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


     

Published online 29 September 2005
Published in Soil Sci Soc Am J 69:1703-1710 (2005)
DOI: 10.2136/sssaj2004.0345
© 2005 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 Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
Related Collections
Right arrow Agricultural Pesticides
Right arrow Fate
Right arrow Solute Transport Models

Soil Chemistry

Sorption-Desorption Behavior of Atrazine in Soils Irrigated with Reclaimed Wastewater

Yaron Droria, Zeev Aizenshtatb and Benny Chefetza,*

a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
b Casali Institute of Applied Chemistry, The Hebrew Univ. of Jerusalem, Jerusalem 91904, Israel

* Corresponding author (chefetz{at}agri.huji.ac.il)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A time-dependent sorption–desorption study of atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine) was performed to reveal its behavior in wastewater- and freshwater-irrigated soils. Atrazine sorption affinity to the freshwater-irrigated soils was significantly higher than to their wastewater-irrigated counterparts. The calculated C-normalized partition coefficient (KOC) values for the freshwater-irrigated soils were 70.7 and 34.2 L kg–1 organic C and the values for the wastewater-irrigated soils were 52.6 and 29.9, respectively, for the two tested soils. These differences did not result from atrazine interactions with dissolved organic matter (DOM) in the wastewater-irrigated soils. All desorption isotherms exhibited hysteresis. Lower desorption was exhibited for the 14-d sorption–desorption experiments, even though the sorption-affinity parameters were similar for the 2- and 14-d sorption periods. Higher desorption hysteresis was observed for the longer sorbate-sorbent contact time for both soils, with desorption decreasing for decreased atrazine loading. The similar atrazine-sorption parameters obtained for the two sorption periods and the increasingly difficult desorption with increasing contact time suggest that the sorption process occurs in two stages. First, sorption occurs to readily available sites; at a later stage, the sorbed molecules interact with the soil organic matter (SOM) or the SOM-mineral complexes such that their desorption ability is significantly hindered. The higher sorption potential and the lower desorption ability of the freshwater-irrigated soils suggest that the SOM which develops in the wastewater-irrigated soils delays or hinders atrazine interaction with sites that are naturally present in these soils.

Abbreviations: AHI, apparent hysteresis index • DOM, dissolved organic matter • HOC, hydrophobic organic compound • KD, partition coefficient • KF, Freundlich distribution coefficient • KFOC, Freundlich carbon-normalized distribution coefficient • KOC, carbon-normalized partition coefficient • SOM, soil organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CROP IRRIGATION with treated wastewater is on the rise in arid and semiarid zones. Recycled wastewater is an important source for irrigation due to increases in both freshwater shortages and treated-wastewater quality. But, treated wastewater contains higher levels of suspended and DOM and inorganic matter than freshwater (Feigin et al., 1991; Fine et al., 2002). Therefore, long-term irrigation with wastewater can affect the physical, chemical, and biological properties of these soils. Several recent reports have demonstrated wastewater irrigation's effect on soil hydraulic conductivity, permeability, infiltration, sodium-adsorption ratio, and microbial activity (Ovreas et al., 1998; Tarchitzky et al., 1999; Beaulieu et al., 2000; Mamedov et al., 2000). Moreover, the introduction of relatively high levels of DOM during long-term irrigation with wastewater can increase the amount and change the properties of the natural SOM. For instance, irrigation with wastewater for a period of 25 yr increased the organic C level in a soil by 50% (Friedel et al., 2000). In addition, Gonzalez-Vila et al. (1995) reported that irrigation with olive mill effluents increased and altered the composition of the soil lipid fraction. In another report, changes in the chemical properties of fulvic acid were obtained after irrigation with wastewater (Boyd and Sommers, 1990). Application of sewage sludge has also been reported to increase the amount of aromatic compounds in the SOM, due to enhanced microbial activity and degradation of readily degradable DOM and SOM fractions (Kalbitz et al., 2003).

Hydrophobic organic compounds (HOCs) tend to sorb to soils mainly via interactions with SOM. Many reports have shown a positive correlation between the organic C content and the sorption potential of HOCs (Means et al., 1980; Garbarini and Lion, 1986; Kleineidam et al., 1999). Therefore, SOM is considered to significantly affect the fate of HOCs in the environment. The ability of SOM to sorb and, more importantly, to desorb HOCs in the soil profile significantly affects the transport of these pollutants toward groundwater. In addition to the amount of SOM, the sorption affinity, as well as the desorption potential of HOCs have been reported to be controlled by the nature and chemical properties of SOM. Generally the uptake of HOCs increases as the polarity of the sorbent (i.e., SOM) decreases (Grathwohl, 1990; Torrents et al., 1997). Other studies have suggested that aromatic and/or aliphatic domains of SOM govern HOC sorption (Chefetz et al., 2000; Ahmad et al., 2001; Gunasekara and Xing, 2003).

Sorption of HOCs to the amorphous domain of SOM yields a linear isotherm, which is concentration-independent and desorption hysteresis is not expected (Xing and Pignatello, 1997; Weber et al., 1998). However, in many cases, significant deviations between sorption and desorption isotherms have been observed (Huang et al., 1997; Huang and Weber, 1998; Weber et al., 1998; Ran et al., 2003). The reasons for sorption–desorption hysteresis are not yet fully understood: they can result from different kinetics of the sorption and desorption processes (Altfelder et al., 2000), entrapment of the sorbate (Guthrie et al., 1999), or specific sorbate-sorbent interactions. In recent years, it has been suggested that sorption nonlinearity and sorption–desorption hysteresis result from the heterogeneous structure of SOM (Pignatello and Xing, 1996; Huang et al., 2003). Natural SOM is composed of two types of sorption domains: the first consists of gel-like or rubber-like organic matter and the second is characterized by more condensed or glassy-type structures. The latter domain is responsible for increased desorption hysteresis with increasing contact time (Weber et al., 1998).

The influx of relatively high concentrations of DOM into the soil, as results from irrigation with recycled wastewater, can have a major influence on interactions of non- and weakly polar organic compounds and the soil matrix. Organic chemicals originally present in the wastewater such as polychlorinated biphenyls, polycyclic aromatic hydrocarbons, organochlorine pesticides, and pharmaceutical compounds have been identified in groundwater in areas irrigated with wastewater or sludge (Muskat et al., 1993; Kolpin et al., 2002). Moreover, Graber et al. (1995) have reported enhanced atrazine transport in soil irrigated with secondary effluents. These authors suggested that complexation of atrazine with the wastewater DOM results in a higher atrazine concentration in the aqueous phase, which is therefore transported deeper into the soils irrigated with the secondary effluents.

The effects of long-term irrigation with wastewater on the physicochemical nature of SOM and its sorptive capabilities are not yet fully understood. Several studies (Marschner, 1998; Ben-Hur et al., 2003) have investigated the effect of wastewater DOM on the mobility of HOCs, but the role of the solid-phase SOM under these conditions was not emphasized. Atrazine is an extensively used herbicide for weed control on agricultural crops, especially corn (Zea mays L.). It is one of the most commonly used agricultural herbicides in the Middle East and the USA. But due to its extensive application, it is also very widely detected in surface water and groundwater (Meisner et al., 1992). For this reason, atrazine is considered to be an important environmental contaminant. Enhanced groundwater contamination with atrazine as a result of wastewater irrigation can be prevented, through an understanding of its retention in the soil environment. Therefore, the main objective of this study was to investigate the sorption–desorption interactions of atrazine with sandy and clayey soils irrigated with freshwater or treated wastewater for long periods.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils
Topsoils were sampled from citrus and avocado orchards in Basra and Akko, Israel, respectively. At each location, the soils were sampled from two nearby plots, one that was irrigated with treated wastewater and the other with freshwater. In each plot, samples were collected from four locations (20 cm away from the dripper). The 0- to 3-cm organic layer was removed and samples were collected from the 3- to 30-cm depth. The four subsamples were combined to form a composite sample (10 kg). The soils were air-dried, sieved through a 2-mm sieve and stored. The samples were labeled according to their location and irrigation water: Basra wastewater, Basra freshwater, Akko wastewater, and Akko freshwater. Selected properties of the studied soils and the irrigation water are presented in Tables 1 and 2, respectively.


View this table:
[in this window]
[in a new window]
  		Table 1. Selected properties of the Basra and Akko wastewater (WW)- and freshwater (FW)-irrigated soils.

 

View this table:
[in this window]
[in a new window]
  		Table 2. Analysis of the freshwater and wastewater used for irrigation in the field plots at Basra and Akko. The data presents an average (± standard deviation) of 3 to 8 samples taken during 2003.

 
Batch Sorption–Desorption Experiments
Atrazine (98% purity, Agan Co., Ashdod, Israel) aqueous solutions were prepared by adding aliquots from a concentrated HPLC-grade methanol stock of the analyte to a background solution containing 10 mM CaCl2 and 200 mg L–1 NaN3 to maintain a constant ionic strength and to inhibit microbial activity, respectively. Sorbent mass (15 g) was selected to achieve 25 to 70% sorption. Atrazine solutions (25 mL), covering a range of 1 to 20 mg L–1, were added to samples previously weighed into 50-mL Teflon centrifuge tubes (Nalgene, Rochester, NY) with Teflon caps. Sorption of atrazine to the tubes was found to be negligible (<1%). The tubes (three replicates and a blank for each concentration) were agitated end-over-end in the dark at 150 rpm, 25°C, for 2 and 14 d. Next, the tubes were centrifuged (8000 x g, 8 min) and 10 mL of the supernatant were removed using a glass pipette and replaced with fresh background solution. The tubes were then further agitated under the same conditions for the following desorption periods: 1, 2, and 7 d for the 2-d sorption period and 1, 7, and 14 d for the 14-d sorption period. Four sequential desorption steps were performed for each desorption period. Supernatants collected after sorption and each of the desorption steps were filtered (0.45 µm) and transferred to 1.5 mL HPLC vials. Quantitative HPLC analysis was performed using an L-7100 LaChrom HPLC (Merck-Hitachi, Darmstadt, Germany) with a LiChrospher RP-18 column (25 cm x 4 mm, 5 µm) and water/acetonitrile (30/70) as the mobile phase. Atrazine was detected using absorbance at 222 nm and was quantified using external standards.

To evaluate the effects of wastewater DOM on atrazine sorption affinity to the soils, we performed a batch sorption–desorption experiments for 2 d using Basra soils. These experiments were similar to the above described, except that instead of a background solution (10 mM CaCl2 and 200 mg L–1 NaN3), atrazine was added to Basra wastewater irrigation solution (pH 7.9, 48 mg L–1 chemical oxygen demand, 40 mg L–1 total suspended solids, 0.02 M ionic strength; Table 2).

At the end of the longest experiment (14-d sorption–desorption), atrazine was extracted from the soils for recovery analysis. Atrazine extraction was performed using the accelerated solvent-extraction technique (Wenzel et al., 1998). Briefly, the soils (10 g) were extracted three times with 33 mL of dichloromethane/acetone (1:1) in a Dionex ASE 200 (Dionex Corp., Sunnyvale, CA). Extraction cycles (10 min each) were performed under constant pressure (15 MPa) at 100°C. The extracts were concentrated and injected into an HP-5890 GC coupled with an HP-5971 MS for quantitative analysis. The calculated atrazine mass balance suggested a recovery of 87 to 97%.

Data Analysis
The Freundlich parameters (KF and N) were calculated from the logarithmic form of the equation: q = KFxCN, where q is the total sorbed concentration (mg kg–1), C is the solution-phase concentration (mg L–1), KF (mg kg–1)(mg L–1)–N is the Freundlich distribution coefficient, and N is the isotherm nonlinearity factor. Isotherms were plotted (log q vs. log C), and log KF and N were obtained from the fitting. The sorption partitioning coefficient (KD) was calculated for the linear portion of the isotherms (up to a sorbed atrazine concentration of 3.5 mg kg–1 soil). The KD values were normalized to the C level of the soil to obtain the KOC. The amount of sorbed atrazine at each desorption step was calculated according to the equation: qidesorbed = Vw/Ws, where qidesorbed (mg kg–1) is the amount of atrazine released in Stage i, Ci, and Ci–1 (mg L–1) are the solution concentrations in Stage i and the former stage, respectively, r is the fraction of the replaced solution, Vw (mL) is the solution volume and Ws (g) is the sample weight (Kan et al., 1994). The ratio of the Freundlich exponents for desorption and sorption (ND/NS) was calculated and used as the desorption apparent hysteresis index (AHI). Lower index values indicate increased difficulty of the sorbed analyte to desorb from the matrix (Gunasekara and Xing, 2003). Statistical analysis was performed by JMPIN software, version 4.0.4 (SAS Institute Inc., Cary, NC).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sorption
Kinetics experiments were performed to evaluate the sorption equilibrium time for atrazine with the tested soils. Atrazine sorption reached a maximum after 2 d of interaction and did not show a significant increase from that point on. The apparent equilibrium time obtained in this study is within the range reported for atrazine in the literature (Beck and Jones, 1996; Seybold and Mersie, 1996; Lu et al., 2002). Based on the sorption kinetics data, we measured the atrazine sorption–desorption behavior during two sorption periods: 2 and 14 d, with the aim of determining whether increasing sorption contact time affects the desorption behavior of atrazine and to reveal the effects of the irrigation water (wastewater versus freshwater) on this behavior.

All atrazine-sorption isotherms were nonlinear, exhibiting N values between 0.80 and 0.90 (Table 3). For the samples collected from Basra, the N values did not show significant differences between the wastewater- and freshwater-irrigated soils or between the two sorption periods. However, the N values calculated for atrazine-sorption isotherms with Akko soils for the 14-d period were significantly lower than those exhibited by the shorter-time sorption isotherms (2 d). Similar to the Basra soils, the N values obtained for the freshwater- and wastewater-irrigated soils from Akko were within a narrow range. A decrease in N values (isotherm nonlinearity) with increasing contact time has been demonstrated in the literature for several sorbates (Weber and Huang, 1996; Young and Ball, 1999; Lesan and Bhandari, 2003) and is considered to be affected mainly by the nature of the SOM (Pignatello and Xing, 1996). At short sorbate-sorbent interaction periods, the sorbate intrudes or interacts with the rubbery-like matrix of the SOM. At later stages, the sorbate molecules slowly penetrate into the glassy domain of the SOM, resulting in a decrease in the isotherm N value. Results from our study (Basra soils) demonstrate constant nonlinearity values with increased atrazine contact time. Similar results of inconsistent N values with increasing contact time for several organic compounds have been reported by Sharer et al. (2003). The results obtained for the Basra soils are therefore in contrast with the dual-mode sorption mechanism (Xing and Pignatello, 1996). However, with the Akko soils, the isotherm nonlinearity is more apparent with longer soil-atrazine contact time. The SOM in Akko soils (which are characterized by high clay content) is probably in organomineral complexes, which provide more internal and time-dependent adsorption sites. This can result in a decrease in isotherm nonlinearity where these sites were occupied only after longer sorbate-sorbent contact time.


View this table:
[in this window]
[in a new window]
  		Table 3. Atrazine sorption coefficients and Freundlich parameters for Basra and Akko wastewater (WW)- and freshwater (FW)-irrigated soils.

 
Although the N values of the freshwater- and wastewater-irrigated soils were similar, the atrazine-sorption parameters (KD and KF) were significantly higher for the former than for the latter. When these parameters were normalized to the C level of a sample (KOC and KFOC), the differences between the two types of irrigated soils were more pronounced due to the higher C content in the wastewater-irrigated soils (Tables 1 and 3). The freshwater/wastewater soil ratios of atrazine KOC values were similar for Basra soils for the two sorption periods (1.34 and 1.38). For Akko soils, these ratios were lower but showed an increase from 1.14 to 1.26 with increasing sorption time. The atrazine KOC and KFOC values calculated for Basra freshwater and wastewater irrigated soils were approximately two-fold higher than those obtained for the corresponding soils from Akko. This clearly suggests the greater affinity of atrazine to the Basra soil's SOM.

Our data suggest that the wastewater-irrigated soils have significantly lower sorption potential than similar soils that have been irrigated with high-quality water. Graber et al. (1995) have suggested that the lower sorption potential of wastewater-irrigated soils results from the presence of a higher amount of DOM in those soils compared with freshwater-irrigated ones. It was suggested that the high level of DOM results in a reduction in the solute's affinity for the solid matrix due to DOM-solute interactions. To evaluate whether the higher partitioning coefficients observed in our study for the freshwater-irrigated soils versus their wastewater-irrigated counterparts resulted from the above mechanism, we performed an experiment with atrazine and the Basra soils with deionized water or in wastewater solution (Table 2). The data obtained from the atrazine sorption and four desorption steps with the two solutions were statistically similar. Therefore, we conclude that the higher sorption coefficients observed for the freshwater-irrigated soils in our study resulted from the higher sorption affinity of atrazine to the solid SOM in these soils. Similar conclusions were drawn by Seol and Lee (2000) who reported that very high levels of DOM (>150 mg L–1 OC) are needed to significantly reduce triazine herbicides sorption by soils.

Desorption
Desorption behavior can provide additional information on the sorbate-sorbent interactions. In the current study, we examined the desorption behavior and hysteresis of atrazine at different time intervals for the 2- and 14-d sorption periods. The atrazine sorption–desorption isotherms for Basra and Akko soils are presented in Fig. 1 and 2 , respectively. All desorption isotherms exhibited hysteresis. Higher desorption hysteresis (atrazine was less readily desorbed) was observed for the 14-d sorption–desorption cycles for both soils. Although desorption hysteresis was less pronounced in the 2-d cycles, these isotherms exhibited a distinct trend of decreasing desorption (increasing hysteresis) with decreased atrazine loading. The same trend, albeit less pronounced, was exhibited by the 14-d isotherms. The increasingly difficult desorption with decreasing solute concentration can be seen in Fig. 3 , where the AHI is plotted against the initial atrazine concentration. Both types of soils (wastewater- and freshwater-irrigated) exhibited a similar trend of hysteresis, however more pronounced deviation between the soils was shown in the 14-d sorption–desorption cycles. The trend of increasing desorption with increasing atrazine concentration can be explained by the limited number of sites available for high-energy binding. Most of these sites are occupied at low solute concentrations, whereas at high solute concentrations, more molecules are taken up by low-energy binding sites and therefore can more readily desorb.



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 1. Sorption isotherms (filled symbols) and four sequential desorption steps (open symbols) of atrazine measured with Basra soils for 2- and 14-d cycles (bars represent standard deviations).

 


View larger version (25K):
[in this window]
[in a new window]
  		Fig. 2. Sorption isotherms (filled symbols) and four sequential desorption steps (open symbols) of atrazine measured with Akko soils for 2- and 14-d cycles (bars represents standard deviations).

 


View larger version (12K):
[in this window]
[in a new window]
  		Fig. 3. Distribution of the apparent hysteresis index (AHI) vs. initial atrazine concentration for Basra (top) and Akko (bottom) freshwater (open symbols)- and wastewater (filled symbols)-irrigated soils. Circles represent data collected for the 2-d sorption–desorption cycles and squares represent the 14-d sorption–desorption cycles.

 
The data obtained in the present study from both Basra and Akko soils showed that longer sorbate-sorbent interactions result in more pronounced desorption hysteresis (Fig. 1 and 2). A similar trend has been reported by Lesan and Bhandari (2003), who showed that atrazine sorption–desorption to agricultural and woodland soils is time-dependent. In their study, both sorption (KF) and desorption parameters changed with time (sorption affinity and AHI increased). In our study, lower desorption was exhibited for the longer-term desorption experiments although the sorption parameters (KD) were similar for the short (2 d) and long (14 d) sorption experiments. The trend of lower desorption (lower AHI values) with increasing contact time is demonstrated in Fig. 4 , in which we combined the data from all sorption and desorption sets (2-d sorption and 1-, 2-, and 7-d desorption; and 14-d sorption and 1-, 7-, and 14-d desorption). In this figure, the AHI data were plotted against total contact time (sorption plus desorption). These data suggest that with increasing contact time, low-energy-type binding sites are converted to higher-energy sites, or that the bound molecules are transferred to sites (domains) that have lower desorption potential (e.g., a mass-transfer mechanism of molecules from the initial sorption domain to another one).



View larger version (12K):
[in this window]
[in a new window]
  		Fig. 4. Distribution of the apparent hysteresis index (AHI) vs. total atrazine contact time (sorption + desorption) for Basra (top) and Akko (bottom) freshwater (open symbols) and wastewater (filled symbols) irrigated soils. Squares, initial concentration of 1 mg L–1 atrazine; circles, initial concentration of 20 mg L–1 atrazine.

 
The similar atrazine-sorption parameters obtained for the two sorption periods and the increasing difficulty desorbing the molecules with increasing contact time suggest that the sorption process occurs in two stages. At first, sorption occurs to readily available sites; at a later stage, the sorbed molecules interact with the SOM or the SOM-mineral complexes in a way that significantly hinders their ability to desorb. The constant sorption-affinity data suggest that no additional binding sites become available with time. Therefore, the increasingly difficult desorption can be explained by structural changes in the SOM and/or by a stronger interaction between the sorbed molecule and the SOM. For example, atrazine can bind to SOM via H-bonding to more than one site (Welhouse and Bleam, 1993); with increasing interaction time, the sorbed atrazine molecule can form a multiple-center complex with the sorbent by site orientation. This mechanism can significantly reduce the desorption potential of atrazine without any increase in sorption capacity.

Increased sorption–desorption hysteresis with increasing sorption time has been reported in many studies which suggested dual-mode sorption as the interaction mechanism (Weber et al., 1998). However, our data, showing an inconsistent increase in sorption nonlinearity and constant sorption capacity within the tested period, suggest that this mechanism probably does not govern atrazine sorption in our tested soils. On the other hand, the decreasing desorption values with increasing sorption time suggests a continuous mass transfer of sorbed molecules from one domain to another (Altfelder et al., 2000). These latter authors suggested that increased deviation of the sorption–desorption isotherms with increasing equilibrium time results from continuous sorption. The increased deviation of desorption points from the sorption line with increasing desorption time was also presented for atrazine by Ma and Selim (1994), who also concluded that atrazine adsorption is a kinetic process. The mass-transfer mechanism or rate-limited diffusion cannot be the dominant mechanisms controlling sorption–desorption behavior in our study since the sorption parameters remained constant during the tested period and only the desorption potential decreased with time. Moreover, our data collected for the shorter contact time (2 d of sorption and four sequential 1-d desorption steps, i.e., 6 d of contact time) revealed significant desorption hysteresis (AHI of 0.35 for 1 mg L–1 initial atrazine concentration for Basra freshwater irrigated soil; Fig. 4). This suggests the relatively rapid formation of an atrazine fraction, which is highly difficult to desorb. We therefore suggest that the bonded molecules did not diffuse or were not transferred with time to other domains but that the binding sites were chemically or physically converted to either higher-energy-type sorption sites, or that the bound molecules were entrapped by the SOM. This mechanism was also suggested by Sharer et al. (2003), who exhibited a large desorption-resistant fraction of several organic compounds after 1 d of contact time.

The AHI values calculated for the 14-d sorption–desorption set for Akko soils were lower than the corresponding values calculated for Basra soils. Another interesting trend was that at the longer contact time (14 d), sorption–desorption isotherms for the freshwater-irrigated soils exhibited higher hysteresis than for the wastewater-irrigated soils. This trend was exhibited for both Basra and Akko soils and was especially pronounced at the higher atrazine concentration (Fig. 3 and 4), suggesting lower desorption potential of atrazine from the freshwater-irrigated soils at the higher atrazine concentration. We believe that the observed differences in desorption hysteresis between the Basra and Akko soils, and more importantly between the freshwater-and wastewater-irrigated soils, result from the physicochemical nature of the SOM. We speculate that the continuous influx of fresh and relatively polar DOM into the wastewater soils provides a mass of compounds that can physically and chemically bind to the natural SOM. This mechanism may result in coated SOM, which consequently has less available binding sites. The newly formed and relatively young type of SOM in the wastewater-irrigated soil may retard atrazine's uptake by blocking available sorption sites deeper within its structure. The preferential coating of the SOM may cause steric interference for the sorption sites, resulting in lower sorption potential of the wastewater-irrigated soils. The relatively young SOM formed in the wastewater-irrigated soils by the influx of fresh DOM can also enhance desorption, as was suggested by Lesan and Bhandari (2003).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study demonstrates that atrazine sorption to soils is a time-dependent process whose true equilibrium time is much longer than that usually obtained in short sorption experiments. We believe that the decreased desorption with increasing contact time can be explained by a modification of the SOM sorption sites, which enables better sorption and less desorption. The clear trend of higher sorption and lower desorption potential in the freshwater-irrigated soils compared with the wastewater-irrigated soils suggests that the SOM in the latter soils has been chemically and physically modified by the influx of DOM, forming SOM which has less available atrazine-sorption sites. Most of the remaining available sites in these soils are probably present at the surface of the SOM and are therefore readily available for desorption. A more detailed study regarding the nature of the SOM in the wastewater-irrigated soils is needed to reveal the specific interactions of atrazine and other potential pollutants with these soils.


   ACKNOWLEDGMENTS
 
This research was supported by research grants from the Israeli Ministry of Agriculture. The Authors thank Drs. K.D. Wenzel and A. Hubert (UFZ, Center of Environmental Research, Leipzig, Germany) for performing the accelerated solvent-extraction analysis.

Received for publication October 26, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 J. Environ. Qual.Home page
Y. Drori, B. Lam, A. Simpson, Z. Aizenshtat, and B. Chefetz
The Role of Lipids on Sorption Characteristics of Freshwater- and Wastewater-Irrigated Soils
J. Environ. Qual., October 27, 2006; 35(6): 2154 - 2161.
[Abstract] [Full Text] [PDF]

Home page
 Soil Sci.Home page
K. Stimler, B. Xing, and B. Chefetz
Transformation of Plant Cuticles in Soil: Effect on their Sorptive Capabilities
Soil Sci. Soc. Am. J., May 23, 2006; 70(4): 1101 - 1109.
[Abstract] [Full Text] [PDF]

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 Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Drori, Y.
Right arrow Articles by Chefetz, B.
Related Collections
Right arrow Agricultural Pesticides
Right arrow Fate
Right arrow Solute Transport Models

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
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome