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Electrochemically Assisted Coagulation of Wastes Polluted with Eriochrome Black T P. Can˜ izares, F. Martı´nez, J. Lobato, and M. A. Rodrigo* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, UniVersidad de Castilla La Mancha, Campus UniVersitario s/n, 13071 Ciudad Real, Spain

The electrochemically assisted coagulation of wastes polluted with Eriochrome Black T (EBT; an azoic dye) is studied. The pH is the most significant parameter, and good removal efficiencies are only obtained for pH values under 6. The EBT removal percentages increase with the electrical current charge passed up to a given value and then they stabilize. The concentration of electrolyte does not seem to influence the removal efficiencies. However, electrocoagulations in sulfate media achieve better results than those carried out in chloride media. The operation mode (continuous or batch) does not affect the removal of EBT at high concentrations of aluminum dosed. For lower concentrations, the discontinuous process attains better removal of EBT for the same amount of aluminum electrogenerated. Two primary coagulation mechanisms can explain the experimental observations: the binding of aluminum hydroxo cations to EBT anionic sites at strongly acidic pH values, and the adsorption of EBT molecules on the surface of the positively charged aluminum hydroxide precipitate at close-to-neutral pH values. Introduction Dyes are colored substances with complex chemical structures (many functional groups) and high molecular weights. These compounds are also highly soluble in water and persistent, once discharged into a natural environment. Thus, their removal from industrial effluents is a subject of major importance from the environmental point of view. Different methods and some combinations of them are used to treat dyeing wastewater, among them biological treatment,1,2 chemical coagulation,3-5 ozonation,4-6 activated carbon adsorption,6,7 ultrafiltration,8 and electrocoagulation.9-18 Coagulation methods involve the addition of coagulant reagents to the wastewater. These reagents promote the formation of insoluble particles from the dyes, allowing the removal of the pollutants from the waste by the subsequent promotion of the aggregation of the particles (flocculation) and the final separation by means of conventional settling or flotation. In the chemical coagulation process, the addition of hydrolyzing metal salts (of iron (Fe3+) or Al3+) as coagulant reagents is typical, while the electrochemical method involves the in situ generation of coagulants by electrolytic oxidation of an appropriate anode material (e.g., iron or aluminum). The removal of dissolved organic matter by coagulation is widely reported in the literature.19-22 The high molecular weights of the dyes, their complex structures, and especially their high solubilities in water make the dye molecules model compounds in the study of this kind of coagulation.9-14 In this context, these molecules are also used as model compounds of large-molecule pollutants in the study of other treatment techniques such as advanced oxidation processes.23 According to the literature, the primary mechanisms for the removal of dissolved organic matter by coagulation are the following:20 (1) the binding of metal species to anionic sites of the organic molecules, neutralizing their charge and resulting in reduced solubility; (2) adsorption of organic substances on amorphous metal hydroxide precipitate. * To whom correspondence should be addressed. Tel.: +34 902 20 41 00. Fax: +34 926 29 53 18. E-mail: [email protected].

Both mechanisms take place as a consequence of the addition of hydrolyzing metal salts to a waste that contains dissolved organic matter. However, the predominant mechanism will depend on the concentration of metal ions, the pH, and the ratio between the hydroxide and Mn+ ions (OH-/Mn+). According to the literature,20 for strongly acidic pHs the primary species are monomeric hydroxometallic cations. For this reason, only the first mechanism is promoted under this condition. At higher pHs (acidic and close to neutrality), both polymeric hydroxometallic cations and metal hydroxide precipitates coexist. Under these conditions it is reported that many positively charged active sites are found on the surface (as a consequence of the adsorption of hydroxometallic cations). This would give a strong adsorption and also charge neutralization, if there are anionic charges due to ionized functional groups in the organic molecules. Consequently, the second mechanism predominates. For higher pHs the net charge in the surface of the amorphous metal hydroxide precipitate changes from positive to negative. This means that coagulation can only occur if there are positively charged groups in the dissolved organic compound. Finally, in strongly alkaline media monomeric hydroxometallic anions are the primary species. Hence, only the first mechanism can explain the coagulation in this range of pHs. The goal of this paper is to evaluate the electrochemically assisted coagulation (without promoting electroflocculation or electroflotation processes) in a continuous operation bench-scale plant, and to study the influence of the different operating parameters in the electrocoagulation processes. To do this, Eriochrome Black T (EBT) solutions are used as a model pollutant. This compound is an azoic dye that is also called Mordant Black 11 with a lot of uses in the dyeing industry and in analytical chemistry. The structure of EBT is shown in Figure 1. Figure 1 also shows the ionization that the functional groups of this molecule suffer in water as a function of the pH. Experimental Section Experimental Devices. The electrocoagulation experiments have been carried out in a bench-scale plant that can work in both continuous and batch operation modes and has a single

10.1021/ie051432r CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006

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Figure 1. Scheme of the Eriochrome Black T structure. Ionization of EBT in aqueous media as a function of pH.

stainless steel rod stirrer and thermostatized by means of a water bath, which allowed maintaining the temperature at the desired set point. The electrolyte was circulated through the electrolytic cell by means of a peristaltic pump. Continuous Operation Mode Assays. Electrochemical coagulation experiments were carried out in the continuous operation bench-scale plant, under galvanostatic conditions. The electrolyte was pumped into the cell and collected in a tank. Samples were taken at the outlet of the cell, and after 40 min of settling, they were filtered. Subsequently, the absorbance spectrum (using a Shimadzu UV-1603 UV-visible spectrophotomer) and the pH (using an inoLab WTW pH meter) were measured to the filtered liquid. To estimate the removal of EBT, both the absorbance at 550 nm (corrected by the pH value) and the chemical oxygen demand were used (550 nm is the wavelength that shows the maximum absorbance). COD was measured using a HACH DR2000 analyzer (standard method24). The aluminum concentration was measured using a Liberty Sequential Varian inductively coupled plasma according to a standard method24 (plasma emission spectroscopy). Preparative experiments were carried out to determine the amount of solution volume required to reach the steady state. In every experiment this final state was tested. The range of electrical charge passed studied was 0.0047-0.0170 A h dm-3. To determine the influence of the pH, the feeding solution pH was changed in the range 1-11. To determine the influence of the EBT concentration, EBT solutions with concentrations in the range 50-200 mg dm-3 were studied. To test the influence of the supporting electrolyte, sodium sulfate concentrations in the range 100-5000 mg dm-3 were tested. Moreover, some experiments were carried out using NaCl as supporting electrolyte to study the influence of the type of electrolyte present in the solution. Previous to each experiment the electrodes were treated with a solution of 1.30 M HCl to reject any effect due to the different prehistory of the electrodes. The amount of aluminum electrogenerated by the applied current at the different operation conditions was determined carrying out several experiments,25 in which the electrolyte consisted of only Na2SO4, and NaOH or H2SO4 added for any subsequent pH adjustment (without any EBT). Batch Operation Mode Assays. The discontinuous experiments were carried out by recirculating the effluent from the electrochemical cell to the feed tank. The procedures to take and measure the samples were the same as those used for the continuous electrocoagulation experiments. Results and Discussion

Figure 2. Layout of the bench-scale plant used to carry out electrocoagulation experiments. (a) Continuous operation mode. (b) Batch operation mode. (c) Section of the electrochemical cell.

compartment electrochemical flow cell (Figure 2). Aluminum electrodes (HE 18) were used as the anode and cathode. Both electrodes were square (100 mm side) with a geometric area of 100 cm2 each and an electrode gap of 9 mm. The electrical current was applied using a Promax FA-376 dc power supply. The current flowing through the cell was measured with a Keithley 2000 digital multimeter. The electrolyte was stored in a 5000 mL glass tank stirred by a Heidolph RZR 2041 overhead

Dynamic Response of the Continuous Electrocoagulation Process. Figures 3 and 4 show the changes in the main parameters during two typical continuous electrocoagulation experiments. As a result of the electrode dissolution and the water reduction processes, both the aluminum concentration and the pH increase. The increase in the ionic strength reduces the electrolyte resistance and makes the cell potential decrease. Likewise, the presence of aluminum promotes the combination of dye molecules to form insoluble compounds and consequently the color of the solution decreases. Each of these parameters reaches a steady-state value for times lower than 10 times the hydraulic residence time of the cell. The main difference between the experiments is the presence of a minimum in the absorbance in the second one (Figure 4). This minimum does not correspond to a similar change in the total aluminum concentration, and it is obtained in all experiments in which

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Figure 3. Dynamic responses of aluminum concentration, absorbance, pH, and cell potential obtained in a typical electrocoagulation experiment. Temperature, 25 °C; flow rate, 19 dm3 h-1; EBT concentration, 100 mg dm-3; supporting medium, 3000 mg dm-3 Na2SO4; current density, 1.4 mA cm-2; initial pH, 4.

Figure 4. Dynamic responses of aluminum concentration, absorbance, pH, and cell potential obtained in an electrocoagulation experiment. Temperature, 25 °C; flow rate, 19 dm3 h-1; EBT concentration, 100 mg dm-3; supporting medium, 3000 mg dm-3 Na2SO4; current density, 1.4 mA cm-2; initial pH, 9.

the steady-state pH is close to neutrality (range 6-8). However, a similar change is obtained in both pH and conductivity; this observation is indicative of changes in the aluminum species present in the solution (the formation of a precipitate leads to an increase in the cell potential and in the pH). According to the process-dynamic theory, this kind of dynamic response (inverse response) is characteristic of complex processes, and it may be explained by the coexistence of at least two primary coagulation mechanisms. This will be discussed below with more detail. Another important observation is related to the amount of aluminum generated in both cases (experiments shown in Figures 3 and 4). The measured values exceed the values calculated if the process is considered to be only electrochemical (if it is considered that aluminum can only come from anodic oxidation of the aluminum sheet at the current intensity maintained in the cell). This fact is also observed in all the experiments carried out in this work. Thus, Figure 5 shows the

amount of aluminum generated in the electrocoagulation process as a function of the electrical charge passed and of the pH. It can be observed that the experimental values are above the theoretical ones calculated by the Faraday law25,26 in all cases, and that the amount of aluminum generated seems to depend strongly on the pH of the treated solution, as the values measured at alkaline pH are sensibly higher than the ones obtained at acidic and neutral pHs. These strange efficiencies (super-faradaic efficiencies) have been previously reported in the literature,25-27 and they have been explained in terms of the chemical dissolution of the aluminum electrode. Thus, in a previous work of our group25 we determined that the chemical dissolution rate of aluminum sheets is several orders of magnitude higher at alkaline pHs than at neutral or acidic pHs. It was also obtained that, to model the electrodissolution process, it was very important to consider the pH profiles between the anode and the cathode as the electrochemical oxidation and reduction of water can significantly modify the pH on the anode and on the

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Figure 7. Diagram of solubility of monomeric aluminum species as a function of aluminum concentration and pH.

Figure 5. Concentration of aluminum generated in the electrochemically assisted coagulation process as a function of electrical charge passed and pH. Temperature, 25 °C; flow rate, 19 dm3 h-1; supporting medium, 3000 mg dm-3 Na2SO4. (a) Initial pH, 4. s, Faraday’s law; [, experimental values. (b) s, Faraday’s law for 0.5 mA cm-2; [, experimental values for current density 0.5 mA cm-2; ---, Faraday’s law for 1.4 mA cm-2; 0, experimental values for current density 1.4 mA cm-2.

Figure 6. Influence of pH on removal of EBT in the continuous electrocoagulation process. Temperature, 25 °C; flow rate, 19 dm3 h-1; EBT concentration, 100 mg dm-3; supporting medium, 3000 mg dm-3 Na2SO4; current density, 1.4 mA cm-2.

cathode surfaces with respect to the bulk pH, and this significantly affects the chemical dissolution process. This is especially important on the cathode surface, where the pH can become strongly alkaline even in case of acidic pHs in the bulk electrolyte. Influence of Wastewater Characteristics. Figure 6 shows the influence of the pH on the electrochemical decolorization of the azoic dye solution. It can be seen that high removals of pollutant can only be achieved for acidic and near neutral pHs. According to the concentration-pH aluminum species diagram shown in Figure 719 in the acidic range of pHs, monomeric hydroxoaluminum cations are the primary species while at less acidic and near neutral pHs these species coexist with aluminum hydroxide precipitates. According to other works in the literature,20 they also coexist with polymeric hydroxoaluminum species. As a result, positively charged precipitates are formed (aluminum hydroxide with adsorbed hydroxoaluminum cations

on the surface). The charge of this precipitate becomes negative when the pH increases (adsorption of hydroxoaluminum anions). In strongly alkaline media, monomeric hydroxoaluminum anions are the main species in solution. Thus the primary coagulation mechanism in the strongly acidic range of pHs should be the binding of aluminum hydroxo cations to EBT anionic sites. However, the higher efficiency observed in close-to-neutral pHs should be explained in terms of the adsorption of EBT molecules on the surface of the positively charged aluminum hydroxide precipitate that is the predominant species formed at close-to-neutral pH. This mechanism can combine with the binding mechanism, as monomeric and polymeric hydroxo cations are also present in solution. An increase in the bulk pH leads to a precipitate with less positively charged sites, and in a slightly alkaline range of pHs, the predominant species in the media is the negatively charged aluminum hydroxide precipitate. These negative charges repel the ionized sites of the EBT molecules (which have no positively charged groups), and avoid the adsorption of the molecules on the precipitate. This supports the abrupt decrease in the removal of color observed in this range of pH. At strongly alkaline pHs, monomeric anionic species are present, and due to the repulsion forces, no binding with the EBT molecules will occur. It is important to note the big changes in the removal of EBT observed for pHs close to 6. These changes lead to instability in the dynamic responses as can be seen in Figure 8, which shows two different responses achieved for the same experimental conditions. The experiment that reaches a steady-state value of pH less than 6 attains high removals, whereas the other experiment reaching pH above 6 at the steady state obtains lower EBT removals. This supports the importance of the adsorption mechanisms for this range of pHs. In this zone of pH, monomeric and polymeric cationic species coexist with the positively charged precipitates. A slight increase in the pH value can lead to an important increase in the ratio of precipitates (and thus to a decrease in the monomeric and polymeric cations) and also to a lower charge of the precipitates. Consequently, the binding and the adsorption onto the precipitate mechanisms are disfavored. Figure 9 shows the influence of the initial EBT concentration on the efficiency of the continuous electrocoagulation of EBT solutions. It can be observed that the EBT removal is maintained almost constant for low values of EBT concentration and then it decreases. This behavior explains the importance of the charge neutralization in the coagulation of EBT (either by adsorption on positively charged sites or by combination of EBT with cations). If the number of positively charged sites (or cations) is below the stoichiometric requirements of EBT, the dye

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Figure 10. Influence of supporting electrolyte in continuous electrocoagulation experiments. Temperature, 25 °C; flow rate, 19 dm3 h-1; current density, 1.4 mA cm-2; initial pH, 4. (a) EBT concentration, 100 mg dm-3; supporting medium, Na2SO4. (b) Supporting media: [, 3000 mg dm-3 Na2SO4; 0, 3000 mg dm-3 NaCl.

Figure 8. Dynamic responses of absorbance and pH with time obtained in two continuous electrocoagulation experiments. Temperature, 25 °C; flow rate, 19 dm3 h-1; EBT concentration, 100 mg dm-3; supporting medium, 100 mg dm-3 Na2SO4; current density, 1.4 mA cm-2; initial pH, 4.

Figure 9. Steady state of EBT removal achieved in continuous electrocoagulation experiments as a function of EBT concentration. Temperature, 25 °C; flow rate, 19 dm3 h-1; current density, 1.4 mA cm-2; initial pH, 4; supporting medium, 3000 mg dm-3 Na2SO4.

removal percentage decreases as the surplus EBT remains in solution. This also shows that the flocculation process is not very important in the removal of dyes. Thus, in a typical coagulation process26 the higher the pollutant concentration the higher the removal obtained, due to a promotion in the flocculation process (the number of collisions between particles depends on the number of particles). Figure 10 shows the influence of the concentration and type of supporting electrolyte. In case of the supporting electrolyte concentration, it can be seen that the concentration of salt does not seem to play an important role in the electrocoagulation process, and thus the increase in the ionic strength has no influence in the removal of the EBT from solution in the range studied. However, in the comparison of sulfate and chloride media, it can be observed that better results are obtained using Na2SO4 as supporting electrolyte. This can be explained by

taking into account that the species of aluminum formed are influenced by the type of electrolyte present in the solution. Thus it is reported that sulfate promotes the generation of amorphous aluminum hydroxide precipitates and also the formation of sulfate hydroxoaluminum complexed ions.20 These species can differ substantially with respect to those generated in the chloride media and justify the differences observed. Influence of Operating Conditions and Operation Mode. Figure 11a shows the influence of the electrical charge passed (Q) in the steady-state decolorization obtained in the continuous electrocoagulation of EBT solutions. In all these experiments the flow rate (q) was maintained at a constant value. Consequently, to modify the electrical charge passed (I/q), the applied intensity was modified in each experiment (I). Thus, the fluid dynamic conditions can be considered to be maintained almost constant in every case, except for the formation of bubbles due to oxygen or hydrogen evolution. It can be observed that the EBT removal increases with the current charge passed up to a given value (0.008 A h dm-3), and then it stabilizes. Figure 11b shows the influence of the flow rate and that of the current density (both are modified simultaneously) in experiments in which the electrical charge passed is maintained constant. Higher current densities (or lower hydraulic retention times) lead to less efficient processes. Figure 11a suggests that under these conditions the primary coagulation mechanisms can be the binding of the aluminum species to the anionic sites of the EBT molecules to form reduced solubility compounds or the adsorption of the dyes onto positively charged aluminum hydroxide precipitate. Thus the addition of amounts of aluminum below the stoichiometric ratio leads to a decrease in the efficiency. This mechanism also supports that an excess of coagulant reagent does not lead to a decrease in the efficiency of the process. In this context, Figure 11b can be explained in terms of the smaller concentration of aluminum reached with the increasing flow rate, as the smaller hydraulic residence time leads to a small contribution of the chemical dissolution rate and the higher current density to a increase in the water oxidation process and, so, to a decrease in the electrochemical contribution. Figure 12 shows the comparison between the continuous and

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Figure 11. Influence of electrical charge passed and flow on steady-state removal of EBT achieved. (a) Temperature, 25 °C; flow rate, 19 dm3 h-1; EBT concentration, 100 mg dm-3; supporting medium, 3000 mg dm-3 Na2SO4; initial pH, 4. (b) Temperature, 25 °C; EBT concentration, 100 mg dm-3; supporting medium, 3000 mg dm-3 Na2SO4; initial pH, 4; electrical charge passed, 0.008 A h dm-3.

continuous processes is instantaneous (the wastewater only passes through the cell once) and in the batch process is progressive (the wastewater is continuously recycled into the cell and the concentration of aluminum increases continuously with time during an experiment). This means that, for a given dose of aluminum, the formed aluminum species can be different: in the batch process the aluminum species formed can react with the EBT previous to further additions of aluminum (in the following pass of the waste through the cell) and in the continuous process a given body of waste receives abruptly all its corresponding aluminum. Thus, in the latter case there is a higher free aluminum concentration in the moment of addition and thus the aluminum produced can form bigger particles of charged aluminum hydroxide precipitates (the aluminum enmeshed inside the particle is not effective), and the formation of polymeric species is not favored. By contrast, the batch process is going to promote more particles (and so a higher charged surface) and also more polymeric hydroxo cationic species. This means that aluminum is more efficiently used in the batch process. Consequently, the positively charged aluminum species are in a higher ratio in the batch processes and the efficiencies in the low aluminum concentration range are higher. Conclusions The electrochemically assisted coagulation processes can be successfully applied in the treatment of wastes polluted with EBT. The pH is the most significant parameter, and good removal efficiencies are only obtained for pH less than 6. Two primary coagulation mechanisms can explain the experimental observations. Thus, in a strongly acidic range of pHs the primary mechanism should be the binding of aluminum hydroxo cations to EBT anionic sites. However, the higher efficiency observed in close-to-neutral pHs should be explained in terms of the adsorption of EBT molecules on the surface of the positively charged aluminum hydroxide precipitate that is the predominant species formed at close-to-neutral pH. This mechanism can combine with the binding mechanism (as monomeric and polymeric hydroxo cations can also be present in solution). These mechanisms are consistent with the observed decreases in efficiency with the increase in the EBT concentration and with the decrease in the specific electrical charge passed, as both support the existence of a stoichiometric ratio between the pollutant and the coagulant reagent. Likewise, these mechanisms explain the differences that exist between the results obtained in the continuous and the batch processes. Acknowledgment

Figure 12. Comparison between continuous and discontinuous electrocoagulation processes. Temperature, 25 °C; current density, 1.4 mA cm-2; initial pH, 4; supporting medium, 3000 mg dm-3 Na2SO4; EBT concentration, 100 mg dm-3. ([) Discontinuous process: volume, 1.5 dm3; time, 18 min. (0) Continuous process: flow rate, 19 dm3 h-1.

discontinuous electrocoagulation of EBT synthetic wastewaters as a function of the aluminum concentration generated in the processes. It can be observed that both processes obtain the same removal of EBT for a high concentration of aluminum. However, the efficiencies obtained by the discontinuous process are higher in the range of low aluminum concentration. Conversely, the changes in the pH are similar in both processes. To explain the differences between both operation modes, it has to be considered that the addition of aluminum in the

This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through Project No. CTM2004-03817/TECNO. Literature Cited (1) Kapdan, I. K.; Alparslan, S. Application of anaerobic-aerobic sequential treatment system to real textile wastewater for color and COD removal. Enzyme Microb. Technol. 2005, 36, 273. (2) Alinsafi, A.; da Motta, M.; Le Bonte, S.; Pons, M. N.; Benhammou, A. Effect of variability on the treatment of textile dyeing wastewater by activated sludge. Dyes Pigm. 2006, 69, 31. (3) Georgiou, D.; Aivazidis, A.; Hatiras, J.; Gimouhopoulos, K. Treatment of cotton textile wastewater using lime and ferrous sulphate. Water Res. 2003, 37, 2248. (4) Barredo-Damas, S.; Iborra-Clar, M. I.; Bes-Pia, A.; Alcaina-Miranda, M. I.; Mendoza-Roca, J. A.; Iborra-Clar, A. Study of preozonation influence on the physical-chemical treatment of textile wastewater. Desalination 2005, 182, 267.

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ReceiVed for reView December 22, 2005 ReVised manuscript receiVed March 8, 2006 Accepted March 16, 2006 IE051432R