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Ind. Eng. Chem. Res. 2010, 49, 8726–8732
Uptake of Bromate Ion on Amorphous Aluminum Hydroxide Ramesh Chitrakar,* Keisuke Mizobuchi, Akinari Sonoda,* and Takahiro Hirotsu Health Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan
Batch experiments were performed to examine the BrO3- uptake from dilute aqueous solutions on amorphous aluminum hydroxide (Cl- type), which was synthesized by addition of a 3 mol/dm3 NaOH solution to a 0.1 mol/dm3 AlCl3 solution until the pH reached 5.8. The uptake from a 0.78 µmol/dm3 BrO3- solution (100 µg of BrO3/dm3) was rapid: equilibrium was attained within 3 min. The BrO3- uptake was found to decrease slowly at pH 4-8 and rapidly at pH >8. The isotherm for BrO3- uptake was well fitted with the Freundlich isotherm model. From distribution coefficient (Kd) values determined from mixed solutions containing 0.10, 1.0, and 2.0 mmol/dm3 NaCl, NaBrO3, NaNO3, Na2SO4, and Na2CO3, the selectivity order was reported as NO3- < BrO3- < SO42- at a pH range 4.8-7.9. These results suggest that the BrO3- uptake on the amorphous aluminum hydroxide occurs through an outer-sphere complex reaction. 2. Experimental Section
1. Introduction -
-
Oxo anions such as nitrate (NO3 ), bromate (BrO3 ), and perchlorate (ClO4-) in drinking water pose potential health risks. Therefore, these anions should be controlled. Ozone treatment causes the formation of BrO3- through oxidation of Br- in the source water.1,2 Concentrations of BrO3- in drinking water after ozonation of water containing background Br- ion have been reported as 0.4-100 µg/dm3 (0.003-0.78 µmol/dm3).3–5 Pilotand full-scale drinking water processes have also shown BrO3formation at concentrations as high as 150 µg/dm3.3 The BrO3formation in drinking water depends on the Br- concentration and the pH of the source water and on the amount of ozone used to disinfect the source water.6 Information about BrO3toxicity is available from studies of laboratory animals: exposure to large amounts of BrO3- ions over a long period of time causes kidney damage and cancer in rats.7 Based on such reports, BrO3ion has been classified as a “possible” human carcinogen. A drinking water standard of 0.078 µmol/dm3 (10 µg of BrO3/ dm3) has already been implemented in the USA, U.K., Canada, and Japan.8 Removal of BrO3- ions from the drinking water after ozonation might be necessary to reduce the concentration level of BrO3-. Studies have been undertaken to investigate BrO3uptake from aqueous solutions on activated carbon,9–13 γ-Al2O3,14 zerovalent iron,15 surface modified TiO2,16 ionexchange-membrane bioreactor,5,17 and activated carbon felt electrode.18 In a previous paper,19 we reported the BrO3- uptake from an aqueous solution on crystalline akagane´ite (β-FeOOH, Cl- type). Fortunately, we found selective BrO3- uptake from dilute aqueous solution on amorphous aluminum hydroxide (Cl- type). The objective of the present study is to examine the BrO3uptake from dilute aqueous solutions on amorphous aluminum hydroxide (Cl- type) to decrease the BrO3- concentration to less than 0.078 µmol/dm3, which is the drinking water standard. The mechanism for the BrO3- uptake on amorphous aluminum hydroxide (Cl- type) was investigated. Commercial R-Al(OH)3 was also used for a comparison study. This is the first report describing BrO3- uptake on amorphous aluminum hydroxide (Cl- type). * To whom correspondence should be addressed. Tel.: +81-87-8693511. Fax: +81-87-869-3554. E-mail:
[email protected] (R.C.);
[email protected] (A.S.).
Synthesis of Aluminum Hydroxides. Three samples of aluminum hydroxides were synthesized as reported by Isobe et al.20 A 3.0 mol/dm3 NaOH solution was added to a 0.10 mol/ dm3 AlCl3 solution (2.5 dm3) with stirring until the pH reached a predetermined value (Table 1). The suspension was aged at room temperature for 1 day. The precipitate was separated by centrifugation, washed with deionized water (200 cm3), and dried at 50 °C. After centrifugation, the supernatant solution did not become clear because colloid particles had formed, probably because the amount of water (200 cm3) used for washing the precipitate was insufficient. Accordingly, the sample was ground to particles of ca. 100 µm diameter. The particles were subsequently dispersed in deionized water (5 dm3), separated by decantation, filtered, and air-dried. The samples were designated as AlOH-n, where n denotes the preparation pH. In addition, AlOH-5.8, AlOH-9.4, and AlOH-11 respectively refer to amorphous aluminum hydroxide (Cl- type), boehmite, and bayerite. Commercial R-Al(OH)3 (AlOH-ref) was used (Wako Pure Chemical Ind. Ltd., Japan). Physical Measurement. The samples were characterized using an X-ray diffractometer (RINT 2100; Rigaku Corp.) with Cu KR radiation and a Ni filter. Thermal behavior of the samples was monitored at a heating rate of 10 °C/min in air (TG DTA 2000; MAC Science Co., Ltd.). Chemical Analysis. The sample AlOH-5.8 (0.05 g) was dissolved in a 5 mol/dm3 HCl solution (5 cm3). The Al content was determined using an inductively coupled plasma (ICP) spectrometer (Model SPS 7800; Seiko Instruments Inc., Chiba, Japan). The Cl- content was determined by use of ion chromatography (761 Compact IC; Metrohm AG) after the sample (0.025 g) was dissolved in 3.6 mol/dm3 H2SO4 (1 cm3). Point of Zero Charge. The point of zero charge (PZC) was determined as described in an earlier report.21 By addition of Table 1. List of Aluminum Hydroxides Prepared sample name
preparation conditions
uptake of BrO3-a (mmol/g)
AlOH-5.8 AlOH-9.4 AlOH-11 AlOH-ref
precipitated at pH 5.8 precipitated at pH 9.4 precipitated at pH 11
1.70 0.44 0.08 0.05
a Solution ) 0.10 mol/dm3 NaBrO3, weight ) 0.050 g, volume ) 50.0 cm3, contact time ) 1 day.
10.1021/ie901705n 2010 American Chemical Society Published on Web 08/17/2010
Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010 3
0.10 mol/dm HNO3 or 0.10 mol/dm NaOH with nitrogen gas bubbling, 0.10 mol/dm3 NaNO3 solutions (25 cm3) were adjusted to the initial pH of 4-9. Samples (0.10 g) were then added to aqueous solutions of NaNO3 of predetermined pH values in the quantities prepared as described above. After each addition and equilibration (2 h stirring), the final pH was measured. A blank was also measured using a sample (0.10 g) in deionized water (25 cm3). Uptake of H+ or OH- by the sample was determined by calculating the differences between the test suspension and blank titration volumes at particular pH values. Distribution Coefficient (Kd). The Kd values were determined by equilibrating a sample (0.10 g) in a mixed solution (10.0 cm3) containing 0.10, 1.0, or 2.0 mmol/dm3 of NaCl, NaBrO3, NaNO3, Na2SO4, and Na2CO3 with occasional shaking at room temperature for 1 day. The concentrations of BrO3-, NO3-, and SO42- ions were determined using ion chromatography (761 Compact IC; Metrohm AG) with an eluent solution of 1.7 mmol/ dm3 NaHCO3 and 1.8 mmol/dm3 Na2CO3. The respective Kd values were calculated as follows: Kd (cm3 /g) )
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3
Figure 1. XRD patterns of (a) AlOH-5.8, (b) AlOH-9.4, (c) AlOH-11, and (d) AlOH-ref (gibbsite).
uptake of anion (mmol/g) concn of anion in solution (mmol/dm3)
Time Dependence of BrO3- Uptake. The sample AlOH5.8 (0.250 g) was stirred in a 0.78 µmol/dm3 BrO3- solution (250 cm3) at room temperature. Then 5 cm3 aliquots were sampled at different intervals using a membrane filter of 0.45 µm. Effect of pH on BrO3- Uptake. The sample AlOH-5.8 (0.05 g) was stirred at room temperature for 1 day in a 0.78 µmol/ dm3 BrO3- solution (100 cm3) at different pH values, which were adjusted by the addition of 1 mol/dm3 HCl or NaOH solution. Isotherm of BrO3- Uptake. The sample AlOH-5.8 (0.10 g) was stirred at room temperature for 1 day in 0.16-3.91 µmol/ dm3 BrO3- solution (100 cm3). Effect of Solid-to-Solution Ratio. The sample AlOH-5.8 (0.02-0.12 g) was stirred at room temperature for 1 day in a 0.78 µmol/dm3 BrO3- solution (100 cm3). Effect of Competing Anions. The sample AlOH-5.8 (0.10 g) was stirred at room temperature for 1 day in a 0.78 µmol/ dm3 BrO3- solution (100 cm3) mixed with 0.10-2.0 mmol/dm3 of either NaCl, NaNO3, Na2SO4, or Na2CO3. The uptake of CO32- on Al oxide samples was not determined, because the ion chromatography was used with a buffer solution of Na2CO3 and NaHCO3 for the eluent. Analysis of BrO3- (0.78 µmol/dm3), NO3-, and SO42- ions were determined using ion chromatography (761 Compact IC; Metrohm AG) with an eluent solution of 1.7 mmol/dm3 NaHCO3 and 1.8 mmol/dm3 Na2CO3. Control standards of 0.078 mmol/ dm3 BrO3-, 0.48 mmol/dm3 NO3-, and 0.41 mmol/dm3 SO42were used. The reproducibility for three replicate analyses at concentrations of 0.78-78 µmol/dm3 BrO3-, 1.6-161 µmol/ dm3 NO3-, and 1.4-104 µmol/dm3 SO42- corresponds to a relative standard deviation of 1.0-2.0%.
Figure 2. TG-DTA curves of AlOH-5.8.
3. Results and Discussion Characterization of Materials. Aluminum hydroxides can exist in different phases depending on their preparation methods. The X-ray diffraction (XRD) patterns of samples prepared at different pH values are presented in Figure 1. The sample AlOH5.8 prepared at pH 5.8 is amorphous as reported.20 A poorly crystalline phase of boehmite (γ-AlOOH) and crystalline phase of bayerite (β-Al(OH)3) are respectively observed on the samples of AlOH-9.4 and AlOH-11.20 An XRD pattern of the AlOHref, a commercial one, corresponds to gibbsite (R-Al(OH)3). The BET surface areas were 9, 227, 88, and 5 m2/g for AlOH-5.8, AlOH-9.4, AlOH-11, and AlOH-ref, respectively. The BrO3- uptake from a 0.1 mol/dm3 NaBrO3 solution was preliminarily performed on samples AlOH-5.8, AlOH-9.4, AlOH-11, and AlOH-ref (Table 1). Sample AlOH-5.8, prepared at pH 5.8, showed the highest BrO3- uptake of 1.7 mmol/g; the other samples showed much lower BrO3- uptakes. Sample AlOH-5.8 showed such a high uptake capacity that it was selected for detailed study. The uptake of NO3- determined on sample AlOH-5.8 was also found to be 1.7 mmol/g after treatment with a 0.1 mol/dm3 NaNO3 solution. A similar finding has been reported for amorphous aluminum hydroxide (Cl- type) showing a NO3- uptake of 1.8 mmol/g.22 The thermogravimetric differential thermal analysis (TGDTA) curves of sample AlOH-5.8 are presented in Figure 2. The mass loss observed at temperatures 40-200 °C is caused mainly by physically adsorbed water. The slow dehydration of the sample continues at higher temperatures of 200-600 °C in
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Figure 3. (a) Change in concentration of BrO3- with time on AlOH-5.8 and (b) pseudo-second-order kinetic plot.
the TG curve. It is difficult to directly estimate the content of physically adsorbed water in the AlOH-5.8 from the TG-DTA analysis. Accordingly, we can assume that sample AlOH-5.8 has the chemical formula of Al(OH)3(HCl)x · yH2O. Based on the weight analysis of Al (24.8 wt %) and Cl (6.59 wt %), AlOH-5.8 is expressed as Al(OH)3 · 0.2(HCl) · 1.4H2O. Time Dependence of BrO3- Uptake. The rate of BrO3uptake on AlOH-5.8 is given in Figure 3a. The inset in Figure 3a shows that the rate very rapidly reaches 90% uptake in just 1 min. The uptake was so fast that it attained equilibrium (95%) in only 3 min. The BrO3- uptake was followed by a decrease in solution pH from 5.5 to 4.8 after addition of the sample, suggesting that a hydrolytic reaction of aluminum hydroxide might occur, which decreases the pH during BrO3- uptake. Between pH 4.7 and 6.5, the Al(OH)2+ species predominates.23 The BrO3- uptake on crystalline akagane´ite is also so fast that equilibrium is attained in 1 h with a decrease of solution pH from 5.5 to 3.5.19 Granular ferric hydroxide, which is a poorly crystallized akagane´ite (β-FeOOH) containing Cl- ion in the tunnel site, also showed rapid BrO3- uptake within 60 min from aqueous solutions.24 A kinetic approach was also undertaken to study the BrO3uptake on sample AlOH-5.8 using pseudo-first-order and pseudo-second-order equations.24 The experimental data show a better fit to the pseudo-second-order equation t/qt ) 1/(ksqe2) + t/qe, where qt and qe are the amounts of BrO3- uptake (µmol/ g) at time t and at equilibrium, respectively, and where ks (g/ µmol/min) is the rate constant of the pseudo-second-order kinetics. The slope and intercept in Figure 3b were used to calculate the parameters: qe ) 0.74 µmol/g, ks ) 1.15 × 10-1 g/µmol/min, R2 ) 0.999. The uptake of the anion from the solution phase by the solid phase can be described by four mass transport mechanisms:24 diffusion at bulk space, film diffusion, intraparticle diffusion, and adsorption into interior sites. Generally, the first and fourth steps take place quickly;24 therefore, these two steps might control the rate of BrO3- uptake on sample AlOH-5.8. Distribution Coefficient (Kd). Table 2 shows Kd values of BrO3-, NO3-, and SO42- on the samples AlOH-5.8, AlOH-9.4, AlOH-11, and AlOH-ref, a commercial sample, in a mixed solution containing different concentrations of NaCl, NaBrO3, NaNO3, Na2SO4, and Na2CO3. AlOH-5.8 shows a selectivity order of NO3- < BrO3- < SO42- among the anions studied. The high Kd value of BrO3- at 0.10 mmol/dm3 shows the possibility of uptake of BrO3- ions from dilute aqueous solutions. Results show that the final pH of the solution is lower after uptake of anions, which suggests that some OH- ions are bound on AlOH5.8. The measurements of Cl- concentrations before and after uptake of anions were also performed. The final Cl- concentrations increased to 4.0, 7.0, and 13.5 mmol/dm3 after uptake of
Table 2. Distribution Coefficient (Kd) Values of Anions in Mixed Solutiona 3
Kd (cm /g) mixed solutionb (mmol/dm3) NO3- BrO3- SO42-
sample name
sample phase
pH
AlOH-5.8
Al(OH)3 · 0.2HCl · 1.4H2O (amorphous)
0.10 1.0 2.0
350 120 40
2100 13400 580 7000 180 6700
4.8 (8.9) 5.1 (10.4) 6.5 (10.6)
AlOH-9.4
poorly crystalline boehmite (γ-AlOOH)
0.10 1.0 2.0
50 10 5
300 30 10
4000 3300 150
5.3 (8.9) 6.9 (10.4) 7.9 (10.6)
AlOH-11
crystalline bayerite (β-Al(OH)3)
0.10 1.0 2.0
na na na
na na na
na na na
8.4 (8.9) 9.9 (10.4) 10.2 (10.6)
AlOH-ref
crystalline gibbsite (R-Al(OH)3)
0.10 1.0 2.0
na na na
na na na
na na na
8.7 (8.9) 10.3 (10.4) 10.6 (10.6)
a For Kd values, na ) no adsorption. Parentheses show initial pH of solution. b NaCl, NaNO3, NaBrO3, Na2SO4 and Na2CO3.
anions, with respective initial Cl- concentrations of 0.10, 1.0, and 2.0 mmol/dm3. An excess release of Cl- ions (3.5 mmol/ dm3) during uptake of anions (BrO3-, NO3-, and SO42-) from AlOH-5.8 suggests that OH- ions also exchanged with Cl- ions in AlOH-5.8. All the anions added (even nitrate and bromate) at sufficiently high concentration are able to replace Cl. At pH >6-7 Cl is easily removed. The Kd values of BrO3- on sample AlOH-9.4 are found to be much smaller than those on AlOH-5.8. Neither the AlOH11 nor the AlOH-ref sample shows uptake of the anions at pH 8.4-10.2, strongly suggesting that the OH- groups on the surface of the crystalline bayerite and gibbsite are not accessible for the anions studied. Wijnja and Schulthess25 reported that the presence of CO32- in the aqueous solution enhanced the adsorption of SO42- and SeO42- from the aqueous solution on bayerite for pH 6-8. Reportedly, gibbsite can capture H2AsO4from an aqueous solution at pH 5.5 via bidentate binuclear complex formation (bidentate linkage between AsO4 tetrahedron and two-edge-shared Al(OH)6 octahedron).26 Effect of pH on BrO3- Uptake. The effect of solution pH 4-10 on BrO3- uptake from a 0.78 µmol/dm3 solution was studied for sample AlOH-5.8 (Figure 4). The BrO3- uptake is high at low pH. It decreases gradually with increasing pH up to 7.8. A sharp decrease in the BrO3- uptake is apparent with further increase beyond pH 8. Probably at pH >7.5-8, the particles strongly aggregated may be partially dispersed. The sample AlOH-5.8 has a PZC at pH 8.1. Lower than pH 8.1, electrostatic attraction enhances the BrO3- uptake. As the pH is increased, the sample surface becomes less positive. The decreased electrostatic attraction results in lower uptake, and
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Figure 4. Effect of pH for uptake of BrO3- on AlOH-5.8. (b) BrO3- and (4) Al3+.
low uptake at high pH can be attributed to increased repulsion between the negatively charged bromate species and the negatively charged sample surface. According to Sparks,23,27 the anions NO3-, Cl-, and ClO4- are adsorbed as outer-sphere complexes on the solid surfaces that exhibit a positive charge. A similar interpretation of an outer-sphere complex reaction is applicable for BrO3- uptake on the present sample AlOH-5.8. An outer-sphere complex reaction is usually a rapid process that is reversible.27 Results show that the BrO3- uptake is so fast that it attains 90% adsorption in just 1 min (Figure 3a). Dissolution of Al from the sample was almost zero at pH 6.5-7.5, but it was 0.4 wt % at pH 4 and 1.0 wt % at pH 10. The higher amount of Al dissolution from the sample might also be a reason for the lower BrO3- uptake. Dissolution of Al at high pH occurs because of the formation of dissolved species of Al(OH)4-. For the regeneration experiment, sample AlOH-5.8 (0.20 g) was treated with 100 µmol/dm3 BrO3- (200 cm3). The amount of BrO3- uptake was 90 µmol/g. After treatment of the BrO3exchanged sample with 0.1 mol/dm3 NaCl, the amount of BrO3released was 88 µmol/g, which is almost equal to the uptake amount. The regenerated sample was further treated with a 100 µmol/dm3 BrO3- solution. The amount of uptake was 88 µmol/ g. In further experiments, after treatment of sample AlOH-5.8 with a 0.10 mol/dm3 NaH2PO4, Na2SO4, or Na2CO3 solution, the sample showed low BrO3- uptake, respectively corresponding to 11, 3, or 5 µmol/g, suggesting that regeneration of the sample is not possible. It only shows the possibility of the apparent regeneration of the sample with NaCl solution. Isotherm of BrO3- Uptake. An isotherm is important to describe the BrO3- uptake on sample AlOH-5.8. The relation between the uptake on solid and the concentration in solution is usually described by the Langmuir or Freundlich isotherm equation. The isotherm for BrO3- uptake on AlOH-5.8 was determined at equilibrium pH 4.7 (equilibrium pH was 4.6-4.7 after BrO3- uptake). The experimental data are presented in Figure 5 with their fittings to the Langmuir and Freundlich isotherm models. The data show a better fit to the Freundlich model: log qe ) log KF + (1/n) log Ce, where qe is the uptake of BrO3- at equilibrium (µmol/g), Ce is the equilibrium concentration of BrO3- (µmol/dm3), KF is a measure of uptake capacity, and n is a measure of interaction strength of BrO3ions on the solid (larger n represents higher affinity). The estimated Freundlich parameters were KF ) 7.9, 1/n ) 0.83, and R2 ) 0.98. The Freundlich model includes the assumption that sites of many types are acting simultaneously, and that numerous available sites exist on the solid.28 In our previous study of BrO3- uptake on crystalline akagane´ite, the uptake is
Figure 5. (a) Langmuir and (b) Freundlich isotherms for uptake of BrO3on AlOH-5.8.
Figure 6. Change in concentration of BrO3- with solid-to-solution ratio of AlOH-5.8.
better described by the Langmuir isotherm than by the Freundlich isotherm.19 Akagane´ite has a fixed number of tunnel sites that can be saturated with BrO3-; then qe cannot increase indefinitely with the equilibrium concentration of BrO3-, as the Freundlich model would predict. In such a case, the Langmuir isotherm can be a more appropriate model for the existence of homogeneous sites of BrO3- uptake in crystalline akagane´ite.19 Effect of Solid-to-Solution Ratio. The solid-to-solution ratio is an important parameter for estimating the uptake capacity of a sample for a given initial concentration of anions. The effect of a solid-to-solution ratio of AlOH-5.8 on BrO3- uptake is depicted in Figure 6. The equilibrium pH values were, respectively, 4.7-4.5 for solid-to-solution ratios of 0.20-1.20 g/dm3. The initial concentration of 0.80 µmol/dm3 BrO3- is decreased to 0.23 µmol/dm3 at a solid-to-solution ratio of 0.20 g/dm3. After the solid-to-solution ratio is increased to 0.80 g/dm3, the concentration of BrO3- is further decreased to 0.06 µmol/dm3. In a previous study,19 we were unable to achieve the equilibrium
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Figure 7. Effect of competing anions for uptake of BrO3- on AlOH-5.8. (×) pH, (9) Cl-, (O) NO3-, ([) SO42-, (2) CO32-, and (0) OH-.
concentration of 0.078 µmol/dm3 BrO3- (10 µg of BrO3/dm3) from an initial concentration of 0.78 µmol/dm3 BrO3- on crystalline akagane´ite (Cl- type) under similar experimental conditions. Effect of Competing Anions. The BrO3- uptake on sample AlOH-5.8 is affected when other anions (added as NaCl, NaNO3, Na2SO4, or Na2CO3) with different concentrations coexist with BrO3- in the solutions (Figure 7). The competition between different anions is affected by many factors such as pH, initial concentrations of anions, nature of the anions, and affinity of the anions for the adsorbent.29 A slight effect on uptake of BrO3is observed by the presence of Cl- or NO3- ions in the concentration range that was studied, suggesting that the sample is more selective toward BrO3- than to either Cl- or NO3- ions. Differently from the cases of Cl- and NO3-, the BrO3- uptake is decreased significantly by the coexistence of SO42- or CO32in a concentration higher than 0.40 mmol/dm3, probably because of the competition of SO42- or CO32- for exchange sites of the sample. The coexistence of OH- ions also has a strong effect on BrO3- uptake. The uptake decreases continuously to 0.2 µmol/g with increased initial concentration up to 1.5 mmol/ dm3 OH-; the BrO3- uptake does not occur at 2 mmol/dm3 OH-. A gradual decrease of the uptake is apparent in equilibrium pH from 4.5 to 7.8. The effect of competing anions on BrO3- uptake on AlOH-5.8 is in the order OH- > CO32-, SO42- > NO3-, Clat the initial concentration of 2.0 mmol/dm3, suggesting that OH- ions are very powerful in replacing the initially adsorbed BrO3-.
Mechanism for BrO3- Uptake on Amorphous Aluminum Hydroxide. The sample AlOH-5.8 is amorphous aluminum hydroxide, as presented clearly in Figure 1a. This amorphous aluminum hydroxide is prepared at a pH value that is lower than the point of zero charge (PZC ) 8.1). The Al-Cl molar ratio of sample AlOH-5.8 is 5.0, which agrees with the reported results.22 We designate sample AlOH5.8 as aAlOH (Cl- type). As predicted from aAlOH (OH- type) prepared by dialysis of aAlOH (Cl- type),20 aAlOH (Cl- type) has a complicated structure containing tetrahedral (AlO4) and pentahedral subunits (AlO5) as well as octahedral ones (AlO6), quite different from crystalline aluminum hydroxides of gibbsite, bayerite, and boehmite, which all consist of only the latter subunit. Kabengi et al.22 demonstrated using flow adsorption calorimetry that aAlOH (Cl- type) exhibits reversible and rapid NO3- uptake but a complicated and slow uptake process of oxo anions such as arsenate and phosphate. Ladeira et al.26 reported that crystalline aluminum hydroxide of gibbsite forms an innersphere bidentate binuclear complex with arsenate on the surface from extended X-ray absorption fine structure (EXAFS) and density functional theory. According to the available literature,23,27,29–31 the uptake of anions from aqueous solutions on Fe, Al, Mn, and Ti oxides or hydroxides are explainable as follows: anions might be specifically adsorbed, involving the replacement of OH- or OH2- groups from the surfaces of the solid sample. These reactions are promoted at low pH, which causes OH- groups to accept protons, with the OH2- group being an easier ligand to displace than OH-. Specific adsorption
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is also regarded as inner-sphere adsorption because it involves direct coordination to the surface metal atom. Anions which form inner-sphere complexes are phosphate, molybdate, arsenate, and selenite. Nonspecific adsorption is regarded as outersphere adsorption. It is influenced by the ionic strength of the system. Chloride, nitrate, and perchlorate ions form outer-sphere complexes. Sulfate, selenate, and carbonate ions have intermediate properties. We can expect that BrO3- ions are bound through an outersphere complex reaction on the surface of aAlOH (Cl- type), based on the following results. 1. The amorphous sample AlOH-5.8, i.e., aAlOH (Cl- type), shows greater uptake of BrO3- than crystalline sample AlOH9.4, AlOH-11, or AlOH-ref. The difference in bromate uptake among samples may be Cl- ion, different BET surface area, and crystallinity. 2. The uptakes of BrO3- and NO3- (also SO42-) on aAlOH (Cl- type) are accompanied by release of Cl-, with a decrease in solution pH. 3. The uptake of BrO3- on aAlOH (Cl- type) is so fast as to attain equilibrium in 3 min, which is well consistent with reversible and fast NO3- uptake through outer-sphere complex reaction. 4. The uptake of BrO3- on aAlOH (Cl- type) is affected by the coexistence of OH-, a usual anion, in the solution phase. Reportedly, aAlOH (OH- type) is readily obtained by dialysis of the aAlOH (Cl- type).20 The formation of inner-sphere bidentate binuclear complexes of oxo anions such as H2PO4- and H2AsO4- with zirconium hydrous oxide (ZrO2 · xH2O) is estimated to proceed via attack of a Zr center by a lone pair of single-charged phosphates assisted by withdrawal of an OH group on the Zr by the hydrogen of an OH group on the phosphate. Another attack of the neighboring Zr by a newly generated lone pair on the same phosphate occurs, supported by withdrawal of an OH group on the same Zr by the hydrogen of the remaining OH group on the phosphate.32 This mechanism indicates that ZrO2 · xH2O exhibits a greater uptake of single-charged oxo anions such as H2PO4- and H2AsO4- than double-charged ones such as HPO42and HAsO42- because the double-charged oxo anions cannot form inner-sphere complexes. In addition, the formation of such inner-sphere complexes depends on the electron-acceptor capability of the metal center: Ti < Fe < Al < Zr. According to this model, the bromate ion BrO3- is very unfavorable for formation of similar inner-sphere bidentate binuclear complexes because it has no OH group. It is remarked that the possession of plural OH groups is indispensable in this model. Therefore, the model of this inner-sphere complex also supports the uptake of BrO3- on an AlOH (Cl- type) through the outer-sphere complex reaction described above. Comparison of BrO3- Uptake with Other Materials. BrO3- uptakes from aqueous solution at low concentrations on different materials reported in the literature and their uptake mechanisms are given in Table 3. The BrO3- uptake on crystalline akagane´ite occurs via an ion-exchange mechanism based on the liberation of Cl- from tunnel sites to aqueous solution. An equilibrium concentration of 0.078 µmol/dm3 can be achieved from a 0.156 µmol/dm3 BrO3- solution at a dose of 1.0 g/dm3.19 A study of boehmite-loaded TiO2 shows an equilibrium concentration of 0.30 µmol/dm3 BrO3- from an initial concentration of 1.6 µmol/dm3 at a dose of 1.0 g/dm3 via a reduction process.16 Complete removal of BrO3- can be achieved on zerovalent iron from an initial concentration of 0.78 µmol/dm3 BrO3- at a dose of 25 g/dm3 via a reduction process.15
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Table 3. Uptake of BrO3 on Different Materials sample
initial equilibrium BrO3- concn BrO3- concn dose (µmol/dm3) (µmol/dm3) (g/dm3) mechanism reference
AlOH-5.8 (aAlOH (Cl- type))
0.78
0.04
1.0
outer-sphere complex reaction
present study
crystalline akagane´ite
1.0 0.15
0.38 0.078
1.0 1.0
ion exchange 19
boehmite-loaded TiO2
1.6
0.30
1.0
reduction
16
zerovalent iron activated carbon
0.78 0.39
0 0.078
25.0 reduction 0.10 reduction
15 9
Powdered activated carbon is also effective for decreasing the BrO3- concentration to 0.078 µmol/dm3 from an initial concentration of 0.39 µmol/dm3 at a dose of 0.1 g/dm3 via a reduction process.9 The significance of the present work is that an equilibrium concentration smaller than 0.078 µmol/dm3 BrO3-, the standard for drinking water, is achieved when AlOH5.8 is treated with 0.78 µmol/dm3 BrO3- at a dose of 1.0 g/dm3. 4. Conclusions The BrO3- uptake from aqueous solution on amorphous aluminum hydroxide (AlOH-5.8) occurs through an outer-sphere complex reaction. The sample shows a high Kd value of BrO3(2100 cm3/g) at concentration of 0.10 mmol/dm3 from a mixed solution of salts (NaCl, NaNO3, Na2SO4, and Na2CO3). The sample AlOH-5.8 is a novel ion-exchange material which shows that the initial 0.78 µmol/dm3 BrO3- can be decreased to
