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Efficient removals of Hg and Cd in aqueous solution through NaOH-modified activated carbon fiber
Chemical Engineering Journal xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Efficient removals of Hg and Cd in aqueous solution through NaOHmodified activated carbon fiber Doo-Won Kima, Jae-Hyung Weeb, Cheol-Min Yanga, , Kap Seung Yangb,c, ⁎
⁎
a
Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea Department of Polymer Engineering, Graduate School, School of Polymer Science and Engineering & Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea c Carbon Composite Materials R&D Center, HPK Inc, 109 Banlyong-ro, Deokjin-gu, Jenju-si, Republic of Korea b
HIGHLIGHTS
GRAPHICAL ABSTRACT
activated carbon fiber • NaOH-modified removes to heavy metals, cost-effectively.
NaOH modification induced • The abundant oxygen functional groups in micropore.
metals adsorbed by a polar • Heavy oxygen functional groups and pore filling mechanism.
ARTICLE INFO
ABSTRACT
Keywords: Heavy metal removal in water Activated carbon fiber NaOH modification Adsorption mechanism
This manuscript describes an efficient and cost-effective method to remove heavy metals of Hg and Cd in aqueous solutions via adsorption on activated carbon fibers after modification with NaOH solution (NaACF). The Hg and Cd metals in the aqueous solution exist as Hg(OH)2 and Cd2+ in the experimental condition of pH 6–8. Surface characterization of the NaACF reveals uniform and narrower micropores with an increase in oxygen functional groups of phenol and lactone compared with the original ACF (pACF). The NaACF demonstrates a superior adsorption rate to both aqueous samples of heavy metal compounds. The granular activated carbon (GAC) with diverse pore structures consisting of micropores, mesopores, and macropores adsorbed the heavy metals at a relatively slow rate. The adsorption mechanisms of the heavy metals into NaACF pores are proposed as pore-filling with non-ionic Hg(OH)2 and electron sharing of oxygens in phenolic, lactone, and carboxylic acid groups with ionic Cd2+. The results from continuous feeding are also reported for the sample blend of 10 wt% NaACF and 90 wt% GAC in increasing the cost performance ratio.
1. Introduction Heavy metals such as mercury, cadmium, and lead are widely recognized as hazard to human due to their high toxicity and potential to accumulate in the human body [1,2]. Heavy metals are broadly found in contaminated soil and acidic leachate water from landfills or abandoned factories [3,4]. Historically, “itai-itai” disease, acrodynia (i.e., pink disease), and Minamata disease are well-documented examples of
the devastating effects of cadmium and mercury water pollution. Presently, the presence of heavy metals in drinking water is severely regulated. The maximum permissible limits of mercury and cadmium in drinking by the World Health Organization (WHO)/Ministry of Environment of Republic of Korea water are 0.006/0.001 and 0.003/ 0.005 ppm, respectively. Therefore, removal of trace heavy metals is essential. The heavy metals in aqueous solution exist in various forms of ions or salts such as M2+, M(OH)+, and M(OH)2 (M; heavy metal),
⁎ Corresponding authors at: Department of Polymer Engineering, Graduate School, School of Polymer Science and Engineering & Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea (K.S. Yang). E-mail addresses: [email protected] (C.-M. Yang), [email protected] (K.S. Yang).
https://doi.org/10.1016/j.cej.2019.123768 Received 2 September 2019; Received in revised form 6 December 2019; Accepted 8 December 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Doo-Won Kim, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123768
Chemical Engineering Journal xxx (xxxx) xxxx
D.-W. Kim, et al.
have led to increasing attention to its potential use in air and water adsorbate filters [18]. The surface of ACF is composed of both hydrophobic graphite layers and hydrophilic oxygen functional groups. Oxygen functional groups with electronegative polarity adsorb both aqueous heavy metal compounds and cations rapidly. Chemical oxidation of the carbon surface introduces more hydrophilic surface with an increase of oxygen functional groups. Various reagents such as nitric acid [19,20], sodium hydroxide (NaOH) [20], citric acid [21], and ozone [22] have been used as chemical oxidizers. In particular, NaOH has been reported for effective increases of oxygen functional groups sustaining the pore structure than the other acid oxidizing agents [20,21]. This study focuses on the effect of ACF pore structure and oxygen functional groups on the adsorption of heavy metal compounds in an aqueous solution. In increasing the cost performance ratio, a continuous feeding system was run through the adsorbent of 10 wt% NaACF and 90 wt% GAC and the results were reported. 2. Experimental 2.1. Adsorbents and ACF modification Commercially available GAC derived from coconut shells for water filtration (pore size: 74–173 µm) was used as a base adsorbent without any modifications. The ACF employed was petroleum-pitch based ACF (pACF, A-10) from Osaka Gas. The pACF (2 g) was dispersed in distilled water (100 mL) with 1 N NaOH (25 g, DUKSAN COMPANY, Republic of Korea) and reacted at 80 ℃ for 3 h with mechanical stirring. The samples were then washed with distilled water until the pH is stabilized, then they were dried overnight at 80 ℃ in a vacuum oven. The NaOH-modified ACF was designated as NaACF.
Fig. 1. Schematic diagram of the continuous feeding system of the heavy metal aqueous sample.
which are sensitive to pH [5,6]. The granular activated carbon (GAC) and activated carbon fiber (ACF) are commonly used for adsorbents due to their extremely porous structure that allows it to effective adsorption of heavy metal, gases, organic and inorganic materials. However, the GAC and ACF can be distinctly characterized on their pore structures [7–10]. The GAC is commercially available material for water purification through manufacturing carbon block to remove pollutants in drinking water [11–15]. The carbon block from the GAC is used for filtrations of microorganisms, organic, and inorganic materials. However, the performance of GAC in removing inorganic matter has a slow average adsorption rate that requires improvement [16]. The pore structure of GAC is branched from the stem of macro- (> 50 nm) to meso- (2–50 nm) and finally to micro- (< 2 nm) pores, thus, the smaller size of metal ions or complex compared with the micro-pore are delayed to be reached to the micropores in which the heavy metal compound adsorbed at lower energy state [17]. Conversely, the pores of the ACF consists of micro-pores only. Heavy metal compounds can reach directly into most micro-pores and be adsorbed at a high rate. Recently, the characteristics of the ACF
2.2. Adsorption behaviors 2.2.1. Batch experiment For batch experiments, a 10 ppm solution of each heavy metal was prepared by dissolving CdCl2 or HgCl2 (Merck, Germany) salt in distilled water in individual Teflon beakers. 0.85 g of GAC or pACF was dispersed in 250 mL of heavy metal aqueous solution by stirring at 100 rpm. Then, a 2 mL of the solution was sampled with a syringe at 1, 2, 4, 10, and, 20 min intervals and measured the heavy metal concentration remained via inductively coupled plasma-atomic emission spectroscopy (ICP). The adsorption rate [%] was calculated using equation (1)
Adsorption rate[%] =
(C0
Ct ) C0
× 100
(1)
where, C0 and Ct are the heavy metal concentrations of the samples at
Fig. 2. Characteristics of pores determined with N2 adsorption/desorption isotherms at 77 K; (a) specific surface area, micropore volume and mesopore volume by tplot method and (b) pore size distribution determined by the MP method (0–2 nm) and BJH method (2–200 nm).
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Table 1 XPS data of total atomic composition and oxygen functional groups.
GAC pACF NaACF
Total atomic composition
Oxygen functional group composition
C1s [%]
O1s [%]
Phenol [%]
Lactone [%]
Carboxyl [%]
90.8 91.3 86.7
9.2 8.7 13.3
6.3 5.0 7.4
2.5 2.2 3.4
3.3 3.1 3.1
Fig. 4. FTIR spectra of pACF and NaACF.
Fig. 5. Equilibrium pH of aqueous solution in the presence of GAC, pACF, NaACF and GAC/NaACF (9:1).
sample analysis was then performed at 10, 20, and 30 min via ICP analysis. The adsorption rate was also calculated based on Equation (1). 2.3. Characterizations The pores of the adsorbents were characterized via t-plot, MP and, Barrett, Joyner, and Halenda (BJH) methods for specific surface area and pore size distribution measured by the Micrometrics system (ASPS2020, USA). Analyses of the surface functional groups of the samples were performed with monochromatized AlKα X-rays of X-ray photoelectron spectroscopy (XPS; ESCALAB 250 Spectrometer, VG Scientific, USA). The functional groups were also confirmed via Fouriertransform infrared spectroscopy (FTIR, IRPrestige-21, SHIMADZU, Japan). ICP was employed to analyze heavy metal concentration (OPTIMA 4300 DV, Perkin Elmer, USA).
Fig. 3. O1s XPS spectra of (a) GAC, (b) pACF and (c) NaACF.
time zero and the sampling time t. 2.2.2. Continuous water feeding experiment A continuous feeding experiment was prepared by packing 15 g of adsorbent on the stainless steel porous plate (0.45 μm pore dia.) in a burette, as shown in Fig. 1. A 10 ppm aqueous solution of each heavy metal was fed at a rate of 140 mL/min using a metering pump. Sequential 3
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Fig. 6. Ionic sate of compounds of (a) cadmium [6] and (b) mercury [26] at various pH.
oxygen functional group via the NaOH reaction reported by Tryk and Chiang [22,24]. 3.2. Cadmium and mercury adsorption in batch system The acceptable pH range of drinking water is 6.5–8.5 and 5.8–8.5 by WHO and Ministry of Environment of Republic of Korea, respectively. Fig. 5 illustrate the ionic state of the Cd and Hg compounds at various pH in the aqueous solution. The equilibrium pH of the water in the presence of GAC, pACF, NaACF was in the range pH 6–8 (Fig. 6). On the basis of the results, cadmium (Cd) and mercury (Hg) are recognized to be Cd2+ and Hg(OH)2 at the equilibrium conditions [6,26]. Fig. 7 shows the adsorption rates of Cd2+ into the adsorbent samples. The adsorption in 1 min was 55% for NaACF, 29% for GAC, and 7% for pACF. After 20 min, the NaACF adsorbed 100% of Cd2+, GAC of 55% and pACF up to 8% only. The results imply that the oxygen functional group with lower oxidation was more effective for the adsorption of Cd2+ i.e., in the decreasing order of phenol, lactone, and carboxylic acid (refer Table 1). Though the pACF was characterized as highest SSA and MV, it showed lowest adsorption rate on Cd2+. It represents that the oxygen functional group with more electron pairs to share is more efficient adsorbing Cd2+ than the amount of the surface from the pore volume [27–29]. Fig. 8 shows the drastic effect of micropores of pACF and NaACF in adsorption of the mercury compound. Both pACF and NaACF adsorbed approximately 81% of the mercury compound in 1 min and approximately 98% in 20 min (Fig. 8(a)), on the other hand, GAC adsorbed approximately 16% of it in 1 min, 68% in 20 min, and 99% after 60 min as shown in Fig. 8(b). The behaviors imply that the Hg(OH)2 was rapidly adsorbed into the micropores with uniform, narrow and shallow structures by the pore filling mechanism. Conversely, the adsorption rate of Hg(OH)2 was slowed down in GAC due to the wide pore size range. Therefore, GAC adsorption reached up to 99% after 60 min in the batch system experiment. Consequently, NaACF proved to be a good adsorbent for both mercury and cadmium in aqueous solution in the pH allowed for drinking water.
Fig. 7. Adsorption isotherms of cadmium compound in aqueous batch experiment.
3. Results and discussion 3.1. Pore structure and functional group of GAC and ACF The precursor ACF (pACF) shows larger specific surface area (SSA) and more micropore volume (MV) compared with those of the GAC containing more mesopore volume, as shown in Fig. 2(a). After NaOH modification, the SSA and MV were slightly decreased due to partial blocking of the micro-pore after the introduction of the more functional groups. Fig. 2(b) and inset noticeably shows that GAC contains micropores of 0.6–1.4 nm and also mesopores and macropores in sizes by the BJH characterization. On the other hand, pACF contains uniformly distributed micropores without macrpores [23]. After NaOH modification, the NaACF maintained the micropore structure albeit with slightly smaller pore size compared to pACF. The oxygen functional groups analyzed by the XPS were confirmed as phenol (532.5 eV), lactone (530.8 eV) and carboxyl (534.2 eV) groups in all the samples (Fig. 3(a–c)). The total oxygen content of pACF increased by NaOH modification to be NaACF and the more functional groups introduced were phenol and lactone, indicating more content at less oxidation state (Table 1). Oxygen functional groups are known to be introduced on the edge of the graphitic layer being phenol and lactone [22,24]. In Fig. 4, absorption peak of lactone (1410 cm−1) increased after NaOH modification [20]. In addition, a 2800–3200 cm−1 peak of aliphatic CeH, CeH2, and CeH3 stretching was remarkably observed for NaACF [20,22,25]. It is implied that the reaction between NaOH and the edge of the graphitic layer (aromatic structure) converted the aliphatic structure with the introduction of the
3.3. Continuous water feeding system A continuous water feeding system provides a practical process for providing drinking water. Fig. 1 illustrates the instrumental the set up for adsorption of cadmium and mercury compounds in a continuous feeding system at a 140 mL/min feeding rate for a typical drinking water filtration device. To improve the cost-performance ratio, 10 wt% of more expensive NaACF was mixed with the less expensive GAC. For Cd2+, GAC/NaACF (9:1) showed higher efficiency than only GAC at all the feeding time sequences as shown Fig. 9(a), although the adsorption amount decreased with time for both samples. The adsorption behavior of the Cd2+ on the carbon adsorbent was indicative of a site filling mechanism at the given number of functional groups for both 4
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Fig. 8. Adsorption isotherms of mercury compounds on GAC, pACF and NaACF in batch experiments; (a) up to 20 min (b) extended to 100% adsorption for GAC.
Fig. 9. Adsorption behaviors of GAC and GAC/NaACF (9/1) for (a) cadmium and (b) mercury compounds for the continuous feeding experiments at various length of feeding time.
samples, showing more adsorptive functional groups on 9:1 GAC/ NaACF than on GAC. In 10 min, only GAC shows 93.4% adsorption; on the other hand, GAC/NaACF (9:1) removed 100% with the addition of 10 wt% NaACF. After 30 min, GAC/NaACF (9:1) showed 1.79 times higher adsorption than GAC only. The result supports the hypothesis that NaACF rapidly adsorbed the Cd2+ ion by the electron sharing mechanism due to abundant oxygen functional groups with lower oxidation value. For Hg(OH)2, the adsorption performance of only GAC was superior in the continuous water feeding system as shown in Fig. 9(b). Nevertheless, the GAC/NaACF (9:1) adsorbed 1–2% more mercury compound than only GAC without any significant changes with sampling time. This indicates sufficient remaining micropores for adsorption even after 30 min, as shown in Fig. 9(b). In continuous water feeding system, the removal efficiency of both cadmium and mercury was enhanced by adding 10 wt% of NaACF. 3.4. Hg(OH)2 and Cd2+ adsorption mechanism in the NaACF Fig. 10 illustrates the mechanism of Hg(OH)2 and Cd2+ adsorption in NaACF. The size of the Hg(OH)2 molecule is estimated to be roughly 0.706 nm [30]. As shown in Fig. 2(b), the main pore size of pACF and NaACF is 0.7–0.8 nm, which are large enough for the adsorption of the Hg(OH)2. Additionally, the uniform, narrow, and shallow micropores in the ACF surface could rapidly adsorb the Hg(OH)2 by the pore filling mechanism. The Cd2+ cation strongly interacted with the abundant oxygen functional groups via a cation ion electron sharing mechanism. Thus, the Cd2+ cation effectively adsorbed to NaOH-modified NaACF,
Fig. 10. Schematics of the pore filling mechanism of non-ionic Hg(OH)2 and the electron sharing mechanism of ionic Cd2+ existing in pH 6–8 of the NaACF.
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which contained abundant sites for metal ion adsorption. Accordingly, we propose that the adsorption mechanisms of the Cd2+ and Hg(OH)2 in the GAC and GAC/NaACF (9:1) are likely electron sharing site occupation of the polar oxygen functional groups and pore filling, respectively (Fig. 10). Moreover, our observations suggest that the adsorption of heavy metals is likely to increase with increased NaACF fractions in the filter.
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4. Conclusions In this work, the adsorption rate of Hg(OH)2 and Cd2+ into ACF was improved by NaOH modification (NaACF), as demonstrated by our continuous water feeding system. The NaACF rapidly adsorbed both Hg (OH)2 and Cd2+ on the basis of its uniform, narrow, and shallow micropores and abundant oxygen functional groups by pore filling and electron sharing mechanism, respectively. A efficiency were improved by 10 wt% mixing of NaACF with GAC for continuous water feeding system. 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. Acknowledgements This work was supported by the Basic Science Research Program (NRF2018R1D1A1A02046116) and Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF2016M3A7B4905618). References [1] L. Järup, Hazards of heavy metal contamination, Br. Med. Bull. 68 (2003) 167–182. [2] M.K. Uddin, A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade, Chem. Eng. J. 308 (2017) 438–462. [3] B.J. Alloway, Heavy Metals in Soils: Trace Metals and Metalloids in Soils and their Bioavailability, Springer Science & Business Media, 2012. [4] J. Vymazal, T. Březinová, Accumulation of heavy metals in aboveground biomass of Phragmites australis in horizontal flow constructed wetlands for wastewater treatment: a review, Chem. Eng. J. 290 (2016) 232–242. [5] R. Mesmer, C. Baes, The hydrolysis of cations, Ed. Wi1ey, EUA (1976). [6] S.-M. Lee, C. Laldawngliana, D. Tiwari, Iron oxide nano-particles-immobilized-sand material in the treatment of Cu (II), Cd (II) and Pb (II) contaminated waste waters, Chem. Eng. J. 195 (2012) 103–111. [7] Y. Huang, Electrical and Thermal Properties of Activated Carbon Fibers, Activated Carbon Fiber and Textiles, Elsevier, 2017, pp. 181–192. [8] D.H. Kim, D.W. Kim, B.-H. Kim, K.S. Yang, Y.-K. Lim, E.N. Park, Study of the adsorbent-adsorbate interactions from Cd (II) and Pb (II) adsorption on activated
6
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Efficient removals of Hg and Cd in aqueous solution through NaOHmodified activated carbon fiber Doo-Won Kima, Jae-Hyung Weeb, Cheol-Min Yanga, , Kap Seung Yangb,c, ⁎
⁎
a
Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea Department of Polymer Engineering, Graduate School, School of Polymer Science and Engineering & Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea c Carbon Composite Materials R&D Center, HPK Inc, 109 Banlyong-ro, Deokjin-gu, Jenju-si, Republic of Korea b
HIGHLIGHTS
GRAPHICAL ABSTRACT
activated carbon fiber • NaOH-modified removes to heavy metals, cost-effectively.
NaOH modification induced • The abundant oxygen functional groups in micropore.
metals adsorbed by a polar • Heavy oxygen functional groups and pore filling mechanism.
ARTICLE INFO
ABSTRACT
Keywords: Heavy metal removal in water Activated carbon fiber NaOH modification Adsorption mechanism
This manuscript describes an efficient and cost-effective method to remove heavy metals of Hg and Cd in aqueous solutions via adsorption on activated carbon fibers after modification with NaOH solution (NaACF). The Hg and Cd metals in the aqueous solution exist as Hg(OH)2 and Cd2+ in the experimental condition of pH 6–8. Surface characterization of the NaACF reveals uniform and narrower micropores with an increase in oxygen functional groups of phenol and lactone compared with the original ACF (pACF). The NaACF demonstrates a superior adsorption rate to both aqueous samples of heavy metal compounds. The granular activated carbon (GAC) with diverse pore structures consisting of micropores, mesopores, and macropores adsorbed the heavy metals at a relatively slow rate. The adsorption mechanisms of the heavy metals into NaACF pores are proposed as pore-filling with non-ionic Hg(OH)2 and electron sharing of oxygens in phenolic, lactone, and carboxylic acid groups with ionic Cd2+. The results from continuous feeding are also reported for the sample blend of 10 wt% NaACF and 90 wt% GAC in increasing the cost performance ratio.
1. Introduction Heavy metals such as mercury, cadmium, and lead are widely recognized as hazard to human due to their high toxicity and potential to accumulate in the human body [1,2]. Heavy metals are broadly found in contaminated soil and acidic leachate water from landfills or abandoned factories [3,4]. Historically, “itai-itai” disease, acrodynia (i.e., pink disease), and Minamata disease are well-documented examples of
the devastating effects of cadmium and mercury water pollution. Presently, the presence of heavy metals in drinking water is severely regulated. The maximum permissible limits of mercury and cadmium in drinking by the World Health Organization (WHO)/Ministry of Environment of Republic of Korea water are 0.006/0.001 and 0.003/ 0.005 ppm, respectively. Therefore, removal of trace heavy metals is essential. The heavy metals in aqueous solution exist in various forms of ions or salts such as M2+, M(OH)+, and M(OH)2 (M; heavy metal),
⁎ Corresponding authors at: Department of Polymer Engineering, Graduate School, School of Polymer Science and Engineering & Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea (K.S. Yang). E-mail addresses: [email protected] (C.-M. Yang), [email protected] (K.S. Yang).
https://doi.org/10.1016/j.cej.2019.123768 Received 2 September 2019; Received in revised form 6 December 2019; Accepted 8 December 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Doo-Won Kim, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123768
Chemical Engineering Journal xxx (xxxx) xxxx
D.-W. Kim, et al.
have led to increasing attention to its potential use in air and water adsorbate filters [18]. The surface of ACF is composed of both hydrophobic graphite layers and hydrophilic oxygen functional groups. Oxygen functional groups with electronegative polarity adsorb both aqueous heavy metal compounds and cations rapidly. Chemical oxidation of the carbon surface introduces more hydrophilic surface with an increase of oxygen functional groups. Various reagents such as nitric acid [19,20], sodium hydroxide (NaOH) [20], citric acid [21], and ozone [22] have been used as chemical oxidizers. In particular, NaOH has been reported for effective increases of oxygen functional groups sustaining the pore structure than the other acid oxidizing agents [20,21]. This study focuses on the effect of ACF pore structure and oxygen functional groups on the adsorption of heavy metal compounds in an aqueous solution. In increasing the cost performance ratio, a continuous feeding system was run through the adsorbent of 10 wt% NaACF and 90 wt% GAC and the results were reported. 2. Experimental 2.1. Adsorbents and ACF modification Commercially available GAC derived from coconut shells for water filtration (pore size: 74–173 µm) was used as a base adsorbent without any modifications. The ACF employed was petroleum-pitch based ACF (pACF, A-10) from Osaka Gas. The pACF (2 g) was dispersed in distilled water (100 mL) with 1 N NaOH (25 g, DUKSAN COMPANY, Republic of Korea) and reacted at 80 ℃ for 3 h with mechanical stirring. The samples were then washed with distilled water until the pH is stabilized, then they were dried overnight at 80 ℃ in a vacuum oven. The NaOH-modified ACF was designated as NaACF.
Fig. 1. Schematic diagram of the continuous feeding system of the heavy metal aqueous sample.
which are sensitive to pH [5,6]. The granular activated carbon (GAC) and activated carbon fiber (ACF) are commonly used for adsorbents due to their extremely porous structure that allows it to effective adsorption of heavy metal, gases, organic and inorganic materials. However, the GAC and ACF can be distinctly characterized on their pore structures [7–10]. The GAC is commercially available material for water purification through manufacturing carbon block to remove pollutants in drinking water [11–15]. The carbon block from the GAC is used for filtrations of microorganisms, organic, and inorganic materials. However, the performance of GAC in removing inorganic matter has a slow average adsorption rate that requires improvement [16]. The pore structure of GAC is branched from the stem of macro- (> 50 nm) to meso- (2–50 nm) and finally to micro- (< 2 nm) pores, thus, the smaller size of metal ions or complex compared with the micro-pore are delayed to be reached to the micropores in which the heavy metal compound adsorbed at lower energy state [17]. Conversely, the pores of the ACF consists of micro-pores only. Heavy metal compounds can reach directly into most micro-pores and be adsorbed at a high rate. Recently, the characteristics of the ACF
2.2. Adsorption behaviors 2.2.1. Batch experiment For batch experiments, a 10 ppm solution of each heavy metal was prepared by dissolving CdCl2 or HgCl2 (Merck, Germany) salt in distilled water in individual Teflon beakers. 0.85 g of GAC or pACF was dispersed in 250 mL of heavy metal aqueous solution by stirring at 100 rpm. Then, a 2 mL of the solution was sampled with a syringe at 1, 2, 4, 10, and, 20 min intervals and measured the heavy metal concentration remained via inductively coupled plasma-atomic emission spectroscopy (ICP). The adsorption rate [%] was calculated using equation (1)
Adsorption rate[%] =
(C0
Ct ) C0
× 100
(1)
where, C0 and Ct are the heavy metal concentrations of the samples at
Fig. 2. Characteristics of pores determined with N2 adsorption/desorption isotherms at 77 K; (a) specific surface area, micropore volume and mesopore volume by tplot method and (b) pore size distribution determined by the MP method (0–2 nm) and BJH method (2–200 nm).
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Table 1 XPS data of total atomic composition and oxygen functional groups.
GAC pACF NaACF
Total atomic composition
Oxygen functional group composition
C1s [%]
O1s [%]
Phenol [%]
Lactone [%]
Carboxyl [%]
90.8 91.3 86.7
9.2 8.7 13.3
6.3 5.0 7.4
2.5 2.2 3.4
3.3 3.1 3.1
Fig. 4. FTIR spectra of pACF and NaACF.
Fig. 5. Equilibrium pH of aqueous solution in the presence of GAC, pACF, NaACF and GAC/NaACF (9:1).
sample analysis was then performed at 10, 20, and 30 min via ICP analysis. The adsorption rate was also calculated based on Equation (1). 2.3. Characterizations The pores of the adsorbents were characterized via t-plot, MP and, Barrett, Joyner, and Halenda (BJH) methods for specific surface area and pore size distribution measured by the Micrometrics system (ASPS2020, USA). Analyses of the surface functional groups of the samples were performed with monochromatized AlKα X-rays of X-ray photoelectron spectroscopy (XPS; ESCALAB 250 Spectrometer, VG Scientific, USA). The functional groups were also confirmed via Fouriertransform infrared spectroscopy (FTIR, IRPrestige-21, SHIMADZU, Japan). ICP was employed to analyze heavy metal concentration (OPTIMA 4300 DV, Perkin Elmer, USA).
Fig. 3. O1s XPS spectra of (a) GAC, (b) pACF and (c) NaACF.
time zero and the sampling time t. 2.2.2. Continuous water feeding experiment A continuous feeding experiment was prepared by packing 15 g of adsorbent on the stainless steel porous plate (0.45 μm pore dia.) in a burette, as shown in Fig. 1. A 10 ppm aqueous solution of each heavy metal was fed at a rate of 140 mL/min using a metering pump. Sequential 3
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Fig. 6. Ionic sate of compounds of (a) cadmium [6] and (b) mercury [26] at various pH.
oxygen functional group via the NaOH reaction reported by Tryk and Chiang [22,24]. 3.2. Cadmium and mercury adsorption in batch system The acceptable pH range of drinking water is 6.5–8.5 and 5.8–8.5 by WHO and Ministry of Environment of Republic of Korea, respectively. Fig. 5 illustrate the ionic state of the Cd and Hg compounds at various pH in the aqueous solution. The equilibrium pH of the water in the presence of GAC, pACF, NaACF was in the range pH 6–8 (Fig. 6). On the basis of the results, cadmium (Cd) and mercury (Hg) are recognized to be Cd2+ and Hg(OH)2 at the equilibrium conditions [6,26]. Fig. 7 shows the adsorption rates of Cd2+ into the adsorbent samples. The adsorption in 1 min was 55% for NaACF, 29% for GAC, and 7% for pACF. After 20 min, the NaACF adsorbed 100% of Cd2+, GAC of 55% and pACF up to 8% only. The results imply that the oxygen functional group with lower oxidation was more effective for the adsorption of Cd2+ i.e., in the decreasing order of phenol, lactone, and carboxylic acid (refer Table 1). Though the pACF was characterized as highest SSA and MV, it showed lowest adsorption rate on Cd2+. It represents that the oxygen functional group with more electron pairs to share is more efficient adsorbing Cd2+ than the amount of the surface from the pore volume [27–29]. Fig. 8 shows the drastic effect of micropores of pACF and NaACF in adsorption of the mercury compound. Both pACF and NaACF adsorbed approximately 81% of the mercury compound in 1 min and approximately 98% in 20 min (Fig. 8(a)), on the other hand, GAC adsorbed approximately 16% of it in 1 min, 68% in 20 min, and 99% after 60 min as shown in Fig. 8(b). The behaviors imply that the Hg(OH)2 was rapidly adsorbed into the micropores with uniform, narrow and shallow structures by the pore filling mechanism. Conversely, the adsorption rate of Hg(OH)2 was slowed down in GAC due to the wide pore size range. Therefore, GAC adsorption reached up to 99% after 60 min in the batch system experiment. Consequently, NaACF proved to be a good adsorbent for both mercury and cadmium in aqueous solution in the pH allowed for drinking water.
Fig. 7. Adsorption isotherms of cadmium compound in aqueous batch experiment.
3. Results and discussion 3.1. Pore structure and functional group of GAC and ACF The precursor ACF (pACF) shows larger specific surface area (SSA) and more micropore volume (MV) compared with those of the GAC containing more mesopore volume, as shown in Fig. 2(a). After NaOH modification, the SSA and MV were slightly decreased due to partial blocking of the micro-pore after the introduction of the more functional groups. Fig. 2(b) and inset noticeably shows that GAC contains micropores of 0.6–1.4 nm and also mesopores and macropores in sizes by the BJH characterization. On the other hand, pACF contains uniformly distributed micropores without macrpores [23]. After NaOH modification, the NaACF maintained the micropore structure albeit with slightly smaller pore size compared to pACF. The oxygen functional groups analyzed by the XPS were confirmed as phenol (532.5 eV), lactone (530.8 eV) and carboxyl (534.2 eV) groups in all the samples (Fig. 3(a–c)). The total oxygen content of pACF increased by NaOH modification to be NaACF and the more functional groups introduced were phenol and lactone, indicating more content at less oxidation state (Table 1). Oxygen functional groups are known to be introduced on the edge of the graphitic layer being phenol and lactone [22,24]. In Fig. 4, absorption peak of lactone (1410 cm−1) increased after NaOH modification [20]. In addition, a 2800–3200 cm−1 peak of aliphatic CeH, CeH2, and CeH3 stretching was remarkably observed for NaACF [20,22,25]. It is implied that the reaction between NaOH and the edge of the graphitic layer (aromatic structure) converted the aliphatic structure with the introduction of the
3.3. Continuous water feeding system A continuous water feeding system provides a practical process for providing drinking water. Fig. 1 illustrates the instrumental the set up for adsorption of cadmium and mercury compounds in a continuous feeding system at a 140 mL/min feeding rate for a typical drinking water filtration device. To improve the cost-performance ratio, 10 wt% of more expensive NaACF was mixed with the less expensive GAC. For Cd2+, GAC/NaACF (9:1) showed higher efficiency than only GAC at all the feeding time sequences as shown Fig. 9(a), although the adsorption amount decreased with time for both samples. The adsorption behavior of the Cd2+ on the carbon adsorbent was indicative of a site filling mechanism at the given number of functional groups for both 4
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Fig. 8. Adsorption isotherms of mercury compounds on GAC, pACF and NaACF in batch experiments; (a) up to 20 min (b) extended to 100% adsorption for GAC.
Fig. 9. Adsorption behaviors of GAC and GAC/NaACF (9/1) for (a) cadmium and (b) mercury compounds for the continuous feeding experiments at various length of feeding time.
samples, showing more adsorptive functional groups on 9:1 GAC/ NaACF than on GAC. In 10 min, only GAC shows 93.4% adsorption; on the other hand, GAC/NaACF (9:1) removed 100% with the addition of 10 wt% NaACF. After 30 min, GAC/NaACF (9:1) showed 1.79 times higher adsorption than GAC only. The result supports the hypothesis that NaACF rapidly adsorbed the Cd2+ ion by the electron sharing mechanism due to abundant oxygen functional groups with lower oxidation value. For Hg(OH)2, the adsorption performance of only GAC was superior in the continuous water feeding system as shown in Fig. 9(b). Nevertheless, the GAC/NaACF (9:1) adsorbed 1–2% more mercury compound than only GAC without any significant changes with sampling time. This indicates sufficient remaining micropores for adsorption even after 30 min, as shown in Fig. 9(b). In continuous water feeding system, the removal efficiency of both cadmium and mercury was enhanced by adding 10 wt% of NaACF. 3.4. Hg(OH)2 and Cd2+ adsorption mechanism in the NaACF Fig. 10 illustrates the mechanism of Hg(OH)2 and Cd2+ adsorption in NaACF. The size of the Hg(OH)2 molecule is estimated to be roughly 0.706 nm [30]. As shown in Fig. 2(b), the main pore size of pACF and NaACF is 0.7–0.8 nm, which are large enough for the adsorption of the Hg(OH)2. Additionally, the uniform, narrow, and shallow micropores in the ACF surface could rapidly adsorb the Hg(OH)2 by the pore filling mechanism. The Cd2+ cation strongly interacted with the abundant oxygen functional groups via a cation ion electron sharing mechanism. Thus, the Cd2+ cation effectively adsorbed to NaOH-modified NaACF,
Fig. 10. Schematics of the pore filling mechanism of non-ionic Hg(OH)2 and the electron sharing mechanism of ionic Cd2+ existing in pH 6–8 of the NaACF.
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which contained abundant sites for metal ion adsorption. Accordingly, we propose that the adsorption mechanisms of the Cd2+ and Hg(OH)2 in the GAC and GAC/NaACF (9:1) are likely electron sharing site occupation of the polar oxygen functional groups and pore filling, respectively (Fig. 10). Moreover, our observations suggest that the adsorption of heavy metals is likely to increase with increased NaACF fractions in the filter.
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4. Conclusions In this work, the adsorption rate of Hg(OH)2 and Cd2+ into ACF was improved by NaOH modification (NaACF), as demonstrated by our continuous water feeding system. The NaACF rapidly adsorbed both Hg (OH)2 and Cd2+ on the basis of its uniform, narrow, and shallow micropores and abundant oxygen functional groups by pore filling and electron sharing mechanism, respectively. A efficiency were improved by 10 wt% mixing of NaACF with GAC for continuous water feeding system. 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. Acknowledgements This work was supported by the Basic Science Research Program (NRF2018R1D1A1A02046116) and Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF2016M3A7B4905618). References [1] L. Järup, Hazards of heavy metal contamination, Br. Med. Bull. 68 (2003) 167–182. [2] M.K. Uddin, A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade, Chem. Eng. J. 308 (2017) 438–462. [3] B.J. Alloway, Heavy Metals in Soils: Trace Metals and Metalloids in Soils and their Bioavailability, Springer Science & Business Media, 2012. [4] J. Vymazal, T. Březinová, Accumulation of heavy metals in aboveground biomass of Phragmites australis in horizontal flow constructed wetlands for wastewater treatment: a review, Chem. Eng. J. 290 (2016) 232–242. [5] R. Mesmer, C. Baes, The hydrolysis of cations, Ed. Wi1ey, EUA (1976). [6] S.-M. Lee, C. Laldawngliana, D. Tiwari, Iron oxide nano-particles-immobilized-sand material in the treatment of Cu (II), Cd (II) and Pb (II) contaminated waste waters, Chem. Eng. J. 195 (2012) 103–111. [7] Y. Huang, Electrical and Thermal Properties of Activated Carbon Fibers, Activated Carbon Fiber and Textiles, Elsevier, 2017, pp. 181–192. [8] D.H. Kim, D.W. Kim, B.-H. Kim, K.S. Yang, Y.-K. Lim, E.N. Park, Study of the adsorbent-adsorbate interactions from Cd (II) and Pb (II) adsorption on activated
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