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Adsorption of ciprofloxacin on 2:1 dioctahedral clay minerals
Applied Clay Science 53 (2011) 723–728
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Research Paper
Adsorption of ciprofloxacin on 2:1 dioctahedral clay minerals Chih-Jen Wang a, b, Zhaohui Li a, c,⁎, Wei-Teh Jiang a,⁎ a b c
Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan, ROC Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Geosciences, University of Wisconsin-Parkside, Kenosha, WI 53144, USA
a r t i c l e
i n f o
Article history: Received 12 December 2010 Received in revised form 24 June 2011 Accepted 24 June 2011 Available online 29 July 2011 Keywords: Antibiotics Cation exchange Ciprofloxacin Clay minerals Intercalation
a b s t r a c t Frequent detection of antibiotics in natural environment and wastewater effluent requires systematic investigations of various clay minerals. Dioctahedral 2:1 clay minerals are major constituents of soil and sediments. Interaction of these clay minerals and ciprofloxacin (CIP) was studied. The CIP adsorption capacities on montmorillonite, rectorite, and illite were 1.19, 0.41, and 0.10 mmol/g, corresponding to 1.0, 1.0, and 0.9 CEC. Desorption of the equivalent amounts of exchangeable cations suggested that the cation exchange was mainly responsible for the CIP adsorption on montmorillonite and rectorite. The expansion of the basal spacing and shifts of characteristic FTIR bands indicated the intercalation of CIP ions with tilted orientation in montmorillonite and rectorite. The displacement of exchangeable cations and associated FTIR band shifts also indicated the cation exchange as important for CIP adsorption on illite but hydrogen bonding between CIP carboxylic groups and basal oxygen atoms on external surface apparently made a significant contribution to the adsorption. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluoroquinolones (FQs) were among the most used antibiotics applied to human therapy and veterinary treatment (Picó and Andreu, 2007). Due to the continuous increase of global needs and inappropriate discharge, they were frequently detected in a variety of natural environments. Ciprofloxacin (CIP), one of the commonly used FQs, was found in the wastewater and surface water with concentrations of several hundred ng/L (Golet et al., 2002a; Karthikeyan and Meyer, 2006; Kolpin et al., 2002; Miao et al., 2004; Renew and Huang, 2004). Higher concentrations up to 150 μg/L were even reported in the effluents from hospitals (Hartmann et al., 1999; Martins et al., 2008). In Switzerland, CIP was detected in sewage sludge from several wastewater treatment plants with concentrations ranging from 1.40 to 2.42 mg/kg (Golet et al., 2002b). These results warranted further studies on interactions between FQs and natural materials so that their fate and transport in the environment can be predicted and their removal from the environment can be facilitated. Several studies were conducted to investigate the CIP adsorption on different soils, and a wide range of the adsorption coefficient Kd from 400 to nearly 50,000 L/kg was reported in the literature (Carrasquillo et al., 2008; Córdova-Kreylos and Scow, 2007; Mackay and Seremet, 2008; Nowara et al., 1997; Uslu et al., 2008; Vasudevan et al., 2009). The large variation of Kd was often related to the diverse constituents in soils from different geographic areas. A positive correlation between Kd values and
⁎ Corresponding authors: Tel.: +886 6 2757575x65437; fax: +886 6 2740285. E-mail addresses: [email protected] (Z. Li), [email protected] (W.-T. Jiang). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.06.014
clay contents was revealed for the sediments from San Francisco Bay area (Córdova-Kreylos and Scow, 2007). In acidic to neutral water, the adsorption of CIP was strongly dependent on the cation exchange capacity (CEC) of soils (Carrasquillo et al., 2008; Vasudevan et al., 2009). These discoveries implied that clay minerals with high CECs might have a better CIP removal. Systematic investigations of adsorption capacities and properties of some widely monitored antibiotics including penicillin, tetracycline (TC), oxytetracycline, etc., on a series of 2:1 dioctahedral clay minerals (Kümmerer, 2009; Tolls, 2001) helped better understanding the behaviors and interaction mechanisms between antibiotics and minerals. However, such studies were much too sparse for CIP and other FQs. In this study, three 2:1 dioctahedral clay minerals, montmorillonite, rectorite, and illite, were tested for the adsorption of CIP from acidic solutions. 2. Materials and methods 2.1. Materials Two source clay minerals, montmorillonite (SAz-1) and illite (IMt-2) acquired from the Clay Minerals Society, and a rectorite, obtained from Zhongxiang, Hubei, China, were used as adsorbents. The CECs of SAz-1 and rectorite measured by the ammonia-electrode method were 1.23 (Borden and Giese, 2001) and 0.41 meq/g (Hong et al., 2008), with Ca2+ as the major exchangeable cation (Table 1). For IMt-2 illite, a much lower CEC of 0.11–0.12 meq/g was reported (Jaynes and Bigham, 1986; Li et al., 2003) and its exchangeable cations were Mg2+ and Ca2+. Their specific external surface area and total surface area determined by
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Table 1 Physical and chemical properties of the 2:1 clay minerals and CIP adsorption.
Mineral properties CEC (meq/g) Major exchangeable cations External specific surface area (m2/g) Total specific surface area (m2/g) Adsorption properties CIP adsorption plateau (mmol/g) Max. cation desorption (meq/g) Ca Mg Na K Total Adsorption densitya (Number of CIP molecules/100 nm2) Average distance between CIP molecules (Å)
SAz-1
Rectorite
IMt-2
1.23 Ca 65 715
0.41 Ca 10 363
0.11–0.12 Mg, Ca 22 –
1.19
0.41
0.10
0.97 0.15 0.03 0.01 1.16
0.32 0.03 0.05 0.01 0.41
0.03 0.02 – – 0.05
100 10.0
68 12.1
268 6.1
a Calculated from plateau CIP adsorption and specific surface areas of 715, 363, and 22 m2/g for SAz-1, rectorite, and IMt-2.
nitrogen adsorption and methylene blue adsorption were listed in Table 1 (Blum and Eberl, 2004; Dogan et al., 2006). Ciprofloxacin hydrochloride (CIP HCl) with a purity higher than 99.6% was purchased from Hangzhou Minsheng Pharmaceutical Group Co. Ltd (China). The molecule (Fig. 1a) is 12.2 Å long, 8.0 Å high and 4.1 Å thick (Turel and Golobic, 2003). An intramolecular hydrogen bond exists between the oxygen atom in the ketone group and the hydrogen atom in the carboxylic group (Turel and Golobic, 2003; Vázquez et al., 2001). With pKa1 = 6.1 and pKa2 = 8.7 (Gu and Karthikeyan, 2005), the charges vary with solution pH (Fig. 1b). In this study, the equilibrium solution pH was within 4–5.5, where a large fraction of CIP exists in the cationic form due to the protonation of the amine group in the piperazine moiety. 2.2. Adsorption experiments In the adsorption experiments, 0.2 g of the clay minerals were dispersed in 20 mL CIP solution in a 50 mL centrifuge tube and shaken at 150 rpm for 24 h. The tube was wrapped with aluminum foil to prevent light-induced decomposition. The adsorption kinetics were determined at an initial CIP concentration C0 = 1000 mg/L for SAz-1 and rectorite and 200 mg/L for IMt-2 at equilibrium times of 0.25, 0.5, 1, 1.5, 2, 4, 8, 12, 24 h. For the adsorption isotherm, C0 was in the range of 500–4000 mg/L for SAz-1, 150–2000 mg/L for rectorite, and 100–
1000 mg/L for IMt-2. After mixing, the dispersions were centrifuged at 5000 rpm for 10 min, and the supernatants were passed through a 0.45 μm filter. All experiments were performed in duplicate. 2.3. Methods of analyses The equilibrium CIP concentration Ce in the filtered supernatant was measured by a UV–Vis spectrophotometer (SmartSpec 3000, BioRad Corp.) at the wavelength of 275 nm. A calibration with 5 standards ranging from 0 to 10 mg/L yielded r 2 N 0.99. The amount of CIP adsorbed (Cs) was calculated from the difference between C0 and Ce. The concentration of Na +, K +, Mg 2+, and Ca 2+ released from SAz1 and rectorite during the adsorption experiments was measured by an ion chromatograph (Dionex 100). To detect the much lower concentrations of cations released from IMt-2, an Optima 7000DV ICPOES (Perkin Elmer) was used. XRD analyses of the raw and CIP adsorbed clay minerals were carried out on a D8 ADVANCE diffractometer with a CuKα radiation at 40 kV and 40 mA (Bruker Corp.). The oriented samples were air-dried and scanned from 1°–20° 2θ with a scanning speed of 0.01°/s. The FTIR spectra were acquired by an Equinox 55 Spectrometer (Bruker Corp.) using KBr discs. Absorbance data were collected by accumulating 256 scans at a resolution of 4 cm −1 in the range of 4000–400 cm −1. 3. Results and discussion 3.1. Adsorption kinetics More than 95% of the added CIP was adsorbed within 8 h by all clay minerals (Fig. 2). The pseudo-second-order kinetics was used to fit the experimental data: 2
qt =
kqe t 1+ kqe t
ð1Þ
where k (g/mg-h) is the rate constant of adsorption, qe (mg/g) the amount of CIP adsorbed at equilibrium, and qt (mg/g) the amount of CIP adsorbed at time t. The above equation can be converted into the linear form: t 1 1 = 2+ t qt qe kqe
ð2Þ
where kqe2 (mg/g-h) is the initial rate. All regression coefficients were N0.99. The rate constants were 8.0, 0.2, and 0.3 g/mg-h, and the initial
Fig. 1. Molecular structure of CIP from Cambridge Structure Database (a) and its speciation at different pHs (b).
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Fig. 2. Kinetics of CIP adsorption fitted by the pseudo-second-order equation (dashed curves).
rates were 1.1 × 10 5, 2.5 × 10 2, and 1.1 × 10 2 mg/g-h for SAz-1, rectorite, and IMt-2, respectively. These results indicated a relatively low reaction barrier for CIP adsorption on montmorillonite and thus yielded the adsorption N 99%, even at the short mixing time of 0.25 h. The rate constants and initial rates for CIP adsorption were much higher for the TC adsorption on the same SAz-1 and rectorite (Chang et al., 2009; Li et al., 2010), indicating enhanced adsorption of CIP compared to TC on these clay minerals. The kinetics results also confirmed that 24 h was a sufficient time for CIP adsorption to reach equilibrium. 3.2. Adsorption isotherms The adsorption isotherms of CIP on the clay minerals are shown in Fig. 3. The Langmuir model fitted the three adsorption data better than the Freundlich model. The regression coefficients were N0.995. Cs =
KL Csm Ce 1+ KL Ce
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2005; Zhang and Huang, 2007), aluminum hydroxide (Gu and Karthikeyan, 2005), and kaolinite (Mackay and Seremet, 2008; Pei et al., 2010), which exhibited much lower adsorption capacities of 0.06–0.15 mmol/g, 0.04 mmol/g, and 0.01–0.02 mmol/g at similar pH values. For SAz-1 and rectorite, the occupied areas would be 9 and 4 Å 2 if all adsorbed CIP molecules were assumed to cover the external surface. Compared with 32 Å 2 (the minimal cross-section area when the long axis of CIP molecule is perpendicular to the mineral surface), the much lower areas imply that the CIP molecules were not only adsorbed at the external surface area. When the total surface areas (including the interlayer spaces) were considered, the area occupied per CIP molecule was 100 Å 2 and 147 Å 2 on SAz-1 and rectorite, large enough to bind all CIP molecules at any orientations. This simple estimation demonstrated that the interlayer space was available for CIP adsorption in both minerals. On the contrary, the much lower CIP adsorption of 0.10 mmol/g on IMt-2 corresponded to an area of 37 Å 2 per CIP molecule if only the external surface area was taken into consideration. The protonation of amine groups of the CIP molecules under the experimental conditions and the positive relations between the CIP adsorption capacity and the CEC of the three clays minerals suggested that cation exchange was the principal adsorption mechanism. The CIP adsorption sites were mainly in the interlayer space of montmorillonite and rectorite but only on the external surface of illite. 3.3. Cation desorption The total amounts of displaced cations plotted against the CIP adsorption on SAz-1 and rectorite yielded a linear curve with the slope of 1 and negligible intercept (Fig. 4a), confirming that cation exchange interaction was the dominant adsorption mechanism. The total amounts of displaced cations desorbed at plateau adsorption values
ð3Þ
where KL (L/mg) was the Langmuir coefficient and Csm (mg/g) the adsorption maximum. The CIP adsorption at the plateau was 395, 135, and 33 mg/g, or 1.19, 0.41, and 0.10 mmol/g, corresponding to 1.0, 1.0, and 0.9 CEC for SAz-1, rectorite, and IMt-2. Thus, these clay minerals were effective adsorbents to remove CIP from acidic aqueous solutions (pH 4–5.5) in comparison to goethite (Gu and Karthikeyan,
Fig. 3. Adsorption isotherms of CIP. Solid and dash curves are Langmuir and Freundlich fits to the observed data.
Fig. 4. Relation between desorption of exchangeable cations and CIP adsorption for (a) the SAz-1 montmorillonite and rectorite, (b) the IMt-2 illite.
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stretching vibration at 1624 cm −1, and the coupling of the carboxylic acid C–O stretching and O–H deformation vibration at 1274 cm −1 (Trivedi and Vasudevan, 2007). The absorption band at 1385 cm −1 was due to the protonation of the amine group of the piperazine moiety (Gu and Karthikeyan, 2005). Two changes were found after the adsorption of CIP onto the clay minerals. The shift of the band of the protonated amine group to 1390 cm −1 suggested the presence of electrostatic attraction between the protonated amine group and the surface charges. A theoretical calculation revealed that such interaction was essential for the zwitterionic CIP attaching to the montmorillonite surface to reach an energetically stable configuration (Carrasquillo et al., 2008). In addition, the ketone stretching band shifted to a higher frequency of 1630 cm −1, indicating reinforcement of the ketone C_O bond (Trivedi and Vasudevan, 2007) due to the release of intramolecular hydrogen bonding between the ketone group and the carboxylic group. Thus, a new hydrogen bond between the hydrogen of the carboxylic acid groups and the basal oxygen of the minerals could be formed. This hydrogen bonding mechanism could be responsible for 50% of the adsorbed CIP molecules that were not attributed to the cation exchange. Hydrogen bonding might be an important mechanism for the adsorption of organic cations on 2:1 clay minerals of low CEC.
were close to the CEC of both minerals, indicating almost quantitative displacement of the exchangeable cations by CIP (Table 1). For IMt-2, the amounts of CIP adsorbed were always higher than the amounts of desorbed cations (Fig. 4b). At the adsorption plateau of 0.1 mmol/g, only 0.05 meq/g cations were desorbed. This difference suggested the contribution of further adsorption mechanisms. With the structural formula (Ca0.06 Mg0.09 K1.37)[Al2.69Fe 3+0.76Fe 2+0.06Mntr Mg0.43Ti0.06][Si6.77Al1.23]O20(OH)4 for IMt-2 reported by the Clay Minerals Society, the calculated quantities of exchangeable Mg 2+ and Ca 2+ should be 0.06 and 0.04 meq/g, respectively. However, at the CIP adsorption plateau (Table 1) the desorption of Mg 2+ (0.02 meq/g) was smaller than that of Ca 2+ (0.03 meq/g), indicating that a higher fraction of Mg 2+ remained in the illite. Although the data were limited, the selective desorption of the exchangeable cations could be related to a difference in their polarization power (ionic charge/radius ratio = 2.02 for Ca 2+ and 2.78 for Mg 2+). The higher polarization power of Mg 2+ leads to electrostatic attraction to the negatively charged surfaces, and thus is less likely to be replaced by CIP molecules. For the adsorption of hexadecyl trimethylammonium (HDTMA) (Lee and Kim, 2003) and cationic tetracycline (Li et al., 2010) on a low-charge montmorillonite (SWy-2) with Na + and Ca 2+ as principal exchangeable cations, the preferential release of Na + (polarization power = 1.05) could be caused by the same effect. Other reaction mechanisms, such as formation of surface complexes and the occurrence of cation bridging, were proposed for zwitterionic/anionic FQs adsorption on aluminum and iron (hydr) oxides (Gu and Karthikeyan, 2005) and montmorillonite (Nowara et al., 1997). The deportonated carboxylic group could play a critical role for the interaction with exposed surface metal ions or interlayer cations of the clay minerals. Nevertheless, both mechanisms were believed to be less important since CIP was mainly in the cationic form at low pH and the fraction of positively charged edges that could bind the COO - groups was small.
The d001-value of raw SAz-1 was 15.04 Å, representing a Ca 2+dominated montmorillonite saturated with two layers of interlayer water (Fig. 6). The basal spacing was 9.96 Å for illite IMt-2 and 24.1 Å for rectorite as characteristic of a hydrated Ca-rectorite. CIP intercalation increased the basal spacing of SAz-1 and rectorite to 17.2 and 25.8 Å at CIP adsorption levels of 1.06 and 0.37 mmol/g. For IMt-2, the basal spacing remained the same after CIP saturation.
3.4. FTIR analyses
3.6. CIP conformation and adsorption density
The spectra of pure CIP HCl and the clay minerals showed well resolved vibration bands (Fig. 5). Within 1250–1850 cm −1, the observed band positions of pure CIP were assigned to the C = O stretching vibration of carboxylic acid at 1707 cm −1, the ketone C = O
Based on the XRD and FTIR data, the conformation of adsorbed CIP molecules in the interlayer space of SAz-1 and rectorite was deduced. In acidic conditions, the gallery heights would be 7.23 and 5.80 Å for CIP saturated SAz-1 and rectorite assuming the d001-value of
3.5. XRD analyses
Fig. 5. FTIR spectra of pure CIP and the CIP adsorbed clay minerals. Band positions of pure CIP are marked by solid lines and those after adsorption onto the clay minerals are denoted by dashed lines.
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Fig. 6. XRD patterns showing the 001 reflection of montmorillonite (SAz-1), rectorite, and illite (IMt-2) (gray) and those of the CIP-adsorbed clay minerals as in Fig. 5 (black).
dehydrated montmorillonite and rectorite was 10 and 20 Å. Considering the interaction between the amine group and the mineral surfaces, the CIP molecules intercalated into montmorillonite and rectorite would take a tilted orientation at 36º and 29º to the basal plane (Fig. 7). The density and average distance between the CIP molecules were calculated using from the adsorption capacities and the surface areas of the minerals (Table 1). On every 100 nm 2, the SAz-1 could accommodate 100 molecules, higher than the capacity of rectorite. For IMt-2, the CIP molecules were adsorbed on the external surface. Though the molecular conformations could not be resolved with the XRD and FTIR investigation, the high CIP adsorption density (268 molecules/100 nm 2) and short average distance implied that the CIP molecules might have their long axes nearly perpendicular to the mineral surface so as to minimize the repulsion between the hydrogen atoms of neighboring CIP molecules. At pH 7, aluminum and iron (hydr)oxides (HAO and HFO) absorbed 0.04 and 0.07 mmol/g CIP (Gu and Karthikeyan, 2005). The adsorption was attributed to the formation of surface complexes between the zwitterionic CIP and HAO/HFO surface metal ions. Using the specific surface areas of 386 and 322 m 2/g for HAO and HFO, the adsorption densities were only 6 and 13 CIP molecules/100 nm 2. It appears that the reactive sites on the HAO/HFO surfaces were highly limited. On the contrary, the adsorption of cationic CIP on 2:1 clay minerals were high and positively correlated to the CEC values. However, the properties of exchangeable cations such as the polarization power could also play a significant role on the interpretation or prediction of adsorption behaviors in some cases. These observations are considered fundamental for further understanding of CIP adsorption onto soils and sediments.
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Fig. 7. Schematic models illustrating the conformations of CIP molecules intercalated in montmorillonite (a) and rectorite (b).
4. Conclusions (1) Adsorption of CIP on the montmorillonite, rectorite, and illite followed the Langmuir adsorption isotherms with plateau adsorption values of 1.19, 0.41, and 0.10 mmol/g, close to the CEC values. (2) The protonated amine groups and the carboxylic groups of the CIP molecules were responsible for the electrostatic attraction and hydrogen bonding to the external and internal surfaces of the clay minerals. (3) Stoichiometric relationships between the exchangeable cations displaced and the CIP molecules adsorbed and expansion of the basal spacing by intercalation demonstrated that cation exchange interaction was the dominant mechanism for CIP adsorption onto montmorillonite and rectorite. (4) Illite adsorbed CIP only on the external surfaces by cation exchange and hydrogen bonding. A high fraction of exchangeable Mg 2+ remaining in illite after CIP adsorption may relate to its comparatively large polarization power. Acknowledgments We are grateful to the reviewers' constructive comments. Funding from National Cheng Kung University (NCKU) for the project of Promoting Academic Excellence & Developing World Class Research Centers to support Li's short term visits to NCKU made this publication possible. We also thank the funding from National Science Council (Taiwan) to Jiang under grant NSC98-2116-M-006-005 and to Wang
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Research Paper
Adsorption of ciprofloxacin on 2:1 dioctahedral clay minerals Chih-Jen Wang a, b, Zhaohui Li a, c,⁎, Wei-Teh Jiang a,⁎ a b c
Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan, ROC Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Geosciences, University of Wisconsin-Parkside, Kenosha, WI 53144, USA
a r t i c l e
i n f o
Article history: Received 12 December 2010 Received in revised form 24 June 2011 Accepted 24 June 2011 Available online 29 July 2011 Keywords: Antibiotics Cation exchange Ciprofloxacin Clay minerals Intercalation
a b s t r a c t Frequent detection of antibiotics in natural environment and wastewater effluent requires systematic investigations of various clay minerals. Dioctahedral 2:1 clay minerals are major constituents of soil and sediments. Interaction of these clay minerals and ciprofloxacin (CIP) was studied. The CIP adsorption capacities on montmorillonite, rectorite, and illite were 1.19, 0.41, and 0.10 mmol/g, corresponding to 1.0, 1.0, and 0.9 CEC. Desorption of the equivalent amounts of exchangeable cations suggested that the cation exchange was mainly responsible for the CIP adsorption on montmorillonite and rectorite. The expansion of the basal spacing and shifts of characteristic FTIR bands indicated the intercalation of CIP ions with tilted orientation in montmorillonite and rectorite. The displacement of exchangeable cations and associated FTIR band shifts also indicated the cation exchange as important for CIP adsorption on illite but hydrogen bonding between CIP carboxylic groups and basal oxygen atoms on external surface apparently made a significant contribution to the adsorption. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluoroquinolones (FQs) were among the most used antibiotics applied to human therapy and veterinary treatment (Picó and Andreu, 2007). Due to the continuous increase of global needs and inappropriate discharge, they were frequently detected in a variety of natural environments. Ciprofloxacin (CIP), one of the commonly used FQs, was found in the wastewater and surface water with concentrations of several hundred ng/L (Golet et al., 2002a; Karthikeyan and Meyer, 2006; Kolpin et al., 2002; Miao et al., 2004; Renew and Huang, 2004). Higher concentrations up to 150 μg/L were even reported in the effluents from hospitals (Hartmann et al., 1999; Martins et al., 2008). In Switzerland, CIP was detected in sewage sludge from several wastewater treatment plants with concentrations ranging from 1.40 to 2.42 mg/kg (Golet et al., 2002b). These results warranted further studies on interactions between FQs and natural materials so that their fate and transport in the environment can be predicted and their removal from the environment can be facilitated. Several studies were conducted to investigate the CIP adsorption on different soils, and a wide range of the adsorption coefficient Kd from 400 to nearly 50,000 L/kg was reported in the literature (Carrasquillo et al., 2008; Córdova-Kreylos and Scow, 2007; Mackay and Seremet, 2008; Nowara et al., 1997; Uslu et al., 2008; Vasudevan et al., 2009). The large variation of Kd was often related to the diverse constituents in soils from different geographic areas. A positive correlation between Kd values and
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clay contents was revealed for the sediments from San Francisco Bay area (Córdova-Kreylos and Scow, 2007). In acidic to neutral water, the adsorption of CIP was strongly dependent on the cation exchange capacity (CEC) of soils (Carrasquillo et al., 2008; Vasudevan et al., 2009). These discoveries implied that clay minerals with high CECs might have a better CIP removal. Systematic investigations of adsorption capacities and properties of some widely monitored antibiotics including penicillin, tetracycline (TC), oxytetracycline, etc., on a series of 2:1 dioctahedral clay minerals (Kümmerer, 2009; Tolls, 2001) helped better understanding the behaviors and interaction mechanisms between antibiotics and minerals. However, such studies were much too sparse for CIP and other FQs. In this study, three 2:1 dioctahedral clay minerals, montmorillonite, rectorite, and illite, were tested for the adsorption of CIP from acidic solutions. 2. Materials and methods 2.1. Materials Two source clay minerals, montmorillonite (SAz-1) and illite (IMt-2) acquired from the Clay Minerals Society, and a rectorite, obtained from Zhongxiang, Hubei, China, were used as adsorbents. The CECs of SAz-1 and rectorite measured by the ammonia-electrode method were 1.23 (Borden and Giese, 2001) and 0.41 meq/g (Hong et al., 2008), with Ca2+ as the major exchangeable cation (Table 1). For IMt-2 illite, a much lower CEC of 0.11–0.12 meq/g was reported (Jaynes and Bigham, 1986; Li et al., 2003) and its exchangeable cations were Mg2+ and Ca2+. Their specific external surface area and total surface area determined by
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Table 1 Physical and chemical properties of the 2:1 clay minerals and CIP adsorption.
Mineral properties CEC (meq/g) Major exchangeable cations External specific surface area (m2/g) Total specific surface area (m2/g) Adsorption properties CIP adsorption plateau (mmol/g) Max. cation desorption (meq/g) Ca Mg Na K Total Adsorption densitya (Number of CIP molecules/100 nm2) Average distance between CIP molecules (Å)
SAz-1
Rectorite
IMt-2
1.23 Ca 65 715
0.41 Ca 10 363
0.11–0.12 Mg, Ca 22 –
1.19
0.41
0.10
0.97 0.15 0.03 0.01 1.16
0.32 0.03 0.05 0.01 0.41
0.03 0.02 – – 0.05
100 10.0
68 12.1
268 6.1
a Calculated from plateau CIP adsorption and specific surface areas of 715, 363, and 22 m2/g for SAz-1, rectorite, and IMt-2.
nitrogen adsorption and methylene blue adsorption were listed in Table 1 (Blum and Eberl, 2004; Dogan et al., 2006). Ciprofloxacin hydrochloride (CIP HCl) with a purity higher than 99.6% was purchased from Hangzhou Minsheng Pharmaceutical Group Co. Ltd (China). The molecule (Fig. 1a) is 12.2 Å long, 8.0 Å high and 4.1 Å thick (Turel and Golobic, 2003). An intramolecular hydrogen bond exists between the oxygen atom in the ketone group and the hydrogen atom in the carboxylic group (Turel and Golobic, 2003; Vázquez et al., 2001). With pKa1 = 6.1 and pKa2 = 8.7 (Gu and Karthikeyan, 2005), the charges vary with solution pH (Fig. 1b). In this study, the equilibrium solution pH was within 4–5.5, where a large fraction of CIP exists in the cationic form due to the protonation of the amine group in the piperazine moiety. 2.2. Adsorption experiments In the adsorption experiments, 0.2 g of the clay minerals were dispersed in 20 mL CIP solution in a 50 mL centrifuge tube and shaken at 150 rpm for 24 h. The tube was wrapped with aluminum foil to prevent light-induced decomposition. The adsorption kinetics were determined at an initial CIP concentration C0 = 1000 mg/L for SAz-1 and rectorite and 200 mg/L for IMt-2 at equilibrium times of 0.25, 0.5, 1, 1.5, 2, 4, 8, 12, 24 h. For the adsorption isotherm, C0 was in the range of 500–4000 mg/L for SAz-1, 150–2000 mg/L for rectorite, and 100–
1000 mg/L for IMt-2. After mixing, the dispersions were centrifuged at 5000 rpm for 10 min, and the supernatants were passed through a 0.45 μm filter. All experiments were performed in duplicate. 2.3. Methods of analyses The equilibrium CIP concentration Ce in the filtered supernatant was measured by a UV–Vis spectrophotometer (SmartSpec 3000, BioRad Corp.) at the wavelength of 275 nm. A calibration with 5 standards ranging from 0 to 10 mg/L yielded r 2 N 0.99. The amount of CIP adsorbed (Cs) was calculated from the difference between C0 and Ce. The concentration of Na +, K +, Mg 2+, and Ca 2+ released from SAz1 and rectorite during the adsorption experiments was measured by an ion chromatograph (Dionex 100). To detect the much lower concentrations of cations released from IMt-2, an Optima 7000DV ICPOES (Perkin Elmer) was used. XRD analyses of the raw and CIP adsorbed clay minerals were carried out on a D8 ADVANCE diffractometer with a CuKα radiation at 40 kV and 40 mA (Bruker Corp.). The oriented samples were air-dried and scanned from 1°–20° 2θ with a scanning speed of 0.01°/s. The FTIR spectra were acquired by an Equinox 55 Spectrometer (Bruker Corp.) using KBr discs. Absorbance data were collected by accumulating 256 scans at a resolution of 4 cm −1 in the range of 4000–400 cm −1. 3. Results and discussion 3.1. Adsorption kinetics More than 95% of the added CIP was adsorbed within 8 h by all clay minerals (Fig. 2). The pseudo-second-order kinetics was used to fit the experimental data: 2
qt =
kqe t 1+ kqe t
ð1Þ
where k (g/mg-h) is the rate constant of adsorption, qe (mg/g) the amount of CIP adsorbed at equilibrium, and qt (mg/g) the amount of CIP adsorbed at time t. The above equation can be converted into the linear form: t 1 1 = 2+ t qt qe kqe
ð2Þ
where kqe2 (mg/g-h) is the initial rate. All regression coefficients were N0.99. The rate constants were 8.0, 0.2, and 0.3 g/mg-h, and the initial
Fig. 1. Molecular structure of CIP from Cambridge Structure Database (a) and its speciation at different pHs (b).
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Fig. 2. Kinetics of CIP adsorption fitted by the pseudo-second-order equation (dashed curves).
rates were 1.1 × 10 5, 2.5 × 10 2, and 1.1 × 10 2 mg/g-h for SAz-1, rectorite, and IMt-2, respectively. These results indicated a relatively low reaction barrier for CIP adsorption on montmorillonite and thus yielded the adsorption N 99%, even at the short mixing time of 0.25 h. The rate constants and initial rates for CIP adsorption were much higher for the TC adsorption on the same SAz-1 and rectorite (Chang et al., 2009; Li et al., 2010), indicating enhanced adsorption of CIP compared to TC on these clay minerals. The kinetics results also confirmed that 24 h was a sufficient time for CIP adsorption to reach equilibrium. 3.2. Adsorption isotherms The adsorption isotherms of CIP on the clay minerals are shown in Fig. 3. The Langmuir model fitted the three adsorption data better than the Freundlich model. The regression coefficients were N0.995. Cs =
KL Csm Ce 1+ KL Ce
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2005; Zhang and Huang, 2007), aluminum hydroxide (Gu and Karthikeyan, 2005), and kaolinite (Mackay and Seremet, 2008; Pei et al., 2010), which exhibited much lower adsorption capacities of 0.06–0.15 mmol/g, 0.04 mmol/g, and 0.01–0.02 mmol/g at similar pH values. For SAz-1 and rectorite, the occupied areas would be 9 and 4 Å 2 if all adsorbed CIP molecules were assumed to cover the external surface. Compared with 32 Å 2 (the minimal cross-section area when the long axis of CIP molecule is perpendicular to the mineral surface), the much lower areas imply that the CIP molecules were not only adsorbed at the external surface area. When the total surface areas (including the interlayer spaces) were considered, the area occupied per CIP molecule was 100 Å 2 and 147 Å 2 on SAz-1 and rectorite, large enough to bind all CIP molecules at any orientations. This simple estimation demonstrated that the interlayer space was available for CIP adsorption in both minerals. On the contrary, the much lower CIP adsorption of 0.10 mmol/g on IMt-2 corresponded to an area of 37 Å 2 per CIP molecule if only the external surface area was taken into consideration. The protonation of amine groups of the CIP molecules under the experimental conditions and the positive relations between the CIP adsorption capacity and the CEC of the three clays minerals suggested that cation exchange was the principal adsorption mechanism. The CIP adsorption sites were mainly in the interlayer space of montmorillonite and rectorite but only on the external surface of illite. 3.3. Cation desorption The total amounts of displaced cations plotted against the CIP adsorption on SAz-1 and rectorite yielded a linear curve with the slope of 1 and negligible intercept (Fig. 4a), confirming that cation exchange interaction was the dominant adsorption mechanism. The total amounts of displaced cations desorbed at plateau adsorption values
ð3Þ
where KL (L/mg) was the Langmuir coefficient and Csm (mg/g) the adsorption maximum. The CIP adsorption at the plateau was 395, 135, and 33 mg/g, or 1.19, 0.41, and 0.10 mmol/g, corresponding to 1.0, 1.0, and 0.9 CEC for SAz-1, rectorite, and IMt-2. Thus, these clay minerals were effective adsorbents to remove CIP from acidic aqueous solutions (pH 4–5.5) in comparison to goethite (Gu and Karthikeyan,
Fig. 3. Adsorption isotherms of CIP. Solid and dash curves are Langmuir and Freundlich fits to the observed data.
Fig. 4. Relation between desorption of exchangeable cations and CIP adsorption for (a) the SAz-1 montmorillonite and rectorite, (b) the IMt-2 illite.
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stretching vibration at 1624 cm −1, and the coupling of the carboxylic acid C–O stretching and O–H deformation vibration at 1274 cm −1 (Trivedi and Vasudevan, 2007). The absorption band at 1385 cm −1 was due to the protonation of the amine group of the piperazine moiety (Gu and Karthikeyan, 2005). Two changes were found after the adsorption of CIP onto the clay minerals. The shift of the band of the protonated amine group to 1390 cm −1 suggested the presence of electrostatic attraction between the protonated amine group and the surface charges. A theoretical calculation revealed that such interaction was essential for the zwitterionic CIP attaching to the montmorillonite surface to reach an energetically stable configuration (Carrasquillo et al., 2008). In addition, the ketone stretching band shifted to a higher frequency of 1630 cm −1, indicating reinforcement of the ketone C_O bond (Trivedi and Vasudevan, 2007) due to the release of intramolecular hydrogen bonding between the ketone group and the carboxylic group. Thus, a new hydrogen bond between the hydrogen of the carboxylic acid groups and the basal oxygen of the minerals could be formed. This hydrogen bonding mechanism could be responsible for 50% of the adsorbed CIP molecules that were not attributed to the cation exchange. Hydrogen bonding might be an important mechanism for the adsorption of organic cations on 2:1 clay minerals of low CEC.
were close to the CEC of both minerals, indicating almost quantitative displacement of the exchangeable cations by CIP (Table 1). For IMt-2, the amounts of CIP adsorbed were always higher than the amounts of desorbed cations (Fig. 4b). At the adsorption plateau of 0.1 mmol/g, only 0.05 meq/g cations were desorbed. This difference suggested the contribution of further adsorption mechanisms. With the structural formula (Ca0.06 Mg0.09 K1.37)[Al2.69Fe 3+0.76Fe 2+0.06Mntr Mg0.43Ti0.06][Si6.77Al1.23]O20(OH)4 for IMt-2 reported by the Clay Minerals Society, the calculated quantities of exchangeable Mg 2+ and Ca 2+ should be 0.06 and 0.04 meq/g, respectively. However, at the CIP adsorption plateau (Table 1) the desorption of Mg 2+ (0.02 meq/g) was smaller than that of Ca 2+ (0.03 meq/g), indicating that a higher fraction of Mg 2+ remained in the illite. Although the data were limited, the selective desorption of the exchangeable cations could be related to a difference in their polarization power (ionic charge/radius ratio = 2.02 for Ca 2+ and 2.78 for Mg 2+). The higher polarization power of Mg 2+ leads to electrostatic attraction to the negatively charged surfaces, and thus is less likely to be replaced by CIP molecules. For the adsorption of hexadecyl trimethylammonium (HDTMA) (Lee and Kim, 2003) and cationic tetracycline (Li et al., 2010) on a low-charge montmorillonite (SWy-2) with Na + and Ca 2+ as principal exchangeable cations, the preferential release of Na + (polarization power = 1.05) could be caused by the same effect. Other reaction mechanisms, such as formation of surface complexes and the occurrence of cation bridging, were proposed for zwitterionic/anionic FQs adsorption on aluminum and iron (hydr) oxides (Gu and Karthikeyan, 2005) and montmorillonite (Nowara et al., 1997). The deportonated carboxylic group could play a critical role for the interaction with exposed surface metal ions or interlayer cations of the clay minerals. Nevertheless, both mechanisms were believed to be less important since CIP was mainly in the cationic form at low pH and the fraction of positively charged edges that could bind the COO - groups was small.
The d001-value of raw SAz-1 was 15.04 Å, representing a Ca 2+dominated montmorillonite saturated with two layers of interlayer water (Fig. 6). The basal spacing was 9.96 Å for illite IMt-2 and 24.1 Å for rectorite as characteristic of a hydrated Ca-rectorite. CIP intercalation increased the basal spacing of SAz-1 and rectorite to 17.2 and 25.8 Å at CIP adsorption levels of 1.06 and 0.37 mmol/g. For IMt-2, the basal spacing remained the same after CIP saturation.
3.4. FTIR analyses
3.6. CIP conformation and adsorption density
The spectra of pure CIP HCl and the clay minerals showed well resolved vibration bands (Fig. 5). Within 1250–1850 cm −1, the observed band positions of pure CIP were assigned to the C = O stretching vibration of carboxylic acid at 1707 cm −1, the ketone C = O
Based on the XRD and FTIR data, the conformation of adsorbed CIP molecules in the interlayer space of SAz-1 and rectorite was deduced. In acidic conditions, the gallery heights would be 7.23 and 5.80 Å for CIP saturated SAz-1 and rectorite assuming the d001-value of
3.5. XRD analyses
Fig. 5. FTIR spectra of pure CIP and the CIP adsorbed clay minerals. Band positions of pure CIP are marked by solid lines and those after adsorption onto the clay minerals are denoted by dashed lines.
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Fig. 6. XRD patterns showing the 001 reflection of montmorillonite (SAz-1), rectorite, and illite (IMt-2) (gray) and those of the CIP-adsorbed clay minerals as in Fig. 5 (black).
dehydrated montmorillonite and rectorite was 10 and 20 Å. Considering the interaction between the amine group and the mineral surfaces, the CIP molecules intercalated into montmorillonite and rectorite would take a tilted orientation at 36º and 29º to the basal plane (Fig. 7). The density and average distance between the CIP molecules were calculated using from the adsorption capacities and the surface areas of the minerals (Table 1). On every 100 nm 2, the SAz-1 could accommodate 100 molecules, higher than the capacity of rectorite. For IMt-2, the CIP molecules were adsorbed on the external surface. Though the molecular conformations could not be resolved with the XRD and FTIR investigation, the high CIP adsorption density (268 molecules/100 nm 2) and short average distance implied that the CIP molecules might have their long axes nearly perpendicular to the mineral surface so as to minimize the repulsion between the hydrogen atoms of neighboring CIP molecules. At pH 7, aluminum and iron (hydr)oxides (HAO and HFO) absorbed 0.04 and 0.07 mmol/g CIP (Gu and Karthikeyan, 2005). The adsorption was attributed to the formation of surface complexes between the zwitterionic CIP and HAO/HFO surface metal ions. Using the specific surface areas of 386 and 322 m 2/g for HAO and HFO, the adsorption densities were only 6 and 13 CIP molecules/100 nm 2. It appears that the reactive sites on the HAO/HFO surfaces were highly limited. On the contrary, the adsorption of cationic CIP on 2:1 clay minerals were high and positively correlated to the CEC values. However, the properties of exchangeable cations such as the polarization power could also play a significant role on the interpretation or prediction of adsorption behaviors in some cases. These observations are considered fundamental for further understanding of CIP adsorption onto soils and sediments.
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Fig. 7. Schematic models illustrating the conformations of CIP molecules intercalated in montmorillonite (a) and rectorite (b).
4. Conclusions (1) Adsorption of CIP on the montmorillonite, rectorite, and illite followed the Langmuir adsorption isotherms with plateau adsorption values of 1.19, 0.41, and 0.10 mmol/g, close to the CEC values. (2) The protonated amine groups and the carboxylic groups of the CIP molecules were responsible for the electrostatic attraction and hydrogen bonding to the external and internal surfaces of the clay minerals. (3) Stoichiometric relationships between the exchangeable cations displaced and the CIP molecules adsorbed and expansion of the basal spacing by intercalation demonstrated that cation exchange interaction was the dominant mechanism for CIP adsorption onto montmorillonite and rectorite. (4) Illite adsorbed CIP only on the external surfaces by cation exchange and hydrogen bonding. A high fraction of exchangeable Mg 2+ remaining in illite after CIP adsorption may relate to its comparatively large polarization power. Acknowledgments We are grateful to the reviewers' constructive comments. Funding from National Cheng Kung University (NCKU) for the project of Promoting Academic Excellence & Developing World Class Research Centers to support Li's short term visits to NCKU made this publication possible. We also thank the funding from National Science Council (Taiwan) to Jiang under grant NSC98-2116-M-006-005 and to Wang
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