Synthesis of Polyaluminum Chloride with a Membrane Reactor

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Ind. Eng. Chem. Res. 2004, 43, 12-17

Synthesis of Polyaluminum Chloride with a Membrane Reactor: Operating Parameter Effects and Reaction Pathways Zhiqian Jia,† Fei He, and Zhongzhou Liu* Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing, P. R. China

Polyaluminum chloride (PAC) is an important flocculant in potable water and wastewater treatment. Al13 or Alb is regarded as the most efficient species of PAC. A low base addition rate, small base drop sizes, and sufficient mixing usually favor the formation of PAC with higher Alb contents. In this paper, PAC was originally synthesized with a membrane reactor in which NaOH solution permeated through the micropores of an ultrafiltration membrane into AlCl3 solution gradually to reduce the NaOH droplets size to nanoscale, about 106 times smaller than that of conventional methods, resulting in a great reduction in the local supersaturation and precipitates generated and, subsequently, in an increase in the Alb content. The effects of the membrane molecular weight cutoff (MWCO) and reactant concentration on the species distribution were investigated. It was found that Alb increases with decreasing MWCO and reactant concentration under the experimental conditions used. A new plausible reaction pathway is also proposed that assumes that the hydrolysis and polymerization of Al3+ constitute a complex net of consecutive and parallel reactions and that the species distribution is mainly determined by the reactions kinetics. Introduction When AlCl3 solution is partially neutralized via the addition of base, polyaluminum chloride (PAC) results, which can serve as an important flocculant in potable water and wastewater treatment.1 Al13 {tridecameric polycation, [AlO4Al12(OH)(24+n)(H2O)(12-n)](7-n)+}, is regarded as the most efficient species of PAC because of its higher electric charge and stability; its structure has been confirmed by 27Al NMR spectroscopy.2,3 The reaction between PAC and ferron reagent is also employed to differentiate PAC into three fractions:4 Ala (mononuclear Al), Alb (reactive polynuclear, e.g., Al13), and Alc (larger polynuclear and solid-phase aluminum trihydroxides). It appears that the Alb fraction corresponds to Al13 in freshly prepared solution.5 Therefore, the ferron-Al reaction can be used for the determination of Al13. The formation of Al13 is related to the solution composition and reaction conditions, such as reactant concentration, base addition rate, basicity, mixing, etc. It seems that the formation of Al13 depends critically on an interfacial disequilibrium between the Al-containing solution and the base being injected.6,7 A local highpH zone would favor the formation of the aluminate ion, Al(OH)4-, which would provide the requisite tetrahedral coordination for the core atoms of the tridecamer.8 Generally speaking, at higher reactant concentrations, lower rates of base addition and smaller base drop sizes favor the synthesis of PAC with higher Alb contents. Thus, various methods, e.g., adding base by capillary with an i.d. of 0.1 mm,9 electrodialysis,10 electrolysis,11 etc., have been devised to reduce the base addition rates and drop sizes. * To whom correspondence should be addressed. Tel.: 86010-62849195. E-mail: [email protected]. † Present affiliation: Zhongguancun 1st Street, Haidian District, Photochemistry Department, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.

Here, a novel method is proposed for the synthesis of PAC in which hollow-fiber ultrafiltration (UF) membranes are employed to add aqueous NaOH into AlCl3 solution gradually under the transmembrane pressure. It is well-known that the micropores at the surface of UF membranes are very small, i.e., several nanometers or tens of nanometers in mean diameter. Therefore, the size of the drops of NaOH solution permeating and the local supersaturation can be greatly reduced, resulting in a decrease in the generation of precipitates and an increase in the Alb content. In this paper, the effects of the membrane molecular weight cutoff (MWCO) and reactant concentration on the species distribution were investigated. A new plausible reaction pathway is also proposed. Experimental Section Equipment. The membrane module was fitted with 10 pieces of hollow-fiber UF membranes (produced by Zhongke Membrane R&D Center, Beijing, China), each 1.0 mm in inner diameter and 0.13 m in length. The membrane material and MWCO were as follows: PS/ PDC (30 000 Da) and PS/PDC (10 000 Da). In this experiment, a BT00-300M peristaltic pump (Lan’ge Instrument Company of Baoding, China), an LZB stainless steel glass flowmeter (No. 14 Automatic Factory of Tianjin, China), and a PHS-2C pH-meter (Jingke Leici Instrument Company of Shanghai) were employed. Preparation Method. The reaction was operated in semibatch mode as shown in Figure 1a. NaOH solution in the graduated glass tube entered into the shell side of the membrane module and gradually permeated under the transmembrane pressure into the lumens of the hollow-fiber membrane through the micropores. At the same time, AlCl3 solution in the stirred tank, with an initial volume of 100.0 mL, was pumped into the lumens of the hollow-fiber membrane and reacted with

10.1021/ie030459c CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003

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Figure 1. (a) Experimental setup and (b) cross section of the membrane module.

the permeating NaOH to form PAC (Figure 1b); it was then returned to the stirred tank for circulating. When the basicity (B, molar ratio of NaOH added to initial AlCl3) reached 2.5, the reaction was stopped. The initial velocity (u0) of AlCl3 solution and the transmembrane pressure (∆P0) were adjusted by the pump and the valves, respectively. During the reaction, the temperature of the solution in the stirred tank was controlled by a heat exchanger. The liquid levels of NaOH solution in the graduated tube were recorded at intervals, and the permeation fluxes were calculated. The pH values of the liquid in the stirred tank were also recorded. At the predetermined B values, the liquid in the tank was sampled and aged for 24 h before detection with the ferron reagent. Analysis. The species distribution of PAC was analyzed by the ferron timed-colorimetric assay. Ferron (0.2%, Sigma Chemical Co.), NaAc solution (20%, A.R), and dilute hydrochloride acid (VHCl/VH2O ) 1:9), with the volume ratio of 2.5:2:1, were mixed to yield the working ferron reagent. In the detection of Ala and Alb, the volumes of sample and ferron reagent were chosen such that the molar ratio of the total concentration of ferron to the total concentration of Al ([ferronT]/[AlT]) was kept to nearly 10:1. After sufficient mixing of the sample and the reagent with a magnetic stirrer at 600 rpm for 30 s, the specimen was transferred into a 1.0-cm-path-length glass cuvette and placed in a UV754 UV-vis spectrophotometer (Spectrum Apparatus Ltd. of Shanghai, China). Then, the absorbance increases were monitored at 370 nm for 120.0 min so that the three fractions, Ala, Alb, and Alc, could be operationally defined. Considering that the absorbance increase after 120 min was negligible, the Al reacted before 120.0 min was assumed to represent the concentration of both Ala and Alb ([Ala + Alb]), while the Ala was thought to react completely within 1.0 min.

Before the determination of the total aluminum concentration ([AlT]), the sample was mixed with dilute hydrochloride acid (pH ) 1.5) and heated at 85 °C for 3 h. After the sample had cooled, the absorbance was also detected with ferron assay at 370 nm, and the Alc concentration ([Alc]) was calculated from the difference between total Al concentration ([AlT]) and concentration of Ala and Alb ([Ala + Alb]). TEM (Hitachi H-800, made in Japan)was employed to detect the species morphology after the PAC sample had been dropped onto copper microgrids and then dried at room temperature. Results and Discussion Effects of Membrane MWCO. The experiments were carried out with membranes with MWCOs of 10 000 and 30 000 under the following conditions: T ) 305 K, u0 ) 1.09 m/s, ∆P0 ) 0.0090 MPa, CA0 ) 0.40 mol/L, and CB0 ) 2.0 mol/L, where CA0 and CB0 represent the initial concentrations of the AlCl3 and NaOH solutions, respectively. For MWCO ) 10000 and 30000, the initial permeation fluxes of NaOH solution were 3 × 10-3 and 0.11 mL‚min-1‚cm-2, and the reaction times needed were 215 and 10 min, respectively. In the reaction process, the permeated NaOH solution neutralizes the H+ cations generated by the hydrolysis of AlCl3 and moves the hydrolysis and polymerization balances to the right. In the initial period (0 < B < ∼2-2.5), Ala and Alb with low polymerization degrees are the dominant species, and the hydrolysis and polymerization rates are high. The H+ generated is neutralized quickly by the OH- permeated (the acid-base neutralization rate is about 1.4 × 1011 mol-1‚dm3‚s-1). Because the solution reactions occurring between the ions include diffusion and reaction stages, the overall reaction rate is related to both the diffusion rate to form the encountering pairs and the intrinsic reaction rate; as a result, the rate of

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Figure 2. Effects of membrane MWCO: (a) pH vs B, (b) pressure drop vs B, (c) u vs B, (d) species distribution vs B. ([) MWCO ) 10 000 (O) MWCO ) 30 000. Here, CA,0 ) 0.40 mol/L, CB,0 ) 2.0 mol/L, ∆P0 ) 0.0090 MPa, T ) 305 K, u0 ) 1.06 m‚s-1.

generation of H+ is always lower than its rate of consumption. Therefore, the pH value rises slowly with increasing B (Figure 2a). When B > ∼2-2.5, Alb with a higher polymerization degree (including Al13 and non-Al13 species) and Alc convert into the main species. According to the StokesEinstein equation, the diffusion coefficient is expressed as

D ) kBT/(6πµr)

(1)

where kB is the Boltzmann constant, µ is the viscosity, and r is the radius of the diffusing particles. The large radii of non-Al13 species, along with the high solution viscosity, lead to the decrease in D and then the hydrolysis-polymerization rate. Al13 molecules, which are the main species of Alb, can coordinate with each other slowly by forming oxygen bridges through deprotonation. The solid phase in Alc can also react with OH-, but the rate is low because of the limitations of the diffusion rate of OH- and the solid-liquid interfacial area. The above three reasons probably result in an increase in the difference between the generation rate of H+ and its rate of consumption, and the pH value rises quickly (Figure 2a). As the reaction proceeds, the viscosity of the liquid increases, leading to a slow increase in the pressure drop between the entrance and exit of the membrane lumens, along with a decrease in the velocity of the liquid in the lumens (Figure 2b, c). When the membrane with MWCO ) 30 000 is employed, more Alc and less Alb are produced (Figure 2d), resulting from the higher permeation rate of NaOH solution, the greater NaOH drop size, the higher local pH value, and the precipitation content. There is a maximum of the Alb content at about B ) 2.25, where

the products contain 56% Alb and 20% Alc for MWCO ) 30 000 but 80% Alb together with 4% Alc for MWCO ) 10 000. After that point, the Alb content decreases, the while Alc content rises. After B ) 1.5, the solution pH value for MWCO ) 30 000 is higher than that for MWCO ) 10 000 (Figure 2a) because the consumption rate of OH- anions by Alc is lower than that by Alb. Effects of Reactant Concentrations. In this series of experiments, CA,0 is either 0.40 or 1.0 M , and simultaneously, the relation CB0 ) 5CA0 is maintained . The other conditions are as follows: MWCO ) 10 000, ∆P0 ) 0.0090 MPa, u0 ) 1.09 m/s, and T ) 305 K. With the increase in AlCl3 concentration from 0.40 to 1.0 M, the initial permeation fluxes of the NaOH solution are the same (about 3 × 10-3 mL‚min-1‚cm-2), whereas the initial pH value decreases from 3.2 to 2.5 (Figure 3a), because the initial pH value is mainly determined by the hydrolysis degree of AlCl3 solution, which is related to the AlCl3 concentration and temperature. For CA,0 ) 1.0 mol/L, the Ala content is always higher because of the lower pH value, e.g., at B ) 0.75, Ala is about 74% and 85% for CA,0 ) 0.4 and 1.0 mol/L, respectively. With the increase in CA,0, as well as CB,0, the local pH value and Al(OH)3 supersaturation increase, leading to a decrease in Alb and an increase in Alc. For example, at B ) 2.5, the Alb content is about 76% and 30%, whereas Alc is about 3% and 46%, for CA,0 ) 0.4 and 1.0 mol/L, respectively (Figure 3b). Thus, lower reactant concentrations favor the improvement of Alb under the experimental conditions used. Hydrolysis and Polymerization Pathways. Akitt et al.8 assumed that, when base solution is added to AlCl3 solution, because of the local high pH values, Al(OH)4- can be formed and can function as a precursor of Al13. Bertsch12 thought that the optimum base addi-

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Figure 3. Effects of reactant concentrations: (a) pH vs B, (b) species distribution vs B. (O) CA,0 ) 0.40 mol/L (2) CA,0 ) 1.0 mol/ L. Here, MWCO ) 10 000, CB,0 ) 5CA,0, ∆P0 ) 0.0090 MPa, T ) 305 K.

tion rate, R1, should ensure that the rate of generation of Al(OH)4- would equal its rate of integration with [Al(H2O)6]3+ to form Al13, R2. When R1 is low, R2 is limited by the formation of Al(OH)4-, and less Al13 is formed. Conversely, when the production rate of Al(OH)4exceeds its consumption rate, precipitation will result from the reestablishment of equilibrium between the excess Al(OH)4- and the bulk solution. Bertsch and Parker13 assumed that there several pathways exist in the hydrolysis and polymerization processes. (1) When the NaOH injection rate is too high, a large amount of [Al(OH)3]n and less Al13 are generated because of the high local supersaturation. 2) When the base injection rate is high, Al13 quickly turns into [Al13]n through the ion-bridge formation or deposits on [Al13]n, and then [Al13]n turns into poorly ordered phase. (3) When the hydrolysis is slow, Al13 is the predominant species, and it can decompose into monomer with an octahedral structure. On the other hand, Al13 can also aggregate together to form [Al13]A and then turn slowly into [Al13]n through ion bridging.

Figure 4. Plausible pathways of Al3+ hydrolysis and polymerization.

We propose a new reaction pathway from the viewpoint of hydrolysis and polymerization kinetics and assume that the hydrolysis and polymerization of Al3+ constitute a complex net of consecutive and parallel reactions. The plausible reaction pathways (Figure 4) are interpreted as follows: Al3+ hydrolyzes and produces Al(OH)2+. Then, Al(OH)2+ takes part in two reactions: hydrolysis reaction 2 and polymerization reaction 2′. Because one of the reactants is different in the above two reactions, we name them quasi-parallel reactions. (i) In hydrolysis reaction 2, Al(OH) 2+ hydrolyzes to produce Al(OH)2+. Then Al(OH)2+ takes part in two quasi-parallel reactions: (a) hydrolysis reaction 3, producing Al(OH)3 and (b) Al(OH)2+ self-polymerization and/or polymerization with Al(OH)+ (reaction 3*), producing Al2(OH)42+, Al2(OH)33+, etc. The product of reaction 3, Al(OH)3, takes part in a series of quasi-parallel reactions: (a) Hydrolysis reaction 4 produces Al(OH)4-, which then coordinates with 12 Al(H2O)63+ or 6 dimers through a hydroxyl bridge to form Al13 (reaction 5). Al13 can absorb the anions, such as Cl-, to form double-electronic layers, which make them exist steadily at low basicity. With the increase in basicity, the pH value rises, and the H2O molecules coordinated in the Al13 structure slowly deprotonate, leading to a decrease in the electric charges and the repulsion potential and, consequently, the aggregation of Al13 or even transforms to AlP1 and AlP2 through the chemical bonds, i.e., reaction 6. Herein, AlP1 refers to a lacuna structure of Al13 with one Al(H2O)63+ lost, and AlP2 represents the polymerization product of AlP1.14 (b) Al(OH)3 aggregates and appears as a precipitate, which then slowly transforms from an amorphous structure to an R- or γ-type crystal, i.e., reaction 4*. The products of polymerization reaction 3*, Al2(OH)42+ and Al2(OH)33+, continue to hydrolyze and polymerize (not shown in Figure 4). (ii) Self-polymerization reaction 2′ produces Al2(OH)24+, which continues to hydrolyze (reaction 3′) and produces Al2(OH)33+

2[Al(OH) (H2O)5]2+ H [Al2(OH)2(H2O)10]4+

(2)

[Al2(OH)2(H2O)10]4+ + H2O H [Al2(OH)3(H2O)9]3+ + H3O+ (3) Al2(OH)33+ takes part in two kinds of quasi-parallel reaction as follows: (a) hydrolysis reaction 4′ with generation of Al2(OH)42+, etc., and (b) self-polymerizaton

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and/or polymerization with Al(OH)2+, i.e., reaction 4′′, producing of Al4(OH)66+ and Al3(OH)45+.

[Al2(OH)3(H2O)9]3+ + H2O H [Al2(OH)4(H2O)8]2++ H3O+ (4) 2[Al2(OH)3(H2O)9]3+ H [Al4(OH)6(H2O)18]6+ (5) [Al2(OH)3(H2O)9]3+ + [Al(OH) (H2O)5]2+ H [Al3(OH)4(H2O)14]5+ (6) These products continue to hydrolyze and/or polymerize (not shown in Figure 4). Generally, the hydrolysis reactions can reduce the species charges and increase the hydroxyl group numbers and, consequently, favor further polymerization. The polymerization reactions can increase the positive charges of species and the ratio of the charge to the diameter of the positive ions, which is favorable for further hydrolysis. In the reaction process, the added OH- ions continuously neutralize the H+ generated from the hydrolysis reaction and move the hydrolysis balances to the right. In contrast, the added OH- influences the polymerization reactions relatively slightly. As far as the reaction rates are concerned, the hydrolysis reactions, e.g., 1, 2, 3, 3′, and 4′, are fast, and the polymerization reaction 2′ is somewhat slow. With the increase in the cation size and charge, the diffusion rates drop; the static electric repulsion potential rises; and the rates of the polymerization reactions, e.g., 3* and 4′′, etc., further decrease. Reaction 4* includes fast physical aggregation and subsequently very slow solidsolid reactions. Once Al(OH)4- anions are formed, they soon coordinate with 12 [Al(H2O)6]3+ or 6 dimers through hydroxyl bridges to form Al13.12 It is well-known that the reaction with the higher kinetic rate usually predominates in parallel reactions. Thus, it can be speculated that the species distribution of PAC is mainly determined by the kinetics of the above reactions. In view of the kinetics features, it is not surprising that, in the synthesis of PAC, the predominant species are usually mono-Al, Al2(OH)24+, and Al13, whereas the amounts of Al3(OH)45+, Al4(OH)66+, etc., are often small.5,6,8,11,13 Thus, it seemed that the proposed reaction pathway could interpret the experimental results reported in the literature well. We proposed that the formation of Al13 must include a series of primary reactions because it is impossible that Al(OH)4- coordinates simultaneously with 12 [Al(H2O)6]3+ or 6 dimers, and the formation mechanism still remains to be discovered. Figure 5 shows that, when B ) 1.5, the particles are very small and appear as floccules; when B ) 2.25, precipitates with a mean diameter of about 50 nm result. Species Control. It seemed that the key to improving the Al13 content is to boost reactions 4 and 5 and to inhibit the reaction 4* (Figure 4). This can be fulfilled by the control of the supersaturation level to ensure the formation of a small amounts of Al(OH)3, followed by the ready hydrolysis of Al(OH)3 under the local basic conditions to produce Al(OH)4-. Too high a supersaturation level will result in the generation of great amounts of Al(OH)3 and the formation of precipitates. The reactant concentration, basicity, base addition rate, and mixing conditions all have effects on the supersaturation level and, thereby, the species distribu-

Figure 5. Effects of B on the morphology of PAC: (a) B ) 1.50, (b) B ) 2.25. The PAC solution was prepared under the following conditions: CA,0 ) 0.02 mol/L, CB,0 ) 0.10 mol/L, MWCO ) 10 000, ∆P ) 0.0090 MPa, T ) 303 K.

tion. Higher reactant concentrations will lead to the generation of more Al(OH)3 precipitates (Figure 3 b). On the other hand, if the concentrations are too low, no Al(OH)3 and Al13 will be generated. For example, Bertsch et al.13 added NaOH solution dropwise to Al3+ solutions with concentrations of 3.34 × 10-2 and 3.34 × 10-3 mol/L at the same addition rate (1.2 mL/ min) and found that Al13 was generated at B ) 0.4 and 1.5, respectively. When the Al3+ solution concentration was lowered to 3.34 × 10-4 mol/L, no Al13 was found even at B ) 2.5. Only when the base addition rate was raised by 100 times was Al13 detected. Basicity also determines the supersaturation. At low basicity, the pH value and supersaturation are low, and Ala is the predominant species. At moderate basicity, the increased pH and supersaturation favor the formation of Al13, and Alb converts into the main species. When the basicity is increased further, the overly high supersaturation will result in the generation of a great deal of precipitates, and Alc will become dominant (Figures 2d and 3b). Under high reactant concentrations, lowering the base addition rate and the droplet size can reduce the local supersaturation and the characteristic diffusion time of the reactants, thus inhibiting the precipitation of Al(OH)3 (Figure 2d). Intensifying mixing can reduce the reactant droplet size and the local supersaturation and

Ind. Eng. Chem. Res., Vol. 43, No. 1, 2004 17 Table 1. Comparison of Various Preparation Methods

method

base addition rate AlT (mL‚min-1) (mol‚L-1)

dropping base slowly15 electrodialysis10 electrolysis16 membrane reactora

0.33 ∼0.3

0.00010 0.26 0.225 0.27

B

Ala (%)

Alb (%)

Alc (%)

1.6 1.8 2.5 2.5

28.1 17.1 17.0 10

62.1 9.8 61.2 21.7 71.7 11.3 79 11

Experimental conditions: MWCO ) 10 000, CA,0 ) 0.40 mol/ L, CB,0 ) 2.0 mol/L, V A,0 ) 100.0 mL, ∆P ) 0.0090 MPa, T ) 305 K. a

prompt the mass transfer. It should be noted that, in the stirring tank, the flow state, recycling ratio, velocity distribution, shear rate, stirrer type, paddles, and so on all have influences on the mixing. However, no correlative studies have yet been reported in the literature. In the classical methods, NaOH solution is usually dropped into the AlCl3 solutions with droplet sizes on the order of about millimeters. While in the membrane reactor, the NaOH permeate size can be reduced to nanoscale, about 106 times smaller than that of the conventional method, resulting in an improvement in the micromixing condition and a decrease in the local supersaturation level. Consequently, PAC with higher Alb contents (∼80%) along with lower Alc contents can be obtained even under higher reactant concentration (Table 1). In practical applications of the membrane reactor, the reaction time can be greatly reduced by increasing the membrane areas. The PAC products display excellent coagulation effects compared with the classical commercial products, which will be discussed in a later paper. Conclusions Polyaluminum chloride was synthesized with a novel membrane reactor. It was found that Alb increases with decreasing membrane MWCO and reactant concentration under the experimental conditions. A new reaction pathway was also proposed that assumed that the hydrolysis and polymerization of Al3+ constitute a complex net of consecutive and parallel reactions and that the species distribution is mainly determined by the reactions kinetics. The reaction pathway could be used to interpret the experimental results reported in the literature well. It seemed that the membrane reactor provides a new practical method for the synthesis of PAC with a high Alb content. Acknowledgment The authors are grateful for financial support from the National Natural Sciences Foundation of China (Project No. 50072042) and Chinese Academy of Sciences (Project No. KZ951-A1-201-02, KZ95T-05). Nomenclature AlT ) total aluminum, mol‚L-1 B ) basicity of polyaluminum chloride, i.e., molar ratio of NaOH added to initial AlCl3 CA,0 ) initial concentration of AlCl3, mol‚L-1 CB,0 ) initial concentration of NaOH solution, mol‚L-1 D ) diffusion coefficient, m2‚s-1

kB ) Boltzmann constant Pvac ) average vacuum in the hollow-fiber membrane lumens, Pa r ) radiusradii of the diffusion particles, m T ) temparature, K u ) velocity in the membrane lumens, m‚s-1 u0 ) initial velocity in the membrane lumens, m‚s-1 VA0 ) initial volume of AlCl3 solution, mL VHCl ) volume of hydrochloride acid, mL VH2O ) volume of water, mL Greek Symbols µ ) viscosity of the solution, Pa‚s ∆P0 ) initial transmembrane pressure, Pa

Literature Cited (1) Yunhwer, S.; Dempsey, B. A. Synthesis and speciation of polyaluminum chloride for water treatment. Environ. Int. 1998, 24(8), 899. (2) Akitt, J. W.; Greenwood, N. N.; Khandelwal, B. L.; Lester, G. D.27Al NMR studies of the hydrolysis and polymerization of the hexa-aquo-aluminum(III) cation. J. Chem. Soc., Dalton Trans. 1972, 604. (3) Thompson, A. R.; Kunwar, A. C. Oxygen-17 and aluminum27 NMR spectroscopic investigation of aluminum(III) hydrolysis products. J. Chem. Soc., Dalton Trans. 1987, 2317. (4) Bersillion, J. L.; Hsu P. H.; Fiessinger F. Characterization of hydroxy-aluminum solution. Soil Sci. Soc. Am. J. 1980, 44, 6330. (5) Bertsch P. M. Speciation of hydroxy-aluminum solution by wet chemical and aluminum-27 NMR methods. Soil Sci. Soc. Am. J. 1986, 50, 1449. (6) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. Mechanism of formation of aluminum trihydroxide from Keggin Al13 polymers. J. Colloid Interface Sci. 1987, 117(1), 47. (7) Parker, D. R.; Bertsch P. M. Identification and quantification of the Al13 tridecameric polycation using Ferron. Environ. Sci. Technol. 1992, 26(5), 908. (8) Akitt, J. W.; Farthing, A. New 27Al NMR studies of the hydrolysis of the aluminum(III) cation. J. Magn. Reson. 1978, 32, 345. (9) Vermeulen, A. C.; Geus, J. W.; Stol, R. J.; De Bruyn, P. L. Hydrolysis-precipitation studies of aluminum(III) solution: Titration of acidified aluminum nitrate solution. J. Colloid Interface Sci. 1975, 51(1), 449. (10) Guangjie, L.; Jiuhui, Q.; Hongxiao, T. Electrochemical production of highly effective polyaluminum chloride. Water Res. 1999, 33(3), 807. (11) Akitt J. W.; Farthing, A. Aluminum-27 NMR studies of the hydrolysis of aluminum(III). Part 4. hydrolysis using sodium carbonate. J. Chem. Soc., Dalton Trans. 1981, 1617. (12) Bertsch, P. M. Conditions for Al13 polymer formation in partially neutralized aluminum solutions. Soil Sci. Soc. Am. J. 1987, 51, 825. (13) Bertsch, P. M.; Parker D. R. The Environmental Chemistry of Aluminum; CRC Press: Boca Raton, FL, 1996. (14) Akitt, J. W.; Farthing, A. Aluminum-27 NMR studies of the hydrolysis of aluminum(III). Part 5. Slow hydrolysis using aluminum metal. J. Chem. Soc., Dalton Trans. 1981, 1624. (15) Parker, D. R. Comparison of three spectrophotometric methods for differentiating mono- and polynuclear hydroxyaluminum complexes. Soil Sci. Soc. Am. J. 1988, 52, 67. (16) Guangjie, L. Preparation of highly effective polyaluminum chloride with electrochemical method. Ph.D. Dissertation, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China, 1998.

Received for review May 30, 2003 Revised manuscript received September 15, 2003 Accepted September 28, 2003 IE030459C