Adsorption and Cosorption of Tetracycline and Copper(II) on

Mar 25, 2008 - Land application of wastes generated from concentrated animal feeding operations may result in accumulation of tetracyclines (TCs) and ...
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Environ. Sci. Technol. 2008, 42, 3254–3259

Adsorption and Cosorption of Tetracycline and Copper(II) on Montmorillonite as Affected by Solution pH Y U - J U N W A N G , †,‡ D E - A N J I A , † R U I - J U A N S U N , †,‡ H A O - W E N Z H U , † A N D D O N G - M E I Z H O U * ,† State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China; Graduate School of Chinese Academy of Sciences, Beijing 100039, China

Received October 18, 2007. Revised manuscript received February 04, 2008. Accepted February 06, 2008.

Land application of wastes generated from concentrated animal feeding operations may result in accumulation of tetracyclines (TCs) and metals in agricultural soils. Adsorption of TCs and metals on soil minerals strongly affects their mobility. This study was conducted to evaluate the interaction between tetracycline (TC) and Cu(II) with regard to their adsorption and cosorption on montmorillonite as affected by solution pH. When solution pH was below 6.5, the presence of TC increased Cu(II) adsorption on montmorillonite, which could be due to increasing Cu(II) adsorption via the TC bridge, or due to the stronger affinity of TC-Cu(II) complex to the mineral than Cu2+ ion itself. Zeta potential of the montmorillonite significantly decreased after the adsorption of TC, suggesting a strong interaction between TC and montmorillonite. Addition of Cu(II) ions increased TC adsorption on the mineral in a wide range of pH. The experimental data were well fit with the weighted sum model. The complexes of TC and Cu(II) (CuH2L2+, CuHL+, and CuL) had higher sorption coefficients (Kd) than that of the corresponding TC species (H3L+, H2L, and HL-). Increasing adsorption of TC and Cu(II) on montmorillonite as they coexist in the normal pH environment may thus reduce their mobility.

Introduction Veterinary antibiotics (VAs) are widely used to treat diseases of animals and incorporated into animal feeds to improve growth rate and feed efficiency. In 1996, about 10.2 × 107 kg of antibiotics were used in Europe, and half of them were used as veterinary therapeutics and also as growth promoters (1). In 2000, more than 22.7 × 107 kg of antibiotics were produced in the United States and more than 40% of them were used as feed supplement to enhance animal growth (1). In China, the use of veterinary antibiotics in animal feeds has been regulated since 1989 and only prophalactic antibiotics are permitted as feed additives. The prophalactic antibiotics that are currently registered in China include monensin, salinomycin, destomycin, bacitracin, kitasamysin, * Corresponding author phone: (86) 25-86881180; fax: (86) 2586881000; e-mail: [email protected]. † Institute of Soil Science, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. 3254

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enramycin and virginiamycin. However, other antibiotics such as tetracyclines (TCs), used for therapeutic purposes, are often illegally used as feed additives by some intensive farming operations in China (1). Tetracyclines are poorly absorbed in the digestive tract of animals, and 50–80% is excreted through feces and urine as unmetabolized parent compound (1). Because land application of animal wastes as plant nutrient sources and soil amendments is a common practice in many countries, there is a growing concern about the potential impact of antibiotics on the environment (2–8). Frequent use of antibiotics has also raised concerns about increased antibiotic resistance of microorganisms (3, 9) and the effect of antibiotics on plant growth (10, 11). Tetracyclines are quite persistent in soils and can accumulate with repeated manure applications. A recent survey of the occurrence of various TCs and sulfamethazine (sulfonamide group) in sandy soils fertilized with liquid manure was conducted in northwestern Germany (7). The reported maximum concentrations for the compounds screened in the study were 27 µg kg-1(oxytetracycline), 443 µg kg-1 (tetracycline), 93 µg kg-1 (cholortetracycline), and 4.5 µg kg-1 (sulfamethazine) in the top 0–30 cm soil. At least three of the 14 agricultural fields surveyed had higher concentrations than the trigger value (100 µg kg-1) of European Agency for the Evaluation of Medicinal Products for TCs (7). Elsewhere, Winckler and Grafe (8) found TCs to persist in agricultural soils at concentrations of 450–900 µg kg-1. Kay et al. (12) found that after pig slurry was applied to a field in two consecutive years both sulfachloropyridazine and oxytetracycline concentrations in the soil were found to be 365 and 1691 µg kg-1, respectively. Consequently, the peak concentrations of the two compounds in drain flow were 613.2 and 36.1 µg L-1, respectively. Copper concentration in some animal wastes can also be high, because copper salt is frequently used in animal feed as a growth promoter (13). Nicholson et al. (14) found that Cu concentrations ranged 18–217 mg kg-1 in swine feeds, adjusted depending on the age of the pigs. Similarly, the concentrations ranged 5–234 mg kg-1 in poultry feeds. Pig manures typically contained 360 mg kg-1 of Cu on a dry matter basis, reflecting metal concentrations in the feeds (14). Land application of the wastes containing high Cu can result in elevated concentration of Cu in soils. The Cu concentration in a wheat field sampled in 2005 following 12 years of biosolids application was 4 times as high as the control receiving no biosolids (15). In such cases where Cu is accumulating due to waste application, antibiotics in soils may also be increased, but little is known as to how they interact with each other. Kong et al. (9) studied the effect of the interaction between oxytetracycline (OTC) and Cu(II) on the functional diversity of soil microbial community and found that the coexistence of OTC and Cu(II) significantly decreased Shannon’s diversity index, a parameter commonly used to characterize species diversity in a community, utilization of carbohydrates and carboxylic acids, compared to the situation when only one of the contaminants was present in the soil. Tetracyclines are amphoteric compounds because of the presence of both Lewis base and Lewis acid functional groups, and may exist as a cation (+ 0 0), zwitterions (+ - 0), or a net negatively charged ion (+ - -) at environmentally relevant pH values (1). The values of sorption coefficients Kd+00 on soils or clays were more than an order of magnitude larger than the values for either Kd+-0 and Kd+--. Sorption edges were best described with a model that included cation 10.1021/es702641a CCC: $40.75

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exchange plus surface complexation of zwitterion forms of these compounds. Soil clays, organic matter, and oxides all have the capacity to adsorb TC (16–22). Solution Ca2+ and Na+ as competitive ions influenced TC adsorption to soil or its active constituents. However, the effect of heavy metal ions on TC adsorption has not been studied, while the positions of the donor groups of TCs make it a chelate able to form a complex with numerous metal ions and strongly alter solution TC species (4, 23). This metal complex formation has been reported to inhibit bacterial growth (24). Additionally, complexation between TCs and heavy metals may affect their environmental behaviors (adsorption, desorption, leaching, and degradation), especially at environmentally relevant pH values where common cations found in soils like Ca2+ and Mg2+ have weaker effects on TC adsorption (18, 25). Stability constants for 1:1 complexes of Cu2+ with TCs are much higher than those of Ca2+ and Mg2+ ions with TCs. For example, stability constants for 1:1 complexes of Cu2+, Ca2+, and Mg2+ with OTC are 1012.4 (4), 106.4, and 105.8 (24), respectively. The objectives of this study were to examine the adsorption and cosorption of TC and Cu(II) on montmorillonite as affected by solution pH using batch experiments, and to calculate the species-species sorption coefficients (Kdi) of TC-Cu(II) complex to montmorillonite.

Materials and Methods Mineral and Chemicals. Hydrochloride salt of tetracycline (TC, 96% purity) was purchased from Sigma Co. and used without further purification. All other chemicals, including copper, sodium, and calcium salts, were of reagents grade. The montmorillonite sample used to conduct this study was collected from Suzhou County, Jiangsu Province, China. X-ray diffraction (XRD) analysis indicated that the sample was pure montmorillonite (see Figure S1 in the Supporting Information) with a cation exchange capacity (CEC) of 105.8 cmol kg-1, as measured by the NH4OAc method (26). About 71.5% exchangeable cations are Ca2+ and Mg2+, and another 21% were Na+ and K+. All solutions were prepared with doubledeionized water. Preliminary Experiments to Determine Equilibrium Time. All experiments were conducted in 50 mL clear polyethylene tubes. Preliminary experiments showed the loss of 0.011 mmol L-1TC by sorption to the polyethylene tube after 240 h storage at 25 ( 1 °C and then to a 0.45 µm membrane filter (Cellulose ester, Shanghai ANPEL Scientific Instrument Co. Ltd., Shanghai, China) to be less than 1% (see Figure S2 in the Supporting Information). The preliminary test was much longer than the longest equilibration period used in the formal study. It suggests that the photodegration and the adsorption on containers of TC were negligible during the study. Tetracycline adsorption on montmorillonite was measured as a function of time to determine the duration of time needed to reach its equilibrium. Tetracycline concentrations in the aqueous phase were analyzed at 0.5, 1, 2, 4, 8, 16, 24, and 48 h. The TC concentration in the filtered solutions was determined by UV/vis spectroscopy (UV-9200, Ruili Inc., China). The absorbance of TC compound was measured at 360 nm (18). Over 80% of the sorption occurred in the first 2 h, followed by a slow increase to the maximum over time (see Figure S3 in the Supporting Information). Based on this observation, 16 h was chosen as the equilibration time for this study to ensure adequate time was given to reach equilibrium. Adsorption of Cu(II) on Montmorillonite as Affected by TC and pH. Adsorption isotherms of Cu(II) on montmorillonite with and without TC were performed by adding 0.100 g montmorillonite to 15.0 mL of 0.01 mol L-1 KCl solution in a centrifuge tube. Five mL of 0.01 mol L-1 KCl solution with different concentrations (0, 0.25, 0.5, 1.25, 2.5, 5.0, and

10.0 mmol L-1) of Cu(II) as CuCl2 was added. And then, 5.0 mL of 0.01 mol L-1 KCl solution containing 0 mmol L-1 was added to one set and 5.0 mL of 0.01 mol L-1 KCl solution containing 1.0 mmol L-1 TC was added to another set. The suspension pH was adjusted to 5.50 with 0.01 mol L-1 HCl or NaOH. The final volume of 25 mL resulted in the suspension Cu(II) concentrations of 0, 0.05, 0.1, 0.25, 0.5, 1.0, and 2.0 mmol L-1, and in TC concentrations of 0 or 0.2 mmol L-1. The centrifuge tubes were continuously shaken for 16 h at 25 ( 1 °C, centrifuged at 9000 g for 10 min, and then filtered through a 0.45 µm membrane filter. Solution pH at equilibrium was measured by a pH electrode (Leichi Instruments, Shanghai, China). The Cu(II) concentration in the filtrate was determined by Hitachi 180–80 Atomic Absorbance Spectrometry (AAS) (Hitachi, Tokyo, Japan). The amount of Cu(II) adsorbed was calculated from the difference in concentrations between the initial and equilibrium solution. The experiments were performed in duplicate. The effect of pH on Cu(II) adsorption on montmorillonite as affected by TC was conducted as following: Five mL of 0.01 mol L-1 KCl solution with 1.25 mmol L-1 Cu(II) was added to a 50 mL centrifuge tube containing 0.100 g montmorillonite. Five mL of 0.01 mol L-1 KCl solution with and without 0.5 mmol L-1 TC was added. The suspension pH was adjusted to different values between 3 and 9 using dilute HCl or NaOH. Then, 0.01 mol L-1 KCl solution was added to bring the final volume of 25 mL, which resulted in the suspension Cu(II) concentrations of 0.25 mmol L-1, and in TC concentrations of 0 or 0.1 mmol L-1. The centrifuge tubes were continuously shaken for 16 h at 25 ( 1 °C, centrifuged at 9000 g, and then filtered through a 0.45 µm membrane filter. The Cu(II) concentration in the filtrate was determined by AAS. Free Cu2+ ion concentration was determined by a Cu ion-selected electrode (Model 94–29, Orion Research, Cambridge, MA), and the final solution pH values were measured as the equilibrium solution pH. Other experimental conditions and measurements were the same as described above. Adsorption of TC on Montmorillonite as Affected by Cu(II) and pH. Adsorption isotherms of TC on montmorillonite with and without Cu(II) were performed. Five mL of 0.01 mol L-1KCl solution containing different concentrations (0, 0.2, 0.4, 0.6, 0.8, and 1.0 mmol L-1) of TC was added to each centrifuge tube containing 0.100 g montmorillonite, and then, 5.0 mL of 0.01 mol L-1 KCl solution with and without 1.25 mmol L-1 Cu(II) was added. The final volume of solution was 25 mL by adding 0.01 mol L-1 KCl solution into the centrifuge tube, which resulted in the suspension TC concentrations of 0, 0.04, 0.08, 0.12, 0.16, and 0.2 mmol L-1, and in Cu concentrations of 0 or 0.25 mmol L-1. The centrifuge tubes were continuously shaken for 16 h at 25 ( 1 °C, centrifuged at 9000 g for 10 min, and then filtered through a 0.45 µm membrane filter. One or two drops of 12 mol L-1 HCl were added into the filtrate to adjust solution pH to 1–2 in order to dissociate the complex of TC and Cu(II). The TC concentration in the filtered solutions was determined by UV/vis spectroscopy. The detection limit of TC concentration by UV spectroscopy was 0.0005 mmol L-1, which was much lower than the lowest TC concentration in our experiment. The amount of TC adsorbed was calculated from the difference in concentrations between the initial and equilibrium solution. The experiments were performed in triplicates. Effect of pH on TC adsorption on montmorillonite as affected by Cu(II) was conducted as following: Five mL of 0.01 mol L-1 KCl with 0.5 mmol L-1 TC was added in 50 mL centrifuge tubes containing 0.100 g of the mineral. The suspension pH was adjusted to different values between 3 and 9 with 0.01 mol L-1HCl or NaOH. The final solution volume was 25 mL by adding 0.01 mol L-1 KCl, and the final VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Adsorption isotherms of Cu(II) on montmorillonite with and without 0.2 mmol L-1 TC. Background electrolyte: 0.01 mol L-1 KCl; Equilibrium time: 16 h; Temperature: 25 ( 1 °C. * The two fits were significantly different (p < 0.023). solution TC concentration was 0.1 mmol L-1. Similar adsorption was done with the final Cu(II) concentration of 0.25 mmol L-1. The samples were continuously shaken for 16 h at 25 ( 1 °C, centrifuged, filtered, and analyzed for TC as described above. Solution pH at equilibrium was measured by a pH electrode. Measurement of Zeta Potential of Montmorillonite. Zetapotential measurements were conducted using a JS94H microelectrophoresis instrument (Powereach Instruments, Shanghai, China). The montmorillonite sample was suspended in 0.01 mol L-1 KCl solution (electrolyte), and the aqueous suspension was equilibrated with Cu(II) (CCu(II)0 ) 0.25 mmol L-1), TC (CTC0 ) 0.1 mmol L-1), and both Cu(II) and TC (CCu(II)0 ) 0.25 mmol L-1; CTC0 ) 0.1 mmol L-1) at different pH values for 16 h. The equilibrated slurry was injected into the microelectrophoresis cell using disposable syringes. A minimum of 10 readings were recorded, and the mean value was reported. Prior to each measurement, the electrophoresis cell was thoroughly washed and rinsed with deionized water, followed by rinsing with the sample solution to be measured. Statistical Analysis. The data were analyzed using Microsoft Excel and SAS 8.0. General linear model analysis of variance (t test) was carried out to compare the treatment differences.

Results and Discussion Adsorption of Cu(II) on Montmorillonite as Affected by TC. Adsorption isotherms of Cu(II) on montmorillonite at pH 5.5 with and without TC (0.2 mmol L-1) are shown in Figure 1. The quantity of Cu(II) adsorbed on montmorillonite increased with increasing Cu(II) concentration in the equilibrium solution. When TC was present in the solution, Cu(II) adsorption on montmorillonite was significantly increased. The effect of an organic chemical on heavy metal adsorption may include several different mechanisms: (1) Enhanced sorption by forming ternary surface complexes involving surface groups of soil particles; (2) Reduced metal sorption due to competition between the surface ligands and the dissolved organic ligands for dissolved metal; (3) Competition between organic compounds and metal ions for surface sites; and (4) Metal adsorption affected as a result of changes in solution pH (28). Tetracycline, which has multiple ionizable functional groups with pKa values of 3.57, 7.49, and 9.88, exists predominantly as zwitterions at pH values typical of the natural environment (18, 19, 27). The presence of TC changes solution Cu(II) species, and TC-Cu(II) complex is 3256

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FIGURE 2. Adsorption of Cu(II) (CCu(II)0 ) 0.25 mmol L-1) on montmorillonite as a function of pH with and without 0.1 mmol L-1 TC. Background electrolyte: 0.01 mol L-1 KCl; Equilibrium time: 16 h; Temperature: 25 ( 1 °C. * The two fits were significantly different (p < 0.034). the major species at pH 5.5. Figure 1 suggests that TC-Cu(II) complex has stronger affinity to montmorillonite than Cu2+ ion does. However, some water-soluble organic chemicals, such as dissolved organic matter (DOM), often decrease metal adsorption (29), which is different from TC. The addition of DOM significantly reduced Cd and Zn sorption capacities by a factor of 2.1–5.7 for Cd and 2.3–13.7 for Zn compared to the control receiving no DOM (30), which is due to the fact that the complexes of Cd-DOM are water-soluble with negative charges while the Cu-TC in this study possesses positive charges. Solution pH is one of the most important factors affecting Cu(II) adsorption on minerals. Adsorption of Cu(II) on montmorillonite at different equilibrium solution pH was studied in the presence and absence of TC in order to find out which process controlled Cu(II) adsorption. Figure 2 shows the effect of pH on Cu(II) adsorption on montmorillonite with and without TC (0.1 mmol L-1). It indicates that Cu(II) adsorption on montmorillonite increased with increasing the pH of equilibrium solution. When solution pH reached 6.5, Cu(II) adsorption on montmorillonite maximized. The addition of TC significantly affected the adsorption edge of Cu(II) on montmorillonite. The presence of TC increased Cu(II) adsorption on montmorillonite if the equilibrium solution pH was below 6.5. While the solution pH was above 7.0, TC decreased Cu(II) adsorption on the mineral in comparison with Cu(II) adsorption in the absence of TC at the same pH. This suggests that the formation of TC-Cu(II) complexes affected Cu(II) adsorption on montmorillonite and the effect was a function of equilibrium pH. Tetracycline has a strong complexing capability with metals (27). A Cu ion-selective electrode was used to analyze free Cu2+ ion in the equilibrium solution (see Figure S4 in the Supporting Information). The presence of TC decreased free Cu2+ ion concentration greatly. Cristina et al. (27) potentiometrically studied the stability constants of complexes formed by Cu(II) with three different TCs using an automatic titration system in aqueous medium at 25 °C (I ) 0.1 mol L-1 NaNO3), and found that all TC species in solution have positive, zwitterion and negative charges as H3L+, H2L0, HL- and L2-, which have strong chelating capability with Cu(II) to form complexes. Possible TC-Cu(II) species include CuH2L2+, CuHL+, CuL, CuH4L22+, CuH3L2+, CuH2L2, CuHL2-, and CuL22-. Speciation calculation by the computer program WinSGW (31–33) showed that the predominant complex species between TC and Cu(II) were CuH2L2+ with little coordination at pH < 4; CuHL+ and CuL when pH was between 4 and 8, and CuL at pH > 8 (see Figure S5 in the Supporting Information).

FIGURE 3. Adsorption of TC on montmorillonite with and without 0.25 mmol L-1Cu(II). Background electrolyte: 0.01 mol L-1 KCl; Equilibrium time: 16 h; Temperature: 25 ( 1 °C. * The two fits were significantly different (p < 0.001).

FIGURE 4. The effect of pH on the adsorption of TC (CTC0 ) 0.1 mmol L-1) on montmorillonite with and without 0.25 mmol L-1Cu(II). Background electrolyte: 0.01 mol L-1 KCl; Equilibrium time: 16 h; Temperature: 25 ( 1 °C. * The two fits were significantly different (p < 0.001).

TABLE 1. Freundlich Sorption Model Coefficients for TC Adsorption Isotherms on Montmorillonite in the Absence and Presence of Cu(II)

on the interaction of each form of clay with TC: (1) an interaction between TC and clay due to the ion exchange between the clay surface and the protonated amine group of the TC, (2) complexation reactions between the divalent cations or H+ on the clay and TC, and (3) a mechanism where there is interaction between TC with the exposed Al ions on the edges of clay. Our preliminary experiments showed that when pH was not adjusted (data not shown), introduction of Cu(II) increased the adsorption of TC on montmorillonite, but the pH of the equilibrium solution decreased about 1–2 units. Solution pH strongly affected the adsorption of TC on clay and soil (18, 19). Tetracycline adsorption on montmorillonite in the absence and presence of Cu(II) at different equilibrium solution pH was carried out to investigate whether pH was the only variable affecting TC adsorption in the presence of Cu(II) (Figure 4). The results showed that when Cu(II) was present, the adsorption of TC was affected. When solution pH was below 9.0, Cu(II) increased the adsorption of TC resulting in little TC left in the equilibrium solution. The zeta potential of the montmorillonite surface equilibrated with 0.25 mmol L-1 Cu(II) in the presence and absence of TC (0.1 mmol L-1) varied (see Figure S6 in the Supporting Information). The zeta potential of montmorillonite decreased with increasing solution pH. Montmorillonite displayed net negative charges in the entire experimental pH range. The presence of Cu(II) increased the positive charges of the montmorillonite surface only when pH was below 5.5. Tetracycline is an amphoteric molecule having multiple ionizable functional groups that exist predominantly as zwitterions at pH values typical of the natural environment. These zwitterions adsorbed on the montmorillonite by complexing with cations of the mineral, and correspondingly increased the negative charge of montmorillonite surface. The predominating TC species are H3L+ and H2L0 at pH < 5.5, but the predominant TC species at solution pH between 5.5 and 7 are H2L0, HL-, and L2-, which have strong coordination with metals. At alkaline pH values (pH > 7), where hydroxyl groups (pKa2) become increasingly more negative and the C-4 nitrogen (pKa3) begins to deprotonate, TCs can complex with metal ions easily. The 1:1 and 2:1 metal-TC complexes have been observed spectroscopically (36, 37). Solution Cu(II) reacts with TC to form water-soluble complexes, which have less negative surface charge and are easily adsorbed on soil surface rather than H2L0 and HL- at high pH conditions. In addition, TC adsorption can take place on the sites where Cu(II) was specifically adsorbed at high pH, and acts as a bridge between soil particles and TC. According to Figueroa et al. (18) and Sassman and Lee (19), TC adsorbed on montmorillonite mainly by cation

Cu(II) (mmol L-1)

Kf (mg1-NLNkg-1)

1/N

r2

0 0.25

5230 ( 830 36200 ( 2560

0.62 ( 0.03 0.25 ( 0.02

0.98 0.99

When the solution pH was above 6.5, the complex CuL was neutral and had lower affinity to the montmorillonite surface than free Cu2+ ion, since the montmorillonite surface had high negative charges at such pH. Copper(II) ion can be precipitated at higher pH, but TC complexation with Cu(II) inhibited the precipitation of Cu(II). Hence, the presence of TC increased solution Cu(II) concentration. On the other hand, the presence of TC increased the Cu(II) adsorption at lower pH (pH < 6.5) because more Cu(II) was adsorbed on the sites of montmorillonite via the TC bridge. Under acidic condition, TC is strongly adsorbed on clay surface by ion exchange (34), while Cu(II) can be adsorbed on montmorillonite surface on the sites of montmorillonite where TC had been adsorbed. Another possible reason is that the complex of TC and Cu(II) has higher affinity to montmorillonite surface than Cu2+ ion alone. Adsorption of TC on Montmorillonite as Affected by Cu(II). Adsorption isotherms of TC on montmorillonite with and without Cu(II) are presented in Figure 3. The amounts of TC adsorbed on montmorillonite increased with increasing TC concentration in equilibrium solution. The adsorption data over the range of TC concentrations studied here can be well described by the Freundlich equation: Cs ) KfCe1⁄N or the logarithmic form: log Cs ) log Kf + (1 ⁄ N) log Ce where Cs is the amount of TC adsorbed (mg kg-1 of adsorbent); Ce is the equilibrium concentration of TC (mg L-1); and Kf (mg1-N LN kg-1) and N are the constants that give estimates of the adsorptive capacity and intensity, respectively. Adsorption isotherms of TC on montmorillonite fit Freundlich equations with high correlation coefficients (r2) 0.97–0.99) (Table 1). An examination of the Freundlich Kf values (Table 1) for the clay material shows that montmorillonite has a high adsorptive capacity for TC. Tetracycline can be adsorbed on montmorillonite by ion exchange with H+ and Na+ or by complexing with Ca2+ on the surface of montmorillonite. Sithole and Guy (34, 35) postulated three mechanisms based

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Oklahoma State University for his assistance in improving the manuscript.

Supporting Information Available The Supporting Information includes data for sorption kinetics of TC on montmorillonite, equilibrium free Cu2+ concentrations in solution, TC and Cu(II) speciation calculated by WinSGW, and zeta-potential measurements for montmorillonite after adsorption of Cu(II) and TC. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited FIGURE 5. Measured and predicted sorption coefficient (Kd) of TC on montmorillonite as affected by Cu(II) and pH. exchange, and each ionic species had its own sorption magnitude. Similar to what Figueroa et al. (18) did for TC adsorption on montmorillonite, the following weighted sum model was employed for the present study: Kd )

∑K

di × fi

where Kdi is the individual sorption coefficients (L eq-1) normalized to pH-dependent CEC, and fi is the species fraction for TC, and Σfi ) 1. The regression analysis revealed the Kdi of H3L+, H2L0, HL- and L2-was 23 200 ( 1470, 4240 ( 570, 4930 ( 1060, and 2680 ( 1360 L eq-1, respectively. This indicates that H3L+ had higher affinity and H2L0, HL- and L2- all had lower affinity to the montmorillonite surface. The model fits the data of TC adsorption on montmorillonite very well (Figure 5). Figueroa et al. (18) also used this model to fit TC adsorption on montmorillonite and found the model fit the experimental data well, and the Kd was 54 900 and 3000 L eq-1 for H3L+ and H2L0, respectively. Their Kd values are higher than those in the present study because the montmorillonite in their study had higher CEC than that used by our study. When Cu(II) is present, TC-Cu(II) complexes change the fraction of TC species. Our results showed that the complexes CuH2L2+, CuHL+, and CuL had higher Kd (2.48 ( 0.56 × 106, 4.24 ( 0.21 × 105 and 4.85 ( 0.27 × 105, respectively) than that of H3L+, H2L, and HL-, but CuH4L22+ and CuH3L2+ had lower Kd than L2-. This may explain why Cu(II) increased TC adsorption at lower pH, since the affinity of the complex between TC and Cu(II) to the mineral surface varies with solution pH. Environmental Significance. Land application of wastes generated from concentrated feeding operations can result in high concentrations of TCs and metals accumulating in agricultural soils. When TC and Cu(II) coexist, they affect each other’s adsorption on montmorillonite, one of the highly reactive common soil constituents. Coexistence of TC and Cu(II) enhanced their adsorption on montmorillonite at environmentally relevant pH values, thus reducing their mobility. The adsorption and cosorption of TC and Cu(II) on soil may affect the degradation of TC and/or the transport of TC and Cu(II) in the environment.

Acknowledgments This work was financially supported by the CAS Research Program on Soil Biosystems and Agro-Product Safety (no. CXTD-Z2005-4-1), the National Natural Science Foundation of China (no. 40671095; 20677064), and Natural Science Foundation of Jiangsu Province, China (no. BK2007263). We greatly appreciate the constructive comments and suggestions from three anonymous reviewers and editor to improve our manuscript. We also thank Professor Hailin Zhang from 3258

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