Galvanostatic removal of Pb2+ ions from diluted solutions by the use of a membrane-less flow-through cell with stainless steel wool electrodes

ARTICLE

Galvanostatic removal of Pb²⁺ ions from diluted solutions by the use of a membrane-less flow-through cell with stainless steel wool electrodes

Lucio C. Almeida; Nerilso Bocchi^*; Romeu C. Rocha-Filho; Sonia R. Biaggio

Departamento de Química, Universidade Federal de São Carlos (UFSCar), CP 676, 13560-970 São Carlos-SP, Brazil

ABSTRACT

The removal of Pb²⁺ ions from dilute solutions was investigated under galvanostatic conditions using a membrane-less flow-through cell with stainless steel wool (SSW) electrodes. The response surface methodology (independent variables: electric current and volumetric flow rate) was used to find the optimal conditions for Pb²⁺ removal from 2 L of an aqueous solution of 50 mg L^-1 Pb(NO₃)₂ in 0.10 mol L^-1 NaNO₃ + 0.10 mol L^-1 H₃BO₃ (pH 4.8). The obtained response surfaces revealed that the volumetric flow rate (59-341 L h^-1) does not substantially affect the Pb²⁺ removal process. On the other hand, the electric current increase led to higher Pb²⁺ removal, with significantly improved values of specific energy consumption and current efficiency when compared with those reported in the literature. For an experiment carried out in the optimum-condition region (70 mA and 200 L h^-1), a Pb²⁺ conversion of 99.6% was attained after 90 min of electrolysis. The use of the membrane-less flow-through cell with SSW electrodes, besides being significantly more energy efficient, led to Pb²⁺ removal on both electrodes, but predominantly in the SSW anode, on which more than 95% of the Pb²⁺ ions were converted to PbO₂.

Keywords: electrolytic removal of Pb²⁺ ions, electrochemical treatment, Pb²⁺ wastewater, simultaneous cathodic/anodic Pb²⁺ removal, factorial design, response surface methodology

RESUMO

A remoção de íons Pb²⁺ a partir de soluções diluídas foi estudada, em condições galvanostáticas, usando uma célula de fluxo sem membrana com eletrodos de esponja de aço inoxidável (SSW). A metodologia de superfície de resposta (variáveis independentes: corrente elétrica e vazão) foi usada para obter as condições ótimas para a remoção de Pb²⁺ a partir de 2 L de uma solução de Pb(NO₃)₂ 50 mg L^-1 em NaNO₃ 0,10 mol L^-1 + H₃BO₃ 0,10 mol L^-1 (pH 4,8). A partir das superfícies de respostas, concluiu-se que a vazão (59-341 L h^-1) não influencia significativamente no processo de remoção de Pb²⁺. Por outro lado, o aumento da corrente elétrica leva a uma maior remoção de Pb²⁺, com valores significantemente melhores de consumo de energia específica e eficiência de corrente quando comparados àqueles relatados na literatura. Para um experimento realizado na região de condições ótimas (70 mA e 200 L h^-1), uma conversão de Pb²⁺ de 99,6% foi atingida depois de 90 min de eletrólise. A utilização da célula de fluxo sem membrana com eletrodos de SSW, além de ser energeticamente mais eficiente, levou à remoção de Pb²⁺ em ambos os eletrodos, mais predominantemente no anodo, sobre o qual 95% dos íons Pb²⁺ foram convertidos a PbO₂.

Introduction

Nowadays, environmental agencies of many countries require the treatment of aqueous effluents containing toxic metallic ions, such as Cd²⁺, Cr⁶⁺ and Pb²⁺, to decrease their concentrations to lower values than those permitted for discharge into sewers (few mg L^-1 down to zero, depending on the country). The treatment of these effluents is usually done adjusting their pH or adding a suitable counter-ion to form precipitates. The resultant solid sludge may then be sent to a licensed landfill. Despite the efficiency of this methodology, the possible mobility of those metallic ions may lead to contamination of watercourses.

Electrochemical techniques have been extensively used for the treatment of aqueous effluents because they offer an environmentally and friendly way of removing toxic metallic ions^1-15 and/or organic species^16-22 via redox reactions. The environmental compatibility is the main advantage of electrochemical methods, as the reagent, the electron, is a "clean reagent".^23,24 However, the removal of metallic ions from dilute aqueous solutions is hindered by slow mass-transfer kinetics. Therefore, the use of conventional plane electrodes is ineffective because of low current efficiencies and high energy consumptions.²⁵ This limitation can be overcome by the use of three-dimensional porous electrodes,²⁶ which lead to an enhancement of the mass-transfer rate and allow the use of low current densities, but with a relatively high electric current per unit cell volume.²⁷

Specifically for Pb²⁺ ions, several works on their removal from aqueous solutions using three-dimensional electrodes were reported in the literature. De Leon and Pletcher¹ investigated the influence of different anions at pH 2 (perchlorate, nitrate, tetrafluoroborate, sulfate and chloride) on the kinetics and current efficiency for Pb²⁺ reduction at a reticulated vitreous carbon (RVC) cathode. The reaction was under mass-transport control only in chloride media, and with other anions in solution the reaction exhibited slower kinetics. For perchlorate, nitrate and tetrafluoroborate, the anion effect was attributed to slow steps in the nucleation and/or early growth of the lead deposition on the carbon surface. In sulfate media, it was attributed to surface poisons and formation of a passivating layer. RVC cathodes of 20,45,60 and 80 pores per inch were also employed by Bertazzoli and co-workers^2,3 for Pb²⁺ reduction from a simulated effluent (50 mg L^-1 Pb(NO₃)₂ + 0.5 mol L^-1 H₃BO₃ + 0.05 mol L^-1 NaNO₃, pH 4), at a constant potential under mass-transport control. According to these authors, the flow-through cell used was able to reduce the Pb²⁺ concentration from ca. 50 to 0.1 mg L^-1 during recirculation times ranging from 20 to 40 min, depending on the RVC porosity and flow rate, and higher degrees of Pb²⁺ reduction were attained at higher cathode porosities and flow rates. Using cyclic voltammetry and atomic force microscopy, Carreño et al.⁴ also investigated anion effects on Pb²⁺ removal from aqueous solutions using a RVC electrode. Higher removal efficiencies were achieved in chloride media, and lowest efficiencies were achieved in sulfate media, which were also attributed to the formation of a passivating surface film.

A three-dimensional cathode formed by a stack of copper screens was used by El-Dead et al.⁵ for Pb²⁺ removal from a flowing alkaline solution (3 mol L^-1 NaOH). As expected, the limiting electric current for Pb²⁺ electrodeposition increased with Pb²⁺ concentration, electrolyte flow rate, screen mesh size, and/or cathode thickness. Ragnini et al.⁶ proposed the removal of Pb²⁺ from aqueous solutions (50 mg L^-1 Pb(NO₃)₂ + 0.5 mol L^-1 H₃BO₃ + 0.05 mol L^-1 NaNO₃, pH 4.8) using a flow-trough cell with a recycled niobium felt. These were obtained by pressing niobium tips resulting from machining of electron-beam refined niobium ingots. Operating at constant potential (-0.95 V vs. SCE), the Pb²⁺ concentration was reduced to 0.5 mg L^-1 during recirculation times from 94 to 150 min, depending on the flow rate. As expected, higher degrees of Pb²⁺ removal were attained for higher flow rates. Kelsall and co-workers^7,8 reported results of experiments and modeling of simultaneous recovery of Pb and PbO₂ in aqueous effluents from lead-acid battery recycling plants. From an electrolyte containing 1 mmol L^-1 Pb²⁺ at pH 12, the Pb²⁺ concentration was depleted to less than 0.06 mg L^-1 in a batch recycle reactor system with graphite felt anodes and graphite or titanium felt cathodes. Simultaneous cathodic Pb and anodic PbO₂ electrodeposition resulted in more rapid Pb²⁺ depletion than for either reaction separately. Experimental electric current density-potential and charge efficiency-potential relationships were in broad agreement with model predictions, with near unity current efficiencies for mass-transport controlled PbO₂ deposition from the electrolyte.

As reviewed above, none of the cited works reports the use of stainless steel wool (SSW) as electrode material for removing Pb²⁺ from aqueous effluents. However, Paidar et al.⁹ reported the use of SSW and graphite felt to simultaneously remove Cu²⁺ and Zn²⁺ from diluted solutions, and concluded that SSW was a promising alternative material to graphite felt for the construction of three-dimensional electrodes. Moreover, SSW offers the advantage of simple electrode regeneration by anodic polarization. Using mild-steel wool and a stainless-steel mesh coil for gold electrowinning, Barbosa et al.¹⁰ observed that electrolyte recirculation enhanced gold recovery from diluted liquors. Elsherief¹¹ designed a flow-by cell with a spiral wound steel electrode for Cd²⁺ electrowinning. Using 250 mL of solution and optimized operating conditions, the Cd²⁺ concentration was reduced from 500 to less than 5 mg L^-1 in 90 min.

In previous works, we reported on the prospects of a membrane-compartmentalized flow-through cell with a SSW cathode for Pb²⁺ removal from diluted solutions, under potentiostatic¹² or galvanostatic¹³ conditions. In the first condition, the cell performance was evaluated for three cathode potentials: -0.70, -0.80 and -0.90 V vs. SCE. At -0.90 V vs. SCE and a volumetric flow rate (Q) of 250 L h^-1, the Pb²⁺ concentration was reduced from 50 to 1 mg L^-1 (98% conversion) in a 90 min electrolysis.¹² Using the same electrochemical flow-through cell under galvanostatic conditions, the response surface methodology (RSM) was employed to find the optimum conditions for Pb²⁺ removal from diluted solutions.¹³ From the obtained response surfaces for Pb²⁺ removal efficiency and current efficiency as a function of the electric current (I) and Q, it was concluded that the region around -0.25 A and 250 L h^-1 (optimized condition) was the best one for Pb²⁺ removal, with values of Pb²⁺ removal efficiency and electric current efficiency of 93 and 22%, respectively, after a 30 min electrolysis. Furthermore, a value of Pb²⁺ removal efficiency of almost 99% was achieved after only 40 min of electrolysis.

In this work, the removal of Pb²⁺ ions from diluted solutions was also investigated under galvanostatic conditions, but now using a flow-through cell with both the cathodes and the anode made of SSW. This cell is similar to the flow-through cell used in previous works,^12,13 but with two important changes: the cationic membranes were removed and the two stainless steel plate (SSP) electrodes were covered with SSW and used as cathodes (the SSW electrode previously used as the cathode is now used as the anode). The benefits of such changes promptly became evident on the values of Pb²⁺ fractional conversion (X_Pb²⁺), current efficiency (Φ^e) and specific energy consumption (w). Thus, in order to determine the optimal conditions for the electrolytic removal of Pb²⁺ ions, I and Q were simultaneously investigated as independent variables through a factorial design and also by RSM.^28,29

Experimental

Chemicals

The diluted solution containing Pb²⁺ ions was prepared using analytical-grade chemicals and distilled and deionized water (Milli-Q^® system, Millipore). Lead nitrate (Aldrich), sodium nitrate (Merck) and boric acid (Merck) were used to prepare this solution with the following composition: 50 mg L^-1 Pb(NO₃)₂ in 0.1 mol L^-1 NaNO₃ + 0.10 mol L^-1 H₃BO₃ (pH 4.8). Atomic absorption standard solutions used in the analyses were prepared using a Titrisol standard solution (Merck).

Central composite rotatable design

A central composite rotatable design (CCRD)³⁰ with two independent variables (I and Q) was employed to find the optimum conditions for Pb²⁺ removal. Five levels of each independent variable were chosen and three replications were carried out at the central point, totaling 11 experiments (see Table 1). Based on preliminary results, the ranges of the independent variables were: I from 44 to 86 mA and Q from 59 to 341 L h^-1. The independent variables were coded according to the following equation:²⁸

where x_i, X_i, X_i0 and ΔX_i are the coded level of the ith independent variable, the real value of the ith independent variable, the real value of the ith independent variable in the central point, and the half of the difference between the upper and lower values of the ith independent variable, respectively.

The correlations between the independent variables and the responses were obtained by using second-order models (least-squares method):

where Y is the response, β₀ a constant coefficient, β_i, β_ii and β_ij the coefficients for the linear, quadratic and interaction effects, respectively, x_i and x_j the coded levels for the independent variables, k the number of independent variables and εthe random error.²⁸ In the present case, the mathematical relationships between the coded levels of the independent variables (I ≡ x₁ and Q ≡ x₂) and the responses (X_Pb²⁺, Φ^e and w) were approximated by a second-order polynomial equation:

where Y is the predicted response, β₀ a constant, β₁ and β₂ the linear coefficients, β₁₁ and β₂₂ the quadratic coefficients and β₁₂ the interaction coefficient.

Experiments in the central point were performed in order to estimate the pure error. All the experiments were randomly carried out in order to minimize the effect of unexplained variability on the observed responses due to systematic errors.²³ Finally, the results were analyzed by the least-squares method and response surfaces^28,29 were generated in order to determine the best conditions for Pb²⁺ removal.

Electrolytic system and procedure for Pb²⁺ removal

The whole system for Pb²⁺ removal (Figure 1) consisted of a membrane-less flow-through electrochemical cell (diameter of 150 mm and length of 80 mm), a PVC reservoir, a magnetic recirculation pump, and a flowmeter to control the solution flow rate. The flow-through electrochemical cell (Figure 2) was mounted, in a "sandwich" form, using five PVC circular plates with the following dimensions: inner and outer diameters of 70 and 150 mm, respectively, and thickness of 12 mm. Each cell end contained a cathode made of Amway^® SSW (70 mm of diameter and 6 mm of thickness, ca. 9 g) pressed against an SSP (150 mm × 150 mm × 5 mm). In order to prevent leakage, silicone gaskets, with format identical to that of the PVC plates but thickness of 2 mm, were used between these plates. Two of the PVC plates acted as turbulence promoters, allowing a uniform electrolyte flow distribution and standardization of the mass transport within the cell.⁶ In the central PVC plate, an Amway^® SSW anode (70 mm of diameter and 12 mm of thickness, ca. 18 g) was fixed by pressing against an SSP ring. The distance between each cathode and the anode was 22 mm. As described in detail elsewhere,¹³ their surface areas per unit volume were 2.7 × 10³ and 1.4 × 10³ m^-1, respectively.

The assembled system was loaded with 2 L of the diluted Pb²⁺ solution and the performance of the cell evaluated by CCRD. All experiments on the galvanostatic removal of Pb²⁺ ions were conducted in the recirculation mode, using a PAR 273A potentiostat/galvanostat controlled by the Echem software. The electrolytic solution was periodically sampled and its Pb²⁺ content analyzed by a Varian SpectrAA 200 spectrophotometer with an air/acetylene flame.

Data analysis

In this work, as pointed out above, three parameters were used to evaluate the performance of the electrochemical system in each CCRD point: X_Pb²⁺, Φ^e and w. The value of X_Pb²⁺ was calculated as:

where C₀ is the initial Pb²⁺ concentration and C the Pb²⁺ concentration after a given electrolysis time. The value of Φ^e was calculated as:

where n is the stoichiometric number of electrons for the global reaction of Pb²⁺ conversion, F the Faraday constant and V the volume of electrolytic solution used in each test. Finally, the value of w (in W h kg^-1) was calculated as:

where U_cell is the cell voltage, M the molar mass of lead and 3600 the conversion factor from second to hour.

Results and Discussion

Optimization of Pb²⁺ removal

Initially, a 2³-full-factorial design with three independent variables and two levels for each variable was adopted to evaluate the influence of I, Q and surface area of the cathodes (only SSP or SSW fixed on SSP) on the Pb²⁺ electrolytic conversion (data not shown). From the results thus obtained, it became clear that lower values of w were obtained for the membrane-less flow-through cell operating with the cathodes of higher surface area (SSW fixed on SSP). Thus, with these cathodes, a set of experiments using a CCRD with only two independent variables (I and Q, Table 1) was conducted in order to obtain the optimum conditions for Pb²⁺ removal. Then, using the obtained responses (see Table 1), a multiple linear-regression analysis (least-squares method) was employed to generate three second-order models,³¹ describing the dependence of X_Pb²⁺, Φ^e and w on the two independent variables. The obtained equations for the three adjusted models containing only the statistically significant coefficients (at 95% confidence level) are:

where x₁ and x₂ correspond to the coded levels for I and Q, respectively.

The response surface generated by equation 7 (Figure 3a) shows that, up to a certain point, the increase of the electric current positively influences the Pb²⁺ conversion process, leading to X_Pb²⁺ values higher than 0.98 after a 30 min electrolysis, which is a significant improvement when compared with those obtained in previous works.^12,13 It is worthwhile to point out that two simultaneous processes contribute for Pb²⁺ removal during electrolyses using the membrane-less flow-through cell: Pb²⁺ + 2e^- → Pb on the cathodes and Pb²⁺ + 2H₂O → PbO₂ + 4H⁺ + 2e^- on the anode.

On the other hand, as one would expect, the corresponding values of Φ^e decrease as I is increased, as illustrated in Figure 3b. This occurs because competitive parallel reactions, such as the oxygen, water and nitrate reduction reactions (O₂ + 4H⁺ + 4e^- → 2H₂O,2 H₂O + 2e^- → H₂ + 2OH^- and NO₃^- + H₂O + 2e^- → NO₂^- + 2OH^-, respectively), and the water oxidation reaction (2H₂O → O₂ + 4H⁺ + 4e^-), become important as I is increased.

As one would also expect, increases in I led to higher values of w (Figure 3c), although the highest obtained valueis significantly lower than those reported in the literature for Pb²⁺ removal.^1,4,7,12,13 The value of w depends on the cell voltage, which can be expressed as:²⁴

where Uº_cell is the thermodynamic equilibrium cell voltage, S|h| the sum of the overpotentials at each electrode and ΣIR the sum of all ohmic drops in the cell due to the flow of I through resistances (R). At this point, it is worthwhile to remember that the cationic membranes were removed in the flow-through cell used in the present work. Therefore, the contribution from ΣIR was strongly minimized, thus remaining only the contributions from ohmic drops associated to the electrolyte, the distance between the electrodes and the magnitude of the electric current. It is clear from equation 7, that Q has no significant influence on X_Pb²⁺ (see also Figure 3a), although several other studies showed that the rate of electrochemical reactions under mass transport conditions is dependent on Q.^2,3,6,12 According to Koene and Janssen,³² increases in Q enhance the electrochemical-reaction rate as a consequence of the diffuse layer becoming thinner. In the present case, lower Q values possibly promoted insignificant changes in the mass transport coefficient, while higher ones led to the formation of preferential streaming channels and/or dead zones.³³ Similarly for Φ^e (Figure 3b), the influence of Q is practically insignificant. However, in the case of w (Figure 3c) there is a dependence on Q: for low I values, w increases with Q, while the opposite occurs for high I values. Such behavior is quite complex, being determined by the interplay between U_cell and Q.

The use of RSM also allows to find the region of optimum conditions for Pb²⁺ removal within the experimental ranges. Thus, taking Figure 3a into account and considering that Q affects the value of w, an optimized experiment was carried out by fixing I and Q at 70 mA and 200 L h^-1, respectively. At these conditions, after a 30 min electrolysis the obtained values of X_Pb²⁺ and w were equal to 0.98 and 1.5 kW h kg^-1, respectively, while the value of Φ^e was 0.35. Additionally, as it can be seen in Figure 4, after a 90 min electrolysis, the Pb²⁺ concentration dropped to 0.21 mg L^-1 (X_Pb²⁺ = 0.996). Furthermore, by measuring the mass of the SSW electrodes (anode and cathodes) before and after the experiments, it became clear that more than 95% of the Pb²⁺ ions in solution were removed as PbO₂ deposited on the SSW anode.

Figure 4

Validation of the quadratic models developed

Analysis of variance (ANOVA)³¹ was applied to numerically validate the three quadratic models (X_Pb²⁺, Φ^e, and w) here developed (Table 3 exemplifies the ANOVA for the X_Pb²⁺ model; the data for the Φ^e and w models can be seen in Tables S1 and ^S2 in the Supplementary Information, SI). The F-test (Fisher's distribution) was also used in order to evaluate the model that best fits the population from which the data were sampled. This statistical test is based on the ratio of two scaled sums of squares reflecting different sources of variability.^30,31 With the aid of ANOVA and the F-test, it was found that the regressions for the three models were statistically significant, since the calculated F-ratio for the regressions (19.56 for X_Pb²⁺,38.46 for Φ^e and 48.23 for w) were greater than the values of the critical F-value (5.05 at 95% confidence level). For all models, the calculated values of the F-ratio for lack of fit (1.74 for X_Pb²⁺,9.21 for Φ^e and 7.23 for w) were smaller than the critical ones (19.16 at 95% confidence level), thus indicating that the data are well adjusted to the models. The use of an F-test presupposes that the residuals have a normal distribution.^30,31Figure 5 illustrates a typical plot of the residuals versus the predicted values for the X_Pb²⁺ model; similar plots for the Φ^e and w models can be seen in the SI (Figure S1). The fact that these plots present no systematic errors (all the residuals are randomly distributed around the mean) confirms the good agreement for the models.³¹ The determination coefficients (R²) for the models, which are additional parameters for assessing the fitting quality of the models, were close to unity (0.951 for X_Pb²⁺, 0.975 for Φ^e and 0.980 for w), indicating that the models were statistically significant.³⁰ Finally, probability plots were also obtained to evaluate the statistical significance of the coefficients for the three developed models (X_Pb²⁺, Φ^e and w). Figure 6 illustrates this plot for the X_Pb²⁺ model; as it can be seen, the terms different from zero were statistically significant. Similar results were obtained in the probability plots for the Φ^e and w models, as can be seen in Figure S2 in the SI.

Conclusions

The results here reported for the removal of Pb²⁺ ions from diluted solutions allow to conclude that a membrane-less flow-through cell with stainless steel wool (SSW) as cathodes and anode is significantly more efficient and economical than a previously used flow-through cell with SSW only as cathode.

The use of factorial design along with RSM was useful for the optimization of the electrolytic removal process in a relatively small number of experiments; the conditions predicted by the developed models are in good agreement with the experimental results, as confirmed by variance analysis. The obtained response surfaces from the developed models revealed that Q (in the range 59-341 L h^-1) did not substantially affect the Pb²⁺ removal process. On the other hand, the increase of I led to higher values of Pb²⁺ conversion, at reasonable values of current efficiency and specific energy consumption, especially when compared with those reported in the literature. An experiment carried out within the optimum-conditions region (using 70 mA and 200 L h^-1, [Pb²⁺] = 50 mg L^-1,2 L of solution) yielded a Pb²⁺ conversion of about 99.6%, after 90 min of electrolysis.

Finally, it should be emphasized that the use of a membrane-less flow-through cell with SSW as cathodes and anode, besides being significantly more energy efficient, led to Pb²⁺ removal in both electrodes, but predominantly in the anode. In the experiment mentioned above, more than 95% of the removed Pb²⁺ ions were converted to PbO₂ on the SSW anode.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as a PDF file.

Acknowledgments

We gratefully acknowledge the Brazilian research funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support and scholarships. Additionally, we thank Professor Edenir R. Pereira Filho (Departamento de Química, UFSCar) for discussions on factorial design, and Professor Luis Augusto M. Ruotolo (Departamento de Engenharia Química, UFSCar) for the use of the AA spectrophotometer.

Submitted: March 11,2011

Published online: June 9,2011

FAPESP has sponsored the publication of this article.

Supplementary Information

Table S1 - Click to enlarge

Table S2 - Click to enlarge

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Publication Dates

• Publication in this collection
  20 Sept 2011
• Date of issue
  Sept 2011

History

• Received
  11 Mar 2011
• Accepted
  09 June 2011