Fluoroquinolones antibiotics adsorption onto microporous activated carbon from lignocellulosic biomass by microwave pyrolysis

Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

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Fluoroquinolones antibiotics adsorption onto microporous activated carbon from lignocellulosic biomass by microwave pyrolysis Muthanna J. Ahmed a,b,*, Samar K. Theydan b a b

Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia Department of Chemical Engineering, University of Baghdad, Baghdad, Iraq

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 November 2012 Received in revised form 10 May 2013 Accepted 16 May 2013 Available online 20 June 2013

Microwave-assisted KOH activation has been adopted to prepare activated carbon from a lignocellulosic biomass, Albizia lebbeck seed pods. Fluoroquinolones antibiotics such as ciprofloxacin (CIP) and norfloxacin (NOR) were removed from aqueous solutions by adsorption on prepared carbon. The surface area, micropores volume, and mesopores volume of such carbon were 1824.88 m2/g, 0.645 cm3/g and 0.137 cm3/g, respectively. The effects of pH, adsorbent dose, and contact time on the adsorptive removal process were studied. Maximum removal percentages of 96.12% and 98.13% were achieved for CIP and NOR adsorption, respectively. The best fitting for equilibrium adsorption data of both antibiotics was obtained by the Langmuir isotherm with maximum capacities of 131.14 and 166.99 mg/g for CIP and NOR, respectively. The kinetic data were found to follow closely the pseudo-second order model for both antibiotics. Results of thermodynamic studies showed an endothermic CIP adsorption compared to an exothermic NOR adsorption under examined conditions. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Activated carbon Microwave Potassium hydroxide Seed pods Antibiotics

1. Introduction Fluoroquinolones (FQs) have been classified among the most important synthetic antibiotics used in human and veterinary medicine [1]. One of the widely used FQs is ciprofloxacin (CIP), which has been identified among the top 10 of high priority pharmaceuticals relevant for the water cycle in general. The persistence of such antibiotic in the environment may induce bacterial resistances as well as present a threat to aquatic organism. Norfloxacin (NOR) is another FQs compound with high antibacterial activity against both gram-negative and grampositive bacteria through inhibition of DNA gyrase [2]. Application of manure as fertilizer and leaking from septic systems leads to the direct release of NOR into surface water. The presence of such undegradable compound in wastewater may present a risk to human health. FQs have been detected at levels of up to 0.036 and 0.45 mg/l in surface water and wastewater effluent, respectively [3]. Although the amounts of FQs in the aquatic environment are low, their continuous input and accumulation may increase their concentration which constitute in the long term a potential risk for

* Corresponding author at: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia. Tel.: +60 129177261. E-mail addresses: [email protected] (M.J. Ahmed), [email protected] (S.K. Theydan).

aquatic and terrestrial organisms. Thus, removal of these compounds from the environment is an important case study. Adsorption is the widely used method for removal of a broad range of FQs pollutants due to its simple design, easy operation, and relatively simple regeneration [4]. It has been detected that activated carbon is the efficient adsorbent for removal of FQs as compared to others, because of its large surface area, micro-porous nature, and high adsorption capacity [5]. The main reasons for high cost of commercial activated carbon are the use of non-renewable and expensive precursors like wood and depending on conventional heating production methods [6]. Therefore, in recent years, research has been focused on the use of renewable and cheaper precursors such as agricultural wastes with the aid of low treatment time microwave production technique, in which energy is directly supplied to the carbon bed by dipole rotation ionic conduction inside the particles. Recently, FQs antibiotics have been successfully removed from aqueous solutions by adsorption on activated carbon prepared from various waste biomasses such as date palm leaflets, peat, cyperus alternifolins, lotus stalk, and Trapa natans husk [7–11]. Albizia lebbeck (A.L.) is a common tree in tropical and subtropical regions. It belongs to the pea family (Fabaceae) and produces seed pods which are 20 cm long and 3 cm wide. This high volume waste constitutes an environmental problem and its reutilization is useful. A.L. seed pods are composed of 36.4% cellulose, 18.9% hemicelluose, 13.6% lignin, and 83.1% volatile matter [12]. The high volatile and lignocellulosic contents promote the use of these pods

1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.05.014

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M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

as precursors for production of microporous activated carbon with high surface area which revealed its capability for efficient adsorptive removal of cephalosporin antibiotics [13]. The main aim of this work is to investigate the ability of A.L seed pods-carbon for adsorptive removal of FQs antibiotics represented by CIP and NOR from aqueous solutions. The initial concentrations of both FQs used in this study were in the range from 20 to 100 mg/l as adopted by Conkle et al. [14]. In addition, the equilibrium isotherms, kinetics and thermodynamics data of the adsorption process were also studied to understand the adsorption mechanism. 2. Materials and methods 2.1. Materials A.L. seed pods used for preparation of activated carbon were collected from trees located in the region of Baghdad University premises, Iraq. After the removal of seeds, the pods were first washed with water to get rid of impurities, dried at 110 8C for 24 h, crushed using disk mill, and sieved. Fraction with average particle size of 0.8 mm was selected for the preparation. Potassium hydroxide (purchased from Didactic company, Espana) of purity 99.9% was used as a chemical activator. CIP (supplied by Nanjing Huaxin Biopharm. Company Ltd., China) of purity 99.9% and NOR (supplied by Ajanta Pharma Limited Company, India) of purity 99.9% were used as adsorbates.

[17]. NOVAWin2 data analysis software was used to perform these calculations [18]. The morphology of prepared activated and raw material was examined by scanning electron microscopy SEM (300 K Pixel CMOS, China). 2.3. Adsorptive removal Batch mode adsorption experiments were carried out to show the effects of pH, contact time, and adsorbent dose on the removal percentages of CIP and NOR. Prepared carbon of 250 mm average particle size at different adsorbent dose (0.25–1.25 g/l) was mixed with 20 ml samples of CIP or NOR solutions with different initial concentrations (20–100 mg/l) and different pH (2–12). The mixtures were added to 100 ml Erlenmeyer flasks, and shaken at 200 rpm and 303 K for various contact times (0–150 min). Then, the samples were filtered and the concentrations of CIP or NOR in the filtrate were analyzed by using a UV-Visible Spectrophotometer (Shimadzu UV-160A) at maximum wavelengths of 275 and 272 nm for CIP and NOR, respectively. Each experiment was duplicated under identical conditions, indeed the results of UV analysis were the average of two readings. The removal percentage of each antibiotic was determined by the following equation: Removal percentage ð%Þ ¼

C0  Ce  100 C0

(2)

where C0 and Ce (mg/l) are the initial and equilibrium concentrations of CIP or NOR solution, respectively.

2.2. Activated carbon preparation 2.4. Adsorption isotherms Dried pods (3 g) were mixed with 20 ml of KOH solution of at 1.0 g/g impregnation ratios (weight of activator/weight of dried pods) for 24 h at room temperature. The impregnated samples were next dried at 80 8C until completely dried and stored in a desiccator. For the activation of dried impregnated samples a quartz glass reactor (2.5 cm diameter  12.5 cm length) was used. The reactor was sealed at one end and the other end had a removable cover connected to a stainless steel pipe of 5 mm inside diameter to allow for the escape of the pyrolysis gases. The reactor was placed in a modified microwave heating apparatus (MM717CPJ, China) and held at radiation power of 620 W for 8 min radiation time. At the end of activation the samples were withdrawn from the apparatus and allowed to cool. Then the samples were soaked with 0.1 M HCl solution such that the liquid to solid ratio is 10 ml/g. The mixtures were left overnight at room temperature, and then filtered and subsequently the samples were repeatedly washed with distilled water until the pH of filtrate reach 6.5–7. After that, the samples were dried at 110 8C for 24 h, and subsequently were weighed to determine the yield of the product. Finally the samples were stored in tightly closed bottles. The yield of prepared carbon is defined as the ratio of final weight of the obtained product after washing and drying to the weight of dried precursor initially used. The yield was calculated based on the following equation: Yield ð%Þ ¼

Wf  100 Wo

The relation between the adsorbed amounts of CIP and NOR and their equilibrium concentrations at different temperatures was investigated. 20 ml of CIP or NOR solutions with different initial concentrations (20–100 mg/l) was placed in 100 ml Erlenmeyer flasks. Carbon with average particle size of 250 mm was added to each flask and kept in a shaker of 200 rpm at various temperatures (303–323 K). Other parameters such as pH, contact time and adsorbent dose were kept at their best values. The concentrations of CIP or NOR solutions were similarly measured and the uptake of each component at equilibrium, qe (mg/g), was calculated using the following equation: qe ¼

ðC 0  C e ÞV W

(3)

where V (l) is the volume of solution and W (g) is the weight of prepared carbon. The experimental isotherm data of both antibiotics were correlated by the Langmuir, Freundlich, and Temkin and Phyzev [19–21]. These equations can be written as follows: Langmuir isotherm

Ce 1 1 ¼ þ Ce qe qm K L qm

Freundlich isotherm ln qe ¼ ln K F þ (1)

where Wf and Wo (g) are the weights of carbon product and dried pods, respectively. The characteristics of prepared carbon represented by surface area, micropores and mesopores volumes, and pore size distribution were determined. Surface area was determined from the application of BET equation to the adsorption–desorption isotherm of N2 at 77 K [15]. Micropores volume was determined by applying the Dubinin–Radushkevich equation. The mesopores volume was determined using BJH desorption branch [16] and the pore size distribution was determined from the density functional theory

1 ln C e n

Temkin isotherm qe ¼ B ln A þ B ln C e

(4)

(5) (6)

where qL (mg/g) is the Langmuir maximum uptake of CIP or NOR per unit mass of carbon, KL (l/mg) is the Langmuir constant related to rate of adsorption, KF ((mg/g) (l/mg)1/n) and n are Freundlich constants which give a measure of adsorption capacity and adsorption intensity, respectively, B is the Temkin constant related to adsorption heat and A (l/mg) is the Temkin parameter related to the equilibrium binding energy. Least-squares regression program based on Hooke–Jeeves and Gauss–Newton method was used to analyze experimental data. This program gave the parameters of

M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

each equation and the agreement between experimental and calculated data in terms of correlation coefficient R2. 2.5. Adsorption kinetics In order to study the effect of contact time on adsorbed amounts of CIP and NOR and to determine the equilibrium time, set of batch mode experiments were carried out. The concentrations of CIP or NOR were continuously measured by taking aqueous samples at each time period. The adsorbed amount of CIP or NOR at time t, qt (mg/g), was determined by:

221

Table 1 Characteristics of prepared activated carbon. Characteristic

Value

Yield (%) Surface area (m2/g) Micropores volume (cm3/g) Mesopores volume (cm3/g)

22.48 1824.88 0.645 0.137

3. Results and discussion 3.1. Yield and characteristics

ðC 0  C t ÞV qt ¼ W

(7)

where Ct (mg/l) is the concentration of CIP or NOR solution at time t (min). In order to study the mechanism of adsorption and determining the rate controlling step, the kinetics data were analyzed by pseudo-first order model, pseudo-second order model, and intraparticle diffusion model [22–24]. The mathematical expressions of these models can be written as: Pseudo-first order model lnðqt  qe Þ ¼ lnðqe Þ  K 1 t

Pseudo-second order model

t 1 t ¼ þ qt K 2 qe qe

Intraparticle diffusion model qt ¼ K 3 t 1=2 þ C

(8)

(9)

(10)

where qe and qt (mg/g) are the uptake of CIP or NOR at equilibrium and at time t (min), respectively, K1 (min–1) is the adsorption rate constant, K2 (g/mg min) is the rate constant of second-order equation, K3 (mg/g min1/2) is the intraparticle diffusion rate constant, and C (mg/g) is a constant that gives an idea about the thickness of the boundary layer. The correlations of above models to kinetics data were further validated by the normalized standard deviation, Dq (%), which is defined as: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP h i2 u ðqexp  qcal Þ=qexp t Dq ð%Þ ¼ 100 N1

The yield and characteristics of prepared carbon were evaluated and summarized in Table 1. This table shows 22.48% yield which is similar to those reported for activated carbons prepared from coffee endocarp (22.7%) and pistachio shells (24%) by KOH activation [25,26]. Also, the yield in this study is higher than that reported by Vargas et al. [27] who found that 10.80% maximum yield was obtained for activated carbon from flamboyant pods by NaOH activation. This is probably due the ability of KOH to remove more volatile materials from the particles compared to NaOH. Table 1 shows that KOH activation of A.L. seed pods produce carbon with high surface area of 1824.88 m2/g as compared to that produced from coffee endocarp (893 m2/g) and pistachio shells (1096 m2/g) using the same activator [25,26]. This may be due to the high lignocellulosic content of A.L. seed pods compared to other precursors. SEM images for the A.L. seed pods and prepared carbon are shown in Fig. 1. It can be seen from Fig. 1a, that the surface of raw precursor is smooth and the microwave activation with KOH provided the development of many pores (Fig. 1b). The

(11)

where N is the number of data points, qexp and qcal (mg/g) are the experimental and calculated adsorption capacities, respectively. 2.6. Adsorption thermodynamics For further understanding of the adsorption mechanism of CIP and NOR on prepared carbon, thermodynamic analysis was performed. The thermodynamic parameters represented by the change in free energy (DG), enthalpy (DH), and entropy (DS). These parameters are determined by using the following equations: lnðK d Þ ¼

DS R



DH RT

DG ¼ RT lnðK d Þ

Kd ¼

qe  ðW=VÞ Ce

(12)

(13)

(14)

where R is the universal gas constant (8.314 J/mol K), T is temperature (K), and Kd is the distribution coefficient for the adsorption.

Fig. 1. SEM micrgraphs (300) of A.L. seed pods (a) and prepared carbon (b).

M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

6KOH þ 2C ! 2K þ 3H2 þ 2K2 CO3 Correspondingly, K2CO3 was reduced by carbon to form K, K2O, CO, and CO2 according to the following reactions: K2 CO3 þ 2C ! 2K þ 3CO K2 CO3 ! 2K2 O þ CO2 K2 O þ 2C ! 2K þ CO It was assumed that metallic potassium formed during the gasification process would diffuse into the internal structure of carbon matrix widening the existing pores and created new porosities. Fig. 2 displays the isotherms of N2 adsorption– desorption and pore size distribution for prepared carbon. As shown in Fig. 2a, the isotherm presents a high adsorption at low relative pressure, characteristics of microporous materials, where the adsorption branch resembles that of a type I isotherm in the international union of pure and applied chemistry IUPAC classification. From Fig. 2a it can be also concluded that the isotherm displayed a small hysteresis loop, indicating the presence of very small mesopore volume. Fig. 2b presents the pore size distribution for the activated carbons made in this study. This figure shows that the pore structure consists basically of micropores, which are mainly defined by IUPAC as pores smaller

than 20 A˚ in diameter. This can be also concluded from the results of Table 1, which show a micropore volume of 0.645 cm3/g compared to a mesopore volume of 0.137 cm3/g. This revealed that A.L seed pods are good precursors for preparation of microporous activated carbon which can be used in many industrial applications. 3.2. Adsorptive removal 3.2.1. Effect of pH CIP and NOR removal percentages of prepared carbon as a function of pH (2–12) at different initial concentrations is shown in

a

Removal percentage (%)

development of porosity is associated with gasification according to the following reaction [28]:

125

110

100 mg/l

20 mg/l

CIP

NOR

95

80

65

50 0

3

6

9

12

15

pH

b

150

Removal percentage (%)

222

125

100 mg/l

20 mg/l

CIP

NOR

100

75

50

25 0

0.3

0.6

0.9

1.2

1.5

dose (g/l)

Removal percentage (%)

c 120 100 80 60 40 20

100 mg/l

20 mg/l

CIP

NOR

0 0

40

80

12 0

16 0

20 0

time (min) Fig. 2. Isotherms of N2 adsorption–desorption (a) and pore size distribution (b).

Fig. 3. Effect of pH (a), dose (b), and time (c) on removal percentage of CIP and NOR at 303 K.

M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

223

molecules in the solution is high and hence all adsorbate molecules may interact with the adsorbent and be removed from the solution.

Fig. 3a. For both compounds, adsorption initially increases with increasing pH and reached a plateau then decreased at higher pH. An increase in pH from 2 to 9 at 0.75 g/l dose, 20 mg/l initial CIP concentration and 90 min contact time leads to an increase in CIP removal percentage from 89.611% to 96.12%. While, the increase in pH from 2 to 5 at 0.50 g/l dose, 20 mg/l initial NOR concentration and 60 min contact time causes an increase in NOR removal percentage from 94.78% to 98.12%. Therefore, the values of 9 and 5 are considered to be the best pH for maximum removal of CIP and NOR, respectively. The observed adsorption behavior shown in Fig. 3a can be attributed to a combination of surface charge characteristics of adsorbent and pH-dependent speciation of CIP and NOR. The positively charged adsorbent becomes progressively negatively charged with increasing pH. When pH is above 9 or 5 for CIP or NOR, respectively, the electrostatic repulsion between adsorbent and adsorbate prevailed so that the adsorbed amounts begin to decrease. This has been explained by Sun et al. [9] for CIP removal by activated carbon from cyperus alternifolius precursor. They reported 8.7 as a best pH value, where adsorption initially increased with increasing pH and reached a plateau when solution pH was approaching its pKa2 (8.7) and then decreased at higher pH. Liu et al. [10] showed that maximum adsorption was achieved at pH 5.5 and decreased significantly when pH was higher or lower than this range for NOR removal by adsorption on activated carbon from lotus stalk (LAC). Although both antibiotics have two similar proton-binding sites (carboxyland piperazinyl group) with reported pKa values of 6.22 and 8.51, respectively, there is a difference in pH values which give best adsorbed amounts. This may be confirmed by the fact that the solution pH increased after NOR sorption at pH 4.5–5.5, causing repulsion between adsorbent and adsorbate and the sorption greatly depressed. Accordingly, cation exchange is proposed to be an important mechanism participating in the adsorption of FQs compounds. Fig. 3a also shows that the removal efficiency decreased with increasing initial CIP or NOR concentration. This is probably because at lower initial concentration the ratio of surface active sites to the total adsorbate

3.2.2. Effect of adsorbent dose The relation of prepared carbon dose (0.25–1.25 g/l) and removal percentages of CIP and NOR at different initial concentrations is explained in Fig. 3b. This figure shows the increase of removal percentages with increasing adsorbent dose up to values of 0.75 and 0.5 g/l, for CIP and NOR, respectively and then there values remain constant. Increasing of adsorbent dose from 0.25 to 0.75 g/l leads to an improvement in CIP removal percentage from 79.94% to 96.12% at initial CIP concentration of 20 mg/l, pH of 9, and contact time of 90 min. Also, this figure shows that the increase in dose from 0.25 to 0.5 g/l causes an improvement in NOR removal percentage from 81.27% to 98.13% at initial NOR concentration of 20 mg/l, pH of 5 and contact time of 60 min. Therefore, the values of 0.75 and 0.5 g/l can be considered as the best adsorbent doses which give maximum removal percentages for CIP and NOR, respectively. The increase in removal efficiency with increasing adsorbent dose is probably due to the greater adsorbent surface area and pore volume available at higher adsorbent dose providing more functional groups and active adsorption sites that result in a higher removable percentage [29]. 3.2.3. Effect of contact time The dependence of removal percentage of CIP and NOR on contact time (0–150 min) at various initial concentrations is shown in Fig. 3c. This figure shows that removal percentages increase with time and adsorption reached equilibrium in about 90 and 60 min for CIP and NOR, respectively. Maximum CIP removal percentage of 96.12% is obtained at initial CIP concentration of 20 mg/l, pH of 9 and contact time of 90 min compared to maximum value of 98.13% for NOR at 20 mg/l initial NOR concentration, pH of 5 and 60 min contact time. This figure also shows that rapid increase in removal percentage is achieved during the first 30 min. The fast adsorption at the initial stage may be due to the higher

Table 2 Adsorption isotherm parameters. Adsorbate

Temperature (K)

Langmuir isotherm qm (mg/g)

KL (l/mg)

R2

CIP

303 313 323

108.74 118.05 131.14

3.2713 2.8606 3.1711

0.9999 0.9999 0.9999

NOR

303 313 323

166.99 160.90 153.99

5.1631 4.1006 3.5555

0.9999 0.9999 0.9999

Adsorbate

Temperature (K)

Freundlich isotherm KF ((mg/g) (l/mg)1/n)

R2

n

CIP

303 313 323

6.0682 6.2544 6.7733

3.8643 3.5067 2.9424

0.8060 0.8765 0.9005

NOR

303 313 323

7.5958 7.3205 7.0516

4.2728 4.2385 4.2073

0.8245 0.8200 0.7918

Adsorbate

Temperature (K)

Temkin isotherm A (l/mg)

B

R2

CIP

303 313 323

90.524 69.867 52.834

15.910 18.078 22.767

0.8802 0.9498 0.9803

NOR

303 313 323

216.097 156.785 111.659

22.025 21.706 21.452

0.9125 0.9030 0.8798

M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

224

0.20

antibiotics on activated carbons. Also, Table 2 shows that the Langmuir isotherm gave maximum adsorption capacities of 131.14 and 166.99 mg/g for CIP and NOR, respectively. This result indicates that the uptake of NOR is higher than that of CIP; this is probably because NOR molecule has a more positive surface potential than CIP on the opposite end of its carboxylic group, as explained by Conkle et al. [14] who showed that the distribution of charges in FQs molecules permit an orientation that would allow optimal interactions with sorption sites. The favorability of CIP and NOR adsorption on prepared carbon can be concluded from the results of Freundlich isotherm fitting, shown in Table 2. Values of parameter n > 1 (Table 2) give an indication for the favorability of adsorption. Table 2 also shows that the increase in temperature causes an increase in CIP capacity and a decrease in NOR capacity. These results indicate the endothermic and exothermic natures for CIP and NOR adsorption on prepared carbon, respectively. Similar results were reported by Zhang et al. [31,32] for adsorption of ciprofloxacin and norfloxacin onto modified coal fly ash.

Ce/qe (g/l)

0.15

0.10

0.05 T= 303 K

T= 313 K

T= 323 K

CIP-----NOR

0.00 0

5

10

15

20

25

30

Ce (mg/l) Fig. 4. linear plot of Langmuir isotherm at different temperatures (CIP, pH = 9, dose = 0.75, t = 90 min; NOR, pH = 5, dose = 0.5, t = 60 min).

3.4. Adsorption kinetics The equilibrium time for adsorption of CIP and NOR on prepared carbon has been determined as 90 and 60 min, respectively. The kinetics data within this range of contact time for each compound are fitted with pseudo-first order, pseudo-second order and intraparticle diffusion models, Eqs. (8)–(10). The results are summarized in Table 3 showing the low R2 values of the pseudo-first order equation for both CIP and NOR. Also, for this model, there is a large difference between the experimental and calculated adsorption capacity for both adsorbates represented by Dq (%), indicating a poor pseudo-first order fit to the experimental data. The linear plot of t/qt versus t (Fig. 5a) for pseudo-second order equation is of high R2 values, as shown in Table 4. The better representation of adsorption kinetics was by the pseudo-second order kinetic model and the calculated qe values agree well with the experimental qe values for both FQs antibiotics (Table 3). This suggests that the adsorption of CIP and NOR on prepared carbon follows second-order kinetics. Several authors showed the successful application of pseudo-second order model for representation of experimental kinetics data of both CIP and NOR adsorption on agricultural wastes-based carbons [8,10]. From Table 3, the values of rate constant K2 for CIP and NOR decrease

driving force making fast transfer of adsorbate ions to the surface of prepared carbon particles and the availability of the uncovered surface area and the remaining active sites on the adsorbent [30– 32] reported an equilibrium time of 100 min for adsorption of ciprofloxacin and norfloxacin onto modified coal fly ash. 3.3. Adsorption isotherms Three famous isotherms, namely Langmuir, Freundlich and Temkin isotherms, Eqs. (4)–(6), were used to correlate experimental equilibrium data for adsorption of CIP and NOR on prepared carbon at best conditions and different temperatures. The calculated constants of the three isotherm equations along with R2 values for both antibiotics are presented in Table 2. This table shows that the Langmuir isotherm represented by plot of Ce/qe versus Ce (Fig. 4) is of highest R2 values for both antibiotics at different temperatures. In other words, this result may be due to the homogeneous distribution of active sites on the surface of prepared carbon. Several authors [9,11] showed the successful application of Langmuir isotherm compared to Freundlich and Temkin isotherms to correlate experimental adsorption data of FQs Table 3 Kinetic adsorption parameters. Adsorbate

C0 (mg/l)

Pseudo-first order model qe,exp (mg/g)

qe,cal (mg/g)

K1 (min–1)

R2

Dq (%)

CIP

20 100

26.53 107.0

4.091 4.758

0.0379 0.0239

0.9843 0.9373

2.3614 3.7589

NOR

20 100

39.90 165.02

6.148 7.144

0.0757 0.0537

0.9824 0.9472

2.4522 3.1125

Adsorbate

C0 (mg/l)

Pseudo-second order model qe,exp (mg/g)

qe,cal (mg/g)

K2 (g/mg min)

R2

Dq (%)

CIP

20 100

26.53 107.0

32.19 112.08

0.0014 0.0011

0.9956 0.9989

0.2977 0.1543

NOR

20 100

39.90 165.02

46.49 170.39

0.0019 0.0015

0.9975 0.9957

0.2134 0.2886

Adsorbate

C0 (mg/l)

Intraparticle diffusion model K3 (mg/g min1/2)

R2

Dq (%)

1.841 3.610

0.9455 0.8566

2.9455 4.3756

2.514 2.868

0.9537 0.7581

3.0132 4.5634

qe,exp (mg/g) CIP

20 100

26.53 107.0

NOR

20 100

39.90 165.02

C (mg/g) 8.924 67.98 20.23 117.1

M.J. Ahmed, S.K. Theydan / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 219–226

a

4.0

3.2

20 mg/l

100 mg/l

CIP

NOR

t/qt (min.g/mg)

2.4

1.6

0.8

0.0 0

25

50

75

100

125

t(min)

b

225 20 mg/l

100 mg/l

CIP

NOR

qt (mg/g)

180

135

90

45

0 5.5

4.0

7.0

8.5

t

1/2

10.0

11.5

1/2

(min )

c 5.5

In (Kd)

5.0

4.5

4.0 CIP 3.5 0.003

0.0031

0.0032

0.0033

NOR

0.0034

1/T (1/K) Fig. 5. Plot of pseudo-second order (a) intraparticle diffusion (b), In Kd versus 1/T, (c) at 303 K (CIP, pH = 9, dose = 0.75; NOR, pH = 5, dose = 0.5).

Table 4 Thermodynamics adsorption parameters. Sorbate

CIP NOR

DH (J/mol)

20527.27 30362.73

DS (J/mol)

104.06 58.53

225

with increasing initial concentration. The reason for this behavior may be attributed to the high competition for the sorption surface sites at high concentration which leads to higher sorption rates. Weber and Morris kinetics model has been utilized to understand the mechanism of adsorption process and to determine the rate controlling step which is mainly depends on either surface or pore diffusion. This is a widely used intraparticle diffusion model, Eq. (10), to predict the rate controlling step. The R2 values (Table 3) for this model were lower compared to those obtained from pseudo-first order and pseudo-second order models. Also, there is high deviation between the calculate and experimental values Dq (%) (Table 3). In order to say that the intraparticle diffusion is the rate controlling step, the plot of qt versus t1/2 (Fig. 5b) should be linear and pass through the origin. As can be noticed from this figure, the plot did not pass through the origin and this deviation from the origin or near saturation might be due to the difference in mass transfer rate in the initial and final stages of adsorption. From these results, it can be concluded that intraparticle diffusion is not the dominating mechanism for the adsorption of both CIP and NOR on prepared carbon. 3.5. Adsorption thermodynamics According to Eqs. (12)–(14), the DH and DS parameters for CIP and NOR can be calculated from the slope and intercepts of the plot of In(Kd) versus 1/T (Fig. 5c). The calculated values of DH, DS, and DG are listed in Table 4. The obtained values for Gibbs free energy change (DG) are 11055.51, 11901.79, and 13111.30 J/mol for CIP and 12604.78, 12130.28, and 11475.35 J/mol for NOR adsorption on prepared carbon at 303, 313, and 323 K, respectively. For CIP the increase in DG values with increasing temperature shows an increase in feasibility of adsorption at higher temperatures. On the contrary, for NOR the decrease in DG values with increasing temperature shows a decrease in feasibility of adsorption at higher temperatures. This may be due to the swelling of adsorbent at high temperature giving wider pores and consequently more diffusion of CIP molecules [7]. On the contrary, for NOR which has small molecule as compared to CIP molecule, the wider pores have opposite effect represented by low fraction of narrow pores which are preferred for NOR diffusion. The negative DG values indicate spontaneous nature for the adsorption of both antibiotics. The DH parameters for CIP and NOR adsorption are 20527.27 and 30362.73 J/mol, respectively. These result indicate the endothermic and exothermic natures for CIP and NOR adsorption, respectively. Similar result was reported by El-Shafey et al. [7] who showed endothermic nature for adsorption of CIP on activated carbon prepared from date palm leaflets. The exothermic nature for adsorption of NOR was also concluded by Zhang et al. [33] for adsorption on humic acid extracted from weathered coal. The magnitude of DH gives information on the type of adsorption, which can be either physical or chemical. The enthalpy of adsorption, ranging from 2.1 to 20.9 kJ/mol corresponds to a physical sorption [34]. According to the values of DH obtained in this study, the adsorption of CIP and NOR were taken place via physisorption and chemisorption, respectively. The DS values are 104.06 and 58.53 J/mol for CIP and NOR, respectively. The negative DS value for CIP adsorption suggests a decrease in the randomness at sorbate–solution interface during the adsorption process. 4. Conclusions

DG (J/mol) 303 K

313 K

323 K

11055.51 12604.78

11901.79 12130.28

13111.30 11475.35

Microwave technique has been utilized successfully to prepare activated carbon from A. lebbeck seed pods by KOH activation. Prepared adsorbent showed efficient removal for ciprofloxacin and

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norfloxacin with maximum removal percentages of 96.12% and 98.12%, respectively. Equilibrium adsorption data of ciprofloxacin and norfloxacin were well represented by Langmuir isotherm model, showing maximum adsorption capacities of 131.14 and 166.99 mg/g, respectively. The adsorption kinetic data were well described by the pseudo-second order model for both adsorbates at different initial adsorbate concentrations. Ciprofloxacin adsorption was endothermic compared to exothermic adsorption of norfloxacin on prepared carbon. Acknowledgement We gratefully acknowledge University of Baghdad and Chemical Engineering Department for assist and support of this work. References [1] Pico Y, Andreu V. Fluoroquinolones in soil-risks and challenges. Anal Biochem 2007;387:1287–99. [2] Hirsch R, Ternes TA, Haberer K, Kratz KL. Occurrence of antibiotics in the environment. Sci Total Environ 1999;225:110–8. [3] Batt AL, Kim S, Aga DS. Comparison of the occurrence of antibiotics in four fullscale wastewater treatment plants with varying designs and operations. Chemosphere 2007;68:428–35. [4] Horsing M, Ledin A, Grabic R, Fick J, Tysklind M, Jansen JL, et al. Determination of sorption of seventy five pharmaceuticals in sewage sludge. Water Res 2011;45:4470–82. [5] Carabineiro SAC, Thavorn-amornsri T, Pereira MFR, Serp P, Figneiredo JL. Comparison between activated carbon, carbon xerogel, and carbon nanotubes for the adsorption of antibiotic ciprofloxacin. Catal Today 2012;186:29–34. [6] Auta M, Hameed BH. Preparation of waste tea activated carbon using potassium acetate as an activating agent for adsorption of acid blue 25 dye. Chem Eng J 2011;171:502–9. [7] El-Shafey EI, Al-Lawati H, Al-Sumri AS. Ciprofloxacin adsorption from aqueous solution onto chemically prepared carbon from date palm leaflets. J Environ Sci 2012;24:1579–86. [8] Carabineiro SAC, Thavorn-amornsri T, Pereira MFR, Serp P, Figneiredo JL. Adsorption of ciprofloxacin on surface-modified carbon materials. Water Res 2011;45:4583–91. [9] Sun Y, Yu Q, Gao B, Huang L, Xu X, Li Q. Comparative study on characterization and adsorption properties of activated carbon with H3PO4 and H4P2O7 activation employing cyperus alternifolins as precursor. Chem Eng J 2011;181–182:790–7. [10] Liu W, Zhang J, Zhang C, Ren L. Sorption of norfloxacin by lotus stalk-based activated carbon and iron-doped activated alumina: mechanisms, isotherms and kinetics. Chem Eng J 2011;171:431–8. [11] Huijun X, Weifeng L, Jian Z, Chenglu Z, Liang R. Sorption of norfloxacin from aqueous solution by activated carbon developed from Trapa natans husk. Sci China Chem 2011;54:835–43. [12] Fernandez N, Chacin E, Garcia C, Alastre N, Leal F, Forster CF. The use of seed pods from Albizia lebbeck for the removal of alkyl benzene sulphonates from aqueous solutions. Process Biochem 1996;31:383–7.

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