Environ. Sci. Technol. 2008, 42, 1165–1171
Insights into the Sorption Properties of Cutin and Cutan Biopolymers MICHAL SHECHTER AND BENNY CHEFETZ* Department of Soil and Water Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
Received September 2, 2007. Revised manuscript received November 2, 2007. Accepted November 12, 2007.
Plant cuticles have been reported as highly efficient sorbents for organic compounds. The objective of this study was to elucidate the sorption and desorption behavior of polar and nonpolarorganiccompoundswiththemajorstructuralcomponents of the plant cuticle: the biopolymers cutin and cutan. The sorption affinity values of the studied compounds followed the order: phenanthrene > atrazine > chlorotoluron > carbamazepine. A higher sorption affinity of phenanthrene and atrazine to cutin was probably due to the higher level of amorphous paraffinic carbon in this biopolymer. Phenanthrene exhibited reversible sorption behavior and a high ratio of organic-carbonnormalized distribution coefficient (KOC) to carbon-normalized octanol–water partitioning coefficients (KOWC) with both biopolymers. This suggests that both biopolymers provide phenanthrene with a partition medium for hydrophobic interactions with the flexible long alkyl-chain moieties of the biopolymers. The low KOC/KOWC ratios obtained for the polar sorbates suggest that the polar sites in the biopolymers are not accessible for sorption interactions. Atrazine and carbamazepine exhibited sorption–desorption hysteresis with both sorbents, indicating that both sorbates interact with cutin and cutan via both hydrophobic and specific interactions. In general, the sorptive properties of the studied biopolymers were similar, signifying that the active sorption sites are similar even though the biopolymers exhibit different properties.
Introduction Sorption and desorption interactions of hydrophobic organic compounds (HOCs) are the major processes affecting their fate in the environment. The predominant sorbent for HOCs is the natural organic matter in soils and sediments. Plant cuticular materials are an important precursor for soil organic matter (SOM); these materials tend to accumulate in soils and become incorporated into the soil humus (1, 2). The plant cuticle is a thin, predominantly lipid layer that covers all primary aerial surfaces of vascular plants. Its main functions are to minimize water loss, limit the loss of substances from internal plant tissues, and protect plants against physical, chemical, and biological changes (3). The structure, elemental composition, density, and thickness of the cuticle vary largely among plant species, plant organs, and different growth stages of the plant. The outer surface of the cuticle is covered with epidermal wax, which consists of a complex mixture of long-chain aliphatic and cyclic components. The major structural component of the plant * Corresponding author phone: +972 (8) 948-9384; fax: +972 (8) 947-5181; e-mail:
[email protected]. 10.1021/es702205u CCC: $40.75
Published on Web 01/11/2008
2008 American Chemical Society
cuticle is the cutin biopolymer, which is most often associated with waxes and cuticular polysaccharides. Cutin is defined as an insoluble polymeric network of C16 and C18 fatty acids, hydroxy fatty acids, and epoxy fatty acids, composed primarily of 10,16- and minor amounts of 8,16- and 9,16-dihydroxyhexadecanoic acids (4–6), with molecular sizes varying between 300 and 400 Da (7). The cutin biopolymer provides structural framework and acts as a physical barrier against microbial attack and water loss. Its thickness ranges from 0.5 to 14 µm, with 20 to 600 µg cutin per cm2 of surface area (5). In some plant species, the cuticle may also be composed of an acid- and base-hydrolysis-resistant biopolymer termed cutan (3, 8, 9). Cutan enhances the hydrophobic nature of the cuticle, providing a physiological adaptation for survival under drought conditions (10). Cutan is a highly cross-linked biopolymer (6, 9, 11) consisting of a network of saturated and unsaturated polymethylenic long chains (up to C33) with free carboxylic functionalities (8–10, 12, 13). The structural composition of cutan is not yet well understood. Tegelaar et al. (12) suggested that it is composed of polysaccharide moieties that are covalently attached to polymethylenic units. McKinney et al. (11) proposed a cutan model having a backbone of aromatic rings with hydroxyl groups that form ester linkages with fatty acids of varying chain lengths. Recently, Deshmukh et al. (6) suggested that cutan contains a higher degree of carboxylic and hydroxy functionalities, which form ester linkages with long-chain primary and secondary alcohols. In addition to the importance of plant cuticular matter as a precursor for SOM, plant cuticles have been reported to be highly efficient natural sorbents for HOCs (14–19). In most cases, sorption affinities of HOC with the plant cuticular materials were in the same range as those reported for highly aromatic natural sorbents such as lignin, lignite, wood coal, and humic substances. Little is known about the sorptive properties of the main plant cuticular mattersthe cutin and cutan biopolymers. Therefore, the objective of this study was to elucidate the sorption and desorption behavior of selected environmentally important organic pollutants with these biopolymers to better understand the influence of biopolymer structure and composition on HOC sorption. In order to achieve this, cutin and cutan were isolated from fruits of tomato (cutan-free cuticle) and leaves of Agave americana (cutan-containing cuticle) and purified using a double extraction method. This enabled us to elucidate sorption interaction mechanisms with the pure cutin and cutan biopolymers rather than studying sorption interactions using bulk cuticles (16, 18, 20), mixtures of cuticular fractions (16), and nonpurified fractions such as cutin-lignin (14).
Materials and Methods Isolation and Characterization of Cutin and Cutan Fractions. Cutin and cutan biopolymers were isolated from the fruits of tomato (Lycopersicon esculentum Mill) and leaves of the succulent plant A. americana, respectively. Cuticle sheets were manually peeled from the fresh fruits and leaves after boiling in water. Then the bulk cuticle sheets were soaked in oxalic acid (4 g/L) and ammonium oxalate (16 g/L) solution at 90 °C for 24 h to remove noncuticular residual materials. Waxes were removed by Soxhlet extraction with chloroform/ methanol (1:1, v/v) for 6 h. To obtain the cutin biopolymer, the dewaxed tomato cuticles were hydrolyzed with 6 M HCl (6 h under reflux). To obtain the cutan, the dewaxed A. americana cuticles were saponified (1% w/v KOH in methanol for 3 h at 70 °C) and then hydrolyzed (6 M HCl; 6 h under reflux). All treatments were performed twice to ensure VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Properties of the Sorbates Used in This Study
a
Molecular weight (g/mol).
b
Solubility in water (mg/L). c Octanol-water partitioning coefficient.
complete removal of the desired fraction (14). All purified fractions were washed with deionized water, freeze-dried, ground, and sieved ( atrazine > chlorotoluron > carbamazepine. Carbamazepine and chlorotoluron exhibited statistically similar KOC values for both studied biopolymers, whereas significantly higher KOC values were calculated for atrazine and phenanthrene with the cutin samples as compared to cutan (Table 2). Phenanthrene exhibited relatively linear isotherms with both biopolymers (Freundlich N values of 0.97 and 1.02 for cutin and cutan, respectively). Carbamazepine, atrazine, and chlorotoluron exhibited significantly less linear isotherms with Freundlich N values of 0.85-0.95. In general, a negative correlation has been reported between the polarity of the sorbent and the sorption affinity of nonpolar HOCs such as phenanthrene (16, 17, 28–30). However, in the present study, the sorption affinity of phenanthrene did not correlate with sorbent polarity. Phenanthrene exhibited higher sorption affinity with the cutin although this biopolymer is characterized by a higher polarity index than cutan. Cutin and cutan exhibit polarity values (i.e., (C + H)/N) of 0.2 and 0.11, respectively (18). These values were lower than those previously reported for cuticular fractions (14, 16). In these reports, nonpurified fractions and mixtures of fractions were analyzed, whereas in this study we used the purified cutan and cutin fractions which were isolated from fruits of tomato and leaves of A. americana.
The polarity values obtained for cutin and cutan were lower than the values reported for hydrophobic sorbents such as humic acid, lignin, and kerogen (17, 30, 31). We therefore suggest that both isolated samples (cutin and cutan) can be considered highly nonpolar sorbents; thus the higher polarity index of the cutin did not pose lower sorption affinity of phenanthrene. Furthermore, the polar sites in both sorbents may not be accessible for interactions with the sorbates (16) and they therefore did not reduce phenanthrene sorption as was expected. Our hypothesis that cutin and cutan provide powerful partition media for hydrophobic interactions with phenanthrene is also supported by the high KOC/KOWC ratios (3.83 and 2.34, respectively) calculated for this solute (Table 2). A high KOC/KOWC ratio suggests specific sorbate-sorbent interactions, such as H-bonding and π-π or polar interactions (16). Phenanthrene is expected to have minimal π-π interactions with aromatic moieties of both sorbents due to the low level of aromatic moieties in the studied sorbents. Aromaticity values were only 8% and 2% of the total carbon for cutin and cutan, respectively (18). Moreover, phenanthrene has no active hydrogen for H-bonding interactions. Therefore, we assume that the high KOC/KOWC ratio calculated for phenanthrene results from the highly hydrophobic nature of the biopolymers, provided by their high content of long alkyl moieties (5, 6, 10). These aliphatic structures can provide a dissolution domain for phenanthrene and other polycyclic aromatic hydrocarbons (PAHs) (32). In contrast, the KOC/ KOWC ratios obtained (6 to 17) for the polar compounds (atrazine, carbamazepine, and chlorotoluron) were significantly lower than those calculated for phenanthrene, and they were all less than 1 (Table 2). Chen et al. (16) reported that polar sorbates such as phenol and 1-naphthol exhibit high KOC/KOWC ratios (6 to 17) when interacted with cuticular fractions. It is important to note that in our present study, we used highly purified biopolymers (cutin and cutan), whereas Chefetz (14) and Chen et al. (16) did not use purified fractions. We suggest that the polar sites of the cutin and cutan are not accessible for sorption interactions. In both biopolymers, the polar functionalities (mainly oxygen) are present in ether and ester bonds of long alkyl moieties, and their accessibility for specific interaction is therefore limited. Several studies have reported a positive correlation between the paraffinic carbon content of the sorbent and the sorption affinity of nonpolar HOCs (16, 17, 28–30). Similar to the lack of correlation between the phenanthrene sorption data and the polarity index, phenanthrene sorption affinity values did not correlate with the level of paraffinic carbon. The sorption affinity of phenanthrene with cutan was lower than that with cutin, although the level of paraffinic carbon was 90% for cutan and 76% for cutin (18). We suggest that the higher sorption affinity of phenanthrene to the cutin is related to its higher amount of amorphous paraffinic carbon as compared to the amount of crystalline carbon (26 and 15% of paraffinic carbon, respectively). In contrast, cutan is characterized by a higher proportion of crystalline paraffinic moieties (37%) versus 32% of amorphous carbon. The presence of crystalline and amorphous regions in cuticular materials has a significant impact on their ability to sorb pollutants. While minimal sorption occurs with the crystalline moieties, the amorphous moieties provide sites for partitioning of solutes into the rubbery domains. Thus we believe that the level of amorphous paraffinic carbon, rather than the total level of paraffinic domains, governs sorption of nonpolar HOCs such as phenanthrene. Desorption. Sorption–desorption curves with cutin and cutan are presented in Figures 2 and 3, respectively. The desorption behavior of all solutes was quite dependent on the sorbate’s properties (i.e., different desorption trend for different sorbates) rather than on the sorbent’s structural VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Sorption Parameters of HOCs with Cutin and Cutan Biopolymers sorbent
compounds
KF (( standard error) (mg/kg) · (mg/L)-N
KFOC
N (( standard error)
Freundlich r 2
KOC (L/kg C)
KOC/KOWC
cutin cutin cutin cutin
atrazine carbamazepine chlorotoluron phenanthrene
580 ( 13 115 ( 6 240 ( 30 91980 ( 6680
810 160 340 128600
0.90 ( 0.01 0.88 ( 0.02 0.91 ( 0.03 0.97 ( 0.03
0.998 0.996 0.994 0.992
660 120 245 149500
0.97 0.31 0.70 3.83
cutan cutan cutan cutan
atrazine carbamazepine chlorotoluron phenanthrene
290 ( 10 120 ( 5 180 ( 20 71880 ( 3490
380 150 240 93600
0.95 ( 0.03 0.85 ( 0.03 0.86 ( 0.03 1.02 ( 0.02
0.989 0.996 0.993 0.998
340 110 260 91200
0.50 0.29 0.76 2.34
properties (i.e., similar desorption hysteresis for both sorbents). Sorption–desorption hysteresis can be explained by the dual mode (28). Organic sorbents consist of expanded (rubber-like) and condensed (glass-like) domains. Typically, the rubber-like domains promote linear and reversible isotherms whereas the glass-like domain is responsible for nonlinear isotherms and for pronounced desorption hysteresis. In our study, phenanthrene isotherms with cutin and cutan were linear (N values of 0.97 and 1.02, respectively) and exhibited fully reversible sorption (Figures 2 and 3, respectively). In addition, phenanthrene sorption affinities were similar in single and bisolute sorption studies (phenanthrene-pyrene; data not shown). These findings together with the high KOC/KOWC ratios suggest that phenanthrene
interacts mainly with the flexible long-alkyl-chain moieties in both sorbents. Like phenanthrene, chlorotoluron exhibited similar desorption behavior with both biopolymers. Up to the equilibrium concentration of 1 mg/L, chlorotoluron exhibited statistically similar sorption and desorption curves. Above this concentration, enhanced desorption was exhibited (i.e., desorption curves were below the sorption isotherm). This trend increased with increasing solute equilibrium concentration. In a previous work (18), we reported enhanced desorption of atrazine with bulk and dewaxed cuticle due to a phase transition of the condensed sorption domain in the presence of a high load of competitor (ametryn). In the present study, the experiments were conducted in a singlesolute system, and the concentration of dissolved organic
FIGURE 2. Sorption–desorption of atrazine, carbamazepine, chlorotoluron, and phenanthrene by cutin biopolymer. Each point represents the mean of triplicate measurements; bars represent standard error. Filled symbols are for sorption data, and open symbols are for the desorption points (each desorption isotherm is represented by a different symbol). 1168
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FIGURE 3. Sorption–desorption of atrazine, carbamazepine, chlorotoluron, and phenanthrene by cutan biopolymer. Each point represents the mean of triplicate measurements; bars represent standard error. Filled symbols are for sorption data, and open symbols are for the desorption points (each desorption isotherm is represented by a different symbol). matter (can serve as a third medium in the system) was negligible. It is suggested that enhanced desorption results from a conformational change in the sorbent at high solute loading (33–35). Carbamazepine exhibited sorption–desorption hysteresis with both sorbents although higher desorption hysteresis was observed with the cutan. The desorption hysteresis observed in our study was lower than that reported for carbamazepine with soils (27). In our study, desorption hysteresis increased with decreasing carbamazepine concentration. Moreover, sorption isotherms for this solute were nonlinear, with N values of 0.88 and 0.85 for cutin and cutan, respectively (Table 2). This suggests that carbamazepine probably interacts with cutin and cutan by hydrophobic and specific interactions. The latter binding governs the interactions at low concentrations and therefore results in a lower ability for desorption. The observed desorption trend can be explained by a limited available number of high-energy sites in the biopolymers: 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 (33). The different sorption– desorption behavior of carbamazepine as compared to phenanthrene suggests that they bind to different sorption domains in the biopolymers. Carbamazepine can interact specifically with polar moieties of the sorbents, which can be present in the rubber- or glass-like domains. When sorption was conducted at 60 °C (above the glass transition
temperature, Tg, of the studied biopolymers), a decrease of 20-25% in the sorption affinity of carbamazepine and an increase in the linearity of the isotherms from 0.88 to 1.07 and from 0.85 to 0.94 were observed with cutin and cutan, respectively. Temperature affects sorption behavior as a result of increasing solute solubility and sorbent (biopolymer) mobility (36, 37). Sorption typically becomes more linear with temperatures approaching Tg as the amorphous structures become more rubbery and facilitate partitioning as compared to hole-filling interactions. Therefore our data suggest that carbamazepine is sorbed by specific interactions with the more condensed domains in cutin and cutan. In our previous work, we reported reversible sorption behavior of atrazine with cutan based on a “single-point” desorption isotherm. However, in this study, we report a significant sorption–desorption hysteresis of atrazine to the same sorbent (i.e., cutan) based on four sequential desorption steps. This means that a “single-point” desorption isotherm, which is commonly represented as a desorption isotherm (28, 38, 39), does not represent the desorption behavior of a system, since only one point belongs with a particular starting sorption point (40). This issue is more salient in systems in which desorption behavior is strongly dependent on the solute concentration. A single-point desorption curve of atrazine with cutan reveals no hysteresis, whereas a desorption isotherm based on four sequential desorption steps belonging to a particular starting sorption point shows significant hysteresis. Similar to carbamazepine, atrazine VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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exhibited pronounced desorption hysteresis with both sorbents. However, atrazine’s affinity to cutin was much higher than that to cutan, and the KOC/KOWC ratio with cutin was twice that with cutan (0.97 vs 0.50, respectively, Table 2). Moreover, the KOC/KOWC ratio for atrazine increased with decreasing atrazine concentration, suggesting a higher proportion of specific atrazine-sorbent interactions at low concentration. The higher polarity value for cutin and the higher sorption affinity for atrazine with this sorbent suggest that more sites are available for interactions with atrazine in cutin than in cutan. The easily desorbed molecules at high atrazine concentrations probably interacted nonspecifically whereas at low atrazine concentration more molecules interacted via multicomplexing H-bonding and were therefore more difficult to extract. In summary, in this study we report the sorption behavior of environmentally important organic pollutants to cutin and cutan biopolymers. Our data demonstrate the markedly high sorption capacity of these biopolymers for HOCs due to their hydrophobic nature and the presence of polar sites in their condensed glass-like structures. In general, both sorbents exhibited similar sorptive properties, whereas the higher sorption of phenanthrene and atrazine with cutin was probably due to its more flexible structure and higher number of polar sites. Cutan is less polar and is composed of a higher level of rigid paraffinic moieties, and it is therefore less attractive as a sorbent. Although both cutin and cutan exhibited markedly high sorption affinity for HOCs, it is important to evaluate their sorption potential as natural sorbents in soils.
Acknowledgments This research was supported by a research grant from BARD (No. IS-3385-03), the United States-Israeli Binational Agricultural Research and Development Fund.
Supporting Information Available FTIR spectra of cutin and cutan biopolymers. This material is available free of charge via the Internet at http:// pubs.acs.org.
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