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Published in J. Environ. Qual. 34:552-562 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Organic Compounds in the Environment

Interactions of Organic Compounds with Wastewater Dissolved Organic Matter

Role of Hydrophobic Fractions

Talli Ilania, Elke Schulzb and Benny Chefetza,*

a Department of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
b Department Soil Sciences, UFZ Centre for Environmental Research Leipzig-Halle Ltd., Theodor-Lieser-Str. 4, Halle/Saale 06120, Germany

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

Received for publication September 21, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The role of structural fractions of dissolved organic matter (DOM) from wastewater in the sorption process of hydrophobic organic compounds is still not clear. In this study, DOM from two wastewater treatment plants (Lachish and Netanya, Israel) was fractionated to hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions. The fractions were characterized and their sorptive capabilities for s-triazine herbicides and polycyclic aromatic hydrocarbons (PAHs) were studied. For all sorbates, the binding to the HoN fractions was much higher than to HoA fractions. The HoA fractions were more polar than the HoN fractions, containing a higher level of carboxylic functionalities. However the higher binding coefficients of atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) and ametryn [2-(ethylamino)-4-isopropylamino-6-methyl-thio-s-triazine] obtained for the HoN fractions suggest that their sorption is governed by hydrophobic-like interactions rather than H bonding. The values of binding coefficients of PAHs measured for the HoN fractions were within the range reported for humic acids and much higher than other fractions, suggesting that this fraction plays an important role in the overall sorption of these compounds by DOM. Higher sorption coefficients were measured for the Netanya DOM sample containing higher level of hydrophobic fractions (HoA + HoN) than the Lachish DOM, suggesting that the sorption of hydrophobic organic compounds by DOM is governed by the level of these structural substances. The evaluation of mobility of organic pollutants by wastewater irrigation requires not only assessment of the total carbon concentration but also, more importantly, the content of the hydrophobic fractions.

Abbreviations: DOC, dissolved organic carbon • DOM, dissolved organic matter • FTIR, Fourier transform infrared • HoA, hydrophobic acid • HoN, hydrophobic neutral • KDOC, carbon-normalized partition coefficient • PAH, polycyclic aromatic hydrocarbons • SPME, solid-phase microextraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
RECYCLED WASTEWATER EFFLUENTS are an important source of irrigation water in arid and semiarid regions. The use of the treated effluent in agriculture improves the overall water budget and allows the freshwater to be shifted from agricultural users to domestic and industrial needs. Treated wastewater contains higher concentrations of suspended and dissolved organic and inorganic matter as compared with freshwater. Influx of these components into soils can affect the soil physical and chemical properties, such as hydraulic conductivity, infiltration, and sodium adsorption ratio (Levy et al., 1999; Tarchitzky et al., 1999; Mamedov et al., 2000). Another potential effect of intensive irrigation with treated wastewater is an increase in the level of DOM in the soil profile, as well as possible changes in the chemical properties and composition of the soil organic matter. The influx of relatively high DOM concentrations into soils can affect significantly the fate of nonpolar and polar organic compounds in the soils. Organic chemicals originally present in the wastewater such as polychlorinated biphenyls, PAHs, and organochlorine pesticides have been identified in the ground water in areas irrigated with wastewater or amended with sewage sludge (Muszkat et al., 1993). Moreover, the concentrations of some pesticides were higher, and found deeper in soils irrigated with effluent water as compared with sites irrigated only with high-quality water (Graber et al., 1995). Because many of the pesticides were found in the ground water and/or below the root zone, it was concluded that the transport of these chemicals is assisted by the presence of DOM (Fitch and Du, 1996; Nelson et al., 1998; Graber et al., 2001).

Sorption and transport of many organic chemicals by DOM or soluble humic substances were extensively investigated (Lee and Farmer, 1989; Chin et al., 1997; Raber et al., 1998; Perminova et al., 1999; Cox et al., 2000; Williams et al., 2000; Kopinke et al., 2001; Poerschman and Kopinke, 2001; Kulikova and Perminova, 2002; MacKenzie et al., 2002). However, only a few studies have investigated the effect of DOM from wastewater on the sorption of pesticides and other organic contaminants (Graber et al., 1995; Seol and Lee, 2000, 2001). Still the role of DOM in the sorption and transport of hydrophobic organic compounds is not fully understood. Seol and Lee (2000) suggested that very high concentrations of dissolved organic carbon (DOC; >150 mg/L) are needed to suppress significantly the sorption of triazine herbicides by soils. Spark and Swift (2002) concluded that the DOM from soil has little or no effect on the sorption and transport of some pesticides. Celis et al. (1998) reported that DOM from limed sewage sludge reduces atrazine sorption to soil, but DOM extracted from composted sludge exhibits elevated atrazine sorption to soils due to increased atrazine–DOM–soil interactions. These studies clearly indicate that DOM can affect the sorption of hydrophobic pesticides in soils. However, the diverse affects of the DOM suggest that its chemical nature and composition play an important role in the sorption mechanism.

Wastewater DOM is highly heterogeneous in size and chemical composition. The molecular masses of the DOM range from less than 500 to more than 5000 Da (Imai et al., 2002), and it is comprised of a mixture of humic materials, polysaccharides, polyphenols, proteins, lipids, and heterogeneous molecules. Determination of DOM chemical structure is difficult due to of this complex composition. Thus, preliminary fractionation is essential for better understanding of the variety of different physicochemical compounds comprising DOM (Leenheer, 1981). Resin adsorbents are used for DOM fractionation to hydrophobic and hydrophilic fractions, which are further separated to acid, neutral, and base subfractions. This preparative DOM fractionation appears to be useful for the evaluating the characteristics of effluent wastewater, lake, soil, sludge, and compost DOM (Chefetz et al., 1998a, 1998b, 1998c; Raber et al., 1998; Imai et al., 2002). In this study, we aimed to investigate the mechanism governing the binding of s-triazine herbicides (atrazine and ametryn) and PAHs (fluoranthene, phenanthrene, and pyrene) to hydrophobic structural fractions of wastewater DOM in relation to their physicochemical properties.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Wastewater Sampling and Extraction and Purification of Dissolved Organic Matter
Treated wastewater was sampled from two municipal wastewater treatment plants in Israel, one based on activated sludge treatment (Netanya) and the second on the operation of oxidation ponds (Lachish). Both types of treated wastewater are used for irrigation in agriculture. During sampling, the wastewater was filtered through a 0.45-µm filter (Polypure; Pall Corporation, East Hills, NY) and NaN3 (200 mg/L) was added to the sampling containers to inhibit microbial activity. Samples were placed in the refrigerator immediately on reaching the laboratory and were stored for not more than 3 d before use. A portion of the wastewater was concentrated using a Prep/Scale ultra-filtration system (Millipore, Billerica, MA) with a molecular weight cut-off of 1000 Da. Both the DOM that was not concentrated, and the concentrated DOM (>1000 Da) were fractionated to hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions based on sorption behavior on DAX-8 resin (Sigma-Aldrich, St. Louis, MO) as described in detail by Leenheer (1981) and Chefetz et al. (1998a). The hydrophilic fractions (not adsorbed to the resin) were collected but were not used in this study because the tested sorbates exhibited very low sorption affinities toward these fractions in preliminary experiments. The DOC concentrations were measured immediately after the sampling and fractionation procedure using a combustion TOC analyzer (FormacsHT; Skalar Analytical, Breda, the Netherlands). The HoA and HoN sample fractions were freeze-dried and stored in a desiccator. Dissolved organic matter, HoA, and HoN samples were also obtained from a mature compost of municipal solid waste as described by Chefetz et al. (1998b).

Dissolved Organic Matter Characterization
Freeze-dried samples (DOM, DOM > 1000, HoA, HoA > 1000, HoN, HoN > 1000) were analyzed in triplicate for C, H, and N contents with an EA 1108 elemental analyzer (Fisons Instruments, Milan, Italy). The total acidity of the samples was determined using a procedure described for humic substances (Swift, 1996). The Fourier transform infrared (FTIR) spectra of freeze-dried samples were collected for the wavenumber range of 4000 to 400 cm–1 on a 550 Magna-IR spectrometer (Nicolet Instruments, Madison, WI). Samples were oven-dried at 65°C for 48 h and finely ground before analysis (Chefetz et al., 1998a). Samples were prepared for the analyses by mixing 98 mg of KBr (Sigma) with 2 mg of the freeze-dried material and then compressing the mixture into pellets. To obtain the FTIR spectra, 40 scans were collected; spectra were compared after applying a linear baseline correction at 4000, 2000, 860, and 400 cm–1 as zero absorbance points.

Sorbates
Two s-triazine herbicides, atrazine and ametryn, were obtained from Agan Chemicals (Ashdod, Israel). Three PAHs, phenanthrene, fluoranthene, and pyrene, were purchased from Sigma. Selected properties of the sorbates (>98% purity) are listed in Table 1.


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  		Table 1. Selected physical and chemical properties of the studied sorbates.

 
Sorption Experiments with Triazine Herbicides
The partitioning coefficients of the pesticides to the DOM fractions (bulk > 1000, HoA > 1000, HoN > 1000) were measured using dialysis-bag sorption experiments (Carter and Suffet, 1982; Seol and Lee, 2000). Spectral/Por 6 dialysis bags (Spectrum Laboratories, Rancho Dominguez, CA) with a molecular weight cut-off of 1000 Da were washed with distilled water, 1 M Na2CO3, 1 M NaHCO3, and again in distilled water before use. The DOM solutions at concentrations of 40 to 610 mg DOC/L were prepared by dissolving the freeze-dried samples in 20 mL of 0.1 M NaOH. The solutions were stirred overnight, then the pH values were measured and adjusted to 7.8 (the pH value of the wastewater) using 0.5 M HCl. The volume of the solutions was adjusted to 100 mL with background solution (5 mM CaCl2 and 200 mg/L NaN3). Before using, the solutions were filtered (0.45 µm) and if necessary, the pH was readjusted. Aliquots (5 mL) of the solutions were placed in the dialysis bags. Before the sorption experiments, the bags were dialyzed overnight against background solution to ensure that during the sorption experiments only the >1000-Da DOM fraction is used.

The dialysis bags containing DOM were placed in 40-mL glass test tubes containing 5 mL of background solution with atrazine or ametryn at concentrations ranging from 0 to 20 mg/L. The tubes were agitated in the dark on a platform shaker at 90 rpm for 2 d at 25°C. Data obtained from preliminary experiments indicated that 2 d is sufficient time for sorbate–sorbent equilibrium. Two types of blanks were measured: (i) blank tubes in absence of DOM to verify that pesticide concentrations inside and outside the bag were equal; and (ii) blank tubes with DOM without pesticides for DOC measurements. Sorption to the dialysis bag was found to be negligible. Pesticide concentrations inside and outside the dialysis tubes were determined using L-7100 LaChrom HPLC (Merck-Hitachi, Darmstadt, Germany) equipped with a photodiode array detector. Atrazine and ametryn were detected using a LiChrospher RP-18 column (25 cm x 4 mm, 5 µm), with a 70:30 (v/v) acetonitrile to water mobile phase at a flow rate of 1 mL/min. Both triazines were detected using absorbance at 220 nm and were quantified using external standards. The adsorbed amounts were quantified as the difference between pesticide concentrations inside and outside the dialysis bag. A linear regression between the bound triazine (mg/kg DOC) and the free triazine equilibrium concentration (mg/L) yields the carbon-normalized partition coefficient (KDOC).

Sorption Experiments with Polycyclic Aromatic Hydrocarbons
The binding coefficients of the PAHs to the DOM fractions (HoA, HoN) were measured using solid-phase microextraction (SPME), which is a very useful method for determining sorption coefficients of organic compounds on DOM. A detailed description of the technique is presented by Poerschman and Kopinke (2001). The DOM solutions containing 230 to 1500 mg DOM/L were prepared by dissolving freeze-dried DOM samples in 5 mL of 0.1 M NaOH in 25-mL volumetric flasks, being followed by addition of 10 mL of water. Solutions were sonicated in an ultrasonic bath (Transsonic 460; Elma, Singen, Germany) at a frequency of 35 kHz for 15 to 30 min, then the pH values were adjusted to 7.8 (pH of the wastewater) with 0.5 M HCl and the volume set to 25 mL. A second ultrasonic treatment was applied.

Aliquots (5 mL) of the DOM solutions were placed in SPME tubes (10 mL), which were spiked with the studied PAHs to the final concentration of 30 ng/L. The tubes were sealed with magnetic flange caps (silicone/PTEE) and were agitated at 29°C for 60 min equilibrium. Preliminary experiments suggested a rapid (5–15 min) sorption equilibrium. After the equilibrium time, the solutions were placed in a CTC Combi PAL SPME autosampler (CTC Analytic, Zwingen, Switzerland). Then the tubes were sampled and agitated for additional 15 min at 650 rpm. The PAHs were accumulated on a 65-µm polydimethylsiloxane/divinylbenzene fiber (Supelco, Bellefonte, PA) in the headspace for 5 min. The PAHs were then desorbed at 270°C in the injector of a HP-6890 GC (Hewlett-Packard, Palo Alto, CA), with a column temperature of 100°C. The column (HP-5MS, 30 m x 250-µm i.d., 0.25-µm thickness) temperature was then increased to 210°C at a rate of 30°C/min and to 300°C at a rate of 17°C/min. The PAHs were detected and quantified using MS-5973 (Hewlett-Packard). The binding coefficients were calculated according to the equation:

where n1 and n2 are the amounts of the sorbates extracted by the SPME fiber from solution containing DOM and from solution without DOM, respectively, and CDOC is the DOC concentration (Kopinke et al., 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Wastewater Dissolved Organic Matter Characterization
Dissolved organic matter contains a mixture of low-molecular-weight compounds and chemically heterogeneous macromolecules (Qualls and Haines, 1991). Thus, preliminary fractionation is essential for the better understanding of the contribution of the individual structural components to overall sorption properties. The DOM from the activated-sludge treatment plant in Netanya was composed of 33% hydrophilic and 67% hydrophobic fractions (on a C basis). The content of the hydrophobic fraction (HoA + HoN) was only 38% of the total carbon in the wastewater DOM isolated from the oxidation-pond treatment facility in Lachish. The two samples also differed in their relative contents of the HoA fraction. This fraction made up 60% of the total DOC in the Netanya wastewater vs. only 30% in the Lachish sample. The level of the HoN fraction was similar in the two samples (8–9% of the total DOC). The relative percentage of the HoA in the hydrophobic fraction was 88 and 76% for the Netanya and Lachish samples, respectively, and the relative percentage of the HoN in the hydrophobic fraction was 12 and 23% for the two wastewater samples, respectively. It is suggested that the advanced wastewater purification treatment (activated sludge as compared with oxidation pond) results in a significant reduction of readily degradable compounds (i.e., the hydrophilic fraction), hence the relative enrichment of the hydrophobic fractions in the Netanya treated wastewater. The hydrophobic matter is assumed to be more resistant to microbial degradation. These data are consistent with those presented by Imai et al. (2002). Due to their highly hydrophobic nature and their relative abundance in the wastewater solutions, two fractions (HoA and HoN) were chosen for analysis of their sorptive capabilities.

The main absorbance peaks of the FTIR spectra of the various isolated DOM fractions (Fig. 1 and 2) were in the following wavelength: 3300 to 3430 cm–1 (H bonds, OH groups), 2850 to 2965 cm–1 (C–H stretch of –CH2 and –CH3), 1710 to 1720 cm–1 (C=O of COOH), 1620 to 1660 cm–1 (C=C in aromatic structure, COO–, H-bonded C=O), 1540 to 1580 cm–1 (amide II bonds), 1400 to 1465 cm–1 (C–H deformation of CH2 or CH3 groups), 1375 to 1420 cm–1 (CH3, COO–), 1240 to 1260 cm–1 (aromatic C, C–O stretch), and 1100 to 1000 cm–1 (C–O stretch of polysaccharide) (Baes and Bloom, 1989; Niemeyer et al., 1992).



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  		Fig. 1. Absorbance Fourier transform infrared (FTIR) spectra of the bulk hydrophobic acid fractions (HoA-bulk, right side) and bulk hydrophobic neutral fractions (HoN-bulk, left side) isolated from Netanya (top) and Lachish (middle) treated wastewaters, and from compost water extract (bottom).

 


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  		Fig. 2. Absorbance Fourier transform infrared (FTIR) spectra of the >1000-Da fractions isolated from the wastewater samples; hydrophobic acid fractions (HoA, right side) and hydrophobic neutral fractions (HoN, left side).

 
All three HoA spectra were characterized by a dominant C=O carboxylic vibration peak (1716–1720 and 1652 cm–1), polysaccharide vibration peaks of variable intensities, and minor peaks of methyl and methylene vibrations. The intensity of the carboxyl vibration band made it the governing peak in the spectra of the HoA fractions isolated from compost DOM and Netanya wastewater, but it was shifted to lower frequency (1652 cm–1) in the spectrum of the Lachish HoA fraction. The Lachish HoA fraction was also characterized by a lower acidity value as compared with the fractions isolated from Netanya and compost (Table 2). Moreover, the Netanya and compost HoA samples exhibited similar C levels and H to C atomic ratios (1.29), which were lower than the H to C ratio recorded for the HoA fraction from Lachish (1.42). The higher H to C ratio of the fraction from Lachish suggests its higher aliphaticity. The FTIR spectra and the chemical properties of the HoA fractions (except that from Lachish) were similar to data recorded for other isolated HoA fractions and fulvic acids (Qualls and Haines, 1991; Chefetz et al., 1998c). It has been suggested that the HoA fraction resembles a light fraction of fulvic acid. The spectrum of the Lachish HoA fraction suggests that it has lower carboxylic content and it is richer in polysaccharide and paraffinic structures than the other HoA fractions. The different wastewater treatments resulted not only in DOM containing different contents of HoA (60 vs. 30%) but also HoA fractions exhibiting different chemical properties. Therefore, it is suggested that advanced wastewater treatment increases the level of hydrophobic fractions in the DOM and increases the oxidation stage of these fractions. This hypothesis is supported by the high carboxyl content exhibited by the HoA isolated from the final stages of composting.


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  		Table 2. Selected chemical characterization of the isolated samples: dissolved organic matter (DOM), hydrophobic acid (HoA), and hydrophobic neutral (HoN).

 
In contrast to the FTIR spectra of the HoA fractions, the carboxylic C=O vibration peak is absent in spectra of the HoN fractions, suggesting less of a polar character. As a result, the relative intensities of the paraffinic and aromatic functionalities (peaks at 2960–2850, 1660, and 1440 cm–1) were pronounced significantly. The HoN spectra from the wastewater samples exhibited significantly higher carbohydrate levels than the fraction isolated from the compost water extract. The HoN fractions from the wastewater exhibited lower acidity values than the corresponding HoA samples. The HoN samples from the compost exhibited higher acidity values than other HoN samples and even a higher value than the corresponding HoA sample. The H to C ratio and the FTIR spectra suggest that the HoN fraction is composed of aliphatic (paraffinic) structures with a low level of polar functionalities.

The sorption coefficients of the triazine herbicides were measured using 1000-Da dialysis tubes. We therefore performed FTIR measurements for the HoA > 1000 Da and HoN > 1000 Da samples. The main differences between the spectra of the unfractionated (bulk) and the >1000-Da samples (Fig. 1 and 2, respectively) were a relative increase in peak intensities of the polysaccharides (1000–1100 cm–1) and aromatic C=C (1620 cm–1) vibration bands as compared with the intensities of the carboxyl C=O and saturated C–H stretch vibration peaks. The same trend was observed for all HoA > 1000 Da and HoN > 1000 Da samples, but it was more pronounced for the fractions isolated from the Lachish DOM (Fig. 2). These data suggest that the higher-molecular-weight fraction of both HoA and HoN samples is composed of a polymeric structure of covalently linked carbohydrates. The content of the linked-carbohydrate structures is probably significantly lower in the low molecular weight (<1000 Da) fraction. Similar conclusions have been reported by Guggenberger and Zech (1994). The relative enrichment of the >1000-Da fractions with polysaccharides and aromatic structures will be further discussed in relation to the sorptive properties of this fraction.

Sorption of Triazine Herbicides
Atrazine and ametryn are widely applied herbicides (Barbash et al., 2001) that have different physicochemical properties (Table 1) due to their different substituents (Cl and SCH3, respectively). The substituents affect the basicity of the N atoms and the acidity of the N–H bonds of the molecule, as well as the consequent modes of interaction with sorbents. The pKb value of atrazine is lower than that of ametryn (1.7 and 3.9, respectively); therefore, atrazine is more likely to form relatively stronger H bonds with carboxylic moieties of DOM than ametryn (Chefetz et al., 2004). This can result in a higher sorption affinity for atrazine if the hydrogen bonding is governing the sorbate–sorbent interactions.

The atrazine sorption isotherms are presented in Fig. 3 and the sorption coefficient data are summarized in Table 3. Within the measured atrazine concentrations, all isotherms were linear (r2 > 0.97), suggesting that partitioning is the major sorption mechanism. In all cases, sorption of atrazine to the HoN fraction was greater than to the HoA and unfractionated DOM samples. These data are contradictory to the hypothesis that high content of carboxylic functionalities and high acidity values, as was exhibited by the HoA fractions, would lead to increased sorption due to H bonding. At the pH of the solutions (7.8), the carboxylic groups of the HoA can interact with atrazine by forming H bonds with the hydrogen atom of the side-chain amino group of the triazine molecule. However, it appears that specific atrazine–DOM interactions were dominated by nonspecific (hydrophobic-like) sorbent–sorbate interactions because the sorbents containing higher carboxylic functionalities (i.e., HoA) exhibited lower sorption potential than the HoN fractions. The measured atrazine distribution coefficients for the HoA and HoN fractions were within the range reported for soluble humic substances (Kulikova and Perminova, 2002), but higher than the values reported for DOM from municipal waste effluent and swine effluent (Seol and Lee, 2000). The higher KDOC values recorded in this study probably resulted from the predialysis treatment performed in our study to eliminate the <1000 Da residual DOM fractions. The <1000 Da DOM can reduce the overall measured uptake by a sample.



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  		Fig. 3. Sorption isotherms of atrazine to >1000-Da samples from Netanya and Lachish treated wastewaters, and from compost water extract. Bars represent standard deviation. HoA, hydrophobic acid; HoN, hydrophobic neutral; DOM, dissolved organic matter.

 

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  		Table 3. Partition coefficients of atrazine and ametryn to the isolated >1000-Da sorbents: dissolved organic matter (DOM), hydrophobic acid (HoA), and hydrophobic neutral (HoN).

 
Another interesting observation is that the measured KDOC values for the fractions isolated from the two wastewater samples can be arranged in the following order: HoN > DOM > HoA. For the fractions isolated from the compost water extract, the measured KDOC value of atrazine with HoA was three times higher that those obtained for the DOM. The relatively similar KDOC values calculated for the HoN and HoA samples indicate similar sorptive properties within each group of sorbents. We suggested that the lower polarity of the HoN (i.e., the absence of carboxyl band in the FTIR spectrum) isolated from Lachish DOM resulted in higher KDOC values. The presence of polar functional groups in the HoA fractions probably decreases the overall hydrophobicity of this sorbent, thus reducing its affinity to nonpolar organic chemicals. These findings support the hypothesis that nonspecific hydrophobic binding is the key interaction of atrazine with DOM. Similar conclusions were drawn by Kulikova and Perminova (2002), who investigated atrazine sorption by dissolved humic substances. They demonstrated the importance of aromatic structures for the atrazine sorption to the DOM. Our experiments show very high sorption coefficients for samples rich in paraffinic moieties. Therefore, we speculate that the nonspecific binding of atrazine can be governed by both aromatic and aliphatic domains of the DOM. Moreover, the location of the polar functionalities (linked to aliphatic or aromatic moieties), which decreases the overall hydrophobicity of a sorbent, can be important rather than the total level of the sample aromaticity or aliphaticity.

Atrazine showed higher complexation than ametryn to both the HoA and HoN fractions (Fig. 4 ; Table 3). Ametryn is expected to exhibit lower sorption affinity than atrazine due to its higher aqueous solubility (Sluszny et al., 1999). It is important to note that the partition coefficient values for ametryn to HoA were close to the values obtained for atrazine (except for HoA from Lachish), whereas the KDOC values calculated for ametryn by the HoN samples were significantly lower than the values recorded for the atrazine by the same sorbent. Moreover, a higher ratio (atrazine KDOC to ametryn KDOC) was exhibited by the Lachish HoN sample than the Netanya HoN sample (4.2 versus 3.7, respectively). The lower sorption obtained for the HoA samples as compared with the HoN samples and the higher sorption recorded for HoN samples, which exhibited the less polar functionalities, suggest that the main sorption mechanism of ametryn is hydrophobic interactions.



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  		Fig. 4. Sorption isotherms of ametryn to >1000-Da samples from Netanya and Lachish treated wastewaters, and from compost water extract. Bars represent standard deviation. HoA, hydrophobic acid; HoN, hydrophobic neutral.

 
Several aspects must be taken into consideration when trying to elucidate the sorption of triazines to the bulk (unfractionated to >1000 Da) DOM: (i) the ability of the higher-molecular-weight sorbents to provide more hydrophobic sites; (ii) the different chemical properties of the bulk and >1000-Da DOM samples; and (iii) the relative amount of the higher-molecular-weight fraction in the bulk DOM sample. Our collected data for the PAHs (see below, sorption to the unfractionated and >1000-Da fractions) show that the >1000-Da fractions sorb significantly higher amounts of analytes than the corresponding bulk samples. Therefore, it is speculated that sorption experiments with bulk DOM samples would lead to lower binding coefficients of the triazine herbicides than the measured values, but the differences between the sorbents (HoN vs. HoA) are expected to remain the same (i.e., HoN > HoA).

Sorption of Polycyclic Aromatic Hydrocarbons
The calculated sorption coefficients for the tested PAHs are summarized in Table 4. Based on their hydrophobicity level, pyrene exhibited the highest and phenanthrene the lowest KDOC values. As observed for the triazine herbicides, all tested PAHs exhibited higher sorption affinity toward the HoN fractions than toward the more polar sorbent HoA. Because the sorption experiments were conducted using the SPME technique, and phase separation (bound vs. free analyte) was not required, we used the unfractionated samples (the >1000-Da samples were used for the sorption experiments with the triazines). To reveal if PAH exhibited higher binding affinity toward higher-molecular-weight sorbent, we tested the differences in sorptive capabilities between the >1000-Da and the bulk HoA samples. As expected, the calculated KDOC values of all PAHs were noticeably higher for the higher-molecular-weight sorbents (>1000 Da). For example, the pyrene KDOC values were 7000 versus 4360 L/kg DOC and 23000 versus 5420 L/kg DOC for HoA > 1000 Da and the unfractionated HoA samples isolated from Netanya and Lachish wastewaters, respectively. A similar trend of increasing sorption with increasing molecular weight is also expected for the HoN fractions (Chin et al., 1997). It is worth noting that although the calculated KDOC values for the HoA > 1000 Da fractions were significantly higher than the values recorded with the bulk HoA samples, they were still lower than the KDOC values measured with the bulk HoN samples. This finding highlights the importance of the HoN fraction as a remarkably high-affinity sorbing agent in the DOM mixture.


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  		Table 4. Partition coefficients of phenanthrene, fluoranthene, and pyrene to the isolated bulk hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions.

 
The KDOC values for phenanthrene and fluoranthene with the HoN sample from Netanya were higher than the values calculated for the HoN sample isolated from the Lachish wastewater. The opposite trend was shown for pyrene. The lower oxygen content and higher aliphatic nature of the HoN sample from Lachish probably facilitated the sorption of pyrene, which is larger in size and more hydrophobic than the other solutes. This assumption is supported by the markedly low sorption coefficients calculated for the HoN sample from the compost water extract. The compost HoN sample is characterized by a high level of carboxyl and hydroxyl functionalities, as well as by a higher acidity level (456 cmol/kg, vs. 385 and 151 cmol/kg for HoN from Netanya and Lachish samples, respectively). The HoA sample from the Lachish wastewater exhibited the higher KDOC values for all PAHs than other HoA samples. This sample exhibited the lowest level of total acidity among the HoAs (Table 2). Moreover, the HoA sample from Lachish is characterized by a higher H to C ratio (1.42), as compared with a ratio of 1.29 calculated for the fractions isolated from the compost water extract and Netanya wastewater. These findings support the data obtained for the atrazines; it seems that the presence of polar functional groups in the HoA fractions reduces their affinity to interact with nonionic organic chemicals. It is also suggested that the aliphatic- or paraffinic-like domains within the HoA or HoN sorbents provide a hydrophobic environment, assisting in the sorption of very hydrophobic sorbates (Piccolo et al., 1998).

Kopinke et al. (2001) reported higher sorption coefficients for pyrene to water-soluble humic acids than to fulvic acids. They found that an increase in sorbent aromaticity results in a higher sorption potential and that the more polar sorbent exhibits lower sorption potential. Our data follow the same trend, which is in agreement with the polarity differences between the HoN and HoA sorbents. The pyrene KDOC values measured for the HoA fractions were lower than the values reported for soluble humic and fulvic acids, but the values measured for the HoN fractions were within the range reported for humic acids (Kopinke et al., 2001). By contrast, our KDOC values for all tested PAHs were significantly lower than the values obtained for soil DOM samples using a fluorescence-quenching technique (Raber et al., 1998).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The reported partition coefficients are of major importance for estimation the sorption of hydrophobic organic compounds by wastewater DOM. As indicated by the obtained KDOC values, the hydrophobic neutral (HoN) fraction showed significant sorptive capacity toward organic pollutants. Although this fraction made up a relatively small portion of the DOM (<10%) and is mainly composed from aliphatic moieties, it is a dominant sorbent, especially in DOM, which is characterized by low hydrophobicity. The role of this fraction (HoN) in the mobility and transport of organic pollutants in soils is not yet clear. Guo and Chorover (2003) concluded that the fulvic acid–like hydrophobic and aromatic fractions of DOM are selectively retained in soil columns. Based on its higher hydrophobicity, the HoN fraction is therefore expected to significantly interact with the soil matrix, thereby reducing the mobility of DOM-adsorbed hydrophobic organic compounds.

Another interesting observation was the higher atrazine binding coefficient obtained for the Netanya DOM as compared with the other samples. This resulted from the higher content of the hydrophobic fractions (HoN + HoA) in this sample compared with the Lachish and compost DOM samples. Therefore, although the Netanya wastewater showed lower DOC content than Lachish (12 and 23 mg/L, respectively), higher KDOC values of Netanya sample suggest that the chemical composition and properties of DOM have an important effect on the total DOM's sorptive potential. Hence, the evaluation of the mobility of organic pollutants by wastewater irrigation requires not only the assessment of the total DOC concentration but also, more importantly, the contents of the hydrophobic fractions.


   ACKNOWLEDGMENTS
 
This research was supported by research grants from the International Arid Lands Consortium (IALC), the German Federal Ministry of Education and Research (BMBF, Project #01 SF 0116), and the Andrea and Charles Bronfman Philanthropies (ACBP).


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


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JEQ 2005 34: 403-407. [Full Text]  




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