The sorption interaction of three widely used tetracycline antibiotics, namely, tetracycline, chlortetracycline, and oxytetracycline, under various conditions of time, pH, temperature, ionic strength, and exchangeable cations on illites and kaolinites was examined. First-order reaction indicated diffusion-controlled adsorption, maximum adsorption occurring at pH values of 3.5, 3, and 4 for tetracycline, oxytetracycline, and chlortetracycline, respectively. Adsorption isotherms of “L” type showed that the adsorption of antibiotics was in the order of chlortetracycline > oxytetracycline > tetracycline. Adsorption was maximum for Al-saturated complexes and followed the order of Al- > Na- > K- Ca. The adsorption varied inversely with changes in temperature and ionic strength up to 0.6 after which it became constant. Free energy changes (Δ
The use of antibiotics as veterinary pharmaceuticals has become the integral part of the animal food industry because of their valuable contributions in treating diseases [
As Indian saline soil mainly contains illite and kaolinite clays, the purpose of this work is to study the sorption of tetracycline, oxytetracycline, and chlortetracycline on Morris illite and Bath kaolinite under different physical and chemical conditions along with X-ray diffraction, IR analyses, and thermodynamic parameters in order to evaluate the feasibility of clays to remove tetracyclines from wastewater.
The minerals used in these studies were an illite from Morris, IL, USA, and monomineralic standard kaolinite from Bath, SC, USA. Through treatment with 1 N NaCl, a <2 um fraction of clay samples was purified by sedimentation and centrifugation and converted into Cl− free Na-saturated clay. Homoionic suspensions of K-, Ca-, and Al-illite and kaolinite were obtained from the Na form by the ion exchange technique. The CEC of illite was 20.8 C mol (p+) kg−1 for the illite and 11 C mol (p+) kg−1 for the kaolinite.
The effect of the equilibrium pH on adsorption of tetracycline, oxytetracycline, and chlortetracycline was investigated at pH values ranging between 2 and 11 with a 0.5 increment obtained by adding 0.1 M HCl or 0.1 M NaOH as required on Na-illite and Na-kaolinite. A 10 mL of clay suspension (containing 0.1 g of clay) was mixed with a 15 mL of tetracycline solution (100
Batch sorption experiments were conducted using 10 mL of the appropriate illite or kaolinite suspensions in a large number of glass stoppered tubes, adding 0 to 15 mL of standard tetracyclines solution (concentration 100
For ionic strength experiment, 10 mL of appropriate illite or kaolinite suspensions (containing 0.1 g of clay) were mixed with a 15 mL of tetracyclines solution (100
The concentrations of TC, OTC, and CTC in extracts were analyzed by HPLC using an Agillent 1100 system with an octadecylsilane column (
Metal cations desorbed were estimated in the supernatant solution by atomic absorption spectrophotometer/flame photometer.
The results of adsorption were correlated with X-ray and IR studies. For X-ray analysis, the clay samples and clay-antibiotics complexes were orientated on glass slides and allowed to dry at room temperature. The X-ray patterns were recorded on a General Electric XRD 6 diffraction unit at 2
All the chemicals used were of analytical grade and all the experiments were done in three replicates.
The first-order kinetics model was best fitted for the sorption of tetracycline, oxytetracycline, and chlortetracycline on illites and kaolinites. The first-order kinetics model is given by the following expression:
The influence of pH on the adsorption of tetracyclines on illite (Figure
Sorption of tetracyclines on illite and kaolinite at different pH values.
The data showing the effect of solution ionic strength on sorption of tetracyclines (Table
Adsorption of tetracycline, oxytetracycline, and chlortetracycline on illite and kaolinite saturated with different cations at several ionic strengths (of appropriate salt) at 293 K.
Ionic strength | Saturating |
Amount of tetracycline sorbed |
Amount of oxytetracycline sorbed |
Amount of chlortetracycline sorbed | |||
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Al3+ | 8.62 | 4.65 | 8.85 | 4.78 | 9.03 | 4.92 |
Na+ | 8.41 | 4.54 | 8.67 | 4.65 | 8.82 | 4.78 | |
K+ | 8.29 | 4.41 | 8.52 | 4.52 | 8.65 | 4.65 | |
Ca2+ | 8.15 | 4.25 | 8.41 | 4.40 | 8.49 | 4.52 | |
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Al3+ | 8.23 | 4.32 | 17.10 | 4.52 | 17.32 | 4.74 |
Na+ | 8.04 | 4.18 | 16.84 | 4.38 | 17.10 | 4.50 | |
K+ | 7.95 | 4.07 | 16.66 | 4.25 | 16.90 | 4.40 | |
Ca2+ | 7.77 | 3.96 | 16.40 | 4.14 | 16.62 | 4.25 | |
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Al3+ | 7.96 | 4.02 | 16.70 | 4.26 | 16.85 | 4.45 |
Na+ | 7.84 | 3.90 | 16.44 | 4.12 | 16.62 | 4.26 | |
K+ | 7.68 | 3.75 | 16.25 | 3.92 | 16.40 | 4.10 | |
Ca2+ | 7.52 | 3.58 | 16.08 | 3.78 | 16.22 | 3.88 | |
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Al3+ | 7.15 | 3.76 | 16.35 | 3.90 | 16.55 | 4.12 |
Na+ | 6.95 | 3.62 | 16.12 | 3.75 | 16.28 | 3.88 | |
K+ | 6.74 | 3.45 | 15.90 | 3.55 | 16.04 | 3.65 | |
Ca2+ | 6.50 | 3.32 | 15.68 | 3.45 | 15.84 | 3.54 | |
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Al3+ | 6.75 | 3.58 | 15.75 | 3.70 | 15.94 | 3.88 |
Na+ | 6.44 | 3.46 | 15.62 | 3.55 | 15.80 | 3.65 | |
K+ | 6.32 | 3.28 | 15.54 | 3.40 | 15.65 | 3.43 | |
Ca2+ | 6.18 | 3.14 | 15.38 | 3.22 | 15.50 | 3.38 | |
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Al3+ | 6.24 | 3.44 | 15.18 | 3.54 | 15.32 | 3.62 |
Na+ | 5.92 | 3.28 | 15.06 | 3.36 | 15.20 | 3.55 | |
K+ | 5.78 | 3.14 | 14.92 | 3.24 | 15.06 | 3.34 | |
Ca2+ | 5.58 | 3.07 | 14.80 | 3.12 | 14.92 | 3.26 | |
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Al3+ | 6.14 | 3.32 | 6.36 | 3.38 | 6.48 | 3.47 |
Na+ | 5.86 | 3.15 | 6.02 | 3.20 | 6.10 | 3.29 | |
K+ | 5.74 | 3.07 | 5.84 | 3.10 | 5.98 | 3.16 | |
Ca2+ | 5.56 | 2.95 | 5.72 | 3.02 | 5.86 | 3.10 |
The experimental data of sorption of studied tetracyclines on illite and kaolinite at pH 5-6 in the studied range were best fitted (
Freundlich adsorption isotherm constants of three antibiotics adsorption on illite and kaolinite, saturated with Na-, K-, Ca-, and Al-cations.
Sorbent | Tetracycline | Oxytetracycline | Chlortetracycline | |||||||||||||||
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293 | 308 | 323 K | 293 | 308 | 323 K | 293 | 308 | 323 K | 293 | 308 | 323 K | 293 | 308 | 323 K | 293 | 308 | 323 K | |
Al-illite | 108 | 100 | 94 | 0.880 | 0.855 | 0.825 | 113 | 105 | 99 | 0.895 | 0.865 | 0.840 | 117 | 109 | 103 | 0.915 | 0.885 | 0.865 |
Na-illite | 103 | 97 | 90 | 0.865 | 0.835 | 0.800 | 106 | 101 | 96 | 0.880 | 0.850 | 0.820 | 111 | 104 | 99 | 0.895 | 0.865 | 0.840 |
K-illite | 98 | 93 | 85 | 0.845 | 0.820 | 0.795 | 102 | 98 | 93 | 0.860 | 0.830 | 0.805 | 106 | 100 | 95 | 0.875 | 0.855 | 0.830 |
Ca-illite | 93 | 88 | 80 | 0.835 | 0.805 | 0.785 | 96 | 92 | 85 | 0.845 | 0.820 | 0.805 | 102 | 96 | 90 | 0.860 | 0.835 | 0.815 |
Al-kaolinite | 105 | 98 | 92 | 0.865 | 0.850 | 0.820 | 109 | 103 | 97 | 0.885 | 0.860 | 0.835 | 115 | 107 | 102 | 0.890 | 0.865 | 0.845 |
Na-kaolinite | 101 | 95 | 88 | 0.855 | 0.830 | 0.810 | 103 | 98 | 93 | 0.875 | 0.850 | 0.820 | 110 | 103 | 97 | 0.880 | 0.850 | 0.835 |
K-kaolinite | 96 | 91 | 86 | 0.840 | 0.810 | 0.790 | 101 | 95 | 90 | 0.860 | 0.835 | 0.805 | 105 | 99 | 93 | 0.865 | 0.845 | 0.820 |
Ca-kaolinite | 91 | 86 | 79 | 0.830 | 0.800 | 0.775 | 95 | 90 | 86 | 0.845 | 0.820 | 0.795 | 99 | 93 | 89 | 0.855 | 0.830 | 0.810 |
Adsorption isotherms for sorption of tetracycline, oxytetracycline, and chlortetracycline on illite and kaolinite saturated with different cations.
Figure
Effect of temperature on adsorption of tetracyclines on Na-saturated illites and kaolinites.
The amount of metal cations desorbed (Figure
Amount of total metal cations desorbed as affected by tetracycline sorption on illite and kaolinite.
The thermodynamic parameters of sorption were calculated using following relationship:
The values of
Thermodynamic values of tetracycline, oxytetracycline, and chlortetracycline sorption on Na-illite at different pH values.
Illite
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293 K | 308 K | 323 K | 293 K | 308 K | 323 K | |||
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Kaolinite
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293 K | 308 K | 323 K | 293 K | 308 K | 323 K | |||
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The negative values of overall adsorption heat (
An examination of IR spectra showed a shift of bands at 1620 cm−1 (
The present study reveals that adsorption of three tetracycline antibiotics on illite and kaolinite is influenced by solution pH and solution ionic strength. The sorption obeyed first-order kinetics and equilibrium is attained within 24 h. Sorption of tetracyclines on illite and kaolinite followed Freundlich sorption isotherm. From the thermodynamic parameters, it may be inferred that adsorption of all the studied antibiotics on clay minerals occurs via protonation and/or coordination in between cations of clay and oxygen of >C=O group of tetracyclines. The adsorption is correlated directly with the polarizing power of exchangeable cation and inversely with the temperature and solution ionic strength. The adsorption was in the order CTC > OTC > TC. XRD and IR data showed that the sorption is restricted to the external surfaces of clay.
The author expresses his sincere thanks to The University Grants Commission, New Delhi, India, for the financial assistance.