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Adsorption capacity of bone char for removing fluoride from water solution. Role of hydroxyapatite content, adsorption mechanism and competing anions
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JIEC-1835; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Adsorption capacity of bone char for removing fluoride from water solution. Role of hydroxyapatite content, adsorption mechanism and competing anions N.A. Medellin-Castillo a, R. Leyva-Ramos b,*, E. Padilla-Ortega b, R. Ocampo Perez b, J.V. Flores-Cano b, M.S. Berber-Mendoza a a Centro de Investigacio´n y Estudios de Posgrado, Facultad de Ingenierı´a, Universidad Auto´noma de San Luis Potosi, Av. Dr. M. Nava 8, San Luis Potosi, SLP 78290, Mexico b Centro de Investigacio´n y Estudios de Posgrado, Facultad de Ciencias Quı´micas, Universidad Auto´noma de San Luis Potosi, Av. Dr. M. Nava 6, San Luis Potosi, SLP 78210, Mexico
A R T I C L E I N F O
Article history: Received 21 August 2013 Accepted 30 December 2013 Available online xxx Keywords: Adsorption Bone char Fluoride Hydroxyapatite Water pollution
A B S T R A C T
The adsorption of fluoride from water on bone char (BC) was investigated in this work, and the fluoride adsorption capacity of BC was compared to that of hydroxyapatite (HAP). The adsorption capacity of BC and HAP drastically increased while decreasing the pH from 7.0 to 5.0. Furthermore, the fluoride adsorption on BC was due to its HAP content and was not considerably affected by the presence of the anions Cl, HCO3, CO32, SO42, NO3 and NO2. The mechanism of fluoride adsorption on BC was attributed to electrostatic interactions between surface charge of BC and fluoride ions in solution. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The impact of fluoride concentration in drinking water onto human health has been extensively studied and reviewed in various works [1,2]. A concentration of fluoride between 0.5 and 1.5 mg/L is essential for the human body, especially in preventing the incidence of dental caries; however, long term ingestion of water with excess amount of fluoride, above 2 mg/L, can cause diseases such as dental and skeletal fluorosis, masculine infertility and cancer [2]. Several treatment processes have been applied to remove excess fluoride from drinking water. Some of the common processes are chemical precipitation [3], ion exchange [4], adsorption [5,6], reverse osmosis [4], electrodialysis [7] and nanofiltration [8]. Adsorption is the most suitable process for drinking water treatment because it is relatively simple, economic, and appropriate for small communities [9,10]. During the past 10 years, extensive research has been carried out to find both low-cost and high capacity adsorbents for the removal of fluoride from drinking water. A wide range of
* Corresponding author. Tel.: +52 444 826 2440; fax: +52 444 826 2372. E-mail address: [email protected] (R. Leyva-Ramos).
adsorbents have been developed and tested, including activated alumina [11], natural minerals such as laterite, zeolite, and caolinite [12–14], nanomaterials [15], carbonaceous materials [7,16], bone char (BC) and hydroxyapatite (HAP) [5,17,18]. BC is manufactured by the carbonization of cattle bones and contains around 10% carbon and 90% HAP [19]. The adsorption of fluoride on BC has been investigated in various works [5,6,18,20]. In a previous work [5], it was shown that the capacity of BC for adsorbing fluoride was higher than those of activated alumina and activated carbon. Furthermore, the effect of solution pH and temperature on the adsorption capacity of fluoride onto BC was investigated and it was found that the maximum adsorption took place at pH 3 and decreased nearly 20 times by increasing the pH from 3 to 12. This behavior was attributed to the electrostatic interactions between the surface of BC and fluoride ions in solution. The adsorption capacity was not influenced by temperature in the range from 15 to 35 8C. The anions HCO3, CO32, Cl, NO3, NO2, and SO42 are commonly found in drinking water and these anions can compete with the fluoride for the adsorption sites of BC, affecting the adsorption capacity of BC toward fluoride. The competing effect of these anions on the capacity of BC for adsorbing fluoride has not been investigated in detail. Due to the high toxicity of As(V), the competitive adsorption of As(V) and fluoride from aqueous
1226-086X/$ – see front matter ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.105
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solution on BC has been studied because it is well documented that fluoride and As(V) may be co-occurring in drinking waters in several countries of Latin America [21]. Mlilo et al. [22] determined the adsorption isotherm of fluoride on BC with and without As(V) and concluded that the As(V) did not affect the fluoride adsorption capacity of BC for an initial concentration of As(V) of 0.250 mg/L. Calcium hydroxyapatite, Ca10(PO4)6(OH)2, is an important inorganic material in biology and chemistry [23–25]. Its structure, ion exchange capacity, adsorption affinity, and its capacity to bond with organic molecules of different sizes, have attracted much attention in the last two decades. The adsorption of heavy metals, Pb2+, Cu2+ and Cd2+, from aqueous solutions on HAP has been attributed to the ion exchange of Ca2+ on the HAP by the metal cation in solution [26,27]. Adsorption of fluoride on this adsorbent has been studied in several works [17,27] and it has been found that the mechanism of fluoride adsorption is due to electrostatic attractions and ion exchange processes. BC is mainly composed of HAP and it is essential to evaluate the contribution of this mineral to the adsorption of fluoride on BC. Thus, the main objective of this research was to evaluate the role of HAP on the capacity of BC for adsorbing fluoride from aqueous solution. Furthermore, the mechanism of fluoride adsorption on BC would be elucidated, and the effect of the presence of anions, commonly found in natural waters, on the fluoride adsorption capacity of BC would be analyzed in detail. 2. Materials and methods 2.1. Bone char and hydroxyapatite In this work, the granular BC used is commercially known as Fija Fluor and is manufactured from cattle bones by APELSA, Guadalajara, Mexico. The BC was sieved to an average particle diameter of 0.79 mm, washed repeatedly with deionized water, dried in an oven at 100 8C for 24 h, and stored in plastic containers. Analytical grade hydroxyapatite (purity < 99% according to the manufacturer, Sigma–Aldrich, CAS Number: 1306-06-5) was also used in this study. 2.2. Characterization of bone char and hydroxyapatite The contents of carbon, hydrogen and nitrogen on BC were determined using an Elemental Analyzer, Fisons Carlo Erba, EA1108. The phosphorus weight percentage of BC was evaluated by Inductively Coupled Plasma (ICP) Atomic Emission Spectroscopy using a Thermo Jarrel Ash, model IRIS/AP, instrument. The sample was prepared for ICP analysis using the method described by Medellin-Castillo [28]. The surface charge and point of zero charge (PZC) of BC and HAP were determined by a titration method [5]. The acid stability of BC was determined by a procedure similar to that recommended by Wingenfelder et al. [29]. A BC mass of 0.5 g and 40 mL of solution with an initial pH varying from 0.8 to 12.0 were added to a plastic bottle. The solutions were prepared by mixing proper volumes of 0.1 N HNO3 and NaOH solutions. The BC and acid solution were left in contact for 7 days and periodically mixed in an orbital shaker, and the final pH of the acid solution was measured. Moreover, the concentration of Ca2+ and PO43 dissolved in the acid solutions were determined by atomic absorption spectroscopy and atomic emission spectroscopy. The morphology of BC and HAP particles were observed by means of a scanning electron microscope (SEM), Philips, model XL30. The microscope was equipped with an energy dispersive Si(Li) detector EDAX DX4 for microanalysis. X-ray diffraction (XRD) analysis was used to identify the crystalline species present in both adsorbents employing an X-ray
diffractometer, Rigaku, model DMAX 2000. The textural properties of both materials (specific area, pore volume and average pore diameter) were determined by the N2-BET method using a surface area and porosimetry analyzer, Micromeritics, model ASAP 2010. 2.3. Determination of the concentrations of the anions in water solution The fluoride concentration in an aqueous solution was determined by a potentiometric method using a potentiometer, Orion, model SA720, and a selective electrode for fluoride ion. This method allowed the determination of fluoride concentrations in the range from 0.1 to 10 mg/L. The fluoride calibration curves were prepared with standard solutions of fluoride. Further details of the analytical method can be found elsewhere [28,30]. The concentrations of bicarbonate (HCO3), carbonate (CO32), chloride (Cl), nitrate (NO3), nitrite (NO2), phosphate (PO43), and sulfate (SO42) in water solutions were determined using the methods recommended by APHA [30]. 2.4. Adsorption equilibrium data Experimental adsorption equilibrium data were obtained as follows. A portion of 490 mL of an aqueous solution of a known initial concentration of fluoride or HCO3, CO32, Cl, NO3, NO2, and SO42 was added to the batch adsorber. A given mass of BC or HAP was added to a Nylon basket and placed inside the adsorber. The mass of BC or HAP was 0.5 or 1.0 g, and the initial concentrations of fluoride varied from 1 to 20 mg/L. The adsorber was placed in a constant temperature water bath and the solution was continuously mixed by a magnetic stirrer located just below the water bath. The solution was left in contact with the BC or HAP until equilibrium was reached, which took between 5 and 7 days. The solution pH was measured periodically with a pH-meter and kept constant by adding few drops of 0.01 and 0.05 HNO3 or NaOH solutions as required. The total volume added of HNO3 or NaOH solutions was recorded and considered in the mass balance. The solution was sampled at specific time intervals and the concentration of fluoride was determined for each sample. Equilibrium was reached when the concentrations of two consecutive samples did not change over time. The mass of fluoride or HCO3, CO32, Cl, NO3, NO2, and SO42 adsorbed at equilibrium was calculated by performing a mass balance of fluoride or other anions. The effect of coexisting or competing anions such as chloride on the fluoride adsorption capacity of BC was carried out by determining the adsorption equilibrium data of fluoride at pH = 7.0 and T = 25 8C, and varying the initial concentration of chloride from 20 to 80 mg/L during the adsorption of fluoride. 3. Results and discussion 3.1. Morphology of BC and HAP As seen in Fig. 1(a), the forms and shapes of the BC particles are very irregular and its surface is fractured, rough and porous. The distribution of the particle sizes is not uniform. The morphology of the HAP particles is shown in Fig. 1(b), and the particles presented a layered shape that is typical of HAP. 3.2. Chemical composition The elemental analysis of BC showed that the weight percentages of nitrogen, hydrogen and carbon were 0.95, 0.52 and 6.31%, correspondingly. The carbon content is very similar to the value reported by Wilson et al. [31] for a BC manufactured from cattle bones.
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3
Concentration of calcium, mg/L
1800 1500 1200 900 600 300 0
0
2
4
6
8
10
12
6 8 Final pH b)
10
12
Final pH
Concentration of phosphate, mg/L
a) 80 70 60 50 40 30 20 10 0
Fig. 1. SEM images of BC particles (a) and HAP particles (b).
The weight percentage of phosphorus in the BC was found to be 15.0%. Assuming that all the phosphorus belongs only to the HAP present in the BC, the percentage of HAP in BC can be estimated to be 84.8%. This value is slightly higher than that reported by Wilson et al. [29], who found a HAP weight percentage in a BC of 76%. The difference can be attributed to the cattle bone used as raw material in the manufacture of the BC. 3.3. Acid–base stability of BC The chemical stability of BC was studied by contacting this adsorbent with acid and basic solutions at different pH during 5 days and analyzing the Ca2+ and PO43 concentrations in the solution. The concentrations of calcium and phosphate in the aqueous solution vs. the final pH is plotted in Fig. 2(a) and (b), respectively. In these figures, it can be observed that the Ca2+ concentration is less than 28.2 mg/L at pH > 4 and increased drastically at pH < 3. This behavior is due to the dissolution of HAP and calcium carbonate in the water solution. Moreover, the PO43 concentration varied from 6 to 13 mg/L in a pH range between 4 < pH < 12 and increased drastically at pH < 4, which is a result of the dissolution of the BC. If the Ca2+ concentration was exclusively
0
2
4
Fig. 2. Acid–base stability of the BC at T = 25 8C. (a) Concentration of calcium and (b) concentration of phosphate in solution.
changed by the dissolution of HAP, at any solution pH, the molar ratio of Ca2+/PO43 would be constant to 5/3, according to the chemical formula of HAP, Ca10(PO4)6OH. The molar ratio of Ca2+/ PO43 was 43.1, 1.96 and 0.05 at pH 2.0, 7.0 and 12.0, respectively. This result corroborated that the calcium concentration in aqueous solution was from the dissolution of both HAP and calcium carbonate contained in the BC. 3.4. Textural properties of BC and HAP The textural properties of BC and HAP are shown in Table 1. The textural properties of BC have been reported in various works [32], and the values are very similar to those presented in this work. The results showed that the BC has a greater surface area than that of the HAP. This is probably due to the main components of BC being the HAP, Ca10(PO4)6(OH)2 and carbon. Therefore, the increase in the surface area of BC is essentially due to its carbon content. The surface area of HAP has been reported in the literature [17], and the values are in the range from 45 to 54.2 m2/g.
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Table 1 Textural properties and pHPZC of BC and HAP. Adsorbent
Surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
pHPZC
BC HAP
104 62.9
0.30 0.26
11.1 16.4
8.4 7.0
peaks of the HAP happen at the 2u values of 25.98, 31.78, 408, 46.78, and 49.58 [32]. Comparing the XRD pattern of BC to that of HAP, it can be noted that BC is mainly composed of HAP, but the relative intensities of the characteristic peaks for BC are slightly different from those of the HAP due to the content of HAP in BC. 3.6. Surface characterization of BC and HAP
The N2 adsorption isotherms on BC and HAP were shown to be characteristic of mesoporous solids and corresponded to type IV isotherm [33]. The average pore diameters of BC and HAP were 11.1 and 16.4 nm, respectively. The pore volume distribution of BC and HAP revealed that mesopores (2 nm < pore diameter < 50 nm) represented approximately 46 and 80% of the total pore volume of HAP and BC, respectively. 3.5. XRD analysis of BC and HAP The XRD patterns of the BC and HAP are shown in Fig. 3a and b, respectively. The crystalline species of both materials were identified comparing the characteristic peaks shown in the XRD pattern with the database of the diffractometer (Journal of Crystallographic Powder Diffraction Spectra). The characteristic
The surface charge of the BC and HAP are illustrated in Fig. 4. The pHPZC of the HAP and BC were 7.0 and 8.4, respectively. Similar results have been reported for BC and HAP [34–36]. The pHPZC of the BC was basic because its concentration of basic sites was higher than that of acid sites [5]. As noted in Fig. 4, the surface charge of the BC was increased by raising the ionic strength, but the pHPZC of BC was not dependent on the ionic strength. It was shown that the main constituent of BC was HAP and the surface charge of the BC was mainly due to the interactions between the HAP surface and the ions in the water solution. The functional groups of the HAP surface that affect its surface charge are the phosphates, BBP-OH, and the hydroxyls, BBCa-OH. The positive charge of the BC surface was due to the following protonation reactions: BBP-OH þ Hþ ! BBPOH2 þ
100
HAP
BBCa-OH þ Hþ ! BBCa-OH2 þ
Relative Intensity
80
Bone Char
On the other hand, the negative charge of the BC surface was originated by the following deprotonation reactions:
60 40
BBP-OH ! BBPO þ Hþ BBCa-OH ! BBCa-O þ Hþ
HAP
where BB represents the BC surface. Thus, the protonation reactions predominated at pH < pHPZC, although the deprotonation reactions occurred at pH > pHPZC.
HAP HAP HAP
20
3.7. Adsorption isotherm
0 20
30
40
50
60
70
80
90
2θ a) 100
The Freundlich, Langmuir, and Prausnitz-Radke isotherm models were fitted to the experimental adsorption equilibrium data of fluoride on BC and HAP. These isotherm models can be
HAP
HAP
Relative Intensity
80 Hydroxyapatite
60 HAP
40 HAP HAP
20 0
20
30
40
50
2θ
60
70
80
90
b) Fig. 3. XRD patterns of the BC (a) and the HAP (b).
Fig. 4. Surface charge distribution of BC and HAP.
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Table 2 Values of the parameters for the Langmuir, Freundlich and Prausnitz-Radke adsorption isotherms. Adsorbent
pH
Freundlich 1/n
k (L
Langmuir
1/n1
/mg
g)
1/n
BC
5 7
3.96 2.72
0.31 0.29
HAP
5 7
5.15 2.37
0.35 0.48
%D 6.86 4.39 17.1 11.3
Prausnitz-Radke K (L/mg) 1.55 1.22 1.32 0.18
7.74 5.44 10.9 11.1
%D
a (L/g)
16.4 16.0
36.9 42.0
15.7 16.9
64.4 56.4
b
b
b (L/mg) 7.55 3.07 11.0 23.0
%D
0.79 0.76
9.54 2.15
0.70 0.53
10.5 7.56
the fluoride in aqueous solution is mostly adsorbed on the HAP contained in the BC and not by the other constituents of BC.
represented by the subsequent mathematical relationships: q ¼ kC 1=n
qm (mg/g)
(1) 3.9. Effect of pH on the capacity of BC for adsorbing fluoride
q¼
aC 1 þ bC b
(2)
(3)
where C (mg/L) is the concentration of fluoride at equilibrium and q (mg/g) is the mass of fluoride adsorbed per mass of adsorbent. The parameters K (L/mg) and qm (mg/g) are the Langmuir constants related to the energy of adsorption and maximum adsorption capacity, respectively. The parameters k (L1/n mg11/n g1) and n are the Freundlich constants related to the adsorption capacity and b intensity. The parameters a (L/g), b (L/mg) and b are the Prausnitz-Radke isotherm constants. The isotherm constants were estimated by a least-squares method based on an optimization algorithm and the values are shown in Table 2 along with the average absolute percentage deviation, %D, which was computed from the following equation: N 1X qexp qcal %D ¼ (4) 100 qexp N i¼1
where N is the number of experimental data points, qexp (mg/g) is the experimental mass of fluoride adsorbed, and qcal (mg/g) is the mass of fluoride adsorbed predicted with the adsorption isotherm model. The experimental adsorption equilibrium data were satisfactorily interpreted by the three isotherm models since the average percentage deviations were less than 10.5, 16.9, and 17.1% for the Prausnitz-Radke, Langmuir and Freundlich isotherms, correspondently. As shown in Table 2, the Prausnitz-Radke isotherm best fitted the data for BC and HAP since, in the 3 of the 4 isotherm cases shown in Table 2, the %D of the Prausnitz-Radke isotherm was lower than the %D of Langmuir and Freundlich isotherms. This can be explained by recalling that the Prausnitz-Radke model is a three-parameter isotherm whereas the Langmuir and Freundlich models are two-parameter isotherms. 3.8. Effect of the presence of HAP on the adsorption capacity of BC The experimental adsorption equilibrium data of fluoride on BC and HAP are shown in Fig. 5 at T = 25 8C and pH = 5.0 and 7.0. It can be noted that the fluoride adsorption on HAP is greater than that of the BC. At an equilibrium fluoride concentration of 1.5 mg/L, the ratio between the mass of fluoride adsorbed on the BC and that on the HAP, qBC/qHAP is 0.77 and 0.95 at pH = 5.0 and 7.0, respectively, and at an equilibrium fluoride concentration of 4.0 mg/L, the ratio qBC/qHAP is 0.73 and 0.81 at pH = 5.0 and 7.0, correspondingly. These ratios are very close to the weight fraction of HAP in the BC, which is 0.848 (see Section 3.2). This reveals that the fluoride adsorption capacity of BC is mostly due to its content of HAP. Thus,
In a previous work [5], it was reported that the capacity of BC for adsorbing fluoride was significantly dependent on the solution pH and decreased considerably while raising the pH from 3 to 11. Similar results were found in this work and are displayed in Fig. 5. This behavior was attributed to the electrostatic interactions between fluoride ions in water solution and the surface charge of BC. The surface of BC was positively charged at pH < pHPZC = 8.4 and the F ions in solution were attracted to the surface of BC. The mass of fluoride adsorbed decreased by reducing the pH from 3 to 8.4 because the surface charge of BC decreased by raising the pH, this behavior is depicted in Fig. 4. Additional experiments were carried out to investigate the variation of the solution pH during the adsorption of fluoride on the BC. The experiments consisted on performing adsorption experiments as already described using a BC mass of 1.0 g, various initial fluoride concentrations (1, 5, 10, and 20 mg/L) and various initial pH values (3, 5, 9, and 12). During these experiments, the pH of the solution was not kept constant and the final pH was recorded along with the mass of fluoride adsorbed. As expected, the results revealed that the fluoride adsorption capacity of BC decreased when the pH increased independently on the initial concentration of fluoride (Fig. 6a). Additionally, it was observed that the solution pH varied during the adsorption of fluoride on BC. The initial pHs of the solutions were approximately 3, 5, 7, 9 and 12 and the final pH of the solutions presented an average value of 6.9, 7.9, 7.9, 8.1 and 11.8, respectively. The
14
Mass of fluoride adsorbed, mg/g
q KC q¼ m 1 þ KC
BC, pH = 5 BC, pH = 7 HAP, pH = 5 HAP, pH = 7
12 10 8 6 4 2 0 0
2
4
6
8
10
12
14
Concentration of fluoride at equilibrium, mg/L Fig. 5. Adsorption isotherms of fluoride on BC and HAP. T = 25 8C and pH = 5.0 and 7.0.
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altered by the dissolution of the carbonate and HAP contained in the BC. 3.10. Adsorption mechanism
Fig. 6. Effect of initial pH and initial concentration of fluoride on the adsorption capacity of BC (a) and the final pH (b).
variation of the solution pH during adsorption is shown in Fig. 6b, and the solution pH increased when the experiments were performed at a pH < pHPZC, while it remained almost constant when the experiments were carried out at a pH > pHPZC. At the initial pH of 3 and 5, the pH increased drastically during the adsorption and the magnitude of the increase was almost the same without and with fluoride in the solution. In other words the variation of pH was independent on the initial concentration of fluoride. It is important to mention that the amount of fluoride adsorbed at the initial pH of 3 and 5 is much greater than that at the other pH values. This implies that the pH variation cannot be attributed to the adsorption of fluoride. Thus, the OH ions on the BC surface were not exchanged by the F ions from the solution since the final pH of the solution without fluoride was almost the same as the final pH of the solution with fluoride. The pH variation during the adsorption of fluoride can be explained recalling that the protons are released from or bound to the BC surface due to the protonation/deprotonation reactions described previously. Furthermore, the solution pH can be also
Abe et al. [18] assumed that OH and PO43 ions in BC were the only ionic species that could be exchanged by fluoride ions, and concluded that the ion exchange between fluoride ions from the solution and the OH ions on the BC was not relevant to the adsorption of fluoride. This conclusion is in agreement with the result argued in the previous section. Furthermore, Abe et al. [18] proposed that the ion exchange of fluoride ions with the PO43 ions represented between 18 and 42% of the fluoride adsorbed. In order to corroborate this assumption, the amount of phosphate released from BC during fluoride adsorption and its variation with the final pH of the solution and the initial concentration of fluoride was investigated and the results are graphed in Fig. 7a. In this figure, it can be observed that independently of the final pH, a certain amount of phosphate was always released to the solution. The phosphate released from BC was supposed to be from the dissolution of BC and the fluoride ion exchange. In this last mechanism, the fluoride ions were transferred from the solution to the surface of BC and the phosphate ions were released from the surface of BC to the solution. However, in some experiments, the mass of phosphate released when no fluoride was present, was even higher than that of phosphate released when the fluoride was present in the solution (see Fig. 7a). The dependence of the milliequivalents of phosphate released, QP, with respect to the milliequivalents of fluoride adsorbed, QF, is shown in Fig. 7b. If the adsorption of fluoride on BC was due to ion exchange of phosphate, the QF must be equal to QP. This condition is represented by the straight line (QP = QF) plotted in Fig. 7b. The QP is greater than the QF when the QF is below 0.07 mequiv., and the opposite occurs when the QF is above 0.07 mequiv. The data graphed in Fig. 7b indicated that the ratio of QP/QF ranged from 0.34 to 9.5. There was not relationship between the QF and the QP. Hence, most of the phosphate ions in the solution were dissolved from the BC, but it cannot be argued that the amount of phosphate was exchanged by fluoride, as it was previously concluded [18]. The adsorption of fluoride can be attributed to electrostatic interactions between the surface charge of BC and fluoride ions in the aqueous solution and it can be represented with the following reactions: BBCa-OH2 þ þ F ! BBCa-OH2 F BBPOH2 þ þ F ! BBPOH2 F In the above mechanisms, the interaction between the surface complexes BBCa-OH2+ and BBPOH2+ and the F do not lead to a chemical reaction or chemisorption because it has been demonstrated that the adsorption of F on BC was reversible [5]. 3.11. Effect of competing anions upon the adsorption of fluoride on BC In a previous study [5], the BC was tested to remove fluoride from a sample of actual drinking water and the capacity of BC for removing fluoride was slightly reduced due to the presence of competing ions in the drinking water. The sample of drinking water was from the city of San Luis Potosi, Mexico. It is important to investigate the competitive adsorption of Cl, NO2, NO3, CO32, HCO3, and SO42, which are among the most frequently anions detected in drinking water. The individual adsorption equilibrium data of Cl, NO2, NO3, CO32, HCO3, and SO42 on BC are depicted in Fig. 8. Except for the experimental data of CO32, the Freundlich isotherm fitted
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Amount of phosphate released, meq
0.24
Initial concentration of F 0.0 mg/L 1.0 mg/L 5.0 mg/L 10.0 mg/L 20.0 mg/L
0.20 0.16 0.12 0.08 0.04 0.00
6
7
8
9
10
11
12
Final pH
Fig. 8. Individual adsorption isotherms of anions on BC at T = 25 8C and without control of pH.
a) 0.30 Amount of phosphate released, meq
7
Initial concentration of F 1.0 mg/L 5.0 mg/L 10 mg/L 20 mg/L
0.25 0.20 0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Amount of fluoride adsorbed, meq
b) Fig. 7. Effect of initial pH and amount of fluoride adsorbed on the amount of phosphate released from BC to the solution.
plausibly well the experimental adsorption equilibrium data of the anions from aqueous solution, and the Freundlich constants as well as the average absolute percentage deviation, %D, are given in Table 3. The adsorption equilibrium of CO32 presented a sigmoidal shape (Fig. 8), and this unusual behavior could not be represented by the Freundlich isotherm. As seen in Fig. 8, the anions NO2, NO3, and SO42 were slightly adsorbed on BC, whereas the uptake of Cl was very similar to that
Table 3 Freundlich isotherm parameters for the adsorption of Cl, NO2, NO3, and SO42 from aqueous solution on BC at T = 25 8C and without controlling the solution pH. Anion
k (L1/n mequiv.11/n g1)
1/n
%D
Cl NO2 NO3 SO42
0.60 0.042 0.08 0.07
0.89 0.62 1.25 0.88
6.44 14.1 17.8 24.2
of F in a wide concentration range. In the case of the CO32, the uptake of CO32 was dependent on the concentration of F. The uptake of CO32 was much less than that of F at concentrations of CO32 at equilibrium below 0.2 mequiv./L; however, the uptake of CO32 was comparable to that of F at concentrations greater than 0.3 mequiv./L. In other words, the CO32 anions did not compete with the fluoride ions for the active sites of the BC when the concentration of CO32 at equilibrium was less than 0.2 mequiv./L. In a previous work, it was reported that the presence of CO32 did not affect the fluoride adsorption capacity of BC [5] when the BC was tested in removing fluoride from drinking water drawn from a well in San Luis Potosi, Mexico, and the concentration of CO32 was 0.03 mequiv./L. The results of this work proved that the CO32 did not affect the fluoride capacity of BC because the concentration of CO32 was below 0.2 mequiv./L. From the above arguing, the Cl ions could compete against the F for the active sites of BC. The competitive adsorption of Cl and F was investigated by determining the adsorption equilibrium of fluoride at various initial concentrations of Cl. The mass of F adsorbed in the presence of Cl is graphed in Fig. 9, and it can be observed that the capacity of BC for adsorbing F varies slightly when Cl ions are present in the aqueous solution. Similar result was found by Medellin-Castillo et al. [5]. Furthermore, the fluoride adsorption capacity of BC does not vary with a trend related to the initial concentration of Cl since the adsorption capacity of BC increased or decreased slightly in relation to the fluoride adsorption isotherm. The reason of this behavior is that the affinity of the F toward BC is much greater than that of the Cl. Thus, the F ions do not compete against Cl ions for the basic sites of BC. As seen in Fig. 6, the anion affinity order toward BC decreased in the following order: F > Cl > CO32 > NO2 NO3 HCO3 SO42 for the concentrations of anions less than 0.3 mequiv./L. Several factors influence the anion affinity order, among the most important are hydrated ionic radius, acid dissociation constant and ionic charge [37]. In the case of BC, the accessibility of the anions to the BC pores did not affect the anion affinity because the average pore diameter of BC (11.1 nm) was at least 28 times greater than the hydrated ionic radii of the anions, which ranged from 0.332 to 0.394 nm [38]. Much more experimental information is required to conduct a detail analysis of the anion affinity order.
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8
References
Mass of fluoride adsorbed, mg/g
6 5 4 3 Initial concentration of Chloride Cl- 80 mg/L Cl- 40 mg/L Cl- 20 mg/L Cl- 0 mg/L
2 1 0
0
2 4 6 8 10 Concentration of fluoride at equilibrium, mg/L
Fig. 9. Competitive adsorption isotherms of fluoride and chloride on BC at pH = 7.0 and T = 25 8C.
4. Conclusions The pH variation during the adsorption of fluoride on BC in aqueous solution revealed that the ion exchange reaction between the OH ions on the HAP and F ions in solution is not taking place. The pH increase during the adsorption of fluoride was due to the dissolution of HAP and calcium carbonate contained in BC and the protonation/deprotonation reactions of the active sites of HAP in BC. The fluoride in aqueous solution is mainly adsorbed on the HAP contained in the BC and not on the other constituents. The fluoride ion in the range of concentrations in drinking water could be adsorbed on BC without competition by other anions that are commonly found in natural waters. The mechanism of fluoride adsorption on the BC was attributed to the electrostatic interactions between the surface charge of BC and F ions in solution. Acknowledgments This work was funded by Fondo de Apoyo a la Investigacio´n (FAI), UASLP, and the federal program PROMEP, through grants No.: C12-FAI-03-55.55 and PROMEP/103.5/12/3953, respectively.
[1] S. Ayoob, A.K. Gupta, Environ. Sci. Technol. 36 (2006) 433. [2] D.L. Ozsvath, Rev. Environ. Sci. Biotechnol. 8 (2009) 59. [3] L.D. Benefield, J.F. Junkins, B.L. Weand, Process Chemistry for Water and Wastewater Treatment, Prentice Hall, New Jersey, 1982p. 405. [4] S. Meenakshi, R.C. Maheshwari, J. Hazard. Mater. B137 (2006) 456. [5] N.A. Medellin-Castillo, R. Leyva-Ramos, R. Ocampo-Perez, R.F. Garcia de la Cruz, A. Aragon-Pin˜a, J.M. Martinez-Rosales, R.M. Guerrero-Coronado, L. Fuentes-Rubio, Ind. Eng. Chem. Res. 46 (2007) 9205. [6] W. Ma, F. Ya, R. Wang, Y. Zhao, Int. J. Environ. Technol. Manage. 9 (1) (2008) 59. [7] A. Sivasamy, K.P. Singh, D. Mohan, M. Maruthamuthu, J. Chem. Technol. Biotechnol. 76 (7) (2001) 717. [8] A. Lhassani, M. Rumeau, D. Benjelloun, M. Pontie, Water Res. 35 (2001) 3260. [9] World Health Organization (WHO), Guidelines for Drinking Water Quality, World Health Organization, Geneva, 1993. [10] Y. Ma, F. Shi, X. Zheng, J. Ma, C. Gao, J. Hazard. Mater. 185 (2011) 1073. [11] S. Ayoob, A.K. Gupta, Chem. Eng. J. 133 (2007) 273. [12] A.K. Chaturvedi, K.C. Pathak, V.N. Singh, Appl. Clay Sci. 3 (1998) 337. [13] M. Marathamuthu, A. Sivasamy, Fluoride 27 (2) (1994) 81. [14] M. Sarkar, A. Banerjee, P.P. Pramanick, A.R. Sarkar, Chem. Eng. J. 131 (2007) 329. [15] C.-F. Chang, P.-H. Lin, W. Ho¨ll, Colloid. Surf. A 280 (2006) 194. [16] R. Leyva-Ramos, J.H. Soto-Zun˜iga, J. Mendoza-Barron, R.M. Guerrero-Coronado, Adsorp. Sci. Technol. 17 (7) (1999) 533. [17] X. Fan, D.J. Parker, M.D. Smith, Water Res. 37 (2003) 4929. [18] I. Abe, S. Iwasaki, T. Tokimoto, N. Kawasaki, T. Nakamura, S. Tanada, J. Colloid Interf. Sci. 275 (2004) 35. [19] B.S. Girgis, A.A. Kader, A.N.H. Aly, Adsorpt. Sci. Technol. 15 (1997) 277. [20] M.E. Kaseva, J. Water Health (2006) 139. [21] M.T. Alarcon-Herrera, J. Bundschuh, B. Nath, H.B. Nicolli, M. Gutierrez, V.M. ReyesGomez, D. Nun˜ez, I.R. Martin-Dominguez, O. Sracek, J. Hazard. Mater. 262 (2013) 960. [22] T.B. Mlilo, L.R. Brunson, D.A. Sabatini, J. Environ. Eng.-ASCE 136 (2010) 391. [23] J. Arends, J. Christoffersen, M.R. Christoffersen, H. Eckert, B.O. Fowler, J.C. Heughebaert, G.H. Nancollas, J.P. Yesinowski, S.J. Zawacki, J. Cryst. Growth 84 (3) (1987) 515. [24] R.Z. LeGeros, in: H.M. Myers (Ed.), Monographs Oral Science, Vol. 15, Karger, Basel, 1991, pp. 1–201. [25] J.C. Elliot, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier, Amsterdam, 1994. [26] Y. Takeuchi, H. Arai, J. Chem. Eng. Japan 23 (1) (1990) 75. [27] C. Sairam Sundaram, N. Viswanathan, S. Meenakshi, Bioresour. Technol. 99 (2008) 8226. [28] N.A. Medellı´n-Castillo, Equilibrio y Cinetica de Adsorcion de Fluoruros sobre Carbon de Hueso, (Ph.D. Thesis), Universidad Autonoma de San Luis Potosi, Mexico, 2009. [29] U. Wingenfelder, C. Hansen, G. Furrer, R. Schulin, Environ. Sci. Technol. 39 (2005) 4606. [30] American Public Health Association (APHA), Standard Methods for Examination of Water and Wastewater, 18th ed., Washington, D.C., 1994. [31] J.A. Wilson, I.D. Pulford, S. Thomas, Environ. Geochem. Health 25 (2003) 51. [32] C.W. Cheung, J.F. Porter, G. McKay, Langmuir 18 (2002) 650. [33] J. Rouquerol, F. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London, UK, 1999. [34] L. Wu, W. Forsling, P.W. Schindler, J. Colloid Interf. Sci. 147 (1) (1991) 178. [35] Y. Xu, F.W. Schwartz, S.J. Traina, Environ. Sci. Technol. 28 (8) (1994) 1472. [36] S. Dimovic´, I. Smicˇiklas, I. Plec´asˇ, D. Antonovic´, M. Mitric´, J. Hazard. Mater. 164 (2008) 279. [37] T. Sato, T. Wakabayashi, M. Shimada, Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 89. [38] E.R. Nightingale, J. Phys. Chem. 62 (1959) 1381.
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Adsorption capacity of bone char for removing fluoride from water solution. Role of hydroxyapatite content, adsorption mechanism and competing anions N.A. Medellin-Castillo a, R. Leyva-Ramos b,*, E. Padilla-Ortega b, R. Ocampo Perez b, J.V. Flores-Cano b, M.S. Berber-Mendoza a a Centro de Investigacio´n y Estudios de Posgrado, Facultad de Ingenierı´a, Universidad Auto´noma de San Luis Potosi, Av. Dr. M. Nava 8, San Luis Potosi, SLP 78290, Mexico b Centro de Investigacio´n y Estudios de Posgrado, Facultad de Ciencias Quı´micas, Universidad Auto´noma de San Luis Potosi, Av. Dr. M. Nava 6, San Luis Potosi, SLP 78210, Mexico
A R T I C L E I N F O
Article history: Received 21 August 2013 Accepted 30 December 2013 Available online xxx Keywords: Adsorption Bone char Fluoride Hydroxyapatite Water pollution
A B S T R A C T
The adsorption of fluoride from water on bone char (BC) was investigated in this work, and the fluoride adsorption capacity of BC was compared to that of hydroxyapatite (HAP). The adsorption capacity of BC and HAP drastically increased while decreasing the pH from 7.0 to 5.0. Furthermore, the fluoride adsorption on BC was due to its HAP content and was not considerably affected by the presence of the anions Cl, HCO3, CO32, SO42, NO3 and NO2. The mechanism of fluoride adsorption on BC was attributed to electrostatic interactions between surface charge of BC and fluoride ions in solution. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The impact of fluoride concentration in drinking water onto human health has been extensively studied and reviewed in various works [1,2]. A concentration of fluoride between 0.5 and 1.5 mg/L is essential for the human body, especially in preventing the incidence of dental caries; however, long term ingestion of water with excess amount of fluoride, above 2 mg/L, can cause diseases such as dental and skeletal fluorosis, masculine infertility and cancer [2]. Several treatment processes have been applied to remove excess fluoride from drinking water. Some of the common processes are chemical precipitation [3], ion exchange [4], adsorption [5,6], reverse osmosis [4], electrodialysis [7] and nanofiltration [8]. Adsorption is the most suitable process for drinking water treatment because it is relatively simple, economic, and appropriate for small communities [9,10]. During the past 10 years, extensive research has been carried out to find both low-cost and high capacity adsorbents for the removal of fluoride from drinking water. A wide range of
* Corresponding author. Tel.: +52 444 826 2440; fax: +52 444 826 2372. E-mail address: [email protected] (R. Leyva-Ramos).
adsorbents have been developed and tested, including activated alumina [11], natural minerals such as laterite, zeolite, and caolinite [12–14], nanomaterials [15], carbonaceous materials [7,16], bone char (BC) and hydroxyapatite (HAP) [5,17,18]. BC is manufactured by the carbonization of cattle bones and contains around 10% carbon and 90% HAP [19]. The adsorption of fluoride on BC has been investigated in various works [5,6,18,20]. In a previous work [5], it was shown that the capacity of BC for adsorbing fluoride was higher than those of activated alumina and activated carbon. Furthermore, the effect of solution pH and temperature on the adsorption capacity of fluoride onto BC was investigated and it was found that the maximum adsorption took place at pH 3 and decreased nearly 20 times by increasing the pH from 3 to 12. This behavior was attributed to the electrostatic interactions between the surface of BC and fluoride ions in solution. The adsorption capacity was not influenced by temperature in the range from 15 to 35 8C. The anions HCO3, CO32, Cl, NO3, NO2, and SO42 are commonly found in drinking water and these anions can compete with the fluoride for the adsorption sites of BC, affecting the adsorption capacity of BC toward fluoride. The competing effect of these anions on the capacity of BC for adsorbing fluoride has not been investigated in detail. Due to the high toxicity of As(V), the competitive adsorption of As(V) and fluoride from aqueous
1226-086X/$ – see front matter ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.105
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solution on BC has been studied because it is well documented that fluoride and As(V) may be co-occurring in drinking waters in several countries of Latin America [21]. Mlilo et al. [22] determined the adsorption isotherm of fluoride on BC with and without As(V) and concluded that the As(V) did not affect the fluoride adsorption capacity of BC for an initial concentration of As(V) of 0.250 mg/L. Calcium hydroxyapatite, Ca10(PO4)6(OH)2, is an important inorganic material in biology and chemistry [23–25]. Its structure, ion exchange capacity, adsorption affinity, and its capacity to bond with organic molecules of different sizes, have attracted much attention in the last two decades. The adsorption of heavy metals, Pb2+, Cu2+ and Cd2+, from aqueous solutions on HAP has been attributed to the ion exchange of Ca2+ on the HAP by the metal cation in solution [26,27]. Adsorption of fluoride on this adsorbent has been studied in several works [17,27] and it has been found that the mechanism of fluoride adsorption is due to electrostatic attractions and ion exchange processes. BC is mainly composed of HAP and it is essential to evaluate the contribution of this mineral to the adsorption of fluoride on BC. Thus, the main objective of this research was to evaluate the role of HAP on the capacity of BC for adsorbing fluoride from aqueous solution. Furthermore, the mechanism of fluoride adsorption on BC would be elucidated, and the effect of the presence of anions, commonly found in natural waters, on the fluoride adsorption capacity of BC would be analyzed in detail. 2. Materials and methods 2.1. Bone char and hydroxyapatite In this work, the granular BC used is commercially known as Fija Fluor and is manufactured from cattle bones by APELSA, Guadalajara, Mexico. The BC was sieved to an average particle diameter of 0.79 mm, washed repeatedly with deionized water, dried in an oven at 100 8C for 24 h, and stored in plastic containers. Analytical grade hydroxyapatite (purity < 99% according to the manufacturer, Sigma–Aldrich, CAS Number: 1306-06-5) was also used in this study. 2.2. Characterization of bone char and hydroxyapatite The contents of carbon, hydrogen and nitrogen on BC were determined using an Elemental Analyzer, Fisons Carlo Erba, EA1108. The phosphorus weight percentage of BC was evaluated by Inductively Coupled Plasma (ICP) Atomic Emission Spectroscopy using a Thermo Jarrel Ash, model IRIS/AP, instrument. The sample was prepared for ICP analysis using the method described by Medellin-Castillo [28]. The surface charge and point of zero charge (PZC) of BC and HAP were determined by a titration method [5]. The acid stability of BC was determined by a procedure similar to that recommended by Wingenfelder et al. [29]. A BC mass of 0.5 g and 40 mL of solution with an initial pH varying from 0.8 to 12.0 were added to a plastic bottle. The solutions were prepared by mixing proper volumes of 0.1 N HNO3 and NaOH solutions. The BC and acid solution were left in contact for 7 days and periodically mixed in an orbital shaker, and the final pH of the acid solution was measured. Moreover, the concentration of Ca2+ and PO43 dissolved in the acid solutions were determined by atomic absorption spectroscopy and atomic emission spectroscopy. The morphology of BC and HAP particles were observed by means of a scanning electron microscope (SEM), Philips, model XL30. The microscope was equipped with an energy dispersive Si(Li) detector EDAX DX4 for microanalysis. X-ray diffraction (XRD) analysis was used to identify the crystalline species present in both adsorbents employing an X-ray
diffractometer, Rigaku, model DMAX 2000. The textural properties of both materials (specific area, pore volume and average pore diameter) were determined by the N2-BET method using a surface area and porosimetry analyzer, Micromeritics, model ASAP 2010. 2.3. Determination of the concentrations of the anions in water solution The fluoride concentration in an aqueous solution was determined by a potentiometric method using a potentiometer, Orion, model SA720, and a selective electrode for fluoride ion. This method allowed the determination of fluoride concentrations in the range from 0.1 to 10 mg/L. The fluoride calibration curves were prepared with standard solutions of fluoride. Further details of the analytical method can be found elsewhere [28,30]. The concentrations of bicarbonate (HCO3), carbonate (CO32), chloride (Cl), nitrate (NO3), nitrite (NO2), phosphate (PO43), and sulfate (SO42) in water solutions were determined using the methods recommended by APHA [30]. 2.4. Adsorption equilibrium data Experimental adsorption equilibrium data were obtained as follows. A portion of 490 mL of an aqueous solution of a known initial concentration of fluoride or HCO3, CO32, Cl, NO3, NO2, and SO42 was added to the batch adsorber. A given mass of BC or HAP was added to a Nylon basket and placed inside the adsorber. The mass of BC or HAP was 0.5 or 1.0 g, and the initial concentrations of fluoride varied from 1 to 20 mg/L. The adsorber was placed in a constant temperature water bath and the solution was continuously mixed by a magnetic stirrer located just below the water bath. The solution was left in contact with the BC or HAP until equilibrium was reached, which took between 5 and 7 days. The solution pH was measured periodically with a pH-meter and kept constant by adding few drops of 0.01 and 0.05 HNO3 or NaOH solutions as required. The total volume added of HNO3 or NaOH solutions was recorded and considered in the mass balance. The solution was sampled at specific time intervals and the concentration of fluoride was determined for each sample. Equilibrium was reached when the concentrations of two consecutive samples did not change over time. The mass of fluoride or HCO3, CO32, Cl, NO3, NO2, and SO42 adsorbed at equilibrium was calculated by performing a mass balance of fluoride or other anions. The effect of coexisting or competing anions such as chloride on the fluoride adsorption capacity of BC was carried out by determining the adsorption equilibrium data of fluoride at pH = 7.0 and T = 25 8C, and varying the initial concentration of chloride from 20 to 80 mg/L during the adsorption of fluoride. 3. Results and discussion 3.1. Morphology of BC and HAP As seen in Fig. 1(a), the forms and shapes of the BC particles are very irregular and its surface is fractured, rough and porous. The distribution of the particle sizes is not uniform. The morphology of the HAP particles is shown in Fig. 1(b), and the particles presented a layered shape that is typical of HAP. 3.2. Chemical composition The elemental analysis of BC showed that the weight percentages of nitrogen, hydrogen and carbon were 0.95, 0.52 and 6.31%, correspondingly. The carbon content is very similar to the value reported by Wilson et al. [31] for a BC manufactured from cattle bones.
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3
Concentration of calcium, mg/L
1800 1500 1200 900 600 300 0
0
2
4
6
8
10
12
6 8 Final pH b)
10
12
Final pH
Concentration of phosphate, mg/L
a) 80 70 60 50 40 30 20 10 0
Fig. 1. SEM images of BC particles (a) and HAP particles (b).
The weight percentage of phosphorus in the BC was found to be 15.0%. Assuming that all the phosphorus belongs only to the HAP present in the BC, the percentage of HAP in BC can be estimated to be 84.8%. This value is slightly higher than that reported by Wilson et al. [29], who found a HAP weight percentage in a BC of 76%. The difference can be attributed to the cattle bone used as raw material in the manufacture of the BC. 3.3. Acid–base stability of BC The chemical stability of BC was studied by contacting this adsorbent with acid and basic solutions at different pH during 5 days and analyzing the Ca2+ and PO43 concentrations in the solution. The concentrations of calcium and phosphate in the aqueous solution vs. the final pH is plotted in Fig. 2(a) and (b), respectively. In these figures, it can be observed that the Ca2+ concentration is less than 28.2 mg/L at pH > 4 and increased drastically at pH < 3. This behavior is due to the dissolution of HAP and calcium carbonate in the water solution. Moreover, the PO43 concentration varied from 6 to 13 mg/L in a pH range between 4 < pH < 12 and increased drastically at pH < 4, which is a result of the dissolution of the BC. If the Ca2+ concentration was exclusively
0
2
4
Fig. 2. Acid–base stability of the BC at T = 25 8C. (a) Concentration of calcium and (b) concentration of phosphate in solution.
changed by the dissolution of HAP, at any solution pH, the molar ratio of Ca2+/PO43 would be constant to 5/3, according to the chemical formula of HAP, Ca10(PO4)6OH. The molar ratio of Ca2+/ PO43 was 43.1, 1.96 and 0.05 at pH 2.0, 7.0 and 12.0, respectively. This result corroborated that the calcium concentration in aqueous solution was from the dissolution of both HAP and calcium carbonate contained in the BC. 3.4. Textural properties of BC and HAP The textural properties of BC and HAP are shown in Table 1. The textural properties of BC have been reported in various works [32], and the values are very similar to those presented in this work. The results showed that the BC has a greater surface area than that of the HAP. This is probably due to the main components of BC being the HAP, Ca10(PO4)6(OH)2 and carbon. Therefore, the increase in the surface area of BC is essentially due to its carbon content. The surface area of HAP has been reported in the literature [17], and the values are in the range from 45 to 54.2 m2/g.
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Table 1 Textural properties and pHPZC of BC and HAP. Adsorbent
Surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
pHPZC
BC HAP
104 62.9
0.30 0.26
11.1 16.4
8.4 7.0
peaks of the HAP happen at the 2u values of 25.98, 31.78, 408, 46.78, and 49.58 [32]. Comparing the XRD pattern of BC to that of HAP, it can be noted that BC is mainly composed of HAP, but the relative intensities of the characteristic peaks for BC are slightly different from those of the HAP due to the content of HAP in BC. 3.6. Surface characterization of BC and HAP
The N2 adsorption isotherms on BC and HAP were shown to be characteristic of mesoporous solids and corresponded to type IV isotherm [33]. The average pore diameters of BC and HAP were 11.1 and 16.4 nm, respectively. The pore volume distribution of BC and HAP revealed that mesopores (2 nm < pore diameter < 50 nm) represented approximately 46 and 80% of the total pore volume of HAP and BC, respectively. 3.5. XRD analysis of BC and HAP The XRD patterns of the BC and HAP are shown in Fig. 3a and b, respectively. The crystalline species of both materials were identified comparing the characteristic peaks shown in the XRD pattern with the database of the diffractometer (Journal of Crystallographic Powder Diffraction Spectra). The characteristic
The surface charge of the BC and HAP are illustrated in Fig. 4. The pHPZC of the HAP and BC were 7.0 and 8.4, respectively. Similar results have been reported for BC and HAP [34–36]. The pHPZC of the BC was basic because its concentration of basic sites was higher than that of acid sites [5]. As noted in Fig. 4, the surface charge of the BC was increased by raising the ionic strength, but the pHPZC of BC was not dependent on the ionic strength. It was shown that the main constituent of BC was HAP and the surface charge of the BC was mainly due to the interactions between the HAP surface and the ions in the water solution. The functional groups of the HAP surface that affect its surface charge are the phosphates, BBP-OH, and the hydroxyls, BBCa-OH. The positive charge of the BC surface was due to the following protonation reactions: BBP-OH þ Hþ ! BBPOH2 þ
100
HAP
BBCa-OH þ Hþ ! BBCa-OH2 þ
Relative Intensity
80
Bone Char
On the other hand, the negative charge of the BC surface was originated by the following deprotonation reactions:
60 40
BBP-OH ! BBPO þ Hþ BBCa-OH ! BBCa-O þ Hþ
HAP
where BB represents the BC surface. Thus, the protonation reactions predominated at pH < pHPZC, although the deprotonation reactions occurred at pH > pHPZC.
HAP HAP HAP
20
3.7. Adsorption isotherm
0 20
30
40
50
60
70
80
90
2θ a) 100
The Freundlich, Langmuir, and Prausnitz-Radke isotherm models were fitted to the experimental adsorption equilibrium data of fluoride on BC and HAP. These isotherm models can be
HAP
HAP
Relative Intensity
80 Hydroxyapatite
60 HAP
40 HAP HAP
20 0
20
30
40
50
2θ
60
70
80
90
b) Fig. 3. XRD patterns of the BC (a) and the HAP (b).
Fig. 4. Surface charge distribution of BC and HAP.
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Table 2 Values of the parameters for the Langmuir, Freundlich and Prausnitz-Radke adsorption isotherms. Adsorbent
pH
Freundlich 1/n
k (L
Langmuir
1/n1
/mg
g)
1/n
BC
5 7
3.96 2.72
0.31 0.29
HAP
5 7
5.15 2.37
0.35 0.48
%D 6.86 4.39 17.1 11.3
Prausnitz-Radke K (L/mg) 1.55 1.22 1.32 0.18
7.74 5.44 10.9 11.1
%D
a (L/g)
16.4 16.0
36.9 42.0
15.7 16.9
64.4 56.4
b
b
b (L/mg) 7.55 3.07 11.0 23.0
%D
0.79 0.76
9.54 2.15
0.70 0.53
10.5 7.56
the fluoride in aqueous solution is mostly adsorbed on the HAP contained in the BC and not by the other constituents of BC.
represented by the subsequent mathematical relationships: q ¼ kC 1=n
qm (mg/g)
(1) 3.9. Effect of pH on the capacity of BC for adsorbing fluoride
q¼
aC 1 þ bC b
(2)
(3)
where C (mg/L) is the concentration of fluoride at equilibrium and q (mg/g) is the mass of fluoride adsorbed per mass of adsorbent. The parameters K (L/mg) and qm (mg/g) are the Langmuir constants related to the energy of adsorption and maximum adsorption capacity, respectively. The parameters k (L1/n mg11/n g1) and n are the Freundlich constants related to the adsorption capacity and b intensity. The parameters a (L/g), b (L/mg) and b are the Prausnitz-Radke isotherm constants. The isotherm constants were estimated by a least-squares method based on an optimization algorithm and the values are shown in Table 2 along with the average absolute percentage deviation, %D, which was computed from the following equation: N 1X qexp qcal %D ¼ (4) 100 qexp N i¼1
where N is the number of experimental data points, qexp (mg/g) is the experimental mass of fluoride adsorbed, and qcal (mg/g) is the mass of fluoride adsorbed predicted with the adsorption isotherm model. The experimental adsorption equilibrium data were satisfactorily interpreted by the three isotherm models since the average percentage deviations were less than 10.5, 16.9, and 17.1% for the Prausnitz-Radke, Langmuir and Freundlich isotherms, correspondently. As shown in Table 2, the Prausnitz-Radke isotherm best fitted the data for BC and HAP since, in the 3 of the 4 isotherm cases shown in Table 2, the %D of the Prausnitz-Radke isotherm was lower than the %D of Langmuir and Freundlich isotherms. This can be explained by recalling that the Prausnitz-Radke model is a three-parameter isotherm whereas the Langmuir and Freundlich models are two-parameter isotherms. 3.8. Effect of the presence of HAP on the adsorption capacity of BC The experimental adsorption equilibrium data of fluoride on BC and HAP are shown in Fig. 5 at T = 25 8C and pH = 5.0 and 7.0. It can be noted that the fluoride adsorption on HAP is greater than that of the BC. At an equilibrium fluoride concentration of 1.5 mg/L, the ratio between the mass of fluoride adsorbed on the BC and that on the HAP, qBC/qHAP is 0.77 and 0.95 at pH = 5.0 and 7.0, respectively, and at an equilibrium fluoride concentration of 4.0 mg/L, the ratio qBC/qHAP is 0.73 and 0.81 at pH = 5.0 and 7.0, correspondingly. These ratios are very close to the weight fraction of HAP in the BC, which is 0.848 (see Section 3.2). This reveals that the fluoride adsorption capacity of BC is mostly due to its content of HAP. Thus,
In a previous work [5], it was reported that the capacity of BC for adsorbing fluoride was significantly dependent on the solution pH and decreased considerably while raising the pH from 3 to 11. Similar results were found in this work and are displayed in Fig. 5. This behavior was attributed to the electrostatic interactions between fluoride ions in water solution and the surface charge of BC. The surface of BC was positively charged at pH < pHPZC = 8.4 and the F ions in solution were attracted to the surface of BC. The mass of fluoride adsorbed decreased by reducing the pH from 3 to 8.4 because the surface charge of BC decreased by raising the pH, this behavior is depicted in Fig. 4. Additional experiments were carried out to investigate the variation of the solution pH during the adsorption of fluoride on the BC. The experiments consisted on performing adsorption experiments as already described using a BC mass of 1.0 g, various initial fluoride concentrations (1, 5, 10, and 20 mg/L) and various initial pH values (3, 5, 9, and 12). During these experiments, the pH of the solution was not kept constant and the final pH was recorded along with the mass of fluoride adsorbed. As expected, the results revealed that the fluoride adsorption capacity of BC decreased when the pH increased independently on the initial concentration of fluoride (Fig. 6a). Additionally, it was observed that the solution pH varied during the adsorption of fluoride on BC. The initial pHs of the solutions were approximately 3, 5, 7, 9 and 12 and the final pH of the solutions presented an average value of 6.9, 7.9, 7.9, 8.1 and 11.8, respectively. The
14
Mass of fluoride adsorbed, mg/g
q KC q¼ m 1 þ KC
BC, pH = 5 BC, pH = 7 HAP, pH = 5 HAP, pH = 7
12 10 8 6 4 2 0 0
2
4
6
8
10
12
14
Concentration of fluoride at equilibrium, mg/L Fig. 5. Adsorption isotherms of fluoride on BC and HAP. T = 25 8C and pH = 5.0 and 7.0.
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altered by the dissolution of the carbonate and HAP contained in the BC. 3.10. Adsorption mechanism
Fig. 6. Effect of initial pH and initial concentration of fluoride on the adsorption capacity of BC (a) and the final pH (b).
variation of the solution pH during adsorption is shown in Fig. 6b, and the solution pH increased when the experiments were performed at a pH < pHPZC, while it remained almost constant when the experiments were carried out at a pH > pHPZC. At the initial pH of 3 and 5, the pH increased drastically during the adsorption and the magnitude of the increase was almost the same without and with fluoride in the solution. In other words the variation of pH was independent on the initial concentration of fluoride. It is important to mention that the amount of fluoride adsorbed at the initial pH of 3 and 5 is much greater than that at the other pH values. This implies that the pH variation cannot be attributed to the adsorption of fluoride. Thus, the OH ions on the BC surface were not exchanged by the F ions from the solution since the final pH of the solution without fluoride was almost the same as the final pH of the solution with fluoride. The pH variation during the adsorption of fluoride can be explained recalling that the protons are released from or bound to the BC surface due to the protonation/deprotonation reactions described previously. Furthermore, the solution pH can be also
Abe et al. [18] assumed that OH and PO43 ions in BC were the only ionic species that could be exchanged by fluoride ions, and concluded that the ion exchange between fluoride ions from the solution and the OH ions on the BC was not relevant to the adsorption of fluoride. This conclusion is in agreement with the result argued in the previous section. Furthermore, Abe et al. [18] proposed that the ion exchange of fluoride ions with the PO43 ions represented between 18 and 42% of the fluoride adsorbed. In order to corroborate this assumption, the amount of phosphate released from BC during fluoride adsorption and its variation with the final pH of the solution and the initial concentration of fluoride was investigated and the results are graphed in Fig. 7a. In this figure, it can be observed that independently of the final pH, a certain amount of phosphate was always released to the solution. The phosphate released from BC was supposed to be from the dissolution of BC and the fluoride ion exchange. In this last mechanism, the fluoride ions were transferred from the solution to the surface of BC and the phosphate ions were released from the surface of BC to the solution. However, in some experiments, the mass of phosphate released when no fluoride was present, was even higher than that of phosphate released when the fluoride was present in the solution (see Fig. 7a). The dependence of the milliequivalents of phosphate released, QP, with respect to the milliequivalents of fluoride adsorbed, QF, is shown in Fig. 7b. If the adsorption of fluoride on BC was due to ion exchange of phosphate, the QF must be equal to QP. This condition is represented by the straight line (QP = QF) plotted in Fig. 7b. The QP is greater than the QF when the QF is below 0.07 mequiv., and the opposite occurs when the QF is above 0.07 mequiv. The data graphed in Fig. 7b indicated that the ratio of QP/QF ranged from 0.34 to 9.5. There was not relationship between the QF and the QP. Hence, most of the phosphate ions in the solution were dissolved from the BC, but it cannot be argued that the amount of phosphate was exchanged by fluoride, as it was previously concluded [18]. The adsorption of fluoride can be attributed to electrostatic interactions between the surface charge of BC and fluoride ions in the aqueous solution and it can be represented with the following reactions: BBCa-OH2 þ þ F ! BBCa-OH2 F BBPOH2 þ þ F ! BBPOH2 F In the above mechanisms, the interaction between the surface complexes BBCa-OH2+ and BBPOH2+ and the F do not lead to a chemical reaction or chemisorption because it has been demonstrated that the adsorption of F on BC was reversible [5]. 3.11. Effect of competing anions upon the adsorption of fluoride on BC In a previous study [5], the BC was tested to remove fluoride from a sample of actual drinking water and the capacity of BC for removing fluoride was slightly reduced due to the presence of competing ions in the drinking water. The sample of drinking water was from the city of San Luis Potosi, Mexico. It is important to investigate the competitive adsorption of Cl, NO2, NO3, CO32, HCO3, and SO42, which are among the most frequently anions detected in drinking water. The individual adsorption equilibrium data of Cl, NO2, NO3, CO32, HCO3, and SO42 on BC are depicted in Fig. 8. Except for the experimental data of CO32, the Freundlich isotherm fitted
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Amount of phosphate released, meq
0.24
Initial concentration of F 0.0 mg/L 1.0 mg/L 5.0 mg/L 10.0 mg/L 20.0 mg/L
0.20 0.16 0.12 0.08 0.04 0.00
6
7
8
9
10
11
12
Final pH
Fig. 8. Individual adsorption isotherms of anions on BC at T = 25 8C and without control of pH.
a) 0.30 Amount of phosphate released, meq
7
Initial concentration of F 1.0 mg/L 5.0 mg/L 10 mg/L 20 mg/L
0.25 0.20 0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Amount of fluoride adsorbed, meq
b) Fig. 7. Effect of initial pH and amount of fluoride adsorbed on the amount of phosphate released from BC to the solution.
plausibly well the experimental adsorption equilibrium data of the anions from aqueous solution, and the Freundlich constants as well as the average absolute percentage deviation, %D, are given in Table 3. The adsorption equilibrium of CO32 presented a sigmoidal shape (Fig. 8), and this unusual behavior could not be represented by the Freundlich isotherm. As seen in Fig. 8, the anions NO2, NO3, and SO42 were slightly adsorbed on BC, whereas the uptake of Cl was very similar to that
Table 3 Freundlich isotherm parameters for the adsorption of Cl, NO2, NO3, and SO42 from aqueous solution on BC at T = 25 8C and without controlling the solution pH. Anion
k (L1/n mequiv.11/n g1)
1/n
%D
Cl NO2 NO3 SO42
0.60 0.042 0.08 0.07
0.89 0.62 1.25 0.88
6.44 14.1 17.8 24.2
of F in a wide concentration range. In the case of the CO32, the uptake of CO32 was dependent on the concentration of F. The uptake of CO32 was much less than that of F at concentrations of CO32 at equilibrium below 0.2 mequiv./L; however, the uptake of CO32 was comparable to that of F at concentrations greater than 0.3 mequiv./L. In other words, the CO32 anions did not compete with the fluoride ions for the active sites of the BC when the concentration of CO32 at equilibrium was less than 0.2 mequiv./L. In a previous work, it was reported that the presence of CO32 did not affect the fluoride adsorption capacity of BC [5] when the BC was tested in removing fluoride from drinking water drawn from a well in San Luis Potosi, Mexico, and the concentration of CO32 was 0.03 mequiv./L. The results of this work proved that the CO32 did not affect the fluoride capacity of BC because the concentration of CO32 was below 0.2 mequiv./L. From the above arguing, the Cl ions could compete against the F for the active sites of BC. The competitive adsorption of Cl and F was investigated by determining the adsorption equilibrium of fluoride at various initial concentrations of Cl. The mass of F adsorbed in the presence of Cl is graphed in Fig. 9, and it can be observed that the capacity of BC for adsorbing F varies slightly when Cl ions are present in the aqueous solution. Similar result was found by Medellin-Castillo et al. [5]. Furthermore, the fluoride adsorption capacity of BC does not vary with a trend related to the initial concentration of Cl since the adsorption capacity of BC increased or decreased slightly in relation to the fluoride adsorption isotherm. The reason of this behavior is that the affinity of the F toward BC is much greater than that of the Cl. Thus, the F ions do not compete against Cl ions for the basic sites of BC. As seen in Fig. 6, the anion affinity order toward BC decreased in the following order: F > Cl > CO32 > NO2 NO3 HCO3 SO42 for the concentrations of anions less than 0.3 mequiv./L. Several factors influence the anion affinity order, among the most important are hydrated ionic radius, acid dissociation constant and ionic charge [37]. In the case of BC, the accessibility of the anions to the BC pores did not affect the anion affinity because the average pore diameter of BC (11.1 nm) was at least 28 times greater than the hydrated ionic radii of the anions, which ranged from 0.332 to 0.394 nm [38]. Much more experimental information is required to conduct a detail analysis of the anion affinity order.
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8
References
Mass of fluoride adsorbed, mg/g
6 5 4 3 Initial concentration of Chloride Cl- 80 mg/L Cl- 40 mg/L Cl- 20 mg/L Cl- 0 mg/L
2 1 0
0
2 4 6 8 10 Concentration of fluoride at equilibrium, mg/L
Fig. 9. Competitive adsorption isotherms of fluoride and chloride on BC at pH = 7.0 and T = 25 8C.
4. Conclusions The pH variation during the adsorption of fluoride on BC in aqueous solution revealed that the ion exchange reaction between the OH ions on the HAP and F ions in solution is not taking place. The pH increase during the adsorption of fluoride was due to the dissolution of HAP and calcium carbonate contained in BC and the protonation/deprotonation reactions of the active sites of HAP in BC. The fluoride in aqueous solution is mainly adsorbed on the HAP contained in the BC and not on the other constituents. The fluoride ion in the range of concentrations in drinking water could be adsorbed on BC without competition by other anions that are commonly found in natural waters. The mechanism of fluoride adsorption on the BC was attributed to the electrostatic interactions between the surface charge of BC and F ions in solution. Acknowledgments This work was funded by Fondo de Apoyo a la Investigacio´n (FAI), UASLP, and the federal program PROMEP, through grants No.: C12-FAI-03-55.55 and PROMEP/103.5/12/3953, respectively.
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