2132
Ind. Eng. Chem. Res. 2000, 39, 2132-2139
APPLIED CHEMISTRY Copper Electrodeposition and Oxidation of Complex Cyanide from Wastewater in an Electrochemical Reactor with a Ti/Pt Anode Lidia Szpyrkowicz,*,† Francesco Zilio-Grandi,† Santosh N. Kaul,‡ and Anna M. Polcaro§ Environmental Sciences Department, University of Venice, Dorsoduro 2137, 30123 Venice, Italy, National Environmental Engineering Research Institute, 440020 Nagpur, India, and Department of Chemical Engineering and Materials, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy
The paper presents the results of a study on the simultaneous electrooxidation of cyanides and recovery of copper as a metallic deposition on the cathode from weak concentration rinse wastewater in an electrochemical reactor with a Ti/Pt anode. Both a direct electrooxidation process and an indirect electrooxidation in a chloride-rich medium proved feasible, with direct electrooxidation being preferable because of the lower energy consumption. The results show that the process of the direct electrooxidation under alkaline conditions leads to the formation of an electrocatalytic film on the anode. Simultaneous copper electrodeposition on the cathode is feasible and economically convenient, particularly if conducted at pH 13 or higher. The process can be described by the pseudo-first-order kinetics, with the rate constant for Cu removal equal to 0.013 h-1 for very alkaline conditions. Energy consumed for copper electrodeposition proved to be inversely proportional to the initial Cu concentration. For wastewater bearing 1100 mg dm-3 Cu, 5.46 kWh is needed to eliminate 1 kg of metal. The current efficiency for chemical oxygen demand removal reaching 100% or higher was obtained, indicating reactions other than electrooxidation occurring simultaneously. Under the best conditions, the total cyanide concentration was lowered from 250 to 7.9 mg dm-3. Introduction In copper plating operations an electrolytic bath containing copper and potassium cyanides and potassium carbonate is often preferred to a sulfate bath because of the brightness of the metal deposits and their good adherence. In this case the wastewaters which are generated during the rinsing of worked pieces bear copper cyanide complexes. Conventional treatment of these wastewaters consists of oxidation of the cyanide by chlorine gas or hypochlorite and the subsequent removal of cuprous and cupric ions by precipitation as hydroxides. This method requires careful control of the pH and a dose of hypochlorite in excess of the stoichiometric quantity required by the reaction. The resulting sludge is difficult to handle because it contains copper hydroxides which can be subject to easy dissolution, giving rise to a copper-rich leakage. Electrochemical oxidation, particularly if conducted in a reactor with plate electrodes, enabling the cathode to be reused, can be an interesting alternative. Application of electrochemical reactors for the treatment of industrial wastewater is gaining increasing attention.1-3 The use of electrochemical oxidation for the destruction of cyanides has been studied as an alterna* Corresponding author. Phone: (+39)0412578667. Fax: (+39)0412578591. E-mail:
[email protected]. † University of Venice. ‡ National Environmental Engineering Research Institute. § University of Cagliari.
tive to chemical oxidation since the early 1970s. Most of the studies have focused on the destruction of simple cyanides and on the mechanism of the reaction. It has been shown that direct oxidation is slow on platinum4 and graphite,5 exhibiting low current efficiency. Better results have been obtained using Ni anodes,6 a PbO2coated electrode,7 or a reticulated three-dimensional electrode.8 The reaction pathways and byproducts of cyanide decomposition depend heavily on the OH- concentration of the medium. It is generally accepted that in alkaline conditions cyanide complexed with copper can be electrochemically oxidized, giving carbon dioxide and nitrogen gas:9,10
Cu(CN)n(n-1)- + 2nOH- ) Cu+ + nCNO- + nH2O + 2ne (1) CN- + 2OH- ) CNO- + H2O + 2e
(2)
2CNO- + 4OH- ) 2CO2 + N2 + H2O + 6e
(3)
As can be seen from reactions 2 and 3 the direct oxidation of cyanide to CNO- in alkaline media occurs with consumption of 2 mol of alkalinity for 1 mol of reacted cyanide, producing N2 and CO2 as final products.10 At lower pH the presence of oxalate and of a brown polymer (HCN)x, called “azulmin”,10,11 can also be detected.
10.1021/ie9903137 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/14/2000
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2133
Several research studies have observed that the rate of cyanide decomposition can be favorably influenced by the presence of copper in the solution,12,13 postulating that the film of copper oxides deposited on the anode exhibits electrocatalytic properties. The mechanism of this process was studied by Wels and Johnson14 in cyanide decomposition experiments carried out on the copper oxide film obtained by electrodeposition from a Na3Cu(CN)4 solution. They postulated that anodic conversion of CuII to CuIII provides a favorable mechanism for the oxygen-transfer step which occurs concomitantly with electron transfer in the oxidation of CN- to CNO-. The oxygen-transfer step occurs via generation of the hydroxyl radical (′OH)ads at the CuIII site (S) during anodic discharge of H2O:
S + OH- ) S(′OH)ads + e
(4)
S(′OH)ads + CN- + OH- ) S + CNO- + H2O + e (5)
Figure 1. Schematic view of the experimental setup (1, DC power source; 2, anode; 3, cathode; 4, stirring bar; 5, electrolytic cell; 6, magnetic stirrer). Table 1. Experimental Conditions of Different Runs
The net result of the above reactions is reaction 2. In addition to the above reactions cyanate can also decompose, producing ammonium:6
CNO- + 3H2O ) CO32- + NH4+ + H2O
(6)
The above reaction is believed to be autocatalytic with respect to carbonate ions, which can also be formed independently of reaction 3 by absorption of atmospheric CO2 in the alkaline cyanide solution. The problem of removing copper simultaneously with destruction of its organic complexes, particularly with cyanides, has gained less attention. El-Ghaoui et al.,12 who studied the removal of cyanides from coppercomplexed solutions in a packed-bed glass tower, found the process feasible. However, no data on copper removal were presented. Tan et al.9 in their work on the electrochemical destruction of complex cyanide showed that a direct oxidation of complex copper cyanide occurs at the anode. In electrolysis of the copper cyanide using a two concentric cylindrical electrodes under the conditions of pH equal to 11.0, Tan et al.9 have eliminated 90% of copper from the solution: 60% was recovered as metallic copper at the cathode, and 30% was found in the anodic film. The simultaneous removal of copper and cyanide by an electrochemical process has also been proved feasible by Hwang et al.,10 who examined different end products of cyanide decomposition as a function of the pH. No information on the energy consumption is, however, given. From a practical application point of view it would appear interesting to find the conditions which could enable not only the destruction of cyanides from dilute solutions but also the removal of copper in such a form that it can be reused in the electroplating process, preventing it from becoming a solid waste. In our previous study15 the process of simultaneous cyanide destruction and copper recovery by electrodeposition from diluted wastewater was found to be technically and economically feasible when using stainless steel electrodes. The purpose of the present investigation was to assess the possibility of conducting the process in a reactor equipped with a Ti/Pt anode and a copper cathode, which can be reused in metal-plating
run initial pH initial Cu concn [mg dm-3] cell potential U [V] 1 2 3 4 5 6 7 8 9 10
13.2 12.5 12.5 10.0 13.0 13.0 13.0 13.0 7.9 12.5
360 400 426 650 1100 141 210 650 5000 426
2.6 4.8 4.0 6.3 2.5 2.6 2.6 2.2 4.5 3.4
operations. The reaction kinetics and energy consumption were followed during direct and indirect (in the chloride-rich medium) electrooxidation. Experimental Section Technical-grade potassium cyanide, cuprous cyanide, and potassium hydroxide were used to prepare a stock solution containing 5 kg m-3 of copper at the initial cyanide to copper molar ratio of 0.95:1. The experiments were conducted with different initial cyanide concentrations using samples obtained by dilution of the stock solution. The pH was further adjusted to the values in the range 10.0-13.2 by KOH. Table 1 indicates the experimental conditions of different runs. The experimental conditions were chosen in a way to verify the influence of pH, chlorides, and conductivity on the performance of the reactor. Runs 1-8 were performed on the solutions without the addition of chlorides. In run 3, Na2HPO4 and Na2CO3 were added. In the experiments conducted in the presence of chloride (runs 9 and 10), a chloride concentration equal to 2000 mg dm-3 was obtained by the addition of NaCl. Experiments were conducted in batch in an undivided Pyrex glass cylindrical electrolytic cell, depicted in Figure 1. The cathode was a copper cylinder lining the cell walls and the anode was a Ti/Pt plate placed vertically in the center of the cell at a distance of 2 cm from the bottom. The ratio between the cathode and anode surfaces was 4.34:1. Mixing conditions can significantly influence the formation of an anodic film of copper oxides, as pointed out in our previous work with a 316 stainless steel anode.15 For this reason completely developed turbulent conditions with a Reynolds number (Re) equal to 15 000,
2134
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000
Figure 2. Plot of ln{[COD]t/[COD]0} vs time for runs at different pHs (9, run 10; 0, run 4; ], run 3; 4, run 2; 2, run 7).
but with no vortex formation, were selected for the present experiments. Mixing in the cell was performed by a magnetic stirrer and by gases developed on the electrodes. The reported value of the Re number was calculated at the tip of the stirring bar and characterizes the bottom part of the cell. In the upper part of the vessel the Re number is supposed to be lower, because of dissipation of the energy by small eddies. The use of the magnetic mixing was imposed by the cell geometry. For this kind of mixing, however, characteristic geometric ratios cannot be established, which precludes any further correlation of the shear stress with the formation of the anodic film. All of the experiments were conducted at a constant temperature (25-30 °C). The electric power was supplied by a stabilized power source with current voltage monitoring and control in the range 1-10 V. All experiments were conducted under galvanostatic conditions at an anodic current density of 2 A dm-2. The following parameters were monitored during the runs: pH, temperature, conductivity, and Cu and N-NH3 concentration. The performance of the process was followed by an analysis of COD (chemical oxygen demand) using dichromate, as recommended by Standard Methods.16 Some chosen samples were analyzed for the total cyanide concentration by titration with AgNO3 after distillation of the sample acidified by a 1 + 1 H2SO4. The colorimetric determinations were made by a DR 2000 Hach spectrophotometer, pH by an Orion pHmeter model 420A; Cu by a Perkin-Elmer Analyst 100 atomic absorption spectrophotometer after digestion of the samples with HNO3 acid. Experiments to determine the quantity of electrodeposited copper were performed on a smaller scale using a 140 cm3 cyanide solution volume at pH 13 with Ti/Pt and Cu plate electrodes placed at a distance of 1.8 cm. The ratio of the volume to the electrode surface and current conditions of this experimental setup were maintained at the values relative to the experiments on the larger scale. Results and Discussion Effect of pH on the Process Performance during Direct and Indirect Electrooxidation. The performance of the process was followed by analysis of the copper concentration and COD, the aspecific parameter, which is a measure of the quantity of the compounds present in the solute, which can be oxidized by dichromate under standard reaction conditions. Elimination of COD in the experiments conducted with initial values of pH varying from 10 to 13 is shown in Figure 2. The results are expressed as logarithms of the normalized
concentration. The reported data are relative to the direct electrooxidation, during which it can be assumed, on the basis of the mechanistic study presented in the literature, that cyanides are removed via a sequence of reactions 1, 4, 5, and 3, and to indirect electrooxidation mediated by chlorine species, produced by anodic oxidation of chloride ions. More details on the mechanisms involved in the removal of cyanides by reactions with chlorine species are reported later. As expected, there was a strong influence of the initial pH on the performance of the process. In strongly alkaline conditions, just as occurred in our study with 316 stainless steel anodes, a black film, identified previously as cupric oxide,15 started to deposit on the Ti/Pt anode from the beginning of the run. It did not form stable deposits, and some small pieces were continuously transported into the bulk of the solution where they underwent dissolution, with the film surface being continuously renewed. No other precipitates were observed at this level of pH. When the pH was lowered below 12.5, white precipitates appeared in the bulk of the electrolyte solution. They may be attributed to the formation of oxamides (detected by Hwang et al.10 but at much lower pH) or oxalates. No attempt was made to define the composition of the precipitate, because it disappeared after 15-20 min of electrolysis. At the lowest tested pH (equal to 10) the solution assumed a brownish color, attributed in the experiments of Yoshimura et al.11 and Hwang et al.10 to azulmin production; no film deposition was observed on the anode under these conditions. In the process of electrochemical oxidation, the COD removal rate would be proportional to the pollutant concentration and also to the chlorine/hypochlorite concentration if chloride is used as the supporting electrolyte. In this last case the pollutant removal is due to the indirect oxidation effect of chlorine/hypochlorite. Thus, the kinetics for COD removal in the indirect electrochemical oxidation process can be described:
-
d[COD] ) k[COD][Cl2] dt
(7)
where k is the second-order rate constant (m3 mol-1 s-1) and [Cl2] is the concentration of the chlorine species (mol m-3). Chlorine/hypochlorite, produced by the anodic oxidation of chloride during electrolysis, converts to chloride as pollutants are oxidized. Then chloride is anodically oxidized to form chlorine/hypochlorite, which oxidizes the pollutants again. Assuming stationary conditions under which there is no accumulation of chlorine in the solution and that the rate of its production (proportional to the applied current) and the rate of the consumption are equal, the concentration of chlorine/hypochlorite during electrolysis can be assumed to be constant. Therefore, under conditions when the reaction of the removal of organics is slow, with the mass transfer being faster than the reaction rate, the term of [Cl2] and the rate constant (k) in the above equation can be combined to simplify the equation and a new rate constant (kobs) can be used. Accordingly
-
d[COD] ) kobs[COD] dt
(8)
where kobs is the apparent pseudo-first-order rate constant (s-1).
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2135
Figure 3. Values of the rate constant of COD decomposition (experiments without Cl- ions in the solution; the asterisk refers to the run with an addition of Na2PO4 + Na2CO3).
As can be seen from the reactions describing possible pathways of cyanide destruction, several species present in the solution can contribute to the COD value, viz., cyanide, cyanate, azulmin, and oxalate. Therefore, in the search for the reaction rate, the constant deriving from the kinetic equation should be regarded as an “apparent” one, because it represents an overall value of several single reactions occurring simultaneously. In the present study the experiments were conducted with the anode potential far exceeding the standard reversible potential of reaction 5, quoted in the alkaline solution as +0.97 V,17 and oxygen evolution on the anode also occurring. With it being impossible to relate the rate of cyanide removal to the current density in this case, eq 8 was also used to describe the direct electrooxidation process. When a chart of ln([COD]t/ [COD]0 vs time is plotted, linear plots were obtained for all of the runs, indicating that a pseudo-first-order reaction kinetics, expressed by eq 8 can be used. A rate constant for the removal of COD was calculated with the regression coefficient always above 0.99 (0.94 in the run at pH 10). The highest value of kobs equal to 0.01 h-1 was obtained for a pH of 13.2. As can be seen from Figure 3 the rate constant kobs for COD removal proved to be strongly pH-dependent and its value rose with pH. Taking into account large experimental errors, the exact nature of this relationship is difficult to establish. In the narrow range of the pH between 12.5 and 13, a linear relationship between kobs and pH may be assumed. The rate constants for COD decomposition obtained in the present study were slightly higher than the values determined using stainless steel anodes15 (0.004 and 0.01 h-1 respectively for pH 10 and 13 in the present study and 0.003 and 0.007 h-1 for the same pHs for the stainless steel anode). Figure 3 also shows the results obtained under the conditions when conductivity was increased from 6.9 to 22.0 mS cm-1 by the addition of Na2HPO4 and Na2CO3 with a view to lower energy consumption (point * on the graph). The addition of phosphates caused a slight decrease in the reaction rate, probably because of changes in the double-layer structure of the anode resulting from adsorption of phosphate anions and competition by phosphate for adsorption sites needed by CN- in the catalytic oxidation, as already pointed out by Wels and Johnson.14 Figure 4 shows the instantaneous anodic current efficiency (η) of the oxidation of COD, calculated at discrete time intervals as a function of the COD value of the solution. The data in Figure 4 are relative to the experiments conducted at the very alkaline pH. With COD being a parameter which quantifies the amount
Figure 4. Current efficiency for COD removal (9, run 2; 0, run 8; [, run 1; ], run 7; 2, run 3; 4, run 10).
of oxygen necessary to oxidize the substances present in 1 dm3 of the sample, its mole equivalent is equal to 8.18 The current efficiency related to the elimination of COD was thus calculated:
η)
FCCODV 8Q
(9)
where F is Faraday’s constant (96 487 C equiv-1), CCOD is the change in COD (g dm-3), V is the volume of solution (dm3), and Q is the quantity of electricity (C). All of the runs except runs 10 and 3 were initiated using a Ti/Pt anode from which the cupric oxide produced in the previous run had been removed by gently rinsing with a dilute HNO3 solution. Runs with a catalytic film already present on the anode exhibited high current efficiencies at the very outset of electrolysis. For the runs with an initial COD concentration above 300 mg dm-3, the anodic current efficiency apparently exceeded 100%, indicating the occurrence of some chemical reaction, parallel to electrooxidation. A similar phenomenon was observed by Lin et al.19 during electrodeposition of copper from the cyanide effluent. It can be hypothesized that the chemical reaction contributing to the depletion of COD may be reaction 6, by which cyanides decompose with the generation of ammonium. However, an attempt to include this reaction in the mass balance of carbon and nitrogen showed that this reaction could explain only about 0.3% of carbon removal. The quantity of ammonium ion effectively generated in the process may, however, not correspond to that found in the solution, because under these highly alkaline conditions the phenomenon of ammonia stripping is likely to occur, and this alters the nitrogen balance. The other process which can lead to the depletion of ammonia is its direct electrooxidation on the anode, as is already shown for other types of wastewater.20 Figure 4 also shows that the anodic current efficiency during the treatment of cyanide-bearing wastewater decreases as COD is being depleted. When COD is lowered to about 100 mg dm-3, the current efficiency falls to ca. 20% because of the predominance of the parasitic reaction of water electrolysis. The composition of the electrolyte had a strong effect on the current efficiency. Runs at a high concentration of hydroxyl ions (pH 12.5 and higher, with and without addition of other electrolytes) are represented in Figure 4 by a group of current efficiency curves lying relatively close together. The current efficiency (not shown on the graph) determined for the run which started at pH 10 is about 50% lower for the same values of COD. The
2136
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000
Figure 5. Plot of ln{[Cu]t/[Cu]0} vs time for runs at different pHs (9, run 4; [, run 10; ], run 3; 0, run 2; 4, run 7).
presence of chloride ions or the addition of phosphates both exerted a negative influence on the current efficiency during COD removal. The conditions at which the best results were obtained in terms of COD removal occurred when the initial pH equaled 13.2. A decrease in the rate of COD elimination toward the end of this run can probably be explained by mass-transfer limitation, making it difficult for cyanide to reach the electrode surface. The higher the pH of the electrolyzed solution, the better was also the result in terms of the residual concentration of the cyanides, present after 90 min of electrooxidation. A decrease of COD in 90 min of electrolysis from 510 to 88 mg dm-3 under pH 13.2 was accompanied by a drop in the cyanide concentration from 250 to 7.9 mg dm-3 (with the maximum allowed concentration being equal to 0.5 mg dm-3). For approximately the same initial cyanide concentration and a pH of 12.5, the final cyanide concentration after 90 min of electrolysis was double (15.7 mg dm-3). In the run conducted after chloride ions were added, the cyanide concentration was lowered only to 53.5 mg dm-3. In all of the experiments, elimination of COD was accompanied by a decrease of the pH. This drop was particularly evident when the initial pH value was low (run 4 at pH 10), as could be predicted by the consumption of alkalinity resulting from eqs 2 and 3. In this run pH decreased to 9.5 after 90 min of electrolysis. For the experiments conducted with the initial pH equal to 13, the higher the initial content of cyanides, the more significant was the decrease of pH. For example, for the initial concentration of cyanides equal to 80 mg dm-3, the decrease in pH was equal to 0.02 pH units only, while for 370 mg dm-3, the pH was lowered by 0.1. It was not possible to calculate the ratio between the quantity of cyanides removed and the alkalinity consumed for all of the experiments because the final CNconcentration was not always determined. COD could not be used for this purpose because of the fact that, as mentioned previously, along with cyanides other compounds, intermediate products of their degradation, contribute to the COD value. For the data for which this evaluation was possible, it resulted that the higher the initial pH, the higher was the ratio between the consumed alkalinity and the quantity of destroyed cyanides (with both values defined on a molar basis). This ratio was equal to 2.6 for run 1, 0.33 for run 2, and only 0.02 for run 4, which were respectively characterized by initial pHs of 13.2, 12.5, and 10. The elimination of copper in time for different initial values of pH is shown in Figure 5. The data are presented as logarithms of the normalized concentration. Copper elimination curves indicate that the process seems not to be less sensitive to the initial alkalinity of
Figure 6. Current efficiency for Cu removal (9, run 1; [, run 10; 0, run 2; 4, run 5; 2, run 8; b, run 3; O, run 7).
the solution than the removal of COD. However, the form in which copper was deposited on the cathode depended on pH in a significant way: for pH values of 12.5 and above, the deposit was composed of pure metallic copper (indicating the possibility of its reuse in the electroplating process), while for pH 10, the cathode was covered by a gray deposit (composition not analyzed). Copper removal described by the pseudo-firstorder kinetics resulted in rate constants equal to 0.013, 0.0066, and 0.0032 h-1 for initial pHs of 13.2, 12.5, and 10, respectively. Copper electrodeposition was characterized by a relatively low cathodic current efficiency. Analogously to COD, it proved to be a function of the initial concentration; the highest value of the current efficiency equal to 0.6 was obtained for an initial copper concentration equal to 1100 mg dm-3. The efficiencies for lower initial Cu concentrations are much lower, as can be seen from Figure 6. During the electrolysis two mechanisms contributed to the removal of copper: its electrodeposition on the cathode and formation of a copper oxide film on the anode. The ratio between the quantity of copper eliminated as copper electrodeposited on the cathode and the quantity removed on the anode was substantially the same as the value found in the experiments with the stainless steel anode15 and was equal to 90%. The proportion of copper removed by electrodeposition in our study is double the maximum value of 45% reported by Hwang et al.10 and 50% higher than that in the experiments of Tan et al.9 The reason may be related to the fluodynamic conditions. In fact, Hwang et al.10 used a two-compartment cell with no mixing, which could have prevented the formation of the shear sufficient to export the outer, less adherent layers of anodic copper deposits and their subsequent transportation into the bulk, where they can redissolve. Mixing conditions are not specified in the Tan et al.9 experiments, which precludes this kind of comparison. The pH was also higher in our experiments than in those cited, which may also have contributed to differences in the results. Effect of Cl- on the Process Performance. The addition of chlorides causes a change in the pathway of cyanide destruction.21 In alkaline solutions free chlorine produced on the anode reacts with the hydroxide, producing hypochlorite:
Cl2 + 2OH- ) Cl- + ClO- + H2O
(10)
Both dissolved chlorine and hypochlorite can react with free cyanide following the reactions
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2137
CN- + Cl2 ) CNCl + Cl-
(11)
CN- + ClO- + H2O ) CNCl + 2OH-
(12)
At alkaline pH cyanogen chloride reacts readily, producing cyanate:
CNCl + 2OH- ) CNO- + Cl- + H2O
(13)
2CNO- + 3OCl- + H2O ) 2HCO3 + N2 + 3Cl
-
(14)
The presence of copper makes this pathway more complicated because of the formation of copper-chloride complexes. Some authors22 report on the beneficial role that the dosing of chlorides has on the removal of complex cyanides. Under the conditions of our study, this was not confirmed. The addition of chlorides in the run conducted at an initial pH equal to 10 caused a massive formation of brown precipitate (probably azulmin), which occupied 3/4 of the reactor. No deposition was observed on the anode, and the cathode was covered with white precipitate. In a more alkaline solution (initial pH of 12.5), the scenario was different: the addition of chlorides did not prevent the formation of a catalytic film on the anode and the cathode was uniformly covered by metallic copper. The overall kinetics of COD removal was however lowered, as can be seen from data shown in Figure 2, probably because of a change of the reaction pathway. The current efficiency was also lower (see Figure 4), which may be explained by the cathodic reduction of active chlorine, a phenomenon which cannot be prevented while working in the undivided cell. Finally we conclude that the addition of chloride during the electrochemical destruction of copper-complexed cyanides using an undivided cell reactor is not recommended. The differences in the results regarding chloride addition by comparison with the data of El-Ghaoui and Jansson22 may derive from the fact that during the experiments involving indirect electrooxidation they studied the destruction of simple (NaCN) and noncomplexed cyanides. In any case it is difficult to compare the specific rates of the removal of cyanides obtained in these studies with ours because of the different configuration of the reactor (a trickle tower was used by El-Ghaoui and Jansson22). We can also hypothesize that a lack of backmixing in their configuration could have prevented the chlorine loss reactions which, on the contrary, could have occurred in our experiments. Effect of Electrolyte Composition on Energy Consumption. Power consumption is one of the basic parameters governing the applicability of electrochemical processes to wastewater treatment. It appeared more meaningful to calculate energy consumption with reference to the removal of copper rather than the elimination of COD, considering that the fate of different organic compounds, which may be formed during cyanide destruction, contributing to the COD value, was not investigated. The unit energy consumption E, expressed in kWh necessary to remove 1 kg of pollutant, can be calculated according to
(15)
where U ) cell potential (V), I ) applied current, t ) reaction time (h), and m ) mass of the removed pollutant (kg). When m is substituted with the quantity of eliminated Cu (calculated assuming a pseudo-first-order kinetics, according to data depicted in Figure 5), the equation for energy consumption becomes
E ) 10-3UIt[Cu]0-1(1 - exp(-k′obst))
Cyanate is oxidized to nitrogen and bicarbonate:
-
E ) 10-3UItm-1
(16)
where k′obs is the rate constant for the overall process of Cu depletion, including cathodic electrodeposition and removal as an anodic film and [Cu]0 is the initial concentration of copper (mg dm-3). This equation shows that two parameters can influence energy consumption, viz., the cell potential and the rate constant of the reaction. Lowering of the cell potential can be obtained by the addition of strong electrolytes to the solution. However, as was shown in our study, negative changes in the kinetics of the reaction caused by the addition of phosphate and chloride may lead to a net increase in the unit energy consumption by lowering of the reaction rate. Thus, the appropriateness of adding other electrolytes should always be carefully examined. Figure 7 shows power consumption E, calculated from data obtained during different runs, as a function of the initial copper concentration. As expected, the data show that the pH value at which the process is operating has a considerable influence on the energy consumption. In fact, the higher the pH, the higher is the value of the rate constant and the lower is the cell potential. In our study the best results in terms of energy consumption for the removal of COD and copper were obtained under the conditions of pH equal to or above 13. The energy consumption was inversely proportional to the initial value of the copper concentration, as can also be deduced from eq 16. For the process conducted at a pH of 13, the best-fit method gave the following expression for the unit power consumption, referring to 1 kg of copper removed from the wastewater:
E)
6184 + 0.963 [kWh kg-1 ] [Cu]0
(17)
For example, for wastewater bearing 1100 mg dm-3 of Cu, 5.46 kWh is needed to eliminate 1 kg of metal. The energy consumption obtained during the present study was, however, strongly influenced by the ohmic drop in the cell and should be considered as specific to the experimental setup used. It cannot be applied in the scaling up of the reactor but is a useful indication regarding the influence of the composition of the electrolyte on the performance of the process. Conclusions It is generally known that electrochemical technologies encounter strong resistance in being applied to wastewater treatment. Even the metal-plating industry, in which production is based on electrochemical processes, is reluctant to apply electrochemical oxidation/ reduction to treat the wastewater and uses conventional technologies, with the result being the generation of large quantities of sludge, costly to handle and dispose of because of the presence of heavy metals. The less
2138
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000
Figure 7. Energy consumption for Cu removal under different pH conditions and Cu concentrations (closed symbols, pH 13; the asterisk refers to the run with an addition of Na2PO4 + Na2CO3).
sophisticated the proposed electrochemical process is, the more easily it will be accepted as a substitute for the traditional technology. In view of this, undivided cell electrochemical reactors are potentially attractive because their scheme is very simple. In the case of the present study, cyanide decomposition was the main objective, because the cathode deposition of copper is a well-established process. Thus, to avoid copper electrodeposition influencing the global performance of the process, it was decided to develop the cathode area fourfold by comparison to the anode surface because the process was under kinetic control and was not influenced by mass transfer. The results of the study show that the process of the simultaneous removal of copper by electrodeposition and elimination of COD can be achieved in an electrochemical reactor equipped with a Ti/Pt anode. The performance of the reactor proved better, both in regards to the kinetics and energy consumption, when conducted as a direct electrooxidation, with no addition of chloride and under very alkaline conditions. The electrochemical treatment of copper-complexed wastewater is thus an alternative to chemical oxidation, the latter having the disadvantage of creating hazardous sludge. The process can be described by the pseudo-first-order kinetics, in which the composition of the solution strongly influences the reaction rates. Under very alkaline conditions, which proved to be the most indicated, the rate constant for COD removal was equal to 0.01 h-1. The process performed better when no additional electrolytes were present. Under these conditions a current efficiency reaching 100% or higher was obtained, indicating that chemical processes (probably the autocatalytic reaction of cyanate decomposition in the presence of carbonate ions) were occurring simultaneously to electrooxidation. A low current efficiency for copper elimination, which characterized the reactor performance particularly at the end of electrolysis, probably derived from the fact that at very low pollutant concentration the process became strongly mass-transfercontrolled. A pH equal to 13 or above is also indicated regarding energy consumption, because the concentration of hydroxyl ions showed a strong influence on the values of variables which determine this parameter. The energy consumption for the elimination of copper proved also to be influenced by its initial concentration, showing an inversely proportional relationship. For example, for an initial copper concentration equal to 1100 mg dm-3, the power consumption in the reactor under study was as low as 5.46 kWh for 1 kg of removed copper. The data
relative to energy consumption registered during the present study are specific to the experimental setup used, because they are strongly influenced by the ohmic drop in the cell. They cannot be applied in the scaling up of the reactor but give indications regarding the most suitable composition of the solution, in reference to the pH level and regarding the advisability of adding other electrolytes. A comparison of the results of the present study conducted under strong agitation with the literature data regarding experiments performed without mixing shows that the distribution of the eliminated copper between the anodic film and the metallic cathodic deposit may be related to fluodynamic conditions in the cell. Further studies on the aspect of the influence of the sheer stress on the buildup of the two kinds of deposits are planned to be conducted using a cell of different geometry, which will allow this kind of approach. Acknowledgment Authors are grateful to Dr. Ewa Nowak for help in analytical determinations. This project was supported by Tobaldini SpA (Olmo di Creazzo, Vicenza, Italy). Nomenclature COD ) chemical oxygen demand, g dm-3 [Cu]0 ) copper concentration, mg dm-3 E ) power consumption, kWh kg-1 F ) Faraday’s constant, 96487 C equiv-1 I ) applied current, A kobs ) apparent rate constant for COD removal, h-1 k′obs ) apparent rate constant for Cu removal, h-1 n ) number of resolutions of the stirrer Q ) charge, C t ) reaction time, h U ) cell voltage, V V ) reactor volume, dm3 Greek Symbols η ) current efficiency, % ν ) solution viscosity, m2 s-1 Dimensionless Group Re ) nD2ν1 ) Reynolds number (n ) 600 rpm; D ) 0.04 m; ν ) 1.01 × 10-6 m2 s-1)
Literature Cited (1) Bergmann, H.; Hertwig, H.; Nieber, F. Experimental and theoretical studies on a new type of electrochemical reactor for wastewater treatment. Chem. Eng. Proc. 1992, 31, 195. (2) Buso, A.; Giomo, M.; Boaretto, L.; Sandona´, G.; Paratella, A. New electrochemical reactor for wastewater treatment: electrochemical characterization. Chem. Eng. Proc. 1997, 36, 255. (3) Buso, A.; Giomo, M.; Boaretto, L.; Paratella, A. New electrochemical reactor for wastewater treatment: mathematical model. Chem. Eng. Proc. 1997, 36, 411. (4) Tamura, T.; Arikado, T.; Yoneyama, H.; Matsuda, Y. Anodic oxidation of potassium cyanide on platinum electrode. Electrochim. Acta 1974, 19, 273. (5) Arikado, T.; Iwakura, C.; Yoneyama, H.; Tamura, H. Anodic oxidation of potassium cyanide on the graphite electrode. Electrochim. Acta 1976, 21, 1021. (6) Kelsall, G. H.; Savage, S.; Brandt, D. Cyanide oxidation at Nickel anodes. II. Voltammetry and coulometry of Ni/CN-H2O systems. J. Electrochem. Soc. 1991, 138, 117. (7) Hine, F.; Yasuda, M.; Iida, T.; Ogata, Y. On the oxidation of cyanide solutions with lead dioxide coated anode. Electrochim. Acta 1986, 31, 1389.
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2139 (8) Tissot, P.; Fragniere, M. Anodic oxidation of cyanide on a reticulated three-dimensional electrode. J. Appl. Electrochem. 1994, 24, 509. (9) Tan, T. C.; Teo, W. K.; Chin, D.-T. Electrochemical destruction of complex cyanide. Chem. Eng. Commun. 1985, 38, 125. (10) Hwang, J. Y.; Wang, Y. Y.; Wan, C. C. Electrolytic oxidation of cuprocyanide electroplating wastewaters under different pH conditions. J. Appl. Electrochem. 1987, 17, 684. (11) Yoshmura, S.; Katagiri, A.; Deguchi, Y.; Yoshizawa, S. Studies of the anodic oxidation of the cyanide ion in the presence of the copper ion. IV. The kinetics and mechanism of the decomposition of the intermediate tetracyanocuprate(II) ion. Bull. Chem. Soc. Jpn. 1980, 53, 2437. (12) El-Ghaoui, E. A.; Jansson, R. E.; Moreland, C. Application of trickle tower to problems of pollution control. III. Heavy-metal cyanide solutions. J. Appl. Electrochem. 1982, 12, 75. (13) Ho, S. P.; Wang, Y. Y.; Wan, C. C. Electrolytic decomposition of cyanide effluent with an electrochemical reactor packed with stainless steel fiber. Water Res. 1990, 24, 1317. (14) Wels, B.; Johnson, D. C. Electrocatalysis of anodic oxygen transfer reactions: oxidation of cyanide at electrodeposited copper oxide electrodes in alcaline media. J. Electrochem. Soc. 1990, 137, 2785. (15) Szpyrkowicz, L.; Zilio-Grandi, F.; Kaul, S. N.; Rigoni-Stern, S. Electrochemical treatment of copper cyanide wastewaters using stainless steel electrodes. Water Sci. Technol. 1998, 38, 268. (16) Standard Methods for Examination of Water and Wastewater, 17th ed.; American Public Health Association: Washington, DC, 1989.
(17) Kelsall, G. H. Cyanide oxidation at nickel anodes. I. Theromdynamics of CN-H2O and Ni-CN-H2O systems at 298 K. J. Electrochem. Soc. 1991, 138, 108. (18) Comninellis, Ch. Electrochemical treatment of wastewater containing phenol. Trans. Inst. Chem. Eng., B 1992, 70, 219. (19) Lin, M. L.; Wang, Y. Y.; Wan, C. C. A comparative study of electrochemical reactor configurations for the decomposition of copper cyanide effluent. J. Appl. Electrochem. 1992, 22, 1197. (20) Szpyrkowicz, L.; Naumczyk, J.; Zilio-Grandi, F. Electrochemical treatment of tannery wastewater using Ti/Pt and Ti/Pt/ Ir electrodes. Water Res. 1995, 29, 517. (21) Mantovani, A.; Pace, G.; Berton, A. Oxidation of cyanides: plant evaluation on the basis of kinetic and thermodynamic data of the process. Conference Environment and Resources; Duse, F., Pace, G., Eds.; Bressanone, Italy, 1995; p 13. (22) El-Ghaoui, E. A.; Jansson, R. E. Application of trickle tower to problems of pollution control. II. The direct and indirect oxidation of cyanide. J. Appl. Electrochem. 1982, 12, 69.
Received for review May 3, 1999 Revised manuscript received March 15, 2000 Accepted April 23, 2000 IE9903137
