Effect of nitrogen-containing functional groups

Applied Surface Science 293 (2014) 299–305

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Amination of activated carbon for enhancing phenol adsorption: Effect of nitrogen-containing functional groups Guo Yang a,c,d , Honglin Chen a,∗ , Hangdao Qin a,d , Yujun Feng a,b a

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China Polymer Research Institute, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China College of Materials and Chemical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China d Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China b c

a r t i c l e

i n f o

Article history: Received 13 September 2013 Received in revised form 20 December 2013 Accepted 26 December 2013 Available online 4 January 2014 Keywords: Aminated activated carbon Adsorption Phenol Nitrogen-containing functional groups

a b s t r a c t To study the contribution of different nitrogen-containing functional groups to enhancement of phenol adsorption, the aminated activated carbons (AC) were characterized by N2 adsorption/desorption, XPS, Boehm titration, and pH drift method and tested for adsorption behaviors of phenol. Adsorption isotherm fitting revealed that the Langmuir model was preferred for the aminated ACs. The adsorption capacity per unit surface area (qm /SSABET ) was linearly correlated with the amount of pyridinic and pyrrolic N, which suggested that these two functional groups played a critical role in phenol adsorption. The enhancement of adsorption capacity was attributed to the strengthened ␲–␲ dispersion between phenol and basal plane of AC by pyridinic, pyrrolic N. The adsorption kinetics was found to follow the pseudo-secondorder kinetic model, and intraparticle diffusion was one of the rate-controlling steps in the adsorption process. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Phenolic wastewater, generated from chemicals, pharmaceuticals, petroleum, coal gasification, papermaking, wood, rubber, dye, and pesticide industries, is highly toxic and harmful [1]. Phenols in such wastewaters are the priority pollutants by the US EPA and China since they are not only carcinogenic but also cause unpleasant taste and odor even at low concentrations [2,3]. In China, according to the Chinese integrated wastewater discharge standard (GB 8978-1996), the maximum permitted concentration of volatile phenols is 0.5 mg/L for standard A effluent and 2.0 mg/L for standard C effluent [4]. Consequently, various technologies have been developed to treat phenolic wastewater. Among them, adsorption is an effective technology for removal of phenols from wastewater or other aqueous solutions [5]. Activated carbon (AC) is an excellent adsorbent due to its high surface area, well-developed internal pore structure, and tunable surface chemistry [6,7]. Besides the physical or porous structure [8], the surface chemistry of AC is considered as the main factor in the adsorption mechanism from phenol aqueous solutions [9]. Oxidation, a common modification method, introduces oxygen atoms

∗ Corresponding author at: Chinese Academy of Sciences, Chengdu Institute of Organic Chemistry, No. 16, South Section 2, The 1st Ring Road, Chengdu, Sichuan 610041, PR China. Tel.: +86 28 85232579; fax: +86 28 85232579. E-mail address: [email protected] (H. Chen). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.155

into the carbon matrix to modify the surface chemistry of AC [10]. However, a consistent conclusion has been drawn that AC treated by oxidation shows a significantly acidic character and a low phenol adsorption capacity [11]. The decrease in adsorption capacity can be ascribed to the increase of the oxygen-containing functional groups that attract and localize the ␲-electrons of basal plane on the carbon surface to reduce the interaction between phenolic ring and basal plane [12]. Moreover, these acidic groups, particularly carboxylic groups, create water clusters through H-bonding, thus reducing the accessibility and affinity of the phenol molecules to the inner pore structure [13]. However, the enhanced adsorption of phenols is observed for the AC modified by nitrogenation, another widely used method for modification of surface chemistry of carbon materials. Nitrogenation introduces nitrogen-containing functional groups onto the carbon surface, such as–NH2 , pyridinic, pyrrolic, and quaternary nitrogen groups [14], and the resulting carbon is basic in character. The AC modified with ammonia aqueous solution at room temperature shows a higher adsorption capacity toward 2,4-dichlorophenol [15]. The AC treated by urea at 450 ◦ C exhibits a nearly 50% increase in phenol adsorption [16]. Similarly, the AC modified with ammonia gas at high temperature shows more than 20% improvement in adsorption capacity for phenol [17]. The enhancement of phenols adsorption is associated with the nitrogen-containing functional groups introduced into the carbon surface. It is speculated that pyridinic groups possibly relates to slight increase of phenol adsorption on the nitrogen-rich microporous AC compared with nitrogen-poor

300

G. Yang et al. / Applied Surface Science 293 (2014) 299–305

AC prepared from a mixture of polymer and coal-tar pitch [18]. However, the evidence is still weak and the contribution of different nitrogen-containing groups to the adsorption capacity of AC is not very clear. Therefore, further investigation into the effect of amination on the adsorption behavior is necessary. In the present work, nitrogen-containing functional groups were introduced to the AC structure by thermal treatment under ammonia atmosphere. The isotherm data for adsorption of phenol were fitted with the Langmuir and Freundlich models. The parameter qm /SSABET was created to discuss the correlation between adsorption capacity and nitrogen-containing groups.

2. Material and methods 2.1. Preparation of aminated ACs A wood-based AC (Xinsen Carbon Industry Co. Ltd., China) was crushed, sieved to 15–40 mesh, and then treated with 1 M HCl solution for six hours to remove metal ions. The acid-treated AC was washed with deionized water until circum-neutral pH of the supernatant and dried at 105 ◦ C for 24 h prior to use. This acid-treated AC was denoted as ACA. The aminated ACs were prepared by thermal treatment of ACA at 250, 450, 650, and 850 ◦ C for 2 h in a tube furnace. A mixed gas (ammonia and nitrogen) flowed through the quartz tube from start to end of aminated treatment. The flow rates of ammonia and nitrogen were 20 and 60 mL/min (room temperature), respectively. ACN250 represented ammonia treatment of ACA at 250 ◦ C. 2.2. Porosity of AC The porous properties of the AC samples were determined by N2 adsorption/desorption in a Builder SSA-4200 instrument. The AC samples were separately degassed at 300 ◦ C for 2.5 h in a vacuum environment before measurement. The specific surface area was determined by the BET equation, and the total pore volume was calculated from the near saturation uptake (P/P0 = 0.99). The mesopore volume, mesopore surface area, and pore size distribution were calculated by the BJH method. 2.3. Surface chemistry of AC Boehm titration was conducted to determine the number and type of surface oxygen groups [19]. 0.5 g of sample was placed into 25 mL of the solution (0.05 M of NaOH, Na2 CO3 , or NaHCO3 ) for 24 h to reach equilibrium. The acidic oxygen-containing surface groups were determined by back titration with 0.05 M HCl solution. The total content of basic groups was measured with 0.05 M HCl solution. pHpzc was the pH value when the charge on the AC surface was zero. The pHpzc was determined by the pH drift method. 50 mL of a 0.01 M KNO3 solution was adjusted to successive initial values (pHinitial ) between 2 and 12, by adding either HNO3 or KOH. Then 0.15 g AC was added to the KNO3 solution with a constant stirring at room temperature. The final pH, reached after about 48 h, was noted as pHfinal . The pHpzc is the point when pHinitial was equal to pHfinal . X-ray photoelectron spectroscopy (XPS) was performed on a Kratos XSAM800 equipped with Al K␣ X-ray source (1486.6 eV, anode operating at 12 kV, and 15 mA). The binding energies were calibrated based on the graphite C1s peak at 284.5 eV. The spectra was fitted with a Gaussian (80%)–Lorentzian (20%) mixed function after subtraction of a Shirley background using XPS Peak 4.1 software.

2.4. Adsorption of phenol Batch adsorption experiments were conducted at 30–50 ◦ C by agitating 0.1 g AC with 100 mL phenol solution of desired concentration (100–1000 mg/L) in a stoppered conical flask in a thermostat shaker at 150 rpm for 24 h. When reaching equilibrium, the phenol solution was separated from the adsorbent. The final concentration of phenol was analyzed using an ultraviolet spectrophotometer at a wavelength of 269 nm. The amount of uptake capacity at equilibrium, qe (mg/g), was calculated by Eq. (1) qe =

(C0 − Ce ) × V m

(1)

where qe (mg/g) is the amount adsorbed onto the AC at equilibrium, C0 and Ce (mg/L) are the initial and equilibrium concentrations of the solution, respectively, V (L) is the volume of the solution, and m (g) is the mass of AC. The effect of solution initial pH on phenol adsorption was studied over a pH range of 2–12 at 30 ◦ C. The pH was adjusted using 0.1 M HCl or 0.1 M NaOH. The phenol concentration was fixed at 1000 mg/L, with AC dosage of 0.1 g/100 mL. The kinetics studies were performed at 30 ◦ C, and solution pH was not adjusted. The initial concentration was set at 1000 mg/L, and the samples were separated at predetermined time intervals. 3. Results and discussion 3.1. Characterization of AC The results of porosity characterization are presented in Table 1. The wood-based AC (ACA) was a highly mesoporous carbon with large specific surface area (SSABET , 1618 m2 /g), pore volume (Vt , 1.06 cm3 /g) and high percentage of a mesoporous structure (35% for SSAmeso and 65% for Vmeso ). The higher the aminated temperature was, the lower the SSABET and pore volume were. At an amination temperature of 650 ◦ C, the SSABET and Vt of AC displayed slight reduction; and SSAmeso and Vmeso decreased to 507 m2 /g and 0.52 cm3 /g, respectively. When amination temperature reached 850 ◦ C, an obvious decrease in surface area and pore volume was observed. The results obtained from Boehm titration together with pHpzc are presented in Table 2. The amount of total acid of the aminated AC decreased with increasing amination temperature, and the concentration of carboxyl decreased from 0.349 to 0.082 meq/g. By contrast, a noticeable increase was observed in the concentration of total base with amination temperature increasing. The basic ACs were obtained by amination of the acid parent AC. This was confirmed by the pHpzc that increased from 5.80 for the parent AC to 9.78 for ACN850. The enhancement of alkalinity was attributed to the decrease of oxygen-containing acid groups and the formation of nitrogen-containing basic groups via oxygen-containing groups reacting with ammonia. The deconvoluted XP N1s spectra of the parent and aminated ACs are shown in Fig. 1, and the elemental composition is listed in Table 3. According to the spectra and references [20,21], the N1s was deconvoluted into five different types of N-containing species: N1 (pyridine, ∼398.7 eV), N2 (imine/amide/amine, ∼399.5 eV), N3 (pyrrolic, ∼400.1 eV), N4 (quaternary N, ∼401.3 eV), and N5 (Noxide, ∼403.1 eV). The surface nitrogen content of the aminated ACs increased with amination temperature. The highest content of total nitrogen (2.80%) was observed in ACN850. This showed that high amination temperature was favorable to the formation of nitrogen-containing functional groups on the AC surface. However, the concentration variation of specific nitrogen-containing groups was different from that of total nitrogen. When amination temperature exceeded 450 ◦ C, more N1 and N3 species formed on


301

Table 1 Porous properties of the parent and aminated ACs. AC

SSABET (m2 /g)

SSAmicro (m2 /g)

SSAmeso (m2 /g)

Vt (cm3 /g)

Vmeso (cm3 /g)

Vmicro (cm3 /g)

DBJH (Å)

ACA ACN250 ACN450 ACN650 ACN850

1618 1608 1581 1453 1233

1059 1044 1009 946 790

571 564 559 507 442

1.06 1.06 1.04 0.96 0.82

0.69 0.57 0.58 0.52 0.45

0.49 0.46 0.44 0.37 0.37

40.6 40.0 40.2 40.2 40.2

Table 2 Surface groups from the Boehm titration and pHpzc of the parent and aminated ACs. AC

Carboxy(meq/g)

Lactonic(meq/g)

Phenolic(meq/g)

Total acid(meq/g)

Total base(meq/g)

pHpzc


0.349 0.121 0.071 0.096 0.082

0.428 0.289 0.325 0.260 0.255

0.277 0.428 0.317 0.307 0.291

1.054 0.838 0.713 0.663 0.628

0 0.156 0.295 0.328 0.343

5.80 8.50 9.15 9.40 9.78

Table 3 Surface elementary compositions of the parent and aminated ACs derived from XP spectra (C + O + N = 100 at.%). AC

C

O

N

N1

N2

N3

N4

N5


92.26 92.10 92.13 91.76 90.45

7.39 6.87 6.58 6.41 6.75

0.35 1.03 1.29 1.83 2.80

0.00 0.26 0.20 0.62 1.22

0.00 0.33 0.56 0.00 0.00

0.18 0.24 0.38 0.85 1.11

0.00 0.19 0.15 0.36 0.47

0.17 0.00 0.00 0.00 0.00

the AC surface. Higher amination temperature was beneficial to the generation of N1 and N3 species. N2 type species appeared at low amination temperature (250 ◦ C), and reached maximum value of 0.56% at the amination temperature of 450 ◦ C. Carboxyl groups reacted with NH3 at low temperature to form amide-like species, whereas carboxylic anhydrides were formed via dehydration of two adjacent carboxyl groups and then reacted with NH3 to form N2 groups [14,20]. 3.2. Equilibrium adsorption of phenol Two extensively used models, the Langmuir and Freundlich models were used to describe the relationship between the adsorbate loading on the adsorbent and the liquid phase concentration of adsorbate at constant temperatures. The Langmuir equation was formulated as Eq. (2): qe =

qm KL Ce 1 + KL Ce

(2)

where qm (mg/g) is the maximum adsorption capacity to form a complete monolayer on the surface of adsorbate, and KL (L/mg) is the Langmuir constant. The Freundlich model was an empirical equation assuming heterogeneous adsorptive energies on the adsorbent surface: qe = KF Ce 1/n

(3)

where KF ((mg/g)(mg/L)−1/n ) is the Freundlich constant that represents the quantity of adsorbate adsorbed onto the adsorbent for a unit equilibrium concentration, and n is the dimensionless exponent of the Freundlich equation depicting an indication of how favorable the adsorption process. To evaluate the validity of adsorption equilibrium model, the linear correlation coefficients (R2 ) and the root mean square error (RMSE) were used. The RMSE was defined according to Eq. (4).

N (qe,exp − qe,cal )2

Fig. 1. Deconvoluted XP N1s spectra of the parent and aminated ACs.

RMSE =

1

N

(4)

302


Fig. 2. Langmuir (a) and Freundlich (b) adsorption isotherms for phenol on the parent and aminated ACs at 30 ◦ C (no adjusting pH).

where qe,exp (mg/L) is the adsorption capacity obtained from experiments, while qe,cal is the calculated adsorption capacity, and N is the number of experimental data points. Fig. 2 shows the Langmuir and Freundlich adsorption isotherms of phenol on ACs at 30 ◦ C. The Langmuir and Freundlich model parameters, R2 and RMSE are listed in Table 4. R2 was greater than 0.975 and RMSE varied from 1.58 to 4.18 in the Langmuir model, suggesting that the Langmuir model fitted well to the dada. Nevertheless, the value of RMSE in the Freundlich model was greater than that in the Langmuir model suggesting lower predicting accuracy with the Freundlich model. Therefore, the Langmuir model was reasonably applied in all cases. There was an optimal amination temperature for enhancement of phenol adsorption capacity of the aminated ACs. ACN650 showed the highest adsorption capacity of 243.47 mg/g, increasing more than 20% compared to the parent one. The adsorption capacity followed the sequence: ACN650 > ACN850 > ACN450 > ACN250 > ACA. However SSABET of the aminated ACs decreased with increasing amination temperature (Table 2). The relationship between SSABET and adsorption capacity suggested the nitrogen-containing functional groups introduced by amination played an important role in adsorption of phenol.

3.3. Effect of nitrogen-containing functional groups on adsorption capacity It is difficult to investigate the contribution of different nitrogencontaining groups to enhancement of adsorption capacity when the effects of the specific area and surface functional groups were considered simultaneously. Therefore, a parameter qm /SSABET that represented the adsorption amount per unit surface area was created to exclude the effect of surface area on adsorption capacity. The relationship between the amount of different nitrogen-containing groups (N1, N2, and N3) and qm /SSABET was fitted. The multiple correlation coefficient (R2 ) between the amount of N2 and qm /SSABET was only about 0.3. However, a higher R2 between N1 and qm /SSABET was observed (R2 > 0.90), and a much higher R2 between N3 and qm /SSABET was obtained (R2 > 0.98). Therefore, N1 and N3 species contributed much more to the enhancement of adsorption than N2 species. The relationship between N1 + N3 and qm /SSABET was fitted and shown in Fig. 3. qm /SSABET is linearly correlated with the amount of N1 (pyridine) and N3 (pyrrolic), and R2 was greater than 0.95 at adsorption temperatures of 30, 40, and 50 ◦ C. This linear relationship cannot be considered as conclusive quantitative judgment due to the complexity of adsorption. However, it suggested that pyridinic and pyrrolic N groups were the key

N-containing species for the enhancement of adsorption of phenol on the aminated ACs. Practically, two parallel mechanisms were necessary to be considered in adsorption of phenol: one was related to electrostatic force and the other to ␲–␲ dispersion force [22]. The former was reasonably applied to the solution in which adsorbate was dissociated. The latter mechanism arose from the interactions between the delocalized ␲ electrons in basal planes of AC and the ␲ electrons in the aromatic rings. In the case of phenol adsorption on the aminated ACs, phenol mainly existed as molecules in the adsorption conditions of Table 4; therefore the ␲–␲ dispersive interactions predominated. Nitrogen atoms in pyridinic and pyrrolic groups on the aminated AC supplied their p-electrons to the system of ␲conjugated ring, so the carbons with pyridinic and pyrrolic groups had higher electron density [23]. According to the ␲–␲ dispersion mechanism, the phenol adsorption can be considered as interaction of ␲-electrons between phenolic ring and graphitic structure of AC. Due to the higher electron density of the aminated AC, more phenol molecules adsorbed through ␲-electrons dispersion between phenolic ring and basal plane of the aminated AC. As the amination temperature increased, more pyridinic and pyrrolic N species were incorporated into the AC matrix (Table 3), leading to higher ␲-electrons density on the basal plane of the carbon. Since the stronger ␲–␲ dispersive interactions by these ␲-electrons, more phenol molecules were adsorbed on the AC aminated at

Fig. 3. Correlation between qm /SSABET and amount of N1 + N3 derived from XP N1s at 30, 40, and 50 ◦ C.


303

Table 4 Langmuir and Freundlich parameters for the adsorption of phenol on the parent and aminated ACs at 30 ◦ C. AC


Langmuir model

Freundlich model

qm (mg/g)

KL (L/mg)

R2

RMSE

KF ((mg/g)(L/mg)1/n )

n

R2

RMSE

198.66 202.72 204.49 243.47 228.13

0.0059 0.0063 0.0068 0.0095 0.0073

0.997 0.996 0.975 0.978 0.997

2.13 1.58 1.66 3.13 4.18

12.34 13.44 15.41 22.12 17.96

2.53 2.58 2.70 2.84 2.73

0.989 0.970 0.967 0.960 0.971

5.84 7.45 7.67 10.9 8.73

higher temperature. The delocalization of pyridinic and pyrrolic N was also reported where the aminated AC showed high catalytic activity [24,25]. The extra electrons in the nitrogen species were easily transferred to the adsorbed species and formed intermediates. 3.4. Effect of initial pH on phenol adsorption The adsorption of phenol was highly dependent on the initial pH of phenol solution that affected the surface charge of the aminated AC and the degree of ionization of phenol. The effect of initial pH on the adsorption capacity of ACs for phenol is shown in Fig. 4. A similar trend of pH effect was observed for the adsorption of phenol on the parent and aminated ACs. When the initial pH was lower than 7.0, the adsorption amount of phenol was almost constant; then a significant decrease was observed as the pH was higher than 7.0; finally a lowest adsorption capacity about 90 mg/g was observed at pH 11.2 for all the ACs. The pH effect can be explained from the difference in surface chemistry of ACs and properties of phenol in solution. In acidic solution, the undissociated species of phenol were high and the dispersion interactions predominated [26], which resulted in more phenol molecules adsorbed onto the surface of AC. As the initial pH increased, phenol molecules started to dissociate into phenol anions, the adsorption amount of phenol decreased. When pH >pKa of phenol (pH 9.98), the carbon surface was negatively charged when the initial pH was higher than pHpzc . Because of the electrostatic repulsion between phenol anions and AC surface as well as between phenolate–phenolate anions in solution, the adsorption capacity decreased to a minimum value [27]. Another reason for the decreased adsorption capacity was that phenol anion was more soluble in aqueous solution, and this led to stronger interactions between water and phenol anions that are more difficult to break before adsorption. [28–30].

As seen from Fig. 4, ACN650 and ACN850 displayed a steeper decrease in adsorption of phenol from pH 7.0 to 11.2 compared to the parent AC and ACN250. For the formers, the higher content of nitrogen species caused more charges that are negative when the initial pH of the solution was higher than 7.0, which induced intensive repulsion between the carbon and phenolate. This stronger repulsion resulted in a steeper decrease of phenol adsorption on the AC with higher nitrogen content. 3.5. Adsorption kinetics In the present work, the pseudo-first-order kinetic model, pseudo-second-order model, and intraparticle diffusion kinetic model were used to investigate the adsorption of phenol. To evaluate the goodness of fitting and suitability of the model, the linear correlation coefficient (R2 ) and normalized standard deviation (q) were used in kinetic model study. A higher R2 and lower q denoted better model fitting. The q was calculated as follows:

q(%) =

2

[(qexp − qcal )/qexp ] × 100 N−1

(5)

where N is the number of experimental data points, and qexp and qcal (mg/g) are the experimental adsorption capacity and calculated adsorption capacity, respectively. 3.5.1. Pseudo-first-order kinetic model The pseudo-first-order kinetic model is expressed as follows [31]: ln(qe − qt ) = ln qe − k1 t

(6)

where k1 (1/h) is the first-order rate constant, whereas, qe and qt (mg/g) are the amount of phenol adsorbed at equilibrium and at any time, respectively. According to the plot of ln(qe −qt ) versus t, the k1 and qe were calculated by the slope and intercept. The results of pseudo-first-order kinetic model are listed in Table 5. The R2 and q values ranged from 0.941 to 0.998 and from 18.82 to 36.58%, respectively. Considering the high value of q (18.82–36.58%), the pseudo-first-order kinetic model was therefore less likely to explain the adsorption behavior of ACs. 3.5.2. Pseudo-second-order kinetic model The pseudo-second-order model is described as [32]: 1 t t = + 2 qt q k2 qe e

Fig. 4. Effect of initial pH on phenol adsorption on the parent and aminated ACs at 30 ◦ C (phenol initial concentration = 1000 mg/L).

(7)

where k2 (g/mg h) is the second-order rate constant, and qe and qt (mg/g) are the amount of phenol adsorbed at equilibrium and at any time, respectively. The values of k2 and qe were calculated from the slope and intercept of the linear plot of t/qt versus t. The parameters of the pseudo-second-order kinetic model for the parent and aminated ACs are listed in Table 5. The pseudosecond-order kinetic model yielded lower q values ranging from 2.61 to 5.58% for phenol on the ACs. The R2 values derived from the second-order kinetic model were greater than 0.99. So, the

304


Table 5 Parameters of the pseudo-first-order model and pseudo-second-order model for adsorption phenol on the parent and aminated ACs at 30 ◦ C. AC


qe , exp (mg/g)

164.85 165.35 166.37 212.47 187.74

Pseudo-first-order kinetic model

Pseudo-second-order kinetic model

qe , cal (mg/g)

k1 (1/h)

R2

q(%)

qe , cal (mg/g)

k2 (g/mg h)

R2

q(%)

115.90 132.16 97.39 130.53 114.24

0.770 1.089 0.741 0.658 0.805

0.988 0.998 0.941 0.987 0.974

31.72 18.82 22.91 36.58 35.14

178.57 175.44 175.44 227.27 200.00

0.011 0.015 0.013 0.009 0.012

0.998 0.998 0.999 0.999 0.999

3.85 5.58 2.91 2.68 2.61

Table 6 Intraparticle diffusion model constants and correlation coefficients for adsorption of phenol on the parent and aminated ACs at 30 ◦ C. A0C

kd1 (mg/gh1/2 )

R1 2

kd2 (mg/gh1/2 )

C2

R2 2

kd3 (mg/gh1/2 )

C3

R3 2

q (%)


110.69 125.25 128.67 151.46 141.80

0.983 0.997 0.987 0.961 0.990

44.53 43.71 44.16 71.79 54.41

77.39 84.73 81.94 74.97 85.97

0.978 0.971 0.950 0.994 0.987

3.214 3.025 9.405 13.217 6.554

155.61 157.62 141.77 175.97 169.85

0.758 0.978 0.762 0.963 0.887

3.10 1.37 2.44 2.34 1.46

pseudo-second-order model was the suitable equation to describe the adsorption kinetics of phenol on the ACs. The fact that the kinetics of phenol adsorption on the AC follows the pseudo-second-order kinetic models suggested phenol adsorption probably occurred via sharing of electrons between the phenolic ring and basal plane of AC [33–36]. When nitrogen atoms were inserted into AC, higher ␲-electron density was generated in basal plane of the aminated AC due to the delocalization of free electrons in the pyridinic and pyrrolic N [23]. So, more phenol molecules were adsorbed onto the basal plane of the aminated AC by ␲-electron dispersion. 3.5.3. Intraparticle diffusion kinetic model The intraparticle diffusion kinetic model proposed by Weber and Morris was widely applied to study the adsorption process to predict the rate-controlling step [37]. The rate constant of intraparticle diffusion (kdi ) at the stage i was given by the equation: qt = kdi t 1/2 + Ci

(8) t1/2

where qt is the amount of phenol adsorbed on AC, is the square root of adsorption time, and Ci is the intercept at different stage. The Ci was related to the thickness of the boundary layer. The larger Ci indicated the greater effect of the boundary layer on diffusion. If the

qt versus t1/2 was linear, intraparticle diffusion was involved and if the line passed through the origin, then the intraparticle diffusion was the only rate limiting process [38]. Otherwise, the rate limiting process included other mechanisms besides intraparticle diffusion. The parameters of the intraparticle diffusion model are given in Table 6. Considering that the values of R1 2 and R2 2 were greater than 0.95 and q was low (q < 3.1), the adsorption kinetics data fitted with the intraparticle diffusion model was reasonable. The curvefitting plots of the intraparticle diffusion model are demonstrated in Fig. 5. The intraparticle diffusion plots contained three regions: the sharper stage, the second stage and the third stage. For the parent and aminated ACs, the sharper stage was completed within the first 1 h. This stage was owing to external surface adsorption or instantaneous adsorption. The second stage (from 1.0 to 3.0 h) was the gradual adsorption stage attributable to intraparticle diffusion. The last stage was the final equilibrium stage, where intraparticle diffusion started to slow down. The second and third stage did not pass through the origin, which indicated intraparticle diffusion was not the only rate-controlling step in the adsorption process. 4. Conclusions The ammonia treatment significantly improved the adsorption capacity for phenol on AC, and the AC aminated at 650 ◦ C exhibited the highest uptake. The enhancement of adsorption capacity was attributed to the introduction of nitrogen-containing functional groups by amination. Among these nitrogen-containing groups, pyridinic and pyrrolic groups played an important role in adsorption of phenol: the free electrons in pyridinic and pyrrolic N species delocalized to the basal plane leading to strong interaction between the aminated AC and phenol. pH effect on adsorption was also related to the amount of the introduced nitrogen-containing groups. The pseudo-second-order kinetic model indicated phenol adsorption onto the aminated AC involving the sharing of ␲ electrons between the phenolic ring and basal plane, and intraparticle diffusion was one of the rate-controlling steps in the process. Acknowledgements

Fig. 5. Intraparticle diffusion model for adsorption of phenol on the parent and aminated ACs at 30 ◦ C.

The authors would like to acknowledge the financial support provided by the National Key Scientific and Technology Project for Water Pollution Treatment of China (2008ZX07208-004-1), Youth Foundation of Sichuan Province No. (2012JQ0042) and Sichuan University of Science and Engineering Scientific Research Project (2012KY06).


References [1] H.H.P. Fang, O.C. Chan, Toxicity of phenol towards anaerobic biogranules, Water Res. 31 (1997) 2229–2242. [2] R.K. Singh, S. Kumar, A. Kumar, Development of parthenium based activated carbon and its utilization for adsorptive removal of p-cresol from aqueous solution, J. Hazard. Mater. 155 (2008) 523–535. [3] M. Radhika, K. Palanivelu, Adsorptive removal of chlorophenols from aqueous solution by low cost adsorbent—kinetics and isotherm analysis, J. Hazard. Mater. 138 (2006) 116–124. [4] J. Yin, R. Chen, Y. Ji, C. Zhao, G. Zhao, H. Zhang, Adsorption of phenols by magnetic polysulfone microcapsules containing tributyl phosphate, Chem. Eng. J. 157 (2010) 466–474. [5] M.A. Farajzadeh, M.R. Fallahi, Study of phenolic compounds removal from aqueous solution by polymeric sorbent, J. Chin. Chem. Soc. 52 (2005) 295–301. ´ atkowski, ˛ [6] A. Deryío-Marczewska, J. Goworek, A. Swi B. Buczek, Influence of differences in porous structure within granules of activated carbon on adsorption of aromatics from aqueous solutions, Carbon 42 (2004) 301–306. [7] T.J. Bandosz, Effect of pore structure and surface chemistry of virgin activated carbons on removal of hydrogen sulfide, Carbon 37 (1999) 483–491. [8] F. Caturla, J.M. Martinmartinez, M. Molinasabio, F. Rodriguezreinoso, R. Torregrosa, Adsorption of substituted phenols on activated carbon, J. Colloid Interf. Sci. 124 (1988) 528–534. [9] C. Moreno-Castilla, Adsorption of organic molecules from aqueous solutions on carbon materials, Carbon 42 (2004) 83–94. [10] G.D. Sheng, D.D. Shao, X.M. Ren, X.Q. Wang, J.X. Li, Y.X. Chen, X.K. Wang, Kinetics and thermodynamics of adsorption of ionizable aromatic compounds from aqueous solutions by as-prepared and oxidized multiwalled carbon nanotubes, J. Hazard. Mater. 178 (2010) 505–516. [11] S. Liu, R. Wang, Modified activated carbon with an enhanced nitrobenzene adsorption capacity, J. Porous Mater. 18 (2011) 99–106. [12] B. Li, Z. Lei, X. Zhang, Z. Huang, Adsorption of simple aromatics from aqueous solutions on modified activated carbon fibers, Catal. Today 158 (2010) 515–520. [13] L. Velasco, C. Ania, Understanding phenol adsorption mechanisms on activated carbons, Adsorption 17 (2011) 247–254. [14] R. Arrigo, M. Haevecker, S. Wrabetz, R. Blume, M. Lerch, J. McGregor, E.P.J. Parrott, J.A. Zeitler, L.F. Gladden, A. Knop-Gericke, R. Schloegl, D.S. Su, Tuning the acid/base properties of nanocarbons by functionalization via amination, J. Am. Chem. Soc. 132 (2010) 9616–9630. [15] F. Shaarani, B. Hameed, Ammonia-modified activated carbon for the adsorption of 2,4-dichlorophenol, Chem. Eng. J. 169 (2011) 180–185. [16] G.G. Stavropoulos, P. Samaras, G.P. Sakellaropoulos, Effect of activated carbons modification on porosity, surface structure and phenol adsorption, J. Hazard. Mater. 151 (2008) 414–421. [17] J. Przepiorski, Enhanced adsorption of phenol from water by ammonia-treated activated carbon, J. Hazard. Mater. 135 (2006) 453–456. [18] E. Lorenc-Grabowska, G. Gryglewicz, M.A. Diez, Kinetics and equilibrium study of phenol adsorption on nitrogen-enriched activated carbons, Fuel 114 (2013) 235–243. [19] I.I. Salame, T.J. Bandosz, Surface chemistry of activated carbons: combining the results of temperature-programmed desorption, Boehm, and potentiometric titrations, J. Colloid Interf. Sci. 240 (2001) 252–258. [20] S. Kundu, W. Xia, W. Busser, M. Becker, D.A. Schmidt, M. Havenith, M. Muhler, The formation of nitrogen-containing functional groups on carbon nanotube

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31] [32] [33] [34] [35]

[36]

[37]

[38]

305

surfaces: a quantitative XPS and TPD study, Phys. Chem. Chem. Phys. 12 (2010) 4351–4359. H. Wang, R. Côté, G. Faubert, D. Guay, J.P. Dodelet, Effect of the pre-treatment of carbon black supports on the activity of Fe-based electrocatalysts for the reduction of oxygen, J. Phys. Chem. B 103 (1999) 2042–2049. L.R. Radovic, I.F. Silva, J.I. Ume, J.A. Menéndez, C.A.L.Y. Leon, A.W. Scaroni, An experimental and theoretical study of the adsorption of aromatics possessing electron-withdrawing and electron-donating functional groups by chemically modified activated carbons, Carbon 35 (1997) 1339–1348. V.V. Strelko, V.S. Kuts, P.A. Thrower, On the mechanism of possible influence of heteroatoms of nitrogen, boron and phosphorus in a carbon matrix on the catalytic activity of carbons in electron transfer reactions, Carbon 38 (2000) 1499–1503. ´ G.S. Szymanski, T. Grzybek, H. Papp, Influence of nitrogen surface functionalities on the catalytic activity of activated carbon in low temperature SCR of NOx with NH3 , Catal. Today 90 (2004) 51–59. H. Chen, G. Yang, Y. Feng, C. Shi, S. Xu, W. Cao, X. Zhang, Biodegradability enhancement of coking wastewater by catalytic wet air oxidation using aminated activated carbon as catalyst, Chem. Eng. J. 198–199 (2012) 45–51. Y.M. Tzou, S.L. Wang, J.C. Liu, Y.Y. Huang, J.H. Chen, Removal of 2,4,6trichlorophenol from a solution by humic acids repeatedly extracted from a peat soil, J. Hazard. Mater. 152 (2008) 812–819. O. Hamdaoui, E. Naffrechoux, Modeling of adsorption isotherms of phenol and chlorophenols onto granular activated carbon: part II. Models with more than two parameters, J. Hazard. Mater. 147 (2007) 401–411. R.W. Coughlin, F.S. Ezra, I.I. Salame, T.J. Bandosz, Role of surface acidity in the adsorption of organic pollutants on the surface of carbon, Environ. Sci. Technol. 264 (2003) 307–312. O.P. Mahajan, C. Moreno-castilla, P.L. Walker, Surface-treated activated carbon for removal of phenol from water, Sep. Sci. Technol. 15 (1980) 1733–1752. ˜ D.M. Nevskaia, A. Santianes, V. Munoz, A. Guerrero-Ruíz, Interaction of aqueous solutions of phenol with commercial activated carbons: an adsorption and kinetic study, Carbon 37 (1999) 1065–1074. E. Tütem, R. Apak, Ç.F. Ünal, Adsorptive removal of chlorophenols from water by bituminous shale, Water Res. 32 (1998) 2315–2324. X. Yang, B. Al-Duri, Kinetic modeling of liquid-phase adsorption of reactive dyes on activated carbon, J. Colloid Interf. Sci. 287 (2005) 25–34. Y. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. B.H. Hameed, Equilibrium and kinetics studies of 2,4,6-trichlorophenol adsorption onto activated clay, Colloids Surf., A 307 (2007) 45–52. M.J. Jiménez-Cedillo, M.T. Olguín, C. Fall, Adsorption kinetic of arsenates as water pollutant on iron, manganese and iron–manganese-modified clinoptilolite-rich tuffs, J. Hazard. Mater. 163 (2009) 939–945. E. Bulut, M. Özacar, I˙ .A. S¸engil, Equilibrium and kinetic data and process design for adsorption of congo red onto bentonite, J. Hazard. Mater. 154 (2008) 613–622. Y. Li, Q. Du, T. Liu, J. Sun, Y. Jiao, Y. Xia, L. Xia, Z. Wang, W. Zhang, K. Wang, H. Zhu, D. Wu, Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto graphene, Mater. Res. Bull. 47 (2012) 1898–1904. W.H. Cheung, Y.S. Szeto, G. McKay, Intraparticle diffusion processes during acid dye adsorption onto chitosan, Bioresour. Technol. 98 (2007) 2897–2904.