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High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent
Accepted Manuscript High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent Shibiao Wu, Kaisheng Zhang, Junyong He, Xingguo Cai, Kai Chen, Yulian Li, Bai Sun, Lingtao Kong, Jinhuai Liu PII: DOI: Reference:
S0021-9797(15)30289-7 http://dx.doi.org/10.1016/j.jcis.2015.10.045 YJCIS 20831
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
20 May 2015 14 October 2015 17 October 2015
Please cite this article as: S. Wu, K. Zhang, J. He, X. Cai, K. Chen, Y. Li, B. Sun, L. Kong, J. Liu, High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.10.045
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High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent
Shibiao Wu a, b, Kaisheng Zhang b, Junyong He b, Xingguo Cai b, Kai Chen b, Yulian Li b, Bai Sun b, *, Lingtao Kong b, *, Jinhuai Liub
a
School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui, 230039, China.
b
Nanomaterials and Environmental Detection Laboratory, Institute of Intelligent Machines, Chinese
Academy of Sciences, Hefei, Anhui, 230031, China.
Corresponding Author *
E-mail address: [email protected] (L. Kong); [email protected] (B. Sun); Fax: +86-551-65592420; Tel:
+86-551-65591142.
ABSTRACT A novel adsorbent, hydroxyl aluminum oxalate (HAO), for the high efficient removal of fluoride from aqueous solution was successfully synthesized. The adsorbent was characterized and its performance in fluoride (F-) removal was evaluated for the first time. Kinetic data reveal that the F- adsorption is rapid in the beginning followed by a slower adsorption process; 75.9% adsorption can be achieved within 1 min and only 16% additional removal occurred in the following 239 min. The F- adsorption kinetics was well described by the pseudo second-order kinetic model. The calculated adsorption capacity of this adsorbent for F- by Langmuir model was 400 mg g-1 at pH 6.5, which is one of the highest capabilities of 1
today’s materials. The thermodynamic parameters calculated from the temperature-dependent isotherms indicate that the adsorption reaction of F- on the HAO is a spontaneous process. The FT-IR spectra of HAO before and after adsorbing F- show adsorption mechanism should be hydroxyl and oxalate interchange with F-.
Keywords: Hydroxyl aluminum oxalate; Fluoride; Adsorption; Kinetics; Thermodynamics
1. Introduction
Fluorine is being used in industrial activities with an increasing amount and then introduced into the aquatic environment in fluoride (F-) form as a consequence of its widely use in modern industries. Though fluorine is a trace element necessary to human body for the calcification tooth and skeleton [1], a number of diseases, such as fluorosis of bone and cancer [2], are caused by ingestion of water contaminated with excess fluoride. As with the diseases mentioned above, the best alternative to prevention is keeping from excessive fluoride ingestion. According to WHO (World Health Organization), the permissible upper limit of F- in drinking water is 1.0 mg L-1 [1], and the excessive F- in the water should be removed. There are many fluoride waste water treatment methods, including ion exchange [3], reverse osmosis [4], electro dialysis [5], chemical precipitation [6], membrane separation [7], coagulation sedimentation [8], electro coagulation [9] and adsorption processes [10]. Among the methods, adsorption technology is an economical and efficient method for producing high quality of drinking water. In recent years, a variety of adsorbents like activated alumina, chitosan beads, activated carbon, clay, hydroxyapatite, etc., have been identified as the promising defluoridating agents [11, 12]. 2
However, there is still a great demand for more effective, simple, and low-cost adsorbents for the removal of fluoride. Recent studies revealed that the intercalated layered materials were widespread attentions due to their excellence adsorption and ion exchange performances for adsorbing metal ions, such as Cu2+, Cd2+, Pb2+ and Hg2+ etc., with high selectivity [13-19]. As an important defluoridating material, Al(OH)3 is also a layered hydroxide material [20]. The layers are made of aluminium-oxygen octahedrons [Al(OH)6]3-, in which octahedrons share edges, and construct the infinite hexameric ring sheet layer [21]. The layers can be stacked one above the other in different ways, forming four crystallographic forms: bayerite [22], gibbsite [23], nordstrandite [24] and doyleite [25]. Although the intra layer bonding is strong ion covalent, interlayer bonding is weak van der Waal׳s [20]. So, the interlayer space in Al(OH)3 should be easy to be intercalated by some light-weight-molecule matters. Some light-weight-molecule agents including pyromellitic acid [13], lactic acid [26], citric acid [27] and malic acid [28] have been successfully used to synthesize the intercalated layered materials. In this paper, oxalate was selected to intercalate into the interlayer space in Al(OH)3 by a facile hydrothermal method. The structure of Al(OH)3 layer sheets and the process of oxalate intercalation in Al(OH)3 interlayer space was demonstrated in Fig. 1. The oxalate modified Al(OH)3 was name after hydroxyl aluminum oxalate (HAO) here. The interlayer space in Al(OH)3 expanded by intercalation and the HAO specific surface area would be increased notably. By the way, Al(III) on the sheet layer is of good coordination ability toward F- [29], HAO should be an effective adsorbent for the removal of fluoride from aqueous solution. In present study, the defluoridation capacity and adsorption kinetics of the as-prepared HAO for F- were examined. The effects of pH, initial F- concentration and co-existing anions were investigated. The results show that the adsorbent possesses high selectivity 3
and excellent adsorption capacity for F- removal. The F- removal mechanism was studied by Fourier transform infrared spectroscopy (FT-IR), indicating that the fluoride adsorption mainly resulted from the exchange of oxalic ions and hydroxyl groups on the surface of the HAO with fluoride anions. We anticipate this work could provide a good example for highly efficient and long-term usage of the HAO during removing fluoride from drinking water.
2. Materials and methods 2.1. Materials Aluminium isopropoxide (AIP, Al[OCH(CH3)2]3, 99.99%) was purchased from Sigma-Aldrichas an aluminum precursor. Oxalate dihydrate (OD, AR), sodium hydroxide (NaOH, AR), nitric acid (HNO3, AR), sodiumnitrate (NaNO3, AR), sodium chloride NaCl, AR), sodium sulfate (Na2SO4,AR), sodium carbonate(Na2CO3, AR), sodium bicarbonate (NaHCO3, AR), sodium phosphate (Na3PO4), polyvinyl pyrrolidone (PVP, K30) and sodiumfluoride (NaF, AR) were purchased from Shanghai Chemical Reagents Company. Commercial activated aluminum oxide (AAO) was purchased from Shanghai JIADING Molecular Sieve Plant. Ultra-pure water (18 MΩ) was produced using a PSDK System (ZIHANSHIJI, Beijing, China). All reagents were used without further purification. 2.2 Preparation of adsorbents 0.01 mol of AIP, 0.4 g PVP and 40 mL ultra-pure water were mixed and stirred at 800 rpm by a magnetic stirrer for 12 h until uniform white suspension was obtained. The white suspension was Al(OH)3, which was the hydrolysis product of AIP. 0.005 mol of OD were added in the suspension as chelating reagent, and then the suspension was stirred for another 4 h and was transferred to an auto clave, heated at 200 oC, and maintained at this temperature for 12 h. White precipitate was obtained in 4
the auto clave and was named after HAO in this paper. The diagram of preparation of HAO adsorbent was showed in Fig. 1. Al(OH)3 and HAO were washed with ultra-pure water by suction filtration method. Two filter cakes were dried in a vacuum drying oven at 60 oC for 12 h and all become white powders which can be used for characterization or adsorption. 2.3 Characterization X–ray diffraction (XRD) was measured on a D/max2500 with a Cu Kα source (λ = 1.541 Å). The HAO morphology was investigated by a field-emission SEM (Sirion 200, FEI Company, USA) operated at 5 kV voltages. Transmission electron microscopy (TEM) image was obtained from a JEOL JEM–2010 instrument operated at 100 kV. The molecular structure of the products was detected with Fourier transform infrared (FT-IR) spectra (Nexus-870). Zeta potential was carried out on Malvern Zetasizer-3000HS and the experimental data were the averages of triplicate determinations. Thermo gravimetric analysis (TGA) was performed using a Shimadzu TGA-50H analyzer in N2 surroundings. The specific surface area and gas adsorption isotherm of the samples were tested on a Coulter Omnisorp 100CXBrunauer–Emmett–Teller (BET) by using nitrogen adsorption. 2.4 Batch adsorption experiments 2.4.1 Adsorption kinetics experiments NaF was dissolved in ultra-pure water to prepare F- aqueous solution as simulated water polluted by fluoride. F- solution mixed with an appropriate amount of adsorbent was stirred by an electromagnetic agitator which speed was 800 rounds per minute (rpm) in a sealed plastic conical flask at 298 K and pH = 6.5 for 24 h. The mixture was sampled by syringe on schedule and the sample was separated by syringe-driven filter (0.22 μm). The concentration of F- in the clearliquidwas determined by a fluoride ion selective electrode (F001502; Van London-pHoenixCorp.). The experimental data were the 5
averages of triplicate determinations. 2.4.2 Adsorption isotherms experiments A specified volume of F- aqueous solution with different F- concentration mixed with an appropriate amount of adsorbent was shaken by an oscillator in a sealed test tube at 298, 313 and 338 K for 24 h. The mixed suspensions were shaken at 800 rpm. The initial and final concentrations of Fwere detected as above. Removal rate percentage, equilibrium adsorption capacity (Qe) and distribution coefficient (
) are calculated from the following equations: (1) (2) (3)
where,
is the initial concentration (mg L-1),
the mass of the adsorbent, and
is the equilibrium concentration (mg L-1), m (g) is
(mL) is the volume of the suspension. All the experimental data
were the averages of triplicate determinations. The relative errors of the data were about 5%. 2.4.3 pH and interfering anions influence experiments The effect of pH on the fluoride adsorption was conducted using initial fluoride concentrations of 20 mg L-1 with 1 g L-1 of adsorbent. The effects of (chloride, nitrate, sulfate, bicarbonate, carbonate, and phosphate) on fluoride adsorption were performed with an adsorbent dose of 1 g L-1 and an initial fluoride concentration of 10 mg L-1 at pH = 6.5. The co-existing anions were set at several concentrations of 0.1, 0.5, 1.5, and 2 mM, respectively. The mixed suspensions were shaken at 800 rpm. The treatment of experimental data was the same as the experiments above.
3. Results and Discussion
3.1. Adsorbent characterization 6
The morphology of the as-prepared HAO product was observed by SEM. As shown in Fig. 2A, the hexagonal plate like product was obtained via a simple hydrothermal method. The size of the hexagonal plate is about 1-3 μm in length and 0.1-0.24 μm in thickness. TEM image of an individual HAO plate (Fig. 2B) shows both edge and surface of HAO plate are unsmooth. The inset in Fig. 2B is a corresponding selected-area electron diffraction (SAED) pattern of HAO plate which confirms the HAO plate to be single crystalline. Fig. 3 shows the XRD pattern of Al(OH)3 and HAO prepared in this paper. XRD patterns (red line) show that Al(OH)3 is of bayerite structure (JCPDF No. 08-0096). The HAO XRD pattern has a strong peak at 2θ = 13.6o, which corresponds to d-space value = 0.652 nm (Fig. 1C), suggesting that oxalic acid molecules did not destroy the inherent crystal lattice structure of Al(OH) 3, but intercalated into the aluminum-oxygen octahedrons layers, reacted with Al-OH to form HAO [30] and expanded the distance between layers from 0.471nm to 0.652 nm (Fig. 1C). So, the XRD pattern proved that HAO was synthesized successfully in this paper. The main diffraction peaks of Al(OH) 3 and HAO are all very sharp and strong, indicating the two products are well-crystallized. FT-IR measurements were used in order to confirm the formation of oxalic group in HAO. Comparing two curves in Fig. 4, there are strong 1712 cm−1 peak in HAO curve (black line), and no this peak in Al(OH)3 curve (red line). This proves the formation of oxalic group in HAO. The peak at 1712 cm−1 corresponds to the stretching vibration mode of carbonyl group of oxalicester [31], from which it can be deduced that the reaction of hydroxyl groups in Al(OH)3 with oxalic acid attributes to hydrothermal dehydration. The peak at 3675 cm−1 corresponds to the stretching vibration free hydroxyl which shows many hydroxyl groups in Al(OH)3 are maintained after hydrothermal dehydration reaction. 7
TGA curve of HAO is shown in Fig. 5. At temperatures of 25-200 oC, about 4% weight loss is due to the water of crystallization in the sample. When temperature increases further (up to 800 oC), the weight loss rate is about 54%. The results very close theory weight loss rate (53.4%) of thermal decomposition of Al2(OH)4C2O4·0.5H2O. Therefore, it can be calculated that only one-third of Al-OH in Al(OH)3 have reacted with oxalic acid. The hydrothermal reaction model can be expressed as: 2Al(OH)3(s) + H2C2O4(aq)→Al2(OH)4C2O4·0.5H2O (s) + 1.5H2O(l)
(4)
Nitrogen adsorption-desorption isotherm studies were conducted to investigate the porosity of the HAO. These studies show the existence of mesopores in the sample (Fig. 6). The adsorption-desorption isotherm is classified as type IV (Fig. 6 (A)) [32] with a clear hysteresis loop. The surface area was calculated to be 68.34 m² g-1 by BET technique. The average pore diameter was calculated to be 7.0 nm for the HAO using the Barret-Joyner-Halenda model, which the pore size distributions concentrated in the mesopore range (Fig. 6 (B)). The Zeta potentials of the HAO were measured at varied pH at room temperature (Fig. 7). It can be found that the Zeta potentials are all positive values in pH 2-12 range; moreover, the values only change a little when pH increased from 2.0 to 7.0. However, in pH 7.0-12 range, the Zeta potentials values dramatically decreased. Thus, the HAO adsorbent is positively charged at the entire environmentally relevant pH range, which benefits the adsorption of negatively charged F-. 3.2 Influence of pH value on F- adsorption Solution pH is one of the most important parameters to determine the adsorption property of an adsorbent due to its effect not only on surface charge of the adsorbent but also on the degree of ionization and speciation of adsorbate [33, 34]. Fig. 8 shows the variation of adsorption capacity of fluoride adsorbed by the HAO at various initial pH values. Fluoride adsorption by the HAO is 8
sensitive to pH variations. The F- removal rate shows a decreasing trend when pH rises. The removal rate decreases sharply in pH 2-4 range, and has a slowly decrease in pH 4-10. When pH is in 10-11, the removal rate decreases suddenly again, and pH > 11 removal rate decreases slowly once more. The decrease of the removal rate in alkaline pH may be attributed to the competition for surface adsorption sites between the negative hydroxyl and fluoride anions [33]. 3.3 Adsorption kinetics Due to its special structure and morphology, the prepared HAO was used as an adsorbent for removal of fluoride pollution. The NaF aqueous solution was used as simulating pollutant. The initial F- concentration and volume of NaF solution were 5.0 mg L-1 and 100 mL, respectively. The dosage of HAO was 100 mg. The adsorption properties of the HAO were determined by detecting the decrease of F- concentration on schedule. For comparison, the adsorption properties of Al(OH)3 and commercial activated aluminum oxide (AAO) were also detected on the same condition with the HAO. The experiment data were showed in Fig. 9(A). Horizontal axis represents the reaction time t (min), and vertical axis represents the F- adsorption capacities Q (mg g-1). According to Fig. 9(B), after one minute adsorption, the removal rate of F- adsorbed by HAO, Al(OH)3 and AAO reached to 75.9%, 0.81% and 12.1%, respectively. These facts show the HAO material is a kind of very quick adsorbent for F- removal from water. The adsorption equilibrium time by HAO, Al(OH) 3 and AAO were 240, 360 and 240 min, respectively. The equilibrium removal rates reached to 91.9%, 41.3% and 33.8%, respectively. These results prove that HAO is not only a quick but also a high efficient adsorbent for F- removal from water. The adsorption kinetic data were analyzed using a pseudo-second order kinetics model which is based on the assumption that chemisorptions are the rate determining step [35]. The pseudo-second 9
order kinetic model can be described by Eq. (5): (5) where k2 is the pseudo-second-order rate constant (g mg-1 min-1), and Qe and Q are the F- adsorption capacity at equilibrium and time (t), respectively. A plot of t/Q versus yields the values of Qe and k2. The initial adsorption rate r0 (mg g-1 min-1) can be calculated by using Eq. (6): (6) Fig. 9(C) shows the relation between t/Q and t. After linear regression of the data points in the inset, the obtained parameters Qe (mg g-1), k2 (g mg-1 min-1), r0 (mg g-1 min-1) and R2 (coefficient of determination) are listed in Table 1. Three coefficients of determination (R2) are all very close to 1, which implies that F- captured by the three adsorbents follows the pseudo-second order kinetics model very well. Table 1 Rate constants and correlation coefficients of the pseudo second-order kinetic models. Qe(mg g-1)
k2(g mg-1 min-1)
r0 (mg g-1 min-1)
R2
HAO
4.60
0.0943
2.00
0.9999
Al(OH)3
2.13
0.0151
0.0682
0.9993
AAO
1.71
0.0418
0.122
0.9994
Adsorbent
3.4 Adsorption isotherms Equilibrium adsorption isotherms are usually used to determine the capacities of adsorbents. Fig. 10 shows the isotherms of F- adsorption on three different adsorbents (HAO, Al(OH)3 and AAO). The isotherms are fitted to the Langmuir (7) and Freundlich (8) models [36, 37], respectively (Fig. 10). (7) (8) 10
is the equilibrium concentration of F- in aqueous solution (mg L-1). adsorbed on adsorbents (mg g-1). adsorbents
to
form
a
complete
is the amount of F -
is the maximum amount of F- adsorbed per unit weight of monolayer
coverage
on
the
surface.
represents
the ratio of the rate constants of adsorption and desorption and should vary with temperature.
and
n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. The Langmuir isotherm model is suitable for single layer adsorption onto a surface with a finite number of identical sites and uniform energies of adsorption with no transmigration of adsorbate in the plane of the surface [32]. The Freundlich type model is used when the adsorption process is assumed to take place on a heterogeneous surface that varies with surface coverage [32]. The experimental data were fitted with the Langmuir and Freundlich models (Fig. 10). The relative parameters calculated from the two models were listed in Table 2.
The Al(OH)3 adsorption isotherm
is fitted better by the Freundlich model than by the Langmuir model, suggesting that F- adsorption on this adsorbent is multilayer coverage. In AAO, this is reversed; suggesting that F- adsorption on AAO is monolayer coverage. The HAO adsorption isotherm is fitted by either the Freundlich model (R2 = 0.9994) or the Langmuir model (R2 = 0.9968). According to the Langmuir model, the max value of F- adsorption on HAO is 400.0 mg g-1, which is one of the highest capabilities of today’s materials.
11
Table 2 Parameters for the Langmuir and Freundlich models of F- adsorption on three different adsorbents (HAO, Al(OH)3 and AAO)
Langmuir
Freundlich
Adsorbents Qm(mg g-1)
KL (L mg-1)
R2
KF (mg1–n Ln g-1)
n
R2
HAO
400.0
0.00225
0.9968
1.033
1.081
0.9994
Al(OH)3
18.28
0.0235
0.9057
1.023
1.904
0.9852
AAO
11.82
0.0196
0.990
1.056
2.494
0.986
3.5 Adsorption thermodynamic parameters The thermodynamic parameters (
,
, and
) for F- adsorption on HAO can be
calculated from the temperature dependent adsorption isotherms. It is used to define whether the process is endothermic or exothermic and spontaneous. The effect of temperature on F- adsorption onto the HAO at pH 6.5 is given in Fig. 11. The relative parameters calculated from the two models were listed (Table S1, Supplementary Information). Adsorption capacity is lowest at T = 298 K and highest at T = 328 K, which shows that F- adsorption on the HAO is promoted at higher temperature. The standard free energy change (
) can be calculated from the following equation (9): (9)
where
is the universal gas constant (8.314 J mol-1 K-1), T is the temperature in Kelvin.
adsorption equilibrium constant. Values of coefficient) versus 6.572 at T = 298 K,
are obtained by plotting
(Fig. S1, Supplementary Information) and extrapolating = 6.776 at T = 313 K, and 12
is the
(distribution to zero.
= 6.999 at T = 328 K, respectively.
=
The standard enthalpy change (
) and the standard entropy (
) are then calculated from the
versus 1/T for F- adsorption on HAO in the following relationship (10):
linear plot of
(10) versus 1/T for the adsorption of F- on the HAO at 298, 313, and 328 K are
Linear plot of
given (Fig. S2, Supplementary Information). Detailed processes of calculation of the thermodynamic parameters (i.e.,
,
,
, and
) was described in literature [38]. Table 3 shows the
obtained thermodynamic parameters of F- adsorption on the HAO. From Table 3, it can be found that the entropy change,
= 93.36J mol-1, the enthalpy change
free energy change,
= -16.27 kJ mol-1 at 298 K, -17.67 kJ mol-1 at 313 K, and -19.07 kJ mol-1 at
328 K, respectively. The positive an endothermic process. Negative
= 11.55 kJ mol-1, and the standard
value suggests that F- adsorption on the surface of the HAO is value indicates that the adsorption of F- on the HAO is a
spontaneous process. Table 3 The obtained thermodynamic parameters of F- adsorption on the HAO T(K)
(kJ mol-1)
(J mol-1 K-1)
298 313
(kJ mol-1) -16.27
11.55
93.36
328
-17.67 -19.07
3.6 Adsorption mechanism The FT-IR spectra of the HAO before and after adsorbing F- were showed in Fig. 12. It is found that after F- absorption the strong peaks at 3675 and 1715 cm–1 almost vanished, respectively, indicating adsorption through hydroxyl and oxalate interchange with F- reaction. There is a strong peak at 600 cm-1 emerged after F- adsorption, which can be ascribed to the formation of Al-F 13
stretching vibration [39] and proved the hydroxyl and oxalate interchange with F- reaction mechanics . 3.7 Influence of interfering anions on F- adsorption The existence of various anions in the water environment can present potential competition to Fadsorption, which may greatly affect F- removal [40]. The most abundant inorganic competing anions present in the natural water environment are chloride (Cl -), nitrate ( (
), carbonate (
), phosphate (
), sulfate (
), bicarbonate
) [41]. The impact of these inorganic competing anions
on F- adsorption at pH 6.5 onto the HAO was studied as a function of inorganic competing anion concentrations (Fig. 13). The influence of chloride (Cl -), nitrate ( F- absorption was slight, but interference of bicarbonate (
) and sulfate (
), carbonate (
) anions on
), phosphate (
)
anions was obvious when the later three anions concentration rose. With the later three anions impact, the decrease of the removal rate may be attributed to the rise of pH value, which is resulted from these anions’ hydrolysis. When the oxalate (
) anions concentration raised, the interference over the
adsorption for F- on HAO more serious than the three hydrolytic anions. The decrease of the removal rate may be attributed to the F- and oxalate anions’ competition for adsorption on HAO. The interference of the three hydrolytic anions and oxalate (
) anions on F- removal performance
also proved the adsorption mechanics may be hydroxyl and oxalate interchange with F- on HAO surface in water.
Conclusions
A novel adsorbent of HAO with quick and high efficiency for F- removal was prepared, characterized and applied for the first time. Kinetic study results indicate that the adsorption process 14
followed the pseudo second-order kinetic model, and the F- adsorption is rapid in the beginning, nearly 75.9% adsorption can be achieved within 1 min. The calculated adsorption capacity of the adsorbent for F- at pH 6.5 was 400 mg g-1 from Langmuir model, which is one of the highest capabilities
of
today’s
materials.
The
thermodynamic
parameters
calculated
from
the
temperature-dependent isotherms indicate that the adsorption reaction of F- on the HAO is a spontaneous process. Hydroxyl and oxalate interchange with F- reaction should be the mechanisms for F- removal by the HAO. Results show that product prepared in this paper is a kind of promising adsorbent for F- in aqueous solution.
Acknowledgements
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21177131, 11205204, 61273066, and 21277146), the Postgraduate Academic Innovation and Research
Project
Foundation
of
Anhui
University
(yqh100056),
and
Key Technologies R & D Program of Anhui Province (1501021005).
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[30] D.G. Wang, F. Guo, J.F. Chen, L. Shao, H. Liu, Z.T. Zhang, A two-step way to synthesize nano inner-modified aluminum trihydroxide, Colloids and Surfaces a-Physicochemical and Engineering Aspects, 293 (2007) 201-209. [31] Z.-H. Chang, F. Guo, J.-F. Chen, J.-H. Yu, G.-Q. Wang, Synergistic flame retardant effects of nano-kaolin and nano-HAO on LDPE/EPDM composites, Polymer Degradation and Stability, 92 (2007) 1204-1212. [32] J.N. Tiwari, K. Mahesh, N.H. Le, K.C. Kemp, R. Timilsina, R.N. Tiwari, K.S. Kim, Reduced graphene oxide-based hydrogels for the efficient capture of dye pollutants from aqueous solutions, Carbon, 56 (2013) 173-182. [33] Y. Li, P. Zhang, Q. Du, X. Peng, T. Liu, Z. Wang, Y. Xia, W. Zhang, K. Wang, H. Zhu, D. Wu, Adsorption of fluoride from aqueous solution by graphene, J Colloid Interf Sci, 363 (2011) 348-354. [34] M. Jimenez-Reyes, M. Solache-Rios, Sorption behavior of fluoride ions from aqueous solutions by hydroxyapatite, J Hazard Mater, 180 (2010) 297-302. [35] W.S.W. Ngah, N.F.M. Ariff, A. Hashim, M.A.K.M. Hanafiah, Malachite Green Adsorption onto Chitosan Coated Bentonite Beads: Isotherms, Kinetics and Mechanism, Clean-Soil Air Water, 38 (2010) 394-400. [36] I. Langmuir, The constitution and fundamental properties of solids and liquids Part I Solids, J Am Chem Soc, 38 (1916) 2221-2295. [37] H. Freundlich, Concerning adsorption in solutions., Z Phys Chem-Stoch Ve, 57 (1906) 385-470. [38] S. Wu, K. Zhang, X. Wang, Y. Jia, B. Sun, T. Luo, F. Meng, Z. Jin, D. Lin, W. Shen, L. Kong, J. Liu, Enhanced adsorption of cadmium ions by 3D sulfonated reduced graphene oxide, Chem Eng J, 262 (2015) 1292-1302. 19
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20
Figures
Fig. 1 Structure of Al(OH)3 layer sheets (A) c-direction view, (B) b-direction view, and diagram of preparation of HAO sorbent (C) oxalate intercalation in Al(OH) 3 interlayer space and diagram of preparation of HAO adsorbent. The green layers represent the sheet made of aluminium-oxygen octahedrons.
21
A
Fig. 2 (A) SEM image of HAO. (B) TEM image of a HAO plate, the inset is the corresponding SAED pattern.
22
Fig. 3 XRD pattern of Al(OH)3 and HAO. The vertical lines at the bottom correspond to the standard XRD pattern of Al(OH)3 bayerite structure Al(OH)3 (JCPDF No. 08-0096).
23
Fig. 4 FT-IR spectra of Al(OH)3 and HAO.
24
Fig. 5 TGA curve of HAO.
25
Fig. 6 (A) N2 adsorption–desorption isotherms at 77 K and (B) pore-size-distribution curve of HAO.
26
Fig. 7 Zeta potential of the HAO as a function of pH.
27
Fig. 8 The effect of pH on the amount of F- absorbed on HAO. (T = 298 K, [F-]initial = 20 mg L-1, m/V = 1 g L-1)
28
Fig. 9 The kinetics of the HAO adsorption for F - in water. (T = 298 K, pH = 6.5, m/V = 1 gL-1, [F-]initial = 5mg L-1)
29
Fig. 10 F- adsorption isotherms on HAO, Al(OH) 3 and AAO. (T = 298 K, pH = 6.5, m/V = 1 g L-1). The solid lines are Freundlich model fitting, and the dashed lines are Langmuir model fitting.
30
Fig. 11 F- adsorption isotherms on the HAO at three different temperatures. (T = 298 K, pH = 6.5, m/V = 1 gL-1). The solid lines are Freundlich model fitting, and the dashed lines are Langmuir model fitting.
31
Fig. 12 FT-IR spectra of the HAO before and after adsorbing F-.
32
Fig. 13 The influence of interfering anions on the HAO adsorption for F- in water. (T = 298 K, pH = 6.5, m/V = 1 g L-1, [F-]initial = 20 mg L-1)
33
Graphical Abstract
34
35
S0021-9797(15)30289-7 http://dx.doi.org/10.1016/j.jcis.2015.10.045 YJCIS 20831
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
20 May 2015 14 October 2015 17 October 2015
Please cite this article as: S. Wu, K. Zhang, J. He, X. Cai, K. Chen, Y. Li, B. Sun, L. Kong, J. Liu, High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.10.045
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High efficient removal of fluoride from aqueous solution by a novel hydroxyl aluminum oxalate adsorbent
Shibiao Wu a, b, Kaisheng Zhang b, Junyong He b, Xingguo Cai b, Kai Chen b, Yulian Li b, Bai Sun b, *, Lingtao Kong b, *, Jinhuai Liub
a
School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui, 230039, China.
b
Nanomaterials and Environmental Detection Laboratory, Institute of Intelligent Machines, Chinese
Academy of Sciences, Hefei, Anhui, 230031, China.
Corresponding Author *
E-mail address: [email protected] (L. Kong); [email protected] (B. Sun); Fax: +86-551-65592420; Tel:
+86-551-65591142.
ABSTRACT A novel adsorbent, hydroxyl aluminum oxalate (HAO), for the high efficient removal of fluoride from aqueous solution was successfully synthesized. The adsorbent was characterized and its performance in fluoride (F-) removal was evaluated for the first time. Kinetic data reveal that the F- adsorption is rapid in the beginning followed by a slower adsorption process; 75.9% adsorption can be achieved within 1 min and only 16% additional removal occurred in the following 239 min. The F- adsorption kinetics was well described by the pseudo second-order kinetic model. The calculated adsorption capacity of this adsorbent for F- by Langmuir model was 400 mg g-1 at pH 6.5, which is one of the highest capabilities of 1
today’s materials. The thermodynamic parameters calculated from the temperature-dependent isotherms indicate that the adsorption reaction of F- on the HAO is a spontaneous process. The FT-IR spectra of HAO before and after adsorbing F- show adsorption mechanism should be hydroxyl and oxalate interchange with F-.
Keywords: Hydroxyl aluminum oxalate; Fluoride; Adsorption; Kinetics; Thermodynamics
1. Introduction
Fluorine is being used in industrial activities with an increasing amount and then introduced into the aquatic environment in fluoride (F-) form as a consequence of its widely use in modern industries. Though fluorine is a trace element necessary to human body for the calcification tooth and skeleton [1], a number of diseases, such as fluorosis of bone and cancer [2], are caused by ingestion of water contaminated with excess fluoride. As with the diseases mentioned above, the best alternative to prevention is keeping from excessive fluoride ingestion. According to WHO (World Health Organization), the permissible upper limit of F- in drinking water is 1.0 mg L-1 [1], and the excessive F- in the water should be removed. There are many fluoride waste water treatment methods, including ion exchange [3], reverse osmosis [4], electro dialysis [5], chemical precipitation [6], membrane separation [7], coagulation sedimentation [8], electro coagulation [9] and adsorption processes [10]. Among the methods, adsorption technology is an economical and efficient method for producing high quality of drinking water. In recent years, a variety of adsorbents like activated alumina, chitosan beads, activated carbon, clay, hydroxyapatite, etc., have been identified as the promising defluoridating agents [11, 12]. 2
However, there is still a great demand for more effective, simple, and low-cost adsorbents for the removal of fluoride. Recent studies revealed that the intercalated layered materials were widespread attentions due to their excellence adsorption and ion exchange performances for adsorbing metal ions, such as Cu2+, Cd2+, Pb2+ and Hg2+ etc., with high selectivity [13-19]. As an important defluoridating material, Al(OH)3 is also a layered hydroxide material [20]. The layers are made of aluminium-oxygen octahedrons [Al(OH)6]3-, in which octahedrons share edges, and construct the infinite hexameric ring sheet layer [21]. The layers can be stacked one above the other in different ways, forming four crystallographic forms: bayerite [22], gibbsite [23], nordstrandite [24] and doyleite [25]. Although the intra layer bonding is strong ion covalent, interlayer bonding is weak van der Waal׳s [20]. So, the interlayer space in Al(OH)3 should be easy to be intercalated by some light-weight-molecule matters. Some light-weight-molecule agents including pyromellitic acid [13], lactic acid [26], citric acid [27] and malic acid [28] have been successfully used to synthesize the intercalated layered materials. In this paper, oxalate was selected to intercalate into the interlayer space in Al(OH)3 by a facile hydrothermal method. The structure of Al(OH)3 layer sheets and the process of oxalate intercalation in Al(OH)3 interlayer space was demonstrated in Fig. 1. The oxalate modified Al(OH)3 was name after hydroxyl aluminum oxalate (HAO) here. The interlayer space in Al(OH)3 expanded by intercalation and the HAO specific surface area would be increased notably. By the way, Al(III) on the sheet layer is of good coordination ability toward F- [29], HAO should be an effective adsorbent for the removal of fluoride from aqueous solution. In present study, the defluoridation capacity and adsorption kinetics of the as-prepared HAO for F- were examined. The effects of pH, initial F- concentration and co-existing anions were investigated. The results show that the adsorbent possesses high selectivity 3
and excellent adsorption capacity for F- removal. The F- removal mechanism was studied by Fourier transform infrared spectroscopy (FT-IR), indicating that the fluoride adsorption mainly resulted from the exchange of oxalic ions and hydroxyl groups on the surface of the HAO with fluoride anions. We anticipate this work could provide a good example for highly efficient and long-term usage of the HAO during removing fluoride from drinking water.
2. Materials and methods 2.1. Materials Aluminium isopropoxide (AIP, Al[OCH(CH3)2]3, 99.99%) was purchased from Sigma-Aldrichas an aluminum precursor. Oxalate dihydrate (OD, AR), sodium hydroxide (NaOH, AR), nitric acid (HNO3, AR), sodiumnitrate (NaNO3, AR), sodium chloride NaCl, AR), sodium sulfate (Na2SO4,AR), sodium carbonate(Na2CO3, AR), sodium bicarbonate (NaHCO3, AR), sodium phosphate (Na3PO4), polyvinyl pyrrolidone (PVP, K30) and sodiumfluoride (NaF, AR) were purchased from Shanghai Chemical Reagents Company. Commercial activated aluminum oxide (AAO) was purchased from Shanghai JIADING Molecular Sieve Plant. Ultra-pure water (18 MΩ) was produced using a PSDK System (ZIHANSHIJI, Beijing, China). All reagents were used without further purification. 2.2 Preparation of adsorbents 0.01 mol of AIP, 0.4 g PVP and 40 mL ultra-pure water were mixed and stirred at 800 rpm by a magnetic stirrer for 12 h until uniform white suspension was obtained. The white suspension was Al(OH)3, which was the hydrolysis product of AIP. 0.005 mol of OD were added in the suspension as chelating reagent, and then the suspension was stirred for another 4 h and was transferred to an auto clave, heated at 200 oC, and maintained at this temperature for 12 h. White precipitate was obtained in 4
the auto clave and was named after HAO in this paper. The diagram of preparation of HAO adsorbent was showed in Fig. 1. Al(OH)3 and HAO were washed with ultra-pure water by suction filtration method. Two filter cakes were dried in a vacuum drying oven at 60 oC for 12 h and all become white powders which can be used for characterization or adsorption. 2.3 Characterization X–ray diffraction (XRD) was measured on a D/max2500 with a Cu Kα source (λ = 1.541 Å). The HAO morphology was investigated by a field-emission SEM (Sirion 200, FEI Company, USA) operated at 5 kV voltages. Transmission electron microscopy (TEM) image was obtained from a JEOL JEM–2010 instrument operated at 100 kV. The molecular structure of the products was detected with Fourier transform infrared (FT-IR) spectra (Nexus-870). Zeta potential was carried out on Malvern Zetasizer-3000HS and the experimental data were the averages of triplicate determinations. Thermo gravimetric analysis (TGA) was performed using a Shimadzu TGA-50H analyzer in N2 surroundings. The specific surface area and gas adsorption isotherm of the samples were tested on a Coulter Omnisorp 100CXBrunauer–Emmett–Teller (BET) by using nitrogen adsorption. 2.4 Batch adsorption experiments 2.4.1 Adsorption kinetics experiments NaF was dissolved in ultra-pure water to prepare F- aqueous solution as simulated water polluted by fluoride. F- solution mixed with an appropriate amount of adsorbent was stirred by an electromagnetic agitator which speed was 800 rounds per minute (rpm) in a sealed plastic conical flask at 298 K and pH = 6.5 for 24 h. The mixture was sampled by syringe on schedule and the sample was separated by syringe-driven filter (0.22 μm). The concentration of F- in the clearliquidwas determined by a fluoride ion selective electrode (F001502; Van London-pHoenixCorp.). The experimental data were the 5
averages of triplicate determinations. 2.4.2 Adsorption isotherms experiments A specified volume of F- aqueous solution with different F- concentration mixed with an appropriate amount of adsorbent was shaken by an oscillator in a sealed test tube at 298, 313 and 338 K for 24 h. The mixed suspensions were shaken at 800 rpm. The initial and final concentrations of Fwere detected as above. Removal rate percentage, equilibrium adsorption capacity (Qe) and distribution coefficient (
) are calculated from the following equations: (1) (2) (3)
where,
is the initial concentration (mg L-1),
the mass of the adsorbent, and
is the equilibrium concentration (mg L-1), m (g) is
(mL) is the volume of the suspension. All the experimental data
were the averages of triplicate determinations. The relative errors of the data were about 5%. 2.4.3 pH and interfering anions influence experiments The effect of pH on the fluoride adsorption was conducted using initial fluoride concentrations of 20 mg L-1 with 1 g L-1 of adsorbent. The effects of (chloride, nitrate, sulfate, bicarbonate, carbonate, and phosphate) on fluoride adsorption were performed with an adsorbent dose of 1 g L-1 and an initial fluoride concentration of 10 mg L-1 at pH = 6.5. The co-existing anions were set at several concentrations of 0.1, 0.5, 1.5, and 2 mM, respectively. The mixed suspensions were shaken at 800 rpm. The treatment of experimental data was the same as the experiments above.
3. Results and Discussion
3.1. Adsorbent characterization 6
The morphology of the as-prepared HAO product was observed by SEM. As shown in Fig. 2A, the hexagonal plate like product was obtained via a simple hydrothermal method. The size of the hexagonal plate is about 1-3 μm in length and 0.1-0.24 μm in thickness. TEM image of an individual HAO plate (Fig. 2B) shows both edge and surface of HAO plate are unsmooth. The inset in Fig. 2B is a corresponding selected-area electron diffraction (SAED) pattern of HAO plate which confirms the HAO plate to be single crystalline. Fig. 3 shows the XRD pattern of Al(OH)3 and HAO prepared in this paper. XRD patterns (red line) show that Al(OH)3 is of bayerite structure (JCPDF No. 08-0096). The HAO XRD pattern has a strong peak at 2θ = 13.6o, which corresponds to d-space value = 0.652 nm (Fig. 1C), suggesting that oxalic acid molecules did not destroy the inherent crystal lattice structure of Al(OH) 3, but intercalated into the aluminum-oxygen octahedrons layers, reacted with Al-OH to form HAO [30] and expanded the distance between layers from 0.471nm to 0.652 nm (Fig. 1C). So, the XRD pattern proved that HAO was synthesized successfully in this paper. The main diffraction peaks of Al(OH) 3 and HAO are all very sharp and strong, indicating the two products are well-crystallized. FT-IR measurements were used in order to confirm the formation of oxalic group in HAO. Comparing two curves in Fig. 4, there are strong 1712 cm−1 peak in HAO curve (black line), and no this peak in Al(OH)3 curve (red line). This proves the formation of oxalic group in HAO. The peak at 1712 cm−1 corresponds to the stretching vibration mode of carbonyl group of oxalicester [31], from which it can be deduced that the reaction of hydroxyl groups in Al(OH)3 with oxalic acid attributes to hydrothermal dehydration. The peak at 3675 cm−1 corresponds to the stretching vibration free hydroxyl which shows many hydroxyl groups in Al(OH)3 are maintained after hydrothermal dehydration reaction. 7
TGA curve of HAO is shown in Fig. 5. At temperatures of 25-200 oC, about 4% weight loss is due to the water of crystallization in the sample. When temperature increases further (up to 800 oC), the weight loss rate is about 54%. The results very close theory weight loss rate (53.4%) of thermal decomposition of Al2(OH)4C2O4·0.5H2O. Therefore, it can be calculated that only one-third of Al-OH in Al(OH)3 have reacted with oxalic acid. The hydrothermal reaction model can be expressed as: 2Al(OH)3(s) + H2C2O4(aq)→Al2(OH)4C2O4·0.5H2O (s) + 1.5H2O(l)
(4)
Nitrogen adsorption-desorption isotherm studies were conducted to investigate the porosity of the HAO. These studies show the existence of mesopores in the sample (Fig. 6). The adsorption-desorption isotherm is classified as type IV (Fig. 6 (A)) [32] with a clear hysteresis loop. The surface area was calculated to be 68.34 m² g-1 by BET technique. The average pore diameter was calculated to be 7.0 nm for the HAO using the Barret-Joyner-Halenda model, which the pore size distributions concentrated in the mesopore range (Fig. 6 (B)). The Zeta potentials of the HAO were measured at varied pH at room temperature (Fig. 7). It can be found that the Zeta potentials are all positive values in pH 2-12 range; moreover, the values only change a little when pH increased from 2.0 to 7.0. However, in pH 7.0-12 range, the Zeta potentials values dramatically decreased. Thus, the HAO adsorbent is positively charged at the entire environmentally relevant pH range, which benefits the adsorption of negatively charged F-. 3.2 Influence of pH value on F- adsorption Solution pH is one of the most important parameters to determine the adsorption property of an adsorbent due to its effect not only on surface charge of the adsorbent but also on the degree of ionization and speciation of adsorbate [33, 34]. Fig. 8 shows the variation of adsorption capacity of fluoride adsorbed by the HAO at various initial pH values. Fluoride adsorption by the HAO is 8
sensitive to pH variations. The F- removal rate shows a decreasing trend when pH rises. The removal rate decreases sharply in pH 2-4 range, and has a slowly decrease in pH 4-10. When pH is in 10-11, the removal rate decreases suddenly again, and pH > 11 removal rate decreases slowly once more. The decrease of the removal rate in alkaline pH may be attributed to the competition for surface adsorption sites between the negative hydroxyl and fluoride anions [33]. 3.3 Adsorption kinetics Due to its special structure and morphology, the prepared HAO was used as an adsorbent for removal of fluoride pollution. The NaF aqueous solution was used as simulating pollutant. The initial F- concentration and volume of NaF solution were 5.0 mg L-1 and 100 mL, respectively. The dosage of HAO was 100 mg. The adsorption properties of the HAO were determined by detecting the decrease of F- concentration on schedule. For comparison, the adsorption properties of Al(OH)3 and commercial activated aluminum oxide (AAO) were also detected on the same condition with the HAO. The experiment data were showed in Fig. 9(A). Horizontal axis represents the reaction time t (min), and vertical axis represents the F- adsorption capacities Q (mg g-1). According to Fig. 9(B), after one minute adsorption, the removal rate of F- adsorbed by HAO, Al(OH)3 and AAO reached to 75.9%, 0.81% and 12.1%, respectively. These facts show the HAO material is a kind of very quick adsorbent for F- removal from water. The adsorption equilibrium time by HAO, Al(OH) 3 and AAO were 240, 360 and 240 min, respectively. The equilibrium removal rates reached to 91.9%, 41.3% and 33.8%, respectively. These results prove that HAO is not only a quick but also a high efficient adsorbent for F- removal from water. The adsorption kinetic data were analyzed using a pseudo-second order kinetics model which is based on the assumption that chemisorptions are the rate determining step [35]. The pseudo-second 9
order kinetic model can be described by Eq. (5): (5) where k2 is the pseudo-second-order rate constant (g mg-1 min-1), and Qe and Q are the F- adsorption capacity at equilibrium and time (t), respectively. A plot of t/Q versus yields the values of Qe and k2. The initial adsorption rate r0 (mg g-1 min-1) can be calculated by using Eq. (6): (6) Fig. 9(C) shows the relation between t/Q and t. After linear regression of the data points in the inset, the obtained parameters Qe (mg g-1), k2 (g mg-1 min-1), r0 (mg g-1 min-1) and R2 (coefficient of determination) are listed in Table 1. Three coefficients of determination (R2) are all very close to 1, which implies that F- captured by the three adsorbents follows the pseudo-second order kinetics model very well. Table 1 Rate constants and correlation coefficients of the pseudo second-order kinetic models. Qe(mg g-1)
k2(g mg-1 min-1)
r0 (mg g-1 min-1)
R2
HAO
4.60
0.0943
2.00
0.9999
Al(OH)3
2.13
0.0151
0.0682
0.9993
AAO
1.71
0.0418
0.122
0.9994
Adsorbent
3.4 Adsorption isotherms Equilibrium adsorption isotherms are usually used to determine the capacities of adsorbents. Fig. 10 shows the isotherms of F- adsorption on three different adsorbents (HAO, Al(OH)3 and AAO). The isotherms are fitted to the Langmuir (7) and Freundlich (8) models [36, 37], respectively (Fig. 10). (7) (8) 10
is the equilibrium concentration of F- in aqueous solution (mg L-1). adsorbed on adsorbents (mg g-1). adsorbents
to
form
a
complete
is the amount of F -
is the maximum amount of F- adsorbed per unit weight of monolayer
coverage
on
the
surface.
represents
the ratio of the rate constants of adsorption and desorption and should vary with temperature.
and
n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. The Langmuir isotherm model is suitable for single layer adsorption onto a surface with a finite number of identical sites and uniform energies of adsorption with no transmigration of adsorbate in the plane of the surface [32]. The Freundlich type model is used when the adsorption process is assumed to take place on a heterogeneous surface that varies with surface coverage [32]. The experimental data were fitted with the Langmuir and Freundlich models (Fig. 10). The relative parameters calculated from the two models were listed in Table 2.
The Al(OH)3 adsorption isotherm
is fitted better by the Freundlich model than by the Langmuir model, suggesting that F- adsorption on this adsorbent is multilayer coverage. In AAO, this is reversed; suggesting that F- adsorption on AAO is monolayer coverage. The HAO adsorption isotherm is fitted by either the Freundlich model (R2 = 0.9994) or the Langmuir model (R2 = 0.9968). According to the Langmuir model, the max value of F- adsorption on HAO is 400.0 mg g-1, which is one of the highest capabilities of today’s materials.
11
Table 2 Parameters for the Langmuir and Freundlich models of F- adsorption on three different adsorbents (HAO, Al(OH)3 and AAO)
Langmuir
Freundlich
Adsorbents Qm(mg g-1)
KL (L mg-1)
R2
KF (mg1–n Ln g-1)
n
R2
HAO
400.0
0.00225
0.9968
1.033
1.081
0.9994
Al(OH)3
18.28
0.0235
0.9057
1.023
1.904
0.9852
AAO
11.82
0.0196
0.990
1.056
2.494
0.986
3.5 Adsorption thermodynamic parameters The thermodynamic parameters (
,
, and
) for F- adsorption on HAO can be
calculated from the temperature dependent adsorption isotherms. It is used to define whether the process is endothermic or exothermic and spontaneous. The effect of temperature on F- adsorption onto the HAO at pH 6.5 is given in Fig. 11. The relative parameters calculated from the two models were listed (Table S1, Supplementary Information). Adsorption capacity is lowest at T = 298 K and highest at T = 328 K, which shows that F- adsorption on the HAO is promoted at higher temperature. The standard free energy change (
) can be calculated from the following equation (9): (9)
where
is the universal gas constant (8.314 J mol-1 K-1), T is the temperature in Kelvin.
adsorption equilibrium constant. Values of coefficient) versus 6.572 at T = 298 K,
are obtained by plotting
(Fig. S1, Supplementary Information) and extrapolating = 6.776 at T = 313 K, and 12
is the
(distribution to zero.
= 6.999 at T = 328 K, respectively.
=
The standard enthalpy change (
) and the standard entropy (
) are then calculated from the
versus 1/T for F- adsorption on HAO in the following relationship (10):
linear plot of
(10) versus 1/T for the adsorption of F- on the HAO at 298, 313, and 328 K are
Linear plot of
given (Fig. S2, Supplementary Information). Detailed processes of calculation of the thermodynamic parameters (i.e.,
,
,
, and
) was described in literature [38]. Table 3 shows the
obtained thermodynamic parameters of F- adsorption on the HAO. From Table 3, it can be found that the entropy change,
= 93.36J mol-1, the enthalpy change
free energy change,
= -16.27 kJ mol-1 at 298 K, -17.67 kJ mol-1 at 313 K, and -19.07 kJ mol-1 at
328 K, respectively. The positive an endothermic process. Negative
= 11.55 kJ mol-1, and the standard
value suggests that F- adsorption on the surface of the HAO is value indicates that the adsorption of F- on the HAO is a
spontaneous process. Table 3 The obtained thermodynamic parameters of F- adsorption on the HAO T(K)
(kJ mol-1)
(J mol-1 K-1)
298 313
(kJ mol-1) -16.27
11.55
93.36
328
-17.67 -19.07
3.6 Adsorption mechanism The FT-IR spectra of the HAO before and after adsorbing F- were showed in Fig. 12. It is found that after F- absorption the strong peaks at 3675 and 1715 cm–1 almost vanished, respectively, indicating adsorption through hydroxyl and oxalate interchange with F- reaction. There is a strong peak at 600 cm-1 emerged after F- adsorption, which can be ascribed to the formation of Al-F 13
stretching vibration [39] and proved the hydroxyl and oxalate interchange with F- reaction mechanics . 3.7 Influence of interfering anions on F- adsorption The existence of various anions in the water environment can present potential competition to Fadsorption, which may greatly affect F- removal [40]. The most abundant inorganic competing anions present in the natural water environment are chloride (Cl -), nitrate ( (
), carbonate (
), phosphate (
), sulfate (
), bicarbonate
) [41]. The impact of these inorganic competing anions
on F- adsorption at pH 6.5 onto the HAO was studied as a function of inorganic competing anion concentrations (Fig. 13). The influence of chloride (Cl -), nitrate ( F- absorption was slight, but interference of bicarbonate (
) and sulfate (
), carbonate (
) anions on
), phosphate (
)
anions was obvious when the later three anions concentration rose. With the later three anions impact, the decrease of the removal rate may be attributed to the rise of pH value, which is resulted from these anions’ hydrolysis. When the oxalate (
) anions concentration raised, the interference over the
adsorption for F- on HAO more serious than the three hydrolytic anions. The decrease of the removal rate may be attributed to the F- and oxalate anions’ competition for adsorption on HAO. The interference of the three hydrolytic anions and oxalate (
) anions on F- removal performance
also proved the adsorption mechanics may be hydroxyl and oxalate interchange with F- on HAO surface in water.
Conclusions
A novel adsorbent of HAO with quick and high efficiency for F- removal was prepared, characterized and applied for the first time. Kinetic study results indicate that the adsorption process 14
followed the pseudo second-order kinetic model, and the F- adsorption is rapid in the beginning, nearly 75.9% adsorption can be achieved within 1 min. The calculated adsorption capacity of the adsorbent for F- at pH 6.5 was 400 mg g-1 from Langmuir model, which is one of the highest capabilities
of
today’s
materials.
The
thermodynamic
parameters
calculated
from
the
temperature-dependent isotherms indicate that the adsorption reaction of F- on the HAO is a spontaneous process. Hydroxyl and oxalate interchange with F- reaction should be the mechanisms for F- removal by the HAO. Results show that product prepared in this paper is a kind of promising adsorbent for F- in aqueous solution.
Acknowledgements
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21177131, 11205204, 61273066, and 21277146), the Postgraduate Academic Innovation and Research
Project
Foundation
of
Anhui
University
(yqh100056),
and
Key Technologies R & D Program of Anhui Province (1501021005).
References
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20
Figures
Fig. 1 Structure of Al(OH)3 layer sheets (A) c-direction view, (B) b-direction view, and diagram of preparation of HAO sorbent (C) oxalate intercalation in Al(OH) 3 interlayer space and diagram of preparation of HAO adsorbent. The green layers represent the sheet made of aluminium-oxygen octahedrons.
21
A
Fig. 2 (A) SEM image of HAO. (B) TEM image of a HAO plate, the inset is the corresponding SAED pattern.
22
Fig. 3 XRD pattern of Al(OH)3 and HAO. The vertical lines at the bottom correspond to the standard XRD pattern of Al(OH)3 bayerite structure Al(OH)3 (JCPDF No. 08-0096).
23
Fig. 4 FT-IR spectra of Al(OH)3 and HAO.
24
Fig. 5 TGA curve of HAO.
25
Fig. 6 (A) N2 adsorption–desorption isotherms at 77 K and (B) pore-size-distribution curve of HAO.
26
Fig. 7 Zeta potential of the HAO as a function of pH.
27
Fig. 8 The effect of pH on the amount of F- absorbed on HAO. (T = 298 K, [F-]initial = 20 mg L-1, m/V = 1 g L-1)
28
Fig. 9 The kinetics of the HAO adsorption for F - in water. (T = 298 K, pH = 6.5, m/V = 1 gL-1, [F-]initial = 5mg L-1)
29
Fig. 10 F- adsorption isotherms on HAO, Al(OH) 3 and AAO. (T = 298 K, pH = 6.5, m/V = 1 g L-1). The solid lines are Freundlich model fitting, and the dashed lines are Langmuir model fitting.
30
Fig. 11 F- adsorption isotherms on the HAO at three different temperatures. (T = 298 K, pH = 6.5, m/V = 1 gL-1). The solid lines are Freundlich model fitting, and the dashed lines are Langmuir model fitting.
31
Fig. 12 FT-IR spectra of the HAO before and after adsorbing F-.
32
Fig. 13 The influence of interfering anions on the HAO adsorption for F- in water. (T = 298 K, pH = 6.5, m/V = 1 g L-1, [F-]initial = 20 mg L-1)
33
Graphical Abstract
34
35