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Effect of surfactants on recovery of nickel from nickel plating wastewater by electrowinning
~
Pergamon
0043-1354(95)00008-9
Wat. Res. Vol. 29, No. 8, pp. 1821-1826, 1995 Copyright ~) 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0043-1354/95 $9.50 + 0.00
E F F E C T OF SURFACTANTS ON RECOVERY OF NICKEL F R O M NICKEL PLATING WASTEWATER BY ELECTROWlNNING CHEIN-HO HUANG Chemistry Department, Soochow University, Shih-Lin, Taipei, Taiwan, R.O.C. (First received November 1993; accepted in revised form January 1995) Abstract--The effect of surfactants on the electrowinning efficiency of the recovery of nickel from nickel plating wastewater was studied. Owing to the decrease of the effective electrode area and mass transfer coefficient of solution, the electrowinning efficiency was decreased by the presence of surfactants in wastewater. The higher the surface activity and the longer chain-length of the surfactant, the lower the electrowinning efficiency. This drawback could be eliminated by the addition of boric acid and with a greater rate of flow of solution. An attempt had been made to delineate the effect of boric acid on electrowinning efficiency. Key words--boric acid, electrowinning, nickel plating wastewater, surfactant
INTRODUCTION Besides the restoration of dissolved nickel to metallic form and re-incorporate it into the cycle of material, electrowinning is a practical system to diminish spent concentrate or to reduce rinse-water requirements through closed loop operation. As surfactants (anionic and/or nonionic types) are added intentionally to nickelplating and cleaning baths to improve the quality of the plating surface, the treated wastewater inevitably contains t]hese surfactants. F r o m the resuits of cyclic voltammograms in our ealier work we showed that the presence of surfactants in solution produced a Gibbs surface excess (Richer and Lipkowski, 1986; Wu et al., 1992) at the electrode, and therefore affected the electrowinning efficiency. The surface excess of surfactants adsorbed at the cathode decreases the effective cathode area, causing an effective current density higher than its limiting current density; green nickel hydroxides are formed at the cathode surface. A h m e d and Sedahmed (1989) found that the rate of mass transfer of solution decreased in the presence of surfactants. As the surfactant has a longer molecule length than water it decreases the mass transfer coefficient of solution at the cathode-solution interface and hampers free migration of nickel ions into the cathode surface, resulting in a decreased rate of electroreduction of nickel ion at the cathode. For this reason, the presence of surfactants in wastewater is inevitably deleterious to both the limiting current density and cathode current efficiency (CE). Lower limiting current density and lower C E means a lower electrowinning efficiency. Different surfactan~Ls have different surface activities and molecular chainlengths; therefore the elec-
trowinning efficiency depends to a great extent upon the nature of the surfactants. To increase the electrowinning efficiency, an improved understanding of the effect of surfactants is required. Thus the present study was undertaken to investigate the effect of various surfactants on the recovery of nickel from nickel plating wastewater by electrowinning. Besides increasing the flow rate, the addition of boric acid was found of great benefit to decrease the effect of surfactants in the solution. Therefore the other main object of the present study was to assess the effect of boric acid on electrowiinning efficiency. To correlate these effects of surfactants and boric acid, the amount of hydrogen evolved at the cathode was examined.
EXPERIMENTAL Electrowinning was conducted in 8 x 8 x 20 cm rectangular cell that was filled with I litre of studied solution. Nickel (5/am thick with 3 mmmesh) was prepared as cathode, which was mounted on a plastic (PMMA) frame, and was arranged in parallel with platinum anodes. The platinum anodes had 2 mm mesh. The cathode area was 28 cm 2. These flow-through electrodes increased the effective plating area. Wastewater was continuously recirculated in a closed-loop flow circuit through the cell for 1000 coulombs electrowinning at varied current density by a peristaltic pump. The maximum flow rate of this pump was 2.2 1min- t. From the weight of the cathodic deposit CE was calculated (Afifi et al., 1991). The term "1000 Ni" was used to refer the virgin nickel wastewater that was nickel solution at 1000mg 1-I. Nickel sulfate was used to prepare synthetic wastewater. Surfactants investigated were sodium lauryl sulfate (SLS), sodium dodecylbenzene sulfate (SDS), octylphenol-derived polyoxyethylene containing about 10 ethoxy groups (OPI0), and a series of nonylphenol-derived polyoxyethylene containing about 6, 9, 20, and 25 ethoxy groups, designated
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Chein-Ho Huang
1822
NP6, NP9, NP20, and NP25, respectively. An increased number of ethoxy groups means a longer chain-length of the nonionic surfactant. All electrowinning was conducted at 25°C. To determine the amount of hydrogen evolved during electrowinning,a 10 x 19 x 15 cm rectangular cell was filled with 2 litres of solution and a current of 0.02 A was conducted for 100 C with a l0 cm2 cathode. The hydrogen that evolved from the cathode was collected in a graduated cylinder and the volume determined. The surface morphology of deposited nickel was examined by a scanning electron microscope (SEM).
The effect of surfactants As shown in Table 1, when the operated current density was less than the limiting current density, CE progressively increased by increasing both the cathode current density (CD) and the flow rate. Higher flow rate of solution greatly improved the mass transfer of nickel ion into the cathode surface and enabled nickel to be removed efficiently from the solution. Therefore, when the flow rate of 1000 Ni in the 1 litre cell was 0.7921min-~ the limiting current density was below 20 A m 2, whereas when the flow rate increased to 2.21min -1, the limiting current density was increased to more than 45 A m -2. To increase the CE and the limiting current density, the flow rate 2.21min ~ was selected in this work. Anionic and/or nonionic surfactants are always present in nickel plating and cleaning baths. Besides nickel ion, the rinsewater and wastewater from the nickel plating process inevitably contain these surfactants. Surfactants exhibit greater surface adsorption at the electrodes, resulting in the consequent blockage of growth sites on the cathode. This effect decreases the effective electrowinning area, causing a decrease in the double-layer capacitance and nuclei formation requiring higher energy. When the applied voltage exceeds the hydrogen overvoltage, hydrogen is readily evolved from the cathode and the amount of electroreduced nickel decreases. Concurrently, the pH at the cathode-solution interface also increases and then nickel ion reacts with hydroxyl ion to form green nickel hydroxides at the cathode. Therefore, the greater surface adsorption of surfactants results in both lower CE and limiting current density. Furthermore, a longer molecule-chain of the absorbed surfactant molecule increases the interfacial viscosity near the cathode, resulting in a decreased rate of Table 1. 1000 coulombs electrowinning of 1000Ni in a 1 1 cell
( A m -2) 20 25 30 35 40 45 50
CE(%) 0.7921min -I
l . l l m i n -t
1.911min t
G* G G G G G G
30.8 31.9 G G G G G
31.3 32.3 33.4 35.7 G G G
*G, green deposits at the cathode.
CE(%): [0 g I I boric acid] CE(%): [0.5 g I - i boric acid] Surfactant
_ SDS SLS NP6 NP9 OP10 NP20I NP25t
2 0 A m -2 31.9 28.2 28.5 23.0 21.6 G G G
45Am 47.2 G* G G G G G G
2
20Am 30.7 44.3 25. I 20.8 20.1 27.8 25.9 25.3
2
45Am
2
45.4 57.1 54.6 42.9 40.7 40.5 G G
*G, green deposits at the cathode. t T h e CE of 1.0 g l ~ boric acid in NP20- and NP25-containing solutions at 45 A m 2 were 43.3 and 40.3% respectively.
RESULTS AND DISCUSSION
CD
Table 2. 1000 coulombs electrowinning of 1000Ni containing 100 m g l - ~ surfactant in a I litre cell with 2.2 1 min ~ flow rate
2.21min 31.9 32.7 37.5 40.8 44.3 47.2 G
electroreduction of nickel ion at the cathode. As a result, the adsorbed surfactant produced a lower limiting current density and CE. In this work, both the surface activity and chain-length of the surfactant molecule were the studied surfactant effects. The results of preliminary Hull cell test indicated that the limiting current density of surfactantcontaining 1000 Ni was about 20 A m -2. As shown in Table 1, the limiting current density of surfactant-free 1000 Ni was about 45 A m -2. Therefore, these two limiting current densities, 20 and 45 A m -2, were selected to study the surfactant effect. As shown in Table 2, both the limiting current density and CE were decreased by the presence of these surfactants in the 1000 Ni. All these surfactant-containing solutions had limiting current densities < 45 A m -2, and some were even < 20 A m -2. Because of their mutual negative charges, both anionic SDS and SLS were likely repelled from the cathode, but due to some surface adsorption these two surfactant-containing solutions still had CE smaller than that of surfactant-free solution at 20 A m -2. There was also no electrostatic repulsion between polar groups of nonionic surfactant molecules and the cathode, these adsorbed nonionic surfactants also had longer chain-length than those of anionic surfactants; therefore, these studied nonionic-surfactant-containing-solutionshad lower CE and limiting current densities than those of anionic surfactants. Due to the longer chain-length (greater number of moles of ethylene oxide per mole of surfactant), both NP20- and NP25-containing solutions had limiting current densitites < 20 A m -2. Although OPI0 had smaller chain-length than those of NP20 and NP25, with a higher surface activity (Osipow, 1972), it resulted in its limiting current density still being < 20 A m -2. These phenomena were also observed when electrowinning was conducted at a higher CD. After 1000 C electrowinning at 45 A m -2, due to their lower limiting current density, the cathodes in OP10-, NP20- and NP25-containing solutions had dark green nickel hydroxides. In contrast, due to the decreased surfactant effect, the other four surfactant-containing solutions produced only slight or partial green nickel hydroxide deposits at the
Electrowinning of nickel plating wastewater Table 3. The evolvedhydrogenafter 100C electrowinningin a 2 litre cell Hydrogen gas (ml) Surfactant -
-
SDS SLS NP6 NP9 OP10 NP20 NP25
0g1-1 boric acid 0.48 2.43 0.71 2.30 2.37 2.70 2.79 3.26
0.5gl -I boricacid 1.12 0.53 0.50 0.39 0.91 1.06 0.55 0.75
cathode (Corrigan and Bendert, 1989). And green nickel hydroxides deposited at the cathode exhibited electrowinning failure. As a result, it was difficult in a normal process to reach a practical electrowinning efficiency when the wastewater contained these surfactants. Apart from the elec~Lroreduction of nickel ion and surfactant during electrowinning, most other cathodic currents may be used to reduce the hydronium ion at the cathode. Thus the amount of hydrogen evolution may play an imporant role to be correlated
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with the CE of these surfactant-containing solutions (Chen and Lasia, 1992). If the CE is maintained constant, due to possible electroreduction of surfactant in surfactant-containing solutions, then the remaining current for hydrogen evolution becaomes decreased. Therefore, the amount of evolved hydrogen of a surfactant-containing solution should be less than that of a surfactant-free solution. In contrast, as shown in Table 3, the surfactant-containing baths had greater evolved hydrogen. A possible explanation of these results is that the CE values were decreased by the presence of surfactant, resulting in more remaining cathodic current for hydrogen evolution. As shown in Table 3, solutions containing nonionic surfactants with increased surfactant effect had generally greater hydrogen evolved at the cathode. As a result, the effect of surfactants correlated well with the amount of evolved hydrogen. The results of Table 2 are also explained by the surface morphology of nickel deposits. Typical SEM photographs are shown in Fig. 1. These 1000 Ni and SDS- and SLS-containing solutions had an increased rate of nickel deposition and then produced irregu-
Fig. 1. SEM photograph of nickel deposits obtained in the following solutions containing no boric acid at 20Am-2: (1) IL000Ni; (2) 100mg1-1 SDS in 1000Ni; (3) 100mgl -t SLS in 1000Ni; (4) 100mgl -t NP9 in 1000Ni.
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Chein-Ho Huang Table 4. pH values* of 1000Ni in 1 litre cell
Surfactant ( 1 0 0 m g l s)
Boric acid (gl t)
CD ( A m ~)
pH 0
pHt0o
pHi0~
----SDS or SLS NP6 or NP9 OPI0 NP20 NP25
-0. I 0.5 |.0 0.5 0.5 0.5 0.5 0.5
45 45 45 45 45 45 45 20 20
5.7 5.6 5.5 5.3 5.6 5.6 5.6 5.6 5.6
3.5 3.5 3.5 3.4 3.5 3.5 3.5 3.5 3.5
2.7 2.7 2.7 2.7 2.6 2.7 2.7 2.8 2.8
*pH 0 was the pH before electrowinning, pHt00 and pH ~co0were after 100 an 1000 C electrowinning respectively.
larly shaped and coarse grain nickel deposits, whereas due to a smaller rate or electroreduction of nickel ion, the NP9-containing solution had better diffusion control of nickel ion resulting in a more fine and homogeneous grain nickel deposit.
The effect of boric acid As boric acid is an important component of a typical nickel plating bath it may be present in rinsewater and wastewater. It is also relatively inexpensive, nonvolatile and no additional problem for effluent regulation. Therefore the effect of presence of boric acid is valuable to study. The CE depended strongly on the pH value of solution; e.g. the CE was only 15% when the initial pH of 1000 Ni was decreased to 2.5 by the addition of sulfuric acid. A smaller pH resulted in lower CE, whereas as shown in Table 4 the presence of boric acid decreased only a little pH of studied solutions. Therefore, as shown in Table 5, boric acid decreased the CE at 40 and 45 A m -2, but in its presence the limiting current density of 1000 Ni increased from 45 to 80 A m -2 when the CE also increased from 45.4 to 56.5 %. As a result, the presence of boric acid in nickel wastewater provided a means of increasing the limiting current density and CE, so that a greater rate of recovery of nickel was obtained. When the added concentration of boric acid was increased from 0.5 to 1.0gl -~, the limiting current density was increased only from 80 to 9 0 A m -2, thus 0.5 g l -~ should suffice as an added concentration in succeeding experiments. After 1000 C electrowinning, as shown in Table 4, although the pH of 1000 Ni decreased from 5.7 to 2.7, the pH at the cathode-solution interface invariably Table 5. 1000 coulombs electrowinning of 1000 Ni in a 1 litre cell at 2.2 1 min CE% CD ( A m -2) 40 45 50 60 70 80 90
0 g l -I boric acid
0 . 5 g l -I boric acid
44.3 47.2 G* G G G G
36.2 45.4 49.7 51.1 54.2 56.5 G
*G, green deposits at the cathode.
exceeded that of the bulk solution. With its buffering ability, boric acid decreased the rate of increase of pH at the cathode-solution interface to avoid the tendency of formation of green nickel hydroxides at the cathode, resulting in a higher limiting current density. Concurrently, boric acid decreased slightly the pH of the bulk solution and resulted in only slight loss of its CE. As a result, as shown in Table 5, a higher CE along with a higher limiting current density were obtained when the boric acid buffer was present in the 1000 Ni. Furthermore, as shown in Table 2, except NP20 and NP25, the limiting current densities of these surfactant-containing solutions were also increased to more than 45 A m -2 by the addition of 0.5 g 1-1 boric acid. Because of the greater surfactant effect, more boric acid was needed to increase the limiting current densities of NP20- and NP25-containing solutions to 45 A m -2. Greater hydrogen is produced in a solution of smaller pH. For example, when the initial pH of 1000Ni was decreased to 2.5 by the addition of sulfuric acid, the evolved hydrogen increased from 0.48 to 0.87 ml. Although the pH of 1000 Ni after addition of 0.5 g 1-1 boric acid decreased to only 5.5 (Table 4), due to greater acidity from the added boric acid, the amount of evolved hydrogen increased to 1.12 ml. Consequently, the amount of evolved hydrogen was increased either by the presence of boric acid or surfactant in 1000Ni. Whereas, the results of Table 3 reveal a significantly decreased amount of evolved hydrogen in the presence of both boric acid and surfactant in 1000Ni. Hence the presence of boric acid decreased the amount of evolved hydrogen in the surfactant-containing solutions, resulting in greater recovery of nickel and thus higher CE. So, there is also a clear correlation between the amount of evolved hydrogen and the effect of boric acid on surfactant-containing solutions. Perhaps because of improved diffusion control of nickel ion, as shown in Fig. 2, the results of typical SEM analyses showed that smoother nickel deposits were obtained from boric acid- and surfactant-containing solutions except that of SDS. With an increased rate of nickel deposition, the SDS-containing solution produced an irregularly shaped and coarse grain nickel deposit even in the presence of boric acid. As shown in Table 5, although boric acid improves the limiting current density of 1000 Ni, the CE was decreased by the increased acidity of boric acid at 40 and 45 A m -z. Whereas, as shown in Table 2, besides the increasing of limiting current density, the CE of SDS-containing solutions and SLS-containing solution (45 A m -2) were higher than that of surfactantfree solutions by the presence of boric acid. Furthermore, although the amount of evolved hydrogen was increased by the presence of boric acid in surfactant-free solution, the results of Table 3 also indicated that the amounts of evolved hydrogen in the surfactant-eontaining solutions were decreased by
Electrowinning of nickel plating wastewater
1825
Fig. 2. SEM photograph of nickel deposits obtained in the following solutions containing 0.5 g 1- ~boric acid at 20A nl-2: (1) 100mgl -l SDS in 1000Ni; (2) 100mgl -t SLS in 1000 Ni; (3) 100mgl -a NP9 in 1000Ni; (4) 100mgl -j OP10 and 1000Ni.
the presence of boric acid. Therefore, to delineate the effect of boric acid in the surfactant-containing solutions, besides the inherent buffering effect, the orientational and the conformational variation of adsorbed polar group of surfactant molecule may be important. Owing to lhe considerable flexibility (Kar et al., 1973; Tan and Martic, 1990) about the nature of the adsorbed film at the cathode--solution interface, the orientational and conformational state of adsorbed surfactants should be affected by the pH (Osipow, 1972) at tlle cathode-solution interface. Boric acid provided a buffered concentration of hydronium ion to associate with the adsorbed polar end of surfactant molecule, causing the adsorbed surfactant into an ori,~ntational and conformational state for improved electroreduction of nickel ions and decreased electroreduction of hydronium ions. Thus the decreasing of the efl'ective electrode area and mass transfer coefficient by the presence of surfactants was also moderated. Consequently, as shown in Table 2, the electrowinning ef~ciencies of these surfactantcontaining solutions were increased by the presence of boric acid. For a better understanding of the effect
of boric acid, further work will be conducted in greater detail on this point.
The practical operation When the 1000Ni was changed to 750 or 500 mg 1-~ nickel, after 1000 C electrowinning at 45 A m -E, the CE was decreased from 47.2 to 38.6 or 31.7 respectively in a l litre cell. Since greater solution had greater maintenance of nickel concentration and it could operate at a higher CE. For example, after 1000 C electrowinning and at 45 A m -2, the deposition of nickel from boric acid-containing 1000 Ni occurred in a l0 litre cell at CE 87. As the amount of wastewater of plating shops are always > l0 litres; therefore, with higher flow rate and boric acid, a practical electrowinning efficiency could be obtained in the operation of plating shops. As the nickel concentrations of wastewater are >/1000rag 1-1 in some plating shops in Taiwan, these studied results are useful in the treatment of mixed wastewaters from nickel plating and cleaning baths. In addition, the nickel plating bath contains a lot of organic pollutants (Huang, 1992), surfactants
Chein-Ho Huang
1826
Table 6. 1000C electrowinningof 1000mgl ~Ni in a I litre cell at 2.21min * and 45Am z Nickel source Boric acid (gl i) CE (%) Nickel chloride 0 45.7 Nickel chloride 0.5 41.2 Nickel sulfamate 0 46.1 Nickel sulfamate 0.5 43.9
and brighteners. Therefore, these studied results are also useful in the first rinse tank to maintain the level of nickel concentration at a small concentration so that the supply of fresh water needed is decreased, with significant cost saving for nickel recovery and exceptional water saving of rinsewater. Besides nickel sulfate, the principal source of nickel ion in nickel plating solutions, Table 6 indicated that the CE of both nickel chloride and nickel suifamate solutions were enough to be conducted in a normal electrowinning process. Therefore, this electrowinning process can treat the wastewaters of either Watts or conventional sulfamate baths. Futhermore, owing to the components of both rack-used still plating bath and the barrel-used rotational plating bath are almost same; therefore, the wastewaters from these two plating equipments could be treated by this electrowinning process. An actual wastewater was treated in this work. With the addition of 0.5 g 1- ' boric acid and 1000 C electrowinning at 45 A m -2, 10 litres of wastewater from a local plating shop (initial pH 5.2 and 856mg1-1 nickel) had CE 72. The benefit of using nickel mesh cathode was that the loaded cathodes could be directly reused as anode material in a nickel plating cell; anode stripping was not needed. CONCLUSIONS Because of its buffering capacity, boric acid improved the electrowinning efficiency in the surfactantfree nickel wastewater. Following the decrease of the effective electrode area and mass transfer coefficient, the electrowinning
efficiency was decreased by the presence of surfacrants in nickel wastewater. Besides the buffering ability, perhaps because of altered orientation and conformation of adsorbed surfactant molecules, better electrowinnning efficiencies in the surfactantcontaining nickel wastewater were obtained by the presence of boric acid. The drawback of the decrease of electrowinning efficiency by the presence of surfactants was overcome by the presence of boric acid in nickel wastewater. As a result, when the treated solution contains a greater concentration of nickel, either in nickel wastewater streams or rinsewater, this treatment was economical and advantageous. REFERENCES
Afifi S., Hegazy M. and Donya K. (1991) On the electrowinning of zinc from alkaline zincate solutions J. Electrochem Soc. 138, 1929-1933. Ahmed A. M. and Sedahmed G. H. (1989) Effect of surfactants on the rate of mass transfer at gas-evolving electrodes. J. appl. Electrochem. 19, 219-224. Chen L. and Lasia A. (1992) Influence of the adsorption of organic compounds on the kinetics of the hydrogen evolution on Ni and Ni-Zn alloy electrodes. J. Electrochem. Soc. 139, 1058-1064. Corrigan D. A. and Bendert R. M. (1989) Effects of coprecipitated metal ions on the electrochemistry of nickel hydroxide thin films: cyclic voltammetry in 1 M KOH. J. Electrochem. Soc. 136, 723-728. Huang C. H. (1992) Effective removal of organics from nickel wastewater by modified carbon adsorption. Plating Sur/ace Fin. 79 (4), 50-56. Kar G., Healy T. W. and Fuerstenau D. W. (1973) Effects of alkyl amine surfactants on the corrosion of a rotating cooper cylinder. Corrosion Sci. 13, 375-385. Osipow L. I. (1972) Surface Chemistry, pp. 114-115, 216-221. Krieger, New York. Richer J. and Lipkowski J. (1986) Measurement of physical adsorption of neutral organic species at solid electrodes. J. Electrochem. Soc. 133, 121 128. Tan J. S. and Martic P. A. (1990) Protein adsorption and conformational change on small polymer particles. J. Colloid Interface Sci. 136, 415-431. Wu H. M., Lay M. L. and Huang C. H. (1992) Effects of surfactants on cyclic voltammetric stripping analysis of acid copper sulfate plating baths. Plating Surface Fin. 79 (9), 66-68.
Pergamon
0043-1354(95)00008-9
Wat. Res. Vol. 29, No. 8, pp. 1821-1826, 1995 Copyright ~) 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0043-1354/95 $9.50 + 0.00
E F F E C T OF SURFACTANTS ON RECOVERY OF NICKEL F R O M NICKEL PLATING WASTEWATER BY ELECTROWlNNING CHEIN-HO HUANG Chemistry Department, Soochow University, Shih-Lin, Taipei, Taiwan, R.O.C. (First received November 1993; accepted in revised form January 1995) Abstract--The effect of surfactants on the electrowinning efficiency of the recovery of nickel from nickel plating wastewater was studied. Owing to the decrease of the effective electrode area and mass transfer coefficient of solution, the electrowinning efficiency was decreased by the presence of surfactants in wastewater. The higher the surface activity and the longer chain-length of the surfactant, the lower the electrowinning efficiency. This drawback could be eliminated by the addition of boric acid and with a greater rate of flow of solution. An attempt had been made to delineate the effect of boric acid on electrowinning efficiency. Key words--boric acid, electrowinning, nickel plating wastewater, surfactant
INTRODUCTION Besides the restoration of dissolved nickel to metallic form and re-incorporate it into the cycle of material, electrowinning is a practical system to diminish spent concentrate or to reduce rinse-water requirements through closed loop operation. As surfactants (anionic and/or nonionic types) are added intentionally to nickelplating and cleaning baths to improve the quality of the plating surface, the treated wastewater inevitably contains t]hese surfactants. F r o m the resuits of cyclic voltammograms in our ealier work we showed that the presence of surfactants in solution produced a Gibbs surface excess (Richer and Lipkowski, 1986; Wu et al., 1992) at the electrode, and therefore affected the electrowinning efficiency. The surface excess of surfactants adsorbed at the cathode decreases the effective cathode area, causing an effective current density higher than its limiting current density; green nickel hydroxides are formed at the cathode surface. A h m e d and Sedahmed (1989) found that the rate of mass transfer of solution decreased in the presence of surfactants. As the surfactant has a longer molecule length than water it decreases the mass transfer coefficient of solution at the cathode-solution interface and hampers free migration of nickel ions into the cathode surface, resulting in a decreased rate of electroreduction of nickel ion at the cathode. For this reason, the presence of surfactants in wastewater is inevitably deleterious to both the limiting current density and cathode current efficiency (CE). Lower limiting current density and lower C E means a lower electrowinning efficiency. Different surfactan~Ls have different surface activities and molecular chainlengths; therefore the elec-
trowinning efficiency depends to a great extent upon the nature of the surfactants. To increase the electrowinning efficiency, an improved understanding of the effect of surfactants is required. Thus the present study was undertaken to investigate the effect of various surfactants on the recovery of nickel from nickel plating wastewater by electrowinning. Besides increasing the flow rate, the addition of boric acid was found of great benefit to decrease the effect of surfactants in the solution. Therefore the other main object of the present study was to assess the effect of boric acid on electrowiinning efficiency. To correlate these effects of surfactants and boric acid, the amount of hydrogen evolved at the cathode was examined.
EXPERIMENTAL Electrowinning was conducted in 8 x 8 x 20 cm rectangular cell that was filled with I litre of studied solution. Nickel (5/am thick with 3 mmmesh) was prepared as cathode, which was mounted on a plastic (PMMA) frame, and was arranged in parallel with platinum anodes. The platinum anodes had 2 mm mesh. The cathode area was 28 cm 2. These flow-through electrodes increased the effective plating area. Wastewater was continuously recirculated in a closed-loop flow circuit through the cell for 1000 coulombs electrowinning at varied current density by a peristaltic pump. The maximum flow rate of this pump was 2.2 1min- t. From the weight of the cathodic deposit CE was calculated (Afifi et al., 1991). The term "1000 Ni" was used to refer the virgin nickel wastewater that was nickel solution at 1000mg 1-I. Nickel sulfate was used to prepare synthetic wastewater. Surfactants investigated were sodium lauryl sulfate (SLS), sodium dodecylbenzene sulfate (SDS), octylphenol-derived polyoxyethylene containing about 10 ethoxy groups (OPI0), and a series of nonylphenol-derived polyoxyethylene containing about 6, 9, 20, and 25 ethoxy groups, designated
1821
Chein-Ho Huang
1822
NP6, NP9, NP20, and NP25, respectively. An increased number of ethoxy groups means a longer chain-length of the nonionic surfactant. All electrowinning was conducted at 25°C. To determine the amount of hydrogen evolved during electrowinning,a 10 x 19 x 15 cm rectangular cell was filled with 2 litres of solution and a current of 0.02 A was conducted for 100 C with a l0 cm2 cathode. The hydrogen that evolved from the cathode was collected in a graduated cylinder and the volume determined. The surface morphology of deposited nickel was examined by a scanning electron microscope (SEM).
The effect of surfactants As shown in Table 1, when the operated current density was less than the limiting current density, CE progressively increased by increasing both the cathode current density (CD) and the flow rate. Higher flow rate of solution greatly improved the mass transfer of nickel ion into the cathode surface and enabled nickel to be removed efficiently from the solution. Therefore, when the flow rate of 1000 Ni in the 1 litre cell was 0.7921min-~ the limiting current density was below 20 A m 2, whereas when the flow rate increased to 2.21min -1, the limiting current density was increased to more than 45 A m -2. To increase the CE and the limiting current density, the flow rate 2.21min ~ was selected in this work. Anionic and/or nonionic surfactants are always present in nickel plating and cleaning baths. Besides nickel ion, the rinsewater and wastewater from the nickel plating process inevitably contain these surfactants. Surfactants exhibit greater surface adsorption at the electrodes, resulting in the consequent blockage of growth sites on the cathode. This effect decreases the effective electrowinning area, causing a decrease in the double-layer capacitance and nuclei formation requiring higher energy. When the applied voltage exceeds the hydrogen overvoltage, hydrogen is readily evolved from the cathode and the amount of electroreduced nickel decreases. Concurrently, the pH at the cathode-solution interface also increases and then nickel ion reacts with hydroxyl ion to form green nickel hydroxides at the cathode. Therefore, the greater surface adsorption of surfactants results in both lower CE and limiting current density. Furthermore, a longer molecule-chain of the absorbed surfactant molecule increases the interfacial viscosity near the cathode, resulting in a decreased rate of Table 1. 1000 coulombs electrowinning of 1000Ni in a 1 1 cell
( A m -2) 20 25 30 35 40 45 50
CE(%) 0.7921min -I
l . l l m i n -t
1.911min t
G* G G G G G G
30.8 31.9 G G G G G
31.3 32.3 33.4 35.7 G G G
*G, green deposits at the cathode.
CE(%): [0 g I I boric acid] CE(%): [0.5 g I - i boric acid] Surfactant
_ SDS SLS NP6 NP9 OP10 NP20I NP25t
2 0 A m -2 31.9 28.2 28.5 23.0 21.6 G G G
45Am 47.2 G* G G G G G G
2
20Am 30.7 44.3 25. I 20.8 20.1 27.8 25.9 25.3
2
45Am
2
45.4 57.1 54.6 42.9 40.7 40.5 G G
*G, green deposits at the cathode. t T h e CE of 1.0 g l ~ boric acid in NP20- and NP25-containing solutions at 45 A m 2 were 43.3 and 40.3% respectively.
RESULTS AND DISCUSSION
CD
Table 2. 1000 coulombs electrowinning of 1000Ni containing 100 m g l - ~ surfactant in a I litre cell with 2.2 1 min ~ flow rate
2.21min 31.9 32.7 37.5 40.8 44.3 47.2 G
electroreduction of nickel ion at the cathode. As a result, the adsorbed surfactant produced a lower limiting current density and CE. In this work, both the surface activity and chain-length of the surfactant molecule were the studied surfactant effects. The results of preliminary Hull cell test indicated that the limiting current density of surfactantcontaining 1000 Ni was about 20 A m -2. As shown in Table 1, the limiting current density of surfactant-free 1000 Ni was about 45 A m -2. Therefore, these two limiting current densities, 20 and 45 A m -2, were selected to study the surfactant effect. As shown in Table 2, both the limiting current density and CE were decreased by the presence of these surfactants in the 1000 Ni. All these surfactant-containing solutions had limiting current densities < 45 A m -2, and some were even < 20 A m -2. Because of their mutual negative charges, both anionic SDS and SLS were likely repelled from the cathode, but due to some surface adsorption these two surfactant-containing solutions still had CE smaller than that of surfactant-free solution at 20 A m -2. There was also no electrostatic repulsion between polar groups of nonionic surfactant molecules and the cathode, these adsorbed nonionic surfactants also had longer chain-length than those of anionic surfactants; therefore, these studied nonionic-surfactant-containing-solutionshad lower CE and limiting current densities than those of anionic surfactants. Due to the longer chain-length (greater number of moles of ethylene oxide per mole of surfactant), both NP20- and NP25-containing solutions had limiting current densitites < 20 A m -2. Although OPI0 had smaller chain-length than those of NP20 and NP25, with a higher surface activity (Osipow, 1972), it resulted in its limiting current density still being < 20 A m -2. These phenomena were also observed when electrowinning was conducted at a higher CD. After 1000 C electrowinning at 45 A m -2, due to their lower limiting current density, the cathodes in OP10-, NP20- and NP25-containing solutions had dark green nickel hydroxides. In contrast, due to the decreased surfactant effect, the other four surfactant-containing solutions produced only slight or partial green nickel hydroxide deposits at the
Electrowinning of nickel plating wastewater Table 3. The evolvedhydrogenafter 100C electrowinningin a 2 litre cell Hydrogen gas (ml) Surfactant -
-
SDS SLS NP6 NP9 OP10 NP20 NP25
0g1-1 boric acid 0.48 2.43 0.71 2.30 2.37 2.70 2.79 3.26
0.5gl -I boricacid 1.12 0.53 0.50 0.39 0.91 1.06 0.55 0.75
cathode (Corrigan and Bendert, 1989). And green nickel hydroxides deposited at the cathode exhibited electrowinning failure. As a result, it was difficult in a normal process to reach a practical electrowinning efficiency when the wastewater contained these surfactants. Apart from the elec~Lroreduction of nickel ion and surfactant during electrowinning, most other cathodic currents may be used to reduce the hydronium ion at the cathode. Thus the amount of hydrogen evolution may play an imporant role to be correlated
1823
with the CE of these surfactant-containing solutions (Chen and Lasia, 1992). If the CE is maintained constant, due to possible electroreduction of surfactant in surfactant-containing solutions, then the remaining current for hydrogen evolution becaomes decreased. Therefore, the amount of evolved hydrogen of a surfactant-containing solution should be less than that of a surfactant-free solution. In contrast, as shown in Table 3, the surfactant-containing baths had greater evolved hydrogen. A possible explanation of these results is that the CE values were decreased by the presence of surfactant, resulting in more remaining cathodic current for hydrogen evolution. As shown in Table 3, solutions containing nonionic surfactants with increased surfactant effect had generally greater hydrogen evolved at the cathode. As a result, the effect of surfactants correlated well with the amount of evolved hydrogen. The results of Table 2 are also explained by the surface morphology of nickel deposits. Typical SEM photographs are shown in Fig. 1. These 1000 Ni and SDS- and SLS-containing solutions had an increased rate of nickel deposition and then produced irregu-
Fig. 1. SEM photograph of nickel deposits obtained in the following solutions containing no boric acid at 20Am-2: (1) IL000Ni; (2) 100mg1-1 SDS in 1000Ni; (3) 100mgl -t SLS in 1000Ni; (4) 100mgl -t NP9 in 1000Ni.
1824
Chein-Ho Huang Table 4. pH values* of 1000Ni in 1 litre cell
Surfactant ( 1 0 0 m g l s)
Boric acid (gl t)
CD ( A m ~)
pH 0
pHt0o
pHi0~
----SDS or SLS NP6 or NP9 OPI0 NP20 NP25
-0. I 0.5 |.0 0.5 0.5 0.5 0.5 0.5
45 45 45 45 45 45 45 20 20
5.7 5.6 5.5 5.3 5.6 5.6 5.6 5.6 5.6
3.5 3.5 3.5 3.4 3.5 3.5 3.5 3.5 3.5
2.7 2.7 2.7 2.7 2.6 2.7 2.7 2.8 2.8
*pH 0 was the pH before electrowinning, pHt00 and pH ~co0were after 100 an 1000 C electrowinning respectively.
larly shaped and coarse grain nickel deposits, whereas due to a smaller rate or electroreduction of nickel ion, the NP9-containing solution had better diffusion control of nickel ion resulting in a more fine and homogeneous grain nickel deposit.
The effect of boric acid As boric acid is an important component of a typical nickel plating bath it may be present in rinsewater and wastewater. It is also relatively inexpensive, nonvolatile and no additional problem for effluent regulation. Therefore the effect of presence of boric acid is valuable to study. The CE depended strongly on the pH value of solution; e.g. the CE was only 15% when the initial pH of 1000 Ni was decreased to 2.5 by the addition of sulfuric acid. A smaller pH resulted in lower CE, whereas as shown in Table 4 the presence of boric acid decreased only a little pH of studied solutions. Therefore, as shown in Table 5, boric acid decreased the CE at 40 and 45 A m -2, but in its presence the limiting current density of 1000 Ni increased from 45 to 80 A m -2 when the CE also increased from 45.4 to 56.5 %. As a result, the presence of boric acid in nickel wastewater provided a means of increasing the limiting current density and CE, so that a greater rate of recovery of nickel was obtained. When the added concentration of boric acid was increased from 0.5 to 1.0gl -~, the limiting current density was increased only from 80 to 9 0 A m -2, thus 0.5 g l -~ should suffice as an added concentration in succeeding experiments. After 1000 C electrowinning, as shown in Table 4, although the pH of 1000 Ni decreased from 5.7 to 2.7, the pH at the cathode-solution interface invariably Table 5. 1000 coulombs electrowinning of 1000 Ni in a 1 litre cell at 2.2 1 min CE% CD ( A m -2) 40 45 50 60 70 80 90
0 g l -I boric acid
0 . 5 g l -I boric acid
44.3 47.2 G* G G G G
36.2 45.4 49.7 51.1 54.2 56.5 G
*G, green deposits at the cathode.
exceeded that of the bulk solution. With its buffering ability, boric acid decreased the rate of increase of pH at the cathode-solution interface to avoid the tendency of formation of green nickel hydroxides at the cathode, resulting in a higher limiting current density. Concurrently, boric acid decreased slightly the pH of the bulk solution and resulted in only slight loss of its CE. As a result, as shown in Table 5, a higher CE along with a higher limiting current density were obtained when the boric acid buffer was present in the 1000 Ni. Furthermore, as shown in Table 2, except NP20 and NP25, the limiting current densities of these surfactant-containing solutions were also increased to more than 45 A m -2 by the addition of 0.5 g 1-1 boric acid. Because of the greater surfactant effect, more boric acid was needed to increase the limiting current densities of NP20- and NP25-containing solutions to 45 A m -2. Greater hydrogen is produced in a solution of smaller pH. For example, when the initial pH of 1000Ni was decreased to 2.5 by the addition of sulfuric acid, the evolved hydrogen increased from 0.48 to 0.87 ml. Although the pH of 1000 Ni after addition of 0.5 g 1-1 boric acid decreased to only 5.5 (Table 4), due to greater acidity from the added boric acid, the amount of evolved hydrogen increased to 1.12 ml. Consequently, the amount of evolved hydrogen was increased either by the presence of boric acid or surfactant in 1000Ni. Whereas, the results of Table 3 reveal a significantly decreased amount of evolved hydrogen in the presence of both boric acid and surfactant in 1000Ni. Hence the presence of boric acid decreased the amount of evolved hydrogen in the surfactant-containing solutions, resulting in greater recovery of nickel and thus higher CE. So, there is also a clear correlation between the amount of evolved hydrogen and the effect of boric acid on surfactant-containing solutions. Perhaps because of improved diffusion control of nickel ion, as shown in Fig. 2, the results of typical SEM analyses showed that smoother nickel deposits were obtained from boric acid- and surfactant-containing solutions except that of SDS. With an increased rate of nickel deposition, the SDS-containing solution produced an irregularly shaped and coarse grain nickel deposit even in the presence of boric acid. As shown in Table 5, although boric acid improves the limiting current density of 1000 Ni, the CE was decreased by the increased acidity of boric acid at 40 and 45 A m -z. Whereas, as shown in Table 2, besides the increasing of limiting current density, the CE of SDS-containing solutions and SLS-containing solution (45 A m -2) were higher than that of surfactantfree solutions by the presence of boric acid. Furthermore, although the amount of evolved hydrogen was increased by the presence of boric acid in surfactant-free solution, the results of Table 3 also indicated that the amounts of evolved hydrogen in the surfactant-eontaining solutions were decreased by
Electrowinning of nickel plating wastewater
1825
Fig. 2. SEM photograph of nickel deposits obtained in the following solutions containing 0.5 g 1- ~boric acid at 20A nl-2: (1) 100mgl -l SDS in 1000Ni; (2) 100mgl -t SLS in 1000 Ni; (3) 100mgl -a NP9 in 1000Ni; (4) 100mgl -j OP10 and 1000Ni.
the presence of boric acid. Therefore, to delineate the effect of boric acid in the surfactant-containing solutions, besides the inherent buffering effect, the orientational and the conformational variation of adsorbed polar group of surfactant molecule may be important. Owing to lhe considerable flexibility (Kar et al., 1973; Tan and Martic, 1990) about the nature of the adsorbed film at the cathode--solution interface, the orientational and conformational state of adsorbed surfactants should be affected by the pH (Osipow, 1972) at tlle cathode-solution interface. Boric acid provided a buffered concentration of hydronium ion to associate with the adsorbed polar end of surfactant molecule, causing the adsorbed surfactant into an ori,~ntational and conformational state for improved electroreduction of nickel ions and decreased electroreduction of hydronium ions. Thus the decreasing of the efl'ective electrode area and mass transfer coefficient by the presence of surfactants was also moderated. Consequently, as shown in Table 2, the electrowinning ef~ciencies of these surfactantcontaining solutions were increased by the presence of boric acid. For a better understanding of the effect
of boric acid, further work will be conducted in greater detail on this point.
The practical operation When the 1000Ni was changed to 750 or 500 mg 1-~ nickel, after 1000 C electrowinning at 45 A m -E, the CE was decreased from 47.2 to 38.6 or 31.7 respectively in a l litre cell. Since greater solution had greater maintenance of nickel concentration and it could operate at a higher CE. For example, after 1000 C electrowinning and at 45 A m -2, the deposition of nickel from boric acid-containing 1000 Ni occurred in a l0 litre cell at CE 87. As the amount of wastewater of plating shops are always > l0 litres; therefore, with higher flow rate and boric acid, a practical electrowinning efficiency could be obtained in the operation of plating shops. As the nickel concentrations of wastewater are >/1000rag 1-1 in some plating shops in Taiwan, these studied results are useful in the treatment of mixed wastewaters from nickel plating and cleaning baths. In addition, the nickel plating bath contains a lot of organic pollutants (Huang, 1992), surfactants
Chein-Ho Huang
1826
Table 6. 1000C electrowinningof 1000mgl ~Ni in a I litre cell at 2.21min * and 45Am z Nickel source Boric acid (gl i) CE (%) Nickel chloride 0 45.7 Nickel chloride 0.5 41.2 Nickel sulfamate 0 46.1 Nickel sulfamate 0.5 43.9
and brighteners. Therefore, these studied results are also useful in the first rinse tank to maintain the level of nickel concentration at a small concentration so that the supply of fresh water needed is decreased, with significant cost saving for nickel recovery and exceptional water saving of rinsewater. Besides nickel sulfate, the principal source of nickel ion in nickel plating solutions, Table 6 indicated that the CE of both nickel chloride and nickel suifamate solutions were enough to be conducted in a normal electrowinning process. Therefore, this electrowinning process can treat the wastewaters of either Watts or conventional sulfamate baths. Futhermore, owing to the components of both rack-used still plating bath and the barrel-used rotational plating bath are almost same; therefore, the wastewaters from these two plating equipments could be treated by this electrowinning process. An actual wastewater was treated in this work. With the addition of 0.5 g 1- ' boric acid and 1000 C electrowinning at 45 A m -2, 10 litres of wastewater from a local plating shop (initial pH 5.2 and 856mg1-1 nickel) had CE 72. The benefit of using nickel mesh cathode was that the loaded cathodes could be directly reused as anode material in a nickel plating cell; anode stripping was not needed. CONCLUSIONS Because of its buffering capacity, boric acid improved the electrowinning efficiency in the surfactantfree nickel wastewater. Following the decrease of the effective electrode area and mass transfer coefficient, the electrowinning
efficiency was decreased by the presence of surfacrants in nickel wastewater. Besides the buffering ability, perhaps because of altered orientation and conformation of adsorbed surfactant molecules, better electrowinnning efficiencies in the surfactantcontaining nickel wastewater were obtained by the presence of boric acid. The drawback of the decrease of electrowinning efficiency by the presence of surfactants was overcome by the presence of boric acid in nickel wastewater. As a result, when the treated solution contains a greater concentration of nickel, either in nickel wastewater streams or rinsewater, this treatment was economical and advantageous. REFERENCES
Afifi S., Hegazy M. and Donya K. (1991) On the electrowinning of zinc from alkaline zincate solutions J. Electrochem Soc. 138, 1929-1933. Ahmed A. M. and Sedahmed G. H. (1989) Effect of surfactants on the rate of mass transfer at gas-evolving electrodes. J. appl. Electrochem. 19, 219-224. Chen L. and Lasia A. (1992) Influence of the adsorption of organic compounds on the kinetics of the hydrogen evolution on Ni and Ni-Zn alloy electrodes. J. Electrochem. Soc. 139, 1058-1064. Corrigan D. A. and Bendert R. M. (1989) Effects of coprecipitated metal ions on the electrochemistry of nickel hydroxide thin films: cyclic voltammetry in 1 M KOH. J. Electrochem. Soc. 136, 723-728. Huang C. H. (1992) Effective removal of organics from nickel wastewater by modified carbon adsorption. Plating Sur/ace Fin. 79 (4), 50-56. Kar G., Healy T. W. and Fuerstenau D. W. (1973) Effects of alkyl amine surfactants on the corrosion of a rotating cooper cylinder. Corrosion Sci. 13, 375-385. Osipow L. I. (1972) Surface Chemistry, pp. 114-115, 216-221. Krieger, New York. Richer J. and Lipkowski J. (1986) Measurement of physical adsorption of neutral organic species at solid electrodes. J. Electrochem. Soc. 133, 121 128. Tan J. S. and Martic P. A. (1990) Protein adsorption and conformational change on small polymer particles. J. Colloid Interface Sci. 136, 415-431. Wu H. M., Lay M. L. and Huang C. H. (1992) Effects of surfactants on cyclic voltammetric stripping analysis of acid copper sulfate plating baths. Plating Surface Fin. 79 (9), 66-68.