Comparison of the Aluminum Speciation in ... - ACS Publications

Ind. Eng. Chem. Res. 2006, 45, 8749-8756

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APPLIED CHEMISTRY Comparison of the Aluminum Speciation in Chemical and Electrochemical Dosing Processes Pablo Can˜ izares, Fabiola Martıńez, Carlos Jime´ nez, Justo Lobato, and Manuel A. Rodrigo* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, UniVersidad de Castilla La Mancha, Campus UniVersitario s/n, 13005 Ciudad Real, Spain

The aim of this work is to study the differences between the hydrolyzing aluminum species formed in an aqueous solution when the aluminum is added by aluminum salt solution dosing and when it is supplied by electrodissolution. The dosing of aluminum is the first step in the coagulation processes, and it marks the more important differences between the coagulation and the electrocoagulation processes. It has been found that the speciation of aluminum in an aqueous solution does not depend directly on the dosing technology, but on the total concentration of aluminum and pH. This latter parameter changes in different ways for the solution dosing and the electrochemical dosing technologies, and this can be the main difference between both technologies: the pH value increases during the electrochemical process and decreases during the solution dosing process. In continuously operated processes, and feeding the solution dosing and the electrochemical dosing processes with solutions at different pHs (with the aim to obtain the same pH at the steady state), the results obtained in the speciation were nearly the same. More significant differences have been obtained in the comparison of the dosing processes for the discontinuous operation mode as it is impossible to maintain both the aluminum concentration and the pH at the same value. In the acidic range of pHs, the predominant species are the monomeric cationic hydroxoaluminum species. Increases in the pH lead to the coexistence of these monomeric species with increasing amounts of polymeric cations and precipitates. Under pHs close to neutrality, the predominant species are the aluminum hydroxide precipitates, and increases in the pH lead to the dissolution of the precipitates to form monomeric anionic hydroxoaluminum, which is the predominant species at alkaline pHs. The formation of precipitates is promoted in solutions containing sulfates. The ζ potential has been found to give important information and to depend mainly on the pH: pHs below 8 lead to positive values of the ζ potential, while higher values of pH cause negative ζ potentials. This behavior has been explained in terms of the formation of particles of aluminum hydroxide precipitate and of the adsorption of ionic species on their surface. 1. Introduction Particulate pollutants are present in domestic and industrial wastewaters. Natural waters also contain a wide variety of particulate impurities. To remove these particles, solid-liquid separation processes such as sedimentation, flotation, and/or filtration are used. However, the efficiency of these processes depends on the size of the particles. Thus, colloidal particles cannot be effectively removed, due to their small size. In addition, the surface of these colloids usually is negatively charged, and this causes repulsion forces and restrains the aggregation. To increase the size of the particles, and therefore the efficiency of the separation technologies, the addition of coagulant reagents is required (coagulation/flocculation). The aggregation of colloids can be accomplished by destabilization, reducing the repulsion forces between the particles. In this case, the main mechanisms of destabilization are the compression of the ionic layer due to an increase of the ionic strength and the neutralization of superficial charge by adsorption of counterions onto the colloidal particles. Furthermore, colloidal particles can * To whom correspondence should be addressed. Tel.: +34 902204100. Fax: +34 926 29 53 18. E-mail: [email protected].

be enmeshed in a growing hydroxide precipitate formed in the waste (sweep coagulation). The presence of polymeric species can also lead to the formation of bigger particles through interaction of different active groups of the polymer chains with different colloidal particles (bridge flocculation). Anyhow, the primary mechanisms of coagulation in each case will depend on the type of species formed in the system during the dosing of the coagulant reagents. The dosing of aqueous solutions containing aluminum salts (such as aluminum sulfate, aluminum chloride, and others containing prepolymerized aluminum) has been widely used to remove particulate impurities from supply waters and to remove pollutants from domestic and industrial wastewaters.1 Alternatively to the reagent-solution dosing, the coagulant can be electrogenerated in an electrochemical cell by the electrodissolution of an appropriate anode material (aluminum or iron). In this latter case, the coagulation process is known as electrocoagulation, and many works in the literature describe its performance in the treatment of different types of waters and wastewaters.2-7 In a first approach, when the aluminum ion (Al3+) is added to water (by both conventional dosing and electrochemical dissolution), some ionic monomeric hydrolysis species can be

10.1021/ie060824a CCC: $33.50 © 2006 American Chemical Society Published on Web 11/29/2006

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Figure 1. Diagram of the equilibrium of monomeric aluminum species with the amorphous aluminum hydroxide precipitate as a function of the aluminum concentration and pH.

formed, depending on the pH of the solution, as shown in eqs 1-5.

Al(OH)4- + H+ a Al(OH)3 + H2O

(1)

Al(OH)3 + H+ a Al(OH)2+ + H2O

(2)

Al(OH)2+ + H+ a Al(OH)+2 + H2O

(3)

Al(OH)+2 + H+ a Al+3 + H2O

(4)

Al(OH)3(s) a Al3+ + 3OH-

(5)

In this context, the formation of the aluminum hydroxide, which due to its low solubility leads to the formation of precipitates (eq 5), should also be considered. However, it is known that the aqueous chemistry of aluminum is especially complex, as it involves not only monomeric ionic species and the formation of precipitates, but also oligomeric and polymeric species. Due to this large number of species the quantification of the hydrolysis rate constants is a complicated task, and there are significant differences between the values reported in the literature.1 Normally, in the case of low aluminum dosages, it can be assumed that only monomeric species are generated, and it is possible to plot, as a function of pH, the concentrations of the monomeric ionic species in equilibrium with the amorphous hydroxide precipitate (Figure 1), which reproduces the experimental behavior with a great accuracy. Nevertheless, for nondiluted systems, it is necessary to consider the formation of polymeric species, and consequently, the speciation is more complex. Several analytical techniques have been employed to demonstrate the existence of these polymeric species. According to the literature, the most significant polymer is the tridecamer (Al13O4(OH)247+ or Al13), which has been widely characterized by X-ray8 and by 27Al NMR9-11 methods. Furthermore, 27Al NMR has been combined with other methods such as potentiometric titration12,13 and timed spectroscopy (ferron method)14-17 to characterize the hydrolysis aluminum species. The existence of different polymeric species has also been reported in studies of coagulation data18,19 and light-scattering studies.20 In addition to the influence of the aluminum concentration and pH, the solution matrix can also influence the speciation.

Thus, it is reported that solutions containing sulfate ions promote the formation of aluminum precipitates as compared to those in which chloride is the main anion.21,22 Taking all of this into account, the aim of this work has been to study the differences that exist between the hydrolyzing aluminum species formed in an aqueous solution when the aluminum is added by aluminum salt solution dosing and when it is supplied by electrodissolution. Many works describe the differences observed between coagulation and electrocoagulation, but no work has compared the species present in the solution. In fact, the speciation of aluminum has only been studied for conventional coagulation.9,10,16,18 Hence, the characterization of the species formed in the electrochemical process and the comparison of the species formed in both technologies are subjects of major importance. Due to the difficulties existing in the characterization of the different species of aluminum, in this work only three main categories have been considered: monomeric, polymeric, and precipitated aluminum. Analytical procedures for this speciation are widely discussed in the literature.14-17 To meet the goal of the work, the experiments were planned to meet similar conditions between both technologies, especially in terms of the aluminum dose and pH. For this purpose, the results of a previous study focused on the quantification of the electrodissolution of aluminum in electrocoagulation processes were used.23 The most remarkable point in this previous study is the superfaradic yield obtained in the electrochemical dissolution of aluminum, with values higher than 300% for alkaline pHs. Similar results were obtained by other authors.2,3,5 These results were explained in terms of the simultaneous chemical dissolution of the electrodes (eq 6). In addition to this, the aim of the work

2Al + 6H2O f 2Al3+ + 3H2 + 6OH-

(6)

includes studying the influence of the operation mode in the speciation of aluminum by solution dosing and by electrodissolution. As is known, although most research studies of coagulation are carried out in discontinuous-operation laboratory-scale or bench-scale plants, the full-scale plants normally operate in a continuous-operation mode. For this reason, it is important to establish the differences that can be found between both operation modes, as very few works study the influence of this parameter. 2. Experimental Section Experimental Devices. To characterize the hydrolysis species resulting from the addition of coagulants in the conventional and the electrochemical processes, several chemical and electrochemical experiments have been carried out in two benchscale plants described elsewhere.6,7 The electrochemical experiments have been carried out in a bench-scale plant, with a single-compartment electrochemical flow cell. Aluminum electrodes (HE 18) were used as the anode and cathode. Both electrodes were square in shape with a geometric area of 100 cm2 each and with an electrode gap of 9 mm. The electrical current was applied using a dc power supply, PROMAX FA-376. 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 an overhead stainless steel rod stirrer, Heidolph RZR 2041, thermostated by means of a water bath to maintain the temperature at the desired set point, and circulated through the electrolytic cell by a peristaltic pump.

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Figure 2. Changes in the total aluminum concentration, pH, ζ potential, and aluminum species (expressed as aluminum concentration) with time observed in a discontinuous electrochemical experiment (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; initial pH, 4; current density, 1.4 mA cm-2; volume, 2 dm3): (a) [, total aluminum concentration; (b) [, pH; 0, ζ potential; (c) 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

The chemical experiments have been carried out in a benchscale plant similar to the electrochemical one, where the electrochemical cell has been replaced by a single flow reactor (with the same geometry and without electrodes) and by including an aluminum solution (AlCl3 or Al2(SO4)3) dosage system with a peristaltic pump. Aluminum Speciation. The characterization of the hydrolyzed aluminum species generated has been carried out by the ferron method.14,24 This method consists of the timed spectroscopy monitoring of the aluminum ferron (8-hydroxy-7-iodo-5quinolinesulfonic acid) reaction to form a complex of probable composition17 Al(ferron)3 which has a maximum absorbance of 364 nm. Monomeric species react almost instantaneously with ferron, whereas polymeric species have a much slower reaction rate with this compound. The particles of precipitate practically do not react with ferron. Therefore, this method allows distinguishing among monomeric, polymeric, and precipitate species. The analytical measurement has been carried out by filtering the samples using micropore membranes of 0.45 µm to remove the particles of precipitate. Once the sample is filtered, an aliquot is added to the volume of saturated ferron solution freshly prepared (as ferron is not stable24) so that ferron is in excess at pH 5 in an acetate buffered solution. Immediately, the absorbance of the sample is monitored with time, until a constant value is obtained, which is indicative of the end of the reaction.

Figure 3. Changes in the total aluminum concentration, pH, ζ potential, and aluminum species (expressed as aluminum concentration) with time observed in a discontinuous chemical experiment (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; initial pH, 11; volume, 2 dm3): (a) [, total aluminum concentration; (b) [, pH; 0, ζ potential; (c) 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

By plotting the logarithm of the unreacted aluminum vs time, the ratio of aluminum species that react quickly and slowly with ferron (that is, monomeric and polymeric species) can be estimated. Extrapolation of the linear parts of the curve to zero time yields information on the amount of aluminum that is bound in complexes of different degrees of polymerization.14,17 The measurement of total and soluble aluminum (filtered with 0.45 µm) reports the ratio of soluble and precipitate aluminum. Experimental Procedure. Electrochemical experiments were carried out under galvanostatic conditions to supply aluminum to the system at a constant rate. Previous to every experiment the electrodes were treated with a solution of 1.30 M HCl to reject any effect due to the different prehistories of the electrodes. Solution dosing experiments were carried out by direct pumping of the coagulant solution to the reactor with a constant flow rate. The mixture of the aqueous solution (which represents the wastewater in a coagulation process) and the reagent solution takes place at the inlet of the reactor. To reach, at the end of these experiments, the same concentration of aluminum and pH (as those obtained in the electrochemical experiments), the pH of the feeding aqueous solution was modified at the beginning of each experiment by the addition of HCl, H2SO4, or NaOH. Then several experiments were carried out to find the one in which the final conditions were similar. In the continuous-operation mode, the aqueous solution (which models the wastewater in a coagulation process) was pumped from the feeding tank to the cell (or reactor), and then it was collected in a different tank. In the discontinuous-

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Figure 4. Dynamic responses of the aluminum concentration, pH, ζ potential, and aluminum species (expressed as aluminum concentration) observed in a typical continuous electrochemical experiment (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; initial pH, 4; current density, 1.4 mA cm-2; flow rate, 19 dm3 h-1): (a) [, total aluminum concentration; (b) [, pH; 0, ζ potential; (c) 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

operation mode, the aqueous solution was returned to the feeding tank. In both cases, samples were taken at the outlet of the cell. After the experiments, the samples were monitored by the ferron method, and the concentration of aluminum was measured off-line using an inductively coupled plasma Varian Liberty sequential instrument according to a standard method25 (plasma emission spectroscopy). Samples were previously diluted 50: 50 (v/v) with 4 N HNO3 to ensure the total solubility of aluminum. The ζ potential was measured using a Zetasizer Nano ZS (Malvern, U.K.). 3. Results and Discussion General Behavior Observed in the Chemical and the Electrochemical Discontinuous Dosing of Aluminum. Figures 2 and 3 show the changes with time of the total aluminum concentration, speciation of aluminum (momomeric, polymeric, and precipitate), pH, and ζ potential in an electrochemical and a chemical discontinuous experiment, respectively. As can be observed, the total aluminum concentration increases almost linearly with time, and the planning of the experiments allows similar values of this parameter to be obtained with both dosing technologies. On the contrary, at this point it is important to notice that although the objective of the experiments was to achieve simultaneously similar values of the pH and aluminum concentration, it was impossible to obtain them for the discontinuous-operation mode, due to the very different behavior of the pH in the chemical and the electrochemical experiments: The pH value increases with time in the electrochemical

Figure 5. Dynamic responses of the aluminum concentration, pH, ζ potential, and aluminum species (expressed as aluminum concentration) observed in a typical continuous chemical experiment (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; initial pH, 11; flow rate, 19 dm3 h-1): (a) [, total aluminum concentration; (b) [, pH; 0, ζ potential; (c) 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

experiment since the electrochemical system leads to the formation of aluminum hydroxide as a net final product (eqs 7-9).

anode: Al f Al3+ + 3e-

(7) +

2H2O f O2 + 4H + 4e

-

cathode: H2O + e- f 1/2H2 + OH-

(8) (9)

Conversely, it decreases during the chemical experiment as a consequence of the acid properties of the added aluminum solutions (to obtain similar concentrations of aluminum, AlCl3 solutions were used 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). After several preliminary experiments, and to try to maximize the period in which the pH profiles of both technologies fit, it was decided to treat a NaCl solution with a pH close to 5 by electrochemical technology and a solution with a pH slightly over 10 by reagent solution dosing.


Figure 6. Influence of the total aluminum concentration on the hydrolysis aluminum species formed in chemical and electrochemical continuous experiments for pH values near 7 (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; flow rate, 19 dm3 h-1). Electrochemical experiments: (a) concentration of aluminum species formed, (b) steady-state pHs achieved. Chemical experiments: (c) concentration of aluminum species formed, (d) steady-state pHs achieved. Key: 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

Figures 2c and 3c show the changes in the aluminum speciation. It can be observed that the changes of pH greatly influence the speciation, and very different results are obtained by both processes. In the electrochemical process, the pH increases with an increase of the aluminum concentration (Figure 2b), and these higher values of pH lead to progressive increases in the formation of aluminum precipitate with time (Figure 2c). The amount of polymeric species formed is rather low and almost constant during the entire experiment. Likewise, almost no formation of monomeric species is observed. In the case of the chemical experiment, the initial pH (close to 10) favors the formation of soluble species of aluminum. Thus, this can explain the prevalence of monomeric species predominant under these conditions (Figure 3c). The subsequent decrease in the pH leads to the formation of precipitates, reducing the presence of soluble monomeric and polymeric species. Finally, the pH value stabilizes below 5, producing partial redissolution of the precipitate (which achieves a constant concentration for this pH condition) to form polymeric and especially monomeric species. This explanation is supported by the ζ potential changes. It can be observed that, in the electrochemical process, the value of the ζ potential increases with time, to reach a constant value, this parameter being positive during the entire experiment. This could be indicative of the formation of particles of positively charged precipitate, and according to the literature,1 it could be explained in terms of the adsorption of monomeric and polymeric hydroxo cations on the surface of the precipitates, resulting in positively charged precipitates. In the case of the chemical experiment the ζ potential increases more progressively, from negative to positive values. Zero ζ potential is achieved for a pH value around 8. This behavior could be indicative of the formation of particles of precipitate that are initially negatively charged (due to the adsorption of monomeric hydroxo anions) and that later turn their charge to positive (due to the adsorption of hydroxo cations). Dynamic Responses of Aluminum Species Generated in the Continuous Processes. To characterize the behavior of a continuous system, both the dynamic response (changes in parameters from the start-up to the steady state) and the steadystate behavior must be studied.

To compare the dynamic response of the chemical and the electrochemical continuous dosing processes, some experiments were prepared to obtain similar steady-state concentrations of aluminum and pH values. As has been discussed previously, the changes of pH are different in the chemical and the electrochemical experiments. Thus, to achieve similar pH values in the steady state, the initial pHs of the treated solutions in the chemical and the electrochemical experiments were different. Figures 4 and 5 show the typical dynamic responses observed during two continuous electrochemical and chemical coagulation experiments, respectively. In part a of both figures, it can be observed that in both cases the total aluminum concentration increases to reach the steadystate value and that the time required to achieve these steadystate values is shorter for the chemical dosing experiment. Moreover, changes in the pH similar to those observed in the discontinuous experiments can be observed in the continuous experiments: in the electrochemical process the pH increases during the experiment (Figure 4b), whereas in the chemical one the pH decreases (Figure 5b). In this case (continuous experiments), the pH reaches a constant value corresponding to the steady-state value. Dynamic responses of the ζ potential obtained are also shown in Figures 4b and 5b. It can be observed that, in both cases, the ζ potential increases initially to reach a constant value, which is also similar in the chemical and the electrochemical experiments. Part c of both figures show the changes in the speciation of aluminum. In the electrochemical process (Figure 4c) it can be observed that practically all the aluminum concentration added forms precipitates, the concentrations of monomeric and polymeric species being negligible. In the chemical process the initial pH values (Figure 5b) lead to the formation of a small amount of monomeric and polymeric ions (Figure 5c), which disappear to form precipitate with the subsequent decrease of the pH to the steady-state value. Both experiments achieve similar hydrolysis species formed in the steady-state conditions. This fact supports that only the aluminum concentration and pH are responsible for the speciation and that the method of dosing aluminum only influences the value of both parameters but not directly the speciation.

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Figure 7. Concentration of aluminum species generated in chemical and electrochemical continuous experiments for pHs around 5 (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; flow rate, 19 dm3 h-1). Electrochemical experiments: (a) concentration of aluminum species formed, (b) steadystate pHs achieved. Chemical experiments: (c) concentration of aluminum species formed, (d) steady-state pHs achieved. Key: 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates.

Figure 8. Influence of the pH on the aluminum species formed in chemical and electrochemical continuous experiments for two different concentrations of aluminum (temperature, 25 °C; supporting medium, NaCl, 3000 mg dm-3; flow rate, 19 dm3 h-1): 0, monomeric hydroxoaluminum ions; 2, polymeric hydroxoaluminum ions; [, aluminum hydroxide precipitates (all expressed as percentages of the total aluminum concentration); (a) electrochemical experiments, total aluminum concentration 4 mg dm-3; (b) chemical experiments, total aluminum concentration 4 mg dm-3; (c) electrochemical experiments, total aluminum concentration 15 mg dm-3; (d) chemical experiments, total aluminum concentration 15 mg dm-3.

Influence of the Operating Parameters on the Aluminum Species Generated in the Continuous Processes at the Steady State. Figure 6 shows the influence of the aluminum concentration on the type of species formed in the system (by the chemical and the electrochemical modes of addition), at pH values around 7. It can be observed that precipitates are the main species in both cases and also that very small concentrations of polymeric and monomeric species appeared in the nonelectrochemical dosing method. The concentration of these species is still smaller in the electrochemical dosing method experiments. Figure 7 shows the results obtained in the experiments carried out at pHs around 5, for different aluminum concentrations. In this range of pH, the speciation is completely different and a coexistence of monomeric and polymeric species and precipitates is observed. For low doses of aluminum, monomeric

compounds are the primary species. The importance of these species decreases with an increase in the total concentration of aluminum, and at a high concentration of aluminum, the ratios of both polymeric species and precipitates increase significantly. The speciation is similar in both the electrochemical and the nonelectrochemical dosing technologies, and the small difference observed in the steady-state pH can explain the differences observed. Figure 8 shows the influence of the pH on the hydrolysis species generated in different continuous chemical and electrochemical experiments for values of aluminum added near 4 and 15 mg dm-3. It can be observed that, as reported in the literature, in strongly acidic pHs all the aluminum added to the system is soluble and monomeric cations are the primary species that coexist with a small ratio of polymeric cations. An increase in


Figure 9. ζ potential values obtained in several chemical and electrochemical continuous experiments for aluminum concentrations around 4 and 15 mg dm-3. Electrochemical experiments: 2, total aluminum concentration 4 mg dm-3; [, total aluminum concentration 15 mg dm-3. Chemical experiments: ×, total aluminum concentration 4 mg dm-3; 0, total aluminum concentration 15 mg dm-3.

anions present in the solution (mainly Al(OH)4-) under this pH condition, whereas lower pH values lead to the formation of particles of positively charged precipitate, due to the adsorption of hydroxo cations (monomeric or polymeric) present in the solution. The influence of the type of electrolyte is shown in Figure 10 for several chemical and electrochemical experiments carried out for different aluminum concentrations, in both sulfate and chloride media. In this figure, it can be observed that the presence of sulfate ions promotes the formation of precipitates, as compared to that of the chloride ions, and that a lower ratio of polymeric species is generated. This behavior has been previously reported in the literature1 and explained in terms of the promotion of the generation of amorphous aluminum hydroxide precipitates by sulfate ions. In addition, in the presence of polyaluminum salts, sulfate ions encourage the aggregation of the hydrolyzed aluminum species, leading to the formation of larger size polymer species24,25 that can be easily enmeshed in a growing precipitate, and consequently, they have been quantified as precipitates. Conclusions

Figure 10. Influence of the type of electrolyte on the aluminum species generated in several chemical and electrochemical experiments carried out in sulfate and chloride media (temperature, 25 °C; flow rate, 19 dm3 h-1; steady-state pH, 5): light gray bar, monomeric hydroxoaluminum ions; hatched bar, polymeric hydroxoaluminum ions; dark gray bar, aluminum hydroxide precipitates; (a) chemical experiments, total aluminum concentration 9 mg dm-3; (b) electrochemical experiments, total aluminum concentration 60 mg dm-3.

the pH leads to the formation of precipitates that coexist with polymeric and monomeric cations, the last ones being the predominant species. Values of pH close to neutrality cause the precipitation of most of the aluminum present in the solution. The dissolution of the precipitate takes place for pHs between 8 and 9 to form polymeric species and especially monomeric anions. For strongly alkaline pHs, the monomeric anions are the predominant species, whereas no formation of precipitates is observed under this pH condition. No significant differences are observed between the chemical and the electrochemical experiments when they are carried out under the same experimental conditions (of steady-state pH and aluminum concentration). Steady-state values of the ζ potential obtained in these (chemical and electrochemical) experiments are shown in Figure 9. It can be observed that pH values below 8.5 lead to positive ζ potentials in both chemical and electrochemical experiments, whereas higher pHs lead to negative ζ potentials. This observation is consistent with the formation of negatively charged precipitates for pHs over 8.5, due to the adsorption of hydroxo

From this work, the following conclusions can be drawn. (1) The speciation of aluminum in an aqueous solution does not depend directly on the dosing technology but on the total concentration of aluminum and pH. This latter parameter changes in different ways for the solution dosing and the electrochemical dosing technologies: the pH value increases during the electrochemical process since this process leads to the formation of aluminum hydroxide as a net final product; conversely, it decreases during the solution dosing process as a consequence of the acid properties of the added aluminum solutions (to obtain similar concentrations of aluminum, AlCl3 or Al2(SO4)3 solutions must be used due to the small solubility of aluminum hydroxide). (2) In the continuously operated process, and feeding the solution dosing and the electrochemical dosing processes with model wastewaters of different pHs (with the aim to obtain the same pH at the steady state), the results obtained in the speciation were nearly the same. More significant differences have been obtained in the comparison of the dosing process for the discontinuous process as, in this case, it is not possible to maintain both the aluminum concentration and the pH at the same value. (3) In the acidic range of pHs the primary species are the monomeric cationic hydroxoaluminum species. Increases in the pH lead to the coexistence of these monomeric species with increasing amounts of polymeric cations and precipitates. Under pHs close to neutrality the predominant species are the aluminum hydroxide precipitates, and increases in the pH lead to the dissolution of the precipitates to form monomeric anionic hydroxoaluminum, which is the primary species at alkaline pHs. The formation of precipitates is promoted in solutions containing sulfates. (4) The ζ potential has been found to give important information and to depend mainly on the pH: pHs below 8 lead to positive values of the ζ potential, whereas higher values of pH cause negative ζ potentials. This behavior can be explained in terms of the formation of particles of aluminum hydroxide precipitate, which are positively charged (due to the adsorption of cations from the solution) under pHs lower than 8. Higher values of pH produce a reversal of the charge, leading to negatively charged precipitates (due to anions from the solution) which cause negative ζ potentials.

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Acknowledgment This work was supported by the MCT (Ministerio de Ciencia y Tecnologıá, Spain) and by the EU (European Union) through Project CTM2004-03817/TECNO. Literature Cited (1) Duan, J. M.; Gregory, J. Coagulation by hydrolysing metal salts. AdV. Colloid Interface Sci. 2003, 100, 475. (2) Picard, T.; Cathalifaud-Feuillade, G.; Mazet, M.; Vandensteendam, C. Cathodic dissolution in the electrocoagulation process using aluminium electrodes. J. EnViron. Monit. 2000, 2, 77. (3) Jiang, J. Q.; Graham, N.; Andre´, C. A.; Kelsall, G. H.; Brandon, N. Laboratory study of electro-coagulation-flotation for water treatment. Water Res. 2002, 36, 4064. (4) Mollah, M. Y. A.; Morkovsky, P.; Gomes, J. A. G.; Kesmez, M.; Parga, J.; Cocke, D. L. Fundamentals, present and future perspectives of electrocoagulation. J. Hazard. Mater. B 2004, 114, 199. (5) Szpyrkowicz, L. Hydrodynamic Effects on the Performance of Electro-coagulation/Electro-flotation for the Removal of Dyes from Textile Wastewater. Ind. Eng. Chem. Res. 2005, 44, 7844. (6) Can˜izares, P.; Martıńez, F.; Carmona, M.; Lobato, J.; Rodrigo, M. A. Continuous Electrocoagulation of Synthetic Colloid-Polluted Wastes. Ind. Eng. Chem. Res. 2005, 44, 8171. (7) Can˜izares, P.; Martıńez, F.; Lobato, J.; Rodrigo, M. A. Electrochemically assisted coagulation of wastes polluted with erichrome black T. Ind. Eng. Chem. Res. 2006, 45, 3474. (8) Bottero, J. Y.; Tchoubar, D.; Caw, J. M.; Fiessinger, F. Investigation of the Hydrolysis of Aqueous Solutions of Aluminum Chloride. 2. Nature and Structure by Small-Angle X-ray Scattering. J. Phys. Chem. 1982, 86, 3667. (9) Solomentseva, I. M.; Gerasimenko, N. G.; Barany, S. Surface properties and aggregation of basic aluminium chloride hydrolysis products. Colloids Surf., A 1999, 151, 113. (10) Exall, K. N.; vanLoon, G. W. Effects of raw water conditions on solution-state aluminum speciation during coagulant dilution. Water Res. 2003, 37, 3341. (11) Zhang, P.; Hahn, H. H.; Hoffmann, E.; Zeng, G. Influence of some additives to aluminium species distribution in aluminium coagulants. Chemosphere 2004, 57, 1489. (12) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. Studies of Hydrolyzed Aluminum Chloride Solutions. 1. Nature of Aluminum Species and Composition of Aqueous Solutions. J. Phys. Chem. 1980, 84, 2933. (13) Perry, C. C.; Shafran, K. L. The systematic study of aluminium speciation in medium concentrated aqueous solutions. J. Inorg. Biochem. 2001, 87, 115.

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ReceiVed for reView June 28, 2006 ReVised manuscript receiVed October 10, 2006 Accepted October 21, 2006 IE060824A

Comparison of the Aluminum Speciation in ... - ACS Publications

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