 NaN03
I 0.1
CONCENTRATION
1115
I
0.5
I 1
, 2
(M)
Fig. 3. Purification efficiency vs electrolyte concentration for the bipolar (BP) and monopolar (MP) modes.
will soon drop below the Nernst potential of mercury electrodeposition. The penetration of the elcctrodeposit will be shallow. The removal from solution will not be complete so that a larger part of the constant current will be consumed for water electrolysis. In the bipolar mode, at high concentrations the conductivity of the solution within the pores is sufficiently high that the interfacial potential cannot reach the Nernst values for electrodeposition, a situation that corresponds to the 2 M solution. At intermediate potentials, the ir drop creates the necessary interfacial polarization for the electrodeposition. At further lower electrolyte concentrations, a situation similar to that of the monopolar configuration is established where, because of the high ir drop, only a thin layer of the electrode bed (that facing the counter electrode), has sufficiently negative potential for electrodeposition. In this case the electrical current leaves the poorly conductive solution on one end of the electrode bed, flows through the electrode, and leaves it again at the other end. Changing the supporting electrolyte concentration over a wide range will therefore change the current flow regime from that of total flow through the solution where no electrodic process can take place, to a total flow through the electrode. In this case the interfacial potential at the electrode faces becomes very high so that electrolysis of the solvent becomes the predominant process. For the bipolar case, only intermediate supporting electrolyte concentrations are important for removal of trace impurities. In Fig. 3 the purification efficiency expressed as: loO(C,C,)/CO (C, and Cfare, respectively, the initial and the final concentrations of mercury in the solution), is given as a function of the electrolyte concen-
tration for both cases. It is apparent that the monopolar mode is effective over most of the concentration range where practically lOO”/0 of the mercury is removed from the solution. The efficiency decreases at the lowest concentration of 0.001 M as described above. The bipolar mode shows, as expected, equal efficiencies at intermediate ranges and low ranges but falls down far below the monopolar efficiency at high concentrations. In conclusion, it seems that, at least for the present set of conditions, the bipolar mode shows no advantage over the monopolar mode for the electrochemical removal of trace impurities. The advantage of the bipolar mode lies principally in the simplicity of the cell design where a stack in series of electrically insulated bipolar electrodes needs no electrical connections to the outside, and enables the convenience of using high voltages and low currents. Nevertheless, when the thickness of the bipolar electrode is optimized (a procedure which had not been undertaken in this work), only a narrow range of supporting electrolyte concentration will be allowed to establish the proper potential distribution along the porous electrode. This imposes a serious limitation on the range of usefulness of the bipolar electrode.
REFERENCES 1. F. Goodridge. Elecrrochim. Acta 22, 929 (1977). 2. R. Alkire, .I. electrochem. Sot. 120, 900 (1973). 3. M. Fleischmann and Z. Ibrisagic, J. appl.Electrochem. 10, 151 (1980). 4. Y. Oren and A. Soffer, Electrochim. Actn 28, 1649 (1983).
COMMUNICATION
THE ELECTRODEPOSITION OF TRACE METALLIC IMPURITIES: DEPENDENCE ON THE SUPPORTING ELECTROLYTE CONCENTRATION-A COMPARISON BETWEEN BIPOLAR AND MONOPOLAR POROUS ELECTRODES M. ABDA, Y. OREN and A. SOFFER Chemistry
Division,
Nuclear
(Received
Research
20 May
Center-Negev,
Beer-Sheva,
P.O. Box 9001,
1986, in revised form 26 Nooember
Israel 84190
1986)
Abstract-Heavy metal impurities occur in contaminated water resources and industrial streams in trace amounts and very often in the presence of much higher concentrations of indifferent electrolytes. It is shown that under diffusion controlled conditions, the removal efficiency of metallic contaminants by electrodeposition using monopolar porous electrodes is high as long as the electrolyte concentration is high enough so that the ir drop of the solution in the pores is small. In the bipolar mode, the efficiency decreases considerably in both high and low concentrations.
INTRODUCTION The growing awareness for environment protection and the frequent classification of new materials as potential threats for human’s health, brings about new regulations which enforce very low concentrations of poisonous contaminants in the various types of water resources and industrial streams. Ionic dissolved mercury is considered highly hazardous and as such, its maximum concentration in drinking water is limited to l-2 ppb. Since in most cases the impurities occur in trace amounts in presence of much greater concentrations of inert electrolyte, the study of such systems is worthwhile. In the present communication, the removal of trace concentrations of mercuric ions by a graphite felt electrode from solutions of various supporting elcctrolyte concentrations is described. A comparison of results obtained from monopolar[l] and bipolar[l-3] electrode arrangements is presented. This type of electrode material was shown to be more efficient than others[4].
EXPERIMENTAL Two electrochemical flow cells are shown in Fig. 1 a and b. Each porous electrode is made by stacking layers of graphite felt 1 mm thick each. This arrangement made it easy to analyze sections of the electrodes for deposited mercury after disassembling the cell. In the monopolar mode the anode and cathode are separated by a 1 mm thick porous polypropylene disc through which the solution flows freely. In the bipolar mode, two polypropylene discs are used to separate the two platinum wire current collectors from the bipolar
graphite felt. For thorough comparison, the cathodic felt of the monopolar arrangement had the same length (20 mm) and diameter (7 mm) as the single bipolar electrode. The flow rate was 7 mlmin-‘. The cathode was at the solution inlet side and a constant current of 4 mA was passed. This current is about 3.4 times greater than required for complete removal of mercury from the flowing solution. Solutions containing HgZ + [as Hg(NOs)J and NaN03 as a supporting electrolyte in the concentration ranges 10 ppm and 2-0.001 M, respectively were used.
RESULTS
AND
DISCUSSION
Figure 2 shows the profiles of the mercury electrodeposited along the bipolar electrode bed at several electrolyte concentrations. It is apparent that mercury removal changes from negligible at high electrolyte concentrations, increases at intermediate and decreases again at the lowest concentrations. It is also clear that the depth of penetration varies as the electrolyte concentration changes. The same analysis was done for the monopolar case where it was found that the depth of penetration of mercury is the least for the lowest supporting electrolyte concentration. These results may be interpreted as follows. In the monopolar mode, at high electrolyte concentration, the resistivity of the solution in the pores is low, so that the interfacial potential will be constant throughout the electrode length. In this case the deposition of mercury will be diffusion controlled and the depth of deposited mercury will extend until the removal is complete. At lower concentrations the ir drop will be significant so that the interfacial potential 1113
1114
M. ABDA~~
al. Solution 0llt1er
Solution inlet
I
P;
Pt
POt-0US polyprpylene
separator
separator
I Solution inlet
Fig. 1. Experimental setup. A, monopolar;
B,
bipolar.
_~~~___ Bipolar,
NJNf13
4 m7
(M) \ h
0.5 0.1 0.01
-\ 1
ELECTRODE
Fig. 2. Normalized
LENGTH
@ n
h
(sun)
concentration of deposited mercury us electrode length.
The electrodeposition of trace metallic impurities
0
I 0.001
I 0.01 NaN03
I 0.1
CONCENTRATION
1115
I
0.5
I 1
, 2
(M)
Fig. 3. Purification efficiency vs electrolyte concentration for the bipolar (BP) and monopolar (MP) modes.
will soon drop below the Nernst potential of mercury electrodeposition. The penetration of the elcctrodeposit will be shallow. The removal from solution will not be complete so that a larger part of the constant current will be consumed for water electrolysis. In the bipolar mode, at high concentrations the conductivity of the solution within the pores is sufficiently high that the interfacial potential cannot reach the Nernst values for electrodeposition, a situation that corresponds to the 2 M solution. At intermediate potentials, the ir drop creates the necessary interfacial polarization for the electrodeposition. At further lower electrolyte concentrations, a situation similar to that of the monopolar configuration is established where, because of the high ir drop, only a thin layer of the electrode bed (that facing the counter electrode), has sufficiently negative potential for electrodeposition. In this case the electrical current leaves the poorly conductive solution on one end of the electrode bed, flows through the electrode, and leaves it again at the other end. Changing the supporting electrolyte concentration over a wide range will therefore change the current flow regime from that of total flow through the solution where no electrodic process can take place, to a total flow through the electrode. In this case the interfacial potential at the electrode faces becomes very high so that electrolysis of the solvent becomes the predominant process. For the bipolar case, only intermediate supporting electrolyte concentrations are important for removal of trace impurities. In Fig. 3 the purification efficiency expressed as: loO(C,C,)/CO (C, and Cfare, respectively, the initial and the final concentrations of mercury in the solution), is given as a function of the electrolyte concen-
tration for both cases. It is apparent that the monopolar mode is effective over most of the concentration range where practically lOO”/0 of the mercury is removed from the solution. The efficiency decreases at the lowest concentration of 0.001 M as described above. The bipolar mode shows, as expected, equal efficiencies at intermediate ranges and low ranges but falls down far below the monopolar efficiency at high concentrations. In conclusion, it seems that, at least for the present set of conditions, the bipolar mode shows no advantage over the monopolar mode for the electrochemical removal of trace impurities. The advantage of the bipolar mode lies principally in the simplicity of the cell design where a stack in series of electrically insulated bipolar electrodes needs no electrical connections to the outside, and enables the convenience of using high voltages and low currents. Nevertheless, when the thickness of the bipolar electrode is optimized (a procedure which had not been undertaken in this work), only a narrow range of supporting electrolyte concentration will be allowed to establish the proper potential distribution along the porous electrode. This imposes a serious limitation on the range of usefulness of the bipolar electrode.
REFERENCES 1. F. Goodridge. Elecrrochim. Acta 22, 929 (1977). 2. R. Alkire, .I. electrochem. Sot. 120, 900 (1973). 3. M. Fleischmann and Z. Ibrisagic, J. appl.Electrochem. 10, 151 (1980). 4. Y. Oren and A. Soffer, Electrochim. Actn 28, 1649 (1983).