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Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution
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JECE 495 1–6 Journal of Environmental Chemical Engineering xxx (2014) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
1 2 3
Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution
4 Q1
Fumihiko Ogata a , Yuka Iwata a , Naohito Kawasaki a,b, *
5 6
a b
Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Antiaging Center, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 September 2014 Accepted 5 November 2014
Zeolites were produced by hydrothermal treatment of fly ash for 48, 60, and 72 h (referred to as FA48, FA60, and FA72) in alkaline solution. Physical and chemical analyses were performed on the FA samples. Moreover, the amount of tungsten adsorbed on the FA samples, derived from the adsorption isotherms under different pH conditions, and the effect of the contact time on the adsorption were evaluated. Parent FA consisted primarily of mullite crystals, whereas the hydrothermally treated FA48, FA60, and FA72 samples consisted of phillipsite, zeolite X, and zeolite A, respectively. The specific surface areas and pore volumes of parent FA were smaller than those of FA48, FA60, and FA72. FA48 generated the largest specific surface area and pore volume. The saturated amount of tungsten adsorbed on FA48 was greater than that of the other FA samples. Tungsten adsorption was more effective (larger amount) at pH 2.0– 3.0 than at pH 6.1–6.5 or pH 11.0–12.0. These results suggest that the tungsten was adsorbed on the surface of FA48 through interactions between the electrons of the positively charged FA48 surface and the tungsten anions in solution. Analysis of the equilibrium adsorption data using the Langmuir and Freundlich equations showed that the correlation coefficient of the Freundlich isotherm was higher than that of the Langmuir model. The data obtained in this study fit more adequately to the pseudo-secondorder model than the pseudo-first-order model. Collectively, these results suggest that FA48 is prospectively useful for the adsorption of tungsten from aqueous solutions. ã 2014 Published by Elsevier Ltd.
Keywords: Fly ash Hydrothermally treated in an alkaline solution Tungsten Adsorption
7
Introduction
8
Recently, atomic power, coal, and natural gas have gained consideration as alternatives to petroleum as energy sources. The abundance of coal deposits makes coal advantageous as an energy source relative to the other energy sources [1]. A huge amount of fly ash is generated from electric power plants, and approximately 500 million tonnes of fly ash is discharged per year worldwide. Because fly ash has pozzolanic properties after reaction with lime [2], about 20% of fly ash is used in building-material related applications. However, the remaining fly ash that is disposed in landfills still poses a growing threat to the environment due to its
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* Corresponding author at: Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. Tel.: +81 6 6730 5880x5556; fax: +81 6 6721 2505. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
fine structure and toxic elements [3]. Hence, the development of an effective method for recycling fly ash is of paramount importance. The conversion of fly ash into zeolites is one promising technique for utilization of fly ash, and alkaline hydrothermal synthesis of zeolites from fly ash has been the subject of research interest for over 30 years [4]. Classic alkaline conversion of fly ash into zeolite is based on the combination of different activation solution-to-fly ash ratios, along with variation of the temperature, pressure, and reaction time to obtain different zeolite types. Sodium or potassium hydroxide solutions of different concentrations have been combined under atmospheric and water vapor pressure at temperatures ranging from 80 to 200 C and for periods of 3–48 h in the synthesis of up to 13 different zeolites from the same fly ash sample [5–14]. Zeolites generated from fly ash have a wide range of applications in ion exchange, as molecular sieves, catalysts, and adsorbents [15–17]. As another concern, tungsten is a possible water contaminant that may be present due to mining and industrial activities such as the release of poorly treated effluents from tungsten mines or
http://dx.doi.org/10.1016/j.jece.2014.11.015 2213-3437/ ã 2014 Published by Elsevier Ltd.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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tungsten treatment plants and the smelting of tungsten ores [18]. Tungsten is a valuable resource used to produce cemented carbide tools; however, a lot of tungsten is uselessly lost during manufacturing processes in Japan. Tungsten is one of the stockpiled elements in Japan because its supply depends significantly on imports, and as such, is subject to shortages based on global supply [19]. Japan is one of the main consumers of rare metals (including tungsten); therefore, situations that may prevent importation of rare metals or result in depletion of reserves have the potential to decimate the Japanese manufacturing industry. Based on these considerations, the current objective is to develop and characterize a low-cost adsorbent for recycling tungsten [20] while simultaneously addressing the issue of reusing waste fly ash. The adsorption or recovery of tungsten from aqueous solution using adsorbents (specifically, zeolites produced from waste fly ash) has rarely been studied. If the adsorption of tungsten on zeolite produced from fly ash under different conditions could be achieved, the issues of reuse of fly ash and the adsorption of tungsten from aqueous solution could be simultaneously addressed. Thus, we present the development of an adsorbent by hydrothermal treatment of fly ash in alkaline solution under different conditions and investigate its capability to adsorb tungsten from aqueous solution.
60
Materials and methods
61
Materials
62
Fly ash (FA) was obtained from the Tachibana-wan Thermal Power Station (Shikoku Electric Power, Inc, Japan). The main components of FA are SiO2 and Al2O3, which comprise 70–80% of the total weight. The other minor components of FA are Fe2O3, CaO, MgO, Na2O, K2O, and rare earth metals [19]. Zeolites were produced by hydrothermal treatment of FA in an alkaline solution [21]. FA (0.5 g) was added to 40 mL of a 3.0 mol/L sodium hydroxide solution (Wako Pure Chemical Industries, Co., Ltd., Japan), and the mixture was subsequently heated at 93 C for 48, 60, and 72 h. The suspension was filtered through a 0.45 mm membrane filter (Advantec MFS, Inc., Japan). The residue was washed with distilled water and dried at 110 C for 24 h. The samples generated with treatment for 48, 60, and 72 h are referred to as FA48, FA60, and FA72, respectively [22].
63 64 65 66 67 68 69 70 71 72 73 74 75
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Properties of FA products
77
The morphologies and crystallinities of the treated FA samples were studied using scanning electron microscopy (SEM; JSM-5500LV; JEOL, Japan) and X-ray diffractometry (XRD; MiniFlex II; Rigaku, Japan). The percentage yield of the treated FA samples was calculated using the weight of FA before and after calcination. The ash content was measured using the method reported in a previous study (JIS M8812). The surface functional groups (i.e., acidic and basic functional groups) were determined by using the method reported by Boehm [23,24]. The specific surface area, pore volume, and mean pore diameter were measured using a specific surface analyzer, NOVA4200e instrument (Yuasa Ionic, Japan), and nitrogen adsorption/ desorption isotherm analysis, respectively. The pH values of the solutions containing the FA samples were measured using the following method: FA (0.1 g) was added to 50 mL of distilled water (pH 7.0) and maintained at 25 C for 2 h. The suspensions were subsequently filtered using a 0.45 mm membrane filter. The solution pH was measured using a digital pH meter (Mettler, Toledo, Japan). The pHpzc of the samples was measured by the method reported by Faria and co-workers [25].
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
Isotherms for adsorption of tungsten on treated FA samples at different pH
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The adsorbent (0.05 g) was added to 50 mL of 5–50 mg/L tungsten solution (Na2WO4, model solution) (pH 2.0–3.0, pH 6.1–6.5, and pH 11.1–12.0), which was adjusted by hydrochloric acid and sodium hydroxide solution (Wako Pure Chemical Industries, Co., Ltd., Japan). The suspension was shaken at 100 rpm for 48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu, Japan). The amount of tungsten adsorbed was calculated by using Eq. (1):
99
X¼
ðC o C e ÞV M
98
100 101 102 103 104 105 106 107 108
(1)
where X is the amount adsorbed (mg/g), C0 is the concentration prior to adsorption (mg/L), Ce is the concentration after adsorption (mg/L), V is the volume of the solvent, and M is the mass of the adsorbent (g).
110 109 111
Saturated amount of tungsten adsorbed on FA samples
114
The adsorbent (0.25 g) was added to 100 mL of 250 mg/L (pH 2.0) tungsten solution (Na2WO4, model solution). The suspension was shaken at 100 rpm for 48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using ICP-AES. The amount of tungsten adsorbed was calculated by using Eq. (1).
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Effect of contact time on the adsorption of tungsten on FA samples
121
The adsorbent (0.05 g) was added to 50 mL of 50 mg/L (pH 2.0) tungsten solution (Na2WO4, model solution). The suspension was shaken at 100 rpm for 0.5–48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using ICP-AES. The amount of tungsten adsorbed was calculated by using Eq. (1).
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Results and discussion
128
Properties of FAs
129
SEM images of the adsorbents are presented in Fig. 1. Original FA consisted of spherical particles of various diameters, whereas zeolite crystals were produced in the case of FA48, FA60, and FA72. The XRD patterns of the adsorbents (Fig. 2) show that the parent FA consisted primarily of mullite crystals (3Al2O32SiO2), while the FA48, FA60, and FA72 samples consisted of phillipsite, zeolite X, and zeolite A, respectively [26,27]. These results indicate that hydrothermal treatment of FA in alkaline solution transformed the FA into phillipsite, zeolite X, and zeolite A, based on the treatment conditions. Table 1 shows the chemical properties of the original and treated FA samples. The respective percentages ash in FA, FA48, FA60, and FA72 were 95.6, 89.7, 84.5, and 88.3%. A previous study reported that zeolite produced by hydrothermal treatment of FA in alkaline solution contained water in its pores. In this study, the water molecules in the FA48, FA60, and FA72 pores were evaporated upon heating prior to analysis; heating contributed to the decreased ash content of the FA48, FA60, and FA72 samples [28]. The amount of acid consumed by FA (0.54 mmol/g) was greater than that consumed by FA48, FA60, and FA72 (0.34–0.44 mmol/g) in analysis of the content of basic sites, whereas the amount of based consumed by FA48, FA60, and FA72 (3.22–3.40 mmol/g) was greater than that consumed by parent FA (0.28 mmol/g). These results suggest a decrease in the number of
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Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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3
Fig. 1. SEM images of adsorbents.
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acidic functional groups or an increase in the number of basic functional groups due to treatment of FA in alkaline solution. Table 2 shows the physical properties of the original and treated FA samples. The specific surface area and pore volume of FA (2.7 m2/g and 12.7 mL/g) were smaller than those of FA48, FA60, and FA72 (23.3–45.3 m2/g and 57.6–71.5 mL/g). On the other hand, the mean pore diameter of FA (189.8 Å) was larger than that of FA48, FA60, and FA72 (6.31–9.89 nm). In general, based on their volume, pores are divided into micropores (0.5 nm < d 2 nm), mesopores (2 nm < d 50 nm), and macropores (d > 50 nm). Moreover, increasing the specific surface area decreases the mean pore diameter [29]. Conformance to this trend was observed in this study. The pHpzc is the pH at which the net particle charge becomes zero, and is an important parameter in describing surface behavior. The pHpzc of
250000
䕔 䕰䕔 䕰 䕰 䕰 䕿 䕿 䕰 䕰
䕔 䕰䕿 䕰
200000
Intensity y (cps)
154
䕔
䕧䕧
FA72
䕧 䕧 䕔 䕔
䕧
150000
䕧 䕧䕿
100000
䕻 䕧䕔 䕧 䕧 䕧
䕧 䕧 䕔䕧 䕧 䕧䕧
䕔 䕔䕧 䕿 䕔 䕿䕧 䕧
䕿 䕔 䕧
䕧 䕧 䕧
䕧 䕔
䕧
FA60
䕧
䕔
䕻
䕔 䕔 䕔䕔
10
20
30
40
Amount of tungsten adsorbed on FA samples
176
Isotherms for adsorption of tungsten on the FA samples in different pH solutions are shown in Fig. 4. The amount of tungsten adsorbed on parent FA was smaller than that adsorbed on FA48, FA60, and FA72. Moreover, the amount adsorbed at pH 2.0–3.0 was greater than that at pH 6.1–6.5 or pH 11.0–12.0. The solution pH after adsorption was <9.40 (initial pH 2.0–3.0), which indicates that the surface of FA48 was positively charged (pHpzc of FA48 = 9.40). These results suggest that tungsten was absorbed on the surface of FA48 through interactions between the electrons of the positively charged FA48 surface and the tungsten anions in solution. In comparison, the pHpzc of FA is 7.94, but the specific surface area of FA is very small compared to that of FA48. Thus, the amount of tungsten adsorbed was related not only to chemical
177
Table 1 Chemical properties of FAs.
䕔 䕔 䕔
䕔
FA
0 0
167
FA48
䕻
50000
FA, FA48, FA60, and FA72 was 7.94, 9.40, 9.81, and 9.83, respectively (Fig. 3). The surface functional groups, specific surface area, pore volume, and pHpzc are closely related to the adsorption capability. These results suggest that hydrothermal treatment of FA in alkaline solution led to significant pore development in the FA48, FA60, and FA72 samples, resulting in novel adsorbents. Moreover, the specific surface area of FA48 was the largest of the four FA samples; thus the required treatment time, and therefore the cost, for producing FA48 is reduced compared to other treatments.
50
Samples
Yield (%)
Acidic consumptions (m mol/g)
Basic consumptions (m mol/g)
Ash (%)
pH
FA FA48 FA60 FA72
81.8 86.2 97.8
0.54 0.34 0.44 0.35
0.28 3.35 3.22 3.40
95.6 89.7 84.5 88.3
7.2 10.1 10.4 10.1
60
2θ (deg) 䕔: Mullite, 䕻: Quartz, 䕧: ZeoliteX, 䕿: Phillipsite , 䕰: Zeolite A Fig. 2. XRD patterns of adsorbents.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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Table 2 Physical properties of FAs. Samples Specific surface area (m2/g)
50 Pore volume (mL/g)
Mean pore diameter (Å)
FA FA48 FA60 FA72
190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206
2.7 45.3 23.3 36.7
0.3 3.2 1.2 1.2
6.5 55.9 37.7 38.5
5.9 12.4 25.7 20.3
12.7 71.5 57.6 60
Amouunt adsorbbed (mg/g g)
r 2 1 nm 1 < r 2 25 nm r < Total 25 nm 189.8 63.1 98.9 65.4
interaction but also to physical interaction. The saturated amount of tungsten adsorbed on FA, FA48, FA60, and FA72 was 1.2, 72.6, 72.5, and 60.6 mg/g, respectively. Correlation coefficient of relationship between the saturated amount adsorbed and specific surface area, micropores, mesopores, macropores, total pore volume, and mean pore diameter is 0.852, 0.695, 0.932, 0.756, 0.973, and 0.928, respectively (data not shown). These results indicated that the pore size of adsorbent (especially, total pore volume and mean pore diameter) was very important factor than specific surface area for adsorption of tungsten from aqueous solution. Equilibrium data, commonly known as the adsorption isotherms, are the main medium to investigate the adsorption mechanism. Two traditional adsorption isotherm models, i.e., Langmuir and Freundlich, are used to describe the equilibrium between adsorbed tungsten and the adsorbent [30,31]. The Langmuir isotherm model can be expressed as: qe ¼
K L qmax C e ð1 þ K L qmax Þ
(2)
207 208
The linear form can be expressed as: Ce 1 Ce ¼ þ qe K L qmax qmax
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where Ce is the equilibrium concentration of tungsten ions (mg/L), qe is the equilibrium capacity of tungsten ions on the adsorbent (mg/L), qmax is the maximum adsorption capacity (mg/g) of the adsorbent, and KL is the Langmuir adsorption constant (L/mg). Thus, qmax and KL can be calculated from the linear plots of Ce/ qe versus Ce [32]. The Freundlich isotherm is an empirical equation. It can be used to describe heterogeneous systems. The non-linear form of the
14 12 10 pHFFinal
210 209 211
(3)
8 6 FA : 7.94 FA48 : 9.40 FA60 : 9.81 FA72 : 9.83
4 2
2
4
6
8
10
12
40
30
30
20
20
10
10
0 500
10
20
30
40
0 0 50
50
FA48
40
10
20
30
40
50
40
50
FA72
40
30
30
20
20
10
10
0
FA60
0
10
0 0 40 30 50 10 20 Equilibrium concentration (mg/L)
20
30
䖃: pH2.0-3.0, 䕕: pH6.1-6.5, 䕺: pH11.0-12.0
Fig. 4. Adsorption isotherms of tungsten on FAs at different pH solutions.
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Freundlich model is: qe ¼ K F C ne
(4) 220 219
and the linear form is expressed as: logqe ¼ logK F þ ð1=nÞlogC e
(5)
where KF is an indicator of the adsorption capacity (mg/g) and n is a constant for the adsorption intensity. KF and n can be determined from the linear plots of log qe versus log Ce. The calculated parameters of the Langmuir and Freundlich isotherms for the adsorption of tungsten on the adsorbent are summarized in Table 3. The correlation coefficient of the Freundlich isotherm (0.855–1.000) was higher than that of the Langmuir model 0.766–0.985. In a previous study, tungsten was found to be easily adsorbed on the FA surfaces when 1/n was in the range of 0.1–0.5, but was not easily adsorbed on the FA surfaces when 1/n > 2 [33]. In this study, tungsten was easily adsorbed onto the FA surfaces when 1/n was in the range 0.12–0.49. The qmax value for FA48 (Langmuir constant = 62.34 mg/g) was greater than that of FA (11.25 mg/g), FA60 (43.45 mg/g), and FA72 (45.23 mg/g), which indicates that FA48 exhibits good adsorption capacity for tungsten.
䖃: FA, 䕕: A-FA48, 䕧䠖A-FA60 ,䕰 : A-FA72
222 223 224 225 226 227 228 229 230 231 232 233 234 235
236
Fig. 5 shows the effect of contact time on the adsorption of tungsten on the surface of the FA samples. The equilibrium adsorption concentrations were reached within 20 h. The amount of tungsten adsorbed on FA48 was greater than that adsorbed on FA or FA72. Thus, FA48 can be produced from waste FA at low cost, and is prospectively useful for adsorption of tungsten. The kinetics of tungsten adsorption was analyzed using Lagergen’s pseudo-first-order and Ho’s pseudo-second-order
237
Samples
Freundlich constants logKF 1/n r
Langmuir constants qmax (mg/g) KL(L/mg)
r
FA FA48 FA60 FA72
0.73 1.25 1.32 1.3
11.25 62.34 43.45 45.23
0.8 0.766 0.985 0.984
14
pHInitial
221
Effect of contact time on the adsorption of tungsten on FAs
Table 3 Freundlich and Langmuir constants for adsorption of tungsten on FAs at pH 2.0–3.0.
0 0
50
FA
40
0.12 0.49 0.36 0.43
0.876 0.855 1 0.999
10.45 3.04 1 1.17
Fig. 3. Determination of the pHpzc of the FAs using the pH drift method.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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Am mount ad dsorbed ((mg/g)
50 40 30 20 10 0
0
500
1000 1500 2000 2500 3000 Elapsed time (min)
䖃: FA, 䕕: FA48, 䕧䠖FA60 ,䕰 : FA72
Fig. 5. Effect of contact time on the adsorption of tungsten on FAs.
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models, which are extensively used to describe the kinetics of adsorption of solutes from a liquid solution [34]. The pseudo-first order equation is as follows: dqt ¼ k1 ðqe qt Þ dt
249 248 250 251 252 253
(6)
where qt (mg/g) and qe (mg/g) are the amounts of tungsten ions adsorbed at time t and at equilibrium, respectively. k1 (1/hr) is the rate constant of the pseudo-first order adsorption model. Integrating Eq. (6) with the boundary conditions t = 0 to t = t and qt = 0 and qt = qt gives the linearized form: lnðqe qt Þ ¼ lnqe k1 t
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If the adsorption kinetics follows a pseudo-first order model, the plot of ln (qe qt) versus t should be linear. On the other hand, the pseudo-second order model can be represented in the following form: dqt ¼ k2 ðqe qt Þ2 dt
260 259 261 262
(8)
where k2 is the rate constant of the pseudo-second order model (g/ mg hr). After integrating Eq. (8) for boundary conditions qt = 0 at t = 0 and qt = qt at t = t, the following equation can be obtained: t 1 t ¼ þ qt k2 q2e qe
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Thus, the constants qe and k2 can be determined experimentally from the slope and intercept of the plot of t/ qt versus t. The correlation coefficients and rate constants, k1 and k2, evaluated according to these two models are listed in Table 4. The experimentally determined values (qe, exp) and those calculated using the pseudo-second-order model (qe, cal) were, within the range of error, the same. From the correlation coefficients, the pseudo-second-order kinetic model (r = 0.982–0.998) more adequately described the adsorption of tungsten on the surface of the FA samples than the pseudo-first-order model.
Table 4 Fitting results of kinetic data using pseudo-first and pseudo-second order model. Samples
FA FA48 FA60 FA72
qe,exp
Conclusions
274
Zeolites were produced by hydrothermal treatment of fly ash (FA) for 48, 60, and 72 h (referred to as FA48, FA60, and FA72) in alkaline solution. The amount of acid consumed by FA (0.54 mmol/g) was greater than that consumed by FA48, FA60, and FA72 (0.34–0.44 mmol/g) in analysis of the content of basic sites, whereas the amount of based consumed by FA48, FA60, and FA72 (3.22–3.40 mmol/g) was greater than that consumed by parent FA (0.28 mmol/g). Treatment for 48 h yielded the zeolite with the largest specific surface area and pore volume (FA48; 45.3 m2/g and 12.7 mL/g). The amount of tungsten adsorbed on parent FA was smaller than that adsorbed on FA48, FA60, and FA72. Moreover, the amount adsorbed at pH 2.0–3.0 was greater than that at pH 6.1–6.5 or pH 11.0–12.0. Based on Langmuir and Freundlich modeling of the equilibrium adsorption data, the correlation coefficient of the Freundlich isotherm (0.855–1.000) was higher than that of the Langmuir model 0.766–0.985. Equilibrium adsorption of tungsten on FA samples was achieved within 20 h. The pseudo-second-order model more accurately described the experimental data than the pseudo-first-order model. The results of this study show that the FA-based zeolites generated from waste FA are good adsorbents for removal of tungsten from aqueous solutions, and this protocol is a prospective means of addressing the dual objective of tungsten removal and utilization of FA waste.
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References
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276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299
(7)
254 255
5
qe, cal (mg/g)
r
PSOM k2*1000 (g/mg h)
qe ,cal (mg/g)
r
(mg/g)
PFOM k1*1000 (1/h)
2.24 44.9 41.5 45.2
0.1 6 3.1 3.1
0.3 4.7 29 14
0.1 0.4 1 0.8
4.1 2.3 0.2 0.7
2.6 45 42 46
1 1 1 1
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1 2 3
Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution
4 Q1
Fumihiko Ogata a , Yuka Iwata a , Naohito Kawasaki a,b, *
5 6
a b
Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Antiaging Center, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 September 2014 Accepted 5 November 2014
Zeolites were produced by hydrothermal treatment of fly ash for 48, 60, and 72 h (referred to as FA48, FA60, and FA72) in alkaline solution. Physical and chemical analyses were performed on the FA samples. Moreover, the amount of tungsten adsorbed on the FA samples, derived from the adsorption isotherms under different pH conditions, and the effect of the contact time on the adsorption were evaluated. Parent FA consisted primarily of mullite crystals, whereas the hydrothermally treated FA48, FA60, and FA72 samples consisted of phillipsite, zeolite X, and zeolite A, respectively. The specific surface areas and pore volumes of parent FA were smaller than those of FA48, FA60, and FA72. FA48 generated the largest specific surface area and pore volume. The saturated amount of tungsten adsorbed on FA48 was greater than that of the other FA samples. Tungsten adsorption was more effective (larger amount) at pH 2.0– 3.0 than at pH 6.1–6.5 or pH 11.0–12.0. These results suggest that the tungsten was adsorbed on the surface of FA48 through interactions between the electrons of the positively charged FA48 surface and the tungsten anions in solution. Analysis of the equilibrium adsorption data using the Langmuir and Freundlich equations showed that the correlation coefficient of the Freundlich isotherm was higher than that of the Langmuir model. The data obtained in this study fit more adequately to the pseudo-secondorder model than the pseudo-first-order model. Collectively, these results suggest that FA48 is prospectively useful for the adsorption of tungsten from aqueous solutions. ã 2014 Published by Elsevier Ltd.
Keywords: Fly ash Hydrothermally treated in an alkaline solution Tungsten Adsorption
7
Introduction
8
Recently, atomic power, coal, and natural gas have gained consideration as alternatives to petroleum as energy sources. The abundance of coal deposits makes coal advantageous as an energy source relative to the other energy sources [1]. A huge amount of fly ash is generated from electric power plants, and approximately 500 million tonnes of fly ash is discharged per year worldwide. Because fly ash has pozzolanic properties after reaction with lime [2], about 20% of fly ash is used in building-material related applications. However, the remaining fly ash that is disposed in landfills still poses a growing threat to the environment due to its
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* Corresponding author at: Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. Tel.: +81 6 6730 5880x5556; fax: +81 6 6721 2505. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
fine structure and toxic elements [3]. Hence, the development of an effective method for recycling fly ash is of paramount importance. The conversion of fly ash into zeolites is one promising technique for utilization of fly ash, and alkaline hydrothermal synthesis of zeolites from fly ash has been the subject of research interest for over 30 years [4]. Classic alkaline conversion of fly ash into zeolite is based on the combination of different activation solution-to-fly ash ratios, along with variation of the temperature, pressure, and reaction time to obtain different zeolite types. Sodium or potassium hydroxide solutions of different concentrations have been combined under atmospheric and water vapor pressure at temperatures ranging from 80 to 200 C and for periods of 3–48 h in the synthesis of up to 13 different zeolites from the same fly ash sample [5–14]. Zeolites generated from fly ash have a wide range of applications in ion exchange, as molecular sieves, catalysts, and adsorbents [15–17]. As another concern, tungsten is a possible water contaminant that may be present due to mining and industrial activities such as the release of poorly treated effluents from tungsten mines or
http://dx.doi.org/10.1016/j.jece.2014.11.015 2213-3437/ ã 2014 Published by Elsevier Ltd.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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tungsten treatment plants and the smelting of tungsten ores [18]. Tungsten is a valuable resource used to produce cemented carbide tools; however, a lot of tungsten is uselessly lost during manufacturing processes in Japan. Tungsten is one of the stockpiled elements in Japan because its supply depends significantly on imports, and as such, is subject to shortages based on global supply [19]. Japan is one of the main consumers of rare metals (including tungsten); therefore, situations that may prevent importation of rare metals or result in depletion of reserves have the potential to decimate the Japanese manufacturing industry. Based on these considerations, the current objective is to develop and characterize a low-cost adsorbent for recycling tungsten [20] while simultaneously addressing the issue of reusing waste fly ash. The adsorption or recovery of tungsten from aqueous solution using adsorbents (specifically, zeolites produced from waste fly ash) has rarely been studied. If the adsorption of tungsten on zeolite produced from fly ash under different conditions could be achieved, the issues of reuse of fly ash and the adsorption of tungsten from aqueous solution could be simultaneously addressed. Thus, we present the development of an adsorbent by hydrothermal treatment of fly ash in alkaline solution under different conditions and investigate its capability to adsorb tungsten from aqueous solution.
60
Materials and methods
61
Materials
62
Fly ash (FA) was obtained from the Tachibana-wan Thermal Power Station (Shikoku Electric Power, Inc, Japan). The main components of FA are SiO2 and Al2O3, which comprise 70–80% of the total weight. The other minor components of FA are Fe2O3, CaO, MgO, Na2O, K2O, and rare earth metals [19]. Zeolites were produced by hydrothermal treatment of FA in an alkaline solution [21]. FA (0.5 g) was added to 40 mL of a 3.0 mol/L sodium hydroxide solution (Wako Pure Chemical Industries, Co., Ltd., Japan), and the mixture was subsequently heated at 93 C for 48, 60, and 72 h. The suspension was filtered through a 0.45 mm membrane filter (Advantec MFS, Inc., Japan). The residue was washed with distilled water and dried at 110 C for 24 h. The samples generated with treatment for 48, 60, and 72 h are referred to as FA48, FA60, and FA72, respectively [22].
63 64 65 66 67 68 69 70 71 72 73 74 75
76
Properties of FA products
77
The morphologies and crystallinities of the treated FA samples were studied using scanning electron microscopy (SEM; JSM-5500LV; JEOL, Japan) and X-ray diffractometry (XRD; MiniFlex II; Rigaku, Japan). The percentage yield of the treated FA samples was calculated using the weight of FA before and after calcination. The ash content was measured using the method reported in a previous study (JIS M8812). The surface functional groups (i.e., acidic and basic functional groups) were determined by using the method reported by Boehm [23,24]. The specific surface area, pore volume, and mean pore diameter were measured using a specific surface analyzer, NOVA4200e instrument (Yuasa Ionic, Japan), and nitrogen adsorption/ desorption isotherm analysis, respectively. The pH values of the solutions containing the FA samples were measured using the following method: FA (0.1 g) was added to 50 mL of distilled water (pH 7.0) and maintained at 25 C for 2 h. The suspensions were subsequently filtered using a 0.45 mm membrane filter. The solution pH was measured using a digital pH meter (Mettler, Toledo, Japan). The pHpzc of the samples was measured by the method reported by Faria and co-workers [25].
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Isotherms for adsorption of tungsten on treated FA samples at different pH
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The adsorbent (0.05 g) was added to 50 mL of 5–50 mg/L tungsten solution (Na2WO4, model solution) (pH 2.0–3.0, pH 6.1–6.5, and pH 11.1–12.0), which was adjusted by hydrochloric acid and sodium hydroxide solution (Wako Pure Chemical Industries, Co., Ltd., Japan). The suspension was shaken at 100 rpm for 48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu, Japan). The amount of tungsten adsorbed was calculated by using Eq. (1):
99
X¼
ðC o C e ÞV M
98
100 101 102 103 104 105 106 107 108
(1)
where X is the amount adsorbed (mg/g), C0 is the concentration prior to adsorption (mg/L), Ce is the concentration after adsorption (mg/L), V is the volume of the solvent, and M is the mass of the adsorbent (g).
110 109 111
Saturated amount of tungsten adsorbed on FA samples
114
The adsorbent (0.25 g) was added to 100 mL of 250 mg/L (pH 2.0) tungsten solution (Na2WO4, model solution). The suspension was shaken at 100 rpm for 48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using ICP-AES. The amount of tungsten adsorbed was calculated by using Eq. (1).
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Effect of contact time on the adsorption of tungsten on FA samples
121
The adsorbent (0.05 g) was added to 50 mL of 50 mg/L (pH 2.0) tungsten solution (Na2WO4, model solution). The suspension was shaken at 100 rpm for 0.5–48 h at 25 C. The sample was filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using ICP-AES. The amount of tungsten adsorbed was calculated by using Eq. (1).
122
Results and discussion
128
Properties of FAs
129
SEM images of the adsorbents are presented in Fig. 1. Original FA consisted of spherical particles of various diameters, whereas zeolite crystals were produced in the case of FA48, FA60, and FA72. The XRD patterns of the adsorbents (Fig. 2) show that the parent FA consisted primarily of mullite crystals (3Al2O32SiO2), while the FA48, FA60, and FA72 samples consisted of phillipsite, zeolite X, and zeolite A, respectively [26,27]. These results indicate that hydrothermal treatment of FA in alkaline solution transformed the FA into phillipsite, zeolite X, and zeolite A, based on the treatment conditions. Table 1 shows the chemical properties of the original and treated FA samples. The respective percentages ash in FA, FA48, FA60, and FA72 were 95.6, 89.7, 84.5, and 88.3%. A previous study reported that zeolite produced by hydrothermal treatment of FA in alkaline solution contained water in its pores. In this study, the water molecules in the FA48, FA60, and FA72 pores were evaporated upon heating prior to analysis; heating contributed to the decreased ash content of the FA48, FA60, and FA72 samples [28]. The amount of acid consumed by FA (0.54 mmol/g) was greater than that consumed by FA48, FA60, and FA72 (0.34–0.44 mmol/g) in analysis of the content of basic sites, whereas the amount of based consumed by FA48, FA60, and FA72 (3.22–3.40 mmol/g) was greater than that consumed by parent FA (0.28 mmol/g). These results suggest a decrease in the number of
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Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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116 117 118 119 120
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3
Fig. 1. SEM images of adsorbents.
155 156 157 158 159 160 161 162 163 164 165 166
acidic functional groups or an increase in the number of basic functional groups due to treatment of FA in alkaline solution. Table 2 shows the physical properties of the original and treated FA samples. The specific surface area and pore volume of FA (2.7 m2/g and 12.7 mL/g) were smaller than those of FA48, FA60, and FA72 (23.3–45.3 m2/g and 57.6–71.5 mL/g). On the other hand, the mean pore diameter of FA (189.8 Å) was larger than that of FA48, FA60, and FA72 (6.31–9.89 nm). In general, based on their volume, pores are divided into micropores (0.5 nm < d 2 nm), mesopores (2 nm < d 50 nm), and macropores (d > 50 nm). Moreover, increasing the specific surface area decreases the mean pore diameter [29]. Conformance to this trend was observed in this study. The pHpzc is the pH at which the net particle charge becomes zero, and is an important parameter in describing surface behavior. The pHpzc of
250000
䕔 䕰䕔 䕰 䕰 䕰 䕿 䕿 䕰 䕰
䕔 䕰䕿 䕰
200000
Intensity y (cps)
154
䕔
䕧䕧
FA72
䕧 䕧 䕔 䕔
䕧
150000
䕧 䕧䕿
100000
䕻 䕧䕔 䕧 䕧 䕧
䕧 䕧 䕔䕧 䕧 䕧䕧
䕔 䕔䕧 䕿 䕔 䕿䕧 䕧
䕿 䕔 䕧
䕧 䕧 䕧
䕧 䕔
䕧
FA60
䕧
䕔
䕻
䕔 䕔 䕔䕔
10
20
30
40
Amount of tungsten adsorbed on FA samples
176
Isotherms for adsorption of tungsten on the FA samples in different pH solutions are shown in Fig. 4. The amount of tungsten adsorbed on parent FA was smaller than that adsorbed on FA48, FA60, and FA72. Moreover, the amount adsorbed at pH 2.0–3.0 was greater than that at pH 6.1–6.5 or pH 11.0–12.0. The solution pH after adsorption was <9.40 (initial pH 2.0–3.0), which indicates that the surface of FA48 was positively charged (pHpzc of FA48 = 9.40). These results suggest that tungsten was absorbed on the surface of FA48 through interactions between the electrons of the positively charged FA48 surface and the tungsten anions in solution. In comparison, the pHpzc of FA is 7.94, but the specific surface area of FA is very small compared to that of FA48. Thus, the amount of tungsten adsorbed was related not only to chemical
177
Table 1 Chemical properties of FAs.
䕔 䕔 䕔
䕔
FA
0 0
167
FA48
䕻
50000
FA, FA48, FA60, and FA72 was 7.94, 9.40, 9.81, and 9.83, respectively (Fig. 3). The surface functional groups, specific surface area, pore volume, and pHpzc are closely related to the adsorption capability. These results suggest that hydrothermal treatment of FA in alkaline solution led to significant pore development in the FA48, FA60, and FA72 samples, resulting in novel adsorbents. Moreover, the specific surface area of FA48 was the largest of the four FA samples; thus the required treatment time, and therefore the cost, for producing FA48 is reduced compared to other treatments.
50
Samples
Yield (%)
Acidic consumptions (m mol/g)
Basic consumptions (m mol/g)
Ash (%)
pH
FA FA48 FA60 FA72
81.8 86.2 97.8
0.54 0.34 0.44 0.35
0.28 3.35 3.22 3.40
95.6 89.7 84.5 88.3
7.2 10.1 10.4 10.1
60
2θ (deg) 䕔: Mullite, 䕻: Quartz, 䕧: ZeoliteX, 䕿: Phillipsite , 䕰: Zeolite A Fig. 2. XRD patterns of adsorbents.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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Table 2 Physical properties of FAs. Samples Specific surface area (m2/g)
50 Pore volume (mL/g)
Mean pore diameter (Å)
FA FA48 FA60 FA72
190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206
2.7 45.3 23.3 36.7
0.3 3.2 1.2 1.2
6.5 55.9 37.7 38.5
5.9 12.4 25.7 20.3
12.7 71.5 57.6 60
Amouunt adsorbbed (mg/g g)
r 2 1 nm 1 < r 2 25 nm r < Total 25 nm 189.8 63.1 98.9 65.4
interaction but also to physical interaction. The saturated amount of tungsten adsorbed on FA, FA48, FA60, and FA72 was 1.2, 72.6, 72.5, and 60.6 mg/g, respectively. Correlation coefficient of relationship between the saturated amount adsorbed and specific surface area, micropores, mesopores, macropores, total pore volume, and mean pore diameter is 0.852, 0.695, 0.932, 0.756, 0.973, and 0.928, respectively (data not shown). These results indicated that the pore size of adsorbent (especially, total pore volume and mean pore diameter) was very important factor than specific surface area for adsorption of tungsten from aqueous solution. Equilibrium data, commonly known as the adsorption isotherms, are the main medium to investigate the adsorption mechanism. Two traditional adsorption isotherm models, i.e., Langmuir and Freundlich, are used to describe the equilibrium between adsorbed tungsten and the adsorbent [30,31]. The Langmuir isotherm model can be expressed as: qe ¼
K L qmax C e ð1 þ K L qmax Þ
(2)
207 208
The linear form can be expressed as: Ce 1 Ce ¼ þ qe K L qmax qmax
212 213 214 215 216 217
where Ce is the equilibrium concentration of tungsten ions (mg/L), qe is the equilibrium capacity of tungsten ions on the adsorbent (mg/L), qmax is the maximum adsorption capacity (mg/g) of the adsorbent, and KL is the Langmuir adsorption constant (L/mg). Thus, qmax and KL can be calculated from the linear plots of Ce/ qe versus Ce [32]. The Freundlich isotherm is an empirical equation. It can be used to describe heterogeneous systems. The non-linear form of the
14 12 10 pHFFinal
210 209 211
(3)
8 6 FA : 7.94 FA48 : 9.40 FA60 : 9.81 FA72 : 9.83
4 2
2
4
6
8
10
12
40
30
30
20
20
10
10
0 500
10
20
30
40
0 0 50
50
FA48
40
10
20
30
40
50
40
50
FA72
40
30
30
20
20
10
10
0
FA60
0
10
0 0 40 30 50 10 20 Equilibrium concentration (mg/L)
20
30
䖃: pH2.0-3.0, 䕕: pH6.1-6.5, 䕺: pH11.0-12.0
Fig. 4. Adsorption isotherms of tungsten on FAs at different pH solutions.
218
Freundlich model is: qe ¼ K F C ne
(4) 220 219
and the linear form is expressed as: logqe ¼ logK F þ ð1=nÞlogC e
(5)
where KF is an indicator of the adsorption capacity (mg/g) and n is a constant for the adsorption intensity. KF and n can be determined from the linear plots of log qe versus log Ce. The calculated parameters of the Langmuir and Freundlich isotherms for the adsorption of tungsten on the adsorbent are summarized in Table 3. The correlation coefficient of the Freundlich isotherm (0.855–1.000) was higher than that of the Langmuir model 0.766–0.985. In a previous study, tungsten was found to be easily adsorbed on the FA surfaces when 1/n was in the range of 0.1–0.5, but was not easily adsorbed on the FA surfaces when 1/n > 2 [33]. In this study, tungsten was easily adsorbed onto the FA surfaces when 1/n was in the range 0.12–0.49. The qmax value for FA48 (Langmuir constant = 62.34 mg/g) was greater than that of FA (11.25 mg/g), FA60 (43.45 mg/g), and FA72 (45.23 mg/g), which indicates that FA48 exhibits good adsorption capacity for tungsten.
䖃: FA, 䕕: A-FA48, 䕧䠖A-FA60 ,䕰 : A-FA72
222 223 224 225 226 227 228 229 230 231 232 233 234 235
236
Fig. 5 shows the effect of contact time on the adsorption of tungsten on the surface of the FA samples. The equilibrium adsorption concentrations were reached within 20 h. The amount of tungsten adsorbed on FA48 was greater than that adsorbed on FA or FA72. Thus, FA48 can be produced from waste FA at low cost, and is prospectively useful for adsorption of tungsten. The kinetics of tungsten adsorption was analyzed using Lagergen’s pseudo-first-order and Ho’s pseudo-second-order
237
Samples
Freundlich constants logKF 1/n r
Langmuir constants qmax (mg/g) KL(L/mg)
r
FA FA48 FA60 FA72
0.73 1.25 1.32 1.3
11.25 62.34 43.45 45.23
0.8 0.766 0.985 0.984
14
pHInitial
221
Effect of contact time on the adsorption of tungsten on FAs
Table 3 Freundlich and Langmuir constants for adsorption of tungsten on FAs at pH 2.0–3.0.
0 0
50
FA
40
0.12 0.49 0.36 0.43
0.876 0.855 1 0.999
10.45 3.04 1 1.17
Fig. 3. Determination of the pHpzc of the FAs using the pH drift method.
Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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G Model
JECE 495 1–6 F. Ogata et al. / Journal of Environmental Chemical Engineering xxx (2014) xxx–xxx
Am mount ad dsorbed ((mg/g)
50 40 30 20 10 0
0
500
1000 1500 2000 2500 3000 Elapsed time (min)
䖃: FA, 䕕: FA48, 䕧䠖FA60 ,䕰 : FA72
Fig. 5. Effect of contact time on the adsorption of tungsten on FAs.
245 246 247
models, which are extensively used to describe the kinetics of adsorption of solutes from a liquid solution [34]. The pseudo-first order equation is as follows: dqt ¼ k1 ðqe qt Þ dt
249 248 250 251 252 253
(6)
where qt (mg/g) and qe (mg/g) are the amounts of tungsten ions adsorbed at time t and at equilibrium, respectively. k1 (1/hr) is the rate constant of the pseudo-first order adsorption model. Integrating Eq. (6) with the boundary conditions t = 0 to t = t and qt = 0 and qt = qt gives the linearized form: lnðqe qt Þ ¼ lnqe k1 t
256 257 258
If the adsorption kinetics follows a pseudo-first order model, the plot of ln (qe qt) versus t should be linear. On the other hand, the pseudo-second order model can be represented in the following form: dqt ¼ k2 ðqe qt Þ2 dt
260 259 261 262
(8)
where k2 is the rate constant of the pseudo-second order model (g/ mg hr). After integrating Eq. (8) for boundary conditions qt = 0 at t = 0 and qt = qt at t = t, the following equation can be obtained: t 1 t ¼ þ qt k2 q2e qe
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Thus, the constants qe and k2 can be determined experimentally from the slope and intercept of the plot of t/ qt versus t. The correlation coefficients and rate constants, k1 and k2, evaluated according to these two models are listed in Table 4. The experimentally determined values (qe, exp) and those calculated using the pseudo-second-order model (qe, cal) were, within the range of error, the same. From the correlation coefficients, the pseudo-second-order kinetic model (r = 0.982–0.998) more adequately described the adsorption of tungsten on the surface of the FA samples than the pseudo-first-order model.
Table 4 Fitting results of kinetic data using pseudo-first and pseudo-second order model. Samples
FA FA48 FA60 FA72
qe,exp
Conclusions
274
Zeolites were produced by hydrothermal treatment of fly ash (FA) for 48, 60, and 72 h (referred to as FA48, FA60, and FA72) in alkaline solution. The amount of acid consumed by FA (0.54 mmol/g) was greater than that consumed by FA48, FA60, and FA72 (0.34–0.44 mmol/g) in analysis of the content of basic sites, whereas the amount of based consumed by FA48, FA60, and FA72 (3.22–3.40 mmol/g) was greater than that consumed by parent FA (0.28 mmol/g). Treatment for 48 h yielded the zeolite with the largest specific surface area and pore volume (FA48; 45.3 m2/g and 12.7 mL/g). The amount of tungsten adsorbed on parent FA was smaller than that adsorbed on FA48, FA60, and FA72. Moreover, the amount adsorbed at pH 2.0–3.0 was greater than that at pH 6.1–6.5 or pH 11.0–12.0. Based on Langmuir and Freundlich modeling of the equilibrium adsorption data, the correlation coefficient of the Freundlich isotherm (0.855–1.000) was higher than that of the Langmuir model 0.766–0.985. Equilibrium adsorption of tungsten on FA samples was achieved within 20 h. The pseudo-second-order model more accurately described the experimental data than the pseudo-first-order model. The results of this study show that the FA-based zeolites generated from waste FA are good adsorbents for removal of tungsten from aqueous solutions, and this protocol is a prospective means of addressing the dual objective of tungsten removal and utilization of FA waste.
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References
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276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299
(7)
254 255
5
qe, cal (mg/g)
r
PSOM k2*1000 (g/mg h)
qe ,cal (mg/g)
r
(mg/g)
PFOM k1*1000 (1/h)
2.24 44.9 41.5 45.2
0.1 6 3.1 3.1
0.3 4.7 29 14
0.1 0.4 1 0.8
4.1 2.3 0.2 0.7
2.6 45 42 46
1 1 1 1
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Please cite this article in press as: F. Ogata, et al., Properties of novel adsorbent produced by hydrothermal treatment of waste fly ash in alkaline solution and its capability for adsorption of tungsten from aqueous solution, J. Environ. Chem. Eng. (2014), http://dx.doi.org/10.1016/j. jece.2014.11.015
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