Single-solute and bi-solute sorption of phenanthrene and pyrene onto pine needle cuticular fractions

Environmental Pollution 158 (2010) 2478e2484

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Single-solute and bi-solute sorption of phenanthrene and pyrene onto pine needle cuticular fractions Yungui Li, Baoliang Chen*, Lizhong Zhu Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China

Cellulose components play a regulating role in the relative contribution of aromatic cores and aliphatic moieties to sorption of pine needle cuticular fractions.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 August 2009 Received in revised form 14 March 2010 Accepted 16 March 2010

To better understand interaction mechanisms of pine needles with persistent organic pollutants, singlesolute and bi-solute sorption of phenanthrene and pyrene onto isolated cuticular fractions of pine needle were investigated. The structures of cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy and solid-state 13C NMR. Polymeric lipids (cutin and cutan) exhibited notably higher sorption capabilities than the soluble lipids (waxes), while cellulose showed little affinity with sorbates. With the coexistence of the amorphous cellulose, the sorption of cutan (aromatic core) was completely inhibited, so the cutin components (nonpolar aliphatic moieties) dominated the sorption of bulk needle cuticle. By the consumption of the amorphous cellulose under acid hydrolysis, sorption capacities of the de-sugared fractions were dramatically enhanced, which controlled by the exposed aromatic cores and the aliphatic moieties. Furthermore, the de-sugared fractions demonstrated nonlinear and competitive sorption due to the specific interaction between aromatic cores and polycyclic aromatic hydrocarbon. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbon Pine needle cuticle Sorption Aromatic core Aliphatic phase

1. Introduction Evergreen coniferous forests play a critical role in the global transport of persistent organic pollutants (POPs) due to the high degree of forestation and high leaf area index of conifers (Kylin and Sjodin, 2003; Hellstrom et al., 2004). Their canopies act as a buffer pool for POPs and could significantly enhance the precipitation of POPs from atmosphere to ecosystems and soils, termed the forest filter effect (Su and Wania, 2005; Su et al., 2007; Nizzetto et al., 2008). Numerous studies have demonstrated that pine needles present high affinity with organic pollutants (Kylin and Sjodin, 2003; Nizzetto et al., 2006), and then function as the main contributor for trapping atmospheric organic pollutants. Pine needles were widely used as good passive samplers to monitor airborne organic pollutants and to assess the transport of atmospheric POPs on a global and regional scale (Eriksson et al., 1989; Tremolada et al., 1996; Lehndorff and Schwark, 2009a,b). Recently, Wang et al. (2009) suggested that soils sequestered PAHs mainly via dry/wet deposition of particles and pine needles trapped PAHs mainly from the gas phase. Therefore, the

* Corresponding author. E-mail address: [email protected] (B. Chen). 0269-7491/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2010.03.021

distribution of polycyclic aromatic hydrocarbons (PAHs) in soils and pine needles could be used to infer the information of gasparticle partitioning. Nevertheless, the interaction mechanisms of pine needles cuticle with hydrophobic pollutants are still not well understood. The hydrophobic epicuticular waxes of plant were usually assumed to be the main partition medium for organic contaminants (Schreiber and Schonherr, 1992; Ratola et al., 2009). However, some studies emphasized that the extractable lipids (i.e., waxes) were not sufficient to illustrate plant uptake, and highlighted the dominant role of the polymeric lipids (i.e., cutin and cutan) in the affinity of plant cuticles with organic pollutants (Chen et al., 2005, 2008a; Shechter and Chefetz, 2008; Li and Chen, 2009). Ratola et al. (2009) reported that the total concentration of PAHs was much higher in needles than that in barks, considering the same tree. But, the reason for such distinctive behavior of plant uptake is not elucidated. The main objective of the current study is to elucidate the effect of structural characteristics on the sorption properties of pine needle cuticular fractions. Single-solute and bi-solute sorption of phenanthrene and pyrene onto isolated pine needle cuticular fractions were conducted. The cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy (FTIR) and solid-state 13C NMR.

Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

2.1. Isolation of pine needle cuticular fractions Pine needle cuticular fractions were isolated by a similar method that used in an earlier study (Chen et al., 2008a). Pine needles were collected from the campus woods in Zhejiang University, China on April 2008. Cuticles were peeled manually from the pine needles, then incubated in a solution of oxalic acid (4 g/L) and ammonium oxalate (16 g/L) at 90
C for 24 h and washed with deionized distilled water to remove any residual mesophyll materials and the used chemicals. Isolated pine needle cuticle (PNC1) was oven-dried at 60
C, ground, and sieved (<0.18 mm) before the sequential or individual extraction procedure. Firstly, extractable lipids (waxes) were removed from PNC1 by Soxhlet extraction with chloroform/methanol (1:1) at 70
C for 24 h, yielding dewaxed-fraction (PNC2). Secondly, to remove the depolymerizable lipid fraction (i.e., cutin), PNC2 was saponified with 1% potassium hydroxide in methanol for 3 h at 70
C under refluxing and stirrer-spinning conditions, producing the non-saponifiable fraction (PNC3). Thirdly, acid hydrolysis were conducted in 6 mol/L HCl solution with refluxing for 6 h at 100
C to eliminate the polysaccharides component from the non-saponifiable fraction (PNC3), dewaxed-fraction (PNC2), and bulk sample (PNC1), producing cutan fraction PNC4, dewaxedede-sugared fraction (PNC5), and de-sugared fraction (PNC6), respectively. The residues were separated from the basic or acidic solution by filtration, and then were washed with a mixed solution of methanol and deionized distilled water (V/V, 1:1) to adjust these fractions to neutral conditions and to remove dissolved organic matter (e.g., cutin monomer and carbohydrates) sorbed by these residues. All samples were oven-dried at 60
C, ground, and sieved (<0.18 mm) before analysis and sorption experiments. The yields of cuticle fractions (PNC2ePNC6) were all calculated to the percentage contents of bulk cuticle (PNC1). 2.2. Characterization of the pine needle cuticular fractions Elemental (C, H, N) analysis was conducted via an EA 112 CHN elemental analyzer (Thermo Finnigan), while the oxygen content was calculated by the mass difference. The H/C and (O þ N)/C atomic ratios were calculated to evaluate the aliphatic characteristics and polarity of the isolated pine needle cuticular fractions, respectively. FTIR spectra were recorded in the 4000e600 cm1 region by a Bruker Vector 22 attenuated total reflectance-Fourier transform infrared (ATR-FTIR) with a resolution of 4.0 cm1. Solid-state cross-polarization magic angle-spinning and total-sideband-suppression 13C NMR spectra (CPMAS-TOSS) were obtained with a Bruker AVANCE-400WB spectrometer (BIOSPINAG, Germany) operated at the 13C frequency of 100.6 MHz. The instrument was run under the following conditions: contact time, 2 ms; spinning speed, 10 kHz; 90
1H pulse, 4.1 ms; acquisition delay, 4 s; line broadening, 35 Hz; and number of scans, 10 000. Within the 0e220 ppm chemical shift range, C atoms were assigned to paraffinic carbons (0e50 ppm); substituted aliphatic carbons (50e109 ppm); aromatic carbons (109e163 ppm); carboxyl carbons (163e190 ppm); and carbonyl carbons (190e220 ppm). 2.3. Single-solute and bi-solute sorption experiment Phenanthrene and pyrene were chosen as model polycyclic aromatic hydrocarbons (PAHs) for their abundance in the pine needles (Lehndorff and Schwark, 2004; Ratola et al., 2009; Wang et al., 2009), and their respective physicochemical properties are as follows: molecular weight of 178.2 and 202.3 g/mol; aqueous solubility of 1.29 and 0.135 mg/L (25
C); and octanolewater partition coefficient (Kow) of 28840 and 80000. Batch phenanthrene and pyrene sorption by all isolated cuticular fractions (PNC1ePNC6) was performed at 25  0.5
C as described elsewhere (Chen et al., 2008a). In brief, the initial concentrations were ranging from 5 to 950 mg/L for phenanthrene, and from 0.5 to 95 mg/L for pyrene. The background solution included 0.01 mol/L CaCl2 and 200 mg/L NaN3, at pH ¼ 7. Each isotherm contained ten concentration points with a constant solidesolution ratio, and each point including the control (with sorbent and without sorbate) and calibration (with sorbate and without sorbent), was run in duplicate. Certain amount (1e5 mg) sorbent was placed into the 8 or 40 mL screw cap vials and then full-filled with sorbate solution to minimize the evaporation part and ensure 30e80% removal rate of sorbate. After 3 days equilibration, the solution was separated by centrifugation at 4000 rpm for 15 min, and 0.5 mL aliquots were mixed with 0.5 mL acetonitrile for high performance liquid chromatography (HPLC) analysis. Phenanthrene and pyrene concentrations were measured with an Agilent 1200 HPLC fitted with G1321A fluorescence detector and Agilent Eclipse XDB-C 18 column (4.6 mm  250 mm  5 mm). Injection volumes of 15 mL, a mobile phase of 90% acetonitrile/10% water with a flow rate of 1 mL/min, an excitation wavelength 244 nm with emission wavelength of 360 nm for phenanthrene, and an excitation wavelength 237 nm with emission wavelength of 385 nm for pyrene were used. Because of minimal mass loss, the sorbed solute was calculated by the aqueous concentration difference. Competitive sorption experiments for all cuticular fractions were conducted with a similar procedure to single-solute sorption experiments at 25  0.5
C. A bi-solute sorption system was designed: pyrene was used as the primary solute with various concentrations ranging from 0.5 to 95 mg/L, and phenanthrene as a competitor was fixed at one initial concentration (i.e., 0.5 mg/L).

Table 1 Relative mass fractions of the pine needle cuticular and their elemental analysis and atomic ratios. Samplea

Yield, %wtb C, %wt H, %wt N, %wt O, %wtc (N þ O)/C H/C

PNC1 100 PNC2 94.66 PNC3 83.13 PNC4(Cutan) 35.23 PNC5 47.09 PNC6 49.80 d 5.34 Wax d 11.53 Cutin d 47.90 Sugar

50.23 48.04 44.81 57.93 58.53 61.50 71.49 79.45 37.94

6.69 6.36 5.96 5.37 5.95 6.48 14.15 8.49 6.62

0.16 0.23 0.26 0.26 0.28 0.26 0.01 0.00 0.13

42.93 45.37 48.97 36.43 35.24 31.76 14.34 12.60 55.31

0.64 0.71 0.83 0.48 0.46 0.39 0.15 0.11 1.10

1.59 1.58 1.59 1.11 1.21 1.26 2.36 1.27 2.08

a PNC: pine needle cuticular fraction. The number in the name of each sample was identified as follows: “1” for bulk cuticle, “2” for dewaxed, “3” for non-saponifiable, “4” for cutan, “5” for dewaxed-hydrolyzed, “6” for de-sugared. b The yields of all fraction were calculated to the percentage contents of bulk cuticle (PNC1). c Oxygen content was calculated by the mass difference. d Calculated results based on mass balance.

2.4. Data analysis Freundlich model (Q ¼ Kf CeN) was used, where Q is the amount sorbed per unit weight of sorbent, mg/kg; Ce is the equilibrium concentration, mg/L; Kf [(mg/kg)/ (mg/L)N] is the Freundlich capacity coefficient, and N (dimensionless) describes the isotherm curvature. Regression parameters (logKf and N) were calculated using the logarithmic form of the equation. Sorption coefficient (Kd) was calculated from the slope of the linear isotherms, and Koc value was obtained by normalizing Kd to the carbon level (foc) of the sorbent.

3. Results and discussion 3.1. Characterization of the pine needle cuticular fractions The mass fractions and elemental composition of isolated pine needle cuticular fractions (PNC1ePNC6) are list in Table 1. The basic compositions of cuticle were soluble lipids (waxes), deploymerizable insoluble lipids (cutin), non-saponifiable biopolymers (cutan), as well as polysaccharides. Cutan (35.2%) and sugar (47.9%) were the dominant constitutes of pine needle cuticle, while waxes and cutin only amounted to 5.34 and 11.5%, respectively. The aromaticity (H/C ¼ 1.59) and polarity index [(O þ N)/C ¼ 1.10] of pine needle cuticle were much higher than fruit cuticles (Chefetz, 2003; Chen et al., 2005; Li and Chen, 2009; Shechter et al., 2006). After acid hydrolysis, polarity of all samples dropped markedly and

3350

2927 2856

1370 1270 1613 1446 1734

780

PNC6

Absorbance

2. Materials and methods

2479

PNC5 PNC4 PNC3 PNC2

1024 PNC1

1311

1506 3600

3200

2800 1800

1600

1400

1200

1041 1000

800

-1

Wavenumber, cm

Fig. 1. Fourier transform infrared spectra of pine needle cuticular fractions.

600

2480

Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

Fig. 2. Solid-state

13

C NMR spectra of pine needle cuticular fractions.

aromaticity rose notably due to the enhancement of cutan content, which is consistent with the previous report (Li and Chen, 2009). The FTIR spectra between 4000 and 600 cm1 for PNC1ePNC6 are shown in Fig. 1. Various function groups were observed, including eOH (3350 cm1), eCH2- (2856, 2927, 1446, 1450, 1370 and 1380 cm1), ester C]O (1734 cm1), aromatic C]C and C]O (1613 and 1600 cm1), aromatic CO- and phenolic eOH (1270 cm1), CeOeC (1041, 1024 cm1 and 780 cm1), aromatic CeH (1056 cm1). Pine needle cuticle was dominated by CeOeC bands due to its high sugar content. Aliphatic eCH2- and ester C]O (1734 cm1) in pine needle cuticles were abundant, and gradually eliminated with the lipids extraction process, i.e., from PNC1 to PNC3. After waxes, cutin and amorphous cellulose removal, aromatic C]C and C]O (1613 cm1), CeH (1506 cm1) and aromatic CO- and phenolic eOH (1270 cm1) were preserved in PNC4, suggesting that aromatic bands were the major functional groups in cutan fraction. Peaks of eOH (3350 cm1) and CeOeC (1041, 1024 cm1 and 780 cm1) of carbohydrates were sharply reduced after acid hydrolysis. The preserved broad peaks at 1041 and 1024 cm1 in PNC4ePNC6 indicated that carbohydrates in

Table 2 Integration results of solid-state sample

PNC1 PNC2 PNC3 PNC4 PNC5 PNC6 a b c d

13

needle cuticle couldn’t be removed completely by HCl hydrolysis. Rosenberg et al. (2003) demonstrated that the removal carbohydrates by HCl hydrolysis were mainly non-structural and hemicellulosic plant carbohydrates (hemicellulose). These observations are in line with the composition and elemental analysis. To better understand the structure and conformation of pine needle cuticle, solid-state 13C NMR spectra and C-containing functional group contents are presented in Fig. 2 and Table 2. According to Table 2, the distinct structural characteristics of pine needle cuticle focused on its much higher polar C (80.1%) and aromatic C (21.2%) than fruit plant cuticles (Chen et al., 2005; Shechter et al., 2006). Similar to spruce needles (Kogel-Knabner et al., 1994), the NMR spectra of pine needle cuticle was dominated by polysaccharide signal (consistent with FTIR spectra). The alkyl-C (0e50 ppm) level was about 9.32% with major peaks at 27 ppm (methylene carbons) and 33 ppm (crystalline aliphatic carbons), which was much lower than that of tomato and pepper cuticles (Chefetz et al., 2000; Chen et al., 2005; Shechter et al., 2006). The O-alkyl-C (50e109 ppm region) was the most abundant moieties of pine needle cuticle, up to 56.36%, exhibited sharp peaks at 72 ppm (C-2, C-3, and C-5 in cellulose) and 105 ppm (anomeric carbon in carbohydrates). Signals located at 56 ppm were the most characteristic functional group for lignin, whereas peaks at 83 ppm and 89 ppm were assigned to amorphous and crystalline celluloses, respectively. The aromatic C content (109e163 ppm) of pine needle cuticle was relatively high (21.2%) in comparison with that of pepper cuticle (8.5%, Chen et al., 2005) due to the cutan content difference (35.23% vs 7.9% for pine needle vs pepper cuticle). Signals in paraffinic carbon region (0e50 ppm) and carboxylic ester C (163e190 ppm) declined markedly after lipids extraction. The peaks at 33 ppm (crystalline aliphatic carbons) reduced sharply after wax removal, while signals toward 30 ppm (amorphous carbons) and 173 ppm (carboxyl/amide carbons) nearly disappeared after saponification treatment. These observations suggested that waxes were the main contributor of crystalline C, and cutin presented rubber-like nature with amorphous carbons. Acid hydrolysis effectively removed hemicellulose fractions in all hydrolyzed samples were supported by the dramatically reduction of carbohydrate C (61e96 ppm, Table 2) and the sharp decrease of the peaks around 72 ppm (Fig. 2). The intensity of peak at 83 ppm (amorphous celluloses) dropped much greatly in comparison with peak at 89 ppm (crystalline celluloses), indicating that amorphous celluloses (hemicellulose) were easier to hydrolyze than crystalline celluloses. The notably increasing intensity at 147 ppm (phenolic carbons) was attributed to the cleavage of aryl ether in lignin. Similar to the humufication process of plant residue, the level of paraffinic C (0e50 ppm) and aromatic C (109e163 ppm) increased rapidly after de-sugared treatment (Kelleher et al., 2006). Plotting total aromatic C amount (y) with cutan content (x), a positive linear relationship (y ¼ 1.99x þ 3.94, R2 ¼ 0.91) was observed, suggesting

C NMR spectra.

Distribution of C chemical shift, ppm(%) 0e50

50e61

61e96

96e109

109e145

145e163

163e190

190e220

9.32 8.51 3.57 16.14 18.80 23.32

2.54 3.19 2.98 6.18 4.51 4.85

42.37 53.19 59.52 19.92 18.80 18.66

11.44 13.83 13.10 7.97 6.39 7.46

10.59 9.04 8.93 25.30 20.49 21.27

10.59 6.38 6.55 18.92 18.42 12.87

8.05 3.19 3.57 2.79 6.02 4.29

5.08 2.66 1.79 2.79 6.58 7.28

aliphatic C: total aliphatic carbon region (0e109 ppm). aromatic C: total aromatic carbon region (109e163 ppm). polar C: total polar C region (50e109 and 145e220 ppm). aliphatic polar C: polar C in aliphatic region (50e109 ppm).

Aliphatic C(%)a

Aromatic C(%)b

Polar C(%)c

Aliphatic polar C(%)d

65.68 78.72 79.17 50.20 48.50 54.29

21.19 15.43 15.48 44.22 38.91 34.14

80.08 82.45 87.50 58.57 60.71 55.41

56.36 70.21 75.60 34.06 29.70 30.97

Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

3.3. Effects of composition on pine needle uptake The accumulation behavior of pine needle with organic pollutants has been mainly attributed to the partition of the outer waxy layer in pine needle cuticle (Schreiber and Schonherr, 1992; Ratola et al., 2009). Nevertheless, sorption coefficients (Kd) just changed a little after waxes removal (PNC2/PNC1 ¼ 1.0 and 0.91 for phenantheren and pyrene, respectively). These phenomena demonstrated that the waxy layer was an effective sorption medium for PAHs and its sorption capacity was presumably similar to that of the bulk cuticle, but this waxy layer wasn’t the main contributor for PAHs accumulation in bulk pine needle cuticle. Furthermore, Kd dropped markedly after saponification (PNC3/PNC2 ¼ 0.38 and 0.37 for phenanthrene and pyrene, respectively), suggesting that cutin was the dominant contributor for the overall sorption of bulk needle cuticle. The dewaxedede-sugared fraction (PNC5) presented the highest sorption capability (Kd, 42860 and 278900 L/kg for phenanthrene and pyrene) among all cuticular fractions. However, it is hard to investigate the sorption capacity of cutin directly when the selected cuticle consisted of cutan fraction, because cutan component couldn’t be intactly removed without disturbing the chemical structure of cutin by enzymatic or chemical methods. Therefore, to assess the sorption properties of cutin, sorption coefficients (Kd) were indirectly estimated through mass balance, 0 0 þ Kd;cutin  fcutin , where Kd,5, Kd,cutan, and i.e., Kd;5 ¼ Kd;cutan  fcutan Kd,cutin are sorption coefficients of the dewaxedede-sugared fraction 0 0 and fcutin are the (PNC5), cutan (PNC4) and cutin respectively; fcutan relative mass contents of cutan and cutin in PNC5 (i.e., 75.34% and 24.66%). The calculated Kd,cutin were 72 560 and 484 300 mL/g,

7500

6000

Sorbed amount (mg/kg)

Single-solute sorption isotherms of phenanthrene and pyrene to the isolated cuticular fractions were conducted (see Fig. 3). Isotherms fit well to the Freundlich model, and the regression parameters are listed in Table 3. Sorption coefficients (Kd and Koc) and Koc/Kowc ratios were also calculated in Table 3, where the Kowc is the carbon-normalized Kow (Kowc ¼ Kow/foc, foc is the percentage of carbon content of octanol, i.e., 73.8%). For phenanthrene and pyrene, isotherms of bulk pine needle cuticle (PNC1), dewaxed fractions (PNC2) and non-saponifiable fraction (PNC3) were practically linear with Freundlich N values of 0.95e1.03, suggesting their sorption mechanisms were dominated by partition. However, all de-sugared samples (PNC4ePNC6) exhibited more nonlinear isotherms in comparison with their precursor samples (PNC3ePNC1). Sorption coefficients (Kd) of bulk needle cuticle were 6660 and 40720 L/kg for phenanthrene and pyrene, respectively. The Kd value of phenanthrene by pine needle cuticle (6660 L/kg), was much lower than that of fruit cuticles and potato periderm (Kd of 16225e53948 L/kg, Li and Chen, 2009), but a little higher than that of the bulk root tissue of switchgrass (Kd of 3974 L/kg, Chen and Schnoor, 2009). The relative lower partition coefficient of pine needle cuticle may be attributed to its relative higher polysaccharides content (47.9%) in comparison with fruits cuticle and potato periderm. A negative linear relationship between Kd and polysaccharides content was also observed (Kd ¼ 1351x þ 73965, R2 ¼ 0.99) among all these selected plant skins, confirming the crucial role of polysaccharides in governing PAHs sorption to plant cuticles.

A

PNC1 PNC2 PNC3 PNC4 PNC5 PNC6

4500

3000

1500

0 0.0

0.1

0.2

0.3

0.4

0.5

Equilibrium concentration (mg/L) 250

B

200

Sorbed pyrene amount (mg/kg)

3.2. Sorption in a single-solute system

respectively, for phenanthrene and pyrene, which are more than 10 times higher than those of bulk cuticle. In view of the mass content of cutin (11.53%) in pine needle cuticle, it was concluded that cutin was the key contributor for the overall sorption of bulk cuticle.

150

PNC1 without phen PNC2 without phen PNC3 without phen PNC1 with phen PNC2 with phen PNC3 with phen

100

50

0 0.000

0.002

0.004

0.006

0.008

Equilibrium concentration (mg/L)

C Sorbed pyrene amount (mg/kg)

that cutan was the main aromatic constitute of cuticlular fractions. All these pronounced changes would significantly affect their affinities with organic pollutants considering the crucial role of aliphatic and aromatic moieties as sorption domains for organic pollutants (Chefetz and Xing, 2009).

2481

1500

1000

PNC4 without phen PNC5without phen PNC6 without phen PNC4with phen PNC5 with phen PNC6 with phen

500

0 0.000

0.002

0.004

0.006

0.008

Equilibrium concentration (mg/L) Fig. 3. Single-solute and bi-solute sorption isotherms to pine needle cuticular fractions [A: single-solute sorption isotherms of phenanthrene (phen); B: single-solute and bisolute sorption isotherms of pyrene to PNC1-PNC3 with the absence and presence of phen (0.5 mg/L) as a competitor. C: single-solute and bi-solute sorption isotherms of pyrene to PNC4ePNC6 with the absence and presence of phen (0.5 mg/L) as a competitor].

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Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

Table 3 Sorption regression parameters of phenanthrene (Phen) and pyrene to pine needle cuticular fractions, and their sorption coefficients (Kd and Koc). Organic pollutants

Sorbenta

logKfb

Nb

Freundlich R2

Kd (L/kg)c

Linear R2

Koc(L/kg)

Koc/Kowcd

Phen

PNC1 PNC2 PNC3 PNC4 PNC5 PNC6 PNC1 PNC2 PNC3 PNC4 PNC5 PNC6 PNC1 PNC2 PNC3 PNC4 PNC5 PNC6

3.823  0.014 3.815  0.029 3.368  0.014 4.396  0.024 4.503  0.021 4.456  0.017 4.628  0.095 4.644  0.076 4.089  0.081 4.977  0.062 5.269  0.063 5.3126  0.105 4.531  0.041 4.512  0.063 4.033  0.047 5.095  0.131 5.332  0.048 5.012  0.053

0.992  0.011 1.008  0.022 0.947  0.011 0.790  0.013 0.856  0.011 0.853  0.010 1.013  0.031 1.032  0.025 0.990  0.028 0.833  0.019 0.917  0.018 0.965  0.003 0.969  0.013 0.975  0.020 0.972  0.016 0.987  0.043 0.992  0.015 0.892  0.016

0.999 0.996 0.999 0.998 0.999 0.999 0.993 0.995 0.994 0.996 0.997 0.991 0.999 0.997 0.998 0.987 0.998 0.997

6600  98 6587  102 2507  28 33137  1294 42859  768 38836  812 40720  1121 36975  780 13530  255 211677  7385 278891  5582 263274  7584 39989  603 37243  504 12167  241 104403  241 209930  5214 180646  4054

0.998 0.998 0.999 0.987 0.997 0.996 0.993 0.996 0.997 0.990 0.996 0.993 0.998 0.998 0.996 0.974 0.994 0.996

13140 13710 5595 57200 73230 63150 81070 76970 30190 365400 476500 428100 79610 77520 27150 180200 358700 293700

0.35 0.36 0.15 1.51 1.93 1.67 0.96 0.91 0.36 4.34 5.66 5.08 0.95 0.92 0.32 2.14 4.26 3.49

Pyrene without phen

Pyrene with 0.5 mg/L phen

a

The meaning for the sorbent name was presented in Table 1. The Freundlich parameters (Kf and N) were calculated using the logarithmic form of the equation Q ¼ Kf CN e , where Q is the amount sorbed per unit weight of sorbent, mg/ kg; Ce is the equilibrium concentration, mg/L; Kf [(mg/kg)/(mg/L)N] is the Freundlich capacity coefficient; and N (dimensionless) describes the isotherm curvature. R is regression coefficient. c Kd is the sorption coefficient (Kd ¼ Q/Ce), calculated from the slope of linear equation. d Koc is the carbon-normalized sorption coefficient (Koc ¼ Kd/foc), and Kowc is the carbon-normalized Kow (Kowc ¼ Kow/foc, foc is the percentage of carbon content of octanol, i.e., 73.8%). The octanol-water partition coefficient is 28,000 for phenanthrene and 80,000 for pyrene. b

Another polymeric lipids, cutan (PNC4) also presented dramatically high sorption capacities (33 140 and 211700 L/kg for phenanthrene and pyrene), which were about five times larger than that of wax and bulk cuticle. Considering the strong affinity of cutan, the non-saponifiable fraction (PNC3) was expected to present high sorption coefficient for its relatively high cutan content (up to 42.38%). However, experimental results showed that sorption capacities of PNC3 for the selected sorbates were the lowest among all fractions, which are more than 10 times lower than those of cutan. These observations indicated that the potential sorption capacities of cutan were completely restricted by the coexistence of sugar fractions, because cutan components may be dispersed among these hydrophilic domains (Chen and Schnoor, 2009). Therefore, cutin and waxes were the effective storage mediums of PAHs in the bulk cuticle, and cutin acted as the key contributor (>90%). However, cutan couldn’t make real contribution to the overall sorption with the presence of amorphous cellulose. Without the presence of the amorphous cellulose, the cutan components exhibited powerful affinity with PAHs, supported by the dramatically enhancement of sorption capacities and more nonlinear sorption isotherms for all hydrolyzed cuticular fractions (PNC4ePNC6). After sugar removal, sorption coefficients (Koc) of pyrene increased more greatly than those of phenanthrene (i.e., PNC4/PNC3 ¼ 12 vs 10, PNC5/PNC2 ¼ 6.2 vs 5.3, PNC6/PNC1 ¼ 5.3 vs 4.8 for pyrene vs phenanthrene), which could be attributed to the more strong specific interaction between cutan (aromatic cores) and pyrene. 3.4. Role of aliphaticity and aromaticity in pine needle uptake The relative role of aliphatic and aromatic moieties in the sorption of organic compounds was well reviewed by Chefetz and Xing (2009). A general positive trend was observed by plotting the Koc values of phenanthrene with aliphaticity, and the strong positive correlation between the sorption affinity and aromaticity was also highlighted. However, as the author concluded there were no significant correlations between either aromaticity or aliphaticity and sorption coefficients. In this study, a positive linear correlation between Koc values and nonpolar aliphatic C content

(0e50 ppm) was presented in Fig. 4A. As expected, sorption affinity increased with the increase of aromatic C content (109e163 ppm) (Fig. 4B). However, as mentioned above, the high sorption capacity of aromatic domains was supposed to be completely inhibited by the coexistence of cellulose, so it is hard to predict the sorption affinity by aromaticity when the samples consisted of large amounts of amorphous cellulose. For these samples, nonpolar aliphatic C was main sorption contributor, and the sorption affinity was assumed to increase with increasing aliphaticity. After the removal of amorphous cellulose, the aromatic domains became accessible for sorption, and sorption affinity was expected to increase with increasing either aliphaticity or aromaticity. This is why a strong relationship between sorption affinity and aromaticity was observed when a sample was modified to remove the polar functional groups (Johnson et al., 1999; Tang and Weber, 2006). For example, with the decomposition process, the sorption of plant cuticle would be governed by the preserved condensed cutan mediums (Stimler et al., 2006; Shechter and Chefetz, 2008). Chen and Schnoor (2009) also reported that lipids (i.e., suberin and waxes) were the main storage mediums for bulk root of switchgrass, but after the removal of amorphous cellulose, the sorption behavior would be mainly dominated by the aromatic lignin rather than lipids. This suggested that the relative role of aromatic and aliphatic moieties was regulated by the amorphous cellulose component. With the presence of amorphous cellulose, sorption was mainly contributed by nonpolar aliphatic moieties; without the presence of amorphous cellulose, both aromatic and aliphatic moieties were effective in sorption. But cellulose itself played a negative role on the sorption, supported by the negatively linear relationship between the sorption affinity and carbohydrate C (61e96 ppm) content (Fig. 4C). 3.5. Sorption isotherms in a bi-solute system Noncompetitive and linear isotherms of pyrene were performed for PNC1ePNC3 (Fig. 3B and Table 3), and the sorption coefficients (Koc) were quite similar in single-solute and bi-solute systems. Nevertheless, strong competitive sorption was observed between phenanthrene and pyrene for all de-sugared fractions (PNC4ePNC6),

Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

resulting in a decrease of 25e51% in the sorption coefficients (Koc). For de-sugared fractions (except PNC6), the linearity of isotherms of pyrene increased with the presence of phenanthrene as a competitor, e.g., Freundlich N values approaching 1 (Shechter et al., 2006). The nonlinear and strongly competitive sorption behaviors presented by de-sugared fractions were also supposed to be the result of the

Sorption conefficient (Koc, L/kg)

A

500000

Phenanthrene Pyrene 400000

300000

y = -98275 + 25707x R = 0.94 200000

y = -12613 + 3787x R = 0.93

100000

0 0

5

10

15

20

25

2483

specific interaction (mainly pep electron interaction) between the sorbates and the exposed aromatic cores (Shechter et al., 2006; Chen and Schnoor, 2009). Since cutan was assumed to be the main aromatic contributor of cuticular fractions, the adsorption amount contributed by specific interaction was expected to rise with the increase of cutan content, i.e., PNC4 > PNC5 > PNC6. To demonstrate this hypothesis, adsorption isotherms separated from the total sorption of de-sugared cuticular fractions were obtained by a modifiedmethod used by a previous study (Chen et al., 2008b). In detail, the adsorption amounts (QA) of de-sugared fractions were calculated as QA ¼ QTeQP ¼ QTeKPCe, where QT is the total amount of the pyrene sorbed onto the cuticular fractions; QA and QP are the amounts contributed by adsorption and partition, respectively; KP is the partition coefficient. Considering the linear isotherm in the bi-solute system, the sorbed amount of pyrene was deemed to be contributed by partition. Hence, QA ¼ QTeKPCe, where QT and Ce were the sorption experimental data in the single-solute system, while KP was the slope of the sorption isotherm of pyrene in the bi-solute system. Plotting the calculated QA value with corresponding Ce, the separated adsorption isotherms of pyrene to de-sugared fractions were presented in Fig. 5A. As expected, the sequence of adsorption amounts were consistent with the order of cutan content, i.e., PNC4 > PNC5 > PNC6, during the experimental concentration range. The maximum adsorption amounts at 0.0005, 0.005, and 0.05 mg/L

Paraffinic carbon (%)

A

500000

400000

1500

1200

Phenanthrene Pyrene

Adsorped amount (Q , mg/kg) A

Sorption conefficients (Koc, L/kg)

B

300000

y = -173655 + 14761x R = 0.92 200000

y = -24922 + 2217x R = 0.97

100000

900

600

PNC4 PNC5 PNC6

300

0 10

20

30

40

0 0.00

50

B Adsorped amount (Q , mg/kg) A

Sorption conefficients (Koc, L/kg)

1200

Phenanthrene Pyrene

300000

y = 610586- 10380x R = 0.96

200000

y = 92341 - 1544x R = 0.96

100000

0.03

0.04

0.05

0.06

1500

500000

400000

0.02

Equilibrium concentration (mg/L)

Aromatic carbon (%)

C

0.01

Ce 0.0005 mg/L Ce 0.005 mg/L Ce 0.05 mg/L

y = -636 +20.0x R = 0.99

900

600

y = -448 + 9.59x R = 0.93 300

y = -115 +2.17x R = 0.94

0 10

20

30

40

50

60

0 60

70

80

90

100

110

Cutan content (%)

Carbohydrate carbon (%) Fig. 4. Relationships between Koc and paraffinic C (0e50 ppm, A), aromatic C (109e163 ppm, B), or carbohydrate C (61e96 ppm, C) of pine needle cuticular fractions.

Fig. 5. The separated adsorption isotherms of pyrene to de-sugared cuticular fractions (A), and the correlation of the maximum adsorption amounts (QA) with cutan content of de-sugared cuticular fractions (B).

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Y. Li et al. / Environmental Pollution 158 (2010) 2478e2484

pyrene were linearly correlated with cutan content of the de-sugared fractions (see Fig. 5B). These in turn confirmed that pep electron interaction between the aromatic cutan and PAHs was the key contributor for nonlinear adsorption. 4. Conclusions Polymeric lipids of pine needle cuticle, i.e., cutin and cutan, exhibited notably higher sorption abilities than soluble lipids (waxes), while cellulose showed little affinity with PAHs. However, the powerful sorption capacity of aromatic cutan was completely depressed by coexisting amorphous cellulose so that it couldn’t make real contribution for the overall sorption. Therefore, the aliphatic moieties (i.e., waxes and cutin) made a real contributor to the overall sorption of bulk pine needle cuticle. After removal of amorphous cellulose, aromatic cutan became effective sorption medium to PAHs, and the de-sugared fractions demonstrated nonlinear and competitive sorption isotherms. Cellulose was concluded to be the key regulator, which governed the relative role of aromatic and aliphatic moieties as the sorption medium for organic pollutants to pine cuticular fraction. These observations further help to understand the role of evergreen coniferous forests in global transport of persistent organic pollutants. Acknowledgements This project was supported by National Natural Science Foundation of China (20977081, 20737002, and 40671168), and Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 200765). References Chefetz, B., Xing, B., 2009. Relative role of aliphatic and aromatic moieties as sorption domains for organic compounds: a review. Environ. Sci. Technol. 43, 1680e1688. Chefetz, B., Deshmukh, A.P., Hatcher, P.G., Guthrie, E.A., 2000. Pyrene sorption by natural organic matter. Environ. Sci. Technol. 34, 2925e2930. Chefetz, B., 2003. Sorption of phenanthrene and atrazine by plant cuticlar fractions. Environ. Toxic. Chem. 22, 2492e2498. Chen, B., Schnoor, J., 2009. Role of suberin, suberan, and hemicellulose in phenanthrene sorption by root tissue fractions of switchgrass (Panicum virhatum). Environ. Sci. Technol. 43, 4130e4136. Chen, B., Johnson, E.J., Chefetz, B., Zhu, L., Xing, B., 2005. Sorption of polar and nonpolar aromatic organic contaminants by plant cuticular materials: the role of polarity and accessibility. Environ. Sci. Technol. 39, 6138e6146. Chen, B., Li, Y., Guo, Y., Zhu, L., Schnoor, J., 2008a. Role of the extractable lipids and polymeric lipids in sorption of organic contaminants onto plant cuticles. Environ. Sci. Technol. 42, 1517e1523. Chen, B., Zhou, D., Zhu, L., 2008b. Transitional adsorption and partition of nonpolar and polar aromatic contaminants by biochars of pine needles with different pyrolytic temperatures. Environ. Sci. Technol. 42, 5137e5143. Eriksson, G., Jensen, S., Kylin, H., Strachan, W., 1989. The pine needles as monitor of atmospheric pollution. Nature 341, 42e44.

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