Geoderma 145 (2008) Contents lists available at ScienceDirect. Geoderma. journal homepage:

Transcription

1 Geoderma 145 (2008) Contents lists available at ScienceDirect Geoderma journal homepage: Sorption of organic compounds to humin from soils irrigated with reclaimed wastewater Yaron Drori a, Zeev Aizenshtat b, Benny Chefetz a, 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 Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel article info abstract Article history: Received 17 October 2007 Received in revised form 11 February 2008 Accepted 26 February 2008 Available online 14 April 2008 Keywords: Desorption Lipid Wastewater Humin Humin is an important fraction of soil organic matter that is known to tightly bind many organic compounds. The objective of this study was to evaluate the role of humin as a natural sorbent in soils irrigated with freshwater and treated wastewater. To meet this goal, the sorption desorption behavior of phenanthrene, atrazine and chlorotoluron was studied with humin isolated from Basra and Akko soils sampled from plots irrigated with treated wastewater or freshwater. For the Basra samples, the sorption affinities (K OC values) of all tested compounds were higher for the humin sample isolated from the freshwater-irrigated soil than for that from the wastewater-irrigated soil. The opposite trend was recorded for the Akko humin samples. Desorption hysteresis was observed with atrazine but not with chlorotoluron or phenanthrene. Lipid extraction significantly decreased phenanthrene sorption affinities (K OC values) for both Basra humin samples. For the humin isolated from the Akko freshwater-irrigated sample, lipid removal significantly decreased the sorption linearity, but sorption affinity was less affected. For the humin isolated from the Akko wastewaterirrigated sample, lipid extraction did not affect sorption affinity or linearity. Our data suggest that humin lipids constitute a powerful sorption domain and probably block pores or other high-energy adsorption sites within the humin matrix; therefore, their removal promotes desorption hysteresis. Our data show significantly small differences in sorption affinity between the humin and the source soil samples. We therefore suggest that the role of humin as a natural sorbent in soils should be studied on humin samples isolated under mild conditions to avoid the removal of matrix components and alteration of the humin Elsevier B.V. All rights reserved. 1. Introduction Agricultural soils play an important role in controlling the fate of organic compounds introduced into the environment due to cultivation. The fate (e.g., transport, accumulation, degradation, transformation, evaporation) and activity (e.g. toxicity, bioavailability) of soil organic compounds are mainly influenced by the soil sorption potential. This soil property is primarily regulated by both the level and the physico-chemical nature of the soil organic matter (SOM) (Huang and Weber, 1997; Chiou et al., 1998). Therefore, knowledge of the effect of human activities such as irrigation with reclaimed wastewater on the level and sorptive properties of SOM in agricultural soils is important. SOM is a complex mixture of molecules with varying physicochemical properties, chemical structures, sizes and functionalities. Traditionally, alkaline solution (0.1 M NaOH) is used to extract and separate SOM into humic acid, fulvic acid and humin fractions on the basis of their solubility (Stevenson, 1994). The role of humic and fulvic acids as natural sorbents for hydrophobic organic compounds (HOCs) Corresponding author. Fax: address: chefetz@agri.huji.ac.il (B. Chefetz). has been extensively studied (Gauthier et al., 1987; Kopinke et al., 2001; Xing, 2001; Marschner et al., 2005; Simpson, 2006; Yu et al., 2006; Wen et al., 2007). To date, little information is available on the sorptive capabilities of the humin fraction, even though it typically represents between 20 and 50% of the SOM (Rice and MacCarthy, 1989; Stevenson, 1994; Rice, 2001; Yang et al., 2004; Nichols and Wright, 2006) and is known to tightly bind many HOCs (Nearpass, 1976; Chiou et al., 2000). Humin is defined as the humus fraction which is insoluble in aqueous solution at any ph. Operationally, humin is the residual organic matter remaining in the soil after removal of humic and fulvic acids (Stevenson, 1994; Rice, 2001). To separate humin from the inorganic minerals in soils, samples are treated with HF (Stevenson, 1994). Alternatively, Rice and MacCarthy (1989) used the methyl isobutyl ketone (MIBK) method to isolate humin. However, these methods affect the physico-chemical nature of the isolated humin due to its separation from the mineral components, or to the harsh acid treatment that can hydrolyze specific components in the humin. The objective of this study was to evaluate the role of humin as a natural sorbent in soils irrigated with treated wastewater as compared to soils irrigated with freshwater. To meet this goal, the sorption desorption behaviors of non-polar (phenanthrene) and low-polar (atrazine and /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.geoderma

2 Y. Drori et al. / Geoderma 145 (2008) Table 1 Properties of Basra and Akko freshwater (FW)- and wastewater (WW)-irrigated soils Basra Akko FW WW FW WW Texture Sandy loam Sandy loam Clay Clay (13% clay) (19% clay) (53% clay) (55% clay) Cation-exchange capacity 12±0.5 14±0.0 27±1.5 29±0.6 (cmol/kg) Specific surface area 47±1.3 72± ± ± 3.9 (m 2 /g) ph 7.9± ± ± ±0.1 Organic carbon (OC) content (%) 0.38± ± ± ±0.03 Humic acid (% of soil OC) 57.3± ± ± ±2.2 Fulvic acid (% of soil OC) 17.3± ± ± ±2.3 Humin (% of soil OC) 25.4± ± ± ±0.3 Humin lipids (% of humin OC) 10.3± ± ± ±0.0 ±standard deviations. chlorotoluron) compounds were studied with near-natural humin; HF- and solvent-extraction treatments were not applied, in order to avoid destruction of the humin mineral matrix. An alternative, milder method of humin isolation was used. 2. Materials and methods 2.1. Soils Soils were sampled from a citrus orchard in Basra, Israel, and an avocado orchard in Akko, Israel. The soils (Rhodoxeralf from Basra and Fluvent from Akko) were collected from two nearby plots, one which had been irrigated with treated wastewater (for more than 25 and 10 years in Basra and Akko, respectively) and the other with freshwater. The top organic layer (0 3 cm) was removed and samples were collected from depths of 3 to 30 cm. For each of the four locations (Basra and Akko freshwater- and wastewater-irrigated plots), mixed samples were prepared from five sub-samples. The soils were airdried and sieved through a 2-mm sieve. The bulk properties of the studied soils were measured by the standard soil-testing methods (Sparks et al., 1996). Selected properties of the studied soils and the irrigation water are presented in Tables 1 and 2, respectively Humin isolation Humic and fulvic acids were removed from the studied soils according to the procedure described by Swift (1996). Briefly, soils were treated with 1 M HCl to remove CaCO 3, and then agitated with 0.1 M NaOH under a N 2 atmosphere for 24 h to extract and remove humic and fulvic acids. The NaOH treatment was repeated five times, until the supernatant was colorless. Then, the residual mineral humin matter was further washed with distilled water to reduce the electrical conductivity of the supernatant to less than 0.05 ds/m and to increase the ph to values higher than 5.5. Samples were lyophilized and the organic carbon (OC) content in the humin samples was measured with an elemental analyzer (Thermo-Finnigan). The OC content of the humin samples was 0.1 and 0.09% for Basra freshwaterand wastewater-irrigated soils, respectively and 0.59 and 0.28% for Akko freshwater- and wastewater-irrigated soils, respectively. To study the effects of lipid removal on the sorption ability of the humin matrix, free lipids were extracted from the humin sub-samples by Soxhlet extraction with benzene:methanol (3:1, v/v) for 16 h (Drori et al., 2006) Batch sorption desorption experiments Selected chemical and physical properties of the studied sorbates are presented in Table 3. Aqueous solutions of atrazine and chlorotoluron (98% purity, Agan Co., Ashdod, Israel) were prepared in a background solution containing 10 mm CaCl 2 and 100 mg/l HgCl 2 to maintain a constant ionic strength and to inhibit microbial activity during the experiment (Chefetz et al., 2006). Aqueous solutions of phenanthrene (N96%, Sigma, St. Louis, MO) were prepared by adding aliquots from a concentrated HPLC-grade methanol stock solution to a background solution. Methanol concentration was maintained at less than 0.01% (v/v) to avoid co-solvent effects. Atrazine solutions (0.2 to 20 mg/l) were added to humin samples (2 3 g) pre-weighed into 30- ml Teflon centrifuge tubes (Nalgene, Rochester, NY) with Teflon caps. Chlorotoluron solutions (0.25 to 45 mg/l) were added to 30-mL glass centrifuge tubes (Kimble, Vineland, NJ) with Teflon caps, containing 2 g of humin samples. Phenanthrene solutions (0.5 to 900 μg/l) were added to 200 mg of humin samples pre-weighed into 30-mL glass centrifuge tubes with Teflon caps. In all cases, the sorbent weight was set to maintain 30 to 70% sorption. The tubes (three replicates and a blank for each concentration) were agitated end-over-end in the dark (25 C) at 150 rpm until equilibrium was reached. Sorption desorption equilibrium time was determined for each sorbent sorbate pair in preliminary experiments conducted for 30 d. For Basra humin, sorption equilibrium time was 14, 7 and 14 d for atrazine, chlorotoluron and phenanthrene, respectively; for Akko humin samples, sorption equilibrium time was 21, 4 and 7 d, respectively. The tubes with the equilibrated samples were centrifuged, and 50% of Table 2 Analysis of the freshwater and wastewater used for irrigation in the field plots at Basra and Akko Basra Akko Fresh water Reclaimed wastewater Fresh water Reclaimed wastewater Average Min. Max. Average Min. Max. Average Min. Max. Average Min. Max. Electrical conductivity (ds/m) ph COD (mg/l) BOD (mg/l) TSS (mg/l) Sodium adsorption ratio (cmol /L) N NO 3 (cmol/l) N NH + 4 (cmol/l) P (cmol/l) Cl (cmol/l) HCO 3 (cmol/l) Na + (cmol/l) Ca 2+ and Mg 2+ (cmol/l) K + (cmol/l) The data represent average, maximum and minimum of samples taken between 1998 and 2008.

3 100 Y. Drori et al. / Geoderma 145 (2008) Table 3 Selected properties of the sorbates used in this study Atrazine Chlorotoluron Phenanthrene Formula C 8 H 14 ClN 5 C 10 H 13 ClN 2 O C 14 H 10 Structure Water solubility (mg/l) log K OW pka Vapor pressure (kpa) the supernatant was removed using a glass pipette and replaced with fresh sorbate-free background solution. Then the tubes were further agitated under the same conditions to perform desorption for the time determined in the kinetics tests. Three sequential desorption steps were performed. An aliquot of the supernatant solution at every sorption or desorption step was used to determine solution concentration in an L-7100 LaChrom HPLC (Merck-Hitachi, Darmstadt, Germany) with a LiChrospher RP-18 column (25 cm 4.6 mm, 5 μm). An isocratic mobile phase of water/acetonitrile was applied (30/70 for atrazine, 40/ 60 for chlorotoluron and 15/85 for phenanthrene). Atrazine and chlorotoluron were detected using a photodiode array detector at 222 and 248 nm, respectively. Phenanthrene was detected by fluorescence detector (excitation at 244 nm and emission at 360 nm). All sorbates were quantified using external standards. All the target analytes exhibited negligible (b0.5%) sorption to the tubes or loss due to volatilization. Mass-balance analysis of the tested analytes after the last desorption step exhibited between 86 and 100% recovery for the lowest and highest concentrations used, respectively Data analysis The Freundlich parameters (K F and N) were calculated using the Freundlich equation: q=k F C e N, where q is the total sorbed amount (mg/kg), C e is the analyte equilibrium concentration (mg/l), K F (mg/ kg)(mg/l) N is the Freundlich distribution coefficient, and N is a Fig. 1. Atrazine sorption desorption isotherms with humin samples isolated from Basra and Akko freshwater- and wastewater-irrigated soils. Filled symbols are for sorption data, open symbols are for desorption data; bars represent standard errors.

4 Y. Drori et al. / Geoderma 145 (2008) Fig. 2. Chlorotoluron sorption desorption isotherms with humin samples isolated from Basra and Akko freshwater- and wastewater-irrigated soils. Filled symbols are for sorption data, open symbols are for desorption data; bars represent standard errors. correction factor. Values for the carbon-normalized Freundlich distribution coefficient (K F OC) were calculated by normalizing K F to the OC content of each sorbent. Distribution coefficients (K D ) and carbon-normalized distribution coefficients (K OC ) were calculated at C e /S w values of 0.01, 0.05 and 0.20 (S w is the analyte's aqueous solubility). The ratio of the Freundlich exponents for desorption and sorption isotherms was calculated and used as a desorption hysteresis index (HI), with lower index values indicating increased difficulty of the sorbed analyte to desorb from the matrix (Gunasekara and Xing, 2003). Statistical analysis (All Pairs, Tukey Kramer, α=0.05) was performed with JMPIN software, version (SAS Institute Inc., Cary, NC). 3. Results and discussion Humin content was higher in the freshwater-irrigated samples than in the wastewater-irrigated samples of both soils. Humin made up 25 and 18% of the total OC amount of the Basra freshwater- and wastewater-irrigated soils, respectively (Table 1). A higher amount of humin and more pronounced differences between samples were measured for the Akko site, where humin made up 60 and 28% of the total OC of the freshwater- and wastewater-irrigated soils, respectively. The measured humin amounts were within the typical range reported for various soils and sediments (Kohl and Rice, 1998; Rice and MacCarthy, 1989; Stevenson, 1994; Yang et al., 2004). Akko freshwater-irrigated soil exhibited the lowest amount of humic acid (22%) and the highest amount of humin, whereas the content of fulvic acid was similar to that in the other tested soils (18%). Humin samples were also analyzed for their lipid content. Lipids made up 10 and 8% of the total OC in the humin samples isolated from Basra freshwater- and wastewater-irrigated soils, respectively. In the Akko samples, lipid concentrations were 1.5 and 10% of the humin OC in the freshwaterand wastewater-irrigated soils, respectively. Tremblay et al. (2005) reported lipid contents amounting to 5% of the total humin OC. Kohl and Rice (1998) reported bound lipids constituting 7 to 9% OC of the different humin samples. It is worth noting that humin lipids made up only 21 and 7% of the total extractable lipids in Basra freshwater- and wastewater-irrigated soils, respectively (Drori et al., 2006). This suggests that most of the extractable lipids in Basra soils were removed from the soil during the alkaline extraction Sorption to humin samples Sorption isotherms of atrazine, chlorotoluron and phenanthrene to humin samples are presented in Figs. 1 3, respectively. With the Basra samples, the OC-normalized coefficients (K OC, Table 4) ofalltested compounds were higher in the humin sample isolated from the freshwater-irrigated soil than in that from the wastewater-irrigated soil. An opposite trend was observed with Akko humin samples: higher K D and K OC values were observed for the humin from the wastewaterirrigated soil as compared with that from the freshwater-irrigated soil. The K OC values exhibited for the humin samples were much higher than those recorded for the bulk soils (Drori et al., 2005, 2006), a trend which has been reported in several studies (Gunasekara and Xing, 2003; Kang

5 102 Y. Drori et al. / Geoderma 145 (2008) Fig. 3. Phenanthrene sorption desorption isotherms with humin samples isolated from Basra and Akko freshwater- and wastewater-irrigated soils. Filled symbols are for sorption data, open symbols are for desorption data; bars represent standard errors. and Xing, 2005; Oren and Chefetz, 2005; Bonin and Simpson, 2007). It was concluded that the high sorption to humin is related to its extraction procedure. In soil, humin is the organic matter complexed to mineral surfaces (Rice, 2001) and it is surrounded by humic and fulvic acids, making it less accessible as a sorbent. Therefore, when the humic and fulvic fractions are removed and the minerals destroyed, greater sorption to humin is expected. It is important to note that in our study, the higher sorption with the humin samples was less significant than that reported by Gunasekara and Xing (2003) and Bonin and Simpson (2007). This is probably because we did not use HF treatment and therefore did not alter the humin mineral complexes. In addition to the higher sorption affinity with the isolated humin samples, their sorption isotherms exhibited a higher degree of nonlinearity than the isotherms measured with the bulk soils (Drori et al., 2005, 2006). Atrazine exhibited Freundlich N values of 0.89 and 0.88 with the bulk Basra soils whereas N values of 0.73 and 0.79 were obtained for the humin samples. Chlorotoluron exhibited Freundlich N values of 0.9 and 0.86 with the bulk Basra soils whereas N values of 0.73 and 0.8 were obtained for the humin samples. The largest differences between the soil and corresponding humin were obtained with phenanthrene. In this case, Freundlich N values decreased from 0.91 and 0.87 with the bulk Basra soil samples to 0.61 and 0.75 with the humin samples isolated from the freshwater- and wastewater-irrigated soils, respectively. It has been suggested that the lower degree of isotherm linearity obtained with humin samples is related to the more condensed and rigid structure of the humin organic matter relative to the bulk SOM (Gunasekara and Xing, 2003; Kang et al., 2003; Kang and Xing, 2005; Simpson et al., 2007). High sorption affinity and a nonlinear sorption isotherm are typical for condensed or glassy organic matter with specific sorption sites(pignatello and Xing, 1996; Huang and Weber, 1997; Xing and Pignatello, 1997). The high sorption capability and the nonlinear isotherms obtained with the humin samples emphasize the importance of humin as a natural sorbent for HOCs in soils (Xie et al., 1997; Chefetz et al., 2000; Xing, 2001). However, it is important to reiterate that although the humin samples exhibited high sorptive capacity, it is suggested that in the bulk soil, humin presents fewer available sorption sites due to its association with minerals and its interactions with the humic and fulvic acids. Although with the humin samples all sorbates exhibited lower N values than with the corresponding untreated soils, different trends were observed between the soils. In the Basra soil, lower N values were obtained with the humin sample isolated from the freshwaterirrigated soil; in contrast, in the Akko humin samples (except for chlorotoluron), lower N values were obtained with the sample isolated from the wastewater-irrigated soil. These differences, together with the opposite trend in sorption affinities between the humin isolated from the freshwater- and wastewater-irrigated samples of the two soils (Basra and Akko), suggest that long-term irrigation with treated wastewater affected the sorptive properties of the humin in the two studied sites in opposite ways. This might relate to the different properties of the tested soils (e.g., clay versus sandy) and qualitative differences in the wastewater for irrigation (Table 2) Phenanthrene sorption to lipid-extracted humin samples Lipids are one of the SOM fractions that play an important role in HOC sorption. They can act as an absorbent domain for HOCs or they

6 Y. Drori et al. / Geoderma 145 (2008) Table 4 Organic carbon-normalized sorption parameters for atrazine, chlorotoluron and phenanthrene with humin samples from Basra and Akko freshwater (FW)- and wastewater (WW)-irrigated soils FW Basra WW FW Akko WW Atrazine K F OC (mg/kg) (mg/l) N 920 (12) 390 (43) 190 (0) 350 (3) r N 0.73 (0.02) 0.79 (0.05) 0.89 (0.01) 0.76 (0.03) K OC (L/kg) K OC (L/kg) K OC (L/kg) Chlorotoluron K F OC (mg/kg) (mg/l) N 1550 (30) 650 (11) 270 (16) 900 (29) r N 0.73 (0.02) 0.80 (0.01) 0.81 (0.02) 0.83 (0.01) K OC (L/kg) K OC (L/kg) K OC (L/kg) Phenanthrene K F OC (mg/kg) (mg/l) N 31,050 (670) 33,560 (3750) 19,140 (1370) 18,490 (630) r N 0.61 (0.01) 0.75 (0.00) 0.90 (0.02) 0.75 (0.02) K OC (L/kg) 187, ,140 30,340 58,480 K OC (L/kg) 99,870 70,980 25,830 39,1108 K OC (L/kg) 53,310 47,470 21,990 26,150 Phenanthrene with lipid-extracted humin K F OC (mg/kg) (mg/l) N 17,260 (1450) 15,560 (960) 10,510 (120) 17,170 (3090) r N 0.57 (0.03) 0.71 (0.05) 0.77 (0.02) 0.74 (0.03) K OC (L/kg) 123,660 58,660 29,670 56,950 K OC (L/kg) 62,140 36,910 20,640 37,450 K OC (L/kg) 31,230 23,230 14,360 24,630 Standard deviation values are presented in parentheses. calculated at C e /S w = 0.01, where C e and S w are the equilibrium concentration and aqueous solubility, respectively. C e /S w = C e /S w =0.2. can compete with HOCs for sorption sites. The lipid level in our humin samples (Table 1) was higher than the level (5%) reported for soils (Stevenson, 1994). Therefore, we speculated that lipids would have a major effect on the overall sorption behavior of HOCs with humin, and that lipid removal would enable us to reveal the sorptive properties of the core humin. In the Basra samples, lipid extraction significantly decreased phenanthrene sorption affinities, whereas sorption linearity (N value) was only slightly decreased (Table 4). For the humin isolated from the freshwater-irrigated sample, K F OC values decreased by 44% due to lipid removal. A larger decrease (54%) was obtained for the lipid-extracted humin from the counterpart wastewater sample. This suggests that the extracted lipids constitute a significant sorption domain within the humin structure. It is important to note that lipid removal resulted in a decrease in sorption affinity that was much larger than the relative proportion of lipids in the Basra humin samples (8 10% of the humin OC). Moreover, lipid removal increased the differences in sorption affinity between the freshwater- and wastewater-irrigated samples. The K OC (at C e /S w =0.01) ratio between the humin from the freshwater- and wastewater-irrigated soils was 1.76 for the bulk humin and 2.1 for the lipid-extracted humin samples. Although lipid removal affected the sorption affinity, it did not significantly affect the heterogeneity of the sorption process (minor effects on sorption linearity). In the Akko humin samples, lipid removal affected phenanthrene sorption differently with the different humin samples. Although lipids made up only 1.5% of the OC in the humin isolated from the Akko freshwater-irrigated sample, their removal significantly decreased the sorption-isotherm linearity (N value decreased from 0.90 to 0.77), but sorption affinity was less affected. In the humin isolated from the Akko wastewater-irrigated sample, lipids made up 10% of the total OC but did not affect sorption affinity or linearity. Kohl and Rice (1999) and Tremblay et al. (2005) reported that lipid removal from soils and humin samples decreases the sorption nonlinearity and significantly increases the sorption affinity of phenanthrene. Similarly, Wang and Xing (2007) reported that lipid removal increases sorption affinities of phenanthrene by humic acids and humin. It was concluded that lipids compete with phenanthrene for specific sorption sites. In our previous study with the bulk Basra soils (Drori et al., 2006), we reported a moderate increase (25%) in phenanthrene sorption affinity due to lipid removal in the freshwaterand wastewater-irrigated soils. We concluded that in the bulk soils (used to isolate humin in this study), the partitioning domains for phenanthrene were not a limiting parameter for phenanthrene binding and therefore phenanthrene sorption was not significantly affected by lipid removal. Moreover, we reported that lipids were better competitors for sorbates capable of polar interactions than for phenanthrene. In this study with Basra humin samples, lipid removal significantly decreased sorptive binding of phenanthrene. Therefore, we conclude that lipids in the Basra humin samples act as sorption domains (sorbents) for phenanthrene and not as competitive agents. Since sorption linearity was only slightly decreased by lipid removal, we assume that these lipids contain heterogeneous sorption sites similar to the bulk humin. In addition, lipids were not a major factor in the higher sorption ability observed for the humin isolated from the freshwater-irrigated soil. In the Akko wastewater-irrigated humin sample, lipids (10% of the total humin OC) were poor sorption agents since their removal only slightly affected the OC-normalized sorption affinity and/or linearity (Table 4). This suggests that these lipids were in a crystalline form having limited sorption capability, similar to cuticular waxes (Shechter et al., 2006). The opposite trends observed in our study (lipid removal decreases K OC ) vs. that of Wang and Xing (2007) (lipid removal increases K OC ) are probably due to the different origin and nature of the samples. In this study, lipids were removed from humin, which represents a highly stable, recalcitrant and highmolecular-weight fraction of SOM, isolated from well-developed soils, whereas in the report by Wang and Xing (2007), lipids were extracted from a peat soil which is considered a geologically young material. Therefore, the natures of the core sorbents (lipid-extracted materials) and lipids were different, and they therefore reacted differently (as sorption domain or as competitor for phenanthrene). Removal of the lipids from the Akko freshwater-irrigated humin sample (1.5% of the total humin OC) significantly increased the heterogeneity of the sorption domains. The decrease in sorption linearity after their removal suggests that the lipids were acting as a high-affinity partitioning domain. These differences in the nature of the lipids resulted in an increase in the differences in the sorptive properties of the Akko humin samples after their removal. The total level of lipids extracted from the humin samples was much lower than that extracted from the bulk soils (Drori et al., 2006). Therefore, most of the lipids were removed from the soil during the alkaline extraction with the humic and fulvic acids. These findings are in line with the relatively polar character of the lipids reported in our previous study (Drori et al., 2006), and suggest that the humin lipids have a much more hydrophobic character than the bulk lipids and can therefore act as an efficient sorption domain in the humin Desorption Desorption behavior can provide additional information on sorbate sorbent interactions. In the current study, we examined the desorption isotherms of atrazine, chlorotoluron and phenanthrene with the humin samples (Figs. 1 3, respectively). Desorption

7 104 Y. Drori et al. / Geoderma 145 (2008) hysteresis of atrazine was observed with all tested humin samples (Fig. 1). Sorption desorption hysteresis can be explained by the dual mode model (Huang and Weber, 1997) which suggests that organic sorbents consist of an expanded (rubber-like) domain which promotes linear and reversible sorption isotherms, and a condensed (glass-like) domain that facilitates nonlinear isotherms and promotes desorption hysteresis (Weber et al., 1998). In our study, atrazine sorption isotherms were nonlinear (with N values of ) and desorption isotherms exhibited increasing hysteresis with decreasing atrazine concentration. For example, the calculated HI values decreased (i.e., increasing desorption hysteresis) from 0.78 and 0.72 to 0.17 and 0.12 with decreasing atrazine concentration in humin samples from the freshwater- and wastewater-irrigated Basra soils, respectively. This suggests that atrazine interacts with humin via hydrophobic and specific interactions. The latter binding governs the interactions at low concentrations and therefore results in a lower desorption ability. The observed desorption trend can be explained by a limited number of available high-energy sites in the humin samples: these sites are occupied at low solute concentration, whereas at high solute concentration, more molecules are taken up by low-energy binding sites and therefore can readily desorb. Similar findings (i.e., desorption hysteresis and increasing hysteresis with decreasing solute concentrations) were observed with atrazine and the bulk soils (Drori et al., 2005). However, the extent of the desorption hysteresis was much more pronounced (especially at high atrazine concentration) with the bulk soils, suggesting that humic acid is probably the SOM fraction responsible for the low desorption ability of atrazine in these soils. With the Basra samples, sorption desorption hysteresis of atrazine was similar for the two humins. However with the Akko samples, desorption isotherms for the freshwater-irrigated humin sample exhibited higher hysteresis than for the wastewater-irrigated sample. This was especially pronounced at high atrazine concentration. In this case, the calculated HI values for high and low atrazine concentrations were 0.3 and 0.24, respectively, with humin from the freshwater-irrigated soil, and 0.5 and 0.34, respectively, with humin from the wastewater-irrigated soils. In contrast to the pronounced hysteresis observed with atrazine, chlorotoluron did not exhibit desorption hysteresis. Statistically, all desorption isotherms were similar to the source sorption isotherm. Spurlock (1995) reported that specific interactions between phenylurea herbicides and SOM are dominant at low sorbed-phase concentrations but become less important relative to nonspecific interactions with increasing concentrations. This mechanism, as was observed with atrazine, results in desorption hysteresis. The nonhysteretic behavior of chlorotoluron with humin in our study suggests that chlorotoluron molecules have a lower ability to form specific bonds with the humin, as indicated by their lower H-bonding energy relative to atrazine (Chefetz et al., 2004). Since chlorotoluron is less polar than atrazine, it probably readily interacts with the less polar and more hydrophobic sorption domains of the humin. Phenanthrene exhibited interesting desorption behavior (Figs. 3 and 4). With the Basra humin samples (Fig. 3), phenanthrene exhibited fully reversible isotherms, even though sorption isotherms were nonlinear with N values of 0.61 and Statistically, the desorption isotherms were similar to the source sorption isotherms; the calculated K F OC values for the desorption isotherms were 30,340 and 33,640 (mg/kg)(mg/l) N, Fig. 4. Phenanthrene sorption desorption isotherms with lipid-extracted humin samples isolated from Basra and Akko freshwater- and wastewater-irrigated soils. Filled symbols are for sorption data, open symbols are for desorption data; bars represent standard errors.

8 Y. Drori et al. / Geoderma 145 (2008) respectively. This suggests that the nonlinear sorption obtained with the humin samples does not necessarily result from a hole-filling mechanism. It could result from a surface interaction, the surface-sorbed molecules probably desorbing more readily. Similarobservations of nonlinear sorption and negligible desorption hysteresis have been reported for phenanthrene with peat (Ran et al., 2002) and sediment humin (Oren and Chefetz, 2005). With the Akko humin samples (Fig. 3), sorption desorption hysteresis was observed only at the low solute concentrations (HI values were 0.02 and 0.06). In general, the desorption isotherms of the humin samples were similar for the freshwater- and wastewater-irrigated soils. However, a different desorption behavior was observed for phenanthrene with the lipid-extracted humin samples (Fig. 4). Although lipid level was similar in both Basra humin samples (8 and 10%), lipid removal resulted in more pronounced desorption hysteresis with the humin sample from the wastewater-irrigated soil, whereas only minor effects were observed with its freshwater-irrigated counterpart. For the high solute concentration, the HI values were 0.35 and 0.86 for the humin samples, respectively. Lipid removal from the Akko samples significantly enhanced desorption hysteresis. In this case, HI values were between 0.11 and This suggests that the nature of the lipids in the two systems (Basra humin from the freshwater- and wastewater-irrigated soils) is different. In addition to their role as sorbents, lipids in the humin from the Basra wastewater-irrigated soil were probably blocking or competing for high-energy sorption sites. Their removal therefore made those sites accessible and facilitated desorption hysteresis. When lipids were removed, phenanthrene molecules could penetrate into the newly exposed pores within the more condensed humin or humin mineral structure. In this environment, solute molecules are less likely to desorb to the aqueous phase under the same conditions as in the sorption step (Weber et al., 1998). Humin lipids in the freshwater-irrigated sample were probably acting as a separate sorption domain, and their removal therefore did not affect phenanthrene desorption behavior with the core humin sorption domain. In both Akko humin samples, lipid removal significantly increased the desorption hysteresis, suggesting that lipids probably prevent phenanthrene molecules from penetrating into the pores within the humin or humin mineral structure. 4. Conclusions Our aim here was to study the role of humin as a natural sorbent in agricultural soils and to evaluate the effects of irrigation with treated wastewater on the sorptive properties of this SOM fraction. Our data show opposite trends for the soils from the two studied sites. With the Basra samples, higher sorption affinity was recorded with the humin isolated from the freshwater-irrigated soil, whereas with the Akko samples, higher HOC affinity was measured with the humin isolated from the wastewater-irrigated soil. Moreover, with the Basra samples, lipid removal decreased the sorption affinity of phenanthrene, whereas with Akko samples, lipid removal did not affect phenanthrene sorption affinities (K OC ). The difference in the observed trends might relate to the different properties of the tested soils (e.g., clay versus sandy), the different periods of wastewater irrigation (10 versus more than 25 years) and the different quality of the irrigation water (Table 2). We believe that the nature and properties of the humin at the two soil sites differ and the irrigation with wastewater therefore did not have similar effects on the sorptive properties of the humin. In contrast to other reports, our data with the Basra samples showed that lipid removal induces a decrease in the sorption affinity of phenanthrene with the tested humins. This suggests that humin lipids act as a sorption domain, rather than as competitors of phenanthrene. In some cases, these lipids block or hinder the accessibility of phenanthrene molecules to pores or other high-energy adsorption sites. Therefore, lipid removal induces desorption hysteresis. In this study, we used a minimal, mild treatment to isolate the humin, instead of the HF treatment which is widely used to remove soil minerals and to increase OC concentration of the residual humin. Humin samples isolated by HF treatment exhibit significantly higher HOC sorption affinity relative to the original soil. Moreover, the sum of the sorptions to the isolated humic fractions is greater than sorption to the source soil. Our data show significantly smaller differences in sorption affinity between the humins and their respective bulk soil samples. Similar to Bonin and Simpson (2007), we suggest that clay minerals play an important role in HOC sorption to humin samples, and sorption should therefore be measured with humin samples isolated by mild treatment to avoid removal or alteration of the humic-mineral matter complexes. Acknowledgements This research was supported by a research grant from the Israeli Ministry of Agriculture and Rural Development. The Authors thank Anat Lowengart-Aycicegi and Jorge Tarchitzky (The Agricultural Extension Service, Ministry of Agriculture and Rural Development, Israel) for providing the water analysis data. References Bonin, J.L., Simpson, M.J., Variation in phenanthrene sorption coefficients with soil organic matter fractionation: the result of structure or conformation? Environ. Sci. Technol. 41, Chefetz, B., Deshmukh, A., Hatcher, P.G., Guthrie, E.A., Pyrene sorption by natural organic matter. Environ. Sci. Technol. 34, Chefetz, B., Bilkis, Y., Polubesova, T., Sorption desorption behavior of triazine and phenylurea herbicides in Kishon river sediments. Water Res. 38, Chefetz, B., Stimler, K., Shechter, M., Drori, Y., Interactions of sodium azide with triazine herbicides: effect on sorption to soils. Chemosphere 65, Chiou, C.T., McGroddy, S.E., Kile, D.E., Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments. Environ. Sci. Technol. 32, Chiou, C.T., Kile, D.E., Rutherford, D.W., Sheng, G., Boyd, S.A., Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fractions: potential sources of the sorption nonlinearity. Environ. Sci. Technol. 34, Drori, Y., Aizenshtat, Z., Chefetz, B., Sorption-desorption behavior of atrazine in soils irrigated with reclaimed wastewater. Soil Sci. Soc. Am. J. 69, Drori, Y., Lam, B., Simpson, A., Aizenshtat, Z., Chefetz, B., The role of lipids on sorption characteristics of freshwater- and wastewater-irrigated soils. J. Environ. Qual. 35, Gauthier, T.D., Seltz, W.R., Grant, C.L., Effects of structural and compositional variations of dissolved humic materials on pyrene K OC values. Environ. Sci. Technol. 21, Gunasekara, A.S., Xing, B., Sorption and desorption of naphthalene by soil organic matter: importanceof aromaticandaliphaticcomponents. J. Environ. Qual. 32, Huang, W., Weber, W.J., A distributed reactivity model for sorption by soils and sediments. 10. Relationships between sorption, hysteresis, and the chemical characteristics of organic domains. Environ. Sci. Technol. 31, Kang, S., Xing, B., Phenanthrene sorption to sequentially extracted soil humic acids and humins. Environ. Sci. Technol. 39, Kang, S., Amarasiriwardena, D., Veneman, P., Xing, B., Characterization of ten sequentially extracted humic acids and a humin from a soil in western Massachusetts. Soil Sci. 168, Kohl, S.D., Rice, J.A., The binding of contaminants to humin: a mass balance. Chemosphere 36, Kohl, S.D., Rice, J.A., Contribution of lipids to the nonlinear sorption of polycyclic aromatic hydrocarbons to soil organic matter. Org. Geochem. 30, Kopinke, F.-D., Georgi, A., Mackenzie, K., Sorption of pyrene to dissolved humic substances and related model polymers. 1. Structure-property correlation. Environ. Sci. Technol. 35, Marschner, B., Winkler, R., Jödemann, D., Factors controlling the partitioning of pyrene to dissolved organic matter extracted from different soils. Eur. J. Soil Sci. 56, Nearpass, D.C.,1976. Adsorption of picloram by humic acids and humin. Soil Sci.121, Nichols, K.A., Wright, S.F., Carbon and nitrogen in operationally defined soil organic matter pools. Biol. Fertil. Soils 43, Oren, A., Chefetz, B., Sorption desorption behavior of polycyclic aromatic hydrocarbons in upstream and downstream river sediments. Chemosphere 61, Pignatello, J.J., Xing, B., Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30, Ran, Y., Huang, W., Rao, P.S.C., Liu, D., Sheng, G., Fu, J., The role of condensed organic matter in the nonlinear sorption of hydrophobic organic contaminants by a peat and sediments. J. Environ. Qual. 31, Rice, J.A., Humin. Soil Sci. 166, Rice, J.A., MacCarthy, P., Isolation of humin by liquid liquid partitioning. Sci. Total. Environ. 81/82, Shechter, M., Xing, B., Kopinke, F.-D., Chefetz, B., Competitive sorption desorption behavior of triazine herbicides with plant cuticular fractions. J. Agric. Food Chem. 54,

9 106 Y. Drori et al. / Geoderma 145 (2008) Simpson, M.J., Nuclear magnetic resonance based investigations of contaminant interactions with soil organic matter. Soil Sci. Soc. Am. J. 70, Simpson, A.J., Song, G., Smith, E., Lam, B., Novotny, E.H., Hayes, M.H.B., Unraveling the structural components of soil humin by use of solution-state nuclear magnetic resonance spectroscopy. Environ. Sci. Technol. 41, Sparks, D.L., Page, A.L., Helmke, P.A., Loeppert, R.H., Soltanpour, P.N., Tabatabi, M.A., Johnston, C.T., Sumner, M.E., Methods of soil analysis. Part 3. Chemical Methods. Soil Science Society of America Inc., Madison, WI. Spurlock, F.C., Estimation of humic-based sorption enthalpies from nonlinear isotherm temperature dependence: theoretical development and application to substituted phenylureas. J. Environ. Qual. 24, Stevenson, F.J., Extraction, fractionation, and general chemical composition, Humus Chemistry - Genesis, Composition, Reactions. John Wiley & Sons Inc., New York. Swift, R.S., Organic matter characterization. In: Sparks, D.L., et al. (Ed.), Methods of Soil Analysis. Part 3. Chemical Methods. Soil Science Society of America Inc., Madison, WI. Tremblay, L., Kohl, S.D., Rice, J.A., Gagne, J.-P., Effects of lipids on the sorption of hydrophobic organic compounds on geosorbents: a case study using phenanthrene. Chemosphere 58, Wang, X., Xing, B., Roles of acetone-conditioning and lipid in sorption of organic contaminants. Environ. Sci. Technol. 41, Weber, W.J., Huang, W., Yu, H., Hysteresis in the sorption and desorption of hydrophobic organic contaminants by soils and sediments. 2. Effect of soil organic matter heterogeneity. J. Contam. Hydrol. 31, Wen, B., Zhang, J.-J., Zhang, S.-Z., Shan, X.-Q., Khan, S.U., Xing, B., Phenanthrene sorption to soil humic acid and different humin fractions. Environ. Sci. Technol. 41, Xie, H., Guetzloff, T., Rice, J.A., Fractionation of pesticide residues bound to humin. Soil Sci. 162, Xing, B., Sorption of anthropogenic organic compounds by soil organic matter: a mechanistic consideration. Can. Soil Sci. 81, Xing, B., Pignatello, J.J., Dual-mode sorption of low polarity compounds in glassy poly(vinyl chloride) and soil organic matter. Environ. Sci. Technol. 31, Yang, Z., Singh, B.R., Sitaula, B.K., Fractions of organic carbon in soils under different crop rotations, cover crops and fertilization practices. Nutr. Cycl. Agroecosy. 70, Yu, Z., Sharma, S., Huang, W., Differential roles of humic acid and particulate organic matter in the equilibrium sorption of atrazine by soils. Environ. Toxicol. Chem. 25,