ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 4, pp. 693−698. © Pleiades Publishing, Ltd., 2015. Original Russian Text © T.A. Kenova, I.S. Vasil’eva, V.L. Kornienko, 2015, published in Zhurnal Prikladnoi Khimii, 2015, Vol. 88, No. 4, pp. 646−651.

VARIOUS TECHNOLOGIES

Removal of Heavy Metal Ions from Aqueous Solutions by Electrocoagulation Using Al and Fe Anodes T. A. Kenova, I. S. Vasil’eva, and V. L. Kornienko Institute of Chemistry and Chemical Technology, Siberian Branch, Russian Academy of Sciences, Akademgorodok 60, str. 24, Krasnoyarsk, 660036 Russia e-mail: [email protected] Received April 27, 2015

Abstract—Electrocoagulation treatment of model wastewater solutions to remove jointly present Cu2+ and Ni2+ ions was studied. The influence of the current density, concentration of impurities, anode material, and structure of the precipitate of coagulant metal hydroxides on the efficiency of removal of the heavy metal ions was examined. The decisive factor under the chosen process conditions is the anode material. The electrocoagulation efficiency is considerably higher when using the aluminum anode, compared to the iron anode. DOI: 10.1134/S1070427215040242

addition, the precipitate is formed in the course of electrocoagulation in considerably smaller amount and is more compact and better filterable. It should be noted that the electrocoagulation has also certain drawbacks, the main of which are the need for replacing soluble anodes and for increasing the electrical conductivity of solutions in cases when it is insufficient [10, 11]. The electrocoagulation consists in the in situ generation of the coagulant via electrochemical oxidation of the soluble anodes, usually those made of iron or aluminum, and formation of ОН– ions and hydrogen at the cathode. Anode:

Solution of problems of environment protection and potable water saving requires the development of efficient and environmentally safe methods for treatment of industrial wastewaters to remove anthropogenic impurities [1]. Wastewaters from the majority of industrial enterprises contain highly toxic components such as cyanides, thiocyanates, simple and complex ions of heavy metals (Cd, Cr, Cu, Ni, Zn, Pb, etc.), and various organic impurities. To reduce the concentration of metals in wastewaters, various methods are used depending on the kind and content of ecopollutants: chemical precipitation, adsorption, flotation, membrane filtration, electrochemical treatment, and biodegradation [2–6]. Electrochemical methods are among the safest and simplest methods for wastewater treatment to recover valuable components, to eliminate harmful components, including decomposition of the majority of organic and inorganic impurities, and to remove heavy metal ions [4, 7–9]. The electrocoagulation method for wastewater treatment is among such methods. It allows not only the content of harmful impurities in the wastewater to be considerably reduced, but also the secondary contamination of the wastewater to be avoided. In

Fe(s) → ← Fe2+ + 2e,

(1)

Al(s) → ← Al2+ + 3e,

(2)

Cathode: – H2O + e → ← 1/2H2 + OH .

(3)

In the course of dissolution, the ions of these metals undergo hydrolysis. The degree of hydrolysis depends on the concentration of metal ions, pH of solution, and concentration and chemical nature of the impurities [12]. The forming hydroxides occur in the solution in 693

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EXPERIMENTAL

(a)

τ, min E, %

(b)

τ, min Fig. 1. Efficiency E of the removal of Cu2+ and Ni2+ ions as a function of the electrocoagulation time τ. Anode: (a) aluminum and (b) iron; the same for Fig. 3. Current density, mA cm–2: (1, 1') 15, (2, 2') 30, (3, 3') 45, and (4, 4') 60. (1–4) Cu2+ and (1'–4') Ni2+.

the form of an amorphous suspension and of monomeric or polymeric complexes, which can bind pollutants via complexation and electrostatic interaction. These processes cause destabilization of impurities, leading to their efficient removal [1, 13–15]. In addition, the following processes are possible: migration of charged particles to electrodes of opposite sign and their aggregation due to charge neutralization, formation of hydroxides of impurity metals near the cathode surface as a certain pH value is reached, and subsequent adsorption and coprecipitation with the coagulant metal [16]. Despite numerous studies on the use of the electrocoagulation method, it is necessary to study it further with the aim to intensity the purification parameters depending on the wastewater composition. In this study, we examined the influence of the anode material, current density, and concentration of the jointly present copper and nickel ions on the kinetics and efficiency of the electrocoagulation from model solutions.

The electrocoagulation was performed in a cylindrical cell with a solution volume of 45 mL with stirring on a magnetic stirrer. Aluminum and iron plates were used as soluble anodes. The pretreated electrodes were arranged in a cell. The interelectrode spacing was 0.5 cm, and the anode surface area was 2.0 cm2. The electrolysis current in the galvanostatic mode was set with a P-5848 potentiostat. The current density on the electrodes was 15, 30, 45, and 60 mA cm–2. Model solutions were prepared using copper (CuSO4·5H2O) and nickel (NiSO4·7H2O) salts (chemically pure grade). Because real wastewaters in most cases have relatively high electrical conductivity, the conductivity of the model solutions was increased by adding K2SO4 to a concentration of 0.5 M. The pH of the solution was in the range 5.3–5.8 depending on the concentration of the salts. The pH was measured with a Multitest pH meter. The initial concentration of Cu2+ and Ni2+ was 100, 250, and 500 mg L–1 for each ion. The concentration of metal ions in the solution was determined by the atomic absorption method. It is known that the electrocoagulation efficiency is influenced by numerous factors, such as the initial pH of the solution, current density, time, concentration and composition of impurities, and anode material [12]. One of the most important parameters that can be directly controlled in the course of electrocoagulation is the working current density. The current density determines the amount of the produced coagulant and the rate of the generation of gas bubbles. It also influences the mass transfer in the solution and at the electrodes. Figures 1a and 1b show how the efficiency of the Cu2+ and Ni2+ removal using the (a) aluminum and (b) iron anodes depends on time at different current densities. As expected, the efficiency of the removal of the metal ions from the solution increases as the current density is increased. After 30-min electrolysis, the efficiency of the copper removal using the iron anode was 45, 60, and 76% at a current density of 30, 45, and 60 mA cm–2, respectively. The efficiency of the nickel removal was considerably lower in this case: 10, 19.6, and 30.8%, respectively. In electrocoagulation with an aluminum anode, the removal efficiency increased to 30 and 76% for Cu2+ and to 25 and 68% for Ni2+ at current densities of 15 and 45 mA cm–2, respectively. An increase in the electrocoagulation time to 60 min leads to an increase

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in the process efficiency in both cases. The copper ions are removed from the solution virtually completely (the residual Cu2+ concentration did not exceed 0.2 mg L–1), whereas the efficiency of the Ni2+ removal by electrocoagulation with an iron anode was from 30 to 68% depending on the current density. On the other hand, with an aluminum anode, the Cu2+ and Ni2+ ions were removed at approximately equal rate, with 96–99.96% efficiency of the process, except the experiment at a low current density (15 mA cm–2), in which the removal efficiency was 50 and 45% for Cu2+ and Ni2+, respectively. As the current density is increased, the anode dissolution rate and the concentration of the coagulant metal in the solution increase. Simultaneously, the rate of formation of the hydroxide ion at the cathode increases, which leads to a faster increase in pH of the solution and, correspondingly, in the amount of the precipitate of iron or aluminum hydroxide. As a consequence, the impurities are removed more efficiently. The rate of the reduction of the impurity metals on the cathode and of their precipitation in the form of the corresponding hydroxides near the cathode surface as a result of formation of local zones with high pH values also increase with increasing current density. Thus, the removal of copper and nickel ions was complete in electrocoagulation with an aluminum anode, whereas with an iron anode the residual concentration of Ni2+ in the solution was relatively high. Analysis of the pH variation in time at different current densities (Fig. 2) shows that, in the initial period of the process, pH in the bulk of the solution does not noticeably change relative to the initial value with the iron anode (5.63–5.75) and slightly decreases with the aluminum anode (to 5.1–5.2). It is known that both soluble monomeric and polymeric complexes bearing different charge [Al(OH)2+, 4+, Al(OH)2+, Al2(OH)24+, Al(OH)2–, Al(OH)52–, Al7(OH)17 7+ 5+ Al 13О 4(OH) 24 , Al 13(OH) 34 ] and insoluble Al(OH) 3 species can occur in the solution depending on pH and concentration of the coagulant metal [12, 17]. When aluminum is dissolved in accordance with reaction (2), Al3+ and ОН– are generated in a molar ratio of 1 : 3; for the polymeric species precipitating at pH 5–6, this ratio is lower than 3 (between 2 and 2.5) [17]. Probably, in the initial period of the electrolysis, when the concentration of aluminum ions is low, the reactions of the formation and precipitation of Cu(OH)2 and Ni(OH)2, exerting a buffer effect on pH, occur along

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Fig. 2. pH of the solution as a function of the electrocoagulation time τ at different current densities. Current density, mA cm–2: (1) 15, (2, 2') 30, (3, 3') 45, and (4') 60. Anode: (1–3) aluminum and (2'–4') iron.

with the formation of polymeric aluminum species. The variation of the solution pH with time and with an increase in the current density and, correspondingly, the impurity removal efficiency are determined by the kinetics and equilibria of the reactions occurring in this complex system. An increase in the Al3+ and ОН– concentrations shifts the equilibrium toward formation of insoluble amorphous Al(OH)3, which removes the impurity metal ions from the solution by coprecipitation. In the course of iron oxidation, iron is mainly present in the solution in the form of Fe2+ ions, because pH of the solution is yet insufficient for the formation of Fe2+ hydroxide. A part of the ions can be oxidized to Fe3+ with oxygen dissolved in the electrolyte. Hydrolysis of trivalent iron occurs via formation of Fe(OH)2+ and Fe(OH)2+ intermediates to Fe(OH)3 [12]. The formation of these species, along with hydrolysis and precipitation of impurity metals near the cathode, can exert a buffer effect on pH. With increasing electrocoagulation time, the concentration of iron and hydroxide ions increases. At pH < 7, the oxidation of Fe2+ with the dissolved oxygen occurs slowly, and the ОН– ions formed at the cathode are consumed incompletely. In this case, the impurity ions can be removed via formation of their hydroxides. Further increase in pH leads to the formation and precipitation of bivalent iron in the form of the hydroxide, along with the precipitation of Fe(OH)3, and to an increase in the electrocoagulation efficiency. An increase in the current density leads to an increase in pH to a lesser extent than in the electrocoagulation with aluminum. It should be

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E, %

(a)

τ, min

(b)

τ, min

Fig. 3. Efficiency E of the removal of Cu2+ and Ni2+ ions as a function of the electrocoagulation time τ at different initial concentrations of the ions. Ion concentration, mg L–1: (1, 1') 100, (2, 2') 250, and (3, 3') 500. (1–3) Cu2+ and (1'–3') Ni2+.

noted that practically complete removal of nickel ions at this concentration was reached only after 3-h electrolysis. Apparently, under these conditions the efficiency of the nickel hydroxide formation and its coprecipitation with iron hydroxides is considerably lower [Ni(OH)2 starts to form at pH > 6] than the efficiency of the removal of Cu2+ ions, which are completely removed within 1 h. An increase in the concentration of heavy metal ions leads to a decrease in the efficiency of their removal from the solution. Figures 3a and 3b show how the initial concentration of the Cu2+ and Ni2+ ions influences the electrocoagulation efficiency at a current density of 30 mA cm–2. The efficiency of the 1-h process with the Fe anode increased from 60 to 89% for copper and from 2 to 25% for nickel as their concentration was decreased from 500 to 100 mg L–1 (Fig. 3b). In electrocoagulation with the Al anode, the efficiency was 63–100% for Cu2+ and 47–100% for Ni2+ (Fig. 3a). In the course of electrocoagulation at high content of heavy metal ions, we observed the copper deposition on an iron anode, particularly noticeable at a concentration of 500 mg L–1. It should be noted that the copper deposition on the anode was observed throughout the process at all the current densities. Partial shielding of the anode surface by the copper metal deposit leads to deceleration of the iron oxidation, to a decrease in the Fe2+ concentration in the solution, and, correspondingly, to a decrease in the amount of the precipitate of iron hydroxides. In addition, at sufficiently high concentration of ions in the solution, they can be involved in competing reactions of cathodic reduction, thus decreasing the fraction of current utilized for the formation of hydroxide ions. Variation of pH near the cathode influences the rate of formation of copper and

nickel hydroxides and the intensity of their coprecipitation with iron hydroxides. As the electrocoagulation time and current density are increased, the copper metal layer is detached from the anode surface both mechanically, owing to stirring, and by the released oxygen, which, in turn, leads to acceleration of the Fe2+ formation, to its hydrolysis, and to a decrease in the Ni2+ concentration in the solution. High efficiency of the electrocoagulation with the aluminum anode may be due both to higher adsorption power of precipitates of polynuclear aluminum complexes and to rapid increase in pH of the solution, causing hydrolysis of impurity metals. It should be noted that a large amount of Fe2+ ions remains in the solution after the electrocoagulation with the iron anode, removal of the impurities, and separation of the precipitated hydroxides. To remove Fe2+ ions, it is necessary either to increase рН and precipitate iron in the form of Fe(OH)2 or to increase the rate of the oxidation to Fe3+, whose hydroxide is considerably less soluble [18]. The latter approach can be implemented by introducing an oxidant or performing additional aeration of the solution in the course of electrocoagulation. To check the possibility of removing iron ions from the solution and examine the adsorption ability of trivalent iron precipitates, we performed a series of electrocoagulation experiments using hydrogen peroxide as oxidant. The presence of hydrogen peroxide in the solution leads to faster oxidation of bivalent iron ions to Fe3+ and, correspondingly, to more intense formation of a suspension of amorphous Fe(H2O)3(OH)3 [19]. When performing experiments without hydrogen peroxide, we observed the formation of a black-green precipitate, whereas in the experiments with the addition of Н2О2

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the precipitate had brown color characteristic of iron(III) hydroxides. Experiments on removal of heavy metal ions in the course of electrocoagulation with the addition of hydrogen peroxide were performed under identical conditions (Cu2+ and Ni2+ concentrations, pH, current density). The required amount of hydrogen peroxide was calculated from the stoichiometry of reaction (4) taking into account the fact that iron passes into the solution in the form of Fe2+ in an amount determined by the Faraday law. Hydrogen peroxide was added into the solution in portions at equal time intervals: 3Fe2+ + 2H2O2 → 2Fe3+ + 2H2O + O2.

(4)

The dependences of the efficiency of the copper and nickel removal on the process parameters are ambiguous. For example, the above-discussed dependence of the process efficiency on the current density is observed only at metal ion concentrations in the solutions of 100 mg L–1. At a concentration of 250 mg L–1, the efficiency of the Cu2+ removal is higher at a lower current density, and the efficiency of the nickel removal is independent of the current density and does not exceed 20%. At the metal ion concentration increased to 500 mg L–1, the efficiency both for copper and for nickel is independent of the current density. Similar trends were also observed with variation of the initial concentration of the impurities. It should be noted, however, that in all these experiments the efficiency of the removal of the copper and nickel ions was considerably lower than in the electrocoagulation experiments performed without adding Н2О2. For example, at с = 250 mg L–1, the efficiency of the copper removal in 1 h was 64, 55.5, and 62% at the current densities of 30, 45, and 60 mA cm–2 and more than 99% at the same current densities in the course of electrocoagulation with hydrogen peroxide and without it, respectively. For nickel, the efficiency was 30, 55.2, and 68%, respectively, without hydrogen peroxide and as low as 6–11% with the addition of hydrogen peroxide. Although the results obtained are difficult to interpret unambiguously, it can be assumed that the adsorption power of hydrated trivalent iron oxides is considerably lower than that of the hydrolysis products of Fe2+ and Fe3+ present simultaneously. The residual concentration of iron in the filtrate after performing the electrocoagulation with the addition of

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hydrogen peroxide is 3–5 times lower than without Н2О2, but is higher by an order of magnitude than the residual Al3+ concentration. Higher efficiency of the removal of heavy metal ions without hydrogen peroxide can be attributed to the fact that the electrocoagulation can be accompanied, along with the formation of iron(II) and (III) hydroxide precipitates, by the formation of complex hydrated compounds of type [xFeII(6–x)Fex(OH)12]x+[(Ax/n·yH2O]x–, which have considerably larger specific surface area and are more reactive. The probability of the formation of such compounds, termed “green rust,” is confirmed by the data of [20, 21]. Moreno et al. [20] studied in detail the electrocoagulation with the iron anode and suggested the mechanism and electrode reactions accounting for the observed phenomena. Thus, the data that we obtained on the electrocoagulation treatment of solutions to remove heavy metal ions (Cu2+, Ni2+) suggest a complex mechanism of the interaction of products formed by hydrolysis of the coagulant metal with the impurities present in the solution. CONCLUSIONS (1) The efficiency of the removal of copper and nickel ions is largely determined by the character of the hydroxide precipitates obtained, whose adsorption ability is determined by the process conditions and by the soluble anode material. (2) The jointly present Cu 2+ and Ni 2+ ions are removed from solution most completely when the electrocoagulation is performed with an aluminum anode. The removal is due to hydration of metal ions, their adsorption on active sites of polynuclear complexes, and coprecipitation with Al(OH)3. (3) In electrocoagulation with an iron anode, the efficiency of the nickel removal is considerably lower than that of the copper removal and depends on the concentration of the ions in the solution. At high Cu2+ concentrations, the formation of coagulant metal ions is inhibited by the copper deposition on the anode. (4) The efficiency of the removal of copper and nickel ions is considerably lower when performing the electrocoagulation with the addition of hydrogen peroxide, but the residual concentration of iron ions in the solution is appreciably lower in this case.

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