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Synthesis of novel zeolites produced from fly ash by hydrothermal treatment in alkaline solution and its evaluation as an adsorbent for heavy metal removal
Journal of Environmental Chemical Engineering 8 (2020) 103687
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Synthesis of novel zeolites produced from fly ash by hydrothermal treatment in alkaline solution and its evaluation as an adsorbent for heavy metal removal
T
Yuhei Kobayashia, Fumihiko Ogataa, Takehiro Nakamuraa, Naohito Kawasakia,b,* a b
Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Antiaging Center, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite Fly ash Alkaline hydrothermal treatment Adsorption Heavy metal
In this study, zeolites (FA6, FA12, and FA24) were prepared from JIS Type-II fly ash (FA) by alkaline hydrothermal treatment, and their physicochemical characteristics were investigated. In addition, their adsorption capability for heavy metals ions (Hg2+ and Pb2+) was evaluated. Zeolites have Hydrosodalite and/or Zeolite P structure, and the specific surface area of FA24 was greater than that of FA6 and FA12. A higher amount of heavy metals ions was adsorbed by FA24 than by FA6 and fA12. After adsorption using FA24, the binding energy of Pb2+ was detected, on the other hand, the binding energy of Hg2+ was not detected under our experimental conditions. These results suggest that FA24 surface properties shows a low adsorption capability of Hg2+. In addition, ion exchange with sodium ions in the adsorbent was also related to the adsorption capability of heavy metals ions (correlation coefficient: 0.965–0.973). A pH of approximately 5 was found to be optimal for the adsorption of heavy metals ions under our experimental conditions.
1. Introduction The water environment is facing a severe problem of heavy metals ions contamination, which strongly affects both aquatic and human health. Heavy metals ions such as Hg2+ and Pb2+ are toxic and can induce immense damage to kidney function, immune system, and central nervous system [1,2]. Therefore, the World Health Organization has restricted the concentration for Hg2+ and Pb2+ in drinking water to below 0.01 and 0.001 mg/L, respectively [1,3,4]. Moreover, low cost materials that have been investigated for their potential for application as adsorbents for removal of heavy metals ions such as Hg2+ and Pb2+ from aqueous solutions including biomass [5], fly ash (FA) [6–8] and natural materials like clay or zeolite [9,10]. Among them, adsorption techniques such as ion exchange using FA are simple and cost-effective. Coal is an important energy source in Japan because of low cost, large deposits, and stability [11,12]. A large amount of coal ash is discharged from electric power plants and other factories that use coal. About 40% of coal FA is reused as raw material for manufacturing cement and concrete, but the residue is mainly used for landfills [13,14]. Many researchers have previously reported that the conventional technique of producing zeolites from coal FA is an effective method to use coal FA [15–18]. Zeolite has a porous structure in three dimensional
⁎
crystal lattices, which exhibit good adsorption and ion exchange potential for cations, including zeolite [19]. Additionally, the synthesis of zeolite produced from FA is strongly influenced by the treatment conditions (such as the concentration of the alkaline solution, temperature, reaction time, and ratio of volume of alkaline solution and mass of ash) and the type or components (physicochemical properties) of FA [20]. Particularly, physicochemical properties of FA generated from electric power plants in terms of composition, surface chemistry and reactivity are of very important in the development of the conventional technique of producing zeolite from FA. Additionally, the evaluation of prepared zeolite characteristics is useful information for removal of heavy metals ions [21]. We previously reported the properties of novel zeolite produced from FA by alkaline thermodynamic treatment and its adsorption (recovery) capability of rare metals from aqueous solution [22,23]. However, few researchers have studied Hg2+ and Pb2+ adsorption by zeolite prepared from coal FA under different conditions [6–8]. Moreover, the adsorption mechanism of Hg2+ and Pb2+ has not been elucidated in previous studies. Therefore, if the adsorption of heavy metals ions by prepared zeolite could be explored, the value and applicability of the prepared zeolite would drastically increase. In this study, we investigated the physicochemical properties of
Corresponding author at: Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
https://doi.org/10.1016/j.jece.2020.103687 Received 30 September 2019; Received in revised form 21 December 2019; Accepted 12 January 2020 Available online 13 January 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 8 (2020) 103687
Y. Kobayashi, et al.
Fig. 1. XRD patterns and SEM images of FA, FA6, FA12, and FA24. : Mullite, : Quartz, : Hydrosodalite, : Zeolite P.
novel zeolite prepared from FA by alkaline hydrothermal treatment and its adsorption capability of heavy metals ions (Hg2+ and Pb2+) from aqueous solution. The adsorption capacity was examined via bath experiments. The effect of pH, temperature, and contact time on the adsorption was analyzed.
solutions. The suspensions were shaken at 100 rpm for 24 h at 25 °C. Subsequently, the sample was filtered through a 0.45-μm membrane filter, and the filtrate was analyzed via inductively coupled plasma optical emission spectrometry iCAP-7600 (ICP-OES, Thermo Fisher Scientific Inc., Japan). The amount of Hg2+ and Pb2+ adsorbed was calculated by Eq. (1):
2. Materials and methods
q = V (C0 – Ce)/W
2.1. Materials
where q is the amount adsorbed (mg/g), C0 is the initial concentration, Ce is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of the adsorbent (g).
The sample solutions of Hg2+ and Pb2+ were prepared using mercury standard solution (HgCl2 in 0.1 mol/L HNO3, FUJIFILM Wako Pure Chemical Co., Japan) and lead standard solution (Pb(NO3)2 in 0.1 mol/ L HNO3, FUJIFILM Wako Pure Chemical Co., Japan), respectively. FA was obtained from the Tachibana-wan Thermal Power Station (Shikoku Electric Power, Inc., Japan). Zeolites were prepared by hydrothermal treatment of FA in an alkaline solution [23]. Briefly, FA (3.0 g) was added to 240 mL of 3 mol/L sodium hydroxide solution (FUJIFILM Wako Pure Chemical Co., Japan). The suspensions were heated at 93 °C for 6, 12, and 24 h. Subsequently, the suspensions were filtered through a 0.45 μm membrane filter (Advantec MFS, Inc., Japan). The residue was washed with distilled water and dried at 50 °C for 24 h (each sample was denoted as FA6, FA12, and FA24). All reagents (special reagent grade) were purchased from FUJIFILM Wako Pure Chemical Co., Japan.
(1)
2.4. Effect of contact time, pH, and temperature on the adsorption of Hg2+ and Pb2+ First, to evaluate the contact time, FA24 (0.01 g) was added to 50 mL of 10 mg/L Hg2+ and Pb2+ solution. The suspension was shaken for 0.5, 1, 3, 6, 9, 12, 15, 18, 24, 30, 36, 42, and 48 h at 100 rpm at 25 °C. Second, to evaluate pH, a number of samples were prepared by adding 0.01 g of FA24 to 50 mL of Hg2+ and Pb2+ solution that had concentrations ranging from 10, 30, and 50 mg/L. The suspension pH was adjusted using hydrochloric acid or sodium hydroxide solution to maintain the pH in a range of 2.0–5.0 or 2.0–7.0 for Hg2+ and Pb2+ solution, and then, the sample solutions were shaken at 100 rpm for 24 h at 25 °C. Finally, to evaluate the temperature, FA24 (0.01 g) was added to 50 mL of Hg2+ and Pb2+ solution that had concentrations ranging from 10 to 50 mg/L. The suspension was shaken at 100 rpm for 24 h at 7, 25, and 45 °C. The amount of Hg2+ and Pb2+ adsorbed in each case was calculated using Eq. (1).
2.2. Physicochemical properties Morphology and X-ray diffraction (XRD) analysis were performed using the SU1510 (SEM, Hitachi Ltd., Japan, measurement condition is 15 eV) and MiniFlex II (XRD, Rigaku, Japan), respectively. Fourier transform infrared (FT-IR) spectra of the adsorbents were measured using a FT-IR-460Plus (JASCO, Co., Japan). The cation exchange capacity (CEC) was determined by the Japanese Industrial Standard method (JIS K 1478: 2009). In addition, the specific surface area, pore volume, and mean pore diameter were measured with a specific surface analyzer, NOVA4200e instrument (Yuasa Ionic, Japan). Electron spectroscopy was performed using an X-ray photoelectron spectroscopy system, AXIS-NOVA (Shimadzu Co., Ltd., Japan). The pH of the solution was measured with a digital pH meter (HORIBA, Ltd, Japan).
3. Results and discussion 3.1. Physicochemical properties of FAs The XRD patterns and SEM images of FA, FA6, FA12, and FA24 are shown in Fig. 1. FA comprised spherical particles of various diameters and mainly mullite (3Al2O3 ∙ 2SiO2) and quartz (SiO2). Zeolite crystals were produced on the FA surface after alkaline hydrothermal treatment. The structures of hydrosodalite (Na4(AlSiO4)3OH) and zeolite P (Na6(AlO2)6(SiO2)10 ∙ 15H2O) for FA6, FA12, and FA24 were newly generated, indicating that FA was converted into zeolite under these experimental conditions. Previous studies have reported that the alkaline hydrothermal reaction for synthesizing zeolite has three steps. The dissolution step of Si and Al from FA; the condensation step of an
2.3. Adsorption capability of Hg2+ and Pb2+ Adsorbent (0.01 g) was added to 50 mL of 10 mg/L Hg2+ and Pb2+ 2
Journal of Environmental Chemical Engineering 8 (2020) 103687
Y. Kobayashi, et al.
Fig. 2. FT-IR spectra of FA, FA6, FA12, and FA24.
zeolite from FA were different in the previous experiment) [7]. These results suggest that the alkaline hydrothermal treatment in this study was useful for producing zeolites to remove heavy metals ions from aqueous solutions.
amorphous aluminosilicate intermediate (gel-like substance), in which the amorphous aluminosilicate intermediate is formed by the reaction of Si and Al with FA on the solid-liquid interface; and the crystallization step of aluminosilicate gel to form zeolite crystals [24]. FT-IR spectra of FA, FA6, FA12, and FA24 are shown in Fig. 2. No characteristic peaks were found in FA, whereas eOH stretching vibration (3527–3541 cm−1), HeOeH bending (1653–1654 cm−1), SieOeSi or AleOeSi asymmetric stretching (977–983 cm−1), and AleOeAl stretching vibration (566 and 457–458 cm−1) were found in FA6, FA12, and FA24. In alkaline hydrothermal reaction, zeolite was produced from Si and Al released from the FA skeleton. Therefore, we can confirm the Si(Al)eOeSi(Al) asymmetric stretching in FA6, FA12, and FA24. Additionally, the bands of HeOeH bending indicate that water molecules existed in the zeolite surface and/or the cavities in zeolite [7,25,26]. Physicochemical properties of the adsorbents are summarized in Table 1 and Fig. S1. Firstly, the CEC value of FA was approximately 30.0 cmol/kg. The value of CEC of FA6, FA12, and FA24 was 4–6 times higher than that of FA, indicating that alkaline hydrothermal treatment affects the CEC value. The CEC value is an index for the adsorption capability of zeolite; therefore, FA24 showed high potential for removal of heavy metals ions from aqueous solutions. In addition, the specific surface area, pore volume, and mean pore diameter were in the order FA < FA6 < FA12 < FA24. The specific surface area, pore volume, and mean pore diameter of FA24 was approximately 30 times, 98 times, and 2.5 times higher than those of FA. In addition, the specific surface area of FA24 (43.1 m2/g) in this study was greater than that of zeolite synthesized from FA previously (12.7 m2/g, the synthesis conditions of
3.2. Amount of Hg2+ and Pb2+ adsorbed by FAs The amounts of Hg2+ and Pb2+ adsorbed onto FA, FA6, FA12, and FA24 are shown in Fig. S2. The amount of Hg2+ and Pb2+ adsorbed was in the order FA < FA6 < FA12 < FA24, and the amount of Pb2+ adsorbed onto FA24 (25.5 mg/g) was higher than that of Hg2+ adsorbed onto FA24 (7.5 mg/g). The ion radius of Pb2+ (1.02 × 10−10 m) is similar to that of Hg2+ (1.19 × 10−10 m). However, the adsorption capability using FA24 was quite different under our experimental condition. These results suggest that the solvation of hydroxyl ions (solvent effect) affect the interaction between Pb2+ or Hg2+and FA24 structure. Moreover, the physicochemical properties of the adsorbent strongly affect the adsorption capability. Therefore, we evaluated the relationship between the amount of Hg2+ or Pb2+ adsorbed and the physicochemical properties of FAs (Table 2). The amount of Hg2+ and Pb2+ adsorbed was related to the CEC value (correlation coefficient: 0.901–0.992), specific surface area (correlation coefficient: 0.949–0.999), pore volume (correlation coefficient: 0.967–0.998), and mean pore diameter (correlation coefficient: 0.884–0.973), respectively. Additionally, FA24, which exhibited the highest adsorption capacity for Hg2+ and Pb2+ in aqueous solutions, was selected for the following experiments in this study. The determination of experiment condition (the optimizing of experiment condition) for field application
Table 1 Chemical and physical properties of FA, FA6, FA12, and FA24. Adsorbents CEC (cmol/kg)
pH5 pH10
Specific surface area (m2/g) Pore volume (μL/g) Mean pore diameter (Å)
r≦10 Å 10 < r≦250 Å Total
FA
FA6
FA12
FA24
30.0 31.0 1.4 0.1 2.0 2.2 56.7
120.0 160.0 31.2 0.3 103.5 140.3 131.2
133.0 170.0 36.4 0.7 136.3 183.4 135.5
137.0 184.0 43.1 1.1 170.0 214.6 141.0
Table 2 Correlation coefficient of relationship between amount of Hg2+ and Pb2+ adsorbed and the properties of adsorbents. Adsorbates CEC (cmol/kg) Specific surface area (m2/g) Pore volume (μL/g) Mean pore diameter (Å)
3
pH5 pH10
Hg2+
Pb2+
0.901 0.901 0.949 0.967 0.884
0.991 0.992 0.999 0.998 0.973
Journal of Environmental Chemical Engineering 8 (2020) 103687
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Fig. 3. Relationship between amount of Hg2+ and Pb2+ adsorbed onto FA24 and amount of Na+ released.
is difficult, because a lot of factors affect with each other. Therefore, fundamental studies (effect of temperature, contact time, and pH) using FA24 were demonstrated from the following section.
ln (qe – qt)/qe = - k1t 2
t/qt = 1/(k2qe ) + t/qe
Fig. S3 shows the adsorption isotherms of Hg2+ and Pb2+. The amount adsorbed onto FA24 was higher than that adsorbed onto FA. The amounts of Hg2+ and Pb2+ adsorbed onto FA24 were 22.4 and 30.7 mg/g, respectively (initial concentration: 50 mg/L). Previous studies have reported that the amount of Hg2+ adsorbed onto zeolite is approximately 0.06 mg/g [6] and that of Pb2+ adsorbed onto natural zeolite is 13.02–23.03 mg/g [27]. Therefore, the zeolite produced from FA in alkaline hydrothermal treatment (FA24) is a useful adsorbent for the removal of heavy metals ions from aqueous solution. Next, we evaluated the adsorption mechanism of heavy metals ions onto FA24. First, the relationship between the amount of Hg2+ or Pb2+ adsorbed and the amount of Na+ released was investigated (Fig. 3). The correlation coefficients of the amount of Hg2+ and Pb2+ adsorbed and the amount of Na+ released were 0.973 and 0.965, respectively. This result indicates that the amount of heavy metals ions adsorbed was related to ion exchange with Na+ in FA24. As mentioned earlier (Table 2), the correlation coefficient of the relationship between the amount adsorbed and the CEC value is 0.901–0.992, which supported that ion exchange for the removal of heavy metals ions occurred in this study. In addition, previous studies reported trends similar to those observed in this study [7,28]. In previous studies, the amount of cations (Na+ in this study) released from zeolite were not quantitatively analyzed; therefore, our results provide useful information to elucidate the adsorption mechanism of heavy metals ions using FA24. Additionally, the binding energy of the FA24 surface for Hg2+ or Pb2+ before and after adsorption was measured (Fig. 4). Before adsorption, Si and Al peaks were detected, and after adsorption of Pb2+, we could observe new Pb (4f) peaks at 136.6 and 141.5 eV, which indicate that Pb2+ is adsorbed onto the FA24 surface. In contrast, Hg peaks were not detected after adsorption, which suggests that FA24 surface properties show a low adsorption capability of Hg2+ in aqueous solution. From Fig. 3, the linear slops for Hg2+ and Pb2+ adsorption is 0.12 and 0.55, respectively. Therefore, the adsorption mechanism of Pb2+ is different to that of Hg2+ in this study. 3.4. Effects of contact time on Pb
2+
and Hg
(3)
where qe and qt correspond to the amount adsorbed at equilibrium and at time t (mg/g), respectively; moreover, k1 is the pseudo-first-order rate constant (1/h) and k2 is the pseudo-second-order rate constant (g/ mg/h). The kinetics fitting parameters are summarized in Table 3. In general, the pseudo-second-order model showed a significantly higher correlation coefficient (0.962 and 0.975 for Hg2+ and Pb2+) than the pseudo-first-order model (0.303 and 0.302 for Hg2+ and Pb2+) in all cases, suggesting a chemical adsorption process for Hg2+ and Pb2+ onto FA24 [31]. The value of the calculated (qe, cal) in the pseudosecond-order model was closer than that obtained from the experiment (qe, exp). Additionally, the chi-square analysis (χ2) was conducted for the evaluation of kinetic models to avoid including error. The low value of χ2 indicates that the data from kinetic model is similar to the experimental data [32]. The χ2 values of kinetic models were also shown in Table 3. The χ2 value of the pseudo-second-order model was smaller than that of the pseudo-first-order model. These results suggested that the obtained results were considered to be a better much for the pseudo-second-order model. Collectively, chemisorption is possibly the dominant mechanism in the adsorption process of Hg2+ and Pb2+ using FA24 [33]. However, previous study reported the wide spread misuse of a second-order kinetic model [34]. Therefore, the kinetic data were applied to the following Eq. (4).
3.3. Adsorption isotherms of Hg2+ and Pb2+ onto FA and FA24
2+
(2)
1/Ct = 1/C0 + k2’t
(4)
where C0 and Ct correspond to the concentration of a reactant at time 0 and t, respectively. k2’ is the reaction rate constant. The correlation coefficient ( r2) of Hg2+ and Pb2+ is 0.195 and 0.200, respectively (Fig. S5). This result indicated that this reaction does not follow second-order kinetics under our experiment. Finally, we determined that it is not suitable for applying the pseudo-second-order equation in this case. 3.5. Effects of pH on Pb2+ and Hg2+ adsorption The effect of pH on Pb2+ and Hg2+ adsorption using FA24 is shown in Fig. S6. The amount of Pb2+ adsorbed showed similar trends to that of Hg2+ adsorbed in this study. At low pH (approximately 2.0), there was competition between H+ ions in the solution and Pb2+ or Hg2+ ions accompanied by an increase in the positive charge of the FA24 surface (the pHpzc of FA24 is 9.9) [8]. At an acidic pH (approximately 5.0), Pb2+ and Hg2+ are more mobile and can be adsorbed by clay
adsorption
Fig. S4 shows the effect of contact time on the adsorption of Pb2+ and Hg2+ onto FA24. Equilibrium adsorption was attained within 4 h. The kinetics data were fitted using the following Langergren’s pseudofirst-order Eq. (2) and Ho’s pseudo-second-order Eq. (3) [29,30] 4
Journal of Environmental Chemical Engineering 8 (2020) 103687
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Fig. 4. Binding energy of Hg2+ or Pb2+ before and after adsorption.
mineral (such as phyllosilicates) [7,8]. H+ or Na+ ions in SiOH(Na) or AlOH(Na) showed ion exchange ability with Pb2+ and Hg2+ in acidic solutions. In contrast, the predominant species of mercury in solution is Hg (OH)2 at around pH 5.0, and at a pH between 2 and 6, small amounts of Hg(OH)+ are present [6]. Additionally, the comparative dispersal of Pb2+ species in solution showed that Pb2+ mainly existed in the form of Pb(OH)+, Pb3(OH)42+, and Pb4(OH)44+ at pH > 6.2 [1,35,36]. At higher pH values, heavy metal hydroxides are the main species, and metal ions are removed via precipitation.
Table 4 Langmuir constants for Hg2+ or Pb2+ adsorption onto FA24. Samples
Hg2+
Pb2+
3.6. Effects of temperature on Pb2+ and Hg2+ adsorption
7℃ 25 ℃ 45 ℃ 7℃ 25 ℃ 45 ℃
Ws (mg/g)
a (L/mg)
r
χ2
30.8 25.5 23.1 31.7 38.0 40.5
0.07 0.24 0.35 0.17 0.11 0.16
0.979 0.972 0.861 0.908 0.989 0.974
0.607 0.081 18.553 0.707 0.152 15.078
Pb2+ and Hg2+ onto FA24. The correlation coefficients of the Langmuir equation were 0.861–0.989. Thus, the adsorption of heavy metals ions onto FA24 surface is attributed to monolayer adsorption. Additionally, the value of Ws for Pb2+ adsorption increased with increasing temperatures. However, the value of Ws for Hg2+ adsorption slightly increased with increasing temperature, which suggests that the adsorption capacity is not clearly different between adsorption temperatures 7–45 °C. Moreover, the chi-square analysis (χ2) was also conducted for the evaluation of isotherm models (Table 4). The χ2 value of the Langmuir model at 7 and 25 °C was smaller compared to 45 °C, which indicating that the adsorption isotherms data of Pb2+ and Hg2+ onto FA24 are difficult to much fit the Langmuir model at high adsorption temperature.
Adsorption isotherms of Pb2+ and Hg2+ onto FA24 at different temperatures in a single solution system are shown in Fig. S7. The amount of Hg2+ adsorbed slightly increased and that of Pb2+ adsorbed clearly increased with increasing temperatures. These results indicate that the adsorption process of heavy metals ions is endothermic. Additionally, the discussion about the binding energy of Hg2+ was described in previous section. The peak of Hg2+ was not detected after adsorption, which suggests that a part of Hg2+ was taken into the FA24. From the Fig. S6, the adsorption temperature weakly affected the adsorption capacity of Hg2+ compared to Pb2+. The decrease or little increase in adsorption with increasing temperature, suggest weak adsorption interaction between adsorbent surface and the metal ion, which supports physisorption. Therefore, the physisorption might be related to the adsorption of Hg2+ under our experimental condition. To understand the adsorption mechanism and the relationship between adsorbate and adsorbent in the liquid medium at equilibrium, the Langmuir (5) equations was used [37]. Ce/qe = 1/Wsa + Ce/Ws
Langmuir constants
4. Conclusions We prepared a novel adsorbent (FA24) produced from FA for the removal of Hg2+ and Pb2+ from aqueous solution. The physicochemical properties of FA24 were drastically changed after alkaline hydrothermal treatment. The adsorption capability of Hg2+ and Pb2+ onto FA24 was higher than that onto FA and was related to the CEC (r = 0.901–0.991), specific surface area (r = 0.949–0.999), and pore volumes (r = 0.967–0.998). In addition, ion exchange with sodium ions in FA24 was an important factor for removal of heavy metals ions. Hg2+ and Pb2+ were optimally adsorbed under acidic conditions (pH of 5),
(5)
where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), and Ws and a are the Langmuir constants related to the monolayer adsorption capacity and energy of adsorption, respectively [38]. Table 4 summarizes the Langmuir constants for the adsorption of Table 3 Fitting results of kinetic data using PFOM and PSOM in single solution system. Samples
Hg2+ Pb2+
qe,
exp
5.6 18.1
(mg/g)
PFOM
PSOM
k1 (1/hr)
qe,
0.011 0.019
2.1 5.4
cal
(mg/g)
r
χ2
k2 (g/(mg hr))
qe,
0.303 0.302
8.83 × 102 2.73 × 103
0.244 0.067
4.1 15.0
5
cal
(mg/g)
r
χ2
0.962 0.975
0.991 4.112
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and FA24 could rapidly adsorb Hg2+ and Pb2+ (the adsorption equilibrium was achieved within 4 h) under these experimental conditions. Moreover, the amounts of adsorbed Hg2+ and Pb2+ by FA24 increased with adsorption temperature, indicating an endothermic adsorption process. Collectivily, it is suggested that FA24 prospectively useful adsorbent for removal of heavy metals ions from aqueous solutions.
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CRediT authorship contribution statement Yuhei Kobayashi: Investigation, Visualization, Writing - original draft. Fumihiko Ogata: Writing - review & editing, Project administration. Takehiro Nakamura: Investigation. Naohito Kawasaki: Conceptualization, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2020.103687. References [1] World Health Organization Geneva, WHO Environmental Health Criteria 1 Mercury, WHO, 1976, pp. 1–131. [2] World Health Organization Geneva, WHO Environmental Health Criteria 3 Lead, WHO, 1977, pp. 1–79. [3] D.M. Nanicuacua, M.G. Segatelli, M.Z. Corazza, C.R. Teixeira Tarley, Assessment of organosilane-functionalized nano-carbon black for interference-free on-line Pb(ii) ion enrichment in water, herbal medicines and environmental samples, Anal. Methods 8 (2016) 2820–2830. [4] M. Liu, B. Zhang, H. Wang, F. Zhao, Y. Chen, Q. Sun, Facile crosslinking synthesis of hyperbranch-substrate nanonetwork magnetite nanocomposite for the fast and highly efficient removal of lead ions and anionic dyes from aqueous solutions, RSC Adv. 6 (2016) 67057–67071. [5] R. Bun-ei, N. Kawasaki, F. Ogata, T. Nakamura, K. Aochi, S. Tanada, Removal of lead and iron ions by vegetable biomass in drinking water, J. Oleo Sci. 55 (2006) 423–427. [6] V. Somerset, L. Petrik, E. Iwuoha, Alkaline hydrothermal conversation of fly ash precipitates into zeolites 3: the removal of mercury and lead ions from wastewater, J. Environ. Manage. 87 (2008) 125–131. [7] K. He, Y. Chen, Z. Tang, Y. Hu, Removal of heavy metal ions from aqueous solution by zeolite synthesized from fly ash, Envrion. Sci. Pollut. Res. 23 (2016) 2778–2788. [8] M. Mhamdi, N.H. Galai, M.E. Elaloui, M. Trabelsi-Ayadi, Adsorption of lead onto smectite from aqueous solution, Environ. Sci. Pollut. Res. 20 (2013) 1686–1697. [9] N.V. Medvidovic, J.P.M. Trgo, Column performance in lead removal from aqueous solutions by fized bed of natural zeolite-clinoptilolite, Sep. Purif. Technol. 49 (2006) 237–244. [10] A. Cincotti, N. Lai, R. Orru, G. Cao, Sardinian natural clinoptilolites for heavy metals and ammonium removal: experimental and modeling, Chem. Eng. J. 84 (2001) 275–282. [11] Agency for Natural Resources and Energy, https://www.enecho.meti.go.jp/about/ special/johoteikyo/sekitanyakuwari.html, 2018 (Accessed 5 December 2019). [12] U.S. Energy Information Administration, International Energy Outlook 2010, (2010).
6
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Synthesis of novel zeolites produced from fly ash by hydrothermal treatment in alkaline solution and its evaluation as an adsorbent for heavy metal removal
T
Yuhei Kobayashia, Fumihiko Ogataa, Takehiro Nakamuraa, Naohito Kawasakia,b,* a b
Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Antiaging Center, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite Fly ash Alkaline hydrothermal treatment Adsorption Heavy metal
In this study, zeolites (FA6, FA12, and FA24) were prepared from JIS Type-II fly ash (FA) by alkaline hydrothermal treatment, and their physicochemical characteristics were investigated. In addition, their adsorption capability for heavy metals ions (Hg2+ and Pb2+) was evaluated. Zeolites have Hydrosodalite and/or Zeolite P structure, and the specific surface area of FA24 was greater than that of FA6 and FA12. A higher amount of heavy metals ions was adsorbed by FA24 than by FA6 and fA12. After adsorption using FA24, the binding energy of Pb2+ was detected, on the other hand, the binding energy of Hg2+ was not detected under our experimental conditions. These results suggest that FA24 surface properties shows a low adsorption capability of Hg2+. In addition, ion exchange with sodium ions in the adsorbent was also related to the adsorption capability of heavy metals ions (correlation coefficient: 0.965–0.973). A pH of approximately 5 was found to be optimal for the adsorption of heavy metals ions under our experimental conditions.
1. Introduction The water environment is facing a severe problem of heavy metals ions contamination, which strongly affects both aquatic and human health. Heavy metals ions such as Hg2+ and Pb2+ are toxic and can induce immense damage to kidney function, immune system, and central nervous system [1,2]. Therefore, the World Health Organization has restricted the concentration for Hg2+ and Pb2+ in drinking water to below 0.01 and 0.001 mg/L, respectively [1,3,4]. Moreover, low cost materials that have been investigated for their potential for application as adsorbents for removal of heavy metals ions such as Hg2+ and Pb2+ from aqueous solutions including biomass [5], fly ash (FA) [6–8] and natural materials like clay or zeolite [9,10]. Among them, adsorption techniques such as ion exchange using FA are simple and cost-effective. Coal is an important energy source in Japan because of low cost, large deposits, and stability [11,12]. A large amount of coal ash is discharged from electric power plants and other factories that use coal. About 40% of coal FA is reused as raw material for manufacturing cement and concrete, but the residue is mainly used for landfills [13,14]. Many researchers have previously reported that the conventional technique of producing zeolites from coal FA is an effective method to use coal FA [15–18]. Zeolite has a porous structure in three dimensional
⁎
crystal lattices, which exhibit good adsorption and ion exchange potential for cations, including zeolite [19]. Additionally, the synthesis of zeolite produced from FA is strongly influenced by the treatment conditions (such as the concentration of the alkaline solution, temperature, reaction time, and ratio of volume of alkaline solution and mass of ash) and the type or components (physicochemical properties) of FA [20]. Particularly, physicochemical properties of FA generated from electric power plants in terms of composition, surface chemistry and reactivity are of very important in the development of the conventional technique of producing zeolite from FA. Additionally, the evaluation of prepared zeolite characteristics is useful information for removal of heavy metals ions [21]. We previously reported the properties of novel zeolite produced from FA by alkaline thermodynamic treatment and its adsorption (recovery) capability of rare metals from aqueous solution [22,23]. However, few researchers have studied Hg2+ and Pb2+ adsorption by zeolite prepared from coal FA under different conditions [6–8]. Moreover, the adsorption mechanism of Hg2+ and Pb2+ has not been elucidated in previous studies. Therefore, if the adsorption of heavy metals ions by prepared zeolite could be explored, the value and applicability of the prepared zeolite would drastically increase. In this study, we investigated the physicochemical properties of
Corresponding author at: Faculty of Pharmacy, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
https://doi.org/10.1016/j.jece.2020.103687 Received 30 September 2019; Received in revised form 21 December 2019; Accepted 12 January 2020 Available online 13 January 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 8 (2020) 103687
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Fig. 1. XRD patterns and SEM images of FA, FA6, FA12, and FA24. : Mullite, : Quartz, : Hydrosodalite, : Zeolite P.
novel zeolite prepared from FA by alkaline hydrothermal treatment and its adsorption capability of heavy metals ions (Hg2+ and Pb2+) from aqueous solution. The adsorption capacity was examined via bath experiments. The effect of pH, temperature, and contact time on the adsorption was analyzed.
solutions. The suspensions were shaken at 100 rpm for 24 h at 25 °C. Subsequently, the sample was filtered through a 0.45-μm membrane filter, and the filtrate was analyzed via inductively coupled plasma optical emission spectrometry iCAP-7600 (ICP-OES, Thermo Fisher Scientific Inc., Japan). The amount of Hg2+ and Pb2+ adsorbed was calculated by Eq. (1):
2. Materials and methods
q = V (C0 – Ce)/W
2.1. Materials
where q is the amount adsorbed (mg/g), C0 is the initial concentration, Ce is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of the adsorbent (g).
The sample solutions of Hg2+ and Pb2+ were prepared using mercury standard solution (HgCl2 in 0.1 mol/L HNO3, FUJIFILM Wako Pure Chemical Co., Japan) and lead standard solution (Pb(NO3)2 in 0.1 mol/ L HNO3, FUJIFILM Wako Pure Chemical Co., Japan), respectively. FA was obtained from the Tachibana-wan Thermal Power Station (Shikoku Electric Power, Inc., Japan). Zeolites were prepared by hydrothermal treatment of FA in an alkaline solution [23]. Briefly, FA (3.0 g) was added to 240 mL of 3 mol/L sodium hydroxide solution (FUJIFILM Wako Pure Chemical Co., Japan). The suspensions were heated at 93 °C for 6, 12, and 24 h. Subsequently, the suspensions were filtered through a 0.45 μm membrane filter (Advantec MFS, Inc., Japan). The residue was washed with distilled water and dried at 50 °C for 24 h (each sample was denoted as FA6, FA12, and FA24). All reagents (special reagent grade) were purchased from FUJIFILM Wako Pure Chemical Co., Japan.
(1)
2.4. Effect of contact time, pH, and temperature on the adsorption of Hg2+ and Pb2+ First, to evaluate the contact time, FA24 (0.01 g) was added to 50 mL of 10 mg/L Hg2+ and Pb2+ solution. The suspension was shaken for 0.5, 1, 3, 6, 9, 12, 15, 18, 24, 30, 36, 42, and 48 h at 100 rpm at 25 °C. Second, to evaluate pH, a number of samples were prepared by adding 0.01 g of FA24 to 50 mL of Hg2+ and Pb2+ solution that had concentrations ranging from 10, 30, and 50 mg/L. The suspension pH was adjusted using hydrochloric acid or sodium hydroxide solution to maintain the pH in a range of 2.0–5.0 or 2.0–7.0 for Hg2+ and Pb2+ solution, and then, the sample solutions were shaken at 100 rpm for 24 h at 25 °C. Finally, to evaluate the temperature, FA24 (0.01 g) was added to 50 mL of Hg2+ and Pb2+ solution that had concentrations ranging from 10 to 50 mg/L. The suspension was shaken at 100 rpm for 24 h at 7, 25, and 45 °C. The amount of Hg2+ and Pb2+ adsorbed in each case was calculated using Eq. (1).
2.2. Physicochemical properties Morphology and X-ray diffraction (XRD) analysis were performed using the SU1510 (SEM, Hitachi Ltd., Japan, measurement condition is 15 eV) and MiniFlex II (XRD, Rigaku, Japan), respectively. Fourier transform infrared (FT-IR) spectra of the adsorbents were measured using a FT-IR-460Plus (JASCO, Co., Japan). The cation exchange capacity (CEC) was determined by the Japanese Industrial Standard method (JIS K 1478: 2009). In addition, the specific surface area, pore volume, and mean pore diameter were measured with a specific surface analyzer, NOVA4200e instrument (Yuasa Ionic, Japan). Electron spectroscopy was performed using an X-ray photoelectron spectroscopy system, AXIS-NOVA (Shimadzu Co., Ltd., Japan). The pH of the solution was measured with a digital pH meter (HORIBA, Ltd, Japan).
3. Results and discussion 3.1. Physicochemical properties of FAs The XRD patterns and SEM images of FA, FA6, FA12, and FA24 are shown in Fig. 1. FA comprised spherical particles of various diameters and mainly mullite (3Al2O3 ∙ 2SiO2) and quartz (SiO2). Zeolite crystals were produced on the FA surface after alkaline hydrothermal treatment. The structures of hydrosodalite (Na4(AlSiO4)3OH) and zeolite P (Na6(AlO2)6(SiO2)10 ∙ 15H2O) for FA6, FA12, and FA24 were newly generated, indicating that FA was converted into zeolite under these experimental conditions. Previous studies have reported that the alkaline hydrothermal reaction for synthesizing zeolite has three steps. The dissolution step of Si and Al from FA; the condensation step of an
2.3. Adsorption capability of Hg2+ and Pb2+ Adsorbent (0.01 g) was added to 50 mL of 10 mg/L Hg2+ and Pb2+ 2
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Fig. 2. FT-IR spectra of FA, FA6, FA12, and FA24.
zeolite from FA were different in the previous experiment) [7]. These results suggest that the alkaline hydrothermal treatment in this study was useful for producing zeolites to remove heavy metals ions from aqueous solutions.
amorphous aluminosilicate intermediate (gel-like substance), in which the amorphous aluminosilicate intermediate is formed by the reaction of Si and Al with FA on the solid-liquid interface; and the crystallization step of aluminosilicate gel to form zeolite crystals [24]. FT-IR spectra of FA, FA6, FA12, and FA24 are shown in Fig. 2. No characteristic peaks were found in FA, whereas eOH stretching vibration (3527–3541 cm−1), HeOeH bending (1653–1654 cm−1), SieOeSi or AleOeSi asymmetric stretching (977–983 cm−1), and AleOeAl stretching vibration (566 and 457–458 cm−1) were found in FA6, FA12, and FA24. In alkaline hydrothermal reaction, zeolite was produced from Si and Al released from the FA skeleton. Therefore, we can confirm the Si(Al)eOeSi(Al) asymmetric stretching in FA6, FA12, and FA24. Additionally, the bands of HeOeH bending indicate that water molecules existed in the zeolite surface and/or the cavities in zeolite [7,25,26]. Physicochemical properties of the adsorbents are summarized in Table 1 and Fig. S1. Firstly, the CEC value of FA was approximately 30.0 cmol/kg. The value of CEC of FA6, FA12, and FA24 was 4–6 times higher than that of FA, indicating that alkaline hydrothermal treatment affects the CEC value. The CEC value is an index for the adsorption capability of zeolite; therefore, FA24 showed high potential for removal of heavy metals ions from aqueous solutions. In addition, the specific surface area, pore volume, and mean pore diameter were in the order FA < FA6 < FA12 < FA24. The specific surface area, pore volume, and mean pore diameter of FA24 was approximately 30 times, 98 times, and 2.5 times higher than those of FA. In addition, the specific surface area of FA24 (43.1 m2/g) in this study was greater than that of zeolite synthesized from FA previously (12.7 m2/g, the synthesis conditions of
3.2. Amount of Hg2+ and Pb2+ adsorbed by FAs The amounts of Hg2+ and Pb2+ adsorbed onto FA, FA6, FA12, and FA24 are shown in Fig. S2. The amount of Hg2+ and Pb2+ adsorbed was in the order FA < FA6 < FA12 < FA24, and the amount of Pb2+ adsorbed onto FA24 (25.5 mg/g) was higher than that of Hg2+ adsorbed onto FA24 (7.5 mg/g). The ion radius of Pb2+ (1.02 × 10−10 m) is similar to that of Hg2+ (1.19 × 10−10 m). However, the adsorption capability using FA24 was quite different under our experimental condition. These results suggest that the solvation of hydroxyl ions (solvent effect) affect the interaction between Pb2+ or Hg2+and FA24 structure. Moreover, the physicochemical properties of the adsorbent strongly affect the adsorption capability. Therefore, we evaluated the relationship between the amount of Hg2+ or Pb2+ adsorbed and the physicochemical properties of FAs (Table 2). The amount of Hg2+ and Pb2+ adsorbed was related to the CEC value (correlation coefficient: 0.901–0.992), specific surface area (correlation coefficient: 0.949–0.999), pore volume (correlation coefficient: 0.967–0.998), and mean pore diameter (correlation coefficient: 0.884–0.973), respectively. Additionally, FA24, which exhibited the highest adsorption capacity for Hg2+ and Pb2+ in aqueous solutions, was selected for the following experiments in this study. The determination of experiment condition (the optimizing of experiment condition) for field application
Table 1 Chemical and physical properties of FA, FA6, FA12, and FA24. Adsorbents CEC (cmol/kg)
pH5 pH10
Specific surface area (m2/g) Pore volume (μL/g) Mean pore diameter (Å)
r≦10 Å 10 < r≦250 Å Total
FA
FA6
FA12
FA24
30.0 31.0 1.4 0.1 2.0 2.2 56.7
120.0 160.0 31.2 0.3 103.5 140.3 131.2
133.0 170.0 36.4 0.7 136.3 183.4 135.5
137.0 184.0 43.1 1.1 170.0 214.6 141.0
Table 2 Correlation coefficient of relationship between amount of Hg2+ and Pb2+ adsorbed and the properties of adsorbents. Adsorbates CEC (cmol/kg) Specific surface area (m2/g) Pore volume (μL/g) Mean pore diameter (Å)
3
pH5 pH10
Hg2+
Pb2+
0.901 0.901 0.949 0.967 0.884
0.991 0.992 0.999 0.998 0.973
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Fig. 3. Relationship between amount of Hg2+ and Pb2+ adsorbed onto FA24 and amount of Na+ released.
is difficult, because a lot of factors affect with each other. Therefore, fundamental studies (effect of temperature, contact time, and pH) using FA24 were demonstrated from the following section.
ln (qe – qt)/qe = - k1t 2
t/qt = 1/(k2qe ) + t/qe
Fig. S3 shows the adsorption isotherms of Hg2+ and Pb2+. The amount adsorbed onto FA24 was higher than that adsorbed onto FA. The amounts of Hg2+ and Pb2+ adsorbed onto FA24 were 22.4 and 30.7 mg/g, respectively (initial concentration: 50 mg/L). Previous studies have reported that the amount of Hg2+ adsorbed onto zeolite is approximately 0.06 mg/g [6] and that of Pb2+ adsorbed onto natural zeolite is 13.02–23.03 mg/g [27]. Therefore, the zeolite produced from FA in alkaline hydrothermal treatment (FA24) is a useful adsorbent for the removal of heavy metals ions from aqueous solution. Next, we evaluated the adsorption mechanism of heavy metals ions onto FA24. First, the relationship between the amount of Hg2+ or Pb2+ adsorbed and the amount of Na+ released was investigated (Fig. 3). The correlation coefficients of the amount of Hg2+ and Pb2+ adsorbed and the amount of Na+ released were 0.973 and 0.965, respectively. This result indicates that the amount of heavy metals ions adsorbed was related to ion exchange with Na+ in FA24. As mentioned earlier (Table 2), the correlation coefficient of the relationship between the amount adsorbed and the CEC value is 0.901–0.992, which supported that ion exchange for the removal of heavy metals ions occurred in this study. In addition, previous studies reported trends similar to those observed in this study [7,28]. In previous studies, the amount of cations (Na+ in this study) released from zeolite were not quantitatively analyzed; therefore, our results provide useful information to elucidate the adsorption mechanism of heavy metals ions using FA24. Additionally, the binding energy of the FA24 surface for Hg2+ or Pb2+ before and after adsorption was measured (Fig. 4). Before adsorption, Si and Al peaks were detected, and after adsorption of Pb2+, we could observe new Pb (4f) peaks at 136.6 and 141.5 eV, which indicate that Pb2+ is adsorbed onto the FA24 surface. In contrast, Hg peaks were not detected after adsorption, which suggests that FA24 surface properties show a low adsorption capability of Hg2+ in aqueous solution. From Fig. 3, the linear slops for Hg2+ and Pb2+ adsorption is 0.12 and 0.55, respectively. Therefore, the adsorption mechanism of Pb2+ is different to that of Hg2+ in this study. 3.4. Effects of contact time on Pb
2+
and Hg
(3)
where qe and qt correspond to the amount adsorbed at equilibrium and at time t (mg/g), respectively; moreover, k1 is the pseudo-first-order rate constant (1/h) and k2 is the pseudo-second-order rate constant (g/ mg/h). The kinetics fitting parameters are summarized in Table 3. In general, the pseudo-second-order model showed a significantly higher correlation coefficient (0.962 and 0.975 for Hg2+ and Pb2+) than the pseudo-first-order model (0.303 and 0.302 for Hg2+ and Pb2+) in all cases, suggesting a chemical adsorption process for Hg2+ and Pb2+ onto FA24 [31]. The value of the calculated (qe, cal) in the pseudosecond-order model was closer than that obtained from the experiment (qe, exp). Additionally, the chi-square analysis (χ2) was conducted for the evaluation of kinetic models to avoid including error. The low value of χ2 indicates that the data from kinetic model is similar to the experimental data [32]. The χ2 values of kinetic models were also shown in Table 3. The χ2 value of the pseudo-second-order model was smaller than that of the pseudo-first-order model. These results suggested that the obtained results were considered to be a better much for the pseudo-second-order model. Collectively, chemisorption is possibly the dominant mechanism in the adsorption process of Hg2+ and Pb2+ using FA24 [33]. However, previous study reported the wide spread misuse of a second-order kinetic model [34]. Therefore, the kinetic data were applied to the following Eq. (4).
3.3. Adsorption isotherms of Hg2+ and Pb2+ onto FA and FA24
2+
(2)
1/Ct = 1/C0 + k2’t
(4)
where C0 and Ct correspond to the concentration of a reactant at time 0 and t, respectively. k2’ is the reaction rate constant. The correlation coefficient ( r2) of Hg2+ and Pb2+ is 0.195 and 0.200, respectively (Fig. S5). This result indicated that this reaction does not follow second-order kinetics under our experiment. Finally, we determined that it is not suitable for applying the pseudo-second-order equation in this case. 3.5. Effects of pH on Pb2+ and Hg2+ adsorption The effect of pH on Pb2+ and Hg2+ adsorption using FA24 is shown in Fig. S6. The amount of Pb2+ adsorbed showed similar trends to that of Hg2+ adsorbed in this study. At low pH (approximately 2.0), there was competition between H+ ions in the solution and Pb2+ or Hg2+ ions accompanied by an increase in the positive charge of the FA24 surface (the pHpzc of FA24 is 9.9) [8]. At an acidic pH (approximately 5.0), Pb2+ and Hg2+ are more mobile and can be adsorbed by clay
adsorption
Fig. S4 shows the effect of contact time on the adsorption of Pb2+ and Hg2+ onto FA24. Equilibrium adsorption was attained within 4 h. The kinetics data were fitted using the following Langergren’s pseudofirst-order Eq. (2) and Ho’s pseudo-second-order Eq. (3) [29,30] 4
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Fig. 4. Binding energy of Hg2+ or Pb2+ before and after adsorption.
mineral (such as phyllosilicates) [7,8]. H+ or Na+ ions in SiOH(Na) or AlOH(Na) showed ion exchange ability with Pb2+ and Hg2+ in acidic solutions. In contrast, the predominant species of mercury in solution is Hg (OH)2 at around pH 5.0, and at a pH between 2 and 6, small amounts of Hg(OH)+ are present [6]. Additionally, the comparative dispersal of Pb2+ species in solution showed that Pb2+ mainly existed in the form of Pb(OH)+, Pb3(OH)42+, and Pb4(OH)44+ at pH > 6.2 [1,35,36]. At higher pH values, heavy metal hydroxides are the main species, and metal ions are removed via precipitation.
Table 4 Langmuir constants for Hg2+ or Pb2+ adsorption onto FA24. Samples
Hg2+
Pb2+
3.6. Effects of temperature on Pb2+ and Hg2+ adsorption
7℃ 25 ℃ 45 ℃ 7℃ 25 ℃ 45 ℃
Ws (mg/g)
a (L/mg)
r
χ2
30.8 25.5 23.1 31.7 38.0 40.5
0.07 0.24 0.35 0.17 0.11 0.16
0.979 0.972 0.861 0.908 0.989 0.974
0.607 0.081 18.553 0.707 0.152 15.078
Pb2+ and Hg2+ onto FA24. The correlation coefficients of the Langmuir equation were 0.861–0.989. Thus, the adsorption of heavy metals ions onto FA24 surface is attributed to monolayer adsorption. Additionally, the value of Ws for Pb2+ adsorption increased with increasing temperatures. However, the value of Ws for Hg2+ adsorption slightly increased with increasing temperature, which suggests that the adsorption capacity is not clearly different between adsorption temperatures 7–45 °C. Moreover, the chi-square analysis (χ2) was also conducted for the evaluation of isotherm models (Table 4). The χ2 value of the Langmuir model at 7 and 25 °C was smaller compared to 45 °C, which indicating that the adsorption isotherms data of Pb2+ and Hg2+ onto FA24 are difficult to much fit the Langmuir model at high adsorption temperature.
Adsorption isotherms of Pb2+ and Hg2+ onto FA24 at different temperatures in a single solution system are shown in Fig. S7. The amount of Hg2+ adsorbed slightly increased and that of Pb2+ adsorbed clearly increased with increasing temperatures. These results indicate that the adsorption process of heavy metals ions is endothermic. Additionally, the discussion about the binding energy of Hg2+ was described in previous section. The peak of Hg2+ was not detected after adsorption, which suggests that a part of Hg2+ was taken into the FA24. From the Fig. S6, the adsorption temperature weakly affected the adsorption capacity of Hg2+ compared to Pb2+. The decrease or little increase in adsorption with increasing temperature, suggest weak adsorption interaction between adsorbent surface and the metal ion, which supports physisorption. Therefore, the physisorption might be related to the adsorption of Hg2+ under our experimental condition. To understand the adsorption mechanism and the relationship between adsorbate and adsorbent in the liquid medium at equilibrium, the Langmuir (5) equations was used [37]. Ce/qe = 1/Wsa + Ce/Ws
Langmuir constants
4. Conclusions We prepared a novel adsorbent (FA24) produced from FA for the removal of Hg2+ and Pb2+ from aqueous solution. The physicochemical properties of FA24 were drastically changed after alkaline hydrothermal treatment. The adsorption capability of Hg2+ and Pb2+ onto FA24 was higher than that onto FA and was related to the CEC (r = 0.901–0.991), specific surface area (r = 0.949–0.999), and pore volumes (r = 0.967–0.998). In addition, ion exchange with sodium ions in FA24 was an important factor for removal of heavy metals ions. Hg2+ and Pb2+ were optimally adsorbed under acidic conditions (pH of 5),
(5)
where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), and Ws and a are the Langmuir constants related to the monolayer adsorption capacity and energy of adsorption, respectively [38]. Table 4 summarizes the Langmuir constants for the adsorption of Table 3 Fitting results of kinetic data using PFOM and PSOM in single solution system. Samples
Hg2+ Pb2+
qe,
exp
5.6 18.1
(mg/g)
PFOM
PSOM
k1 (1/hr)
qe,
0.011 0.019
2.1 5.4
cal
(mg/g)
r
χ2
k2 (g/(mg hr))
qe,
0.303 0.302
8.83 × 102 2.73 × 103
0.244 0.067
4.1 15.0
5
cal
(mg/g)
r
χ2
0.962 0.975
0.991 4.112
Journal of Environmental Chemical Engineering 8 (2020) 103687
Y. Kobayashi, et al.
and FA24 could rapidly adsorb Hg2+ and Pb2+ (the adsorption equilibrium was achieved within 4 h) under these experimental conditions. Moreover, the amounts of adsorbed Hg2+ and Pb2+ by FA24 increased with adsorption temperature, indicating an endothermic adsorption process. Collectivily, it is suggested that FA24 prospectively useful adsorbent for removal of heavy metals ions from aqueous solutions.
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