Ecotoxicology (2011) 20:1141–1147 DOI 10.1007/s10646-011-0665-6
Tetracycline adsorption on kaolinite: pH, metal cations and humic acid effects Yanping Zhao • Jinju Geng • Xiaorong Wang Xueyuan Gu • Shixiang Gao
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Accepted: 22 March 2011 / Published online: 3 April 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Contamination of environmental matrixes by human and animal wastes containing antibiotics is a growing health concern. Because tetracycline is one of the most widely-used antibiotics in the world, it is important to understand the factors that influence its mobility in soils. This study investigated the effects of pH, background electrolyte cations (Li?, Na?, K?, Ca2? and Mg2?), heavy metal Cu2? and humic acid (HA) on tetracycline adsorption onto kaolinite. Results showed that tetracycline was greatly adsorbed by kaolinite over pH 3–6, then decreased with the increase of pH, indicating that tetracycline adsorption mainly through ion exchange of cations species and complexation of zwitterions species. In the presence of five types of cations (Li?, Na?, K?, Ca2? and Mg2?), tetracycline adsorption decreased in accordance with the increasing of atomic radius and valence of metal cations, which suggested that outer-sphere complexes formed between tetracycline and kaolinite, and the existence of competitor ions lead to the decreasing adsorption. The presence of Cu2? greatly enhanced the adsorption probably by acting as a bridge ion between tetracycline species and the edge sites of kaolinite. HA also showed a major effect on the adsorption: at pH \ 6, the presence of HA increased the adsorption, while the addition of HA showed little effect on tetracycline adsorption at higher pH. The soil
Y. Zhao J. Geng X. Wang X. Gu (&) S. Gao (&) State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, People’s Republic of China e-mail:
[email protected] S. Gao e-mail:
[email protected] Y. Zhao e-mail:
[email protected]
environmental conditions, like pH, metal cations and soil organic matter, strongly influence the adsorption behavior of tetracycline onto kaolinite and need to be considered when assessing the environmental toxicity of tetracycline. Keywords Tetracycline Adsorption Kaolinite Copper Cations Humic acid
Introduction Tetracyclines (TCs) are one of the most widely-used antibiotics in the world. They are used extensively for disease control and in livestock feed for several decades due to their great therapeutic values (Chen and Huang 2010; Sarmah et al. 2006). It was estimated that 12.6 thousand metric tons of antibiotics were sold in the United States for animal use in 2007. The average usage of veterinary antibiotics has reached approximately 6,000 tons annually in China (Zhao et al. 2010). Most antibiotics, are poorly adsorbed or fully metabolized by animal body, and 30–90% of antibiotics are excreted into the environment via urine and feces (Bound and Voulvoulis 2004). Residues of antibiotics, including TCs discharged from municipal wastewater treatment plants and agricultural runoff are frequently detected in surface water, groundwater, soils, and sediments (Hamscher et al. 2002; Kolpin et al. 2002). Environmental concentrations of antibiotics are typically below the threshold levels to exhibit medicinal treatment effects on bacterial populations and other at-risk species. However, chronic exposure to low levels of antibiotics alone or along with other toxicants may still exert pressure on the evolution of antibiotic resistant bacteria which in turn may minimize the effectiveness and therapeutic value of antibiotics (Chen and Huang 2010; Kim and Carlson
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2007). Tetracycline residues in excreta of treated animals may pose a threat of contaminating groundwater and surface water (Kulshrestha et al. 2004). Therefore, it is important to understand the factors that affect the mobility of antibiotics in soils when we need to minimize their transport to groundwater and to prevent runoff resulting from nonpoint source pollution. Tetracycline is an amphoteric molecule having multiple ionizable functional groups (Fig. 1a), that exist predominantly as zwitterions (Fig. 1b) in typical environmental pH range. It can undergo protonation-deprotonation reactions and present different ionic species depending on the solution pH in which tetracycline is dissolved (Fig. 1b). It has been reported that tetracycline can be highly adsorbed by clay minerals (Figueroa et al. 2004; Pils and Laird 2007; Wang et al. 2008), oxide minerals (Figueroa and Mackay 2005; Gu and Karthikeyan 2005), organoclays (Sithole and Guy 1987b), and humic substances (Gu and Karthikeyan 2008; Gu et al. 2007; Kulshrestha et al. 2004), which directly influenced its transportation, transformation and bioavailability to organisms. Because the transport of tetracycline in the environment is of great concern, the interaction between tetracycline and soil clay minerals has been well studied,
Fig. 1 Structure of tetracycline. The regions framed by dashed lines represent the structural moieties associated with the three acidic dissociation constants (pKa) (a) and pH-dependent speciation of tetracycline molecular (b)
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especially in the last decade. Normally the adsorption of tetracycline onto clays exhibits Langmuir type isotherms (Li et al. 2010; Sithole and Guy 1987a) or Freundlich type (Kulshrestha et al. 2004) and greatly influenced by pH and ionic strength, e.g. decreased as the ionic strength and pH increased (Figueroa et al. 2004; Kulshrestha et al. 2004; Li et al. 2010; Sithole and Guy 1987a; Wang et al. 2010). The adsorption normally involved two types of mechanisms depending on pH condition. The first is cation exchange reactions between the clay surfaces and the protonated amine group on the tetracycline under acidic condition (Figueroa et al. 2004; Kulshrestha et al. 2004; Li et al. 2010; Pils and Laird 2007; Sithole and Guy 1987a; Wang et al. 2010). The second is surface complexation of zwitterions onto the clay surfaces accompanied with proton uptake which is more favorable on acidic and neutral condition (Figueroa et al. 2004; Sithole and Guy 1987a). Kulshrestha (2004) also suggested it was a hydrophobic mechanisms. With the presence of divalent cations, the sorption was enhanced and a surface-bridging mechanism may be involved especially in neutral or alkaline condition (Figueroa et al. 2004; Pils and Laird 2007; Wang et al. 2008). Soil humic substances (HA) also could interfere with the interaction between tetracycline and clay surfaces. It was reported that with a large amount of HA, the sorption was reduced either because HA masked the sorption sites or inhabit interlayer diffusion of tetracyclines (Kulshrestha et al. 2004; Pils and Laird 2007). However, most studies on the interaction between tetracycline and clay minerals focused on the smectite, which has high surface area and cation exchange capacity (CEC). Kaolinite, a common soil 1:1 type clay mineral in tropical or subtropical area, gained less attention. Compared to smectite, it bears much less constant negative charges and CEC. Kaolinite also has no interlayer surfaces, hence has much less surface area. Figueroa (2004) investigated the adsorption interactions of three tetracycline (oxytetracycline, chlortetracycline, tetracycline) with montmorillonite and kaolinite, and found that sorption capacity of montmorillonite was much higher than that of kaolinite. However effects of ionic strength, coexistence of various types of cations and soil humic substance onto the interaction between the tetracycline and kaolinite are not well studied. Whether the adsorption mechanisms postulated for smectites were suitable for kaolinite needs to be confirmed. The objectives of this study were to investigate the effects of five background electrolyte cations (Li?, Na?, K?, Ca2? and Mg2?), the heavy metal cation (Cu2?) and a model humic acid compound (HA) on tetracycline sorption to kaolinite as a function of pH.
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Materials and methods Materials and chemicals Hydrochloride salt of tetracycline (96% purity) and humic acid (HA) were purchased from Sigma-Aldrich Co. (St. Louis, MO) and stored at -20°C. They were used without further purification. Acetonitrile (HPLC grade) and oxalic acid (99% purity) were also from Sigma-Aldrich. All other chemicals, including iron, lithium, sodium, potassium, copper, calcium and magnesium salts, were A.R. grade from Sinopharm Chemical Reagent Co. (Shanghai, China). The sample of kaolinite used in this study was obtained from the south of Nanjing, China. The preparation of sorbents were conducted using the method described by Gu et al. (Gu and Evans 2008). Briefly, samples were dispersed in deionized water and filtered through a 53 lm (270 mesh) sieve to remove most of the sand fraction. The clay fraction, \2 lm diameter particles, was separated by sedimentation technique, then acid washed to remove dissolving impurities. The sample was then neutralized to its natural pH and excess salts were removed by dialysis method. The clay was then freeze-dried and kept in plastic bottles for further use. The clay sample was confirmed to be kaolinite by X-ray diffraction analysis (D/MAX-RA, RigaKa, Japan) (Fig. 2). The specific surface of the sample was determined by the N2/BET method (ASAP 2020, Micromeritics, USA) and found to be 22.3 m2 g-1. The CEC measured by Ba2?/ NH4? exchange at pH 8 was 17.5 cmol kg-1 (McKeague 1978). Adsorption kinetics For kinetic studies, 0.06 g of kaolinite and 15 ml of 0.01 M NaNO3 solution with initial concentration of tetracycline at 0.1125 mM (50 mg l-1) were combined in each 20 ml glass vial. The vials were wrapped with aluminum foils to
Fig. 2 X-ray diffraction patterns of kaolinite
Fig. 3 Adsorption kinetics of tetracycline onto kaolinite. The solid line is pseudo-second-order fit to the observed data. The insert stands for the goodness of fit
prevent light induced decomposition and mixed on a reciprocal shaker at 150 rpm for 2, 4, 6, 8, 10, 21.5, 23, 28, 33, 45, 57 and 69 h at pH 5. The tetracycline concentration in the filtered solutions was determined by HPLC with a UV detector (Agilent 1100) (Gu and Karthikeyan 2005). Over 90% of the sorption occurred in the first 4 h, followed by a slow increase to the maximum over time (Fig. 3). The kinetic adsorption was described a pseudo-second-order model, which has been widely applied for the adsorption of pollutants from aqueous solutions in recent years (Ho 2006). Based on this observation, 24 h was chosen as the equilibration time in this study to ensure adequate time was given to reach equilibrium. Batch adsorption experiments Background electrolyte cations, including Li?, Na?, K?, Ca2? and Mg2?, heavy metal Cu2? and humic acid (HA) effects on tetracycline adsorption onto kaolinite as a function of pH were assessed using batch adsorption experiments. Stock solutions of tetracycline were prepared by dissolving it in 0.01 M LiNO3, NaNO3, KNO3, Ca (NO3)2 or Mg (NO3)2. Stock and working solutions were prepared freshly within 1 h before use. All adsorption experiments were conducted in 20 ml glass vials equipped with polytetrafluoroethylene-lined screw caps. The effect of background cations was examined as follows. A mass of 10 mg kaolinite was added to a total of 10 mM background electrolyte (0.01 M LiNO3, NaNO3, KNO3, Ca (NO3)2 and Mg (NO3)2). 0.54 ml tetracycline stock solution was spiked to each tube to give an initial tetracycline concentration of 0.1125 mM and a solid-towater ration of 1 g l-1, while the pH varied from 3 to 9 in 0.3 unit increments with the addition of small volumes of
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0.1 M HNO3 or 0.02 M NaOH. N2 was purged to remove dissolved oxygen to prevent possible oxygen-mediated degradation of tetracycline (Chen et al. 2008; Ji et al. 2009). Sample tubes were wrapped in Al foil to prevent exposure to light and shaken at 25 ± 0.5°C for a period of 24 h. At the end of the equilibration time, tubes were centrifuged for 30 min at 2,880 g. Then, the supernatant was filtered through 0.22 lm Nylon filters. Concentrations of tetracycline in supernatant were analyzed by HPLC. To take account for solute loss from processes other than adsorbent sorption, working calibration curves were built separately receiving the same treatment and conditions (pH, temperature, etc.) as the adsorption samples but without adsorbent. The amount of tetracycline adsorbed was calculated by subtracting the equilibrium tetracycline concentration from the initial concentration. Final pH values were measured immediately at the end of the equilibrated time using an Orion 8272 PerpHect Ross SureFlow electrode. Effect of Cu2? on the adsorption as a function of pH was conducted similar as the method described above. The background electrolyte was 0.01 M NaNO3, and Cu2? was spiked to give an initial concentration of 0.1 mM. Effect of HA on the tetracycline sorption was conducted in three HA concentration, 5, 50 and 100 ppm, respectively. HA stock solution was prepared by adding HA dry powder to pH 10.0 NaOH solution to get a 1,000 ppm bulk solution. The adsorption edge experiment was conducted as a similar manner as for cation effect. The background electrolyte for HA effect experiment was 0.01 M NaNO3 solution.
Results and discussion Background electrolytes effects Five types background electrolyte cations were investigated in this study, including three monovalent cations (Li?, Na? and K?) and two divalent cations (Ca2? and Mg2?) and their concentrations were all 0.01 M. Results showed that the adsorption trends of all five cations over the studied pH range 3–9 were all decreased as pH increased (Fig. 4). It was consistent with the results reported in previous studies about the adsorption of tetracyclines onto clays (Figueroa et al. 2004; Parolo et al. 2008; Pils and Laird 2007; Wang et al. 2010). At about pH 3, tetracycline was almost 100% sorbed by kaolinite in all types of electrolytes. With the increasing of pH, the adsorption started to decrease. However, the adsorption decreased in accordance with the increase of atomic radius and valence of metal cations, e.g. the decreasing degree followed an order: Li? \ Na? \ K? \ Ca2? \ Mg2?.
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Fig. 4 Adsorption edges of tetracycline onto kaolinite in five different metal cation electrolytes (Li?, Na?, K?, Ca2? and Mg2?).(Suspension density = 1.0 g l-1. Total concentration of tetracycline = 0.1125 mM, T = 25°C, Equilibration time = 24 h)
Tetracycline is an amphoteric molecule with multiple ionizable functional groups (Fig. 1a). In aqueous solutions three different groups of the molecule can undergo protonation-deprotonation reactions depending on the solution pH. The overall charge of tetracycline could be positive (pH \ 3.3), neutral (3.3 \ pH \ 7.68), or negative (pH [ 7.68) (Fig. 1b). Kaolinite is a 1:1 dioctahedral aluminosilicate with basal surface sites, on which the surface charge is always considered to be negative, and edge sites where the charge can be either positive or negative, effectively depending on the pH (Carty 1999). The portion of edge surfaces of kaolinite is relatively high, about 12–34% (Brady et al. 1996). Results showed that cation exchange is the dominant adsorption mechanism of tetracyclines adsorption onto kaolinite. At pH \ 3.3, the predominant form of tetracycline is cation, which can be adsorbed by the negatively charged surface sites of kaolinite. With pH increasing, tetracycline molecular gradually becomes neutral or negatively charged through deprotonation, and the charge repulsion is expected where the sorbate and sorbent were both negatively charged. Hence the adsorption of tetracycline onto kaolinite appeared to decrease as the pH increased. However, although tetracycline showed less adsorption as the pH increased, the sorption amounts did not exhibit a characteristic shape ‘‘edge’’ that would be expected if only the cationic species interacted with the clay surface, which was also found in previous studies (Figueroa et al. 2004; Parolo et al. 2010; Parolo et al. 2008). Figueroa et al. (2004) suggested that the cation was still a large contributor to overall tetracyclines sorption even when the dominant solution phase species was the zwitterions. They also suggested that zwitterionic species of tetracycline may interact with the surface or edge
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sites of kaolinite through surface complexation mechanism. In a recent study (Li et al. 2010), it was found that the adsorption of tetracycline onto smectites was accompanied by simultaneous H? uptake. The additionally adsorbed H? could serve as counterions to partially offset the negative charges on the tricarbonyl or phenolic diketone functional groups, which meant cation exchange may still work even under neutral pH conditions. Results in this study also indicated that zwitterions of tetracycline may also interact with kaolinite surfaces. Results in the present study also found that tetracycline adsorption decreased in accordance with the increase of atomic radius and valence of cations over the pH range from 3 to 9 (Fig. 4), indicating that the adsorption was greatly influenced by the type of electrolyte cations. The adsorption mechanism was more like an outer-sphere cation exchange reaction between tetracycline and the surface of kaolinite. Considering the structure of kaolinite, the adsorption may mainly occur at the permanently negatively charged surface sites. Therefore the adsorption was greatly influenced by the ionic strength and electrolyte types. Higher ionic strength, larger ionic radius and higher cation valence would be expected to decrease the adsorption. On the contrary, it was different from the phenomenon found for oxides (Gu and Karthikeyan 2005). In our another similar study for goethite, the background electrolyte showed almost no effect on the adsorption, indicating an inner-sphere surface complexation happened. However, one interesting phenomenon observed in this study was that the ion bridging effect of divalent electrolyte cation for smectite was not observed for kaolinite. Figueroa et al. (2004) reported that the presence of bivalent cation (Ca2?) could compete with tetracycline adsorption to montmorillonite at acidic pHs, but promote the adsorption when pH [ 7, suggesting that divalent metal cations may enhance the adsorption by bridging bonds under alkaline condition. Similar results were also found for soil clays (Pils and Laird 2007). However, it was observed in this study that the presence of Ca2? and Mg2? inhibited tetracycline adsorption relative to Li?, Na? and K? over the whole adsorption pH range, indicating that ion bridging effect of Ca2? and Mg2? at the alkaline condition did not happen for kaolinite, or at least not significant for kaolinite. Divalent cations, like Ca2? and Mg2?, have a two-side effect on the adsorption. They can decrease the adsorption by increasing the ionic strength because they have higher valence. On the other hand, they may enhance the adsorption by acting as bridging ions. Compared to smectites, kaolinites have lower surface area, CEC values, and surface potential: their attraction capability to cations, especially at alkaline conditions, is much less than smectites, which may reduce the possibility of forming surface ternary bridging species.
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Cu (II) effect Adsorption of tetracycline onto kaolinite in the absence and presence of 0.1 mM Cu2? was also studied as a function of pH. The copper concentration was chosen as about 1:1 molar ratio to tetracycline concentration. Results (Fig. 5) showed that the presence of Cu significantly promoted the tetracycline adsorption over the adsorption pH range from 3 to 10, especially when pH [ 7 (Fig. 5). For instance, tetracycline adsorption was increased by 6.7 mmol kg-1 and 19.6 mmol kg-1 with the presence of 0.1 mM Cu2? at pH 3.3 and pH 9.5 respectively compared without Cu2?. The results were consistent with the previous studies (Jia et al. 2008; Wang et al. 2008). As discussed above, divalent cations have two mechanisms of influencing adsorption. However, the Cu2? concentration in this study was much lower than the background electrolyte. Therefore, its influence to ionic strength was almost negligible. It mainly acted as a bridging ion to enhance the adsorption. At pH 3–4, Cu2? adsorbed to kaolinite mainly through outer-sphere complexes at the permanently negative charged sites (Carty 1999; Li et al. 2010; Parolo et al. 2008). The weak bonds between kaolinite surfaces and Cu only resulted in a small degree of enhancement of the adsorption. At pH [ 5, Cu2? could form strong inner-sphere complexes (:SOCu?) with variable charged edge sites of kaolinite(Grossl et al. 1994; Swedlund et al. 2009), while Ca2? and Mg2? only form weak outer-sphere complexes. Accordingly, the strong surface complexes of kaolinite and Cu would favor the formation of ternary surface species :SOCuTC. The adsorption of tetracycline onto kaolinite reached a maximum at about pH 7. At pH [ 7, the adsorption of tetracycline started to decline even in the presence of Cu, this
Fig. 5 The effect of pH on the adsorption of tetracycline (C0 = 0.1125 mM) onto kaolinite with and without 0.1 mM Cu 2?. (Suspension density = 1.0 g l-1, background electrolyte = 0.01 M NaNO3, T = 25°C, equilibration time = 24 h)
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may be because the variable charged edge sites of kaolinite becomes more negative charged (Li et al. 2010) and tetracycline molecule is prevented from approaching kaolinite surfaces by electrostatic repulsion. Therefore, the adsorption started to decrease. HA effect Adsorption of tetracycline onto kaolinite in the presence of 5 ppm, 50 ppm and 100 ppm HA exhibited a strong pHdependence in studied pH range from 3 to 10 (Fig. 6). The presence of 5 ppm HA, which was 1:10 ratio to tetracycline concentration, showed little effect on the adsorption. While, with the presence of 50 ppm and 100 ppm HA, significant enhancement of tetracycline adsorption was observed at pH \ 6. At pH 4, the presence of HA increased the adsorption of tetracycline from 111.5 mmol kg-1 (no HA) to 122.2 mmol kg-1 (with 50 ppm HA) and 130.4 mmol kg-1 (with 100 pm HA). As pH increased above 6, HA almost has no effect on the tetracycline adsorption. The adsorption edges were difficult to tell from each other (Fig. 6). At pH \ 6, the predominant forms of tetracycline are cationic and zwitterionic species which can complex with deprotonated sites on HA (mainly carboxylic groups) through ion exchange or hydrogen bond. It has been reported that natural organic matter has strong affinity to the surface of clay and metal oxide particles (Tombacz et al. 2004). Organic matter can complex with clays through water bridging, ligand exchange, hydrogen bonding, polyvalent cations bridging, etc. (Manjaiah et al. 1998). Previous studies showed that tetracycline could be sorbed by HA and the sorption was pH-dependent, which was consistent with complexation between the cationic/zwitterionic
Fig. 6 The effects of HA (5 ppm, 50 ppm and 100 ppm) on the adsorption of tetracycline onto kaolinite. (Suspension density = 1.0 g l-1, background electrolyte = 0.01 M NaNO3, T = 25°C, equilibration time = 24 h)
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tetracycline species and deprotonated sites in HA (mainly carboxylic functional groups) (Gu et al. 2007). Accordingly, in the tetracycline-HA-kaolinite ternary system, tetracycline can either complex with HA, or interact with surfaces of kaolinite. Hence, HA may act as a bridge, similar as Cu2?, between kaolinite and tetracycline at acidic conditions, resulting in the increased tetracycline sorption. On the other hand, tetracycline may also interact with HA through hydrophobic partition. Furthermore, in acidic condition, HA would tend toward precipitation and we are not surprised if the co-precipitate with tetracycline happened. When pH above 6, the dominant form of tetracycline is anion, and both HA and kaolinite are more negatively charged as pH increasing. The electrostatic repulsion was expected in this case, so there was less HA complexing with kaolinite and tetracycline, which resulted in the little effect of HA on the tetracycline adsorption onto kaolinite under alkaline condition.
Conclusions The results of this study indicated that the background electrolyte cations, heavy metals and organic matters (model humic acid) significantly influenced the adsorption of tetracycline onto kaolinite over a wide pH range. With the presence of background metal cations, the adsorption of tetracycline decreased in accordance with the increase of atomic radius and valence of metal cations, which suggested that outer-sphere complexes formed between tetracycline and kaolinite. Ca2? and Mg2? did not show the bridge ion effect for kaolinite; this may be because the surface charge of kaolinite was relatively low. By comparison, the presence of a multivalent heavy metal, Cu2?, increased tetracycline adsorption onto kaolinite by bridge bonding because Cu2? could form strong inner-sphere complexes with kaolinite surfaces. The presence of HA ([5 ppm) enhanced the interaction between tetracycline and kaolinite when pH \ 6, while it showed no effect on the adsorption at higher pH. This study suggested that in a soil mainly composed by kaolinite, tetracycline will be strongly adsorbed in acidic pH condition, while neutral and alkaline condition will be in favor of its mobility. Low ionic strength, multivalent heavy metals and high content of HA will help tetracycline to be fixed in soil. These environmental factors need to be considered while assessing its bioactivity and risk to organisms. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No. 20807022), and the Key Special Program on the Science and Technology for the Pollution Control and Treatment of Water Bodies (No. 2009ZX07210-004, 2008ZX07316-004).
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