Defluoridation by freshly prepared aluminum hydroxides

Chemical Engineering Journal 175 (2011) 144–149

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Defluoridation by freshly prepared aluminum hydroxides Ruiping Liu a , Wenxin Gong a,b , Huachun Lan a,b , Yuping Gao a,c , Huijuan Liu a,∗ , Jiuhui Qu a a

State key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Graduate School, Chinese Academy of Sciences, Beijing 100039, China c School of Civil and Environmental Engineering, University of Science and Technology, Beijing 100083, China b

a r t i c l e

i n f o

Article history: Received 30 June 2011 Received in revised form 15 September 2011 Accepted 17 September 2011 Keywords: Defluoridation In-situ Al2 O3 ·xH2 O Zeta potential Al species distribution Solid-phase Alc

a b s t r a c t The exposure to fluoride via drinking water is a great issue globally. This study investigates the adsorptive capability of the freshly prepared aluminum hydroxide, i.e., in-situ Al2 O3 ·xH2 O, towards fluoride. The maximum adsorption of above 110 mg F/g Al is observed in pH ranges from 5.0 to 7.2. The adsorption equilibrium is achieved within 120 min, and pseudo-second-order model may well describe the adsorption kinetics (R2 = 0.999), indicating the involvement of chemisorptions in. The characteristics of low particle diameter, high surface area, and the surface reactivity of the amorphous in-situ Al2 O3 ·xH2 O enables its superior to remove fluoride for the in-situ Al2 O3 ·xH2 O. pH impacts the distribution of Al species and the quantity of solid-state Al available for fluoride. The ratios of monomeric Al, i.e., Ala , are below 4% in pH from 6 to 8, and then increase to 19.2% at pH 4 and to 28.9% at pH 10. Alc , showing adverse trends to Ala , is the main species for fluoride removal. In cases that the amount of in-situ Al2 O3 ·xH2 O available is the same, surface charge is dominant to affect the removal of fluoride. FTIR indicates the replacement of surface hydroxyl groups by fluoride. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Over 300 million individuals are exposed to fluoride via drinking water globally, especially in developing countries, among which more than one-third are suffering from dental and skeletal fluorosis. There have been proposed different technologies for defluoridation including (electro) coagulation [1], precipitation [2], adsorption [3,4], reverse osmosis [5], and the Nalgonda process [6]. However, many initiatives on defluoridation of water have resulted in frustration and failure [7]. There is a great deal of engineering are poorly operated in practice due to the limited removal efficiency, high cost, complicated operation, and labored maintenance. Adsorption exhibits good feasibility and advantages in distributed rural areas because it requires little expertise and minimum labored-operation. Different adsorbents such as activated alumina [8], rare earth oxides [9], and low-cost adsorbents [10] (e.g., bone charcoal, calcite, clay charcoal, tree bark, saw dust, rice husk, ground nut husk), have been investigated for defluoridation. Tripathy and Raichur studied the removal of fluoride by manganese dioxide-coated activated alumina (MCAA), and reported the Langmuir constant of Q to be 0.1701 mg/g and 0.159 mg/g at pH 4 and 7, respectively [4]. Meenakshi et al. reported the maximal sorption capability of 0.782 mg/g for the mechanochemically activated kaolinites [11]. Fan et al. investigated

∗ Corresponding author. Tel.: +86 10 62849151; fax: +86 10 62849160. E-mail address: [email protected] (H. Liu). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.09.083

the adsorption capability of several low cost materials, i.e., Hydroxyapatite, Fluorspar, Quartz activated using ferric ions, Calcite, and Quartz towards fluoride, and reported the highest sorption capability of 4.54 mg/g for Hydroxyapatite [10]. However, the use of these adsorbents is generally restricted by the low adsorbing capability, high empty bed contact time (EBCT), and frequent-regeneration. The mixed metal oxides with significantly higher efficacy to remove fluoride have also been developed. Deng et al. successfully developed a novel Mn–Ce oxide adsorbent with high sorption capacities towards fluoride, and the powdered and granular adsorbent were 79.5 and 45.5 mg/g respectively at the equilibrium fluoride concentration of 1 mg/L [12]. The modified alumina adsorbent with calcium oxide or manganese oxide has also been reported before [13]. Unfortunately, the high cost for adsorbents preparation and the complicated regeneration procedures also inhibits the largescale application of these novel adsorbents in practice. A novel adsorbent (i.e., FMBO-Diatomite) was successfully developed in our previous study to remove arsenic, and this is prepared by the in-situ coating of ferric and manganese binary oxide (FMBO) onto diatomite [14]. The in-situ preparation and regeneration method maintains the surface adsorption activity as much as possible, and overcomes several drawbacks involves in the convention regeneration method such as time-consuming, low-operability, the production of highly basic wastewater. This strategy may be feasible to develop another novel adsorbent for defluoridation if the component with good activity towards fluoride is used. Aluminum hydroxide, which is commonly formed in alum coagulation or electro coagulation process, shows good adsorptive

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Table 1 Kinetic equations tested to describe time-dependent fluoride adsorption to the in-situ Al2 O3 ·xH2 O. Kinetic models

Equation

Parameters

Pseudo-first-order Elovich Pseudo-second-order

qt = qmax − exp(ln(qmax ) − kt) qt = a + k ln(t) qt = qmax + qmax /(kqmax t − 1)

qt is the amount of fluoride adsorbed at time t. qmax is the maximum amount of fluoride adsorbed at equilibrium. k and a are constants

capability towards fluoride [1,15]. The freshly prepared aluminum hydroxide, i.e., in-situ Al2 O3 ·xH2 O, which is formed through the enhanced hydrolysis of aluminum ion (Al3+ ), may be an adoptable component to be coated onto porous carriers. However, to our best knowledge, there are little studies on the adsorptive ability of in-situ Al2 O3 ·xH2 O towards fluoride. The objectives of this study were to: (1) evaluate the adsorption capability of in-situ Al2 O3 ·xH2 O towards fluoride at different conditions; (2) illustrate the main characteristics of the in-situ Al2 O3 ·xH2 O at different pH ranges, i.e., the Al species distribution, zeta potential, and residual Al levels; (3) propose the dominant reactions involves in the removal of fluoride by the in-situ Al2 O3 ·xH2 O. 2. Materials and methods 2.1. Experimental methods All chemicals were of analytical-reagent grade and were used without further purification. The raw water was prepared by adding sodium fluoride (NaF) into deionized water in polyethene vessels to get desired concentrations of fluoride. Sodium nitrate (NaNO3 ) and sodium bicarbonate (NaHCO3 ) were also added to obtain required ionic strength and alkalinity, and pH was adjusted thereafter to desired values by hydrochloric acid (HCl) and sodium hydroxide (NaOH). The in-situ Al2 O3 ·xH2 O was prepared by reactions between aluminum chloride (AlCl3 ) and NaOH. The stock solution of these two reagents was well agitated soon after being mixed and was then dosed immediately. Batch adsorption experiments were conducted in capped vessels with continuous rotary shaking (120 rpm) at 25 ± 1 ◦ C for 2 h. Preliminary kinetics study showed that the contact time of 2 h was sufficient to achieve adsorption equilibrium. Samples were filtered through 0.45-␮m membrane filters to analyze the concentrations of fluoride and Al concentrations in the filtrate. The equilibrium pH was also measured thereafter, and showed the slight pH variation of ±0.2 after adsorption. The removal of fluoride by AlCl3 coagulation was also compared to evaluate the removal capability of the in-situ Al2 O3 ·xH2 O. The aforementioned procedures were preceded at the same Al dosages. The experiments on the removal of fluoride by AlCl3 and the in-situ Al2 O3 ·xH2 O were performed in duplicates. 2.2. Characterization and analytical methods The X-ray diffraction (XRD) pattern of the in-situ Al2 O3 ·xH2 O was determined using a X’Pert PRO Powder diffractometer machine (PANalytical Co.). Data were collected at 40 keV using a graphite curved-crystal monochromator between 10◦ and 90◦ 2 in 0.02◦ steps. The specific surface area was measured by nitrogen adsorption using the BET method with a Micromeritics ASAP 2000 (Micromeritics Co., USA) surface area analyzer. The samples were analyzed using a scanning electron microscope (SEM) (S-3000N, Hitachi Co., USA). Before analysis, the samples were sputter coated (Quorum Polaron SC7620 Minisputter Coater) with gold/palladium (45 s) in order to reduce the charging effect in the microscope. Power samples of the

in-situ Al2 O3 ·xH2 O were freeze-dried before the above noted analysis. pH was measured with a precise pH meter (720A, Thermo Orion, USA). The concentrations of fluoride were determined by a selective electrode for fluoride ions. The electrokinetic potential ( potential) of the in-situ Al2 O3 ·xH2 O was measured with a Zeta Potential Analyzer, model Zetasizer2000 (Malvern Co.) in triplicate. The average volumetric particle diameter (AVPD) and PSD were determined with a Mastersizer2000 Laser Particle Size Analyzer (Malvern Co.). Total Al concentrations (AlT ) were measured with an inductively coupled plasma optical emission spectrometer (ICP-OES) (Optima 2000, PerkinElmer, UK). The ferron colorimetric method was used to analyze initial Al species distribution of the coagulants. The absorbance increase was monitored for 120 min to operationally define three fractions of Ala , Alb , and Alc , which corresponded to monomeric, medium polymer, and larger polymer species and/or solid-phase Al(OH)3 , respectively. The reaction time of Ala -ferron was 1 min, and the Al species reacting with the ferron reagent before 120 min represented [Ala + Alb ], then Alc was obtained by AlT minus Ala and Alb . 2.3. Adsorption kinetic modeling The time-dependent experimental data from the adsorption of fluoride onto the in-situ Al2 O3 ·xH2 O were fit to three different kinetic models of pseudo-first-order equation, Elovich model, and pseudo-second-order (Table 1) [16]. These kinetic models were fit to the fluoride adsorption data using non-liner regression analysis. The conformity between experimental data and the model-predicted values was evaluated by the coefficient of determination (R2 ). 3. Results and discussions 3.1. Evaluation of adsorption capability of in-situ Al2 O3 ·xH2 O towards fluoride 3.1.1. The removal of fluoride at different Al dosages in near neutral pH ranges To optimize the doses of the in-situ Al2 O3 ·xH2 O to remove fluoride in practice, Fig. 1 presented the removal efficiency of fluoride with elevated dosages of in-situ Al2 O3 ·xH2 O in near neutral pH conditions of 6.0, 7.0, and 7.5. With the Al dosages increasing from 13.5 to 108 mg/L as Al, the removal efficiency of fluoride increased from 47.8% to 96.7% at pH 6, from 39.2% to 90.3% at pH 7.0, and from 35.1% to 88.9% at pH 7.5. The elevated doses of the in-situ Al2 O3 ·xH2 O provided more adsorptive sites available, and increased the removal of fluoride thereafter. Additionally, the removal of fluoride was favored at lower pH, which was as high as 93.8% at 54 mg/L of insitu Al2 O3 ·xH2 O as Al at pH 6.0, and decreased to 75.1% at pH 7.0 and to 72.6% at pH 7.5 accordingly. The effect of pH on the removal of fluoride by the in-situ Al2 O3 ·xH2 O will be discussed detail later. The adsorption by in-situ Al2 O3 ·xH2 O was observed to be superior to remove fluoride than AlCl3 coagulation (Fig. S1). Moreover, the difference of fluoride removal efficiency at a certain pH, i.e., DF |pHi , was developed and described in Eq. (1): DF |pHi = E(Al2 O3 · xH2 O)|pHi − E(AlCl3 )|pHi

(1)

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0.12

0.10

80

0.08

Qe (mg/g)

F- removal efficiency (%)

100

60

0.06

40

20

pH6.0 pH7.0

0.04

pH7.5

0.02

Fluoride removal capability Pseudo-first-order Pseudo-second-order Elovich

0.00

0 0

20

40

60

80

100

120

140

0

Al dosage (mg/L)

where E(Al2 O3 ·xH2 O)|pHi and E(AlCl3 )|pHi corresponded to the removal efficiency of fluoride by in-situ Al2 O3 ·xH2 O and AlCl3 at pHi , respectively. DF |pHi values were consistently positive with the elevated Al dosages, and this indicated the better capability of removing fluoride for the in-situ Al2 O3 ·xH2 O than AlCl3 . DF |pHi values were observed to be dependent on pH conditions. The DF |pH7.5 values at Al dosages of 13.5 and 108 mg/L were 8.4% and 10.3% whereas the DF |pH6.0 values decreased to lower values of 4.5% and 2.7% (Fig. S1). 3.1.2. The removal of fluoride at different pH In a wide pH ranges from 3.0 to 11.0, the in-situ Al2 O3 ·xH2 O showed significantly different adsorptive capability towards fluoride (Fig. 2). The in-situ Al2 O3 ·xH2 O exhibited maximum removal capability towards fluoride in weak acidic pH conditions. Quantitatively, the removal efficiency of fluoride was higher than 60% in weak acidic pH ranges from 7.0 to 5.0, and then decreased from 63.1% to 18.9% ([F− ]0 = 4 mg/L) and from 68.2% to 38.1% ([F− ]0 = 10 mg/L) with pH decreasing from 5.0 to 3.0 accordingly. The increase of solution pH also inhibited the removal of fluoride

[F]0= 10 mg/L [F]0= 4 mg/L

F- removal (mg/g Al)

120

2

3

4

5

Time (h)

Fig. 1. The fluoride removal efficiency with elevated dosages of the in-situ Al2 O3 ·xH2 O at pH 6, 7.0, and 7.5 [experimental conditions: [F]0 = 10 mg/L].

140

1

100

Fig. 3. The removal of fluoride by the in-situ Al2 O3 ·xH2 O with prolonged time and the modeling of adsorption kinetic (experimental conditions: [F]0 = 10 mg/L, the dosage of the in-situ Al2 O3 ·xH2 O = 81 mg/L as Al, pH 7.0).

by in-situ Al2 O3 ·xH2 O. Moreover, with pH increasing from 7.0 to 11.0, the removal efficiency of fluoride decreased from 67.4% to 11.8% ([F− ]0 = 4 mg/L) and from 87.8% to 20.5% ([F− ]0 = 10 mg/L) accordingly. The alum speciation was strongly pH dependent. The alumina dissolves to soluble Al species at either low or high pH conditions. Theoretical calculations showed that alumina starts to dissolve to alum ions (Al3+ ) at pH < 5.5 and to aluminates ([Al(OH)4 ]− ) at pH > 8, as indicated in Eqs. (2) and (3). Consequently, the observed lower removal capacity towards fluoride in Fig. 2 was not only due to the pH effects, but also to the dissolution of alumina and the less alumina available for fluoride adsorption. The species distribution of alum at different pH would be discussed later. Al(OH)3 + 3H+ = Al3+ + 3H2 O −

Al(OH)3 + H2 O = [Al(OH)4 ] + H

(2) +

(3)

In weak acidic conditions, the in-situ Al2 O3 ·xH2 O showed the maximum adsorption capability of 117.6 mg F/g Al (10 mg/L of [F− ]0 , 81 mg/L as Al, [F− ]equilibrium = 0.48 mg/L) and 128.0 mg F/g Al (4 mg/L of [F− ]0 , 27 mg/L as Al, [F− ]equilibrium = 0.54 mg/L), respectively. To enable the quantitative comparison of adsorption capability with other adsorbents, the in-situ Al2 O3 ·xH2 O was assumed to be in the form of Al2 O3 ·3H2 O (i.e., 2Al(OH)3 ) after drying. Then the aforementioned adsorption capability per dry-weight of the adsorbent was calculated to be 40.66 mg F/g Al2 O3 ·3H2 O ([F− ]equilibrium = 0.48 mg/L) and 44.28 mg F/g Al2 O3 ·3H2 O ([F− ]equilibrium = 0.54 mg/L), respectively.

80 60 40 20 0 2

4

6

8

10

12

pH Fig. 2. The removal capability of the in-situ Al2 O3 ·xH2 O towards fluoride in pH ranges from 3 to 11 [experimental conditions: (a) [F]0 = 4 mg/L, the dosage of the in-situ Al2 O3 ·xH2 O = 27 mg/L as Al; (b) [F]0 = 10 mg/L, the dosage of the in-situ Al2 O3 ·xH2 O = 81 mg Al/L].

3.1.3. Adsorption kinetics Adsorption kinetics is crucial to the design of adsorption reactors for defluoridation. The adsorption of fluoride onto the in-situ Al2 O3 ·xH2 O was observed to be fast (Fig. 3). The removal efficiency of fluoride achieved to 75.8% after 10 min, to 78.5% after 30 min and to 86.0% after 120 min, and then remained at this steady state with prolonged contact time. Three different kinetic models of pseudo-first-order equation, Elovich model, and pseudo-second-order, as indicated in Table 1, were used to evaluate the kinetic mechanism that controlled the adsorption processes of fluoride on the in-situ Al2 O3 ·xH2 O. The kinetic parameters obtained from the models were shown in Table 2. Through comparing the regression coefficient values, it was found that the adsorption kinetic data of fluoride

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Table 2 The fitted kinetic model parameters for the adsorption of fluoride on the in-situ Al2 O3 ·xH2 O. Pseudo-first-order

Elovich

q = qmax − exp(ln(qmax ) − kt)

y = a + k ln(t)

Pseudo-second-order q = qmax + qmax /(kqmax t − 1)

qmax (mmol g−1 )

k (mmol g−1 h−1 )

R2

a

k

R2

qmax

K

R2

0.10

12.62

0.996

0.10

0.01

0.988

0.11

−218.01

0.999

onto the in-situ Al2 O3 ·xH2 O were fitted well with these three models, and the R2 values were calculated to be 0.996, 0.988, and 0.999, respectively. Comparatively, among these models the pseudo-second-order model was best to describe the adsorptive kinetics, and this indicated the involvement of chemisorptions in the adsorption of fluoride onto the in-situ Al2 O3 ·xH2 O.

pH 10. The ratios of Alb were in ranges from 8.6% to 13.5% in wide pH ranges from 4 to 10. It was noted that the high soluble Al levels were observed in strong acidic and basic pH conditions, which rarely occurred in natural pH conditions. On the other hand, the dissolution of alum oxide and the levels of soluble Al in neutral pH range was low, which was valuable for its application to remove fluoride from natural water.

3.2. Characterization of the in-situ Al2 O3 ·xH2 O 100 In situ Al2O3

Ala

80

AlCl3

60 40 20

Ratio of different Al species (%)

0 100

Alb

80 60 40 20 0 100

Alc

80 60 40 20 0 1000

Soluble Al (mg/g)

The in-situ Al2 O3 ·xH2 O exhibited high adsorptive capability and rapid adsorption velocity towards fluoride. This may be first attributed to its low particle diameter and the high surface area of in-situ Al2 O3 ·xH2 O. The XRD pattern of the in-situ Al2 O3 ·xH2 O showed no obvious crystalline peak, and this indicated that this alumina exists mainly in amorphous form. SEM image also indicated course surfaces with plentiful pores on the surfaces of the in-situ Al2 O3 ·xH2 O (Fig. S2). The amorphous phase with low crystalline may be responsible for its high surface area. The BET surface area was determined to be as high as 118.24 m2 /g for this in-situ Al2 O3 ·xH2 O. Generally, amorphous alumina exhibits high surface area and a large number of surface active sites, which decreases greatly with the increase of crystallinity. Moreover, PSD analysis indicated that its particle diameter was ranged from 0.8 to 200 ␮m with the AVPD value of 24.46 ␮m. The aforementioned characters provided much sites available to benefit the adsorption of fluoride and were valuable for its high removal capacity towards fluoride. Additionally, the preparation procedures of the in-situ Al2 O3 ·xH2 O avoided the sintering process, which was often used to prepare other adsorbents such as activate alumina. This preparation method kept the surface reactivity towards fluoride as much as possible. In practice, the in-situ Al2 O3 ·xH2 O may be in-situ coated onto porous carriers to greatly simplify the granulation of adsorbent, and quickly restores the adsorptive capacity of in-situ Al2 O3 ·xH2 O towards fluoride. During Al coagulation, there produces different incomplete hydrolysis products such as Al(OH)2+ , Al(OH)2 + , and Al2 (OH)2 4+ [17]. These species also exhibit high reactivity towards fluoride, but unfortunately form soluble Al–F species to a large extent, especially in cases of insufficient alkalinity. The formation of soluble Al–F species cannot achieve the removal of fluoride. Comparatively, the enhanced hydrolysis of Al3+ during preparing the in-situ Al2 O3 ·xH2 O minimized the formation of soluble Al species and the Al–F species thereafter. Furthermore, the solubility of the in-situ Al2 O3 ·xH2 O was pH dependent, and the residual Al concentrations increased significantly in pH ranges of below 5 and above 9 in equations noted above (Fig. 4d). This indicated the dissolution of the in-situ Al2 O3 ·xH2 O and the formation of soluble Al species, i.e., the monomeric Al, the small, median, and large polymeric Al at low pH, and the Al(OH)4 − at high pH. The Al species distribution of the in-situ Al2 O3 ·xH2 O at different pH was illustrated in Fig. 4a–c. In the acidic region, the ratios of Ala increased with lower pH and reached to as high as 19.2% at pH 4. The elevated pH in basic region also increased Ala ratio and achieved to the maximal value of 28.9% at pH 10. The ratios of Alc showed adverse trends to Ala , which showed stable values of near to 85% in pH 6 to 8, and decreased to 69.9% at pH 4 and to 61.4% at

800 600 400 200 0 3

4

5

6

7

8

9

10

11

pH Fig. 4. Al species distribution of the in-situ Al2 O3 ·xH2 O and AlCl3 in pH from 4 to 10 and the corresponsive soluble Al concentrations of the in-situ Al2 O3 ·xH2 O.

148

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2.0

10 1.5

5 0

ΔF (mg/L)

Zeta potential (mV)

15

-5 -10

1.0

0.5 -15 3

4

5

6

7

8

9

10

y=-0.0448x-0.118

11

2

R =0.9633

pH

0.0

Fig. 5. Zeta potential analysis of the in-situ Al2 O3 ·xH2 O in equilibrium pH ranges from 3.1 to 10.9.

3.3. Mechanisms involve in the removal of fluoride by the in-situ Al2 O3 ·xH2 O Results above indicated that pH impacted the removal of fluoride by the in-situ Al2 O3 ·xH2 O, and the weak acidic pH was optimum to remove fluoride (Fig. 2). The soluble Al concentrations were lowest in this pH range, and provided most insoluble Al species available to adsorb fluoride (Fig. 4d). However, there also showed obvious difference in fluoride removal at the same levels of solid-phase Al. The removal efficiency of fluoride at pH 6.2 was 95.2% (10 mg/L of [F− ]0 , 81 mg/L as Al) and 86.4% (4 mg/L of [F− ]0 , 27 mg/L as Al), which decreased to lower values of 88.3% and 67.4% (Fig. 2) at pH 7.2. It was indicated in Fig. 4 that the residual Al concentrations and the ratios of different Al species were in the same level under these conditions. Consequently, it was assumed that another mechanism dominated in the removal of fluoride by the in-situ Al2 O3 ·xH2 O under these conditions. It was observed in Fig. 5 that the zeta potential at pH 6 was higher than that at pH 7. Furthermore, the removal efficiency of fluoride was observed to be positively correlated with the zeta potential of the in-situ Al2 O3 ·xH2 O in wide pH ranges from 3 to 11 (Fig. S3). Surface charge may dominate in the removal of fluoride by the insitu Al2 O3 ·xH2 O. To further demonstrate the significance of surface charge on the removal of fluoride, four kinds of ions (i.e., Ca2+ , Cl− , SO4 2− , and PO4 3− ) at three concentration levels (2, 5, and 10 mM) were introduced to evaluate their effects on the zeta potential and the removal of fluoride. Fig. S4 indicated that chloride and sulfate ions showed slight effect on the removal of fluoride, and that phosphate indicated strong side effects. As for the removal of fluoride, it was noted that the levels of phosphate in natural underground water were generally low, and its adverse effect may be largely ignored in practice. Additionally, Fig. S4 also showed that these anions decreased zeta potential of the in-situ Al2 O3 ·xH2 O to different extent accordingly. Fig. 6 indicated that the more significant decrease of zeta potential () was negatively correlated with

-30

-20

-10

0

ΔZeta (mV) Fig. 6. Relativity between  and F− in the removal of fluoride by the insitu Al2 O3 ·xH2 O in the presence of different ions (experimental conditions: [F]0 = 10 mg/L, the dosage of the in-situ Al2 O3 ·xH2 O = 81 mg/L as Al, pH 6.0).

the removed fluoride (F− ) by the in-situ Al2 O3 ·xH2 O (R2 = 0.963). These results indicated the significance of surface charge as the initiating step of the removal process on the removal of fluoride by the in-situ Al2 O3 ·xH2 O. FTIR analysis also provided valuable information on the interactions between fluoride and the in-situ Al2 O3 ·xH2 O. Fig. 7 illustrated the FTIR spectra of the in-situ Al2 O3 ·xH2 O and those after adsorbing fluoride at different ratios of F to Al (mg/mg) of 1: 5 and 1: 1. As for these spectra, the peaks at 3493 cm−1 and 1650 cm−1 were respectively ascribed to the stretching collision and deformation of water molecules, and there showed no obviously variation in these two peaks. In wavenumber ranges from 1340 to 1600 cm−1 , the in-situ Al2 O3 ·xH2 O showed a peak at 1529 cm−1 and a broad band at 1454 cm−1 . These two peaks corresponded to the stretching and bending modes of Al–O [18]. After adsorbing fluoride, these bands disappeared and there showed another remarkable absorption peaks at 997 cm−1 . This may be attributed to the substitution of surface hydroxyl groups on the in-situ Al2 O3 ·xH2 O by fluoride [19,20]. Additionally, the in-situ Al2 O3 ·xH2 O showed a peak at 623 cm−1 , which was assigned to the stretching collision of the Al–O groups [19]. This band showed no obvious shift to lower wavenumbers. However, the formation of surface complexes (e.g., Al–O–F) was 1.60

3493

623

1.40 997

1.20

Absorbance

Comparatively, AlCl3 showed significant variation of Al species distribution with lower ratios of insoluble Alc in a wide pH range [17]. These Al species in neutral pH ranges enables the high efficiency of removing fluoride for the in-situ Al2 O3 ·xH2 O. Zeta potential analysis showed that the in-situ Al2 O3 ·xH2 O exhibited positive surface charge at pH < 7.2, with the maximal value of +14.7 mV at pH 6.2 (Fig. 5). The elevated pH decreased the zeta potential, which was observed to be as low as −14.1 mV at pH 10.9. It was noted that the zeta potential corresponded to solid-state Alc species, rather than the soluble Al species of Ala and Alb . The surface charge of the in-situ Al2 O3 ·xH2 O impacted the adsorption of negatively charged fluoride greatly.

-40

1.00

F Al = 0

0.80

F Al = 1 5

1529

F Al = 1 1

0.60

1650 1454

0.40 0.20 0.00 4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1) Fig. 7. FTIR analysis of the in-situ Al2 O3 ·xH2 O at different ratios of F to Al (mg/mg) (experimental conditions: the dosage of the in-situ Al2 O3 ·xH2 O was 50 mg/L as Al, pH 6).

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proposed in considering the occurrence of chemisorptions, as indicated by the high R2 value in pseudo-second-order model. The interfacial interactions of chemisorptions are the final step in the removal of fluoride by the in-situ Al2 O3 ·xH2 O. The replacement of surface hydroxyl groups by fluoride and the formation of Al–O–F complexes might involve in the adsorption of fluoride onto the in-situ Al2 O3 ·xH2 O, as being described in Eq. (4).

Appendix A. Supplementary data

[Alx Oy (OH)z ](OH− )3x−2y−z + (3x − 2y − z)F−

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→ [Alx Oy (OH)z ](F− )3x−2y−z + (3x − 2y − z)OH−

(4)

These Al–O–F complexes combined with the insoluble in-situ Al2 O3 ·xH2 O to achieve fluoride removal. Comparatively, the formation of soluble Al–F complexes occurred during AlCl3 coagulation, which decreased the capability to remove fluoride to some extent. 4. Conclusions The in-situ Al2 O3 ·xH2 O exhibits high adsorptive capability and rapid adsorption velocity towards fluoride, owing to its higher surface area and stronger activity than other common adsorbents. Additionally, while preparing the in-situ Al2 O3 ·xH2 O, the enhanced hydrolysis of Al3+ favors the formation of insoluble Alc species and minimizes the soluble Al–F complexes mostly. This effect enables better capability than AlCl3 coagulation with respect to the removal of fluoride. Moreover, the removal of fluoride by the in-situ Al2 O3 ·xH2 O is pH dependent, and the highest removal efficiency in weak acidic conditions. The ratio of insoluble Alc , i.e., the solid-state Al species available for fluoride removal, dominates in the removal of fluoride. The surface charge of the in-situ Al2 O3 ·xH2 O also affects to a large extent in cases that the insoluble Alc is in the same level. The replacement of surface hydroxyl groups by fluoride and the formation of Al–O–F complexes might be observed in FTIR analysis. The in-situ Al2 O3 ·xH2 O may be in-situ coated onto porous carriers such as diatomite to develop a novel adsorbent for defluoridation. To regenerate the used adsorbent with fluoride saturated on its surface, a new layer of active in-situ Al2 O3 ·xH2 O can also be in-situ coated onto the surface of the old layer to simplify the regeneration of used adsorbents, and to quickly restore the adsorptive capacity towards fluoride accordingly. Acknowledgments This work was supported by the key project of National “863” High-tech R&D Program of China (2009AA062905) and the Funds for Creative Research Groups of China (Grant No. 50921064). Appreciation is also extended to the Special Co-construction Project of Beijing Municipal Commission of Education for financial support.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2011.09.083. References