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Adsorption characteristics of tetracycline by two soils: assessing role of soil organic matter.

Introduction

Tetracycline has been extensively used in human infection medicines, veterinary medicines, animal growth promoters, and prophylactics because it has broad-spectrum antimicrobial activity against a variety of pathogenic bacteria (Zhang et al. 2008). Tetracycline antibiotics are ranked second in production and usage worldwide, and first in China (Gu and Karthikeyan 2005). Most antibiotics have entered the soil environment through municipal effluents, sewage sludge, solid wastes, and manure applications (Zhou et al. 2006, 2007). Between 25 and 75% (Kulshestra et al. 2004), even 70-90% (Halling-Sorensen 2000; Jjemba 2002), of tetracyclines administered to animals are excreted via urine or faeces in the active form. Thus, the agricultural practice of using manure or other wastes as low-cost fertilisers may be of ecotoxicological concern (Zhang et al. 2008; Zhou et al. 2008), especially in soils, which can have high tetracycline residues by receiving frequent waste discharge.

Investigation of adsorption characteristics of antibiotics in soils is of great importance environmentally, because such a process is associated with the ecotoxicity, degradation, transportation, and bioaccumulation of antibiotics in the soil environment. Association of antibiotics with mineral particles and soil organic matter (SOM) will determine their transporting ability in the surface runoff, leaching ability through soils, and mobility in aquifers. Bioassay studies also revealed that antibiotics lose their antibacterial activity when they are adsorbed on soils (Koschorreck et al. 2002). However, compared with other well known xenobiotics such as pesticides, PAHs, and PCBs, there is very limited information available on the fate and transformation of antibiotics in soil/water environments (Gu and Karthikeyan 2005). As one of the most important antibiotics, tetracycline adsorption in soil would be more complicated than other xenobiotics due to its physiochemical properties. Different cationic forms (+ 00, + - 0, + - -) of tetracycline occur at different pH values. Tetracycline carries a positive charge throughout the environmentally relevant pH range, and consists of hydrophilic compounds, and hence has high water solubility. The high polarity (e.g. log [K.sub.ow] = -1.97 to -0.47) and aqueous solubility (0.52--117mM) are also pH-dependent. Several investigations had shown that there is a high adsorption of tetracycline by reference soil components, such as clays (Kulshestra et al. 2004) and hydrous oxides of soil (Figueroa and Mackay 2005; Gu and Karthikeyan 2005). However, the adsorption characteristics of the antibiotics in soil environment are not fully elucidated.

Most adsorption studies on antibiotics have focused on individual soil component including reference clays (Kulshestra et al. 2004), humic acids (Gu et al. 2007), clay-humic complexes (Pils and Laird 2007), Al/Fe hydrous oxides (Gu and Karthikeyan 2005), and organic matter (Kulshestra et al. 2004). These investigations showed that the adsorption of antibiotics is dependent on pH, cation exchange capacity (CEC) (Sassman and Lee 2005), Al/Fe hydrous oxides (Gu and Karthikeyan 2005), and clay components (Kulshestra et al. 2004) in soil. The presence of other cations from buffer salts or ionic strength adjustment could alter solid-solution distributions of tetracycline by competing for negative charge sites on clays. The mechanisms of tetracycline adsorption on clays and Al/Fe hydrous oxides involve the formation of complexes between tetracycline and clays or hydrous oxides (Gu and Karthikeyan 2005). The adsorption of oxytetracycline, as one of the tetracycline antibiotics, can occur via physical mechanisms such as hydrogen bonding, van der Waals forces, and/or chemical mechanisms including cationic exchanges, protonation, electrostatic interactions, coordination, and complexation (Kulshestra et al. 2004). Furthermore, the adsorption of tetracycline can be characterised by 2 processes of different kinetics: a fast initial adsorption to outer surfaces, followed by a slow penetration by slow diffusion into interlayers between clay minerals and micropores (Kulshestra et al. 2004). The adsorption of tetracycline with individual clays and Fe oxides can well fit the Langmuir isotherm (Figueroa and Mackay 2005), but Gu and Karthikeyan (2005) reported that the Freundlich isotherm could better describe the adsorption of tetracycline on Al/Fe hydrous oxides.

Ultimately, the purpose of studying on the adsorption of tetracycline on soil components is to identify the main components of whole soils that can sorb tetracycline and to understand at a mechanistic level how different soil--environment conditions affect the adsorption of tetracycline by whole soils. However, such information is rather limited. Sassman and Lee (2005) studied the adsorption of 3 tetracycline antibiotics by several soils, and showed that all were highly sorbed, especially in acidic and clay-rich soils. Moreover, the adsorption tended to decrease with an increase in pH. Figueroa and Mackay (2005) investigated the adsorption of oxytetracycline on Fe oxide-rich soils and showed that the adsorption of oxytetracycline on Georgeville and Orangeburg Ultisol soils decreased with an increase in pH. Chemical digestion treatments were used to deduce that soil adsorption occurred by complexation of oxide coatings with clays and quartz grains.

SOM is one of the main adsorbents for most contaminants in the soil environment (Sun et al. 2005; Yu and Zhou 2005). Tetracycline can strongly chelate with monovalent and multivalent cations in the soil environment with high organic matter (Kulshestra et al. 2004). Therefore, it is of great interest to understand how SOM can affect the mobility of tetracycline in soil environment, minimise its transport into the groundwater, and avoid surface runoff resulting in nonpoint source pollution. However, previous studies have focused mostly on the sorption of tetracycline as a function of clays (Kulshestra et al. 2004), hydrous oxides (Figueroa and Mackay 2005; Gu and Karthikeyan 2005), CEC, and pH (Sassman and Lee 2005). The effect of organic matter on the sorption of tetracycline in soil environments has been rarely studied, and more work is needed to elucidate sorption behaviours of tetracycline in various soils with different organic matter contents.

The major goal of this study, therefore, is to understand the role of SOM in influencing on the environmental fate and transformation of tetracycline in soil. Specific emphases include: (1) to investigate the effect of SOM on the adsorption kinetics of tetracycline; and (2) to examine the effect of SOM on the adsorption isotherm of tetracycline in cinnamon soil and red soils.

Materials and methods

The tested chemical

Tetracycline hydrochloride (99% purity, analytical grade) was bought from the Sigma Co. in St. Louis, USA. The standard reference material of tetracycline hydrochloride was obtained from the Institute of Veterinary Drug Control in Beijing, China. They were stored at 4[degrees]C.

The molecular structure of tetracycline antibiotics is depicted in Fig. 1. It is tetracycline when [R.sub.1] = H, [R.sub.2] = C[H.sub.3], [R.sub.3] = OH, and [R.sub.4] = H. There are tricarbonylamide, phenolic diketone, and dimethylamine groups in tetracycline antibiotics, which can contribute to a marked pH-dependent speciation affecting aqueous solubility and lipophilicity. Tetracycline has 3 p[K.sub.a] values (3.3, 7.68, 9.69), causing its occurrence as a cationic, zwitterionic, and anionic species under acidic, moderately acidic to neutral, and alkaline conditions, respectively (Gu and Karthikeyan 2005). Usually, tetracycline occurs predominantly as zwitterions at the typical pH values of the natural environment (Sassman and Lee 2005). The important physicochemical properties of tetracycline are: molecular weight (MW) (444.43), molecular formula ([C.sub.22][H.sub.24][O.sub.8][N.sub.2]), aqueous solubility (0.52-117mM), and log [K.sub.ow] (-1.97 to -0.47).

[FIGURE 1 OMITTED]

Tested soils and treatments

The 2 surface (0-0.20 m) soils used in this study were as follows. Cinnamon soil (Alfisol) samples were collected from a forest park in the Tianjin Economic-Technological Development Area (TEDA) in northern China. Red soil (Ultisol) samples were collected in the suburb of Guilin, Guangxi Municipality in south-eastern China. There was no antibiotic application in these sampling sites before the collection of soil samples. These soil samples were air-dried, gently crushed to pass through a 2-mm sieve, thoroughly mixed, and stored in closed containers at the room temperature before their use.

Some selected properties of the soils used in this study are listed in Table 1. Cinnamon soil is rich with SOM and clays, and the content of Fe oxides in red soil is high. The soils were treated using 6% [H.sub.2][O.sub.2] at a minimum amount to remove SOM (Kaiser and Zech 2000; Kahle et al. 2004). This procedure was repeated several times until the air bubble could not be released again when having added fresh 6% [H.sub.2][O.sub.2]. The treated soils were then dried by the water-bath method. After that, ~90% of organic carbon in soils was removed. Thus, the treated soil samples can be basically considered as "organic matter free". A mass balance indicated that <1.0% of clay minerals in the soils were lost during SOM removal.

Adsorption kinetic experiments

To investigate the interaction of tetracycline with different soils and the effect of SOM on the adsorption kinetics, tetracycline adsorption v. time was measured in darkness at 25 [+ or -] 1[degrees]C for 24 h. The initial tetracycline concentration was 1.25, 5.0, and 25.0mg/kg, respectively, and the suspension was kept at a constant pH (7.0) and its ionic strength was 0.01 mol/L of Ca[Cl.sub.2]. About 2.5g of air-dried, radiated soil (natural or SOM-free soil) was mixed with 20mL 0.01M Ca[Cl.sub.2] and 5 mL of tetracycline solution in 50-mL glass test tubes and agitated in a rotary shaker for 24 h. Supernatants were taken at specified time intervals (3 min, 8 min, 15 min, 30 min, 45 min, 1 h, 2.5 h, 5 h, 7h, l 1 h, 18 h, 24h) and centrifuged immediately at 4000 r.p.m, for 20 min. The concentration of tetracycline in the supernatant was measured by HPLC, and the sorbed amounts were calculated by the subtraction method. All adsorption kinetic experiments were carried out in triplicate.

Batch adsorption experiments

Batch adsorption experiments were performed as a function of SOM according to the OECD technical guideline (No. 106) (OECD 2000). Glass centrifuge tubes were covered with Al foils to prevent exposure to light. Suspensions were equilibrated at 25 [+ or -] 1[degrees]C.

A Ca[Cl.sub.2] solution was used to minimise the suspension of soil particles and to simulate natural soil water despite the fact that tetracycline can form complexes with divalent cations such as calcium ions. The Ca[Cl.sub.2] (0.01 M) solution was made as a solvent, then 5 concentrations of tetracycline (6.25, 12.5, 25.0, 62.5, and 125mg/L) were made in the 0.01M Ca[Cl.sub.2] solvent. Triplicate samples with 2.5 g of air-dried soil, 20 mL of Ca[Cl.sub.2] solution, and 5 mL of tetracycline solution were made for each soil at each concentration to reach a final volume of 25 mL with tetracycline concentrations of 1.25, 2.50, 5.00, 12.5, and 25.0mg/L, respectively, in the test glass tubes. This concentration range was chosen because soils receiving wastes from swine production operations have been reported to contain concentrations of this magnitude (Hamscher et al. 2002; Kay et al. 2004). A blank for each soil and a control for each concentration were made. Blanks contained 25 mL of Ca[Cl.sub.2] solution and 2.5g of each soil, and the control contained 20mL of Ca[Cl.sub.2] solution and 5 mL tetracycline solution in a test glass tube without soil samples. All samples were agitated for 24 h in the rotary shaker. After centrifugation at 4000 r.p.m. for 20 min, the supernatants were decanted into wide-mouth amber jars. One drop of 6 mol/L HCl solution was added, and the pH was increased from 2 to 3 in order to minimise degradation and epimerisation of tetracycline and its adsorption to silanol groups that may be present in glassware (Sassman and Lee 2005). Subsequently, supematants were filtered through a syringe filter with 0.45 [micro]m pore size. The concentration of tetracycline was determined by HPLC with the UV detection. The equilibrium concentrations of the non-adsorbed portion of tetracycline were also used to validate adsorption isotherms.

Analytical methods

The supernatant solution was filtered through a syringe filter with 0.45 gm pore size and then condensed with a rotatory evaporator (German Heidolph) at 40[degrees]C. Finally, the supernatant solution was condensed to 2mL in dark glass vials. The concentration in supernatant solution was measured by reverse-phase HPLC with a 46 x 255mm Waters ODS-C18 (5 [micro]m) column followed by UV detection at 360nm. The mobile phase was a mixture of 0.01 M oxalic acid-acetonitril (75:25, v/v) in an equilibrium system at a flow rate of 1.0 mL/min. The retention time was 4.56m in. The limits of detection and quantification for tetracycline were 0.01 mg/L when the sample was uncondensed.

Statistical analysis

Statistical analysis was used to investigate the effect of SOM on adsorption kinetics and adsorption isotherms. Significant difference tests at P=0.05 or 0.01 were performed by analysis of variance (ANOVA) using SPSS 13.0 for Windows. Adsorption data were fitted by Sigma Plot 8.02.

Results

Adsorption kinetics of tetracycline by two natural soils

Adsorption of tetracycline on cinnamon soil and red soil was measured as a function of time in order to determine an equilibration time that would be close to the equilibrium with minimal transformation or irreversible binding under different initial aqueous concentrations. The loss of tetracycline from solution in this study was only through adsorptive action by the soils, because the data from blanks indicated that there was no adsorption of tetracycline on glass centrifuge tubes. Tetracycline concentrations were analysed in the aqueous phase at different equilibrium time intervals, and the sorbed concentration in soil was estimated by the loss of tetracycline from the aqueous phase relative to applied aqueous concentrations (by subtraction).

The adsorption kinetics of tetracycline on the 2 soils are shown in Fig. 2; the adsorption of tetracycline could be divided into a 2-stage reaction including rapid adsorption and slow adsorption. During the initially rapid adsorption phase, the amount of adsorbed tetracycline increased rapidly. Rapid adsorption took place within a few minutes in the 2 soils. The time span of the slow adsorption was likely to be in the range of several months or even years. The rapid interaction is most probably attributed to chemical reaction and film-diffusion processes, and the slow one was caused by diffusion into micropores of inorganic minerals and organic components, and sites of slow reactivity and surface precipitation. In the experiment, the adsorption of tetracycline on the 2 soils was largely rapid, because >98% of tetracycline adsorption occurred within only 3 min with 1.25, 5.00, and 25.0mg/kg of tetracycline. During the slow adsorption process, it took longer (11 h) for the slow adsorption process to attain the equilibrium in cinnamon soil than in red soil (only 5 h). The results also showed that there was only a minor difference between the amount of tetracycline adsorbed by soils after 11h (or 5h) and 24h. Thus, 24h could be suggested as a sufficient equilibration time for tetracycline adsorption. This is basically consistent with the OECD technical guideline.

As shown in Fig. 2, the concentration (Cs) of tetracycline adsorbed to the soil at equilibrium was 12.37, 49.76, and 249.4 mg/kg for cinnamon soil, when the initial concentration of tetracycline in solution was 1.25, 5.00, and 25.0mg/kg, respectively. Furthermore, Cs for red soil was 12.37, 49.79, and 249.7 mg/kg, respectively, for the 3 tetracycline concentrations. The adsorption of tetracycline on the 2 soils was the same (Cs=12.37mg/kg) when the concentration of tetracycline in solution was 1.25 mg/kg; however, there was a difference between the 2 soils with an increase in the initial tetracycline concentration (from 5.0 to 25.0mg/kg). Cs in cinnamon soil was 0.021 and 0.320mg/kg, respectively, lower than that in red soil under 5.0 and 25.0mg/kg of initial tetracycline solution. It can be inferred that the adsorption of tetracycline on red soil at its equilibrium was stronger than on cinnamon soil at the same initial tetracycline concentration.

[FIGURE 2 OMITTED]

Models describing tetracycline adsorption on soils are necessary and useful tools. The Elovich equation, the exponent equation, the first-order equation, and the diffusion equation are often used to simulate the chemical kinetics process of compound adsorption:

Elovich equation: y = a + blnt (1)

Exponent equation: y = [at.sup.b] (2)

First-order equation: y - a(1 - [e.sup.-bt]) (3)

Diffusion equation: y = a + b[t.sup.1/2] (4)

where t is time (h), a and b are model constants, y is the concentration (mg/kg) of tetracycline sorbed. The 4 models describing the adsorption of tetracycline in the 2 soils with different initial tetracycline concentrations are listed in Table 2. It was observed that the 4 equations can all simulate the kinetic process between Cs and time (h) under 3 tetracycline concentrations in the 2 soils, because all correlation coefficients were significant (P<0.05), except for the first-order equation describing adsorption kinetics of 1.25 mg/kg tetracycline in cinnamon soil. Usually, the more optimal the model, the higher is the [R.sup.2] value. Thus, the fitted equations were in the sequence: Elovich equation > exponent equation > diffusion equation>first-order equation according to the order of average [R.sup.2] from 0.429 * to 0.898 **. The Elovich equation and the exponent equation were the best fitted kinetics describing Cs changes with time (h) in the 2 soils.

Effect of SOM on tetracycline adsorption kinetics

The effect of SOM on the equilibrium time of tetracycline adsorption in the 2 soils was investigated at 1.25, 5.0, and 25.0 mg/L of tetracycline in solution (shown in Fig. 3). The adsorption process of tetracycline affected by SOM also consisted of a 2-stage reaction: an initially rapid adsorption, followed by a slow adsorption whether organic matter occurred or not. During the slow adsorption process, it took a longer (1 l-h) slow adsorption process to attain equilibrium for natural cinnamon soil. However, it took only 7h for SOM-free cinnamon soil. In particular, the adsorption equilibrium time of tetracycline in red soil was only 5 h, whether SOM was removed or not under the 3 tetracycline concentrations (Fig. 3). This phenomenon showed that SOM affected the equilibrium time of tetracycline adsorption with cinnamon soil to a great extent, but not with red soil. The results also showed that 24 h could also be suggested as a sufficient equilibration time for the 2 SOM-free soils.

[FIGURE 3 OMITTED]

Based on the above data shown in Fig. 3, the removal of SOM can reduce Cs in the 2 soils. SOM in cinnamon soil reduced Cs with 0.032, 0.288, and 0.732 mg/kg, when the initial concentration of tetracycline in solution was 1.25, 5.0, and 25.0mg/kg, respectively. The significant (P<0.05) difference between Cs of natural cinnamon soil and the SOM-free cinnamon soil was observed by variance analysis. However, Cs of red soil decreased 0.003, 0.011, and 0.086mg/kg, respectively, after its SOM was removed. Moreover, there was no significant (P>0.05) difference between Cs of the natural red soil and the SOM-free red soil. The above results showed that the decreased Cs in cinnamon soil was much greater than that in red soil after SOM was removed, which could be caused by the difference between SOM concentrations in cinnamon soil (3.86%) and red soil (1.06%).

The 4 models including the Elovich equation, the exponent equation, the diffusion equation, and the first-order equation were also used to simulate the kinetics process of compound adsorption on the SOM-free soils with different tetracycline concentrations (Table 3). The results showed that all 4 equations could simulate the kinetics process between Cs and time (h) under the 3 tetracycline concentrations in the 2 SOM-free soils, because all correlation coefficients were significant (P< 0.05). The fitted equations were in the sequence: exponent equation > Elovich equation > diffusion equation > first-order equation according to the order of [R.sup.2] value ranging from 0.483 ** to 0.882 **. The exponent equation and the Elovich equation were the best fitted kinetics describing the Cs changes with time (h) in the 2 SOM-free soils. As shown in Table 2, the average [R.sup.2] values in the exponent equation and the Elovich equation decreased for the SOM-free soils compared with the natural soils, while the average [R.sup.2] values in the first-order equation and the diffusion equation increased. The results also showed that the exponent equation and the Elovich equation could better simulate the relationship between Cs and time (h), especially for the natural soils.

Adsorption isotherms in 2 natural soils

The adsorption isotherms of tetracycline by the 2 natural soils are shown in Fig. 4, and the soil-water distribution coefficients ([K.sup.d], the ratio between adsorbed tetracycline and the concentration of tetracycline in solution) are summarised in Table 4. Over the whole range of tested tetracycline concentrations from 1.25 to 25.0mg/kg in the 2 soils, the 2 isotherms were S-shaped. The sorbed tetracycline concentration in red soil was always higher than that in cinnamon soil at the range of tested concentrations. The [K.sup.d] values ranging from 93.13 to 1341L/kg showed strong adsorption of tetracycline in the 2 soils, and increased with an increase in tested aqueous concentrations, but Kd in cinnamon soil was always lower than that in red soil at same concentration of tetracycline in solution.

Laboratory adsorption isotherm data were used in mathematical models as indicators of tetracycline mobility to predict tetracycline availability and potential groundwater contamination. The Freundlich equation, the linear equation, the Langmuir equation, and the Temkin equation (as shown in Table 5) were also used to describe the adsorption of tetracycline in soils. In all the isotherms, Cs is also the sorbed concentration (mg/kg) on soil at equilibrium; Ce is the concentration in the solution (mg/L); and others are adsorption isotherm parameters, which are of different meaning in different isotherms. Among the 4 isotherms, only the Freundlich isotherm can well simulate the relationship between Cs and Ce, because all [R.sup.2] values were significant (P<0.01). The Freundlich constants [K.sup.F] and n denote the adsorption capacity and the adsorption intensity of tetracycline on soils, respectively.

Effect of SOM on adsorption isotherms

SOM can primarily control the adsorption of some organic compounds in soil. Figure 5 showed that the removal of SOM can decrease Cs in different soils, according to the adsorption isotherm curves before and after removing SOM. Even though SOM was removed, the amount of tetracycline sorbed on red soil was still higher than that on cinnamon soil at the same aqueous concentration of tetracycline.

The [K.sup.d] values of SOM-free soils are also summarised in Table 4. It was showed that [K.sup.d] in the SOM-free soils increased with an increase in the tested aqueous tetracycline concentrations (from 0 to 25.0 mg/kg). The range of [K.sup.d] was 74.30-879.3L/kg, which indicated strong adsorption of tetracycline on the 2 SOM-free soils. [K.sup.d] of the natural soils was always higher than that of the SOM-free soils, and [K.sup.d] difference (the difference between [K.sup.d] of a natural soil and SOM-free soil) in cinnamon soil was higher than that in red soil at the same concentration of tetracycline in solution. Moreover, [K.sup.d] in the SOM-free cinnamon soil was always lower than that in the SOM-free red soil at the same concentration of tetracycline in solution.

The adsorption of tetracycline by the SOM-free soils was also well described by the Freundlich equation ([R.sup.2] significant, P<0.01) (Table 4). SOM exerted different effects on adsorption by different types of soils. There was a difference in change in [K.sub.F] and n values of the 2 soils before and after [H.sub.2][O.sub.2] treatment. [K.sub.F] decreased considerably after the removal of SOM. However, n in red soil decreased and n in cinnamon soil increased after the removal of SOM (Table 5).

Discussion

Unlike well-studied, non-polar organic compounds, tetracycline antibiotics have multiple ionisable functional groups. Because of this, the adsorption of tetracycline on soils is complicated, and until now, the adsorption mechanism has not been well known. Adsorption of tetracycline antibiotics on some model soil components has been investigated, for example, reference clays (Kulshestra et al. 2004), and Al/Fe hydrous oxides (Gu and Karthikeyan 2005). However, we were interested to understand adsorption interactions between tetracycline antibiotics and whole soils that are aggregates of various sorbents and have wider environmental significance. SOM is the main adsorbent for most contaminants in the soil environment (Sun et al. 2005). Tetracycline antibiotics strongly chelate with monovalent and multivalent cations in the SOM (Pankaj 2004). In fact, in natural soils, SOM can combine with minerals to form organic-mineral complexes. Also, soil aggregates take part in soil processes as a whole (Pan 2000) and result in differences from the pure SOM or its individual components. Some earlier studies also suggested that soil aggregates may play a key role in the sequestration of organic contaminants, probably by providing tortuous sorptive and/or desorptive pathways (Zhou 1995; Sun et al. 2005).

[FIGURE 4 OMITTED]

Soil components such as clays (Sun et al. 2005; Pils and Laird 2007), Al/Fe hydrous oxides (Gu and Karthikeyan 2005), and organic matter (Pils and Laird 2007; Gu et al. 2007) strongly affected kinetic processes of tetracycline adsorption (Gu and Karthikeyan 2005). The above results indicate that the equilibrium of tetracycline adsorption was attained more rapidly in red soil (5 h) than that in cinnamon soil (11 h) (Fig. 1), although the 2 soils had the same pH (7), temperature (25[degrees]C), and levels of ionic strength (0.01 M Ca[Cl.sub.2]) when the kinetic adsorption experiment was carried out. To a certain extent, soil properties could affect the adsorption of tetracycline. The chemical analysis showed 48.9% clays and 3.86% organic matter in cinnamon soil, and only 25.6% clays and 1.06% organic matter in red soil. However, Al/Fe hydrous oxides in red soil were richer than those in cinnamon soil. Thus, adsorption of tetracycline with clays and organic matter was dominant in cinnamon soil, while adsorption with Al/Fe hydrous oxides was dominant in red soil. The X-ray diffraction analysis showed that tetracycline was sorbed in the interlayers of separate clays after the adsorption reaction reached its equilibrium (Pils and Laird 2007). In fact, it was more difficult for tetracycline to diffuse into the interlayers of clays and organic components in cinnamon soil because clay-humic complexes between clays and organic matter could inhibit the adsorption speed of tetracycline with clays. Therefore, the slow adsorption stage of tetracycline with cinnamon soil needed a longer time to attain its equilibrium. This result was confirmed when equilibrium time was shortened to 7 h after SOM was removed (Fig. 3), because clay-humic complexes did not occur after the removal of SOM. Adsorption interactions of tetracycline antibiotics with soil components such as organic matter and quartz were thought to be negligible in some soils such as red soil (Figueroa and Mackay 2005). The sharp red colour indicates iron oxide coating of clays, SOM, and silicate surfaces due to high concentration of Al/Fe hydrous oxides (Figueroa and Mackay 2005), which would cause Al/Fe hydrous oxides to be the dominant component for the adsorption of tetracycline antibiotics in red soil. The adsorption mechanism of tetracycline antibiotics on red soil was through transportation to oxide surfaces through surface complexation (Figueroa and Mackay 2005) with the rapid speed. For example, adsorption of tetracycline on Al/Fe hydrous oxides was a fast process, attaining its equilibrium after only 8 h (Gu and Karthikeyan 2005). Thus, the adsorption speed for red soil was rapid, and it took only 5 h to attain equilibrium when surface complexation to oxide surfaces was the most important mechanism of tetracycline adsorption on red soil. However, equilibrium time was longer (11h) for cinnamon soil due to the mechanisms of tetracycline diffusing into the interlayers between clays and organic components. Based on these results, the adsorption equilibrium time and Cs on soil depended on the soil properties, especially the quantity and type of Al/Fe hydrous oxides, clays, and SOM.

[FIGURE 5 OMITTED]

The adsorption of tetracycline with clays was dominant in cinnamon soil, and clay-humic complexes were destroyed after SOM in soil was removed. Thus, tetracycline can enter interlayers of separate clays in the SOM-free soil more easily than in soil with SOM. This may be responsible for the shorter time of tetracycline adsorption equilibrium in the SOM-free cinnamon soil (7 h) than that in the natural soil (11 h). Cinnamon soil with high SOM (3.86) had a strong adsorption of tetracycline. Thus, Cs would decrease greatly after SOM in the soil was removed. In contrast, SOM in red soil could weakly take part in adsorption of tetracycline because of the action of Fe oxides with high concentrations of coated clays, SOM, and silicate surfaces. More Al/Fe hydrous oxides were exposed to soil surfaces when the soil was treated by removing organic matter, because the complexation between SOM and hydrous oxides was destroyed although some SOM formed a small quantity of the complexation. On the basis of the results, adsorption with Al/Fe hydrous oxides was dominant in red soil whether SOM occurred or not, because SOM in red soil did not affect the adsorption equilibrium time (5 h) and weakly affected Cs under different tetracycline concentrations. The adsorption interactions of tetracycline antibiotics with soil components including organic matter were thought to be negligible in red soil (Figueroa and Mackay 2005). It was also proven in this study. In highly weathered soils, Al/Fe hydrous oxides can account for as much as 50% of the total soil mass (Gu and Karthikeyan 2005). Thus, SOM could not play an important role in adsorption of tetracycline for highly weathered soils. In this study, similar results were found, because Al/Fe hydrous oxides in red soil were up to 49.97% of the total soil mass. Moreover, the adsorption of tetracycline on the 2 soils was rapid, and >98% of added tetracycline in solution was sorbed within only 3 min in the natural and SOM-free soils with 1.25, 5.0, and 25.0mg/kg of tetracycline.

Generally speaking, clay minerals and SOM are rich with negative charges, whereas Al/Fe hydrous oxides are rich with positive charges. Tetracycline-containing zwitterions at pH 7 have good adsorption on soils through cationic and anion exchanges. Adsorption of tetracycline on soils was very strong with high [K.sub.d] in all the tested soils (Table 4). There was a high [K.sub.d] for excess adsorption sites in soils, and [K.sub.d] increased with an increase in the tested concentration (1.25-25.0mg/kg) of tetracycline in solution. There were much higher [K.sub.d] values in red soil, which were attributed to more excess adsorption sites in red soil than in cinnamon soil when the initial concentration of tetracycline was the same.

Freundlich isotherm equations which resemble the S-type curves were the most fitting model for adsorption of tetracycline on the 2 natural soils. The results from the batch adsorption experiments indicated that adsorption of tetracycline on red soil ([K.sub.F]=6.819E+09) was relatively stronger than that on cinnamon soil ([K.sub.F]=20 520). The normalisation of [K.sub.F] using the content of organic matter in soil gave [K.sub.OM] values of 1.767E+111/kg for red soil and 1.936E+061/kg for cinnamon soil.

Based on the Gibbs equation, the standard molar Gibbs free energy change is calculated as:

[DELTA][G.sup.0] = -RT ln[K.sub.oc] (5)

where R is the universal gas constant [8.314 J/(mol x K)] and T is absolute temperature (K). Using all these experimental data in the integrated forms of the Gibbs adsorption equations, the standard molar free energy change [DELTA][G.sub.0] was calculated in kJ/mol unit. The [DELTA][G.sup.0] values were -32.77 kJ/mol for cinnamon soil and -67.58 kJ/mol for red soil. According to the threshold (40kJ/mol) suggested by Carter et al. (1995), physical adsorption in cinnamon soil and chemical adsorption in red soil could be identified as the dominant mechanisms of tetracycline adsorption. The negative values of [DELTA][G.sup.0] for tetracycline adsorption on soil are the indication of a spontaneous process. In other words, the adsorption forces of tetracycline on soil are strong enough to break the potential barrier.

It was shown that [K.sub.d] of SOM-free soils was lower than that of natural soils, because SOM as one of the most important sorbents was removed from soils. The fact that SOM affected [K.sub.d] of cinnamon soil more strongly than that of red soil could be explained by the higher SOM concentration in cinnamon soil than red soil. SOM and clays were the main sorbents in cinnamon soil, while Al/Fe hydrous oxides were the main sorbents in red soil. Therefore, SOM affected adsorption of tetracycline on cinnamon soil more strongly than that on red soil.

The Freundlich isotherm equation was also the best fitting model to describe adsorption of tetracycline on the SOM-free soils. The [K.sub.F] values decreased after the removal of SOM. This was attributed to the effect of tetracycline sorbed by SOM. However, n of red soil decreased and n of cinnamon soil increased after the removal of SOM (Table 5), which showed that removal of SOM affected the adsorption intensity of the soils differently. Even though SOM was removed, the amount of tetracycline sorbed on red soil was higher than on cinnamon soil. Among the tested soils, those with high content of Al/Fe hydrous oxides had the high adsorption affinities.

Conclusions

The adsorption of tetracycline on red soil was stronger and more rapid than on cinnamon soil. Different dominant components for sorbing tetracycline in the 2 soils caused different equilibrium times and Cs. Al/Fe hydrous oxides as dominant components in red soil could quickly sorb tetracycline through the mechanism of surface complexation, whereas SOM and clays in cinnamon soil slowly sorbed tetracycline through diffusion into the interlayers of SOM and clays. Greater adsorption capacity and adsorption intensity in red soil could be caused by different soil properties, because red soil could have more adsorption sites than cinnamon soil.

SOM affected kinetics of tetracycline adsorption in cinnamon soil more intensively than in red soil, because SOM was the dominant component for adsorption in cinnamon soil, but not in red soil. The equilibrium time of tetracycline adsorption on soil was shortened and Cs decreased in cinnamon soil when SOM was removed from soil. The exponent equation and the Elovich equation could well simulate the relationship between Cs and the equilibrium time (h), especially for natural soils; >98% of tetracycline in solution could be sorbed within only 3 min in natural and SOM-free soils.

The Freundlich isotherm equation which resembles the S-type curve was the best fitting model for the 2 soils whether SOM was removed or not. However, [K.sub.F] decreased after SOM was removed. SOM affected n of different soils diversely. In red soil, n decreased. In contrast, n increased in cinnamon soil. Soils with high Al/Fe hydrous oxides had high adsorption affinities. According to the standard molar free energy change [DELTA][G.sup.0], physical adsorption and chemical adsorption were identified as the dominant adsorption mechanisms for cinnamon soil and red soil, respectively.

Acknowledgments

This work was financially supported by the International Foundation for Science (Sweden) (grant No. AC/19097). The authors also thank the Ministry of Education, People's Republic of China, for financial support as a grand fostering project (grant No. 707011) and the National Natural Science Foundation of China as a general project (grant No. 20777040).

Manuscript received 3 May 2008, accepted 27 November 2008

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Yanyu Bao (A), Qixing Zhou (A,B,C), and Yingying Wang (A)

(A) Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China.

(B) Key Laboratory of Terrestrial Ecological Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China.

(C) Corresponding author. Email: zhouqixing2008@163.com
Table 1. Selected soils properties

 [Fe.sub.2] [Al.sub.2]
 [O.sub.3] [O.sub.3]
 SOM CEC
Soil type pH (%) (cmo/kg) (g/kg)

Cinnamon soil 6.23 3.86 19.88 20.55 40.54
Red soil 5.52 1.06 13.69 398.7 101.1

 Texture (%)
 Clay Silt Sand
Soil type (<0.001 mm) (0.001-0.01 mm) (>0.01 mm)

Cinnamon soil 48.90 36.20 14.90
Red soil 25.60 45.20 29.20

Table 2. Parameters for kinetic models of tetracycline adsorption
on soils

[+ or -] Standard error (n = 12), [[R.sup.2].sub.0.01] = 0.468,
[[R.sup.2].sub.0.05] = 0.306; * P < 0.05, ** P < 0.01 for
correlation coefficient

 a

 Elovich equation
 y = a + blnt

Cinnamon soil 1.25mg/kg 12.32 [+ or -] 0.001
Red soil 1.25 mg/kg 12.31 [+ or -] 0.007
Cinnamon soil 5 mg/kg 49.54 [+ or -] 0.005
Red soil 5 mg/kg 49.58 [+ or -] 0.031
Cinnamon soil 25mg/kg 248.5 [+ or -] 0.022
Red soil 25 mg/kg 248.7 [+ or -] 0.155

Average --

 b [R.sup.2]

 Elovich equation y = a + blnt

Cinnamon soil 1.25mg/kg 0.016 [+ or -] 0.001 0.981 **
Red soil 1.25 mg/kg 0.026 [+ or -] 0.004 0.825 **
Cinnamon soil 5 mg/kg 0.075 [+ or -] 0.002 0.989 **
Red soil 5 mg/kg 0.101 [+ or -] O.Ol6 0.805 **
Cinnamon soil 25mg/kg 0.316 [+ or -] 0.011 0.988 **
Red soil 25 mg/kg 0.498 [+ or -] 0.079 0.801 **

Average -- 0.898 **

 a

 Exponent equation
 y = [at.sup.b]

Cinnamon soil 1.25mg/kg 12.32 [+ or -] 0.001
Red soil 1.25 mg/kg 12.31 [+ or -] 0.007
Cinnamon soil 5 mg/kg 49.54 [+ or -] 0.005
Red soil 5 mg/kg 49.58 [+ or -] 0.031
Cinnamon soil 25mg/kg 248.5 [+ or -] 0.022
Red soil 25 mg/kg 248.7 [+ or -] 0.156

Average --

 b [R.sup.2]

 Exponent equation y = [at.sup.b]

Cinnamon soil 1.25mg/kg 0.0013 [+ or -] 0.0001 0.981 **
Red soil 1.25 mg/kg 0.0021 [+ or -] 0.0003 0.823 **
Cinnamon soil 5 mg/kg 0.0015 [+ or -] O.0001 0.989 **
Red soil 5 mg/kg 0.0020 [+ or -] 0.0003 0.804 **
Cinnamon soil 25mg/kg 0.0013 [+ or -] 0.0000 0.987 **
Red soil 25 mg/kg 0.0020 [+ or -] 0.0003 0.800 **

Average -- 0.897 **

 First-order equation
 y = a([l - e.sup.-bt])

Cinnamon soil 1.25mg/kg 12.33 [+ or -] 0.009
Red soil 1.25 mg/kg 12.34 [+ or -] 0.012
Cinnamon soil 5 mg/kg 49.60 [+ or -] 0.040
Red soil 5 mg/kg 49.68 [+ or -] 0.047
Cinnamon soil 25mg/kg 248.8 [+ or -] 0.168
Red soil 25 mg/kg 249.2 [+ or -] 0.233

Average --

 First-order equation
 y = a([l - e.sup.-bt])

Cinnamon soil 1.25mg/kg 106.7 [+ or -] 10.21 0.277
Red soil 1.25 mg/kg 89.04 [+ or -] 5.725 0.543 **
Cinnamon soil 5 mg/kg 102.6 [+ or -] 9.338 0.314 *
Red soil 5 mg/kg 88.81 [+ or -] 5.567 0.563 **
Cinnamon soil 25mg/kg 106.1 [+ or -] 9.399 0.311 *
Red soil 25 mg/kg 88.95 [+ or -] 5.527 0.566 **

Average -- 0.429 *

 Diffusion equation
 y = a + [bt.sup.1/2]

Cinnamon soil 1.25mg/kg 12.29 [+ or -] 0.007
Red soil 1.25 mg/kg 12.28 [+ or -] 0.019
Cinnamon soil 5 mg/kg 49.41 [+ or -] 0.028
Red soil 5 mg/kg 49.44 [+ or -] 0.079
Cinnamon soil 25mg/kg 248.0 [+ or -] 0.133
Red soil 25 mg/kg 248.1 [+ or -] 0.395

Average --

 Diffusion equation
 y = a + [bt.sup.1/2]

Cinnamon soil 1.25mg/kg 0.0185 [+ or -] 0.003 0.807 **
Red soil 1.25 mg/kg 0.0250 [+ or -] 0.008 0.496 **
Cinnamon soil 5 mg/kg 0.0880 [+ pr -] 0.012 0.852 **
Red soil 5 mg/kg 0.0977 [+ or -] 0.033 0.473 **
Cinnamon soil 25mg/kg 0.3636 [+ or -] 0.055 0.814 **
Red soil 25 mg/kg 0.4804 [+ or -] 0.163 0.464 *

Average -- 0.651 **

Table 3. [R.sup.2] Values for different kinetic models of
tetracycline adsorption on SOM-free soils

[[R.sup.2].sub.0.01] = 0.468, [[R.sup.2].sub.0.05] = 0.306;
* P < 0.05, ** P < 0.01 for correlation coefficient

 Elovich Exponent
 equation equation
 y = a + blnt y = [at.bup.b]

SOM-free cinnamon soil 1.25 mg/kg 0.946 ** 0.964 **
SOM-free red soil 1.25mg/kg 0.839 ** 0.838 **
SOM-free cinnamon soil 5 mg/kg 0.945 ** 0.945 **
SOM-free red soil 5mg/kg 0.802 ** 0.801 **
SOM-free cinnamon soil 25 mg/kg 0.924 ** 0.924 **
SOM-free red soil 25mg/kg 0.820 ** 0.819 **
Average 0.879 ** 0.882 **

 First-order Diffusion
 equation equation
 y = a(l - y = a +
 [e.sup.-bt]) [bt.sup.1/2]

SOM-free cinnamon soil 1.25 mg/kg 0.364 * 0.726 **
SOM-free red soil 1.25mg/kg 0.546 ** 0.516 **
SOM-free cinnamon soil 5 mg/kg 0.425 * 0.710 **
SOM-free red soil 5mg/kg 0.553 ** 0.473 **
SOM-free cinnamon soil 25 mg/kg 0.463 * 0.688 **
SOM-free red soil 25mg/kg 0.549 ** 0.494 **
Average 0.483 ** 0.607 **

Table 4. Adsorption distribution coefncients ([K.sub.d]) for
tetracycline in untreated and treated soils

 [K.sub.d]
 Solution [K.sub.d] without
 conc. with SOM [K.sub.d]
Soil type (mg/kg) SOM (L/kg) difference

Cinnamon soil 1.25 93.13 74.30 18.83
 2.50 131.1 95.90 35.24
 5.00 210.9 94.42 116.4
 12.5 383.6 126.9 256.7
 25.0 412.9 186.2 226.7

Red soil 1.25 93.19 91.05 2.138
 2.50 134.9 132.0 2.891
 5.00 231.8 220.4 11.34
 12.5 526.4 489.6 36.85
 25.0 879.3 674.7 204.6

Table 5. Parameters for adsorption isotherms of different soils

[+ or -] Standard error (n = 5), [[R.sup.2.sub.0.01] = 0.841,
[[R.sup.2].sub.0.05] = 0.658; * P < 0.05, ** P < 0.01 for
correlation coefficient

 Freundlich adsorption
 isotherms: Cs = [K.sub.F]
 [C.sub.e.sup.n]

 n

Cinnamon soil 1.564 [+ or -] 0.227
SOM-free cinnamon soil 1.832 [+ or -] 0.215
Red soil 4.805 [+ or -] 0.562
SOM-free red soil 2.693 [+ or -] 0.441

Average --

 Freundlich adsorption isotherms:
 Cs = [K.sub.F] [C.sub.e.sup.n]

 [K.sub.F] [R.sup.2]

Cinnamon soil 20 520 [+ or -] 13 830 0.933 **
SOM-free cinnamon soil 9719 [+ or -] 4415 0.976 **
Red soil 6.819E+09 [+ or -] 25 943 751 0.964 **
SOM-free red soil 1.823E+06 [+ or -] 27 220 0.918 **

Average -- 0.948 **

 Linear adsorption isotherms:
 Cs = a + [K.sub.d] Ce

 a

Cinnamon soil -59.52 [+ or -] 6.019
SOM-free cinnamon soil -17.84 [+ or -] 3.956
Red soil -117.0 [+ or -] 10.94
SOM-free red soil -105.1 [+ or -] 9.200

Average --

 Linear adsorption isotherms:
 Cs = a + [K.sub.d] Ce

 [K.sub.d] [R.sup.2]

Cinnamon soil 5020 [+ or -] 296.9 0.920 **
SOM-free cinnamon soil 1623 [+ or -] 97.45 0.872 **
Red soil 8987 [+ or -] 638.8 0.440
SOM-free red soil 7929 [+ or -] 509.0 0.663 *

Average -- 0.724 *

 Langmuir adsorption
 isotherms:
 Ce/Cs = 1/K Sm + Ce/Sm

 1/Sm

Cinnamon soil -0.Ol0 [+ or -] 0.522
SOM-free cinnamon soil -0.006 [+ or -] 0.286
Red soil -0.057 [+ or -] 1.757
SOM-free red soil -0.028 [+ or -] 1.034

Average --

 Langmuir adsorption isotherms:
 Ce/Cs = 1/K Sm + Ce/Sm

 1/K Sm [R.sup.2]

Cinnamon soil 0.0008 [+ or -] 0.022 0.264
SOM-free cinnamon soil 0.0013 [+ or -] 0.026 0.870 **
Red soil 0.0017 [+ or -] 0.044 0.745 *
SOM-free red soil 0.0011 [+ or -] 0.031 0.428

Average -- 0.577

 Temkin adsorption isotherms:
 Cs = a + KlogCe

 a

Cinnamon soil 516.2 [+ or -] 30.97
SOM-free cinnamon soil 276.5 [+ or -] 16.52
Red soil 638.4 [+ or -] 46.68
SOM-free red soil 619.6 [+ or -] 41.34

Average --

 Temkin adsorption isotherms:
 Cs = a + KlogCe

 K [R.sup.2]

Cinnamon soil 273.8 [+ or -] 17.48 0.679 *
SOM-free cinnamon soil 152.5 [+ or -] 10.20 0.569
Red soil 338.4 [+ or -] 26.04 0.320
SOM-free red soil 330.6 [+ or -] 23.24 0.460

Average -- 0.507
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Author:Yanyu, Bao; Qixing, Zhou; Yingying, Wang
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:May 1, 2009
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