Preparation of Enteromorpha prolifera-based cetyl trimethyl ammonium bromide-doped activated carbon and its application for nickel(II) removal

Ecotoxicology and Environmental Safety 104 (2014) 254–262

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Preparation of Enteromorpha prolifera-based cetyl trimethyl ammonium bromide-doped activated carbon and its application for nickel(II) removal Man Wang, Fang Hao, Gang Li, Ji Huang, Nan Bao, Lihui Huang n Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, China

art ic l e i nf o

a b s t r a c t

Article history: Received 19 August 2013 Received in revised form 16 January 2014 Accepted 30 January 2014 Available online 15 April 2014

Activated carbon was prepared from Enteromorpha prolifera (EP) by H3PO4 activation in the presence of doped cetyl trimethyl ammonium bromide (CTAB), producing EPAC–CTAB. The thermal decomposition process of the activated carbon substrate was identified by thermo-gravimetric analysis. Scanning electron microscope (SEM), N2 adsorption/desorption, Fourier transform infrared spectroscopy (FTIR), Boehm titration, and X-ray Photoelectron Spectroscopy (XPS) were employed to characterize the physicochemical properties of native EPAC and EPAC–CTAB. EPAC–CTAB exhibited smaller surface area (689.0 m2/g) and lower total pore volume (0.361 cm3/g) than those of EPAC (1045.8 m2/g and 1.048 cm3/ g), while the number of acidic groups, oxygen and nitrogen groups on the surface of EPAC–CTAB increased through CTAB doping. The batch kinetics and isotherm adsorption studies of nickel(II) onto the adsorbents were examined and agreed well with the pseudo-second-order model and the Langmuir model. The maximum adsorption capacity determined from the Langmuir model was 16.9 mg/g for EPAC and 49.8 mg/g for EPAC–CTAB. Under acidic condition, the adsorption of nickel(II) onto EPAC and EPAC– CTAB was hindered due to ion competition and electrostatic repulsion. The results indicated that using CTAB as a dopant for EPAC modification could markedly enhance the nickel(II) removal. & 2014 Published by Elsevier Inc.

Keywords: Enteromorpha prolifera Cetyl trimethyl ammonium bromide Activated carbon Nickel(II)

1. Introduction Metallic nickel (Ni) and its compounds are detrimental to humans and ecological environment because they are toxic, nonbiodegradable and accumulative through the food chain (Ghaedi et al., 2011; Kwon and Jeon, 2013; Lu et al., 2009). They are widely present in effluents of stainless steel and nickel alloys, foundries, batteries, electroplating, ceramics, and pigments industries (Srivastava et al., 2006). Nickel is regarded as a powerful carcinogenic agent that exists as Ni2 þ in aquatic systems (Borba et al., 2006). Thus, development of effective methods for Ni2 þ removal from these industries effluents is of great interest. Activated carbon (AC) has emerged to be a good adsorbent attributed to its large specific surface area, favorable functional groups and prominent adsorption capacity (Arami-Niya et al., 2010; Babel and Kurniawan, 2003), and adsorption by ACs has been proven to be a promising alternative for Ni2 þ removal (Basso et al., 2002; Demirbaş et al., 2002; Lam et al., 2007).

n

Corresponding author. Fax: þ86 531 88364513. E-mail addresses: [email protected], [email protected] (L. Huang). http://dx.doi.org/10.1016/j.ecoenv.2014.01.038 0147-6513 & 2014 Published by Elsevier Inc.

To enhance the capability of AC to adsorb heavy metals, many functional/surface modification methods have been introduced, including chemical treatment, physical treatment and surface modification (Mu and Tang, 2002; Nadeem et al., 2006). Among these methods, various surfactants were used to modify the surface chemical properties (surface charge and surface functional groups) of AC to substantially boost it adsorptive capacity for heavy metal ions (Ahn et al., 2009; Choi et al., 2009; Nadeem et al., 2009). Since surfactants are amphiphilic with both hydrophobic and hydrophilic groups, making them adsorb onto AC. However, the surfactant-modified AC was traditionally obtained by impregnating in surfactant-containing solutions. To our best knowledge, use of cetyl trimethyl ammonium bromide (CTAB), a kind of cation surfactant, as a dopant to modify AC has not been studied thus far. Many inexpensive raw materials have been employed as precursors for preparation of AC, such as palm shells (Lim et al., 2010), sawdust (Gong et al., 2005), kenaf (Cuerda-Correa et al., 2008) and waste tea (Yagmur et al., 2008). Enteromorpha prolifera (EP) is a widely available marine algae, which is rich in functional groups consisting of hydroxyl, carboxyl, and sulfate (Li et al., 2011). Due to these characteristics, it is advantageous to use EP biomass residues as the precusor for producing AC, beneficial use of EP can also reduce waste and alleviate water-pollution.

M. Wang et al. / Ecotoxicology and Environmental Safety 104 (2014) 254–262

In this study, activated carbons were derived from EP by H3PO4 activation with or without CTAB, and were denoted as EPAC–CTAB and EPAC, respectively. The physicochemical properties, such as morphology, porosity and surface characteristics of EPAC–CTAB and EPAC were systematically investigated. Batch adsorption experiments were conducted under various conditions (contact time, initial concentration, solution pH and ionic strength) to gain insights into the adsorption mechanisms of Ni2 þ onto EPAC and EPAC–CTAB, and to explore the use of doped CTAB for performance enhancement.

2. Materials and methods 2.1. Materials Enteromorpha prolifera (EP) was collected from Qingdao city, Shandong province in east China. All the chemical reagents used were of analytical grade. Distilled water was used to prepare all the experimental solutions. Stock solution of standard reagent was prepared by dispersing nickel chloride hexahydrate (NiCl2  6H2O) in distilled water. 2.2. Preparation of Enteromorpha prolifera derived activated carbons EP was rinsed with distilled water to remove impurities, and dried at 80 1C for 24 h. EP was then grinded and sieved into particle sizes of less than 0.25 mm. About 5 g of EP was immersed in H3PO4 solution (40 wt%) at an EP/H3PO4 ratio of 1/2 (w/ w) with or without doping 2 mmol cetyl trimethyl ammonium bromide (CTAB). The varied amounts of CTAB were studied from 0.5 mmol to 16 mmol to determine the optimized preparation conditions. Then the mixtures were dried at 80 1C for 9 h to evaporate moisture, switched to a muffle furnace and heated at 450 1C and maintained for 1 h. After being cooled down naturally, the produced carbonization materials were washed with hot water and distilled water for several times until pH of supernatant liquid was close to neutral, followed by filtration, drying at 105 1C for 8 h and crushing with a grinder. Fine powders with sizes in the range 0.075–0.109 mm (140–200 mesh) with standard sieves were collected and stored in a desiccator for subsequent analysis. Activated carbons prepared from EP by impregnation of H3PO4 with or without CTAB were denoted as EPAC–CTAB and EPAC, respectively.

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the elemental composition and binding state of activated carbons were analyzed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) equipped with an Mg Ka X-ray source. The CasaXPS software was employed to subtract the Shirley background, analyze the composition and deconvolute the XPS peaks. The C 1s peak position located at 284.5 eV was used as calibration to position the other peaks.

2.5. Equilibrium adsorption experiments Adsorption kinetic experiments were conducted in 1-L beakers and the concentration of Ni2 þ solution was chosen as 30 mg/L. The breakers were placed on electromagnetic stirring plates (HJ-3, Jintan Medical Instrument Corporation) at 20 1C with a speed of 125 rpm. Once the two adsorbents at the specified concentration of 0.7 g/L were added, time measurements began. Solution samples were extracted at pre-specified time intervals, filtered by a 0.45 μm millipore membrane filter, and then the resulted filtrate concentrations were measured by a UV–visible spectrophotometer (UV-754, Shanghai, China) at maximum wavelength of 530 nm (China Bureau of Environmental Protection, Water and Wasterwater Monitor and Analysis Method, fourth ed., China Environmental Science Press, Beijing, 2004.) In the process of extraction, water and small molecules were allowed to pass through millipore membrane, which were driven by the pressure difference on both sides of the membrane, while large-size substances are prevented in the side of the membrane. The effects of initial concentration, pH and ionic strength on Ni2 þ adsorption onto adsorbents were conducted in a batch of adsorption experiments. To initiate the experiments, 0.035 g of EPAC or EPAC–CTAB was added to a series of parallel 100-mL conical flasks that contained 50 mL of Ni2 þ solutions (30 mg/L). Afterwards, the capped flasks were agitated in an isothermal water-bathing shaker with a speed of 125 rpm at 20 1C for 24 h until the equilibrium was reached, followed by filtration and analysis as described earlier. The pH levels were adjusted to 2–8 by 0.1 M HCl and 0.1 M NaOH solutions and measured with a pH meter (pHS-3C, Shanghai, China). The adsorption experiments were also carried out under various initial concentrations (20–60 mg/L) and different ionic strengths (0, 0.1 and 0.5 M NaCl). The Ni2 þ solutions without adsorbents were also used as the control condition. All the experiments were run in duplicate. The amount of Ni2 þ uptake, q (mg/g), and the removal efficiency (removal percent) were calculated according to the following equations: q¼

ðc0  cÞv w

removal percent ¼

ð1Þ c0  c c

ð2Þ 2þ

2þ

2.3. Thermo-gravimetric—differential thermal analysis (TG-DTA)

where c0 is the initial Ni concentration (mg/L); c is the Ni concentration at time t or at equilibrium (mg/L); v represents the solution volume (L) and w is the mass of the adsorbent used (g).

The thermal behavior of the EP after H3PO4 or the mixture of H3PO4 and CTAB impregnation (EP–H3PO4 and EP–H3PO4–CTAB) was evaluated by simultaneous thermo-gravimetric (TG) and differential thermal analysis (DTA) using a thermogravimetric analyzer (TGA-50 analyzer). The two samples were heated from ambient to 450 1C and maintain at that temperature for 1 h, with a heating rate of 10 1C/min under a nitrogen stream (100 mL/min).

3. Results and discussion 3.1. Thermo-gravimetric-differential thermal analysis EP–H3PO4 and EP–H3PO4–CTAB

2.4. Characterization methods The pore structures of EPAC and EPAC–CTAB were characterized nitrogen adsorption/desorption isotherms at 77 K by a surface area analyzer (QUADRASORB SI, Quantachrome Corporation, USA). The specific surface area (SBET) and pore size distributions of activated carbons were calculated by using the Brunauer–Emmett– Teller (BET) model and the Density Function Theory (DFT) method. The total pore volume (Vt) was determined from the amount of nitrogen absorbed at P/P0 around 0.95. The external surface area (Sext), the micropore surface area (Smic) and the micropore volume (Vmic) were obtained by the t-plot method. The mean pore diameter (Dp) was calculated by Dp ¼ 4Vt/SBET. The surface textures of the two carbons were observed by using a scanning electron microscope (S4800, SEM Hitachi, Japan). The surface chemical characteristics were investigated using a Fourier transform infrared radiation (FTIR) spectrometer (Fourier-380 FTIR, USA) with a spectra range of 400–4000 cm  1. The pH at the point of zero charge (pHpzc) was defined as the pH at which the zeta potential on adsorbent surface was zero, which was determined from batch equilibrium method investigated elsewhere (Ghaedi et al., 2012). Boehm titration was one of most widely used methods that differentiate and quantify the amount of acidic groups on the carbon surface (Boehm, 1994). A series of 0.25 g of carbon samples were placed in 50 mL of the following solution: NaOH (0.1 N), Na2CO3 (0.1 N), NaHCO3 (0.1 N), and then the mixtures were placed in a constant temperature water bath (SHA-B, Shanghai, China) at 20 1C with an agitation speed of 125 rpm for 48 h. The method was based on the assumption that NaOH neutralized carboxyl, lactone and phenolic groups, Na2CO3 neutralized carboxyl and lactone, and NaHCO3 only neutralized carboxyl groups. In addition,

In the thermal process, the role of H3PO4, as an activation agent, has important functions in facilitating formation of activated carbon. It promotes depolymerization and dehydration, and particle swelling of the starting material (Liu et al., 2012a). Fig. 1 (a) and (b) illustrates the TG and DTA curves of EP–H3PO4 and EP– H3PO4–CTAB. The TG and DTA curves showed the similarities of samples during the first two stages of the thermal process. The initial weight loss (WL) of ten percent occurred below 100 1C, which was likely attributable to evaporation of volatile compounds and moisture. The second WL of 22 percent for EPAC and 27.5 percent for EPAC–CTAB in the 100–360 1C range and a sharp WL at about 150 1C were caused by H3PO4 accelerating the bond cleavage reactions accelerated by H3PO4, leading to hydrolysis of lignocelluloses and formation of cross-linked porous structure between organic matters and the acid (Suárez-Garcıá et al., 2004). As for EPAC, the apparent WL of 8 percent at 360–450 1C may ascribe further decomposition of the lignocellulosic materials. As for EPAC–CTAB, WL of 7 percent at 360–425 1C with a maximal WL rate at 405 1C and WL of 15.5 percent at 425–450 1C were likely due to decomposition of the organic substances facilitated by CTAB.

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450

100

10%

400

90

40%

22%

300 250

70

200

27.5% 7%

60

150

50

Temperature (°C)

Weight (%)

350 80

100

15.5%

50

40 0

10

20

30

40

50

60

70

80

90

0 100

Time (min)

3.4. Adsorption kinetics

-0.01 450 0.00

360°C 400 100°C

350

0.02

300

0.03

250 200

0.04 150

Temperature (°C)

dw/dt (%/min)

0.01

0.05 403°C

100

450°C

0.06

50 150°C

0.07 0

10

20

30

40

50

60

functional groups than EPAC, which implied that several functional groups of EPAC–CTAB emerged in the activation process. The bands located at about 3420, 1558, 1416, 1180 cm  1 in the FTIR spectra of both carbons correspond to the –OH (hydroxyl groups), C¼ O (carboxyl or lactone groups), O–H (carboxylic groups) and C–O (ester groups), respectively (Liu et al., 2011). For EPAC–CTAB, the bands detected at 1341–1308 cm  1 could be attributed to C–N, and the band at 1043 cm  1 to C–O in phenol. The results of Boehm titration for amounts of surface acidic groups of EPAC and EPAC–CTAB are listed in Table 1. As shown, there were more acidic functional groups in EPAC–CTAB than in EPAC, revealing that doped CTAB could introduce more acidic groups, which corresponded well with the results of the FTIR spectra.

70

80

90

0 100

Time (min) Fig. 1. TG (a) and DTA (b) curves for the pyrolysis of EPAC and EPAC–CTAB.

3.2. Surface morphology and textural structure The surface morphological structure of EPAC and EPAC–CTAB are depicted in Fig. 2(A) (100  and 1000  magnification). Surfaces of the carbons showed well-developed porous structure and had distinct cavernous porosity. It could also be seen that the surface of the EPAC was quite irregular and rough, while EPAC– CTAB had a smooth surface. The N2 adsorption/desorption isotherms and pore size distribution of EPAC and EPAC–CTAB are shown in Fig. 2(B). According to the International Union of Pure and Applied Chemistry (IUPAC) classification (Ryu et al., 1999), a large proportion of physisorption isotherms may be divided into the six different types of pore structure. It could be seen from Fig. 2(B) that the nitrogen isotherms for both adsorbents fitted the type IV curve with a hysteresis loop, indicating that the capillary of mesopores was agglomerated. The distributions of pore sizes for both adsorbents showed that the majority pore sizes are located in region of 0–10 nm, illustrating that the two adsorbents were essentially micro-mesoporous structures. The structure parameters of EPAC and EPAC–CTAB, including SBET, pore area, pore volume and Dp, are summarized in Table 1. As shown, EPAC–CTAB had much lower values of SBET (689.0 m2/g) and Vt (0.361 cm3/g) than those of EPAC (1045.8 m2/g and 1.048 cm3/g). The Vmic/Vt values of EPAC and EPAC–CTAB were 25.57 percent and 43.17 percent, indicating that EPAC–CTAB had a larger fraction of micropores. 3.3. Surface chemistry of activated carbons The FTIR spectra (400–4000 cm  1) of EPAC and EPAC–CTAB are shown in Fig. 2(C). They showed that EPAC–CTAB contained more

Fig. 3(a) shows the effect of contact time on the adsorption of Ni2 þ onto the EPAC and EPAC–CTAB. It illustrated that almost 80– 90 percent of the adsorption capacity was achieved within the second hour of contact for both adsorbents. The adsorption increased gradually at the initial stage, and then fell off as time proceeded. It was clear that adsorption capacity of Ni(II) onto EPAC–CTAB (42.8 mg/g) was twice more than that on EPAC (16.0 mg/g). To have a better understanding of the adsorption mechanism and attain reasonable equilibrium time, the experimental kinetic data for adsorption Ni2 þ onto EPAC and EPAC–CTAB were examined by applying three commonly used theoretical models: pseudo-first-order (Miyake et al., 2013; Zeinali, et al.), pseudosecond-order (Ghaedi et al.; Ho and McKay, 1999), and intraparticle diffusion (Kavitha and Namasivayam, 2007). The equations of the three models are shown below lnðqe  qt Þ ¼ ln qe  k1 t

ð3Þ

t 1 t ¼ þ qt k2 qe 2 qe

ð4Þ

qt ¼ kpi t 1=2 þ C i

ð5Þ

where k1 is the pseudo-first-order adsorption rate constant (mg/ (g min)); k2 is the rate constant of the pseudo-second-order (g/ (mg min)); kpi represents the intra-particle diffusion rate constant in stage i (mg/(g min1/2)); Ci refers to the thickness of boundary layer; qt and qe denote the amount of Ni2 þ absorbed at time t and equilibrium (mg/g), respectively. The correlation coefficients (R2) and parameters from the two models are listed in Table 2. As shown, the pseudo-second-order model fitted with the experimental kinetics data well with high correlation coefficients (R2 40.999), compared to the pseudo-firstorder model. The calculated qe (qe,cal) from the pseudo-secondorder model (16.07 mg/g) was much closer to the experimental data (qe,exp) (16.04 mg/g). The pseudo-second-order model assumes that the adsorption process is influenced by chemical adsorption (Ho and McKay, 1999), which is in accordance with electrostatic interaction or cation exchange mechanism proposed later. The intra-particle diffusion model is used to further delineate the adsorption kinetic mechanisms and determine the rate limiting steps. The intra-particle diffusion model for adsorption is multi-linearity, revealing that there are two or more steps occurred in the process (McKay et al., 1985). Fig. 3(b) showed that the first stage can be mainly ascribed to a large quantity of Ni2 þ diffusion from the solution to exterior surface of the two adsorbents. The second stage represented the adsorption rate was gradually lower. The third stage was perceived as the final

M. Wang et al. / Ecotoxicology and Environmental Safety 104 (2014) 254–262

257

0.35

a2

0.30 0.25

3

Pore volume (cm /nm/g)

a1

0.20 0.15 0.10

EPAC EPAC-CTAB

0.05 0.00 0

5

10

15

20

Pore width (nm)

b1

b2

95

876

745 90

Transmittance %

85

1341 1308

80 75 70

3420 1043

EPAC EPAC-CTAB

65

1414 1179

60

1560

55 400

800

1200

1600

2000

2400

2800

3200

3600

4000

-1

Wavenumber (cm )

Fig. 2. (A) SEM micrographs (left: 100  ; right: 1000  magnification) of EPAC (a1, a2) and EPAC–CTAB (b1, b2), (B) Pore size distributions and nitrogen adsorption/ desorption isotherms (inset) for EPAC and EPAC–CTAB and (C) FTIR spectra of EPAC and EPAC–CTAB.

Table 1 Porous structure parameters and amounts of surface functional groups of EPAC and EPAC–CTAB. Samples

SBETa (m2/g)

Sextb (m2/g)

Smicc (m2/g)

Vmicd (cm3/g)

Vte (cm3/g)

Vmic/Vt (percent)

Dpf (nm)

Carboxyl (mmol/g)

Lactone (mmol/g)

Phenolic (mmol/g)

Total acidic (mmol/g)

EPAC EPAC– CTAB

1045.8 689.0

547.7 281.2

498.1 407.8

0.268 0.156

1.048 0.361

25.57 43.17

4.01 2.10

0.063 0.219

0.541 0.468

0.583 1.217

1.187 1.904

a

BET surface area. external surface area. micropore surface area. d micropore volume. e total pore volume. f mean pore diameter. b c

equilibrium due to very few Ni2 þ ions left in the solution with a decrease in the pore space and an increase in electrostatic repulsion on the adsorbent surface (Ghaedi et al., 2013). The related parameters including kpi, Ci and R2 of these corresponding stages are summarized in Table 2. The Ci increased and the kpi decreased as the adsorption proceeded, which implied that intra-particle diffusion played a significant part in the adsorption progress.

The Langmuir isotherm model is characterized by a steep rise that approaches a plateau because of the formation of a monolayer. This empirical model can be applied to describe homogeneous adsorption and it assumes: (1) monolayer adsorption occurred at limited specific homogenous sites and (2) no force between the adsorbed molecules exists and (3) the reaction is dynamic (Langmuir, 1916; Roosta et al., 2014b). The linear form of the Langmuir isotherm is shown below (Roosta et al., 2014a)

3.5. Effects of initial concentration and ionic strength on Ni2 þ adsorption

ce 1 1 ¼ þ ce qe qm kL qm

Two widely employed models, the Langmuir and Freundlich isotherms, are selected to analyze the experimental data for the adsorption of Ni2 þ onto EPAC and EPAC–CTAB.

ð6Þ

where ce denotes the equilibrium Ni2 þ concentration (mg/L); qe is the amount of Ni2 þ adsorbed at equilibrium (mg/g); qm represents the maximum adsorption capacity (mg/g); kL is the Langmuir constant (L/mg).

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16

50

15 40

14 13

qe (mg/g)

qt (mg/g)

30

20

12 11 10

10

EPAC EPAC-CTAB

0M 0.1 M 0.5 M

9 8

0 0

5

10

15

20

7

25

20

30

40

t (h)

50

60

ce (mg/L)

50

50 48

40

46 44

qe (mg/g)

qt (mg/g)

30

20

42 40 38

0M 0.1 M 0.5 M

10

EPAC EPAC-CTAB

36 34

0

32 0

1

2

3

t

1/2

4

5

0

5

1/2

10

15

20

25

30

35

40

ce (mg/L)

(h )

Fig. 3. Adsorption kinetics for Ni2 þ adsorption onto EPAC and EPAC–CTAB (a), and intraparticle diffusion equation fit of the adsorption kinetics (b) (c0 ¼ 30 mg/L; pH¼ 5.50 7 0.03; temperature¼ 207 1 1C).

Fig. 4. (Supplementary material)- Adsorption isotherms of Ni2 þ for (a) EPAC and (b) EPAC–CTAB at three different ionic strengths fit by Langmuir model (solid lines) and Freundlich model (dashed lines) and the pH in the solution after equilibrium (c0 ¼ 30 mg/L; dosage¼ 35 mg/50 mL; temperature ¼ 207 1 1C; pH¼ 5.50 7 0.03; t¼ 24 h).

Table 2 Kinetic model parameters for Ni2 þ adsorption on EPAC and EPAC–CTAB. Kinetic models

Parameters

Adsorbents EPAC

EPAC–CTAB

Pseudo-first-order model

qe,exp (mg/g) qe, cal (mg/g) k1 (mg/g min) R2

16.04 5.67 0.2294 0.9284

42.84 13.72 0.2579 0.9170

Pseudo-second-order model

qe,exp (mg/g) qe, cal (mg/g) k2 (mg/g min) R2

16.04 16.07 0.2756 0.9997

42.84 42.92 0.1324 0.9998

Intra-particle diffusion model

kp1 (mg/(g min1/2)) C1 R2 kp2 (mg/(g min1/2)) C2 R2 kp3 (mg/(g min1/2)) C3 R2

33.38 0.296 0.9795 3.996 8.872 0.9469 0.7830 12.62 0.9347

62.74 2.250 0.9329 7.619 26.06 0.9547 0.6272 39.92 0.9525

The Freundlich isotherm model can be used for non-ideal adsorption based on assumption of multilayer adsorption on heterogeneous surfaces, which the binding strength decreases as

degree of site occupation increases (Freundlich and Heller, 1939; Roosta et al., 2014c). It can be expressed as follows: 1 ln qe ¼ ln kF þ ln ce n

ð7Þ

where kF refers to the empirical constant of adsorption capacity (mg/g (L/mg)1/n); n is an empirical constant. The adsorption isotherms of Ni(II) onto EPAC and EPAC–CTAB fitted by Langmuir and Freundlich equations are presented in Fig. 4 (Supplementary material). The parameters of Langmuir and Freundlich models that fitted to the adsorption equilibrium data are provided in Table 3. The results revealed that experimental data were better fitted by Langmuir model with the values of correlation coefficient (R2 40.997) than by Freundlich model. It implied that monolayer adsorption played a dominant role in the adsorption of Ni2 þ onto the carbons and tended to be homogeneous. Furthermore, the values of n for the Freundlich isotherm were more than one, implying that the Ni2 þ could be adsorbed on EPAC/EPAC–CTAB favorably (Crini et al., 2007). As it can be seen from Fig. 4 (Supplementary Material) that increasing ionic strength from 0 to 0.5 M results in a slightly decrease in Ni2 þ adsorption on the adsorbents. On the basis of

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Table 3 Isotherm parameters for removal of Ni2 þ by EPAC and EPAC–CTAB at different ionic strengths. Isotherms

Parameters

Langmuir

qm (mg/g) kL (L/mg) R2

Freundich

kF ((mg/g(L/mg)1/n) n R2

EPAC

EPAC–CTAB

0

0.1 M

0.5 M

0

0.1 M

0.5 M

16.89 0.1732 0.9974

14.64 0.1064 0.9992

15.85 0.0414 0.9997

49.75 0.9710 0.9992

47.62 0.4930 0.9999

46.95 0.3354 0.9997

6.710 4.724 0.9939

5.424 4.762 0.9702

2.396 2.623 0.9900

33.06 8.711 0.9865

28.90 7.722 0.9819

25.44 6.557 0.9630

investigation by Lützenkirchen (1997), two different surface complexes containing inner sphere and outer sphere can form during adsorption. Insensitivity to the ionic strength has been referred to an inner sphere surface complex, which covalent bonds are established during the adsorbed molecules or ions and the surface functional groups. However, there is a decrease with the ionic strength increasing in the outer sphere surface complexes, which no covalent bonds are formed but ion exchange and electrostatic attraction is primarily responsible for adsorption. The adsorption of Ni2 þ on EPAC/EPAC–CTAB decreased by a very low amount which might arise from the formation of inner sphere complexes, indicating Ni2 þ was primarily covalently bonded onto the adsorbents. In addition, lower removal of Ni2 þ was likely to originate from an increase in competition between the increasing Na þ concentration and Ni2 þ for the adsorption sites on carbons. Thus, electrostatic attraction was partly responsible for the adsorption on EPAC and EPAC–CTAB. Furthermore, (Table 4 Supplementary) summarizes the values of the maximum adsorption capacity of other different adsorbents. A comparison with these adsorbents, such as peanut shells, minotaur (Wilson et al., 2006), modified coir fibers (Shukla et al., 2006), vermiculite, bentonite, zeolite (Katsou et al., 2010), humic acid (El-Eswed and Khalili, 2006), natural kaolinite clay (Jiang et al., 2010), oxidized multiwall carbon nanotubes (Kandah and Meunier, 2007) and hazelnut shell activated carbon, also indicated a high Ni(II) adsorption capacity of EPAC–CTAB and further demonstrated the suitability for Ni(II) removal. 3.6. Effect of initial pH on Ni2 þ adsorption In adsorption of Ni2 þ , solution pH and pHpzc of carbons are considered as important factors influencing the adsorption capacity and adsorption performance, as it might change surface charge properties of the adsorbent and the speciation of nickel. The pHpzc of the samples was 6.12 for EPAC and 5.95 for EPAC– CTAB, confirming the observation on Boehm titrations of the carbons. There are several forms of nickel species (Ni2 þ , Ni (OH) þ , Ni(OH)02, Ni(OH)3 and Ni(OH)24  ) in aqueous solution in the pH range from 2 to 14 as illustrated in Fig. 5(a) (Supplementary material), which are theoretically estimated from the equilibrium constants (log Ka) for Ni(II) hydrolysis reactions (log Ka1 ¼ 4.1, log Ka2 ¼ 8.5, log Ka3 ¼ 11.5 and log Ka4 ¼12) listed in a previous report (Kragten, 1978). The formation of nickel hydroxides (Ni (OH) þ , Ni(OH)02, Ni(OH)3 and Ni(OH)24  ) were observed at pH values 48.0. Therefore, the predominant specie of Ni(II) (Ni2 þ ) adsorption phenomenon in aqueous occurred at pH below 8.0. In Fig. 5(b) (Supplementary material), at low pH value (pH ¼ 2), suppressed uptake of Ni2 þ might be related to strong electrostatic repulsion force between the protonated surface of the EPAC/EPAC– CTAB and Ni2 þ , as well as the fierce competition with H þ cations. There was a steep increase in adsorption capacity of both carbons below pH 5 and a slight increase in the pH range 5–8. As the

solution pH increased and above their pHpzc of the carbons, the extent of increase in nickel adsorption capacity presumably resulted from decreasing electrostatic repulsion and ion competition as well as increasing ion exchange reaction between the surface protons and nickel cations. To evaluate the impact of cation exchange for Ni2 þ adsorption, control experiments without Ni2 þ were prepared under the same pH values. As shown in Fig. 5(c) (Supplementary material), it was clear that the final pH levels of the control samples were more than that with Ni2 þ . Consequently, adsorption of Ni2 þ on EPAC/ EPAC–CTAB in the tested pH range was accompanied by proton release, demonstrating the role of cation exchange mechanism.

3.7. XPS analysis To fully analyze the adsorption mechanisms, XPS analysis was performed on the adsorbents prior to and after Ni2 þ adsorption. The XPS results of EPAC, EPAC–CTAB and EPAC–CTAB–Ni are shown in Fig. 6(a) (Supplementary material), and they showed that carbon, oxygen, nitrogen and nickel were expectedly the primary elements observed from binding energies at 284 eV (C 1s), 399 eV (N 1s), 531 eV (O 1s) and 856 eV (Ni 2p). It was also apparent found that carbon and oxygen were the main elements of EPAC, proving that nitrogen functional groups were incorporated into EPAC. According to the cumulative-area curve, the ratio of each element composition was calculated as C (69.63 percent) and O (30.37 percent) for EPAC, while for EPAC–CTAB: C (50.35 percent), O (41.25 percent) and N (8.4 percent). These results indicated that EPAC–CTAB had a higher content of oxygen groups, which was consistent with the results of FTIR and Boehm titration. In addition, for EPAC–CTAB–Ni, a small but clear peak appeared at binding energy for Ni 2p, suggesting that an amount of nickel were adsorbed after exposure to nickel solution. The Fig. 6(b) (Supplementary material) showed the resolution C 1s spectra of EPAC and EPAC–CTAB systems, comprising three peaks with binding energies of C–C group (284.5 eV), C–O group (285.3– 286.1 eV) and OQC–O group (287.6–288.5 eV) (Liu et al., 2012b). C 1s spectrum of EPAC–CTAB revealed a significant increase in C–O groups and a decrease in C–C groups as compared to EPAC. The details of the fitted results on the binding energy and peaks area (percent) are summarized in (Table 5 Supplementary material). The binding energy (BE) of OQC–O of EPAC–CTAB–Ni surface increased slightly as compared with that on the original EPAC–CTAB, which indicated that O atom was electron donors during the Ni(II) adsorption. This conclusion could also be demonstrated by the dramatic decrease of the peak area (percent) of OQC–O after Ni(II) adsorption. The deconvoluted of N 1s spectrum of EPAC–CTAB and EPAC– CTAB–Ni exhibited that the main peaks, which consisted of binding energies located at 398.3–398.4 eV and 399.8–400.0 eV, corresponded

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1.0

C 1s

0.8

EPAC

O 1s

0.7 0.6 Ni

0.5

2+

C 1s

+

O 1s

Ni(OH)

0

0.4

Ni(OH)2

0.3

Ni(OH)3

0.2

Ni(OH)4

EPAC-CTAB

Intensity

Relative proportion of Ni

2+

species

0.9

-

2-

N 1s

0.1 0.0 2

3

4

5

6

7

8

9

10

11

12

13

C 1s

14

O 1s

EPAC-CTAB-Ni

pH

Ni 2p

N 1s

40

0

100

35

200

300 400 500 600 Binding Energy (eV)

700

800

900

30

qe (mg/g)

25

C 1s EPAC

20 15 10

EPAC EPAC-CTAB

EPAC-CTAB

Intensity

5 0 2

3

4

5

6

7

8

9

pHinitial EPAC-CTAB-Ni 8 7

280 281 282 283 284 285 286 287 288 289 290 291 Binding Energy (eV)

5 N 1s

4

EPAC-CTAB

EPAC EPAC-CTAB EPAC-control EPAC-CTAB-control

3

Intensity

pHfinial

6

2 1 2

3

4

5

6

7

8

EPAC-CTAB-Ni

9

pHinitial Fig. 5. (supplementary material)- Distribution of Ni(II) species as a function of pH (a) and effect of initial pH on the Ni(II) adsorption, (b) and solution pH, (c) by EPAC and EPAC–CTAB (c0 ¼ 30 mg/L; dosage¼35 mg/50 mL; temperature¼207 1 1C; t¼ 24 h).

to N–H and CQN (Milczarek et al., 2011; Zhu et al., 2009) (Fig. 6(c) (Supplementary material) and (Table 5 Supplementary)). It was noteworthy that, although the two samples contained similar nitrogen groups, peak area (percent) and intensity were different. According to the XPS results, the introduced nitrogen groups could be very important for the EPAC–CTAB adsorption process.

395 396 397 398 399 400 401 402 403 404 405 406 407 408 Binding Energy (eV) Fig. 6. (Supplementary material)- Survey of EPAC, EPAC–CTAB and EPAC–CTAB–Ni (a), C 1s XPS spectrum of EPAC, EPAC–CTAB and EPAC–CTAB–Ni (b) and N 1s XPS spectrum of EPAC–CTAB and EPAC–CTAB-Ni (c).

4. Conclusion Activated carbons were prepared from EP by chemical activation with H3PO4 solution (EPAC) or with a mixture of CTAB and

M. Wang et al. / Ecotoxicology and Environmental Safety 104 (2014) 254–262

H3PO4 (EPAC–CTAB). EPAC exhibited 1045.8 m2/g of surface area and 1.048 cm3/g of total pore volume, while EPAC–CTAB had 689.0 m2/g of surface area and 0.361 cm3/g of total pore volume. The number of acidic functional groups and nitrogen groups on the surface of EPAC–CTAB increased when compared to that of EPAC, as confirmed by the FTIR and XPS results. The pseudosecond-order model and Langmuir model describe properly the experimental kinetic data and the adsorption isotherm data of Ni2 þ on the adsorbents. The maximum adsorption capacity, determined from Langmuir model was 16.9 mg/g for EPAC and 49.8 mg/g for EPAC–CTAB. The Ni2 þ uptake was predominantly affected by the solution pH and slightly dependent on ionic strength. The results indicated that using CTAB as a dopant for EPAC modification can markedly enhance the nickel(II) removal from aqueous solutions.

Acknowledgments The authors would like to acknowledge financial support for this work provided by Shandong Province Postdoctoral fund.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ecoenv.2014.01.038.

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