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Electrolytic treatment of mercury-loaded activated carbon from a gas cleaning system
The Science of the Total Environment 261 Ž2000. 195᎐201
Electrolytic treatment of mercury-loaded activated carbon from a gas cleaning system U
L.G.S. Sobrala, , R.L.C. Santosa , L.A.D. Barbosab a
Center for Mineral Technology, Rua 4, Quadra D, Cidade Uni¨ ersitaria, ´ 21941-590, Rio de Janeiro, RJ, Brazil b Federal Uni¨ ersity of Rio de Janeiro, Rio de Janeiro, RJ, Brazil Received 27 September 1999; accepted 4 May 2000
Abstract This study aimed at extracting the adsorbed mercury from the mercury-loaded activated carbon so as to recycling both, the elemental mercury and the carbon, after being reactivated. The process used in this study was the electro-oxidation of the mercury in a reaction system where the loaded carbon is acting as an anode, during the electrolysis of brine, the electrolyte of the cell. 䊚 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Electrolytic treatment; Activated carbon; Electro-oxidation
1. Introduction The pollution of the environment from metals through human industrial activities is assuming a more and more remarkable importance. To such intention, intense studies have been undertaken to evaluate the effect of those metals on living organisms, to monitor the heavy metals concentration in the environment, and to prevent and eliminate the pollution. As the world-wide environmental legislation is
U
Corresponding author. E-mail address: [email protected] ŽL.G.S. Sobral..
becoming more and more restrictive, it is necessary for new studies to be conducted to achieve higher purification levels at an acceptable cost and to develop easier applicable technologies. Among the harmful metals that have a severe impact on ecosystems and, therefore, on human beings, mercury causes serious environmental problems. Mercury remains in the environment and, when exposed to the atmosphere, it is metabolised by bacteria that introduce it into the biological cycle, thus eventually reaching man. The mercury is accumulated in the human organism provoking damage particularly to the digestive, nervous and breathing systems. Elemental mercury is still used in different
0048-9697r00r$ - see front matter 䊚 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 6 3 3 - 1
196
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
industries for various purposes causing damage to the environment. Among them one can mention the chlor-alkali industry that uses, during the electrolysis of brine, a mercury cathode, and in gold prospecting areas for amalgamating gold from gravity concentrates. Regarding the chloralkali industry, nowadays many facilities are using cation exchange membranes, which does not require the use of elemental mercury. As far as gold extraction is concerned, there are available technologies either to avoid mercury volatilisation during the thermal decomposition of the gold amalgam, or to treat the residues from the elutriation operation, allowing the recovery and reuse of it and avoiding its dispersion in the environment. In recent years, the increasing concern about the severe impact of toxic and persistent elements, such as mercury, on ecosystems has become an important issue in environmental legislation. The gas-fired power plant is a widely used technique for energy production, supplying heat and power in several east-European countries. However, during the gas prospecting operation, in deep wells, a considerable mercury concentration is being detected as part of the gas mixture, implying that the straight combustion of such gas is a considerable source of mercury emission. Therefore, in this case, a flue gas cleaning system or emission control measurements have to be strongly considered, or, even better, to pass such gas mixture through an activated carbon column so as to adsorb the elemental mercury vapours and release a mercury-free gas stream. However, once reaching the loading capacity of such carbons another environment problem is introduced. The loaded carbon contains up to 20% wrw of mercury with high conditioning cost. In general, there are different ways to treat mercury-bearing residues, such as: 䢇
䢇
䢇
biological methods ŽMoo-Young, 1992; Yu, 1992.; physical treatments Žmineral-processing techniques. ŽAllen et al., 1988, 1989.; chemical᎐physical treatments Žreagents addition and precipitation. ŽChem. Abs., 81a, 81b;
䢇
Sumitomo, 1973; Gardiner and Munoz, 1971; Flood and Kraynik, 1973; Waltrich, 1972.; and electrochemical treatments ŽSobral et al., 1996..
The electrochemical method is becoming more and more important, as it provides low processing costs, and, in addition, the possibility of recovering the metals under consideration. In this study the electrolytic process was used for removing the elemental mercury from the carbon structure by using it as an anode in a reaction system that can be seen later in the electrolytic experimental procedure.
2. Experimental The Fig. 1, as follows, shows an SEM Žscanning electron microscopy. photograph of an activated carbon loaded with elemental mercury. The white spots indicate elemental mercury being accumulated during the adsorption process. Before going any further, a brief voltametric study had to be carried out so as to better explain the behaviour of elemental mercury during the decontamination process. The present study uses a steady-state linear sweep technique to investigate the mercury electrodeposition process at a vitreous carbon solid electrode from a 10y3 mol dmy3 Hgq2 solution, 1.0 mol dmy3 NaCl as supporting electrolyte. The solutions Želectrolytes . were made up using deionised distilled water and were de-aerated with oxygen-free nitrogen before steady-state polarisation curves were recorded. This study was carried out using a rotating vitreous carbon electrode Žarea s 1.26= 10y5 m2 . embedded in a PTFE holder and attached to a rotating disc assembly. The electrode was polished to a mirror finishing with diamond lapping compound Ž1᎐5-m particle size, Hyprez Five Star Engis Ltd.. using a silk cloth and water-soluble lubricant liquid ŽHyprez Fluid Type W.. A conventional Pyrex cell assembly ŽFig. 2. was used, incorporating a saturated calomel reference electrode separated from the bulk solution by a Lug-
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
197
Fig. 1. An SEM photograph of an activated carbon loaded with elemental mercury.
gin capillary; a large platinum foil in a separate compartment served as a counter electrode. Linear potential scans were generated using a potentiostat ŽEG & G, Princeton Applied Research, Universal Programmer, model 175.. The
Fig. 2. Experimental cell design for experiments with vitreous carbon electrode: Ž1. reference electrode; Ž2. Luggin capillary; Ž3. vitreous carbon electrode Žworking electrode.; Ž4. platinum foil electrode Žcounter electrode.; Ž5. nitrogen bubbler; and Ž6. porous glass sinter.
applied potential and the resulting current were stored in a PC through a data acquisition software ŽLabtech Notebook. using a high-resolution data acquisition board ŽMini-16, Strawberry Tree, Computer Instrumentation & Controls. and subsequently analysed. Electrochemical pre-treatment of glassy carbon electrode ŽGCE. is frequently employed to attain fast kinetics of some electrochemical reactions in aqueous solutions. All factors which affect the surface prior to, during, or even after electrode reaction are of concern when one discusses electrode surface conditions. Probably the most serious problem in solid electrode methodology is associated with understanding the true electrode surface conditions and their possible effect on the electrode process. To eliminate the effects of the roughness build up, because of the deposition of metals on the surface of the glassy carbon electrode, during the experiments under consideration, the following experimental procedure was applied: after each experiment the glassy carbon electrode was
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
198
Fig. 3. Steady-state sweep profile of 1 = 10y3 mol dmy3 HgCl2 , in 0.5 mol dmy3 NaCl supporting electrolyte. Sweep rate 0.050 V sy1 and the potential scan program was: initial potential s 0.00 V ŽSCE., lower limit s y1.00 V ŽSCE., upper limit s q1.00 V ŽSCE. and final potential s 0.00 V ŽSCE..
polished to a mirror finishing and a potential scan applied, for 10 min, at 0.20 V sy1 , in a 0.5 mol dmy3 K2 SO4 solution, from a potential where hydrogen is evolved wy0.40 V ŽSCE.x to a value where the oxidation of water occurs, wq1.80 V ŽSCE.x, i.e. 2Hqq 2e ª H2 2H2 O ª O2 q 4Hqq 4e The cyclic voltammogram of the following figure ŽFig. 3. shows important features: 䢇
䢇
a peak at y0.47 V ŽSCE. resulting from reduction of HgCl42y ions to elemental mercury, and a peak at 0.060 V ŽSCE. resulting from the elemental mercury oxidation back to HgCl42y ions, as depicted in the following reaction:
HgCl42yq 2e m Hgq 4Cly After getting the cathodic peak of HgCl42y reduction, the potential scanning goes until y1.0 V ŽSCE. without any hydrogen evolution since its
cathodic overpotential on the mercury cathodic surface, just generated, is greater than that of any other metal. The large separation Ž; 0.5 V. of peaks may be due to the irreversibility of HgCl42y ions, generated during the dissolution of mercuric chloride ŽHgCl2 ., in dissociating before the reduction reaction takes place. This means that a slower charge-transfer process is occurring. The reaction system depicted in Fig. 4 was used for the electroleaching of the mercury-loaded activated carbon. It is a batch recycle mode reactor where the mercury-bearing activated carbon is used as an anode. As a cathode, a titanium foil was used and the elemental mercury being deposited during the electrolytic process was eventually collected. The electrolyte used in the electro-oxidation tests was a 1-M sodium chloride solution, in which the pH was adjusted to pH 4 with hydrochloric acid. The mercury-loaded carbon was used as an anode, as shown in Fig. 4, with the current being fed through a graphite rod. The mass of carbon used, in each test was up to 150 g and the test’s running time of 6 h with a constant upstream flow rate of 3.3= 10y5 m3 sy1 Ž2 l miny1 .. A plastic screen was used on top of the anodic compartment, in order to avoid the carbon being dragged out from the cell, especially after running this process for a while, as the carbon becomes lighter with time. The electrolytic process starts, once the current is supplied, with the oxidation of the chloride ions to chlorine at the anode surface: 2Clym Cl2 q 2e while at the cathode the water is, initially, reduced producing hydroxyl ions and hydrogen: 2H2 O q 2e m H2 q 2OHy Simultaneously, hypochlorous acid is generated, in bulk, by the chemical reaction of chlorine being formed at the anode with the aqueous phase: Cl2 q H2 O m HClO q Hqq Cly
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
199
Fig. 4. Reaction system used for the electro-oxidation Želectroleaching . tests.
which dissociates into hypochlorite and hydrogen ions. The reaction is directly dependent on the pH. The oxidation power increases as the pH goes down since the generation of HClO is favoured as depicted in the thermodynamic stability diagram of Fig. 5 Žwhich was done using the Outokumpu Research Oy software HSC Chemistry 3.0., which can be confirmed observing the speciation diagram of Fig. 6. As the pH-value goes below pH 5, approximately 100% HclO is generated. y
q
HClO m ClO q H
These hydrogen ions react with the hydroxyl ions, a product of the cathodic reaction, to form water: Hqq OHym H2 O
During the electrolysis, after a while, the production of hypochlorite ions does not increase as expected, which is attributed to the generation of chlorate ions, either chemically: 2HClOq ClOym ClO3yq 2Hqq 2Cly or electrochemically: 6ClOyq 3H2 O m 2ClO3yq 6Hq4Cly q 3r2O2 q 6e Once those chlorine species ŽHClO and ClOy. are produced, the oxidation of elemental mercury takes place according to the following reaction: Hgq 2ClOyq 4Clyq 2H2 O m Cl2 q HgCl42y q 4OHy
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
200
Fig. 6. Speciation of hypochlorite ions and hypochlorous acid as a function of pH.
Fig. 5. Thermodynamic stability diagram for Cl᎐H2 O system at 25⬚C. This diagram was done by using the HSC Chemistry software from Outokumpu Research Oy.
Once produced, the HgCl42y are reduced at the cathode surface HgCl42yq 2e m Hg0 q 4Cly which is the predominant mercury reduction reaction from a mercury solution containing high chloride ion concentration ŽSobral et al., 1996.. For tracing the mercury concentration down during the electro-oxidation process, the electrolyte was sampled every 30 min and analysed for mercury by atomic absorption spectrometry. The activated carbon used before and after the electro-oxidation process was also analysed. The experimental conditions and the results of the electro-oxidation are shown in Table 1.
It was observed, after removing the elemental mercury from the mercury-bearing activated carbon and washing properly, that the specific surface area Ž8050 m2 my3 . was nearly that of a fresh activated carbon with the same particle size. A fresh 8 = 16 mesh activated carbon has a specific surface area in the range of 8000᎐12 000 m2 my3 . However, it is necessary to thoroughly wash down the carbon, once finished the electrolytic process, so as to get rid of sodium chloride solution. This procedure will avoid further NaCl crystallisation on the available surface of the carbon. Once taking this advice into consideration, and, in addition, considering that the just-cleaned carbon has to be reactivated, the recycling of the carbon is strongly recommended.
3. Conclusions Electrochemical technology has made consider-
Table 1 Experimental conditions and results of electro-oxidation Test
Carbona Žanode. Žg.
pH Želectrolyte.
Current ŽA.
Cathode
Hg extraction Ž%.
wHgx b Žppm.
1 2 3 4
138.81 139.71 153.11 152.19
4 4 4 4
0.7 0.7 1.0 1.5
Titanium Steel c Titanium Titanium
70 68 88 98
2.95 1.28 3.76 3.97
a
8 = 16 mesh Ž2.38 mm. activated carbon. Remaining mercury concentration in the electrolyte after the electro-oxidation process. The experimental conditions are those mentioned in the experimental procedures. c Stainless steel. b
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
able advances in the last decade and offers potential as another hydrometallurgical process for treating mercury-bearing residues. The experimental results permit the following conclusions: 1. By choosing suitable operating conditions, it is possible to reduce the mercury concentration to low values with a high extraction efficiency Ž) 99%., considering the utilised residue. It was possible to reduce the residue mercury concentration to less than 3.0 ppm, which is not low enough to be considered suitable for discharging, indicating the necessity of extending the electrolysis time. 2. The final leaching solution should not be discharged since it contains not only high salinity, but also high mercury concentration Ž( 2 ppm., which is not suitable for discharging. It is recommended to re-use such solution back to the residue treatment process. 3. According to the low current densities obtained during the voltammetric study, the use of high surface-area cathodes is recommended. This provides good mass transfer conditions so as to enhance the reactor performance.
201
4. It is recommended to perform the electroleaching process in an acidic range of pH Žbetween pH 4 and 6. so as to enhance the mercury dissolution.
References Allen SJ, McKay G, Khader KYH. J Colloid Interface Sci 1988;126:517᎐524. Allen SJ, McKay G, Khader KYH. J Colloid Interface Sci 1989;45:261᎐302. Chem. Abs., 81a, 82126e. Chem. Abs., 81b, 111188j. Flood DJ, Kraynik CJ. Chem Abs 1973;83:65287g. Gardiner WC, Munoz F. Chem Eng 1971;78:57. Moo-Young M. Waste treatment and recycling. Proceedings of the International Conference on Environmental Biotechnology, Hong Kong, 1992: 11. Sobral LGS, Santos RLC, Hempel M, Thoming J. The elec¨ troleaching of residues containing mercury. Part I: Kinetics aspects. In: Proceedings of the Third International Conference on Clean Technologies for the Mining Industries, Santiago, Chile, 15᎐17 May 1996: 175. Sumitomo Chemical Co. Ltd., German Patent 2 321 196, 1973. Waltrich PE. US Patent 3 704 875, 1972. Yu PHF. Waste treatment and recycling. Proceedings of the International Conference on Environmental Biotechnology, Hong Kong, 1992: 13.
Electrolytic treatment of mercury-loaded activated carbon from a gas cleaning system U
L.G.S. Sobrala, , R.L.C. Santosa , L.A.D. Barbosab a
Center for Mineral Technology, Rua 4, Quadra D, Cidade Uni¨ ersitaria, ´ 21941-590, Rio de Janeiro, RJ, Brazil b Federal Uni¨ ersity of Rio de Janeiro, Rio de Janeiro, RJ, Brazil Received 27 September 1999; accepted 4 May 2000
Abstract This study aimed at extracting the adsorbed mercury from the mercury-loaded activated carbon so as to recycling both, the elemental mercury and the carbon, after being reactivated. The process used in this study was the electro-oxidation of the mercury in a reaction system where the loaded carbon is acting as an anode, during the electrolysis of brine, the electrolyte of the cell. 䊚 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Electrolytic treatment; Activated carbon; Electro-oxidation
1. Introduction The pollution of the environment from metals through human industrial activities is assuming a more and more remarkable importance. To such intention, intense studies have been undertaken to evaluate the effect of those metals on living organisms, to monitor the heavy metals concentration in the environment, and to prevent and eliminate the pollution. As the world-wide environmental legislation is
U
Corresponding author. E-mail address: [email protected] ŽL.G.S. Sobral..
becoming more and more restrictive, it is necessary for new studies to be conducted to achieve higher purification levels at an acceptable cost and to develop easier applicable technologies. Among the harmful metals that have a severe impact on ecosystems and, therefore, on human beings, mercury causes serious environmental problems. Mercury remains in the environment and, when exposed to the atmosphere, it is metabolised by bacteria that introduce it into the biological cycle, thus eventually reaching man. The mercury is accumulated in the human organism provoking damage particularly to the digestive, nervous and breathing systems. Elemental mercury is still used in different
0048-9697r00r$ - see front matter 䊚 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 6 3 3 - 1
196
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
industries for various purposes causing damage to the environment. Among them one can mention the chlor-alkali industry that uses, during the electrolysis of brine, a mercury cathode, and in gold prospecting areas for amalgamating gold from gravity concentrates. Regarding the chloralkali industry, nowadays many facilities are using cation exchange membranes, which does not require the use of elemental mercury. As far as gold extraction is concerned, there are available technologies either to avoid mercury volatilisation during the thermal decomposition of the gold amalgam, or to treat the residues from the elutriation operation, allowing the recovery and reuse of it and avoiding its dispersion in the environment. In recent years, the increasing concern about the severe impact of toxic and persistent elements, such as mercury, on ecosystems has become an important issue in environmental legislation. The gas-fired power plant is a widely used technique for energy production, supplying heat and power in several east-European countries. However, during the gas prospecting operation, in deep wells, a considerable mercury concentration is being detected as part of the gas mixture, implying that the straight combustion of such gas is a considerable source of mercury emission. Therefore, in this case, a flue gas cleaning system or emission control measurements have to be strongly considered, or, even better, to pass such gas mixture through an activated carbon column so as to adsorb the elemental mercury vapours and release a mercury-free gas stream. However, once reaching the loading capacity of such carbons another environment problem is introduced. The loaded carbon contains up to 20% wrw of mercury with high conditioning cost. In general, there are different ways to treat mercury-bearing residues, such as: 䢇
䢇
䢇
biological methods ŽMoo-Young, 1992; Yu, 1992.; physical treatments Žmineral-processing techniques. ŽAllen et al., 1988, 1989.; chemical᎐physical treatments Žreagents addition and precipitation. ŽChem. Abs., 81a, 81b;
䢇
Sumitomo, 1973; Gardiner and Munoz, 1971; Flood and Kraynik, 1973; Waltrich, 1972.; and electrochemical treatments ŽSobral et al., 1996..
The electrochemical method is becoming more and more important, as it provides low processing costs, and, in addition, the possibility of recovering the metals under consideration. In this study the electrolytic process was used for removing the elemental mercury from the carbon structure by using it as an anode in a reaction system that can be seen later in the electrolytic experimental procedure.
2. Experimental The Fig. 1, as follows, shows an SEM Žscanning electron microscopy. photograph of an activated carbon loaded with elemental mercury. The white spots indicate elemental mercury being accumulated during the adsorption process. Before going any further, a brief voltametric study had to be carried out so as to better explain the behaviour of elemental mercury during the decontamination process. The present study uses a steady-state linear sweep technique to investigate the mercury electrodeposition process at a vitreous carbon solid electrode from a 10y3 mol dmy3 Hgq2 solution, 1.0 mol dmy3 NaCl as supporting electrolyte. The solutions Želectrolytes . were made up using deionised distilled water and were de-aerated with oxygen-free nitrogen before steady-state polarisation curves were recorded. This study was carried out using a rotating vitreous carbon electrode Žarea s 1.26= 10y5 m2 . embedded in a PTFE holder and attached to a rotating disc assembly. The electrode was polished to a mirror finishing with diamond lapping compound Ž1᎐5-m particle size, Hyprez Five Star Engis Ltd.. using a silk cloth and water-soluble lubricant liquid ŽHyprez Fluid Type W.. A conventional Pyrex cell assembly ŽFig. 2. was used, incorporating a saturated calomel reference electrode separated from the bulk solution by a Lug-
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
197
Fig. 1. An SEM photograph of an activated carbon loaded with elemental mercury.
gin capillary; a large platinum foil in a separate compartment served as a counter electrode. Linear potential scans were generated using a potentiostat ŽEG & G, Princeton Applied Research, Universal Programmer, model 175.. The
Fig. 2. Experimental cell design for experiments with vitreous carbon electrode: Ž1. reference electrode; Ž2. Luggin capillary; Ž3. vitreous carbon electrode Žworking electrode.; Ž4. platinum foil electrode Žcounter electrode.; Ž5. nitrogen bubbler; and Ž6. porous glass sinter.
applied potential and the resulting current were stored in a PC through a data acquisition software ŽLabtech Notebook. using a high-resolution data acquisition board ŽMini-16, Strawberry Tree, Computer Instrumentation & Controls. and subsequently analysed. Electrochemical pre-treatment of glassy carbon electrode ŽGCE. is frequently employed to attain fast kinetics of some electrochemical reactions in aqueous solutions. All factors which affect the surface prior to, during, or even after electrode reaction are of concern when one discusses electrode surface conditions. Probably the most serious problem in solid electrode methodology is associated with understanding the true electrode surface conditions and their possible effect on the electrode process. To eliminate the effects of the roughness build up, because of the deposition of metals on the surface of the glassy carbon electrode, during the experiments under consideration, the following experimental procedure was applied: after each experiment the glassy carbon electrode was
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
198
Fig. 3. Steady-state sweep profile of 1 = 10y3 mol dmy3 HgCl2 , in 0.5 mol dmy3 NaCl supporting electrolyte. Sweep rate 0.050 V sy1 and the potential scan program was: initial potential s 0.00 V ŽSCE., lower limit s y1.00 V ŽSCE., upper limit s q1.00 V ŽSCE. and final potential s 0.00 V ŽSCE..
polished to a mirror finishing and a potential scan applied, for 10 min, at 0.20 V sy1 , in a 0.5 mol dmy3 K2 SO4 solution, from a potential where hydrogen is evolved wy0.40 V ŽSCE.x to a value where the oxidation of water occurs, wq1.80 V ŽSCE.x, i.e. 2Hqq 2e ª H2 2H2 O ª O2 q 4Hqq 4e The cyclic voltammogram of the following figure ŽFig. 3. shows important features: 䢇
䢇
a peak at y0.47 V ŽSCE. resulting from reduction of HgCl42y ions to elemental mercury, and a peak at 0.060 V ŽSCE. resulting from the elemental mercury oxidation back to HgCl42y ions, as depicted in the following reaction:
HgCl42yq 2e m Hgq 4Cly After getting the cathodic peak of HgCl42y reduction, the potential scanning goes until y1.0 V ŽSCE. without any hydrogen evolution since its
cathodic overpotential on the mercury cathodic surface, just generated, is greater than that of any other metal. The large separation Ž; 0.5 V. of peaks may be due to the irreversibility of HgCl42y ions, generated during the dissolution of mercuric chloride ŽHgCl2 ., in dissociating before the reduction reaction takes place. This means that a slower charge-transfer process is occurring. The reaction system depicted in Fig. 4 was used for the electroleaching of the mercury-loaded activated carbon. It is a batch recycle mode reactor where the mercury-bearing activated carbon is used as an anode. As a cathode, a titanium foil was used and the elemental mercury being deposited during the electrolytic process was eventually collected. The electrolyte used in the electro-oxidation tests was a 1-M sodium chloride solution, in which the pH was adjusted to pH 4 with hydrochloric acid. The mercury-loaded carbon was used as an anode, as shown in Fig. 4, with the current being fed through a graphite rod. The mass of carbon used, in each test was up to 150 g and the test’s running time of 6 h with a constant upstream flow rate of 3.3= 10y5 m3 sy1 Ž2 l miny1 .. A plastic screen was used on top of the anodic compartment, in order to avoid the carbon being dragged out from the cell, especially after running this process for a while, as the carbon becomes lighter with time. The electrolytic process starts, once the current is supplied, with the oxidation of the chloride ions to chlorine at the anode surface: 2Clym Cl2 q 2e while at the cathode the water is, initially, reduced producing hydroxyl ions and hydrogen: 2H2 O q 2e m H2 q 2OHy Simultaneously, hypochlorous acid is generated, in bulk, by the chemical reaction of chlorine being formed at the anode with the aqueous phase: Cl2 q H2 O m HClO q Hqq Cly
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
199
Fig. 4. Reaction system used for the electro-oxidation Želectroleaching . tests.
which dissociates into hypochlorite and hydrogen ions. The reaction is directly dependent on the pH. The oxidation power increases as the pH goes down since the generation of HClO is favoured as depicted in the thermodynamic stability diagram of Fig. 5 Žwhich was done using the Outokumpu Research Oy software HSC Chemistry 3.0., which can be confirmed observing the speciation diagram of Fig. 6. As the pH-value goes below pH 5, approximately 100% HclO is generated. y
q
HClO m ClO q H
These hydrogen ions react with the hydroxyl ions, a product of the cathodic reaction, to form water: Hqq OHym H2 O
During the electrolysis, after a while, the production of hypochlorite ions does not increase as expected, which is attributed to the generation of chlorate ions, either chemically: 2HClOq ClOym ClO3yq 2Hqq 2Cly or electrochemically: 6ClOyq 3H2 O m 2ClO3yq 6Hq4Cly q 3r2O2 q 6e Once those chlorine species ŽHClO and ClOy. are produced, the oxidation of elemental mercury takes place according to the following reaction: Hgq 2ClOyq 4Clyq 2H2 O m Cl2 q HgCl42y q 4OHy
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
200
Fig. 6. Speciation of hypochlorite ions and hypochlorous acid as a function of pH.
Fig. 5. Thermodynamic stability diagram for Cl᎐H2 O system at 25⬚C. This diagram was done by using the HSC Chemistry software from Outokumpu Research Oy.
Once produced, the HgCl42y are reduced at the cathode surface HgCl42yq 2e m Hg0 q 4Cly which is the predominant mercury reduction reaction from a mercury solution containing high chloride ion concentration ŽSobral et al., 1996.. For tracing the mercury concentration down during the electro-oxidation process, the electrolyte was sampled every 30 min and analysed for mercury by atomic absorption spectrometry. The activated carbon used before and after the electro-oxidation process was also analysed. The experimental conditions and the results of the electro-oxidation are shown in Table 1.
It was observed, after removing the elemental mercury from the mercury-bearing activated carbon and washing properly, that the specific surface area Ž8050 m2 my3 . was nearly that of a fresh activated carbon with the same particle size. A fresh 8 = 16 mesh activated carbon has a specific surface area in the range of 8000᎐12 000 m2 my3 . However, it is necessary to thoroughly wash down the carbon, once finished the electrolytic process, so as to get rid of sodium chloride solution. This procedure will avoid further NaCl crystallisation on the available surface of the carbon. Once taking this advice into consideration, and, in addition, considering that the just-cleaned carbon has to be reactivated, the recycling of the carbon is strongly recommended.
3. Conclusions Electrochemical technology has made consider-
Table 1 Experimental conditions and results of electro-oxidation Test
Carbona Žanode. Žg.
pH Želectrolyte.
Current ŽA.
Cathode
Hg extraction Ž%.
wHgx b Žppm.
1 2 3 4
138.81 139.71 153.11 152.19
4 4 4 4
0.7 0.7 1.0 1.5
Titanium Steel c Titanium Titanium
70 68 88 98
2.95 1.28 3.76 3.97
a
8 = 16 mesh Ž2.38 mm. activated carbon. Remaining mercury concentration in the electrolyte after the electro-oxidation process. The experimental conditions are those mentioned in the experimental procedures. c Stainless steel. b
L.G.S. Sobral et al. r The Science of the Total En¨ ironment 261 (2000) 195᎐201
able advances in the last decade and offers potential as another hydrometallurgical process for treating mercury-bearing residues. The experimental results permit the following conclusions: 1. By choosing suitable operating conditions, it is possible to reduce the mercury concentration to low values with a high extraction efficiency Ž) 99%., considering the utilised residue. It was possible to reduce the residue mercury concentration to less than 3.0 ppm, which is not low enough to be considered suitable for discharging, indicating the necessity of extending the electrolysis time. 2. The final leaching solution should not be discharged since it contains not only high salinity, but also high mercury concentration Ž( 2 ppm., which is not suitable for discharging. It is recommended to re-use such solution back to the residue treatment process. 3. According to the low current densities obtained during the voltammetric study, the use of high surface-area cathodes is recommended. This provides good mass transfer conditions so as to enhance the reactor performance.
201
4. It is recommended to perform the electroleaching process in an acidic range of pH Žbetween pH 4 and 6. so as to enhance the mercury dissolution.
References Allen SJ, McKay G, Khader KYH. J Colloid Interface Sci 1988;126:517᎐524. Allen SJ, McKay G, Khader KYH. J Colloid Interface Sci 1989;45:261᎐302. Chem. Abs., 81a, 82126e. Chem. Abs., 81b, 111188j. Flood DJ, Kraynik CJ. Chem Abs 1973;83:65287g. Gardiner WC, Munoz F. Chem Eng 1971;78:57. Moo-Young M. Waste treatment and recycling. Proceedings of the International Conference on Environmental Biotechnology, Hong Kong, 1992: 11. Sobral LGS, Santos RLC, Hempel M, Thoming J. The elec¨ troleaching of residues containing mercury. Part I: Kinetics aspects. In: Proceedings of the Third International Conference on Clean Technologies for the Mining Industries, Santiago, Chile, 15᎐17 May 1996: 175. Sumitomo Chemical Co. Ltd., German Patent 2 321 196, 1973. Waltrich PE. US Patent 3 704 875, 1972. Yu PHF. Waste treatment and recycling. Proceedings of the International Conference on Environmental Biotechnology, Hong Kong, 1992: 13.