Marine Pollution Bulletin xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Influence of the pore structure and surface chemical properties of activated carbon on the adsorption of mercury from aqueous solutions Xincheng Lu, Jianchun Jiang ⇑, Kang Sun, Jinbiao Wang, Yanping Zhang Institute of Chemical Industry of Forest Products, CAF, Nanjing 210042, Jiangsu Province, China National Engineering Lab. for Biomass Chemical Utilization, Nanjing 210042, Jiangsu Province, China Key and Open Lab. on Forest Chemical Engineering, SFA, Nanjing 210042, Jiangsu Province, China Key Lab. of Biomass Energy and Material, Nanjing 210042, Jiangsu Province, China
a r t i c l e
i n f o
Keywords: Activated carbon Mercury Adsorption Pore structure Surface functional groups Adsorption mechanism
a b s t r a c t Reactivation and chemical modification were used to obtain modified activated carbons with different pore structure and surface chemical properties. The samples were characterized by nitrogen absorption–desorption, Fourier transform infrared spectroscopy and the Bothem method. Using mercury chloride as the target pollutant, the Hg2+ adsorption ability of samples was investigated. The results show that the Hg2+ adsorption capacity of samples increased significantly with increases in micropores and acidic functional groups and that the adsorption process was exothermic. Different models and thermodynamic parameters were evaluated to establish the mechanisms. It was concluded that the adsorption occurred through a monolayer mechanism by a two-speed process involving both rapid adsorption and slow adsorption. The adsorption rate was determined by chemical reaction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Mercury and its related compounds discharged into industrial waste water and natural water bodies are serious concerns for all living organisms and the environment due to their high toxicity, volatility and long persistence time (Pvalish et al., 2004; Pacyna et al., 1995; Park et al., 2008). Although the use of mercury has decreased in industry, its overall emission is still a health hazard. The toxicological effects of mercury depend upon the type and amount of the mercury compound and its mode of entry into the body, which can cause emotional and other mental deterioration, blindness, loss of consciousness, involuntary mobilization, etc. The World Health Organization recommends a maximum intake of 1 lg/L and 0.3 mg/week as the maximum acceptable concentration in drinking water (Forster and Wase, 1997). Therefore, mercury-control techniques for aqueous solutions have become a research focus. These techniques include precipitation, ion exchange, reduction, reverse osmosis, adsorption, etc. Among these, adsorption is one of the most reliable technologies for removing mercury from water (Mohan et al., 2001; Granite et al., 2007; Miretzky and Cirelli, 2009; Feng et al., 2004). Activated carbons have been proven to be effective adsorbents due to their developed internal pore structure, huge surface area
⇑ Corresponding author. Tel.: +86 25 85482484; fax: +86 25 85482485. E-mail address: [email protected] (J. Jiang).
and the presence of surface functional groups (Saha et al., 2001; Lu et al., 2012; Liu et al., 2007). Treatments such as thermal treatment and chemical treatment can modify the pore structure and the chemical nature of the activated carbon. The modification of activated carbon is an attractive route towards the novel application of these materials as both liquid-phase and gas-phase adsorbents, as well as catalyst supports (Han et al., 2003; Abdel-Nasser, 2003; Lu et al., 2011; Lin et al., 2006). In recent years, activated carbon has been used as an effective adsorbent to remove mercury. In particular, there are a great number of studies regarding the mercury-adsorption performance of activated carbon impregnated with sulfur, halogens, etc. (Graydon et al., 2009; Wenguo et al., 2006; Lee et al., 2004; Hu et al., 2009; Liu, 2008). In spite of the great number of experiments on this subject, the application of activated carbon to control mercury contamination is limited by a lack of understanding of the effect of pore structure and surface chemical properties on mercury-adsorption performance. The aim of this work is to investigate the influence of pore structure and chemical properties on the adsorption performance of activated carbon for mercury and also to gain a better understanding of the adsorption mechanism. To this aim, a comprehensive experimental analysis of mercury adsorption onto activated carbons was carried out to identify a potential correlation between adsorption capacity and the properties of activated carbon. Adsorption models were also used to investigate the adsorption mechanism.
0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.11.007
Please cite this article in press as: Lu, X., et al. Influence of the pore structure and surface chemical properties of activated carbon on the adsorption of mercury from aqueous solutions. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.11.007
2
X. Lu et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx
2. Material and methods
2.4. Adsorption procedure
2.1. Materials
The adsorption experiments were carried out in batches in a thermostatic oven that can control the temperature between 10 and 60 °C. Each sample consisted of a 300 ml mercury solution in contact with 1.0 g activated carbon. Mercury solutions were prepared by dissolving a given quantity of mercuric chloride in distilled water. The concentration of mercury solution was calculated by Inductively Coupled Plasma Emission Spectrometry (ICP). The adsorption tests were carried out in a volumetric flask using a horizontal axis mechanical agitator. The preliminary kinetic tests were used to obtain the equilibrium time, and this equilibration time was adopted for all the following tests. The adsorption quantity of mercury on activated carbon was calculated by the following Eq. (1).
2.2.1. Regulation of pore structure AC was heated in a nitrogen atmosphere to an activated temperature of 850 °C at a heating rate of 10 °C/min. After reaching the activation temperature, nitrogen gas was stopped and water vapor was used (1.20 g/min) to activate AC for 60 min. The prepared sample was marked as AC1.
2.2.2. Surface modification A known amount of AC1 was oxidized at 25 °C using H2O2 (30%) at a ratio of 1:2 and 2:5 (m:v). The reaction mixture was treated in two steps. In the first step, the mixture was heated at 70 °C for 4 h, and in the second step, the mixture was subjected to ultrasound in an ultrasonic oscillator for 4 h. The reaction solution was removed, and the oxidized sample was washed until it was stable at a pH of approximately 7. The washed samples were then dried for 4 h at a temperature of 110 °C and marked as AC1-1 and AC1-2.
2.3. Characterization methods 2.3.1. Surface area and pore size distribution The specific surface area (SBET) and porosity of the samples were determined by nitrogen adsorption–desorption isotherms measured in a Micromeritics ASAP 2020 apparatus. Adsorption of N2 was performed at 77 K. Before analysis, the samples were degassed under N2 flow at 350 °C for 2 h in a vacuum at 27 Pa. The SBET of the prepared activated carbons was estimated by the BET method using N2 adsorption isotherm data. The micropore volumes were calculated from the amount of N2 adsorbed at a relative pressure of 0.1, and the mesopore volumes were calculated by subtracting the amounts adsorbed at a relative pressure of 0.1 from those at a relative pressure of 0.95 (Rodriguez-Reinoso et al., 1982). The Barret–Joyner–Halenda (BJH) model was employed to calculate the pore-size distribution of the samples.
2.3.2. Acidic functional groups The amounts of acidic functional groups were determined by the Bothem method. This was achieved by accurately weighing 1.00 g of sample into four conical flasks with stoppers. 25 mL of 0.1 M NaHCO3, 0.05 M Na2CO3, 0.1 M NaOH or 0.1 M C2H5ONa solution was added. The mixtures were shaken for 24 h, filtered and back titrated using 0.1 M hydrochloric acid. Phenolphthalein was used as the indicator. The number of basic sites was calculated from the amount of hydrochloric acid that reacted with the sample.
qe ¼ ðC o C e Þ V=m
ð1Þ
where qe is the adsorption quantity (mg/g), Co and Ce are the initial and equilibrium concentration of mercury in solution (mg/L), V is the volume (mL) and m is the weight of activated carbon (g). 3. Results and discussion 3.1. Surface area and pore size distribution 3.1.1. Adsorption–desorption isotherms The analysis of the nitrogen adsorption–desorption isotherms of the samples is shown in Fig. 1. As shown in Fig. 1, all the nitrogen adsorption–desorption isotherms of the samples belong to type IV. In the low-pressure region where p/p0 6 0.1, the nitrogen uptake was significant and the adsorption capacity increased rapidly with increasing pressure. Adsorption saturation was reached quickly, which indicated that samples contained micropores. In the highpressure region, a hysteresis loop was generated by the misalignment of the adsorption and desorption curves, which is caused by capillary condensation of the adsorbate in the mesoporous material. The type of hysteresis loop was H4, suggesting the existence of a slit-shaped mesoporous material. AC1 had a higher adsorption platform compared to AC, which was due to the increased surface area of AC1. For physical activation, H2O can be reacted with C to generate micropores and develop the pore structure. After H2O activation, AC1 had an increased surface area and high pore volume. However, the adsorption platform of modified samples (i.e., AC1-1 and AC1-2) was
300
3
2.2. Sample preparation
Quantity Absorbed (cm /g)
Coconut activated carbon (SBET 797 m2/g, Vtot 0.389 cm3/g) was used as the starting material, which was produced by MU LIN SEN Activated Carbon Co., Ltd. Before the modification processes, activated carbon was washed with deionized water, dried at 110 °C for 5 h and marked as AC.
250
AC
200
AC1 AC1-1 AC1-2
2.3.3. Fourier transform infrared (FTIR) spectroscopy Fourier transform infrared spectroscopy (FTIR) analysis of samples was performed on a Nicolet DXC20 FTIR spectrometer. Samples of particle size
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Influence of the pore structure and surface chemical properties of activated carbon on the adsorption of mercury from aqueous solutions Xincheng Lu, Jianchun Jiang ⇑, Kang Sun, Jinbiao Wang, Yanping Zhang Institute of Chemical Industry of Forest Products, CAF, Nanjing 210042, Jiangsu Province, China National Engineering Lab. for Biomass Chemical Utilization, Nanjing 210042, Jiangsu Province, China Key and Open Lab. on Forest Chemical Engineering, SFA, Nanjing 210042, Jiangsu Province, China Key Lab. of Biomass Energy and Material, Nanjing 210042, Jiangsu Province, China
a r t i c l e
i n f o
Keywords: Activated carbon Mercury Adsorption Pore structure Surface functional groups Adsorption mechanism
a b s t r a c t Reactivation and chemical modification were used to obtain modified activated carbons with different pore structure and surface chemical properties. The samples were characterized by nitrogen absorption–desorption, Fourier transform infrared spectroscopy and the Bothem method. Using mercury chloride as the target pollutant, the Hg2+ adsorption ability of samples was investigated. The results show that the Hg2+ adsorption capacity of samples increased significantly with increases in micropores and acidic functional groups and that the adsorption process was exothermic. Different models and thermodynamic parameters were evaluated to establish the mechanisms. It was concluded that the adsorption occurred through a monolayer mechanism by a two-speed process involving both rapid adsorption and slow adsorption. The adsorption rate was determined by chemical reaction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Mercury and its related compounds discharged into industrial waste water and natural water bodies are serious concerns for all living organisms and the environment due to their high toxicity, volatility and long persistence time (Pvalish et al., 2004; Pacyna et al., 1995; Park et al., 2008). Although the use of mercury has decreased in industry, its overall emission is still a health hazard. The toxicological effects of mercury depend upon the type and amount of the mercury compound and its mode of entry into the body, which can cause emotional and other mental deterioration, blindness, loss of consciousness, involuntary mobilization, etc. The World Health Organization recommends a maximum intake of 1 lg/L and 0.3 mg/week as the maximum acceptable concentration in drinking water (Forster and Wase, 1997). Therefore, mercury-control techniques for aqueous solutions have become a research focus. These techniques include precipitation, ion exchange, reduction, reverse osmosis, adsorption, etc. Among these, adsorption is one of the most reliable technologies for removing mercury from water (Mohan et al., 2001; Granite et al., 2007; Miretzky and Cirelli, 2009; Feng et al., 2004). Activated carbons have been proven to be effective adsorbents due to their developed internal pore structure, huge surface area
⇑ Corresponding author. Tel.: +86 25 85482484; fax: +86 25 85482485. E-mail address: [email protected] (J. Jiang).
and the presence of surface functional groups (Saha et al., 2001; Lu et al., 2012; Liu et al., 2007). Treatments such as thermal treatment and chemical treatment can modify the pore structure and the chemical nature of the activated carbon. The modification of activated carbon is an attractive route towards the novel application of these materials as both liquid-phase and gas-phase adsorbents, as well as catalyst supports (Han et al., 2003; Abdel-Nasser, 2003; Lu et al., 2011; Lin et al., 2006). In recent years, activated carbon has been used as an effective adsorbent to remove mercury. In particular, there are a great number of studies regarding the mercury-adsorption performance of activated carbon impregnated with sulfur, halogens, etc. (Graydon et al., 2009; Wenguo et al., 2006; Lee et al., 2004; Hu et al., 2009; Liu, 2008). In spite of the great number of experiments on this subject, the application of activated carbon to control mercury contamination is limited by a lack of understanding of the effect of pore structure and surface chemical properties on mercury-adsorption performance. The aim of this work is to investigate the influence of pore structure and chemical properties on the adsorption performance of activated carbon for mercury and also to gain a better understanding of the adsorption mechanism. To this aim, a comprehensive experimental analysis of mercury adsorption onto activated carbons was carried out to identify a potential correlation between adsorption capacity and the properties of activated carbon. Adsorption models were also used to investigate the adsorption mechanism.
0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.11.007
Please cite this article in press as: Lu, X., et al. Influence of the pore structure and surface chemical properties of activated carbon on the adsorption of mercury from aqueous solutions. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.11.007
2
X. Lu et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx
2. Material and methods
2.4. Adsorption procedure
2.1. Materials
The adsorption experiments were carried out in batches in a thermostatic oven that can control the temperature between 10 and 60 °C. Each sample consisted of a 300 ml mercury solution in contact with 1.0 g activated carbon. Mercury solutions were prepared by dissolving a given quantity of mercuric chloride in distilled water. The concentration of mercury solution was calculated by Inductively Coupled Plasma Emission Spectrometry (ICP). The adsorption tests were carried out in a volumetric flask using a horizontal axis mechanical agitator. The preliminary kinetic tests were used to obtain the equilibrium time, and this equilibration time was adopted for all the following tests. The adsorption quantity of mercury on activated carbon was calculated by the following Eq. (1).
2.2.1. Regulation of pore structure AC was heated in a nitrogen atmosphere to an activated temperature of 850 °C at a heating rate of 10 °C/min. After reaching the activation temperature, nitrogen gas was stopped and water vapor was used (1.20 g/min) to activate AC for 60 min. The prepared sample was marked as AC1.
2.2.2. Surface modification A known amount of AC1 was oxidized at 25 °C using H2O2 (30%) at a ratio of 1:2 and 2:5 (m:v). The reaction mixture was treated in two steps. In the first step, the mixture was heated at 70 °C for 4 h, and in the second step, the mixture was subjected to ultrasound in an ultrasonic oscillator for 4 h. The reaction solution was removed, and the oxidized sample was washed until it was stable at a pH of approximately 7. The washed samples were then dried for 4 h at a temperature of 110 °C and marked as AC1-1 and AC1-2.
2.3. Characterization methods 2.3.1. Surface area and pore size distribution The specific surface area (SBET) and porosity of the samples were determined by nitrogen adsorption–desorption isotherms measured in a Micromeritics ASAP 2020 apparatus. Adsorption of N2 was performed at 77 K. Before analysis, the samples were degassed under N2 flow at 350 °C for 2 h in a vacuum at 27 Pa. The SBET of the prepared activated carbons was estimated by the BET method using N2 adsorption isotherm data. The micropore volumes were calculated from the amount of N2 adsorbed at a relative pressure of 0.1, and the mesopore volumes were calculated by subtracting the amounts adsorbed at a relative pressure of 0.1 from those at a relative pressure of 0.95 (Rodriguez-Reinoso et al., 1982). The Barret–Joyner–Halenda (BJH) model was employed to calculate the pore-size distribution of the samples.
2.3.2. Acidic functional groups The amounts of acidic functional groups were determined by the Bothem method. This was achieved by accurately weighing 1.00 g of sample into four conical flasks with stoppers. 25 mL of 0.1 M NaHCO3, 0.05 M Na2CO3, 0.1 M NaOH or 0.1 M C2H5ONa solution was added. The mixtures were shaken for 24 h, filtered and back titrated using 0.1 M hydrochloric acid. Phenolphthalein was used as the indicator. The number of basic sites was calculated from the amount of hydrochloric acid that reacted with the sample.
qe ¼ ðC o C e Þ V=m
ð1Þ
where qe is the adsorption quantity (mg/g), Co and Ce are the initial and equilibrium concentration of mercury in solution (mg/L), V is the volume (mL) and m is the weight of activated carbon (g). 3. Results and discussion 3.1. Surface area and pore size distribution 3.1.1. Adsorption–desorption isotherms The analysis of the nitrogen adsorption–desorption isotherms of the samples is shown in Fig. 1. As shown in Fig. 1, all the nitrogen adsorption–desorption isotherms of the samples belong to type IV. In the low-pressure region where p/p0 6 0.1, the nitrogen uptake was significant and the adsorption capacity increased rapidly with increasing pressure. Adsorption saturation was reached quickly, which indicated that samples contained micropores. In the highpressure region, a hysteresis loop was generated by the misalignment of the adsorption and desorption curves, which is caused by capillary condensation of the adsorbate in the mesoporous material. The type of hysteresis loop was H4, suggesting the existence of a slit-shaped mesoporous material. AC1 had a higher adsorption platform compared to AC, which was due to the increased surface area of AC1. For physical activation, H2O can be reacted with C to generate micropores and develop the pore structure. After H2O activation, AC1 had an increased surface area and high pore volume. However, the adsorption platform of modified samples (i.e., AC1-1 and AC1-2) was
300
3
2.2. Sample preparation
Quantity Absorbed (cm /g)
Coconut activated carbon (SBET 797 m2/g, Vtot 0.389 cm3/g) was used as the starting material, which was produced by MU LIN SEN Activated Carbon Co., Ltd. Before the modification processes, activated carbon was washed with deionized water, dried at 110 °C for 5 h and marked as AC.
250
AC
200
AC1 AC1-1 AC1-2
2.3.3. Fourier transform infrared (FTIR) spectroscopy Fourier transform infrared spectroscopy (FTIR) analysis of samples was performed on a Nicolet DXC20 FTIR spectrometer. Samples of particle size
