Enhanced Adsorption for P-Chlorophenol by Ammonia Modified Activated Carbon

Guo Yang 1, a, Jie Yan2, b , Hu Yang3, c and Chenhong Zhang4, d 1,2,3,4 College of Materials and Chemical Engineering, Sichuan University of Science and

Engineering, Zigong 643000, People’s Republic of China.

a yangguo_81@163.com, b yanjie0813@gmail.com, c yanghu67@163.com, d zch2305@163.com

Keywords: Chlorophenol; Ammonia; Adsorption; Activated carbon

Abstract. To investigate the influence of N-containing surface functional groups on adsorption capacity of AC, the activated carbon (AC) was modified with ammonia gas at 650 °C. Surface functional groups were quantitatively analyzing by Boehm titration. The effect of temperature on adsorption capacity suggested higher temperature is favourable. The influence of pH indicated the adsorption was favorable in acidic solution. The adsorption isotherms were fitted by Langmuir model and Freundlich model. Moreover, kinetic studies showed the adsorption of phenol onto adsorbents was followed by pseudo-second-order kinetic model. The adsorption capacity of ammoniated AC for p-chlorophenol was greatly improved.

Introduction

In the past few decades, owing to scarcity of water resources and industrialization development, there is growing public attention on the phenolic wastewater. Various methods are applied to removal of phenols from wastewater such as chemical oxidation [1], membrane filtration [2], biological degradation [3], electrochemical oxidation [4], solvent extraction [5], precipitation [6] and adsorption [7]. Among these methods, adsorption with AC is considered as an efficient technique for the removal of phenols from wastewater [8].

Generally, the AC surface consists of the carbon basal planes and various functional groups. These groups exhibit acidic, basic and neutral characteristics on the surface of AC, which significantly impact on the adsorption capacity [9]. Hence, the groups are vital for the adsorption process [10]. In the case of adsorption from phenolic solution, the O-containing groups play an important role in the adsorption of phenol. More O-containing groups led to decreasing the amount of adsorption and blocking chemisorption sites [11-12]. Moreover, these groups improved the hydrophilicity of AC surface resulted in weak interaction between adsorbent and phenol [13].

To improve the alkaline of adsorbent, the N-containing functional groups are introduced into the AC structure. The common method for incorporating N into AC is treatment with ammonia and urea solution at higher temperatures [14]. Moreover, calcining AC in the atmosphere of ammonia at the different temperatures resulted in a formation of new N-containing groups [15,16]. These incorporated N-containing groups having the free pair of electrons increase the electron donor capacity of adsorbents. Consequently, introduction of N atom into the matrix makes AC more alkalic resulted in the increase adsorption for phenols.

In this study, the AC modified with ammonia gas at 650 °C was selected to study the influence of N-containing groups on adsorption capacity for p-chlorophenol. The influence of temperature and solution pH on adsorption capacity for p-chlorophenol was investigated. Surface acidity/basicity of AC was determined in terms of the Boehm titration and pHpzc. To understand the interaction between surface properties and adsorption capacity, the isotherms data for adsorption of p-chlorophenol were fitted to Langmuir and Freundlich models. The kinetics were fitted with the pseudo-first order and pseudo-second-order models.

Advanced Materials Research Vol. 663 (2013) pp 807-812Online available since 2013/Feb/13 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.663.807

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Experimental

Preparation of Modified AC. The commercial AC purchased from Sinopharm Chemical Reagent Co., China, was demineralized with HCl acid (1 mol/L) for several hours to remove ash as well as residual physically adsorbed chemicals and then was washed until circum-neutral pH of the supernatant, oven dried at 105 °C for 24 h and kept in a desiccator. The obtained carbon material was labelled with GAC. To obtain the N-containing AC, the GAC suffered from calcination in an atmosphere of ammonia at temperature of 650 °C. The modified carbon with ammonia was denoted as NAC.

Boehm Titration and pHpzc. Boehm titration was carried out to determine the number and type of the surface oxygen groups [17]. The 0.500 g of the AC samples was placed into 25 mL of the solution (0.05 mol/L of NaOH, Na2CO3 and NaHCO3) for 24 h to reach equilibrium. The acidic O-containing surface groups of AC were determined by back titration with HCl solution (0.05 mol/L). The total content of basic groups was measured with 0.05 mol/L HCl solution. The pHpzc was determined by pH drift method [18].

Adsorption of P-Chlorophenol. Batch adsorption experiments were carried out by adding 0.10 g AC into 250 mL-Erlenmeyer flasks where 100 mL of p-chlorophenol solutions with different initial concentration (50-800 mg/L) were placed at 25 °C. The finial concentration of p-chlorophenol was analyzed using an ultraviolet spectrophotometer at the wavelength of 280 nm. The amount of adsorption capacity at equilibrium, qe (mg/g), was calculated according to equation (1):

0( )ee

C C Vq

M

− ×=

(1)

where qe (mg/g) is the amount of p-chlorophenol adsorbed onto the AC at equilibrium; C0 and Ce (mg/L) are the initial and equilibrium concentration of the solution respectively. V (L) is the volumes of the solution, and M (g) is the mass of AC.

The effect of solution pH on the p-chlorophenol removal was investigated at 3-11. The pH was adjusted using 0.1 mol/L HCl or 0.1 mol/L NaOH. The initial concentration of p-chlorophenol was 800 mg/L, with AC dosage of 0.1 g/100 mL at 25 °C.

The kinetic studies were performed at 25 °C, solution pH was not adjusted; the initial concentration was set 800 mg/L, and the samples were separated at predetermined time intervals. The uptake of phenol at time t, qt (mg/L), was calculated by the following equation (2):

0( )tt

C C Vq

M

− ×=

(2)

where Ct is the concentration of the phenol (mg/L) in solution at time t.

Results and Discussion

Boehm Titration and pHpzc. The results obtained from Boehm titration together with pHpzc are presented in Table 1. As expected, carboxyl groups noticeably reduce in ammoniated AC, because of the ammonia gas reaction with carboxylic acid. It is apparent that after ammoniation, the total base increased significantly. This result indicates that more N-containing groups were formed on the AC surface. Moreover, the higher pHpzc for NAC also support this conclusion.

Table 1. The Boehm titration and pHpzc of nitrogen-containing activated carbon samples

Sample Carboxy

(meq/g)

Lactonic

(meq/g)

Phenolic

(meq/g)

Total acid

(meq/g)

Total base

(meq/g) pHpzc

GAC 0.349 0.428 0.277 1.054 0.009 5.80

NAC 0.096 0.260 0.307 0.663 0.328 9.40

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Equilibrium Adsorption of Phenol.

Effect of Temperature on Phenol Adsorption. Figure 1 shows the effect of temperature on adsorption of p-chlorophenol at 25, 40 and 55 °C. The adsorption capacities of two AC samples were increased with rising temperature. The result suggests that the adsorption was an endothermic process, and higher temperature was favorable to the adsorption process. This phenomenon may be resulted from the improved temperature led to an increase in the rate of diffusion of solute molecular across the external boundary layer and internal pore of AC as a consequence of less mass transfer resistance [19].

Fig. 1. Effect of solution temperature on p-chlorophenol uptake on AC samples at 25 °C.

Effect of pH on Phenol Adsorption. As showed in Figure 2, the adsorption capacity of GAC and NAC strongly depends on the pH of solution; and a decreasing adsorption capacity of AC was observed with increasing solution pH value. At lower pH, the total surface charge of AC was positive, whereas, at higher pH; it would be negative. In acidic solution, the concentration of the unionized phenol molecules was high, and the dispersion interaction between phenol and AC predominated [20]. Thus, in lower pH solution, the strong electrostatic attractions between the p-chlorophenol molecule and adsorption sites on AC surface resulted in superior adsorption capacity. However, in a basic solution, phenol was dissociated into the anion. The repulsion between the negative surface charge of AC and the phenolate anions is strong, which lead to a lower adsorption capacity [21].

Fig. 2. Effect of solution pH on p-chlorophenol on AC samples at 25 °C

Adsorption Isotherms. The Langmuir model is reasonably applied to monolayer sorption on homogeneous surface and is expressed as follows [22]:

1m L e

e

L e

q K Cq

K C=

+ (3)

where Ce (mg/L) is the equilibrium concentration of p-chlorophenol, qe (mg/g) is the amount of AC adsorbed per unite mass of p-chlorophenol. qm (mg/g) is the maximum adsorption capacity; KL is Langmuir constant. Freundlich model assumes heterogeneous adsorptive energies on the adsorbent surface, which is expressed as [23]:

Advanced Materials Research Vol. 663 809

1/ne F eq K C= (4)

where Ce (mg/L) is the equilibrium concentration of p-chlorophenol, qe (mg/g) is the amount of AC adsorbed per unite mass of p-chorophenol. KF( (mg/g)(mg/L)-1/n) is the Freundlich constant which represents the amount of adsorbate adsorbed onto adsorbent for a unit equilibrium concentration; moreover, n is the dimensionless exponent of Freundlich depicting an indication of how favorable the adsorption process.

The Langmuir and Freundlich adsorption constants evaluated from the isotherms with the R2 are listed in Table 4. As shown in Table 4, the R2 range from 0.97-0.99 in Langmuir model. So, the Langmuir equation gives better fitting and higher accuracy, and it is reasonably applied in all cases.

Table 1. Langmuir and Freundlich parameters for the adsorption of phenol on GAC and NAC at different temperature

Samples

Temp. (°C)

Langmuir model Freundlich model qm (mg/L) KL (L/mg) R2 KF ((mg/g)(L/mg)1/n) n R2

GAC 25 239.89 0.0042 0.989 5.934 0.53 0.985 40 254.65 0.0044 0.998 6.673 0.52 0.983 55 263.58 0.0051 0.991 8.407 0.50 0.978 NAC 25 286.68 0.0084 0.994 15.67 0.44 0.967 40 300.43 0.0091 0.988 17.42 0.43 0.968 55 313.17 0.0100 0.970 19.76 0.42 0.966

Adsorption Kinetics. The kinetic of adsorption depicts the rate of p-chlorophenol uptake on the AC. The pseudo-first-order model [24] was formulated as follows:

1ln( ) lne t eq q q k t− = − (5)

where k1(1/h) is the first-order rate constant, qe and qt (mg/g) are the amount of phenol adsorbed at equilibrium and at any time. According to the plot of ln (qe-qt) versus t, the k1 and qe are calculated by the slope and intercept.

The pseudo-second-order model [25] is described as:

22

1

t e e

t t

q k q q= +

(6)

where k2 (g/mg h) is the second-order rate constant, whereas, qe and qt (mg/g) are the amount of phenol adsorbed at equilibrium and at any time. The values of k2 and qe are calculated from the slope and intercept of the linear plot of t/qt versus t.

To evaluate the goodness of fitting and suitability of the model, The ∆q (%) are used in kinetic model study. A lower ∆q denote better model fitting. The ∆q is calculated as follows:

2exp exp[( ) / ]

(%) 1001

calq q qq

N

−∆ = ×

− (7)

where N is the number of data point, qexp and qcal (mg/g) are the experimental and calculated adsorption capacity.

The adsorption kinetics of p-chlorophenol was displayed in Fig. 3. The initial adsorption rate for p-chlorophenol on GAC and NAC was low. This indicated the intensitive stir may accelerate adsorption. Table 2 listed the parameters of pseudo-first-order kinetic model and pseudo-second-order kinetic model for GAC and NAC. The correlation coefficients were greater than 0.95 in two models. Nevertheless, comparing with the ∆q of pseudo-first-order kinetic model, the pseudo-second-order kinetic model yielded the lower ∆q value. Consequently, the adsorption of p-chlorophenol onto the adsorbents could be accurately described by pseudo-second-order kinetic model. This indicated that modification with ammonia gas did not alter its adsorption mechanism.

810 Health, Structure, Material and Environment

Fig. 3. Adsorption kinetics s of p-chlorophenol on adsorbents at 25 °C

Table 2. Comparision of the pseudo-first-order model and pseudo-second-order model for adsorption p-chlorophenol on GAC and NAC at 25 °C

qe, exp (mg/g)

Pseudo-first-order kinetic model Pseudo-seconed-order kinetic model

qe, cal

(mg/g) k1 (1/h)

R2 ∆q (%) qe, cal (mg/g)

k2 (g/mg h)

R2 ∆q (%)

GAC 175.51 254.98 0.826 0.991 45.85 181.82 0.0072 0.992 4.45 NAC 236.27 297.85 0.935 0.986 23.63 243.90 0.0044 0.986 3.63

Conclusions

The present work shows that the modification with ammonia gas improves the basicity of AC resulting in enhancement of adsorption capacity. This may be due to the increased dispersive force between delocalized π electrons in basal planes and the free electrons of the aromatic ring. The acidic solution is favor to adsorption process because the molecular form is predominant. In the temperature study showed the adsorption of p-chlorophenol on AC is an endothermic process. Langmuir model gave the best correlation for the adsorption of p-chlorophenol onto AC in all cases; the maximum adsorption capacity of p-chlorophenol on NAC was 313.17 mg/L at 55 °C. The adsorption process was found to follow the pseudo-second-order kinetic model.

Acknowledgement

The authors acknowledge the research grant provided by Sichuan University of Science and Engineering under the Science and Technology Innovation Foundation for the College Students (Project No. cx20120312).

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Health, Structure, Material and Environment 10.4028/www.scientific.net/AMR.663 Enhanced Adsorption for P-Chlorophenol by Ammonia Modified Activated Carbon 10.4028/www.scientific.net/AMR.663.807

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