Microsoft word - 28_0304.doc

Effects of pH and Cadmium on Tetracycline Sorption to Soils

Guixiang Zhang, Xitao Liu, Ke Sun, Ye Zhao, Chunye Lin
State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China,
Email [email protected]
Abstract
Batch sorption experiments were conducted to evaluate the sorption of tetracycline (TC) on soils as affected
by pH and cadmium. Sorption isotherms of TC on soils in the presence and absence of Cd (II) were well
fitted with the Freundlich equation. Sorption of TC strongly depended on environmental factors and soils
characteristics. Lower pH facilitated TC sorption through cation exchange mechanism which also took place
at pH above 5.5 where TC existed as zwitterion (H2L0) or anions (HL- and L2-). When pH was above 7,
ligand-promoted dissolution of TC might occur due to the TC weakening the Al-O bond of aluminum oxide
and Fe-O bond of iron oxide. The presence of Cd (II) increased TC sorption on soils, which was resulted
from the decrease of equilibrium solution pH caused by Cd2+ exchange with H+ ions of soil surfaces. The
increase of TC sorption was also related to the formation of TC-Cd complexes and the bridge provided by
Cd2+ between the soil and TC.
Key Words
Sorption, tetracycline, pH; cadmium, soil
Introduction
Sorption is an important process that affects the fate, transportation, bioavailability and toxicity of
contaminants. TC is predominantly sorbed on soil clays and humic substances either mask sorption sites on
clay surfaces or inhibit interlayer diffusion of TC. Sorption of TC decreased with increasing pH and sorption
by clay appeared to be increased in the presence of Ca versus Na (Figueroa et al. 2004; Kulshrestha et al.
2004; Figueroa and MacKay 2005; Pils and Laird 2007). TC has multiple ionizable functional groups
(Sassman and Lee 2005), sorption of TC significantly depends on environment elements and the properties
of soil samples.
On the other hand, soils usually contain other contaminants such as heavy metals, which may affect sorption
of TC. In recent years, some researchers focus on the interaction between metal ions and TC (Wang et al.
2008; Jia et al. 2008). Cadmium (Cd) as a nonessential element can result in adverse effects on animals and
humans (Tu et al. 2007). High contents of Cd may be present in livestock additives because of contamination
of mineral supplements (Nicholson et al. 2003). Thus, Cd can often coexist with TC, which may affect
behavior of TC, however, little attention is paid to this possibly. The objective of this study is to investigate
the sorption isotherms of TC on soils as affected by Cd and pH.
Materials and methods
Chemicals and soils
Tetracycline hydrochloride (98% purity) was obtained from Alfa Aesar. TC stock was prepared in methanol
and stored at 4°C in the dark and refreshed every month. Calcium chloride anhydrous, sodium azide, oxalic
acid dihydrate, CdCl2·5/2H2O, HCl and NaOH were all reagent grade. Acetonitrile and methanol were HPLC
grade. Solutions were prepared with high-purity water (18MΩ, Millipore Simplicity 185).
Three air-dried soils were gently crushed to pass through a 0.25 mm sieve. Three soil samples (0-20 cm)
were collected from East Xinzhuang (Soil 1), North Tanggu Farm (Soil 2) and Northeast Dougu Town (Soil
3) respectively. The properties of soils were shown in Table 1. Cation exchange capacity (CEC) was
determined by the method of ammonium acetate exchange. Organic carbon (OC) contents were determined
using an Elementar Vario EI elemental analyzer (Germany) after the acid-treatment (1 M HCl). Particle size
distributions were determined by laser particle size analyzer (Mastersizer 2000, Malvern Instrument Ltd.
Malvern, UK). The content of total Fe and Al oxides in sediments and soils were determined by HF-HClO4-HCl.
2010 19th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. Table1. Some physical and chemical characteristics of the selected sediment and soil samples.
Samples

Sorption experiments
All experiments were conducted in 30 ml Nalgene polypropylene centrifuge tubes. According to preliminary
experiment, 24 h was chosen as the equilibration time and the loss of TC was negligible. The prepared TC
solution contained 0.01 M CaCl2 to maintain a certain ionic strength and 1.5 mM sodium azide to inhibit
biological activity, and was adjusted to pH 5.5 with HCl or NaOH. Twenty-five milliliter of different
concentrations (from 5 to 150 mg/L) of TC solution with and without 10 mg/L of Cd (II) was added into
each tube containing about 0.1 g soil. Soil 3 was selected to investigate the effect of pH on TC sorption with
and without Cd (II) and were performed with a single TC concentration (30mg/L). Different pHs varying
from 3.5 to 9.5 in one-unit increments were adjusted with 0.1 M HCl or 0.1 M NaOH. Sorbate-free control
and Sorbent-free control tubes were prepared in the same manner. Duplicate experiments were conducted for
all samples.
Concentrations of TC were determined by high-performance liquid chromatography system equipped with a
UV detector (Germany Lumtech K-2600, Lumiere Tech Ltd.) using Inertsil ODS-3 C18 column (5 µm,
250×4.6 mm). Samples were eluted isocratically with a mixture of acetonitrile (23%) and 10 mM oxalic acid
(77%), flowing at 1.0 ml/min. TC was measured by absorption at 360 nm. The amount of TC adsorbed was
calculated by the difference between the amount of TC added initially and that remained in the aqueous after
equilibration.
Results and Discussion
Sorption isotherms
The Freundlich isotherm model commonly used for quantifying equilibrium sorption of hydrophobic organic
compounds (HOCs) by soils has the following forms:
q
where qe is the solid-phase concentration (mg/kg) and Ce is the liquid-phase equilibrium concentration (mg/L). KF is the sorption capacity-related parameter ((mg/kg)/(mg/L)n) and n is the isotherm linearity index. Figure 1 shows the sorption isotherms of TC on soils in the presence and absence of Cd (II) at pH 5.5. The Freundlich Figure 1. Tetracycline sorption isotherms on three soils in the presence and absence of Cd.

equation fits the sorption isotherms of TC with high correlation coefficients (r2= 0.997-0.999) (Table 2),
which suggests that the Freundlich equation can be used to sufficiently describe TC sorption to the soils with
and without Cd (II). The organic carbon-normalized sorption coefficient (KFOC) was calculated by dividing
KF values by the fraction of organic carbon (Foc). The single point Koc (L/kg) was calculated by the following
equation:
K
2010 19th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. Table 2. Freundlich sorption model coefficients for TC adsorption on soils in the presence and absence of Cd (II)
Samples
Ce=0.005 Ce=0.05 Ce=0.5 A95% confidence interval of KF; B 95% confidence interval of n. Cnumber of observations;

The data in Table 2 show that the n values for TC by the soils varied from 0.800 to 0.826 with an average of
0.809. The differences of n values may be caused by different origins of soils. For all soil samples, higher Ce
concentrations would result in lower Koc values because of nonlinear sorption. When Ce was at 0.005 and
0.05 mg/L, the Koc values for the three soils ranged from 1711 to 2439 L/kg with average at 2027 L/kg and
from 1080 to 1633 L/kg with average at 1311 L/kg, respectively. When Ce was at the higher concentration
(0.5 mg/L), the values of Koc were much lower, ranging from 681 to 1094 L/kg.
Effects of Cd and pH on sorption of TC
When pH is 5.5, TC predominantly exists as the zwitterions (Sassman and Lee 2005), Cd (II) increased the
sorption of TC on the three soils (Figure 2). Figure 2 shows that the Kd values was higher at lower pHs,
because the cationic TC can combine with the negatively charged sites on soil surfaces, which increase the
sorption of TC. Kd values of TC for Soil 3 in the presence of Cd (II) at different pHs were also shown in
Figure 2. Cd suppressed the sorption of TC on Soil 3 when pH was below 4.5, this may be due to the
competition of Cd (II) with TC and TC-Cd complexes. On the other hand Cd (II) promoted the sorption of
TC on Soil 3 when pH was above 5.5, this may be due to Cd (II) acting as bridge between TC and Soil 3.
This result was similar to the research on cosorption of TC and Cu (II) on soils conducted by Jia et al.
(2008).
Figure 2. Effects of pH on TC sorption on Soil 3 in the presence and absence of Cd.

Relationship between soil properties and sorption isotherm
The pHs and CEC for three soils were similar, the differences of sorption capacities indicated that other
properties played important roles. This experiment revealed that TC was less favor to be sorbed to Soil 3
which had obviously higher clay than others. A probable reason was that more ligand-promoted dissolution
was occurring during TC sorption to Soil 3. Although Soil 1 and Soil 2 had more iron oxide content, they
had less amount of aluminum oxide than Soil 3 (Table 1), which might result in the higher sorption capacity
for Soil 1 and Soil 2. This result was consistent with sorption of TC to aluminum and iron hydroxides
decreasing with increasing pH when pH was up to 7 (Gu and Karthikeyan 2005).
Possible sorption mechanism of TC in the absence and the presence of Cd(II)
Cation exchange has been reported as an important mechanism for TC sorption at acidic condition. pHs
decreased with increasing concentration of TC (data were not shown), which suggested that cation exchange
took place above pH 5.5, as the negative and positive charges on TCs were spatially separated and they
might play their respective roles similar to that of soil cation and anion exchange sites (Sassman and Lee
2005). When pH was above 7, sorption of TC to aluminum and iron oxides decreased with increasing pH
2010 19th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. because of ligand-promoted dissolution. Ligand-promoted dissolution was more significant for aluminum
oxides than for iron oxides (Gu and Karthikeyan 2005). A possible explanation for the increase of TC
sorption in the presence of Cd (II) could be that the equilibrium solution pHs were lower than those in the
absence of Cd (II). The pH decreased in the equilibrium solution when Cd (II) was added, which was caused
by Cd2+ exchanging with H+ ions of soil surfaces. The cationic Cd could also combine with the negative
charge sites on the soil surfaces, acting as a bridge between TC and soil particles (Pils and Laird 2007; Jia et
al. 2008; Wang et al. 2008). Another reason might be related to the formation of TC-Cd complexes as well.
The predominant TC species at solution pH between 6 and 8 were H2L0, HL-, and L2- (Sassman and Lee
2005), which could strongly combine with the Cd (II) to form CdH2L2+, CdHL+, and CdL complexes. These
TC-Cd complexes had less negative surface charge and more easily adsorbed on soil surfaces than TC itself
at high pH conditions.
Conclusion
Sorption of TC on soils as affected by Cd (II) and pH were investigated in this study. Sorption of TC
decreased with increasing pH, and basic conditions did not facilitate TC sorption, which suggested TC
introduced into alkali soils would increase its environmental risk. When pH was above 7, aluminium and
iron oxides might increase the mobility and bioavailability of TC due to ligand-promoted dissolution in soil
pore waters. Cd enhanced TC sorption on soils at environmentally relevant pH values, thus reducing the
mobility of TC. The results in this study can be potentially valuable in predicting the fate, bioavailability and
risk of TC in soils. In addition, sorption of TC by different components of organic matter in soils needs
further investigations.
Acknowledgements
The authors are grateful to Ministry of Science and Technology (Project No. 2007CB407302) and National
Natural Science Foundation (Project No. 40971058) of China for financial support.
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2010 19th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD.

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