Sorption of naphthalene and phenanthrene by soil humic acids

Environmental Pollution 111 (2001) 303±309

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Sorption of naphthalene and phenanthrene by soil humic acids B. Xing * Department of Plant and Soil Sciences, University of Massachusetts, Stockbridge Hall, Box 37245, Amherst, MA 01003-7245, USA Received 26 August 1999; accepted 15 January 2000

``Capsule'': For both naphthalene and phenanthrene, Freundlich exponents (N) values decreased with increasing aromaticity of six humic acids. Abstract Humic acids are a major fraction of soil organic matter (SOM), and sorption of hydrophobic organic chemicals by humic acids in¯uences their behavior and fate in soil. A clear understanding of the sorption of organic chemicals by humic acids will help to determine their sorptive mechanisms in SOM and soil. In this paper, we determined the sorption of two hydrophobic organic compounds, naphthalene and phenanthrene by six pedogenetically related humic acids. These humic acids were extracted from di€erent depths of a single soil pro®le and characterized by solid-state CP/MAS 13C nuclear magnetic resonance (NMR). Aromaticity of the humic acids increased with soil depth. Similarly, atomic ratios of C/H and C/O also increased with depth (from organic to mineral horizons). All isotherms were nonlinear. Freundlich exponents (N) ranged from 0.87 to 0.95 for naphthalene and from 0.86 to 0.92 for phenanthrene. The N values of phenanthrene were consistently lower than naphthalene for a given humic acid. For both compounds, N values decreased with increasing aromaticity of the humic acids, such an inverse relationship was never reported before. These results support the dual-mode sorption model where partitioning occurs in both expanded (¯exible) and condensed (rigid) domains while nonlinear sorption only in condensed domains of SOM. Sorption in the condensed domains may be a cause for slow desorption, and reduced availability and toxicity with aging. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Humic acids; Sorption; Nonlinear; Organic compounds; Dual-mode

1. Introduction Humic acids are a major fraction of soil organic matter (SOM) which is the dominant sorbent of hydrophobic organic compounds (HOC) in soil. Numerous investigations have demonstrated that sorption of HOC in soils is controlled by SOM unless its content is extremely low (Chiou, 1989; Mader et al., 1997). This is particularly true in water±soil systems because water molecules are preferably adsorbed on mineral surfaces over HOC molecules (Chiou, 1989). Positive correlations between sorption coecients of HOC and SOM contents are also widely reported (Chiou, 1989; Mitra et al. 1999). SOM has been modeled as a dual-mode sorbent (Xing et al., 1996; Xing and Pignatello, 1997, 1998, references therein) or a sorbent with dual-reactive domains (Weber and Huang, 1996a,b; Huang et al., 1997, references therein). These models propose that SOM consists of * Tel.: +1-413-545-4212; fax: +1-413-545-3958. E-mail address: [email protected] (B. Xing).

both expanded (¯exible, rubbery-like) and condensed (rigid, glassy-like) domains. The expanded domains behave as rubbery polymers with linear sorption isotherms while the condensed domains have nonlinear isotherms and competitive sorption (dilute conditions under which solutes are unable to change the property of SOM matrix). The condensed domains can be microscale size of localized areas within SOM. Recent nuclear magnetic resonance (NMR) experiments have provided some spectroscopic evidence for the existence of both ¯exible and condensed segments in SOM (Chien and Bleam, 1998; Xing and Chen, 1999). The two NMR studies (Chien and Bleam, 1998; Xing and Chen, 1999) concluded that aromatic moieties of SOM are likely the condensed (rigid) domains. If that is the case, we would expect a positive relationship between nonlinearity and aromaticity of SOM because nonlinear isotherms occur primarily in condensed domains as discussed above. But such a relationship has not been reported in the literature. In this paper, we determined the sorption of naphthalene and phenanthrene by

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B. Xing / Environmental Pollution 111 (2001) 303±309

six humic acids to study their sorption behavior. These six humic acids were extracted from various depths of a single soil pro®le; thus, they were pedogenetically related. Because of molecular complexity of SOM and possible complication by soil mineral components, humic acid fractions were used instead of whole soil. A better understanding of sorptive behavior in humic acids will help to determine the sorption mechanisms and fate of organic compounds in soil, and provide valuable insight for HOC sequestration in soil with aging (i.e. contact time between HOC and soil). Sequestration of HOC in soil is a critical factor for HOC availability and risk assessment of a contaminated site. 2. Materials and methods An uncultivated soil (a Mollisol) was sampled near the Ellerslie Research Station of University of Alberta, Edmonton, Alberta, Canada. The site is under an open aspen-woodland stand. Eight samples were collected at di€erent depths from a single soil pro®le, three (O1, O2, O3) from organic horizons and ®ve (A1, A2, A3, AB, B) from mineral horizons. We used all three samples from the organic horizons and only three (A1, A2, A3) from mineral horizons in this study. Selected physical and chemical properties and sampling depths are shown in Table 1. These properties were determined using standard soil analytical procedures (Carter, 1993). Some materials were used in our previous work (Xing and Chen, 1999), but the previous paper discussed primarily on the spectroscopic ®ndings for condensed domains in SOM while this work addresses sorptive behavior and mechanisms. [Ring-UL-14C] naphthalene and phenanthrene were purchased from Sigma Chemical Company (St. Louis, MO, U.S.A.), and unlabeled naphthalene and phenanthrene were purchased from Aldrich Chemical Company (Milwaukee, WI, U.S.A.),. Both compounds were used without further puri®cation. These compounds are common organic contaminants in soil and sediment, and have often been used in environmental research. Table 1 Soil propertiesa Horizon

Depth (cm)

Sand (%)

Clay (%)

Organic C (%)

pH

CEC (cmol(+)/kg)

O1 O2 O3 A1 A2 A3

18±13 13±5 5±0 0±10 10±29 29±35

n.d. n.d. n.d. 9 10 9

n.d. n.d. n.d. 48 46 49

49 39 41 5.9 2.1 0.5

7.2 6.9 6.6 6.1 6.6 6.7

139 141 160 54.2 40.6 35.9

a

O, organic horizons; A, surface mineral horizons; 1±3, subhorizons. n.d.=not determined;.

The detail of the humic acid extraction procedure was described by Chen and Pawluk (1995). Brie¯y, soil was mixed with 0.1 M Na4P2O7 solution (1:10 w/v) under N2 and shaken overnight. The alkali-soluble SOM was separated from mineral and humin fractions by high speed (20,000g) centrifugation. Before acidi®cation, the clay was salted out using 0.3 M KCl solution and removed by centrifugation. This process avoids release of Mn and Fe from oxides or other minerals into the humic acids during acidi®cation since high Fe and Mn (paramagnetic elements) contents interfere with 13C NMR analysis. The coagulated clay and organo-clay complex were further treated with 0.1 M Na4P2O7 and 0.3 M KCl under N2 to extract humic acid until a light brown color of extract was reached. The supernatant was combined and acidi®ed with 6 M HCl to pH 1.0 to separate humic acid from fulvic acid. The humic acid was treated with a dilute (0.5% by volume) HF/HCl solution to remove ash, dialyzed in deionized water, freeze-dried, and ground to ®ne powders for elemental and spectroscopic analysis. The elemental content (C, H, N, O) of humic acids was determined using a Carlo Erba CHNS-O EA 1108 elemental analyzer. Elemental measurements were performed immediately after the freeze-drying so that moisture interference (with H and O) would be minimal. Nevertheless, some strongly bound water molecules might still remain in humic acids. The ash content was determined by heating humic acids at 740 C for 4 h. The solid-state 13C NMR spectra of humic acids were obtained using cross-polarization (CP) and magic angle spinning (MAS) techniques. The NMR spectrometer was a Brucker AM 300 instrument, operating with CP (2 ms contact time) and MAS (5 kHz spinning rate) and a HP WP 73A probe, at 75 MHz frequency, with pulse width of 5.50 ms, and 70 ms acquisition time. The rotor was a 4 mm/18 zirconia rotor with a kel-f cap. Within the 0-220 ppm chemical shift range, C atoms were assigned to alkyl C (0±50 ppm), O-alkyl C (50±107), aromatic C (107±165), carboxyl C (165±190 ppm), and carbonyl C (190±220) (Malcom and MacCarthy, 1986; Schnitzer et al., 1991). The spinning sidebands were not corrected because their contributions were small and would cause only minor di€erences in the overall integrated intensities for these humic acids. Sorption experiments were conducted using batch equilibration technique in 8-ml (for naphthalene) or 40ml (for phenanthrene) screw-cap vials (minimal headspace) with Te¯on-lined septa (Xing and Pignatello, 1997). The solution was 0.01 M CaCl2 containing 200 mg/l HgCl2 as biocide to minimize microbial activity. Wolf et al. (1989) reported that HgCl2 was equivalent to three times autoclaving, yet had little impact on soil properties such as cation exchange capacity and pH. The solution-to-solid ratio was 260:1 (oven-dry basis) for naphthalene and 4400:1 for phenanthrene. The initial

B. Xing / Environmental Pollution 111 (2001) 303±309

concentration ranged from 0.008 to 15 mg/ml for naphthalene and from 0.006 to 0.8 mg/ml for phenanthrene. Each isotherm consisted of six to eight concentration points. Each point was run in duplicate and the averages were reported. Two blanks without humic acid were run for each initial concentration. Humic acid suspensions were shaken for 3 days in hematology mixers giving rocking±rotating motions. Then, the vials were centrifuged at 1000g for 20 min and supernatants were sampled for liquid scintillation counting. Because of little sorption by vials and no biodegradation, sorbed naphthalene or phenanthrene by humic acids was calculated by the mass di€erence. The experimental details were reported elsewhere (Xing and Pignatello, 1997; Xing, 1998). Sorption data were ®tted using the Freundlich equation, S=KFCN, where S (mg gÿ1) and C (mg mlÿ1) are sorbed and solution concentrations, respectively, KF (mlN mg1ÿN gÿ1) is the sorption coecient and N (dimensionless) is the exponent. The lower the N, the more nonlinear an isotherm. The parameters, KF and N were determined by linear regression of log-transformed data. Linear ®t of log-transformed data was justi®ed over direct nonlinear curve ®tting in this paper because concentrations were spread evenly over the log scale; thus, the direct nonlinear curve ®tting would underestimate the importance of the low concentration data (Xing, 1998). 3. Results and discussion Selected chemical properties of the six humic acids are shown in Table 2. The carbon content ranged from 50 to 57%, a typical range as reported in the literature. The ash content was very low for all samples (below 0.6%) except for A3 humic acid with 1.8%. The lower the ash content, the better the NMR signals due to the reduced interference from paramagnetic ions. Aromaticity (percent C between 107 and 165 ppm) of humic acids increased from organic horizons to mineral horizons, vice versa for alkyl C (0±50 ppm) and O-alkyl C (50±107 ppm) (Table 3, Fig. 1). Aromaticity of A2 and A3 humic acids was about two times higher than that of the humic acids from organic layers. The 130 Table 2 Elemental content (%), ash content (%), and atomic ratios of the six humic acidsa Horizon

C

H

N

O

Ash

C/H

C/O

O1 O2 O3 A1 A2 A3

53.7 52.2 50.3 54.3 56.8 56.5

4.73 4.14 4.38 3.59 3.04 2.98

2.31 2.85 2.99 2.88 3.51 3.47

38.8 40.2 41.3 38.3 36.2 35.6

0.49 0.26 0.36 0.51 0.63 1.84

0.95 1.05 0.96 1.26 1.55 1.58

1.8 1.7 1.6 1.9 2.1 2.1

a

O, organic horizons; A, surface mineral horizons; 1±3, subhorizons.

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Table 3 Structural carbon distribution (%) of the humic acids extracted from soil horizonsa Horizon Alkyl-C O-alkyl-C Aromatic-C Carboxyl-C Carbonyl-C O1 O2 O3 A1 A2 A3

24 26 25 22 13 10

29 29 30 22 16 14

28 24 25 33 47 51

12 14 14 15 13 14

6 7 6 8 9 11

a Carbon distribution was calculated from solid state 13C CP/MAS NMR spectra; O, organic horizons; A, surface mineral horizons; 1±3, subhorizons.

ppm peak (aromatic region) was bigger and more wellde®ned in mineral horizons than organic horizons (Fig. 1). Atomic ratios of C/H and C/O also increased from organic to mineral horizons, which is consistent with the aromaticity increase with pro®le depth. Freundlich isotherm parameters are presented in Table 4 and isotherms of naphthalene and phenanthrene for A3 humic acid are shown as examples in Fig. 2. Sorption coecients (KF) of phenanthrene ranged from 6300 to 10 200 and naphthalene from 220 to 240. The KF values in this study are comparable with the literature values of similar organic matter (Huang and Weber, 1997; Xing, 1997). Though precise comparison cannot be made between KF values because of their different units as a result of nonlinearity (Chen et al., 1999), it is apparent that KF values of phenanthrene were much larger than naphthalene (Table 4, Fig. 2). This may be attributed (at least partly) to the solubility di€erence between the two compounds (hydrophobic e€ect). Solubility of phenanthrene (1.1 mg lÿ1) is about 30 times lower than naphthalene (31 mg lÿ1). The A2 and A3 humic acids had higher KF values of phenanthrene than other horizons, but this phenomenon was not observed for naphthalene. Higher phenanthrene sorption by A2 and A3 humic acids may be due to the stronger anity to the highly aromatic moieties of these two humic acids (Table 3). Huang and Weber (1997) also reported that phenanthrene sorption was higher in their samples (diagenetically altered organic matter) with higher aromaticity (Tables 1 and 2 in their paper). All isotherms were nonlinear, i.e. N<1. The N values of naphthalene ranged from 0.87 to 0.95 and phenanthrene from 0.86 to 0.92 (Table 4, Fig. 2). All sorption of naphthalene or phenanthrene by humic acids should have occurred in SOM, not by soil minerals because these humic acids were almost totally organic material. The ash (0.3±1.8% ) would be either coated by humic materials if being mineral particles or the sum of all individual cations associated with charged sites and functional groups. Naphthalene or phenanthrene molecules may not be able to see the surface sites of the limited amount of minerals even they are present in these

306

B. Xing / Environmental Pollution 111 (2001) 303±309 Table 4 Freundlich isotherm parameters of the humic acidsa Humic acids

Naphthalene

Phenanthrene

KF

[N]

r2

KF

[N]

r2

O1 O2 O3 A1 A2 A3

223 232 240 220 217 222

0.953‹0.005b 0.969‹0.009 0.969‹0.008 0.916‹0.008 0.893‹0.004 0.870‹0.006

0.999 0.998 0.999 0.998 0.999 0.999

6310 6902 6546 6966 10140 10160

0.923‹0.009 0.932‹0.013 0.926‹0.007 0.881‹0.012 0.868‹0.008 0.861‹0.003

0.999 0.998 0.999 0.998 0.999 0.999

a O, organic horizons; A, surface mineral horizons; 1±3, subhorizons. b Standard deviation.

Fig. 2. Isotherms of the humic acid extracted from A3 horizon.

Fig. 1. Solid-state 13C CP/MAS NMR spectra of the humic acids.

humic acids. In addition, nonpolar molecules such as naphthalene and phenanthrene cannot e€ectively compete with polar water molecules for adsorption on mineral surface. Moreover, water molecules were much more abundant than organic molecules in the experimental systems used in this study. Therefore, isotherm nonlinearity cannot be attributed to mineral surface, but is truly characteristic of SOM. Nonlinear isotherms were also observed for apolar organic chemicals such as toluene, chlorinated benzene, and phenanthrene in other organic sorbents (Huang and Weber, 1997; Xing and Pignatello, 1997; Xing, 1998). The N values of phenanthrene were consistently lower than naphthalene for a given humic acid (Table 4). The value of N can be taken as an index of site energy distribution, i.e. the smaller the N, the more heterogeneous

the sorption sites (Weber et al., 1992). Through siteenergy distribution analysis using the Freundlich isotherm parameters (Carter et al., 1995; Yuan and Xing, 1999), sorption energy of phenanthrene was consistently higher than naphthalene at a given loading rate or sorbed concentration (data not shown). For example, at 600 mg gÿ1, sorption energy of phenanthrene by A3 humic acid was 8.8 kJ molÿ1 and naphthalene 5.8 kJ molÿ1. Sorption energy decreased with increasing loading with a sharp drop at lower loading (i.e. lower sorbed concentrations). The results re¯ect the heterogeneous nature of these humic materials, not a uniform partitioning medium. Nonlinear isotherms in these six humic acids were consistent with dual-mode sorption in SOM (Xing and Pignatello, 1997), caused by the sorption in condensed regions. Using X-ray di€raction and solid-state NMR, Xing and Chen (1999) have observed rigid, condensed aromatic domains in these humic acids, particularly those from mineral horizons. There is other evidence in

B. Xing / Environmental Pollution 111 (2001) 303±309

the literature to support the existence of condensed domains in SOM. For instance, humic acids extracted from various soils have shown the presence of condensed, tightly packed aromatic regions as revealed by the 0.35 nm peak on X-ray di€ractograms (Schnitzer et al., 1991). The N values of naphthalene and phenanthrene were plotted against aromaticity of the six humic acids (Fig. 3). Though the number of data points is limited, it is evident that N values decreased proportionally with increasing aromaticity. Correlation analysis was not performed because of the limited data points. Humic acids at lower depths had higher aromaticity (Table 3, Fig. 1). Humic acids in surface organic horizons would be mainly derived from recent organic matter input (e.g. tree and grass residues) and, thus, younger than that from mineral horizons at lower depths. In a recent review on SOM decomposition and humi®cation, Preston (1996, references therein) described the decomposition sequence of individual SOM components. The most easily metabolizable carbohydrates decompose ®rst, followed by modi®cation and declines of alkyl C (-CH2- peak) and phenolic C (150±160 ppm). As a result, aromatic region becomes dominated as shown by a peak with its maximum around 130 ppm. With further humi®cation, SOM may become highly aromatic, with development of polycondensed rings. The increase of atomic C/H and C/O ratios from organic to mineral horizons (Table 2) is in keeping with the above decomposition and humi®cation sequence. Also, the peak at 115 ppm (unsubstituted, non-condensed ortho phenolic carbons) decreased with depth and disappeared at A2 horizon (Fig. 1). Furthermore, substantial reduction of

Fig. 3. The decrease of Freundlich exponent N with increasing aromaticity.

307

aliphatic C (0±50 ppm), polysaccharide C (50±107 ppm), and phenolic C signals (150 ppm) and concomitant increase of aromatic C signals (Fig. 1, Table 3) re¯ect a higher degree of condensation of humic acids from mineral horizons. Thus, it would be expected that N values of organic horizons were higher than mineral horizons according to the dual-mode sorption model. However, research using SOM samples of diverse origins is required to further elucidate the relationship between nonlinearity and aromaticity. Chiou and Kile (1998) proposed high-surface-area carbonaceous materials (HSACM) in soil such as charcoal to account for isotherm nonlinearity, which is de®nitely a plausible hypothesis. Currently, methods are not available to isolate HSACM and determine their quantity if present in soil and, thus, their direct contribution to sorption and nonlinearity cannot be accurately determined. If sorption is on HSACM surface, it would be fast because physical adsorption on an exposed surface is generally rapid. However, sorption kinetics in SOM is usually slow, from weeks to years (Pignatello and Xing, 1996). Also, isotherm nonlinearity increases with increasing contact time (Weber and Huang, 1996a,b; Huang and Weber, 1998; Xing, 1998), which could not be explained by surface sorption, but is consistent with the dual-mode or dual-reactive domain sorption models. Furthermore, synthetic organic polymers such as polyvinyl chloride and poly(isobutyl methacrylate), have isotherm nonlinearity for HOC (Leboeuf and Weber, 1997; Xing and Pignatello, 1997), but these polymer do not contain any HSACM. Therefore, HSACM is not a priori for isotherm nonlinearity. HSACM could, however, be embedded within SOM through attachment to humic and other natural organic macromolecules during soil genesis and SOM humi®cation or entrapped in humic acids during extraction and precipitation. Then, organic molecules have to di€use through the external SOM to be sorbed on HSACM. This type of HSACM±SOM arrangement conforms to the dual-mode model, and may partly explain the nonlinearity increase with contact time. Again, HSACM is not prerequisite for isotherm nonlinearity as noted above. Nevertheless, HSACM in soil deserves further investigation for its quantity, association with SOM, and contribution to sorption. Inability of interrupting or breaking strong polar contacts between macromolecules in SOM could cause physically limited sorption domains. These domains have been suggested to explain isotherm nonlinearity for HOC (Graber and Borisover, 1998). Strong polar contacts within SOM matrix are not in con¯ict with the dual-mode model. It is expected that polar contacts would increase the degree of cross-linking of macromolecules in SOM, which in turn would lead to more rigid domains. It is well-known that cross-linking increases the glassy transition temperature of polymers.

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B. Xing / Environmental Pollution 111 (2001) 303±309

4. Conclusions Aromaticity of the humic acids used in this study increased with soil depth, indicative of more condensed domains at lower depths (Schnitzer et al., 1991; Xing and Chen, 1999). All sorption isotherms of the six humic acids were nonlinear and the N values of phenanthrene were consistently lower than naphthalene. The KF values of phenanthrene were consistently higher than naphthalene. For both compounds, N values decreased with increasing aromaticity of these humic acids, such a relationship was not reported before. The results in this study lend further support to the heterogeneous nature of SOM, and are in line with the dualmode sorption model. Humic acids of diverse origins and locations need to be used to further test the inverse relationship between Freundlich exponent, N and aromaticity. It should be noted that the characteristics of humic acids change with soil depth (Tables 2 and 3, Fig. 1). These changes a€ect the sorptive behavior of HOC in soil. Thus, soil such as the one used in this study can have variable reactivity with HOC at di€erent depths. Such reactivity variation with depth should be considered in predictive models for fate and transport of HOC, and risk assessment.

Acknowledgements The author thanks Dr. Zhengqi (Victor) Chen for providing the humic acid samples and Mr. Z. Jin for his assistance during the experimental phase of this work. This work was in part supported by the US Department of Agriculture, National Research Initiative Competitive Grants Program (97-35102-4201 and 98-351076319), the Federal Hatch Program (Project No. MAS00773), and a Faculty Research Grant (the University of Massachusetts at Amherst).

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