JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 20 February 2008
Published in J Environ Qual 37:704-711 (2008)
DOI: 10.2134/jeq2007.0030
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Agricola
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Related Collections
Right arrow Humic Substances
Right arrow Pharmaceuticals
Right arrow Soil Chemistry

TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption of the Antibiotic Tetracycline to Humic-Mineral Complexes

Cheng Gua and K. G. Karthikeyanb,*

a Univ. of Wisconsin-Madison, Madison, WI 53706, present address, Dep. of Crop Sciences, Michigan State Univ., East Lansing, MI
b Dep. of Biological Systems Engineering, Environmental Chemistry and Technology Program, 460 Henry Mall, Univ. of Wisconsin-Madison, Madison, WI 53706

* Corresponding author (kkarthikeyan{at}wisc.edu).

Received for publication January 16, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Humic substances, by altering the surface properties and/or competing for available reaction sites, can either suppress or promote sorption of organic compounds to mineral surfaces. Limited literature evidence points to the reduction in sorption of the antibiotic tetracycline to clay minerals in the presence of humic substances. We investigated the surface interaction of Elliott soil humic acid (ESHA) with hydrous Al oxide (HAO) and the effect of this association on tetracycline sorption. Strong interaction between ESHA and HAO led to ESHA-promoted dissolution of HAO and surface charge reversal. The ESHA-HAO sorption–desorption isotherms were successfully described using a modified Langmuir model that accounted for the heterogeneity of HAO surface and ESHA. Ligand exchange was proposed as the major interaction mechanism, and the edge Al atoms on HAO surface were considered as the sorption sites for ESHA macromolecules. ESHA was coated onto HAO to achieve two different organic content (foc) levels of 0.81 and 1.52%. Sorption results were compared for the binary ESHA-tetracycline and HAO-tetracycline systems, and the ternary ESHA-HAO-tetracycline system. The coating of ESHA on HAO significantly suppressed tetracycline sorption levels, attributable to altered HAO surface charge characteristics and/or direct competition between ESHA and tetracycline for potential sorption sites. Higher foc level, besides increasing the extent of sorption suppression, also resulted in greater ionic strength dependence and increased nonlinearity of sorption behavior. It, therefore, appears that the presence of humic substances, in both dissolved and mineral-bound forms, is likely to increase the environmental mobility of tetracycline compounds.

Abbreviations: ESHA, Elliot soil humic acid • HAO, hydrous Al oxide


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
THE previous decade has witnessed an increasing number of publications documenting the presence of antibiotics, potential contaminants, in the environment. For example, members of the tetracycline family of compounds, among the most widely used antibiotics, have been detected in soils (Hamscher et al., 2002; Jacobsen et al., 2004), surface waters (Lindsey et al., 2001; Kolpin et al., 2002; Batt and Aga, 2005) and even ground water (Lindsey et al., 2001; Batt and Aga, 2005; Karthikeyan and Meyer, 2006). Tetracyclines are known to persist and accumulate in soils after repeated usage of liquid manure as fertilizers (Hamscher et al., 2002). However, there is limited information on the retention mechanisms for tetracyclines in subsurface environments.

Both humic substances and soil minerals are important soil components from the standpoint of interaction with organic and inorganic contaminants. Sorption of tetracyclines to humic substances and soil minerals involves diverse mechanisms that respond differently to solution chemistry. For example, cation exchange (Gu et al., 2007), cation bridging (MacKay and Canterbury, 2005), and hydrogen bonding (Sithole and Guy, 1987b) have been proposed for the association of tetracyclines with humic substances. Consequently, the interaction of several antibiotics, including tetracyclines, with humic substances was significantly underestimated by considering only the hydrophobic partitioning mechanism (Tolls, 2001). Similarly, the role of cation exchange and surface complexation during the sorption of tetracyclines to clay (Porubcan et al., 1978; Sithole and Guy, 1987a; Figueroa et al., 2004; Kulshrestha et al., 2004) and oxide minerals (Figueroa and MacKay, 2005; Gu and Karthikeyan, 2005a) has been highlighted as well. Since the soil minerals have a strong affinity for humic substances, humic-mineral complexes are likely to form and exert a strong influence on the reactivity of antibiotics.

Results from binary humic-antibiotic and mineral-antibiotic systems may not be simply combined or extrapolated to understand the reactivity of antibiotics in a complex humic-mineral-antibiotic ternary system. For example, bound humic substances may influence mineral reactivity by altering the type and charge on surface functional groups as well as the available sorption sites. Meanwhile, mineral-bound humic substances can also undergo physiochemical and conformational changes (Kretzschmar et al., 1997; Chorover et al., 1999; Li et al., 2003; Wang and Xing, 2005; Yoon et al., 2005) leading to altered surface properties. Thus, the sorption characteristics of both the minerals and humic substances are modified due to the formation of humic-mineral complexes. Also, the association between humic substances and minerals could occur at potential antibiotic sorption sites on these sorbents (Chorover et al., 1999; Leone et al., 2001). On the other hand, the fractionation and conformational changes in humic substances on sorption to minerals could enhance their reactivity toward certain contaminants; sorption of phenanthrene was significantly enhanced due to the low polarity of humic substances bound to clay minerals (Wang and Xing, 2005). Since previous studies have focused on binary systems, there is a greater need for a detailed investigation of the humic-mineral-antibiotic ternary system.

Preferential interaction between certain components of humic substances (aliphatic vs. aromatic) and hydrous oxides or clay minerals is reported in the literature. It has been suggested that large size hydrophobic humic fractions are preferentially adsorbed to minerals (Gu et al., 1995; Meier et al., 1999; Guo and Chorover, 2003; Wang and Xing, 2005). Using 13C nuclear magnetic resonance (NMR) spectroscopy, Wang and Xing (2005) showed that the aliphatic fractions have a higher affinity for kaolinite and montmorillonite than the aromatic components and humic substances are sorbed onto mineral surfaces at discrete spots. However, McKnight et al. (1992) and Gu et al. (1995) reported preferential sorption of aromatic humic moieties to aluminum (Al) and iron (Fe) oxides. While ligand exchange-surface complexation was proposed as the dominant interaction mechanism for sorption of humic substances on Fe oxide (Gu et al., 1994, 1995), Yoon et al. (2004, 2005) found that weak outer-sphere type complexes would play an important role during the interaction of humic substances with boehmite ({gamma}-AlO(OH)). Lack of an agreement between the above studies could be attributed to differences in the sources of humic substances and inorganic minerals. Other factors, such as the stereochemical arrangement of the functional groups on humic substances may also lead to preferential sorption of certain humic fractions (Gu et al., 1994, 1995).

The objectives of this study, therefore, were to characterize the surface interaction of humic substances with hydrous metal oxides and to investigate the sorption behavior of tetracycline to humic-mineral complexes. As shown in Fig. 1a , tetracycline has multiple ionizable functional groups, which may result in a complex sorption behavior with humic-mineral complexes. Elliott soil humic acid (ESHA) was chosen as the model humic substance; the Elliott silty clay loam is typical of fertile soils present in the midwestern U.S. Hydrous oxide of Al (HAO) was used as a soil mineral representative. In highly weathered soils, HAO can account for up to 50% of the total soil mass (Summer, 2000). As indicated in our previous work, HAO possesses a strong affinity for tetracycline (Gu and Karthikeyan, 2005a). The association of ESHA with HAO was systematically investigated and the sorption characteristics of tetracycline to ESHA-HAO complexes were compared with those for the binary HAO-tetracycline and ESHA-tetracycline systems under identical solution conditions.


Figure 1
View larger version (21K):
[in this window]
[in a new window]

  		Fig. 1. (a) Structure and (b) pH-dependent speciation of tetracycline (TC).

 

   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials
Tetracycline hydrochloride was obtained from Sigma-Aldrich Chemical (St. Louis, MO) and used as received. Important physicochemical properties of tetracycline are (Thiele-Bruhn, 2003; Kulshrestha et al., 2004): molecular mass: 444.43 g mol–1; aqueous solubility: 0.52 to 117 mmol L–1* (*pH-dependent); log Kow: –1.97 to –0.47* (*pH-dependent); and pKa values: 3.3, 7.7, 9.7 (Fig. 1b). Tetracycline stock solution was prepared by dissolving 240.45 mg tetracycline hydrochloride in 100 mL of 0.01 mol L–1 NaCl (pH {approx} 2.7). To this solution radiolabeled 7-3H-tetracycline (specific activity = 5 Ci mmol–1, American Radiolabeled Chemicals Inc.) was added to provide a stock with <1% of the total tetracycline in the 3H labeled form. Reagent ACS-grade NaCl, CaCl2, and NaOH, and trace-metal grade HCl were purchased from Fisher Scientific (Fair Lawn, NJ). Fresh stock solutions were prepared for each experiment.

ESHA (1S102H) was purchased from the International Humic Substance Society (University of Minnesota, Saint Paul, MN) and used without further purification. Table 1 presents the important physicochemical characteristics of ESHA.


View this table:
[in this window]
[in a new window]

  		Table 1. Characterization of Elliott soil humic acid (ESHA).

 
HAO was synthesized by gradual neutralization of a 0.5 mol L–1 AlCl3 solution to pH 7 using 0.5 mol L–1 NaOH (Huang et al., 1977). The suspension was then aged for 48 h at room temperature and centrifuged in 250 mL polypropylene bottles at 10,300 relative centrifugal force (RCF) for 20 min. Supernatant solution was discarded and the precipitate was dialyzed to remove excess Na+ and Cl– and then freeze-dried. HAO was characterized with BET surface area measurement, zeta potential for determination of zero point of charge (ZPC), and X-ray diffraction (XRD) analysis. The BET surface area and pHZPC for HAO are 386 ± 2 m2 g–1 and 9.5, respectively. Broad peaks in XRD spectra confirmed the non-crystalline nature of HAO.

Interaction of ESHA with HAO
The sorption of ESHA on HAO was evaluated following a standard batch sorption technique (Gu et al., 1994). Briefly, for each batch system, 0.03 g (oven dried mass) of HAO was added to a tared 15 mL glass centrifuge tube. The desired pH (6–10) was achieved by adding appropriate amount of 0.01 mol L–1 HCl or NaOH. Stock ESHA (480 mg C L–1; pH {approx} 7.0) was added to provide an initial dissolved organic carbon (DOC) concentration of 48 mg C L–1. Suspensions were equilibrated at 25°C by end-over-end rotation at 7 rpm in the dark for 72 h. All experiments were conducted in duplicate.

For the sorption isotherms, a constant pH of 6.2 was maintained by adding 0.01 mol L–1 NaOH or HCl. ESHA concentrations varied from 5 to 120 mg C L–1. pH was measured immediately at the end of the reaction period by an Accumet AR-50 pH/conductivity meter. Suspensions were centrifuged at 5083 RCF for 20 min. The total organic carbon (TOC) and total soluble Al concentrations in the supernatant were analyzed using a Phoenix 8000 UV-Persulfate TOC analyzer (Tekmar-Dohrmann, Cincinnati, OH) and an atomic absorption spectrometer (AAS; GBC Inc., Australia), respectively. Zeta potential of the solid paste separated by centrifugation was measured using a Brookhaven ZetaPlus analyzer (Brookhaven Instruments Inc., NY).

Desorption isotherms were generated using a decant-refill method with samples from equilibrated sorption systems. Thirteen mL of the supernatant was taken, and the complex was resuspended with the same amount of 0.01 mol L–1 NaCl at a pH value of 6.2. The suspensions were shaken for 24 h at 25°C. The desorption step was repeated twice and TOC concentration in the supernatant after each step was determined.

Preparation of ESHA-HAO Complexes
ESHA-HAO complexes were prepared following the method of Chorover et al. (1999). Three g HAO was reacted with ESHA in 1.5 L of 0.01 mol L–1 NaCl for 72 h at room temperature. Two different initial DOC concentrations of ESHA, 18.4 and 106.7 mg C L–1, were chosen to achieve significantly different humic surface coverages on HAO as determined from the ESHA-HAO sorption isotherm. pH was maintained at 6.2 by the addition of 0.1 mol L–1 HCl or NaOH. After the reaction, the suspensions were centrifuged at 10,300 RCF for 20 min. Supernatant solution was collected for TOC measurement, and the precipitate was resuspended in 500 mL of 0.01 mol L–1 NaCl and reacted at 7 rpm for 24 h. Suspensions were centrifuged again and the washing step was repeated twice. Supernatant samples after each step were analyzed for TOC concentration. The final complexes were freeze-dried. The carbon content (foc) of the ESHA-HAO complexes was calculated based on the supernatant TOC levels remaining after the sorption and subsequent washing steps.

Sorption Experiments
Sorption of tetracycline onto HAO and ESHA-HAO complexes was evaluated using a batch equilibration method, which is similar to the procedure described above. Different amounts of radioactive tetracycline (5 x 10–4 to 1 mmol L–1) were reacted with 0.03 g of HAO or ESHA-HAO complexes (solid-to-solution ratio of 1:500). A constant pH of 6.2 was maintained using appropriate amount of NaOH or HCl added at the beginning of the sorption experiments. The isotherm experiments were conducted at two different ionic strength (I) values of 0.01 and 0.1 mol L–1 (NaCl). After the 24 h equilibration period, which is sufficient for sorption as determined in preliminary experiments, suspension was centrifuged and the supernatant was analyzed for 3H radioactivity (liquid scintillation counting, LSC; Packard Tri-Carb 1600, MA) and Al3+ concentrations (AAS). The amount of tetracycline sorbed onto HAO or ESHA-HAO complexes was determined based on the difference between initial and equilibrium aqueous 3H activities. Control experiments (no HAO or ESHA-HAO complexes) were also conducted, using a similar preparatory scheme as indicated above, to account for losses as sorption to glass tubes and other reactions in solution.

The interaction of tetracycline with ESHA was studied using an equilibrium dialysis method, which is an appropriate method for this system based on our previous study (Gu et al., 2007). Spectra/Pro 6 dialysis membrane with a nominal molecular weight cutoff (MWCO) of 2000 Da was pre-equilibrated in an appropriate background electrolyte for 24 h and then washed thoroughly with MilliQ-grade deionized (DI) water before placement in a 60-mL amber glass bottle filled with background solution of desired I. Proton activity was adjusted with NaOH or HCl to achieve a pH value of 6.2; I was set at 0.01 or 0.1 mol L–1 using NaCl. The dialysis cell was filled with the external solution and a volume of concentrated stock ESHA solution (480 mg C L–1; pH {approx} 7.0) was added to provide a DOC concentration of 24 mg C L–1, and the cell was then sealed. The total suspension mass was 50 g with 10 g inside the dialysis cell. Stock radiolabeled tetracycline solution was added to the external compartment to obtain a total system tetracycline concentration ranging from 5 x 10–4 to 1 mmol L–1. All sorption experiments were performed in duplicate.

ESHA-tetracycline suspensions were agitated in the dark at 25°C in a platform orbital shaker at 60 rpm for 3 d. At the end of the equilibration period, 3H activity of the internal and external solutions was measured by LSC. The quantity of 3H label removed from aqueous solution after interaction with ESHA was determined from the difference between internal (free and ESHA-bound) and external (free) 3H activities (Carter and Suffet, 1982; McCarthy and Jimenez, 1985; Karthikeyan and Chorover, 2000; Buschmann and Sigg, 2004).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Sorption and Desorption of ESHA on HAO
Formation of ESHA precipitates was noticed at pH < 6 in the presence of HAO, which can be attributed to the high solubility of HAO at low pH (Gu and Karthikeyan, 2005a) and the ability of Al to coagulate humic substances (Sposito, 1996; Yuan and Xing, 2001; Lu and Pignatello, 2004). Thus, we focused on the interaction of ESHA with HAO in the pH range of 6 to 10; sorption isotherms for tetracycline presented below were all generated at pH 6.2. Sorption of ESHA to HAO exhibited a strong pH-dependence, with the sorption extent decreasing with increasing pH (Fig. 2 ). Our finding is consistent with other studies on sorption of humic substances to hydrous oxide minerals (Gu et al., 1994; Filius et al., 2000, 2003; Yoon et al., 2004, 2005).


Figure 2
View larger version (8K):
[in this window]
[in a new window]

  		Fig. 2. Interaction of Elliott soil humic acid (ESHA) with hydrous Al oxide (HAO) as a function of pH (ionic strength = 0.01 mol L–1 NaCl; dissolved organic C [DOC] = 48 mg C L–1; equilibration time = 72 h). qe represents the amount of ESHA sorbed to HAO. Mean values and ranges for qe are included (if not shown, range values are within the symbols).

 
Based on potentiometric titration (with NaOH) and subsequent model fits with FITEQL, the carboxylic and phenolic functional group concentration in ESHA was determined to be 8.5 and 2.25 mol kgC–1 (Gu et al., 2007), which agreed well with those reported by Ritchie and Perdue (2003) for the same humic acid type. The decrease in ESHA sorption with increasing pH is partially attributed to electrostatic repulsion (Yoon et al., 2004, 2005), since both the sorbate (ESHA) and the sorbent (HAO; pHZPC = 9.5) become progressively negatively charged with increasing pH. In addition, at lower pH values more carboxylic groups of humic substances are protonated, which reduces charge repulsion among humic functional groups allowing for a condensed conformation, thereby favoring their sorption to mineral surfaces (Kretzschmar et al., 1997). A ligand exchange mechanism may also play an important role especially at pH below the ZPC of hydrous oxides (Gu et al., 1994; Schlautman and Morgan, 1994; Filius et al., 2003).

Interaction of Tetracycline with HAO and ESHA
The isotherms for tetracycline sorption to HAO do not reveal a clear I dependence, especially at lower surface coverage (Fig. 3a ), which is suggestive of the presence of inner-sphere type complexes. We reported earlier that tetracycline forms strong complexes with edge Al atoms on HAO surface through the tricarbonylamide and carbonyl functional groups (Gu and Karthikeyan, 2005a). However, a strong I dependence was noticed for tetracycline sorption to ESHA, with the sorption extent decreasing by more than 50% for a 10-fold increase in background electrolyte concentration (Fig. 3b). Cation exchange was proposed as the dominant sorption mechanism for tetracycline interaction with humic substances (Sithole and Guy, 1987b; Gu et al., 2007). The isotherm data for both the binary systems (i.e., HAO-tetracycline and ESHA-tetracycline) were adequately described using the Freundlich equation (r2 > 0.97), and the parameters are listed in Table 2 . The isotherms for HAO (N = 0.93 at both I) has a more linear trend than those for ESHA (N = 0.81 and 0.75 at 0.01 and 0.1 mol L–1 I). Similar results were obtained for the sorption of phenanthrene to clay minerals and humic acid (Wang and Xing, 2005). Because different batch methods were employed to quantify the sorption of tetracycline to HAO and ESHA, no direct comparison of sorption coefficients (Kf) was performed.


Figure 3
View larger version (14K):
[in this window]
[in a new window]

  		Fig. 3. Isotherms for tetracycline sorption to (a) hydrous Al oxide (HAO), (b) Elliott soil humic acid (ESHA) and (c) ESHA-HAO complexes at two different ionic strength (I) values (pH = 6.2 ± 0.1). Ce denotes the equilibrium tetracycline concentration, qe represents the amount of tetracycline sorbed, and foc indicates the organic carbon content for ESHA loading to HAO. Mean values and ranges for qe and Ce are included (range values if not shown are within the symbols). Model estimates based on a simple additive relationship for the binary ESHA-tetracycline and HAO-tetracycline systems are included in Fig. 3c.

 

View this table:
[in this window]
[in a new window]

  		Table 2. Freundlich parameters{dagger} for tetracycline sorption to hydrous Al oxide (HAO), Elliott soil humic acid (ESHA), and ESHA-HAO complexes.

 
Interaction of Tetracycline with ESHA-HAO Complexes
The two initial ESHA levels (i.e., 18.4 and 106.7 mg C L–1) used to coat the HAO surface yielded significantly different organic carbon contents (foc) of 0.81 and 1.52%, respectively, on HAO. Figure 3c shows the sorption isotherms for tetracycline on ESHA-HAO complexes at 0.01 and 0.1 mol L–1 NaCl I. The isotherm shapes resemble a typical L-type curve, which is normally observed when the adsorbate has a high affinity for the sorbent at low surface coverage and a decreasing affinity with increasing surface coverage (Essington, 2004). The presence of ESHA significantly suppressed tetracycline sorption onto HAO (compare Fig. 3a and 3c), with a greater sorption reduction noticed at the higher foc level (Fig. 3c). As the foc level increased from 0.81 to 1.52%, sorption decreased by as much as 40% (Fig. 3c). Figure 3c also contains predicted sorption estimates for the ESHA-HAO-tetracycline ternary system based on a simple additive model involving the two binary systems: ESHA-tetracycline and HAO-tetracycline. In other words, this prediction is based on the assumption that tetracycline interacts independently with HAO and ESHA. Measured sorption amounts were substantially lower than those levels predicted by simple additive behavior, strongly suggesting the suppression of tetracycline sorption onto HAO by mineral-bound humic substances.

Jones et al. (2005) experimenting with 29 soils, possessing a wide range of physicochemical properties (organic C (OC): 0 to 4%), obtained a negative correlation between percent OC and oxytetracycline sorption, highlighting the inhibitory role of natural organic matter. Pils and Laird (2007) obtained the following order for the extent of sorption of tetracycline and chlortetracycline: clay minerals > humic substances > clay-humic complexes. Sorption reduction of oxytetracycline to montmorillonites in the presence of humic substances was reported by Kulshrestha et al. (2004).

Suppression of tetracycline sorption to HAO in the presence of ESHA observed in this study can be attributed to two factors: direct competition between ESHA and tetracycline for potential sorption sites and/or alteration of HAO surface charge characteristics. For blockage of potential reaction sites in HAO, it is necessary for ESHA to not only sorb at the sites where tetracycline sorption is likely to occur but also for the ESHA-HAO interaction to be fairly strong. Therefore, the interactions between ESHA and HAO were evaluated in detail.

The sorption and desorption isotherms of ESHA on HAO (at pH = 6.2) are shown in Fig. 4 . A modified Langmuir model developed by Gu et al. (1994) was used to study the sorption and desorption isotherms; this complex equation was necessary, since the simple Langmuir form (fit not shown in Fig. 4) described the data only at very low Ce levels. This model (Eq. [1] and [2]) contains a modification of the Langmuir equation to account for the decrease in the binding energy of adsorption with increasing surface coverage on the sorbent. The modified Langmuir model has the following form:

Formula 1[1]

Formula 2[2]
where, qe is the amount of humic substances sorbed onto the hydrous oxide in g C kg–1; K(q) is defined as a surface coverage dependent affinity parameter in L g C–1; qmax is the ESHA sorption capacity in g C kg–1; Ce is the equilibrium ESHA concentration in g C L–1; KL is the Langmuir constant, L g C–1; b is a parameter included to account for the heterogeneous properties of HAO and ESHA. When b = 0, K(q) = KL, and this model becomes the traditional Langmuir isotherm equation. The parameters qmax, KL, and b were estimated by least squares fitting using SYSTAT 10.2 (SYSTAT Software, 2002) and are summarized in Table 3 . The large deviations of KL and b could be attributed either to the limited data points in the isotherms or to errors associated with TOC measurements at lower concentrations (<1 mg C L–1).


Figure 4
View larger version (23K):
[in this window]
[in a new window]

  		Fig. 4. Isotherms for Elliott soil humic acid (ESHA) sorption and desorption to hydrous Al oxide (HAO) (pH = 6.2 ± 0.1; ionic strength = 0.01 mol L–1 NaCl; equilibration time = 72 h). Ce denotes the equilibrium ESHA concentration, Ca is the equilibrium ESHA concentration after the sorption experiment, and qe represents the amount of ESHA sorbed to HAO. Mean values and ranges for qe and Ce are included (range values if not shown are within the symbols).

 

View this table:
[in this window]
[in a new window]

  		Table 3. Model parameters for Elliott soil humic acid (ESHA) sorption and desorption to hydrous Al oxide (HAO).

 
The modified Langmuir model satisfactorily described our experimental data (r2 = 0.92) (Fig. 4), suggesting monolayer sorption of ESHA on HAO. The sorption capacity is 17.91 g C kg–1, which corresponds to 0.046 mg C m–2 (the surface area of HAO is 386 m2 g–1). The surface area normalized sorption capacity is lower than the values reported in the literature; qmax values of 0.199 and 0.3 mg C m–2 at pH 6.3 and 7, respectively, were obtained for fulvic acid sorption to hematite and goethite (Gu et al., 1994; Filius et al., 2000). Differences between humic and fulvic acid (i.e., size, molecular weight, carboxylic and phenolic contents) and lower sorption affinity of HAO for humic substances compared to Fe oxides (Meier et al., 1999) could explain the lower qmax value obtained in our system. Due to the singly coordinated surface hydroxyl groups, the edge Al atoms in HAO are more reactive possessing high Gibbs free energy and are considered the sites for surface complexation interactions with ESHA (Yost et al., 1990; Molis et al., 2000; Duckworth and Martin, 2001; Gu and Karthikeyan, 2005b); the lower site density associated with edge Al atoms could also explain the lower qmax value. The positive value of b indicates that the sorption affinity decreases with increasing ESHA coverage on HAO (Gu et al., 1994).

It is important to note that the edge Al atoms on HAO were proposed as the sorption sites for tetracycline as well (Gu and Karthikeyan, 2005a). Since the edge Al atoms of HAO are proposed as the interaction sites for both ESHA and tetracycline, the association of humic macromolecules would inevitably suppress the sorption of tetracycline. With increasing ESHA surface loading, besides greater tetracycline sorption suppression, significant dependence and decreasing sorption with increasing I value were observed (Fig. 3c). Sorption nonlinearity (higher N values, Table 2) also increased at the higher foc level, highlighting the greater influence of humic substances when present at higher concentrations.

Desorption isotherms provided evidence for the strong nature of interactions (i.e., irreversibility) between ESHA and HAO. The discrepancy between sorption and desorption isotherms provided a clear evidence for hysteretic behavior (Fig. 4), which has also been observed for sorption of humic acids to other oxides and clay minerals (Gu et al., 1994, 1995; Jayasundera and Torrents, 1997; van de Weerd et al., 1999). A sorption–desorption reversibility parameter was introduced and the desorption isotherm was modeled using the following equation given in Gu et al. (1994):

Formula 3[3]
where, qe, K(q), qmax, and Ce are the same as defined earlier. Ca (mg C L–1) is the equilibrium concentration of humic substances after the sorption experiment, and h is defined as the hysteresis coefficient. At h = 0, Eq. [3] is identical to Eq. [1], indicating that sorption–desorption is completely reversible; however, as h approaches unity, qe is independent of Ce, indicating a completely irreversible reaction. Therefore, h can be used to quantify the extent of sorption–desorption hysteresis.

The calculated h values are 0.85, 0.81, and 0.77 for Ca concentrations of 18.84, 44.60, and 87.35 mg C L–1, respectively, suggesting a strong hysteretic sorption–desorption behavior for the ESHA-HAO system (Table 3). The observed hysteretic behavior could be partially attributed to the heterogeneity of ESHA. Since different ESHA macromolecules could have different sorption affinities for HAO, the ESHA component with a higher affinity would be selectively sorbed onto the HAO surface. Consequently, the solution composition of humic substances is likely to be different from the macromolecules retained on HAO (i.e., bound-humics). In other words, the sorption and desorption follow different pathways, which could lead to the observed hysteretic behavior (Gu et al., 1995). Multiple binding sites might be involved during the attachment of ESHA macromolecules on HAO surface (Gu et al., 1994). All the bonds need to be broken simultaneously to detach the molecules from the surface, which would make desorption a slow kinetic process.

The reduction in tetracycline sorption levels to ESHA-HAO complexes could be attributed to the change in HAO surface charge characteristics as well. Sorption of ESHA reversed the surface charge on HAO (Fig. 5 ), which would result in repulsive interactions between the hydrous oxide surface and deprotonated tricarbonylamide and carbonyl groups (Fig. 1) of tetracycline (Gu and Karthikeyan, 2005a). Figure 5 shows the effect of increasing ESHA surface coverage on the surface charge of HAO and the concomitant release of soluble Al. Increasing ESHA sorption levels progressively decreased the zeta potential and a sharp reversal in surface charge (positive to negative) was also noticed (Fig. 5). These observations could support the formation of strong inner-sphere type complexes during ESHA sorption to HAO (Molis et al., 2000; Gu and Karthikeyan, 2005a). The release of Al was barely detectable until a certain qe level (7.59 g C kg–1) was reached. However, when qe exceeded this threshold, soluble Al concentration appeared to be almost linearly related to the amount of ESHA sorbed, indicating that ligand (ESHA)-promoted dissolution of HAO takes place only at the sites where sorption has occurred (Molis et al., 2000). Ligand-promoted dissolution can be considered as a two-step process (Stumm, 1992): initially, ligands are reversibly sorbed onto the mineral surfaces weakening the metal-O bond by destabilizing the metal center in the hydrous oxides. The second step is the detachment of a metal ion (i.e., Al3+) from the surface.


Figure 5
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 5. Surface charge (zeta potential) characteristics of hydrous Al oxide (HAO) and concomitant release of soluble Al as a function of increasing Elliott soil humic acid (ESHA) loading (pH = 6.2 ± 0.1; ionic strength = 0.01 mol L–1 NaCl; equilibration time = 72 h). qe is the amount of ESHA sorbed to HAO. Mean values and ranges for qe, zeta potential, and soluble Al are included (range values if not shown are within the symbols).

 
Therefore, it appears that surface complexation via ligand exchange is the dominant sorption mechanism for ESHA onto HAO. Other mechanisms, (e.g., H-bonding, electrostatic interactions), could be involved as well, although they are not expected to be dominant. The strong interaction between ESHA and HAO, resulting even in altered HAO surface charge characteristics, could be attributed to the observed tetracycline sorption suppression.

Tetracycline(ligand)-Promoted Dissolution
The potential for ESHA-promoted dissolution of HAO was discussed earlier in this section; a similar phenomenon was observed for HAO in the presence of tetracycline in our earlier study (Gu and Karthikeyan, 2005a). It appears that the extent of ESHA coating (organic C loading) significantly affects the tetracycline-promoted dissolution of HAO, as less Al was dissolved from the mineral surface with increasing foc level (Fig. 6 ). Although this observation could be attributed to reduced tetracycline sorption in the presence of ESHA, these data provide additional evidence for competitive interactions between ESHA and tetracycline for sorption sites in HAO.


Figure 6
View larger version (14K):
[in this window]
[in a new window]

  		Fig. 6. Release of soluble Al as a function of increasing tetracycline sorption onto hydrous Al oxide (HAO) and Elliott soil humic acid (ESHA)-HAO complexes (pH = 6.2 ± 0.1; ionic strength = 0.01 mol L–1 NaCl; equilibration time = 24 h). qe represents the amount of tetracycline sorbed. Mean values and ranges for qe and total soluble Al are included (range values if not shown are within the symbols).

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
In natural systems, humic substances are typically strongly associated with inorganic minerals, especially under acidic to neutral pH conditions. Particle-bound humic substances could alter the surface properties of minerals and their ability to sorb organic compounds, such as antibiotics. Therefore, the ternary humic-mineral-antibiotic interactions need to be considered while assessing the environmental fate of these compounds. We observed that the strong affinity between humic substances and hydrous oxides resulted in a significant decrease in the sorption of tetracycline to HAO in the presence of ESHA. Suppression of tetracycline sorption was more pronounced at higher ESHA concentrations. Humic substances could influence tetracycline sorption by directly competing for potential sorption sites and/or by altering the surface charge properties of inorganic minerals. The mobility of tetracyclines and other antibiotics with similar functional group characteristics could be enhanced by the presence of humic substances. It is expected that in organic-rich soil environments, tetracyclines would have a high potential for off-site migration via both surface and subsurface pathways.


   ACKNOWLEDGMENTS
 
This research was supported by the WI Dep. of Natural Resources, WI Groundwater Coordinating Council grant 04-CTP-02 and Water Resources Inst. Project R/UW-CTP-005. The authors thank the assistance of David Rogers (WI State Lab. of Hygiene, Madison, WI) with the TOC measurements.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Agricola
Right arrow Articles by Gu, C.
Right arrow Articles by Karthikeyan, K. G.
Related Collections
Right arrow Humic Substances
Right arrow Pharmaceuticals
Right arrow Soil Chemistry

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome