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Removal of arsenic by iron and aluminium electrochemically assisted coagulation
Separation and Purification Technology 79 (2011) 15–19
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of arsenic by iron and aluminium electrochemically assisted coagulation ˜ Engracia Lacasa, Pablo Canizares, Cristina Sáez, Francisco J. Fernández, Manuel A. Rodrigo ∗ Department of Chemical Engineering, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 20 December 2010 Received in revised form 25 February 2011 Accepted 1 March 2011 Keywords: Electrocoagulation Arsenic Iron electrodes Aluminium electrodes
a b s t r a c t In this work, removal of arsenic by electrocoagulation with iron and aluminium electrodes is studied in a batch bench scale plant. Results demonstrate that both iron and aluminium electrocoagulations are a robust technology capable of removing arsenic down to 10 g dm−3 (the level fixed by most environmental and health agencies). Nevertheless, aluminium electrocoagulation is not as efficient as iron, one for the removal of arsenic when current densities below 2 mA cm−2 are applied. The effect of the current density (from 0.1 to 4.0 mA cm−2 ) is studied with both electrodes. Results show that this parameter influences slightly on the range of pH in which the process works (7–9) and also on the efficiency (always over 99.9%), but it is not a limiting parameter. Results can be easily explained taking into account the solubility of iron, aluminium and arsenates species present in the treated water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Contamination of ground and surface water by arsenic is an environmental concern of the major importance. That pollution mainly arise by natural processes (such as dissolution of arsenic minerals) and/or by anthropogenic activities (such as the uncontrolled mining and metallurgical industrial discharges, the application of arsenic-based pesticides or the dissolution of wood preservatives). Although arsenic polluted waters have been mainly reported in countries such as India or Bangladesh, its occurrence is actually a worldwide concern because it makes unviable the use of a natural water source for human consumption. Both, organic and inorganic species of arsenic may be found in waters. However, inorganic species are the primary ones, depending on the particular speciation upon redox and pH conditions. As(V) is the most stable species under aerobic conditions, existing as arsenates and its protonated forms (AsO4 3− , HAsO4 2− , H2 AsO4 − and H3 AsO4 ). As(III) is the predominant species in anaerobic environment, existing as arsenates and its protonated forms (AsO3 3− , HAsO3 2− , H2 AsO3 − and H3 AsO3 ). Arsenic is a carcinogen compound and its ingestion may affect the gastrointestinal tract, cardiac, vascular and central nervous system. For this reason, the environmental protection agencies throughout the world have suggested that the arsenic content in drinking water should be less than 10 g dm−3 .
∗ Corresponding author. Tel.: +34 902204100; fax: +34 926295318. E-mail address: [email protected] (M.A. Rodrigo). 1383-5866/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.03.005
During the recent years, many technologies have been studied to remove arsenic from waters [1]. Coagulation with iron and aluminium salts in the presence of polyelectrolytes followed by a filtration, adsorption onto different types of solids (activated alumina, activated carbon, activated bauxite, and clay minerals), ion-exchange and membrane technologies (mainly reverse osmosis and electro-dialysis) are among the most reported technologies. However, none of them have been reported in the reference technology. This can be explained because they present important drawbacks, such as the difficulties in meeting the standards of quality required by the environmental agencies, the cost or the lack of robustness. In addition, they perform better for the removal of As(V) than for the removal of As (III), being usually recommended to oxidize As(III) to As(V) by using different types of oxidants (ozone, chlorine, etc.) [2]. Moreover, the dosing of these reagents tends to bring down the water quality because of the residues and byproducts formed. Electrocoagulation or electrochemically assisted coagulation has been recently introduced as a promising technology in the removal of arsenic [3,4]. Several works have studied the effect on the removal of arsenic of the supporting electrolyte [5], cell potential [6], current intensity and/or reactor design [7–9] or electrodes system [10]. Results demonstrate the complexity of the system and the need for additional work to clarify this process. In this context, other recent works about aluminium and iron electrocoagulation try to clarify the complexity of electrocoagulation processes through statistical modeling rather than mechanistic understanding of the processes involved [11,12]. Also, the removal of arsenic from industrial wastes has been studied, focusing the
16
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
attention in this case on the treatability of highly arsenic polluted solutions [13,14]. In this context, the aim of this work is to study the effect of current density on the electrocoagulation with iron and aluminium electrodes and to try to develop a technology suitable for the removal of arsenic down to the limits fixed by health and environmental associations, increasing at the same time the knowledge about the mechanisms involved in the process. 2. Experimental 2.1. Experimental procedure Bench-scale electrocoagulation studies were used to characterize the treatability of synthetic waters. Electrocoagulation experiments were carried out in batch operation mode. Experimental setups and procedures have been widely described elsewhere [15,16]. In these experiments, the coagulant reagent came from the dissolution of iron or aluminium electrodes placed in a single compartment electrochemical flow cell. Both electrodes (anode and cathode) were square in shape (100 cm2 ) and the electrode gap was 9 mm. It is worth to state that in every case the anodic and cathodic materials were the same. This is a normal practice in industrial electrocoagulation processes, because this allows the inversion of the polarity as a response to avoid operation problems, which can be caused by the formation of films of carbonates on the surface of the cathodes, or by the passivation of the anodes. The electrical current was applied using a DC Power Supply FA376 PROMAX. The synthetic wastewater was stored in a glass tank (5000 cm3 ), and recirculated through the electrolytic cell by a peristaltic pump. The synthetic water is composed by sodium arsenate (20 mg As dm−3 ) and a supporting electrolyte to increase its conductivity (1000 mg NaCl dm−3 ). 2.2. Analysis procedure The total arsenic concentration was measured off-line using an inductively coupled plasma mass spectrometry with a quadrupole ICP-MS operated in an He/H2 cell mode (Agilent HP 7500c, University of Oviedo), in order to avoid interferences of 40 Ar35 Cl with 75 As. To ensure the total solubility of arsenic, samples were diluted 1% HNO3 previously. The total aluminium or iron concentration was measured offline using an Inductively Coupled Plasma LIBERTY SEQUENTIAL VARIAN according to a standard method [17] (Atomic Emission Spectroscopy). To determine the total metal concentration, samples were diluted 50:50 (v/v) with 4 N HNO3 to ensure the total solubility of metal. The pH in the aqueous phase was measured using a pH 25 pHmeter (Crison Instruments, Spain). This equipment uses the 50 50 universal pH electrode and it is calibrated regularly with buffer solutions. The zeta potential values were measured in the aqueous solution using a Zetasizer Nano ZS (Malvern, UK). The Zetasizer Nano series calculates the zeta potential by determining the electrophoretic mobility (velocity of a particle in an electric field) and then applying the Henry equation. The electrophoretic mobility is obtained by performing an electrophoresis experiment on the sample and measuring the velocity of the particles using Laser Doppler Velocimetry.
Fig. 1. Evolution of arsenic concentration with electrical charge applied during electrocoagulation process using iron (a) and aluminium (b) electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
iron or aluminium electrodes, at two operating current densities, which define the typical range of current densities employed during electrocoagulation processes. It can be observed that both iron and aluminium electrodes achieve a significant removal of arsenic, being able to reduce the concentration down to five folds and letting to obtain an effluent meeting the standards of quality fixed by most of the environmental agencies. In addition, an electrical charge applied down to 0.5 Ah dm−3 is enough to decrease arsenic concentration under 10 g dm−3 . Then, electrocoagulation is a very efficient process in arsenic removal allowing to obtain highly pure waters. Fig. 2 shows the required dose of iron or aluminium to meet a 0.5 mg dm−3 and a stricter 10 g dm−3 (possible for iron and aluminium electrocoagulation) discharge limit. In the case of iron there is a great influence of the current density, being the process more efficient at lower current densities and exhibiting a maximum in the dosage of iron, which is more clearly observed in the case of the removal down to 0.5 mg As dm−3 and which suggests
3. Results and discussion Fig. 1 shows the changes in the concentration of arsenic when a synthetic groundwater polluted with arsenates (20 mg As dm−3 ) is electrocoagulated in a discontinuous operation mode with both,
Fig. 2. Influence of current density on the required dose of iron to obtain (䊉) 0.5 mg dm−3 and () 10 g dm−3 of arsenic residual, and the required dose of aluminium to meet () 0.5 mg dm−3 and () 10 g dm−3 of arsenic residual.
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
Fig. 3. Evolution of pH (a) and zeta potential (b) with electrical charge applied during electrocoagulation process using iron electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
that, on these conditions, the process is less efficient. On the other hand, in the case of aluminium, there is almost no influence of the current density on the required dosage to reach 0.5 mg dm−3 of As. Nevertheless, to reduce arsenic concentration until the value recommended by the most of environmental agencies aluminium electrocoagulation is less efficient due to higher requirement of metal dissolved in solution. In addition, iron electrocoagulation is more efficient than aluminium one when current densities below 2 mA cm−2 are applied while both electrocoagulation processes have very similar efficiencies working at high values of current density. Profiles of pH and z-potential during electrocoagulation with iron and aluminium are shown in Figs. 3 and 4, respectively, for the two cases studied in Fig. 1. In the case of iron electrocoagulation, it can be observed that pH increases during a first stage and then decreases down to a constant value. At the same time, z-potential decreases rapidly down to a minimum value, and then it increases to finally stabilize. In every case a negative value is obtained although neutrality seems to be closer in the case of lower current density. In the case of aluminium, a similar behaviour is obtained, although the pH range is smaller and final z-potential became almost nil during the experiments. This informs about a nil net charge on the surface of the particles of aluminium precipitates, and this suggests that adsorption of monomeric hydroxoaluminium species is very limited in this particular case. On the contrary, the negative charge observed for the z-potential in the case of iron electrocoagulation informs about the adsorption of hydroxyl ions or monomeric hydroxoferric species. In both cases (iron and aluminium), the minimum in the z-potential may be related to the adsorption of anions of arsenates onto the growing precipitates and the later neutralization of these precipitates with the electrochemical dosing of iron or aluminium reagents. Fig. 5 shows the final pH meets for every operation current density and also the efficiency in the dosing of reagent. It can be clearly observed that pH increases with the operation current density although the increases are less significant for high current densities. This can be explained taking into account production of reagent (Eq. (1)) and the main side reactions of the process: water
17
Fig. 4. Evolution of pH (a) and zeta potential (b) with electrical charge applied during electrocoagulation process using aluminium electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
oxidation and reduction (Eqs. (2) and (3)). Anode :
Al → Al3+ + 3e− +
2H2 O → O2 + 4H + 4e Cathode :
−
or Fe → Fe2+ + 2e−
−
H2 O + e → 1/2H2 + OH
(1) (2)
−
(3)
The higher the current density the greater the amount of hydroxyl anions produced. Some of these anions are neutralized with protons coming from side reaction (2). However, as it is shown in Fig. 5b, efficiencies in the dissolution of metals are very high (in the case
Fig. 5. pH values (a) and reagent dose (b) with current density applied during electrocoagulation process, using iron () or aluminium () electrodes. Discontinuous line corresponds to the theoretical iron (II) and continuous line to the theoretical Al(III).
18
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
Fig. 6. Solubility diagrams for iron (a) and arsenic (b) species during electrocoagulation (, 1 mA cm−2 , , 0.5 mA cm−2 , 䊉, 1.0 mA cm−2 , , 2.0 mA cm−2 , , 3.0 mA cm−2 , and ×, 4.0 mA cm−2 ) process.
of aluminium over 100%) and they only start to decrease for large values of the current density. This explains the smaller increase of the pH observed for large values of the current density. Fig. 6 displays the results of electrolysis at different current densities on a solubility diagram in which the solubility of the four main insoluble iron species is represented. The theoretical solubility curves show the concentrations of the different species plotted as a function of the pH, in the conditions of 25 ◦ C and nil ionic strength. These curves can be found in many works in literature [16]. Despite solubility diagram can merely be taken as a first approximation (because they depend among other factors on the ionic strength, temperature and on the particular species present in the treated water) they can be used in order to guess a mechanistic model of the processes that develops during coagulation and electrocoagulation processes. Moreover, they also can be used to explain the differences obtained between electrocoagulation with iron or aluminium electrodes. As it can be observed, in the case of iron electrocoagulation, solubilities of the two Fe(II) species shown in the graph are very similar, and during a first stage of the treatment they seem to limit the efficiency of the arsenic removal process, because obtained results slide down the solubility curves of Fe(AsO4 )2 and Fe(OH)2 . Increases in iron concentration lead to small decreases in the concentration of arsenic and points lay over the solubility of Fe(II) species. This suggests that Fe(II) is playing an important role on the results, as it was previously suggested by other authors for iron electrocoagulation [18]. Fe(II) is known to be oxidized rapidly to Fe(III) in the conditions used in these experiments. This oxidation acidifies the reaction media explaining the strange form of the pH curve in a second stage (observed for every current density) and also the significant decrease in the concentration of arsenic once points escape from the solubility zone of Fe(II), as Fe(II) species are more soluble than Fe(III) species. In this second zone, further increases in the concentration of iron achieve significant decreases in the concentration of soluble arsenic down to the limit fixed by the solubility of the iron and arsenic species. Curves obtained for every current density are very similar being the only difference their shift towards
Fig. 7. Solubility diagrams for aluminium (a) and arsenic (b) species during electrocoagulation (, 0.5 mA cm−2 , 䊉, 1.0 mA cm−2 , , 2.0 mA cm−2 , , 3.0 mA cm−2 , and ×4.0 mA cm−2 ) process.
higher pHs, due to the non-complete neutralization of the hydroxyl ions produce at the cathode surface, as it was explained previously. Results are not very different in the case of aluminium electrocoagulation (Fig. 7). In this case it can be observed a first step in which adsorption onto aluminium hydroxide is the main removal process and a second stage in which formation of aluminium arsenate seems to be the key process in the removal of arsenic (take in mind that solubility lines are only indicative as they are influenced by many factors). The higher solubility of aluminium arsenate compared to that of iron arsenic salts can explain the better efficiency in the results obtained in iron electrocoagulation above all for low values of current density. In addition, it can be observed a significant change in the pH during aluminium electrocoagulation although less marked than in the case of iron. 4. Conclusions The electrocoagulation is a very efficient technology in the removal of arsenates from water because it let to meet the standards of quality from drinking water, fixed by the most of environmental and health agencies, independently of the current density applied (0.1–4.0 mA cm−2 ) and/or the electrode material used (iron, aluminium). Then, electrocoagulation can be considered a robust technology for arsenate removal as these results are obtained in the whole typical range of operation current densities. However, aluminium electrocoagulation is less efficient for the removal of arsenic than iron electrocoagulation in the case of working at low current densities values due to the higher solubility of aluminium salts. Finally, pHs are maintained close to initial values because the current density slightly has influence on the pH in which the process works. Acknowledgements This work was supported by the MCT (Ministerio de Ciencia y Tecnología, Spain) and by the EU (European Union) through projects CTM2007-60472/TECNO, CTM2010-18833/TECNO and through the project CONSOLIDER-INGENIO 2010 (CSD2006-044).
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
References [1] M. Clara, F. Magalhaes, Arsenic. An environmental problem limited by solubility, Pure Appl. Chem. 74 (2002) 1843–1850. [2] M. Bissen, F.H. Frimmel, Arsenic – a review. Part II. Oxidation of arsenic and its removal in water treatment, Acta Hydrochim. Hydrobiol. 31 (2003) 97–107. [3] P.R. Kumar, S. Chaudhari, K.C. Khilar, S.P. Mahajan, Removal of arsenic from water by electrocoagulation, Chemosphere 55 (2004) 1245–1252. [4] J.R. Parga, D.L. Cocke, V. Valverde, J.A.G. Gomes, M. Kesmez, H. Moreno, M. Weir, D. Mencer, Characterization of electrocoagulation for removal of chromium and arsenic, Chem. Eng. Technol. 28 (2005) 605–612. [5] P. Lakshmipathiraj, S. Prabhakar, G.B. Raju, Studies on the electrochemical decontamination of wastewater containing arsenic, Sep. Purif. Technol. 73 (2010) 114–121. [6] N.S. Kumar, S. Goel, Factors influencing arsenic and nitrate removal from drinking water in a continuous flow electrocoagulation (EC) process, J. Hazard. Mater. 173 (2010) 528–533. ˜ [7] H.K. Hansen, P. Nunez, D. Raboy, I. Schippacasse, R. Grandon, Electrocoagulation in wastewater containing arsenic: comparing different process designs, Electrochim. Acta 52 (2007) 3464–3470. [8] N. Balasubramanian, T. Kojimab, C.A. Bashac, C. Srinivasakannan, Removal of arsenic from aqueous solution using electrocoagulation, J. Hazard. Mater. 167 (2009) 966–969. ˜ C. Montero-Ocampo, A.M. García-Lara, Energy and elec[9] J.F. Martínez-Villafane, trode consumption analysis of electrocoagulation for the removal of arsenic from underground water, J. Hazard. Mater. 172 (2009) 1617–1622.
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[10] J.A.G. Gomes, P. Daida, M. Kesmez, M. Weir, H. Moreno, J.R. Parga, G. Irwin, H. McWhinney, T. Grady, E. Peterson, D.L. Cocke, Arsenic removal by electrocoagulation using combined Al–Fe electrode system and characterization of products, J. Hazard. Mater. 139 (2007) 220–231. [11] N. Balasubramanian, T. Kojima, C. Srinivasakannan, Arsenic removal through electrocoagulation: kinetic and statistical modelling, Chem. Eng. J. 155 (2009) 76–82. ˜ C. Montero-Ocampo, Optimisation of energy consump[12] J.F. Martínez-Villafane, tion in arsenic electro-removal from groundwater by the Taguchi method, Sep. Purif. Technol. 70 (2010) 302–305. [13] N. Balasubramanian, K. Madhavan, Arsenic removal from industrial effluent through electrocoagulation, Chem. Eng. Technol. 24 (2001) 519–521. [14] C.A. Basha, S.J. Selvi, E. Ramasamy, S. Chellammal, Removal of arsenic and sulphate from the copper smelting industrial effluent, Chem. Eng. J. 141 (2008) 89–98. ˜ [15] P. Canizares, F. Martinez, M. Carmona, J. Lobato, M.A. Rodrigo, Continuous electrocoagulation of synthetic colloid-polluted wastes, Ind. Eng. Chem. Res. 44 (2005) 8171–8177. ˜ [16] P. Canizares, F. Martinez, J. Lobato, M.A. Rodrigo, Electrochemically assisted coagulation of wastes polluted with Eriochrome Black T, Ind. Eng. Chem. Res. 45 (2006) 3474–3480. [17] APHA-AWWA-WPCF, Standard Methods for the Examination of Water and Wastewater, seventeenth ed., American Public Health Association, Washington, DC, 1989. [18] H.A.C. Moreno, D.L. Cocke, J.A.G. Gomes, P. Morkovsky, J.R. Parga, E. Peterson, C. Garcia, Electrochemical reactions for electrocoagulation using iron electrodes, Ind. Eng. Chem. Res. 48 (2009) 2275–2282.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of arsenic by iron and aluminium electrochemically assisted coagulation ˜ Engracia Lacasa, Pablo Canizares, Cristina Sáez, Francisco J. Fernández, Manuel A. Rodrigo ∗ Department of Chemical Engineering, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 20 December 2010 Received in revised form 25 February 2011 Accepted 1 March 2011 Keywords: Electrocoagulation Arsenic Iron electrodes Aluminium electrodes
a b s t r a c t In this work, removal of arsenic by electrocoagulation with iron and aluminium electrodes is studied in a batch bench scale plant. Results demonstrate that both iron and aluminium electrocoagulations are a robust technology capable of removing arsenic down to 10 g dm−3 (the level fixed by most environmental and health agencies). Nevertheless, aluminium electrocoagulation is not as efficient as iron, one for the removal of arsenic when current densities below 2 mA cm−2 are applied. The effect of the current density (from 0.1 to 4.0 mA cm−2 ) is studied with both electrodes. Results show that this parameter influences slightly on the range of pH in which the process works (7–9) and also on the efficiency (always over 99.9%), but it is not a limiting parameter. Results can be easily explained taking into account the solubility of iron, aluminium and arsenates species present in the treated water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Contamination of ground and surface water by arsenic is an environmental concern of the major importance. That pollution mainly arise by natural processes (such as dissolution of arsenic minerals) and/or by anthropogenic activities (such as the uncontrolled mining and metallurgical industrial discharges, the application of arsenic-based pesticides or the dissolution of wood preservatives). Although arsenic polluted waters have been mainly reported in countries such as India or Bangladesh, its occurrence is actually a worldwide concern because it makes unviable the use of a natural water source for human consumption. Both, organic and inorganic species of arsenic may be found in waters. However, inorganic species are the primary ones, depending on the particular speciation upon redox and pH conditions. As(V) is the most stable species under aerobic conditions, existing as arsenates and its protonated forms (AsO4 3− , HAsO4 2− , H2 AsO4 − and H3 AsO4 ). As(III) is the predominant species in anaerobic environment, existing as arsenates and its protonated forms (AsO3 3− , HAsO3 2− , H2 AsO3 − and H3 AsO3 ). Arsenic is a carcinogen compound and its ingestion may affect the gastrointestinal tract, cardiac, vascular and central nervous system. For this reason, the environmental protection agencies throughout the world have suggested that the arsenic content in drinking water should be less than 10 g dm−3 .
∗ Corresponding author. Tel.: +34 902204100; fax: +34 926295318. E-mail address: [email protected] (M.A. Rodrigo). 1383-5866/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.03.005
During the recent years, many technologies have been studied to remove arsenic from waters [1]. Coagulation with iron and aluminium salts in the presence of polyelectrolytes followed by a filtration, adsorption onto different types of solids (activated alumina, activated carbon, activated bauxite, and clay minerals), ion-exchange and membrane technologies (mainly reverse osmosis and electro-dialysis) are among the most reported technologies. However, none of them have been reported in the reference technology. This can be explained because they present important drawbacks, such as the difficulties in meeting the standards of quality required by the environmental agencies, the cost or the lack of robustness. In addition, they perform better for the removal of As(V) than for the removal of As (III), being usually recommended to oxidize As(III) to As(V) by using different types of oxidants (ozone, chlorine, etc.) [2]. Moreover, the dosing of these reagents tends to bring down the water quality because of the residues and byproducts formed. Electrocoagulation or electrochemically assisted coagulation has been recently introduced as a promising technology in the removal of arsenic [3,4]. Several works have studied the effect on the removal of arsenic of the supporting electrolyte [5], cell potential [6], current intensity and/or reactor design [7–9] or electrodes system [10]. Results demonstrate the complexity of the system and the need for additional work to clarify this process. In this context, other recent works about aluminium and iron electrocoagulation try to clarify the complexity of electrocoagulation processes through statistical modeling rather than mechanistic understanding of the processes involved [11,12]. Also, the removal of arsenic from industrial wastes has been studied, focusing the
16
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
attention in this case on the treatability of highly arsenic polluted solutions [13,14]. In this context, the aim of this work is to study the effect of current density on the electrocoagulation with iron and aluminium electrodes and to try to develop a technology suitable for the removal of arsenic down to the limits fixed by health and environmental associations, increasing at the same time the knowledge about the mechanisms involved in the process. 2. Experimental 2.1. Experimental procedure Bench-scale electrocoagulation studies were used to characterize the treatability of synthetic waters. Electrocoagulation experiments were carried out in batch operation mode. Experimental setups and procedures have been widely described elsewhere [15,16]. In these experiments, the coagulant reagent came from the dissolution of iron or aluminium electrodes placed in a single compartment electrochemical flow cell. Both electrodes (anode and cathode) were square in shape (100 cm2 ) and the electrode gap was 9 mm. It is worth to state that in every case the anodic and cathodic materials were the same. This is a normal practice in industrial electrocoagulation processes, because this allows the inversion of the polarity as a response to avoid operation problems, which can be caused by the formation of films of carbonates on the surface of the cathodes, or by the passivation of the anodes. The electrical current was applied using a DC Power Supply FA376 PROMAX. The synthetic wastewater was stored in a glass tank (5000 cm3 ), and recirculated through the electrolytic cell by a peristaltic pump. The synthetic water is composed by sodium arsenate (20 mg As dm−3 ) and a supporting electrolyte to increase its conductivity (1000 mg NaCl dm−3 ). 2.2. Analysis procedure The total arsenic concentration was measured off-line using an inductively coupled plasma mass spectrometry with a quadrupole ICP-MS operated in an He/H2 cell mode (Agilent HP 7500c, University of Oviedo), in order to avoid interferences of 40 Ar35 Cl with 75 As. To ensure the total solubility of arsenic, samples were diluted 1% HNO3 previously. The total aluminium or iron concentration was measured offline using an Inductively Coupled Plasma LIBERTY SEQUENTIAL VARIAN according to a standard method [17] (Atomic Emission Spectroscopy). To determine the total metal concentration, samples were diluted 50:50 (v/v) with 4 N HNO3 to ensure the total solubility of metal. The pH in the aqueous phase was measured using a pH 25 pHmeter (Crison Instruments, Spain). This equipment uses the 50 50 universal pH electrode and it is calibrated regularly with buffer solutions. The zeta potential values were measured in the aqueous solution using a Zetasizer Nano ZS (Malvern, UK). The Zetasizer Nano series calculates the zeta potential by determining the electrophoretic mobility (velocity of a particle in an electric field) and then applying the Henry equation. The electrophoretic mobility is obtained by performing an electrophoresis experiment on the sample and measuring the velocity of the particles using Laser Doppler Velocimetry.
Fig. 1. Evolution of arsenic concentration with electrical charge applied during electrocoagulation process using iron (a) and aluminium (b) electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
iron or aluminium electrodes, at two operating current densities, which define the typical range of current densities employed during electrocoagulation processes. It can be observed that both iron and aluminium electrodes achieve a significant removal of arsenic, being able to reduce the concentration down to five folds and letting to obtain an effluent meeting the standards of quality fixed by most of the environmental agencies. In addition, an electrical charge applied down to 0.5 Ah dm−3 is enough to decrease arsenic concentration under 10 g dm−3 . Then, electrocoagulation is a very efficient process in arsenic removal allowing to obtain highly pure waters. Fig. 2 shows the required dose of iron or aluminium to meet a 0.5 mg dm−3 and a stricter 10 g dm−3 (possible for iron and aluminium electrocoagulation) discharge limit. In the case of iron there is a great influence of the current density, being the process more efficient at lower current densities and exhibiting a maximum in the dosage of iron, which is more clearly observed in the case of the removal down to 0.5 mg As dm−3 and which suggests
3. Results and discussion Fig. 1 shows the changes in the concentration of arsenic when a synthetic groundwater polluted with arsenates (20 mg As dm−3 ) is electrocoagulated in a discontinuous operation mode with both,
Fig. 2. Influence of current density on the required dose of iron to obtain (䊉) 0.5 mg dm−3 and () 10 g dm−3 of arsenic residual, and the required dose of aluminium to meet () 0.5 mg dm−3 and () 10 g dm−3 of arsenic residual.
E. Lacasa et al. / Separation and Purification Technology 79 (2011) 15–19
Fig. 3. Evolution of pH (a) and zeta potential (b) with electrical charge applied during electrocoagulation process using iron electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
that, on these conditions, the process is less efficient. On the other hand, in the case of aluminium, there is almost no influence of the current density on the required dosage to reach 0.5 mg dm−3 of As. Nevertheless, to reduce arsenic concentration until the value recommended by the most of environmental agencies aluminium electrocoagulation is less efficient due to higher requirement of metal dissolved in solution. In addition, iron electrocoagulation is more efficient than aluminium one when current densities below 2 mA cm−2 are applied while both electrocoagulation processes have very similar efficiencies working at high values of current density. Profiles of pH and z-potential during electrocoagulation with iron and aluminium are shown in Figs. 3 and 4, respectively, for the two cases studied in Fig. 1. In the case of iron electrocoagulation, it can be observed that pH increases during a first stage and then decreases down to a constant value. At the same time, z-potential decreases rapidly down to a minimum value, and then it increases to finally stabilize. In every case a negative value is obtained although neutrality seems to be closer in the case of lower current density. In the case of aluminium, a similar behaviour is obtained, although the pH range is smaller and final z-potential became almost nil during the experiments. This informs about a nil net charge on the surface of the particles of aluminium precipitates, and this suggests that adsorption of monomeric hydroxoaluminium species is very limited in this particular case. On the contrary, the negative charge observed for the z-potential in the case of iron electrocoagulation informs about the adsorption of hydroxyl ions or monomeric hydroxoferric species. In both cases (iron and aluminium), the minimum in the z-potential may be related to the adsorption of anions of arsenates onto the growing precipitates and the later neutralization of these precipitates with the electrochemical dosing of iron or aluminium reagents. Fig. 5 shows the final pH meets for every operation current density and also the efficiency in the dosing of reagent. It can be clearly observed that pH increases with the operation current density although the increases are less significant for high current densities. This can be explained taking into account production of reagent (Eq. (1)) and the main side reactions of the process: water
17
Fig. 4. Evolution of pH (a) and zeta potential (b) with electrical charge applied during electrocoagulation process using aluminium electrodes. Current density: () 0.5 mA cm−2 , () 3.0 mA cm−2 .
oxidation and reduction (Eqs. (2) and (3)). Anode :
Al → Al3+ + 3e− +
2H2 O → O2 + 4H + 4e Cathode :
−
or Fe → Fe2+ + 2e−
−
H2 O + e → 1/2H2 + OH
(1) (2)
−
(3)
The higher the current density the greater the amount of hydroxyl anions produced. Some of these anions are neutralized with protons coming from side reaction (2). However, as it is shown in Fig. 5b, efficiencies in the dissolution of metals are very high (in the case
Fig. 5. pH values (a) and reagent dose (b) with current density applied during electrocoagulation process, using iron () or aluminium () electrodes. Discontinuous line corresponds to the theoretical iron (II) and continuous line to the theoretical Al(III).
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Fig. 6. Solubility diagrams for iron (a) and arsenic (b) species during electrocoagulation (, 1 mA cm−2 , , 0.5 mA cm−2 , 䊉, 1.0 mA cm−2 , , 2.0 mA cm−2 , , 3.0 mA cm−2 , and ×, 4.0 mA cm−2 ) process.
of aluminium over 100%) and they only start to decrease for large values of the current density. This explains the smaller increase of the pH observed for large values of the current density. Fig. 6 displays the results of electrolysis at different current densities on a solubility diagram in which the solubility of the four main insoluble iron species is represented. The theoretical solubility curves show the concentrations of the different species plotted as a function of the pH, in the conditions of 25 ◦ C and nil ionic strength. These curves can be found in many works in literature [16]. Despite solubility diagram can merely be taken as a first approximation (because they depend among other factors on the ionic strength, temperature and on the particular species present in the treated water) they can be used in order to guess a mechanistic model of the processes that develops during coagulation and electrocoagulation processes. Moreover, they also can be used to explain the differences obtained between electrocoagulation with iron or aluminium electrodes. As it can be observed, in the case of iron electrocoagulation, solubilities of the two Fe(II) species shown in the graph are very similar, and during a first stage of the treatment they seem to limit the efficiency of the arsenic removal process, because obtained results slide down the solubility curves of Fe(AsO4 )2 and Fe(OH)2 . Increases in iron concentration lead to small decreases in the concentration of arsenic and points lay over the solubility of Fe(II) species. This suggests that Fe(II) is playing an important role on the results, as it was previously suggested by other authors for iron electrocoagulation [18]. Fe(II) is known to be oxidized rapidly to Fe(III) in the conditions used in these experiments. This oxidation acidifies the reaction media explaining the strange form of the pH curve in a second stage (observed for every current density) and also the significant decrease in the concentration of arsenic once points escape from the solubility zone of Fe(II), as Fe(II) species are more soluble than Fe(III) species. In this second zone, further increases in the concentration of iron achieve significant decreases in the concentration of soluble arsenic down to the limit fixed by the solubility of the iron and arsenic species. Curves obtained for every current density are very similar being the only difference their shift towards
Fig. 7. Solubility diagrams for aluminium (a) and arsenic (b) species during electrocoagulation (, 0.5 mA cm−2 , 䊉, 1.0 mA cm−2 , , 2.0 mA cm−2 , , 3.0 mA cm−2 , and ×4.0 mA cm−2 ) process.
higher pHs, due to the non-complete neutralization of the hydroxyl ions produce at the cathode surface, as it was explained previously. Results are not very different in the case of aluminium electrocoagulation (Fig. 7). In this case it can be observed a first step in which adsorption onto aluminium hydroxide is the main removal process and a second stage in which formation of aluminium arsenate seems to be the key process in the removal of arsenic (take in mind that solubility lines are only indicative as they are influenced by many factors). The higher solubility of aluminium arsenate compared to that of iron arsenic salts can explain the better efficiency in the results obtained in iron electrocoagulation above all for low values of current density. In addition, it can be observed a significant change in the pH during aluminium electrocoagulation although less marked than in the case of iron. 4. Conclusions The electrocoagulation is a very efficient technology in the removal of arsenates from water because it let to meet the standards of quality from drinking water, fixed by the most of environmental and health agencies, independently of the current density applied (0.1–4.0 mA cm−2 ) and/or the electrode material used (iron, aluminium). Then, electrocoagulation can be considered a robust technology for arsenate removal as these results are obtained in the whole typical range of operation current densities. However, aluminium electrocoagulation is less efficient for the removal of arsenic than iron electrocoagulation in the case of working at low current densities values due to the higher solubility of aluminium salts. Finally, pHs are maintained close to initial values because the current density slightly has influence on the pH in which the process works. Acknowledgements This work was supported by the MCT (Ministerio de Ciencia y Tecnología, Spain) and by the EU (European Union) through projects CTM2007-60472/TECNO, CTM2010-18833/TECNO and through the project CONSOLIDER-INGENIO 2010 (CSD2006-044).
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