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Some of the advantages of the electrocoagulation method are the simple equipment required and the easy automation of the process (18, 30−33); as it ...
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Environ. Sci. Technol. 2006, 40, 6418-6424

Coagulation and Electrocoagulation of Wastes Polluted with Dyes PABLO CAN ˜ I Z A R E S , F A B I O L A M A R T IÄ N E Z , C A R L O S J I M EÄ N E Z , J U S T O L O B A T O , A N D MANUEL A. RODRIGO* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

Dyes are common pollutants in a large variety of industrial wastewaters, and the treatment of these wastes by coagulation has been extensively studied in the literature. This work is focused on the comparison of the efficiencies of the chemical and the electrochemical coagulation processes with hydrolyzing aluminum salts, and it tries to determine the similarities or differences that exist between the two coagulation processes. To do this, Eriochrome Black T solutions were used as a model of dye-polluted wastewater, and experiments of both coagulation technologies were planned to meet the same operation conditions. The pH, the aluminum concentration, the type of electrolyte, and the mode of dosing of aluminum were found to influence the process. Moreover, the speciation of aluminum was found to be the key parameter to explain the results, in terms of the mechanisms previously proposed in the literature for dissolved organic matter coagulation.

Introduction Textile industries generate large volumes of wastewaters polluted with dyes. Due to their high molecular weighs, their complex structures, and especially their high solubilities in water, they persist once discharged into a natural environment. Thus, their removal from industrial effluents is also a subject of the major importance from the environmental point of view. Different treatments and combinations of treatments have been proposed in the literature to effectively manage the textile wastewater. Thus, chemical coagulation (1-8), biological treatment (9, 10), fenton treatment (4, 11), electrochemical oxidation (5, 12), ozonation (6, 7, 13), activated carbon adsorption (8, 13, 14), ultrafiltration (15), and electrocoagulation (16-25) are among the most studied technologies. The removal of dissolved organic matter by coagulation is widely reported in the literature (26, 27). Thus, two mechanisms are supposed to be primary for the removal of dissolved organic matter by coagulation (17, 28, 29): binding of metal species to anionic sites of the organic molecules, neutralizing their charge and resulting in reduced solubility, and the adsorption of organic substances on amorphous metal hydroxide precipitates. The coagulation of dissolved organic matter takes place by the addition of hydrolyzing metal salts to the waste, and although both mechanisms can occur simultaneously, depending on the metal species present in the solution, one of the mechanisms will be the predominant. Moreover, the type * Corresponding author phone: +34 902204100; fax: +34 926 29 53 18; e-mail: [email protected]. 6418

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of species present in the solution will be a function of several parameters, with the most important being the concentration of metal ions, the pH, the ratio between the hydroxyl and metal ions (OH-/ Mn+), and the type of ions present in the solution. In case of addition of aluminum salts as coagulants, monomeric and polymeric ionic hydroxoaluminium species, as well as amorphous aluminum hydroxide precipitates can be formed in the solution, according to the literature (28, 29). Coagulation can be accomplished either by the dosing of a solution containing the coagulant reagent or by the in situ generation of coagulants by electrolytic oxidation of an appropriate anode material (e.g., iron or aluminum) (electrocoagulation). Some of the advantages of the electrocoagulation method are the simple equipment required and the easy automation of the process (18, 30-33); as it does not require any addition of chemicals, the dosing of coagulant reagents depends directly on the cell potential (or current density) applied. Other advantages of electrocoagulation include the promotion in the flocculation process, caused by the turbulence generated by the oxygen and the hydrogen evolution that produces a soft mix, and helps the destabilized particles to link among them and to generate bigger particles (electroflotation). In addition, these oxygen and hydrogen bubbles formed can help to increase the efficiency of the separation process (electroflotation) by carrying the particles to the top of the solution where they can be more easily removed. The results obtained allow classifying electrocoagulation as one of the most promising methods for treating textile wastewater. In this context, the goal of this work is to compare, for both continuous and batch processes, the efficiencies of the chemical and the electrochemical coagulation processes with hydrolyzing aluminum salts, and to determine the similarities or differences that exist between the two coagulation processes. Since the continuous operation mode is more frequently used in full-scale applications, more attention has been paid to it. To meet the objective, experiments of both coagulation technologies have been carried out in the same operation conditions and the results have been interpreted in terms of the mechanisms previously proposed in the literature for dissolved organic matter, in order to determine the advantages and disadvantages of both technologies. To do this, Eriochrome Black T (EBT) solutions were used as model pollutant. This compound is an azoic dye which is also called Mordant Black 11 with a lot of uses in the dyeing industry and in analytical chemistry. This molecule suffers from ionization of several functional groups in water, and its grade of ionization depends on the pH. This report of the work is organized into four different sections. The three first describe the experimental behavior of the coagulation and electrocoagulation systems. The last section (mechanistic model) interprets the results in terms of the aluminum speciation.

Experimental Section Oil/Water Emulsions. The oily phase of the emulsion is composed of a common lubricant oil (Repsol Elite TDI 15W40 provided by Repsol-YPF, Spain) and a soluble oil (SOL 1000 provided by Molydal, France). To prepare the emulsion, the same amounts of both lubricant and soluble oils (50:50 w/w) were mixed and stirred until a homogeneous liquid was obtained. Then, the supporting electrolyte (NaCl or Na2SO4) dissolved in osmotized water was added slowly while the mixture was being stirred to finally obtain the oil-in-water emulsion. 10.1021/es0608390 CCC: $33.50

 2006 American Chemical Society Published on Web 09/08/2006

Experimental Devices. The coagulation experiments were carried out in a bench-scale plant (described elsewhere (34, 35)). In the electrochemical experiments, the coagulant reagent came from the dissolution of aluminum electrodes (type HE 18) placed in a single compartment electrochemical flow cell. Both electrodes (anode and cathode) were square (10 cm sides) and the electrode gap was 9 mm. The electrical current was applied using a DC Power Supply FA-376 Promax. The voltage and the current flowing through the cell were measured with a multimeter (Keithley 2000 Digital Multimeter). The EBT solution was stored in a 5000 cm3 glass tank, stirred by an overhead stainless steel rod stirrer (HeidolpH RZR 2041), and circulated through the electrolytic cell by a peristaltic pump. A thermostated-bath allowed maintaining the temperature at the desired set point. To carry out the chemical coagulation experiments the experimental bench-scale plant was modified by changing the electrochemical flow cell to a single flow reactor (with the same geometry) and by including an aluminum solution (of AlCl3 or Al2(SO4)3) dosage system. Experimental Procedure. Electrochemical coagulation experiments were carried out under galvanostatic conditions. Prior to every experiment the electrodes were treated with a solution of 1.30 M HCl to reject any effect due to the different history of the electrodes. In the continuous operation mode, the EBT solution was pumped from the feed tank to the cell (or reactor) and then it was collected in a different tank. Samples were taken at the outlet of the cell, and after 2400 s of settling they were filtered. Subsequently, the absorbance spectrum (using an UV-visible spectrophotomer Shimadzu UV-1603) and the pH (using an inoLab WTW pH meter) were measured in 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. COD was measured using a Hach DR2000 analyzer. Several discontinuous experiments were carried out by recirculating the effluent from the electrochemical cell to the feed tank. Chemical coagulation experiments followed the same procedure. However, in this case the aluminum was added by dosing a solution containing AlCl3 or Al2(SO4)3. In addition, some discontinuous chemical coagulation experiments were carried out in a standard jar test experimental setup. In these later experiments, a fixed amount of coagulant was added to the EBT solution, and after that the solution was stirred vigorously. The procedures to take and measure the samples were the same as those used for the continuous electrocoagulation experiments. Measurement of the Aluminum Dissolved in the Cell. The accurate measurement of the total aluminum dissolved in the cell in every experiment is difficult due to the formation of two phases after the coagulation-flocculation process. Consequently, many sources of errors can be present in the analyses. To obtain precise data, every experiment (continuous or batch) was repeated maintaining the same operation conditions and changing the EBT solution to an aqueous solution with the same concentration of electrolyte. These experiments were used to quantify the aluminum supplied by the electrochemical system. Results obtained in preliminary experiments demonstrate that the concentration of aluminum is not affected by EBT and then a simple and more accurate measurement is obtained with the procedure proposed. After the experiments, the concentration of aluminum was measured off-line using an inductively coupled plasma Liberty Sequential Varian instrument according to a standard method (36; plasma emission spectroscopy). To determine the total aluminum concentration samples were diluted 50:50 v/v with 4 N HNO3.

FIGURE 1. Dynamic responses of aluminum concentration, pH, and absorbance obtained in several chemical and electrochemical coagulation experiments of solutions containing EBT in sulfate media. Temperature 25 °C; flow rate 19 dm3 h-1; EBT concentration 100 mg dm-3; supporting media Na2SO4 3000 mg dm-3. Electrochemical experiments: current density 1.4 mA cm-2, [ initial pH 4, × initial pH 6. Chemical experiments: 0 initial pH 6, O initial pH 11.

Results and Discussion Dynamic Response of the Continuous Coagulation Processes. Although most of the coagulation studies reported in the literature are carried out in discontinuous-operation labscale or bench-scale plants, the full-scale plants normally operate in a continuous-operation mode. It is assumed that the behavior would be similar but very few works compare the performance of the two operation modes (35). For this reason, one of the goals of this work was to confirm this fact for the treatment of wastes polluted with dyes. In a continuous-operation mode it is important to characterize both the dynamic response (changes in parameters from the startup to the steady-state) and the steadystate values of all parameters. To compare the dynamic response of the chemical and electrochemical coagulation processes, some experiments were prepared to attain the same steady-state concentration of aluminum in the treated waste. Likewise, to know the concentration of aluminum dissolved in the electrochemical experiments, a preliminary study was carried out to characterize the aluminum dissolution process in the electrochemical cell (35, 37). The most remarkable point in this previous study is the super-faradaic yield obtained in the electrochemical dissolution of aluminum, especially for alkaline pHs. Similar results were obtained VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison between the chemical and electrochemical continuous coagulation experiments, influence of the initial EBT concentration. Temperature 25 °C; flow rate 19 dm3 h-1. O Electrochemical experiments: current density 1.4 mA cm-2, initial pH 4. × Chemical experiments: initial pH 6. Supporting media: (a) Na2SO4 3000 mg dm-3, (b) NaCl 2450 mg dm-3.

FIGURE 2. Dynamic responses of aluminum concentration, pH, and absorbance obtained in several chemical and electrochemical coagulation experiments of solutions containing EBT in chloride media. Temperature 25 °C; flow rate 19 dm3 h-1; EBT concentration 100 mg dm-3; supporting media: NaCl 2450 mg dm-3. Electrochemical experiments: current density 1.4 mA cm-2, [ initial pH 4, × initial pH 6. Chemical experiments: 0 initial pH 6, O initial pH 11. by Picard et al. (38). These results can be explained in terms of the simultaneous chemical dissolution of the electrodes (37). Figures 1 and 2 show the typical dynamic responses (changes in parameters from the startup to the steady-state of the experiments) observed during several continuous chemical and electrochemical coagulation experiments, in the coagulation of solutions polluted with EBT, containing sulfate and chloride as supporting electrolyte. In Figures 1a and 2a it can be observed that the time required to achieve the steady-state concentration of aluminum is shorter for the chemical coagulation experiments and that, in both cases, it is lower than 10 times the hydraulic residence time of the reactor. In Figures 1b and 2b it can be observed that the changes in the pH obtained in the chemical and the electrochemical experiments are different. In the electrochemical process the pH increases during the experiments as this process causes the formation of aluminum hydroxide as a net final product (34, 35, 37). On the other hand, the pH decreases in the transitory-state in the chemical process as a consequence of the acid properties of the coagulant added, Al3(SO4)2 or AlCl3 (as it was not possible to add the aluminum concentration formed by the electrochemical procedure as an aluminum hydroxide solution due to its small solubility). 6420

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In Figures 1c and 2c it can be observed that both processes are able to diminish the color of the EBT solutions. However, for similar aluminum concentrations added, the efficiency of the processes depends significantly on the pH achieved in the steady-state. Thus, in sulfate media (Figure 1c) high color removals are obtained by the electrochemical experiment that attained a steady-state pH below 6, and by the chemical experiment in which the steady-state pH is below 5. The worst results were obtained in the electrochemical experiment reaching a pH value higher than 7. In chloride media (Figure 2c) good color removals were attained by chemical and electrochemical experiments that reached steady-state pHs below 6. As it can be observed (Figures 1c and 2c) an inverse response of absorbance with time (successive increases and decreases of the absorbance) is obtained in several experiments, with this behavior being characteristic of complex processes, and it may be explained by the coexistence of the two primary coagulation mechanisms and by the strong influence of the aluminum species (which depends on the pH and aluminum concentration). Influence of the Operating Parameters in the Continuous Coagulation Processes. To study the influence of the different operating parameters in the chemical and the electrochemical continuous coagulation processes, several experiments were planned with the aim to obtain the same steady-state pH and aluminum concentration. Figure 3 shows the influence of the EBT concentration in the continuous chemical and the electrochemical coagulation processes, in sulfate and chloride media for similar conditions of pH and aluminum concentration in the steady state. Two different behaviors can be observed. In the electrochemical process, the removal of EBT decreases with the increase in the EBT concentration, and this effect is more marked in chloride-containing solutions. On the other hand, in the chemical process the removal percentage improves with the increase in the EBT concentration, with this tendency being stronger in sulfate media (lower efficiencies are obtained for low EBT concentrations). Figure 4 shows the influence of the pH in the continuous coagulation processes. It can be observed that the chemical

FIGURE 4. Influence of the steady-state pH achieved in the chemical and electrochemical experiments. Temperature 25 °C; flow rate 19 dm3 h-1; EBT concentration 100 mg dm-3. O Electrochemical experiments: current density 1.4 mA cm-2. × Chemical experiments. Supporting media: (a) and (b) Na2SO4 3000 mg dm-3; (c) and (d) NaCl 2450 mg dm-3.

FIGURE 6. Influence of the supporting electrolyte in the continuous chemical coagulation experiments. Temperature 25 °C; flow rate 19 dm3 h-1; EBT concentration 100 mg dm-3; initial pH 6. Supporting media: 9 Na2SO4, ] NaCl.

FIGURE 5. Comparison of the chemical and electrochemical continuous coagulation processes as a function of the aluminum concentration. Temperature 25 °C; flow rate 19 dm3 h-1; EBT concentration 100 mg dm-3. O Electrochemical experiments: initial pH 4. × Chemical experiments: initial pH 6. Supporting media: (a) Na2SO4 3000 mg dm-3, (b) NaCl 2450 mg dm-3. and the electrochemical technologies yield similar removals of EBT in both sulfate and chloride media. Better results are obtained at slightly acidic and near neutral pHs, while almost no color removal is obtained for pHs values higher than 7. It can be observed that in the acidic range of pH there are two maximums of removal of color, one of them for pHs near 2, and the other one for pH values around 5-6. The variation of the EBT removal with the aluminum concentration for both chemical and electrocoagulation processes in both chloride and sulfate media is shown in Figure 5. It can be observed that the supporting medium does not have a significant influence in the case of the electrocoagulation process, achieving slightly higher values

of EBT removal in sulfate media. A different behavior can be seen when the chemical coagulation is used. At higher values of aluminum concentration the media has no influence, whereas at lower values better results are obtained in chloride media. Figure 6 shows the influence of the concentration of electrolyte in the chemical coagulation process. Similar results are obtained in both media studied, and removal of EBT decreases slightly with the increase in the concentration of supporting electrolyte. Taking into account these results, the compression of the diffuse ionic layer by an increase in the ionic strength can be neglected as an important coagulation mechanism of EBT. Influence of the Operation Mode in Coagulation Processes. The influence of the operation mode in the electrochemical process was studied in a previous work of our group (35). It was found that the discontinuous operation mode yields better removals of EBT for low aluminum concentration, while high aluminum concentrations lead to similar results in both operation modes. In this work, to study the influence of the operation mode and the way of addition of aluminum, several chemical discontinuous experiments were carried out: by a jar test standard method and by recirculating the outlet of the chemical reactor to the feed tank (in the former case the addition of aluminum is abrupt (all the aluminum is added once) while in the latter the aluminum is added progressively VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Influence of the operation mode and aluminum-addition type in the chemical coagulation of EBT in sulfate media. Temperature 25 °C; EBT concentration 100 mg dm-3; supporting media: Na2SO4 3000 mg dm-3. × Continuous experiments, initial pH 6. Chemical discontinuous experiments: O Discontinuous experiments (recirculation to the feed tank), initial pH 10. 2 Discontinuous experiments (jar test) for initial pH 4. 4 Discontinuous experiments (jar test) for initial pH 6. to the waste). Figure 7 shows the results obtained in the different types of chemical coagulation experiments in sulfate media. It can be observed that the highest EBT removals are obtained for the discontinuous jar-test experiments (with abrupt addition of aluminum) for initial pH near 4, as lower aluminum concentrations achieve higher efficiencies. Second, the continuous experiments attain better results than the discontinuous jar-test for initial pH around 6 and the discontinuous experiments with progressive addition. Finally, the lowest efficiencies are reached by the batch process with progressive addition of coagulant. Moreover, all the percentages of removal of EBT are lower than those achieved by the continuous electrocoagulation experiments (see Figure 5a). Mechanistic Model. According to the concentration-pH aluminum-species diagram (35), 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, and they also coexist with polymeric hydroxoaluminium species (29). As a result, positively charged precipitates are formed (aluminum hydroxide with adsorbed hydroxoaluminium cations on surface). The charge of these precipitate become negative when the pH increases (adsorption of hydroxoaluminium anions). In strongly alkaline media, monomeric hydroxoaluminium anions are the main species in solution. In addition to the effect of the pH, the types of aluminum species formed are also influenced by the type of electrolyte present in the solution. Thus, it is reported that sulfate ions promote the generation of amorphous aluminum hydroxide precipitates (29) and consequently the speciation of aluminum can be different in chloride and sulfate media with a higher ratio of insoluble species in the latter case. Finally, the way of addition of aluminum in the coagulation process can influence the species generated in the system (39). Thus, for the same amount of aluminum, in an abrupt addition (such as in the jar test experiments) there is more free aluminum available to form an amorphous precipitate or to form polymeric cationic species. A progressive addition 6422

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of aluminum can lead to the binding of aluminum species initially formed with the EBT molecules previous to further additions, with the formation of the precipitate being less favored in these conditions than in the abrupt addition of aluminum (there is less aluminum available). Taking into account the qualitative speciation of aluminum, the experimental observations obtained in this work can be easily explained. Thus, with regard to the influence of the pH, there are two maximums in the removal of EBT, and they can be explained in terms of the coagulation mechanisms (that are related to the aluminum speciation and the structure of EBT in solution (35). (1) The maximum of color removal observed for pHs close to 2 can be explained by the binding of aluminum monomeric hydroxocations to the negative groups of EBT molecules (-SO3-) and the subsequent formation of reduced solutibility compounds (Figure 8a). (2) The high EBT removals attained for slightly acid pHs can be explained in terms of the binding of EBT anionic groups (-SO3-, -O-) to the positively charged sites (chemisorption) of the aluminum hydroxide precipitates (Figure 8d), and also to the binding of these groups (-SO3-, -O-) to polymeric hydroxoaluminum cations (Figure 8b). In addition, EBT molecules can be enmeshed into growing particles of aluminum hydroxide precipitate (Figure 8c). (3) For pHs above 7 there is no color removal because under these pH conditions the repulsion forces inhibit the adsorption and the binding mechanisms. Thus in this range of pHs the main aluminum species are monomeric hydroxoaluminum anions and negatively charged precipitates (precipitates with anions adsorbed onto their surface), and the EBT molecule is even more negatively charged (35). Regarding the differences obtained in the chemical and the electrochemical continuous methods, they can be explained in terms of the different changes of the pH in both processes: the chemical process tends to diminish the pH of the solution to values near 5, leading to a primary mechanism of binding of the EBT molecules to the cationic aluminum species, while the electrochemical process tends to increase the pH toward neutrality, causing a predominant adsorption (by binding of charged groups of EBT to charged sites of the precipitate) or enmeshment (if the concentration is enough for sweep flocculation) of molecules onto the precipitates. In case of the influence of the EBT initial concentration, the following was observed: (1) In the electrochemical process the color removal decreases with the EBT concentration, and it can be explained by taking into account that in case of the adsorption mechanism, if the number of positively charged sites (of the precipitates) is below the stoichiometric requirements of EBT, no removal of the surplus EBT can occur. In this case, the particles of precipitate are big enough to be separated, so further flocculation processes are not important. (2) In the chemical process the efficiency increases with the EBT concentration, indicating that the flocculation process should play an important role in the process. In this case, the binding of EBT molecules to cationic hydroxoaluminum species (monomeric or polymeric) produces compounds with reduced solubility, but the formation of bigger particles (by the aggregation of small particles) is necessary, and this flocculation process is favored by the number of particles in the system (34). The differences observed in the treatment of solutions containing sulfates and chlorides can be explained by taking into account that the formation of aluminum hydroxide precipitate is more favored in the sulfate medium, leading to the promotion of the adsorption of molecules onto the precipitate in this medium. For this reason, better results are obtained in the electrochemical process for the treatment of solutions in sulfate medium (due to the final pH the

FIGURE 8. Scheme of the coagulation mechanisms of EBT: (a) binding of monomeric hydroxo-aluminum cations with the anionic sites of EBT molecules; (b) binding of aluminum cationic polymeric species with the anionic sites of EBT molecules; (c) enmeshment of EBT molecules in particles of precipitate; and (d) adsorption of EBT molecules onto the surface of positively charged precipitate of aluminum hydroxide. electrochemical coagulation favors the formation of precipitates). In the same way, in chloride medium the binding mechanism is favored (due to the predominance of soluble species of aluminum), and therefore, in the chemical process better results are obtained in the treatment of chloridecontaining solutions. Finally, with regard to the operation mode and the way of addition of aluminum, the differences observed can be explained in terms of the different species of aluminum formed. In the chemical coagulation process the main coagulation mechanism should be binding of the polymeric hydroxoaluminum cations to anionic sites of the EBT molecules. Under the typical pH condition of the chemical experiments (around 5), the monomeric and polymeric cations are the predominant species, although they coexist with precipitates. Hence, in the jar test (batch) experiments the addition of aluminum is carried out in an abrupt way, the waste receives the dose of aluminum at a fixed time. In the batch experiments with recirculation of the effluent to the feed tank, the addition of aluminum is progressive (the wastewater is continuously recycled into the reactor and the concentration of aluminum increases continuously with time during an experiment). This means that for a given dose of aluminum, in the jar test experiments the aluminum produced can form a bigger amount of polymeric species, as there is a higher “free aluminum” concentration in the moment of addition, and consequently, the binding of polymeric cations to EBT molecules can start in the jar test experiments for smaller aluminum concentration added. By contrast, in the batch process with progressive addition of aluminum, 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), so in this case, the monomeric species can predominate with respect to polymeric ones, and the binding of polymeric cations to EBT molecules is less favored. The continuous operation mode is an intermediate situation

between the two batch ways of addition, and intermediate results are obtained. Hence, the speciation of aluminum has been found to be the key parameter to explain the coagulation results, in terms of mechanisms previously proposed in the literature for dissolved organic matter coagulation.

Acknowledgments This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through project CTM2004-03817/TECNO.

Supporting Information Available Table including the most important results of aluminum concentration generated in the continuous electrocoagulation process as a function of the steady-state pH and the electrical charge passed. Figures including diagram of monomeric aluminum species as a function of pH and aluminum concentration, scheme of the structure and ionization of Eriochrome Black T in solution, and scheme of the coagulation and flocculation processes that take place with the binding mechanisms. Influence of the operation mode in the electrochemical process as reported in ref 35. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 7, 2006. Revised manuscript received July 6, 2006. Accepted July 21, 2006. ES0608390