Environ. Sci. Technol. 2008, 42, 4797–4803
Interactions of Hydrophobic Fractions of Dissolved Organic Matter with Fe3+- and Cu2+-Montmorillonite TAMARA POLUBESOVA, YONA CHEN, ROTEM NAVON, AND BENNY CHEFETZ* Department of Soil and Water Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
Received February 5, 2008. Revised manuscript received April 2, 2008. Accepted April 3, 2008.
Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu2+- and Fe3+montmorillonite. Adsorption of both samples was higher on Fe3+montmorillonite than on Cu2+-montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe3+-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe3+montmorillonite surfaces. Adsorption of HoA on Cu2+montmorillonite and HoN on Fe3+- and Cu2+-montmorillonites wasgovernedmainlybyvanderWaalsinteractions.Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe3+-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe3+-montmorillonite similar to goethite.
Introduction
Experimental Section
Dissolved organic matter (DOM) significantly affects solubility, adsorption, and transport of metal ions and organic compounds in the environment (1, 2). Therefore interactions of DOM with clay minerals can affect the fate and reactivity of metal ions and organic pollutants in soils. The main polyvalent cations responsible for the binding of DOM to clays are Ca2+, Cu2+, and Fe3+ (3, 4). Due to heavy anthropogenic impact, Cu2+ and Fe3+ are significant components of the soil organo-clays systems (5, 6). Clay surfaces containing Cu2+ and Fe3+ can result in changes of the adsorption and mobility of organic pollutants in soils and they were found to catalyze the formation of humic and fulvic structures (7). The importance of transition cations for the adsorption of aromatic compounds by mineral surfaces was emphasized by Zhu et al. (8). Functional groups of organic compounds may serve as electron donors and share electrons with transition metal cations (9). The major mechanisms by which DOM adsorbs on mineral oxide surfaces are ligand-exchange-surface complexation, van der Waals interactions, hydrogen bonding, protonation, water-bridging, and cation-bridging (10–12). Studies on DOM interactions with mineral surfaces dem* Corresponding author tel: + 972 (8) 948-9384; fax: + 972 (8) 947-5181; e-mail:
[email protected]. 10.1021/es8003602 CCC: $40.75
Published on Web 05/21/2008
onstrated that adsorptive fractionation of DOM was dependent on the properties of mineral surfaces (13). Iron oxides exhibited a preferential uptake of the high molar mass DOM fractions which were enriched with aromatic and carboxylate groups. Carboxylate groups of the DOM can undergo ligand exchange with surface hydroxyl groups of metal (hydr)oxides to form stable mineral-DOM bonds (13–16). Guo and Chorover (17) reported a retardation of hydrophobic DOM fractions with high molecular weight and high molar absorptivity due to the presence of iron oxides. With respect to montmorillonite, Specht et al. (18) suggested that highmolecular-weight hydrophobic fractions are preferentially adsorbed on mineral surface. Contrary to these conclusions, Chorover and Amistadi (13) showed selective uptake of lowermolecular-weight DOM by montmorillonite without any selectively for aromatic compounds. It was suggested, in accordance with Baham and Sposito (19), that cation- and water-bridging were the dominant mechanisms for DOM retention by montmorillonite. The DOM interactions with smectite surfaces saturated with transition metals are not fully understood. A major obstacle for understanding these interactions has resulted from the fact that the DOM consists of a mixture of compounds (proteins, sugars, humic substances, etc.) exhibiting different physicochemical properties. Hence, fractionation is essential for better insight into the mechanism of DOM binding to clays. The objective of the current research was to elucidate the mechanisms of interactions of the major hydrophobic fractions of DOM (HoA and HoN), with a model systems: montmorillonite enriched with Cu2+ and Fe3+. In our previous studies (20, 21) we emphasized the significant role of these fractions as natural sorbents for organic contaminants. DOM behavior and its environmental fate and impact can be better understood and predicted on the basis of knowledge of the HoA and HoN interactions with clays, and particularly with montmorillonite, which is a major component of clay minerals in soils of arid and semiarid zones of the world (22, 23). Our major hypothesis was that DOM will react with Fe3+ montmorillonite similar to goethite.
2008 American Chemical Society
DOM Preparation. DOM was extracted from composted biosolids (Shacham Givat Ada, Israel). Aqueous DOM extracts were prepared by shaking the compost with distilled water (1:10, solid/water) for 2 h at room temperature. The suspension was centrifuged and the supernatant was filtered through a 0.45-µm membrane. The DOM in the solution was further fractionated to HoA and HoN fractions (20, 21, 24). In brief, the DOM solution was acidified to pH 2.0 with 6 M HCl and loaded onto a column containing Supelite DAX-8 resin (Supelco, Bellefonte, PA). The HoA fraction was displaced from the resin with 0.1 M NaOH. The HoN fraction was desorbed from the resin by Soxhlet extraction with methanol. A portion of the DOM solution was concentrated using a Prep/Scale ultrafiltration system (Millipore Corp., Billerica, MA) with a molecular weight cutoff of 1000 Da. The DOM >1000 Da was fractionated using the abovementioned procedure to obtain the HoA > 1000 Da and HoN > 1000 Da samples. Humic acid (HA) and fulvic acid (FA) were extracted according to standard protocol of the International Humic Substances Society (25) from the same compost as the DOM. DOM Characterization. The freeze-dried samples (HoA, HoN, FA, and HA) were analyzed in triplicate for C, H, N, and S contents with a FLASH EA 1112 elemental analyzer (Thermo Fisher Scientific, The Netherlands). The total acidity as well as phenolic and carboxylic contents of the samples were VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of the Studied Samples: Hydrophobic Acid and Neutral (HoA and HoN, Respectively) DOM Fractions and Humic and Fulvic Acids (HA and FA, Respectively) (%)
HoA HoN HA FA
atomic ratio
mmol/g
C
H
N
H/C
C/N
Oa/C
carboxyl groups
phenol groups
total acidity
53.64 50.50 55.09 41.05
6.02 6.44 6.30 4.77
4.37 5.58 6.61 5.54
1.35 1.53 1.37 1.39
14.32 10.56 9.72 8.65
0.47 0.54 0.42 0.86
3.62 2.19 2.02 6.06
0.37 0.55 0.85 2.53
3.99 2.74 2.87 8.59
a O content was calculated as a difference between the ash-free DOM amount (100%) and percentages of C, N, and H (the amount of S was negligible).
determined using a titration procedure (25). Elemental composition and acidity data are presented in Table 1. Fourier transform infrared (FTIR) analysis was conducted with the initial HoA and HoN, and unbound samples after interactions with Fe3+-montmorillonite. Freeze-dried samples (2 mg) were finely ground and mixed with 98 mg of KBr (IR-grade; Sigma) and compressed into pellets. FTIR absorption spectra (40 scans) were obtained for a wavenumber range of 4000 to 400 cm-1 on a Nicolet 6700 FT-IR (Thermo Scientific, MA) using the OMNIC 7.3 software. Preparations of Clay Sorbents. Wyoming Na-montmorillonite SWy-1 was obtained from the Source Clays Repository (Clay Minerals Society, Columbia, MO). The cation exchange capacity of the clay was 80 cmol/kg consisting of 52% of Ca2+, 26% of Na+, 21% of Mg2+, and 1% of K+ (26). The Cu2+-montmorillonite > crude montmorillonite (Figure 1). We suggest that the observed trend is a result of the combined effects of valency of the exchangeable cation and the low pH of the Fe3+-montmorillonite suspension. The pH values of the blank clay suspensions were 3.95 ( 0.07, 5.76 ( 0.08, and 7.07 ( 0.05 for the Fe3+-, Cu2+-, and crude montmorillonite, respectively. The increase in adsorption of DOM on iron oxides with decrease in pH was demonstrated 4798
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FIGURE 1. Adsorption of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions on Fe3+-, Cu2+-, and crude montmorillonite. The initial pH values of clay suspensions were 3.95 ( 0.07, 5.76 ( 0.08, and 7.07 ( 0.05, respectively. Followed by adsorption of HoA on the studied clays pH changed to 5.03 ( 0.04, 6.21 ( 0.07, and 6.96 ( 0.05, respectively. Followed by adsorption of HoN the pH changed to 4.04 ( 0.06, 5.87 ( 0.07, and 7.1 ( 0.06, respectively. previously (29–32). It was suggested that carboxyl and hydroxyl functional groups can form stable complexes with metal cations with a stronger effect of Fe3+ than that of divalent cations (33, 34). Adsorption isotherms of HoA (Figure 1) on Fe3+- and crude montmorillonite samples were fitted by the Langmuir equation (Table 2). The Langmuir binding coefficient (KL) and maximal adsorption capacity (Smax) for Fe3+-montmorillonite were significantly higher than those calculated for crude montmorillonite (Table 2). This can be explained by stronger surface complexation of carboxyl and hydroxyl functional groups of the HoA with trivalent cations (i.e., Fe3+) as compared with di- or monovalent cations (10). The pH value of the Fe3+-montmorillonite suspension increased from 3.95 ( 0.07 to 5.03 ( 0.04 followed by HoA
TABLE 2. Freundlich and Langmuir Parameters Fitted by the Adsorption Isotherms of DOM Fractions and Humic Materials on Montmorillonite Freundlich exchangeable cation
DOM HoA > 1000 Da HoN > 1000 Da HoN HoA HoN FA HA HoN
KF (mg/kg) · (mg/L)-N
N
Ea
Fe3+
861 ( 202
0.60 ( 0.063 0.961
Fe3+
472 ( 65
1.08 ( 0.041 0.997
Fe3+ Cu2+ Cu2+ Cu2+ Cu2+ crude
447 ( 63 227 ( 55 366 ( 74 1376 ( 356 3367 ( 1186 147 ( 29
1.1 ( 0.05 0.77 ( 0.048 1.03 ( 0.044 0.52 ( 0.078 0.63 ( 0.082 1.06 ( 0.04
0.997 0.992 0.995 0.956 0.943 0.997
Langmuir DOM
exchangeable cation
KL (L/kg)
Smax (mg/kg)
Ea
HoA HoA FA HA
Fe3+ crude Fe3+ Fe3+
0.036 ( 0.008 0.011 ( 0.003 0.049 ( 0.015 0.071 ( 0.014
23342 ( 2045 8269 ( 1210 13015 ( 1554 46510 ( 3529
0.960 0.971 0.907 0.966
a E characterizes the model efficiency. A model efficiency of 1 indicates a perfect fit to the data (27).
and FA adsorption. This suggests an exchange of negatively charged DOM groups on surface hydroxyl groups of montmorillonite surfaces. This mechanism was confirmed for the adsorption of DOM on iron oxide (10, 32). The “saturation” of the surface of Fe3+- and crude montmorillonite samples by the adsorbed HoA units indicates a decrease in surface sites participating in adsorption. Besides, low pH caused protonation of HoA functional groups, which reduced their ability to interact with Fe3+-montmorillonite. At high HoA loading, polar sites of HoA might induce intramolecular repulsion, which can also lead to “saturation” of the surfaces. In contrast to the HoA-Fe3+-montmorillonite system, the pH values did not change followed by adsorption of HoA on the crude montmorillonite (pH of initial colloidal clay was 7.07 ( 0.05). Schlautman and Morgan (35) suggested that at pH >7, adsorption of HA was governed by water- or cationbridging mechanisms. Carboxylate groups of HoA may adsorb on montmorillonite through cation bridging when monovalent exchangeable cations are present and through water bridging when bivalent cations are present (11). Therefore, we suggest that the major mechanism of adsorption of HoA on crude montmorillonite is a water bridging for Ca2+- and Mg2+-occupied sites and cation-bridging for Na+-occupied sites. The Langmuir-type behavior of the isotherm of HoA adsorption on crude montmorillonite can be explained by a decrease in surface bridging sites followed by HoA adsorption. Adsorption isotherms of HoA on Cu2+-montmorillonite as well as adsorption of the large molecular size HoA fraction (>1000 Da) on Fe3+-montmorillonite were nonlinear with Freundlich N values of 0.77 and 0.60, respectively (Table 2). The pH of the Cu2+-montmorillonite system increased from 5.76 ( 0.08 to 6.21 ( 0.07 followed by adsorption of HoA; and the pH of Fe3+-montmorillonite increased from 3.95 ( 0.07 to 4.9 ( 0.06 followed by adsorption of the HoA >1000 Da fraction. The increase in pH due to adsorption suggests that ligand exchange was involved in the adsorption process. Adsorption amounts of HoN were higher than those of HoA for all clays (Figure 1). We speculate that this was due to the higher hydrophobicity and the larger molecular size of HoN as compared to that of HoA (21). The larger molecular
size and higher hydrophobicity of HoN units enhanced its adsorption to Fe3+-montmorillonite. Moreover, at the experimental conditions (pH < 4) approximately half of the HoA carboxylic groups might be protonated and therefore did not participate in the ligand exchange. This suggests that adsorption mechanism of HoA on the Fe3+-montmorillonite is a combination of following interactions: ligand exchange, van der Waals forces, and H-bonding. Preferential adsorption of humic substances and DOM components with higher hydrophobicity and greater molecular weight to soil minerals was previously observed (17, 18). Isotherms of HoN adsorption on mineral surfaces were fitted by the Freundlich equation (Table 2) with the highest binding coefficient for adsorption of the larger molecular weight HoN (>1000 Da) on Fe3+-montmorillonite. It is important to note that the pH values were constant and did not change followed by HoN adsorption by all clays. For HoN, isotherms were linear and binding of HoN to crude montmorillonite was the lowest (Figure 1, Table 2). Lower acidity (Table 1) and larger molecular units of HoN reduced the accessibility of HoN to ligand exchange with the surface of Fe3+-montmorillonite, and van der Waals forces appeared to be the dominant mechanism of HoN adsorption to this mineral since no “saturation” was observed. In this study we used HA and FA as reference materials to evaluate the adsorptive behavior of the investigated DOM samples. Adsorbed amounts of FA and HA on Fe3+- and Cu2+montmorillonite at the studied concentration range were similar to those reported by Theng (36) and Stevenson (4). The binding coefficient of FA to Fe3+-montmorillonite was higher than that for HoA since adsorption of polyelectrolyte is enhanced as the number of potential points of attachment of polyelectrolyte increases (37). This is supported by the higher total acidity of the FA versus the HoA (Table 1). However Smax for HoA was greater than that for FA (Table 2), which is probably due to overall higher polarity of large FA units resulting in stronger intramolecular repulsion for FA as compared with HoA. A similar phenomenon (high adsorption affinity and low surface coverage) was observed for humic substances on hematite and kaolinite (37). The Smax of HA was highest for all three acid components (HA, FA, and HoA) indicating extensive van der Waals binding provided by hydrophobic parts of large HA units. FA and HA adsorption on Fe3+-montmorillonite showed the same “saturation” phenomenon as was observed for HoA adsorption. The pH of the Fe3+-montmorillonite suspension increased from 3.95 ( 0.07 to 5.25 ( 0.06 for FA at maximal adsorption (16 358 mg OC/kg). This trend was similar to the increase in pH observed for HoA. However, approximately the same adsorbed amount of HA (18 078 mg OC/kg) on this surface did not result in an increase of initial pH. Only when HA adsorption was much higher (41 627 mg OC/kg) the pH increased to 4.88 ( 0.49. Since total acidity and amount of carboxyl groups is much lower for HA than that for FA (Table 1), we assumed that the exchange of negatively charged groups of HA with surface hydroxyls of the Fe3+-montmorillonite is less intense than that for FA. Weng et al. (38) demonstrated different adsorption behavior for FA and HA on goethite. These authors suggested that the relatively small molecular size of FA helped the molecules to get closer contact to the surface, whereas the larger size of the HA did not promote such close interactions. In accordance with this model, our results can be explained by exchange of OH groups from the mineral surface by HoA and FA molecules, rather than by larger HA units. This observation indicated the similarity of HoA and FA behavior. A significant increase in pH following adsorption of HoA, FA, and HA was obtained only for the Fe3+-montmorillonite. Chorover and Amistadi (13) reported an increase in pH by up to 0.7 pH units followed by DOM adsorption on goethite, VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Interactions of bulk HoA and HoA >1000 Da with Fe3+-montmorillonite: adsorption isotherms, E2/E3 and ε/ε0 ratios. Left column, hydrophobic acid (HoA); right column, HoA >1000 Da. The initial pH value of clay suspension was 3.95 ( 0.07. Followed by adsorption of bulk HoA and HoA >1000 Da on clay, pH changed to 5.03 ( 0.04 and 4.9 ( 0.06, respectively. whereas they did not observe this trend for Wyoming polyelectrolyte with a large molecular weight. The pH increase montmorillonite. This demonstrates the major effect of from 5.76 ( 0.08 to 6.1 ( 0.08 followed by adsorption of FA exchangeable Fe3+ ion on clay adsorption properties. Adand HA on Cu2+-montmorillonite suggested that ligand sorption isotherms of both HA and FA on Cu2+-montmoexchange might be involved in the adsorption of these rillonite demonstrated nonlinear behavior, which is believed compounds on this mineral. to be due to a combination of surface complexation mechFractionation of DOM Components. One of the objectives anisms with nonspecific van der Waals interactions. The of this study was to find out whether adsorption of DOM highest binding coefficient was obtained for HA as for facilitates fractionation. Our data show that both HoA and 4800
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FIGURE 3. FTIR spectra of the initial and unbound hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions followed by their interactions with Fe3+-montmorillonite.
HoA >1000 Da samples were fractionated due to adsorption on the surface of Fe3+-montmorillonite (Figure 2). A decrease in E2/E3 ratio indicates an increase in the molecular size of the DOM (28). The average E2/E3 ratio decreased from 11.6 to 5.9 and from 10.2 to 4.4 followed by adsorption of bulk HoA and HoA > 1000 Da on Fe3+-montmorillonite. The E2/ E3 data suggested that for both bulk HoA and HoA > 1000 Da samples the larger-size fractions were mainly adsorbed, whereas the lower molecular size fractions remained in solution. Similar to the HoA samples, FA was also fractionated on Fe3+-montmorillonite: the E2/E3 ratio decreased from 10.3 to 3.5 followed by adsorption, which is consistent with the similarity between FA and HoA. Zhou et al. (15) previously demonstrated preferential adsorption of high-molecularweight fractions of FA on goethite at low pH. Certain fractionation of HoN by Fe3+-montmorillonite was also found in this study: E2/E3 ratio decreased from 8.8 to 7.0 following adsorption. The same trend of fractionation (though less significant) of HoA and HoN was noticed for Cu2+- and crude montmorillonite. For Cu2+-montmorillonite, the E2/E3 decreased from 8.6 to 7.3 for HoA and from 11.6 to 8.2 for HoN. For crude-montmorillonite the E2/E3 decreased from 8.6 to 6.3 for HoA and from 11.8 to 7.6 for HoN.c Fractionation of HA was not observed for all clays probably due to large molecular mass and intramolecular interactions of HA composing units. Murphy et al. (37) also reported that fractionation of HA did not occur when it was adsorbed on hematite. Chorover and Amistadi (13) demonstrated that high-molecular-weight components were preferentially adsorbed on goethite whereas low-molecular-weight compounds were adsorbed on montmorillonite. Hunt et al. (16) also observed a decrease in apparent molecular weight of DOM followed by sorption on goethite. Significant HoA and FA fractionation on the surface of Fe3+-montmorillonite indicates the importance of the surface Fe3+ for the properties of clay. Only Fe3+-montmorillonite induced changes in the molar absorptivity (at 280 nm) followed by adsorption of both HoA and HoA >1000 Da. Our data (Figure 2) demonstrate a gradual increase in the ε/ε0 ratio (SUVA ratio) from 0.39 for HoA and from 0.24 for HoA >1000 Da to 1 for both fractions. This suggested that aromatic fractions were preferentially adsorbed on Fe3+-montmorillonite. The same trend was observed for FA: ratio ε0/ε increased from 0.27 to 0.89 followed by adsorption of FA on Fe3+-montmorillonite. Chorover and Amistadi (13) and Zhou et al. (15) also observed preferential adsorption of aromatic components of DOM and FA on goethite. Neither HoN nor HA showed any fractionation based on their aromaticity on the surface of minerals. No preferential adsorption of aromatic components of HoA was observed for Cu2+- and crude montmorillonite. DOM fractionation on the surface of Fe3+-motmorillonite which was obtained in the current research was similar to data reported for DOM fractionation on goethite due to the presence of exchangeable Fe3+ (13, 15). Exchangeable Fe3+ induced selective adsorption of aromatic units of DOM was probably due to an electron transfer mechanism (39). Pinnavaia et al. (40) studied adsorption of aromatic compounds on Fe3+-hectorite and demonstrated electron transfer from an aromatic molecule to a transition metal ion. Cornejo et al. (41) showed the role of surfaceadsorbed iron oxides in the oxidation of hydrocortisone by palygorskite. They emphasized that intimate association of iron with the clay surface appears to be essential for the oxidation of hydrocortisone. Zhu et al. (8) suggested that aromatic rings interact with exchangeable cations of montmorillonite by cation-π bonding. This binding is particularly important for transition metals due to participation of d orbitals. Based on our data we suggest that inner-sphere complexation of HoA and FA with surface-adsorbed iron 4802
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provided close contact and interaction of Fe3+ with the aromatic rings. The lower acidity and larger molecular size of HA (as compared to HoA and FA) reduced the possibility of HA for inner-sphere complexation and fractionation on the surface of Fe3+-montmorillonite. Preferential adsorption of HoN aromatic units was not observed by UV-vis spectroscopy. In this study we also employed FTIR to detect fractionation of the HoA and HoN samples due to their adsorption on Fe3+-montmorilonite (Figure 3). The peak intensity of the carboxyl vibration bands (1712 cm-1) was dominant in the HoA spectra. In contrast, the spectra of the HoN samples were dominated by the 1655 cm-1 peak which is usually assigned to stretching vibrations of CdC (in aromatic or vinyl groups) (42, 43). In addition, the spectrum of the HoN fraction exhibited a pronounced NsH or NdO vibration band at 1558 cm-1, which was missing in the bulk HoA spectrum. This suggests that the two fractions differ in both amount (Table 1) and type of nitrogen groups. In addition, the bulk HoA spectrum demonstrated a higher intensity of vibration bands assigned to sugars or CsO or OsCsC at 1200 cm-1 than the HoN spectrum. Distinct changes in the HoA and HoN spectra resulting from their binding to Fe3+-montmorillonite were observed (Figure 3). The following changes were observed for the HoA: (i) the 1655 cm-1 peak which appeared as a noticeable shoulder in the HoA spectrum decreased significantly in the spectrum of the unbound HoA; (ii) the relative level of parffinic (2930 cm-1 peak) as compared to COOs groups (1712 cm-1 peak) was lower in the unbound HoA fraction (the peak ratios 1712/2930 were 1.2 and 1.5 for the HoA and the unbound samples, respectively); and (iii) the unbound sample was relatively richer in polysaccharides (the peak ratios 1712/1030 were 2.4 and 1.8 for the HoA and the unbound samples, respectively). The following changes were observed for the HoN samples: (i) the 1712 cm-1 peak which appeared as a shoulder in the unbound spectrum was not noticeable in the spectrum of the HoN sample; (ii) the 1558 cm-1 vibration peak decreased significantly in the spectrum of the unbound HoN (the peak ratio 1655/1558 was 1.2 in the HoN spectra and 1.7 in the unbound sample); and (iii) similar to the trend observed for the HoA, the unbound HoN sample was significantly richer in polysaccharides (the peak ratios 1655/1040 were 3.8 and 1.1 for the bulk and the unbound samples, respectively). In general, the FTIR data suggest a preferential adsorption of aromatic moieties of the HoA and HoN samples on the Fe3+-montmorillonite. For both adsorbates, the relative level of the aromatic moieties was reduced in the unbound sample compared to the carboxyl groups. Though UV-vis measurements did not indicate preferential sorption of HoN aromatic compounds by Fe3+-montmorillonite, we can speculate about this phenomenon based on the FTIR data. Polysaccharide components in both samples did not interact with the Fe3+montmorillonite surface. The spectrum of the HoN indicated the presence of significant amount of N-H groups which can undergo protonation at low pH of Fe3+-montmorillonite, and thus enhance adsorption of HoN. Similarly, Evanko and Dzombak (44) emphasized the possible role of N groups in adsorption of DOM on oxides. Our results demonstrated that Fe3+-montmorillonite resulted in major fractionation of DOM hydrophobic components which was not found for Cu2+- and crude montmorillonites. Enhanced DOM fractionation by molecular size and aromaticity on the surface of Fe3+-montmorillonite demonstrated that saturation with Fe3+ significantly changed the adsorptive behavior of the clay surface. We also suggest that due to similarity of adsorption and fractionation of HoA and FA on the surface of Fe3+-montmorillonite, HoA-clay interactions can be used as a model for the FA interactions
on iron-rich clay surfaces. Adsorption and fractionation of DOM fractions by montmorillonite saturated with transition metals demonstrated the importance of surface controlled reactions in the retention of DOM and transport of organic colloids in soils.
Acknowledgments This research was supported by grants from the International Arid Land Consortium (IALC). We thank Ziva Hochman and Maya Kobany for their help in chemical and spectral analyses of the samples.
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