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Adsorption of Norfloxacin from Aqueous


Adsorption

969

Cong-Liang Zhang1 Bao-Ying Li1 Yan Wang1
1

Research Article

College of Chemical Engineering, Zhengzhou University, Zhengzhou, P. R. China.

Adsorption of Norfloxacin from Aqueous Solution onto Modified Coal Fly Ash
Batch adsorption experiments were carried out for the removal of norfloxacin from aqueous solution using modified coal fly ash as adsorbent. The effects of various parameters such as contact time, initial solution concentration and temperature on the adsorption system were investigated. The optimum contact time was found to be 100 min. The equilibrium experimental data can be well fitted by the Freundlich model. Thermodynamic parameters such as DG, DH and DS were also calculated. The negative Gibbs free energy change and enthalpy change indicated the spontaneous and exothermic nature of the adsorption, and the negative entropy change indicated that the adsorption process was aided by decreased randomness.
Keywords: Adsorption, Isotherm, Modified coal fly ash, Norfloxacin Received: May 15, 2009; revised: October 21, 2009; accepted: March 30, 2010 DOI: 10.1002/ceat.200900241

1

Introduction

2
2.1

Experimental
Materials

Norfloxacin, one of the quinolones, which are among the most important classes of synthetic antibacterial agents used in human and veterinary medicines, is active against many pathogenic bacterial species as gyrase inhibitor, which selectively inhibits bacterial DNA synthesis. Quantities of these drugs are potentially excreted as the parent compound or its metabolites and may enter the environment due to the spreading of manure and its slurry on agricultural land, or through direct deposition by grazing livestock [1]. Its properties make it difficult to biodegrade norfloxacin or remove norfloxacin from aqueous solution. Among the techniques for the removal of norfloxacin from wastewater, adsorption has been proved to be an effective and attractive process because of its inexpensive nature and ease of operation [2–6]. However, only limited information on the adsorption behavior of norfloxacin has been reported [7]. In this study, the effects of different parameters, including contact time, initial solution concentration, and temperature, were studied. The isotherms and thermodynamics were also investigated in detail.

Norfloxacin, obtained from Daming Biotech. Co. Ltd., was further purified by recrystallization from aqueous solutions. After filtration and drying, its purity was determined by UV spectrometry (UV-2401PC; Shimadzu) to be 0.996 in mass fraction. The structure of norfloxacin is displayed in Fig. 1. H2SO4 and NaOH used in the experiments were analyticalgrade reagents. The coal fly ash was sampled from the Dengfeng Electric Power Plant in China, mixed with 6 mol/L NaOH in a stainless-steel beaker and maintained at 363 K for 1 h with stirring. The latter mixture was left for 4 h to let the complexes settle down, which were then dialyzed against distilled water, powdered, ground and stored in a desiccator until required. The modified product, i.e. modified coal fly ash (MCFA), contained SiO2 (43.5 %), Al2O3 (17.8 %), CaO (20.7 %), MgO (1.6 %), Fe2O3 (10.8 %), and SO3 (0.8 %), and the loss on ignition (LOI) was 5.8 %. The BET surface area of MCFA was

F N HN

O

COOH


Correspondence: Dr. C.-L. Zhang (zhangcl201@zzu.edu.cn), College of Chemical Engineering, Zhengzhou University, 97th Wenhua Road, Zhengzhou 450002, P. R. China.

N CH3

Figure 1. Molecular structure of norfloxacin.

Chem. Eng. Technol. 2010, 33, No. 6, 969–972

? 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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C.-L. Zhang et al.

found to be 17.5 m2/g (the BET surface area of raw coal fly ash is 5.4 m2/g) by the N2 adsorption isotherm at 77 K, using a QUADRASORB SI automated surface area analyzer (Quantachrome Corporation, USA).

2.2

2.1

2.2

Methods

The MCFA adsorbent (4.0 g) was added to 100 mL norfloxacin solution. The initial concentrations of the norfloxacin solutions used were 40, 60, 80, 100, 120 and 140 mg/L. The initial pH values of the norfloxacin solutions were not adjusted. The adsorption experiments were conducted under constant stirring at controlled temperatures for a certain period. The concentrations of norfloxacin in the residual solutions were analyzed by means of a UV spectrometer (UV-2401PC; Shimadzu) [8]. The wavelength used to analyze the norfloxacin concentrations was 274 nm. The adsorption capacities were calculated according to the mass balance of norfloxacin in the solutions and were represented in units of milligrams of norfloxacin per gram of adsorbent. The adsorption capacities at equilibrium were computed according to Eq. (1): Qe ? ?c0 ? ce ?V=m

Qe (mg/g)

2.0

298K, c0 = 100 mg/L
1.9

40

80

120

160

contact time (min)
Figure 2. Effect of contact time on the adsorption of norfloxacin by MCFA.

(1)

where Qe and ce are the amount adsorbed (mg/g) and the residual concentration (mg/L) at equilibrium, respectively; c0 is the initial concentration of norfloxacin (mg/L); V and m are the volume of the norfloxacin solution (L) and the mass of adsorbent used (g), respectively.

3
3.1

Results and Discussion
Effect of Contact Time

adsorption increased while the percentage adsorption decreased with increasing initial norfloxacin concentration. The number of norfloxacin ions around the absorbent sites of MCFA increased with increasing initial norfloxacin concentration. Hence, the adsorption process was carried out more efficiently, resulting in the increase of the unit adsorption. It can be found that the maximum adsorption capacity of norfloxacin occurred at 298 K, with the adsorption capacities decreasing in the order 298 K > 308 K > 318 K. The decrease in the adsorption capacity upon increasing the temperature indicates that the adsorption was an exothermic process.

Fig. 2 shows the effect of contact time on the adsorption of norfloxacin by MCFA for different initial norfloxacin concentrations. The adsorption was rapid in the initial 100 min; thereafter, the rate of adsorption decreased gradually. At some point in time, when the amount of norfloxacin being adsorbed onto the adsorbent was equal to the amount of norfloxacin being desorbed from the adsorbent, the adsorption process reached a dynamic equilibrium and the adsorption amount remained nearly constant. From contact time curves, it was observed that the equilibrium time was 100 min for all norfloxacin concentrations. This is the reason why the optimum contact time was 100 min in the above batch equilibrium experiments.

2.0

Qe (mg/g)

1.5

298K 308K 318K
1.0

3.2

Effect of Initial Norfloxacin Concentration and Temperature

40

80

120

The effects of the initial norfloxacin concentration and the temperature on the adsorptive removal of norfloxacin by MCFA are shown in Fig. 3. It can be observed that the unit

c0 (mg/L)
Figure 3. Effect of initial norfloxacin concentration and temperature on the adsorption process.

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Chem. Eng. Technol. 2010, 33, No. 6, 969–972

Adsorption

971

3.3

Adsorption Equilibrium of Norfloxacin on MCFA

Adsorption isotherms were measured for norfloxacin on MCFA at 298, 308 and 318 K, and the results are shown in Fig. 4. It was found that the adsorption capacity of MCFA increased as the concentration of norfloxacin in water increased, and the adsorption capacity tended to decrease with increasing temperature. The adsorption isotherms were simulated by the Freundlich model [9] (Eq. 2): logQe ? logKf ? 1 logce n (2)

where ce1 and ce2 are the equilibrium concentrations of solutions (mg/L) at the same Qe, at the absolute temperatures T1 (K) and T2 (K), respectively. DH is the enthalpy change in adsorption (J/mol) and R is the gas constant (8.314 J/ K mol). If the adsorption can be simulated by the Freundlich model, the Gibbs free energy change for the adsorption process is obtained with a derivative equation [11] (Eq. 4): DG ? ?nRT (4)

where Kf and n are parameters indicating the capacity and intensity of adsorption, respectively. The corresponding parameters of MCFA are summarized in Tab. 1. Tab. 1 shows that the experimental data fit well to the Freundlich model. The value of Kf decreases with increasing temperature, and it is obvious that low temperature is helpful for adsorption and that the adsorption mechanism is physical adsorption. In order to further understand the adsorption mechanism of norfloxacin on MCFA, thermodynamic analysis was performed. The isosteric enthalpies of adsorption were calculated with a derivative of the Van’t Hoff equation [10] (Eq. 3):     T1 T2 c DH ? R ln e1 (3) T2 ? T1 ce2

where DG is the Gibbs free energy change in adsorption (J/mol) and n is the corresponding parameter of the Freundlich equation. The adsorption entropies were calculated according to the Gibbs-Helmholtz equation [12] (Eq. 5): DS ? ?DH ? DG?=T (5)

2.0

where DS is the entropy change in adsorption (J/mol K). The calculated isosteric enthalpy changes (DH), Gibbs free energy changes (DG) and entropy changes (DS) of MCFA are presented in Tab. 2. Based on the analysis of the results in Tab. 2, the negative values of the Gibbs free energy changes at all temperatures indicate that the adsorption process is favorable and spontaneous. The negative values of all the enthalpy changes imply that the adsorption process of norfloxacin on MCFA is exothermic. Compared with the movement of norfloxacin in solution, the movement of norfloxacin absorbed on MCFA was greatly restricted, which is suggested by the negative values of all the entropy changes.

Qe (mg/g)

4
1.5

Conclusions

298K 308K 318K
1.0

0

20

40

60

ce (mg/L)
Figure 4. Adsorption isotherms of norfloxacin on MCFA at 298, 308 and 318 K.

Batch adsorption experiments were carried out for the removal of norfloxacin from aqueous solution using MCFA as adsorbent. The effects of various parameters such as contact time, initial solution concentration and temperature on the adsorption system were investigated. The optimum contact time was found to be 100 min. The equilibrium experimental data at 298, 308 and 318 K can be well fitted by the Freundlich model. Thermodynamic parameters such as DG, DH and DS were also calculated. The negative Gibbs free energy change and enthalpy change indicated the spontaneous and exothermic nature of the adsorption, and the negative entropy change indicated that the adsorption process was aided by decreased randomness.

Table 1. Freundlich parameters for norfloxacin on MCFA at various temperatures.
Temperature [K] 298 308 318 Isotherm equation log Qe = 0.2507 log ce – 0.0612 log Qe = 0.2515 log ce – 0.0827 log Qe = 0.2583 log ce – 0.1308 Kf 0.8686 0.8266 0.7399 n 3.989 3.976 3.871 r2 0.9972 0.9971 0.9980

Acknowledgement
This research was financially supported by the Henan Province Natural Science Fund of P. R. China for basic research under Grant No. 0611033400. The authors have declared no conflict of interest.

Chem. Eng. Technol. 2010, 33, No. 6, 969–972

? 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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C.-L. Zhang et al.

Table 2. Thermodynamic parameters for norfloxacin on MCFA.
Qe [mg/g] DH [kJ/mol] 298 K 0.9 1.2 1.5 1.8 2.0 –24.28 –22.95 –21.92 –21.08 –20.59 –9.883 DG [kJ/mol] 308 K –10.18 318 K –10.23 298 K –48.31 –43.85 –40.39 –37.57 –35.93 DS [J/mol K] 308 K –45.78 –41.46 –38.12 –35.39 –33.80 318 K –44.18 –40.00 –36.76 –34.12 –32.58

Symbols used
c0 ce DG DH Kf m n Qe r R DS T V [mg/L] [mg/L] [J/mol] [J/mol] [–] [g] [–] [mg/g] [–] [J/K mol] [J/mol K] [K] [L] initial concentration of norfloxacin residual concentration at equilibrium Gibbs free energy change in adsorption enthalpy change in adsorption parameter indicating the capacity of adsorption mass of adsorbent used parameter indicating the intensity of adsorption residual amount adsorbed at equilibrium correlative coefficient for the Freundlich model gas constant entropy change in adsorption absolute temperature volume of norfloxacin solution

References
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