Environ Sci Pollut Res (2014) 21:3318–3330 DOI 10.1007/s11356-013-2266-9

RESEARCH ARTICLE

The effect of structural compositions on the biosorption of phenanthrene and pyrene by tea leaf residue fractions as model biosorbents Zemin Xi & Baoliang Chen

Received: 19 July 2013 / Accepted: 21 October 2013 / Published online: 14 November 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract To enhance the removal efficiency of polycyclic aromatic hydrocarbons (PAHs) by natural biosorbent, sorption of phenanthrene and pyrene onto raw and modified tea leaves as a model biomass were investigated. Tea leaves were treated using Soxhlet extraction, saponification, and acid hydrolysis to yield six fractions. The structures of tea leaf fractions were characterized by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The amorphous cellulose components regulated the sorption kinetics, capacity, and mechanism of biomass fractions. The adsorption kinetics fit well to pseudo-second-order model and isotherms followed the Freundlich equation. By the consumption of the amorphous cellulose under acid hydrolysis, both the aliphatic moieties and aromatic domains contributed to total sorption, thus sorption capacities of the de-sugared fractions were dramatically increased (5–20-fold for phenanthrene and 8–36-fold for pyrene). All de-sugared fractions exhibited non-linear sorption due to strong specific interaction between PAHs and exposed aromatic domains of biosorbent, while presenting a relative slow rate because of the condensed domain in de-sugared samples. The availability of strong sorption phases (aromatic domains) in the biomass fractions were controlled by polar polysaccharide components, which were supported by the FTIR, CHN, and SEM data. Responsible editor: Leif Kronberg Z. Xi : B. Chen Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China Z. Xi : B. Chen (*) Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Zhejiang University, Hangzhou, Zhejiang 310058, China e-mail: [email protected]

Keywords Tea leaves . Biosorbent . Modification . Polycyclic aromatic hydrocarbon . Structural effect . Wastewater treatment

Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants containing two or more fused aromatic rings of carbon and hydrogen atoms, which are mainly formed from the incomplete combustion of hydrocarbons and biomass, and have been widely detected at elevated concentrations in storm water runoff and wastewater streams (Chen et al. 2004; Zhu et al. 2004; Huang et al. 2006; Dai et al. 2011). PAHs, typical persistent organic pollutants (POPs), have attracted lots of research attention because of their strong hydrophobicity, tendency of bioaccumulation and biomagnification, and cancerogenic properties (Tao et al. 2006). Due to their low solubility and resistance to biodegradation, some conventional physicochemical techniques including ozone oxidation (Chu et al. 2010), photocatalysis (Woo et al. 2009), and biodegradation (Haritash and Kaushik 2009) are not effective to remove PAHs from water. Sorption is an economical and effective treatment method for removing PAHs and active carbon was widely used as a conventional sorbent (Ma and Zhu 2006; Crisafully et al. 2008). However, high cost limits its application in largescale wastewater treatment (Huang et al. 2006; Li et al. 2010a, b). Biosorption is a branch of biotechnology that uses dead or inactive biomass from various origins to reduce chemical concentrations in the aqueous compartment (Volesky 2003). Biosorbents such as microorganism (e.g., fungi, yeast, and bacteria), algae, and wood fiber were investigated for their ability to remove heavy metal, dyes, pesticides, and organic pollutants (Aksu 2005; Eberhardt et al. 2006; Chen et al. 2010; Chen and Ding 2012; Ding et al.

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2013; Sulaymon et al. 2013; Giri et al. 2013; Nigam et al. 2013), and the biosorption of PAHs from aqueous solution has received more and more attention (Chan et al. 2006; Chung et al. 2007; Chen et al. 2011; Olivella et al. 2013a, b). Recently, the removal of PAHs has been focused on microorganism and algae biomass, while plant residues as a biosorbent for PAHs removal need more studies (Huang et al. 2006; Boucher et al. 2007; Li et al. 2010a; Chen and Schnoor 2009; Chen et al. 2011; Ding et al. 2013). Using plant residues as biosorbents for wastewater treatment has increasingly attracted attention due to their relative high sorption affinity with POPs, ubiquity in the environment, and their easy modification into higher sorption capacity materials (Ho et al. 2005; Huang et al. 2006; Li et al. 2010a; Olivella et al. 2013b). Olivella et al. (2011, 2013a, b) found that cork, a natural raw material, presented high affinity with PAHs and could be used as a potential biosorbent for wastewater treatment. Tea is the most widely consumed beverage in the world with a global production of about 3.5 million tons per year (Yoshita et al. 2009). After consumption, tea residues are usually discarded directly into the environment as biomass waste (Sakasegawa and Yatagai 2005). Using waste tea leaves as biosorbents to remove heavy metals (Ahluwalia and Goyal 2005; Park et al. 2008; Mondal 2009; Gupta et al. 2009; Yoshita et al. 2009), dyes (Li et al. 2010c; Hameed 2009), and organic pollutants (Lin et al. 2007) from aqueous solution have been reported. Lin et al. (2007) found that the raw tea leaves and brewed tea leaves displayed relative high sorption affinity with phenanthrene. However, the effects of structural characteristics (e.g., polarity and aromaticity) on POP sorption by the biomass fractions are still unavailable. More studies are needed to account for the differences in sorption behavior of biomass fractions (Wang et al. 2007; Olivella et al. 2013a). Lignin is assumed to be the main storage medium of organic pollutants to raw and treated biomass (Huang et al. 2006; Li et al. 2010a). The importance of aromatic structures of lignin for HOC affinity has been reported (Wang et al. 2007), and biosorbents with higher lignin contents show higher affinity with POPs (Mackay and Gschwend 2000; Crisafully et al. 2008). Recently, Li et al. (2010a) reported that the powerful sorption potential of lignin was seriously restricted by the coexisting polysaccharide (polar component) and the sorption capacity of modified biomass residues can be notably enhanced with the removal of sugar. Furthermore, the effects of organic components in biomass residues on the sorption behaviors of organic pollutants are significantly important in providing a theoretical basis for the modification of effective biosorbents. The main objective of the current study is to elucidate the dependence of PAH removal by plant biomass fractions on the chemical structures of natural organic materials. To this end, tea leaf was selected as a model biomass and modified via three chemical treatments including Soxhlet extraction, saponification, and acid hydrolysis to obtain different components

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of biomass. The raw and modified tea leaf fractions were characterized by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Sorption kinetics of raw and modified biomass samples was also conducted. Phenanthrene and pyrene were chosen as the model PAHs to determine the relationship between PAHs properties and their sorption behavior on tea leaf residues because they are widely spread in the wastewater and are of relative importance in stormwater runoff.

Materials and methods Tea leaf fractions preparation The tea leaves were collected from Meijiawu in Zhejiang province of China on November 2011. The leaves were washed thoroughly with deionized distilled water to remove dust and soluble impurities, and were then oven-dried for 12 h at 70 °C, ground, sieved less than 0.154 mm, and named T1. The biomass waste was treated by Soxhlet extraction, saponification, and acid hydrolysis, using a reported method (Chen et al. 2008). Firstly, the extractable lipids were removed from T1 by Soxhlet extraction with chloroform/methanol (1:1) at 70 °C for 6 h, and the extracted residue was named dewaxed fraction (T2). Secondly, T2 was saponified with 1 % potassium hydroxide in methanol for 3 h at 70 °C under refluxing and stirrer-spinning conditions, removing the depolymerizable lipid fraction and producing the non-saponifiable fraction (T3). Thirdly, acid hydrolysis, conducted in 6 mol/L HCl solution with refluxing for 6 h at 100 °C, was used to eliminate the polysaccharides component from the samples of T3, T2, and T1, producing dewaxed–non-saponifiable–de-sugared fraction (T4), dewaxed–de-sugared fraction (T5), and desugared fraction (T6), respectively. All residues were separated from the basic or acidic solution by filtration and then washed with a mixed solution of methanol and deionized distilled water (v/v, 1:1) or only deionized distilled water to adjust these fractions to neutral conditions and to remove dissolved organic matter sorbed by these residues. All samples were oven-dried at 60 °C, ground, and sieved less than 0.154 mm before analysis and sorption experiments. Characterization of the tea leaf fractions Elemental (C, H, N) analysis was conducted via a Flash EA 1112 CHN elemental analyzer (ThermoFinnigan), while the oxygen content was calculated by the mass difference. The atomic ratio of H/C and (O + N)/C was calculated to measure the aromaticity and polarity of samples. FTIR spectra of the samples were obtained for a wave number range of 4,000 to 400 cm−1 on a Nicolet 6700 fourier transform infrared (FTIR) with a resolution of 4.0 cm−1. Tea fraction samples (2 mg)

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were mixed with 98 mg KBr (ensuring 20–80 % transmittance rate) and compressed into pellets, and then FTIR analysis was conducted. The tea leaf biomass and modified fractions were examined with a Hitachi S4800 scanning electron microscopy (Japan) under high vacuum condition and at an accelerating voltage of 3.0 kV in order to observe the microstructure of tea fractions. For SEM, all samples were sputtered with gold in a sputter coater before being examined. Batch sorption experiment Phenanthrene and pyrene were chosen as representative polycyclic aromatic hydrocarbons (PAHs) due to their different molecular properties and ubiquity in the environment. Their physicochemical properties are listed in Table 1. The sorption kinetics of phenanthrene and pyrene to tea leaf fraction were conducted at 25±0.5 °C. The solid-to-liquid ratio was 1–5 mg/8 mL for phenanthrene to achieve 30–80 % removal of sorbate. Similarly, for pyrene, the ratio was 1.5–3 mg/8 mL for T1, T2, and T3 and 1 mg/40 mL for T4, T5, and T6. For sorption kinetic experiments, the selected initial concentration was 1 mg/L for phenanthrene and 0.1 mg/L for pyrene; the sampling time ranged from 0.5 to 131 h for phenanthrene and from 0.5 to 77 h for pyrene. The sorption isotherms of phenanthrene and pyrene to the six tea leaf fractions were conducted. In brief, the initial concentrations were ranging from 0.008 to 1 mg/L for phenanthrene and from 0.001 to 0.1 mg/L for pyrene. The background solution (pH = 7) contained 0.01 mol/L CaCl2 in deionized distilled water to stimulate the actual soil solution, with 200 mg/L NaN3 as a biocide to restrain the growth of the bacteria. Each isotherm contained ten concentration points; each point, including blanks (i.e., without tea leaves), was run in duplicate. Certain amount (1–5 mg) sorbent was placed into the 8- or 40-mL screw cap vials and then full-filled with sorbate solution to minimize evaporation and ensure 30–80 % removal rate of sorbate, sealed with aluminum paper, and then agitated in the dark for 3 days at 25±0.5 °C to reach apparent equilibrium (prior tests indicated that sorption equilibrium was achieved in less than 2 days). After 3 days of equilibration, the solution was separated by centrifugation at 3,500 rpm for 15 min, and 0.5 mL supernatant was mixed with 0.5 mL acetonitrile for highperformance liquid chromatography (HPLC) analysis.

PAHs

MFa

MWb

S w (mg/L)c

K owd

Phenanthrene Pyrene

C14H10 C16H10

178.2 202.3

1.29 0.135

28,000 80,000

MF molecular formula

b

MW molecular weight

c

S w aqueous solubility at 25 °C

d

K ow octanol–water partition coefficient

Data analysis The Freundlich parameters (log K f and N) were calculated using the logarithmic form of the equation log Q = log K f + N log C e by OriginPro8.5, where Q is the amount sorbed per unit weight of sorbent (mg/kg), C e is the equilibrium concentration in the aqueous solution (mg/L), K f [(mg/kg)/(mg/L)N ] is the Freundlich capacity coefficient, and N (dimensionless) describes the isotherm curvature. Sorption coefficients (K d) were calculated from the slope of the linear isotherms. K oc values were calculated by normalizing K d to the carbon level (f oc ) of each tea leaf fraction. The pseudo-first-order Lagergren model is expressed as log⋅ðqe −qt Þ ¼ log⋅qe −k 1 t=2:303

ð1Þ

where q e and q t are the amounts sorbed per unit weight of sorbent (mg/kg) at equilibrium and at time t (h), respectively; and k 1 (h−1) is the constant of pseudo-first-order rate. The pseudo-second-order kinetic model is expressed as t=qt ¼ 1=k 2 qe 2 þ t=qe

ð2Þ

where k 2 (kg mg−1 h−1) is the constant of pseudo-second-order rate.

Results and discussion Characterization of the tea leaf fractions

Table 1 The physicochemical properties of selected PAHs

a

Phenanthrene and pyrene concentrations were measured by an Agilent 1200 HPLC fitted with G1321A fluorescence detector and Agilent Eclipse XDB-C 18 column (4.6 mm×250 mm× 5 μm). Injection volumes of 15 μL, 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 sorption by the vials was negligible and the losses from evaporation, biodegradation, and photodegradation were insignificant, the sorbed solute was calculated by the aqueous concentration difference of sorbate between the control and calibration.

The elemental composition of the tea leaf fractions (T1–T6) are presented in Table 2. The main chemical compositions of tea leaves were flavanols (25 wt.%), protein (15 wt.%), polysaccharides (13 wt.%), lignin (6 wt.%), as well as ash (5 wt.%) (Balentine et al. 1997). With the removal of extractable lipids and polymer lipids step by step (T1 → T2 → T3), the carbon content of tea leaf fractions gradually decreased, while the oxygen content and nitrogen content increased

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Table 2 The elemental analysis and atomic ratios of the tea leaf fractions Samplesa

C (wt.%)

H (wt.%)

N (wt.%)

O (wt.%)b

H/C

O/C

(N+O)/C

T1 T2 T3 T4 T5 T6

47.69 44.82 42.03 57.07 60.80 66.39

6.51 6.07 5.58 4.94 5.63 6.83

3.71 4.27 4.74 2.67 1.37 1.36

42.09 44.84 47.65 35.32 32.20 25.42

1.64 1.63 1.59 1.04 1.11 1.23

0.66 0.75 0.85 0.46 0.40 0.29

0.73 0.83 0.95 0.50 0.42 0.30

T tea leaf fraction. The number in the name of each sample was identified as follows: “1” for bulk tea leaves, “2” for dewaxed, “3” for non-saponifiable, “4” for dewaxed–non-saponifiable–de-sugared, “5” for dewaxed–de-sugared, and “6” for de-sugared a

b

Oxygen content was calculated by the mass difference

correspondingly. The calculated polarity index (O + N)/C increased from 0.73 for T1 to 0.83 for T2 and 0.95 for T3. The decrease of H/C ratio indicated that the aromaticity increased to some extent after wax extraction and saponification treatment. After acid hydrolysis (T3 → T4; T2 → T5; T1 → T6), the polarities of all samples dropped markedly and aromaticity enhanced notably due to the removal of polysaccharides (polar components). As a result, the dewaxed–non-saponifiable–de-sugared fraction (T4) exhibited the highest aromaticity in comparison with the other fractions. The polarity index [(O + N)/C = 0.73] of bulk tea leaves were higher than that of pine bark (Li et al. 2010b), wood chip, pine needle (Chen et al. 2011), and exhausted coffee waste (Pujol et al. 2013). The FTIR spectra for T1–T6 are shown in Fig. 1. Various functional groups were observed, including –OH (3,366 cm−1), –CH2– (2,925, 2,854, 1,448, 1,377, and 1,323 cm−1), ester C=O (1,705, 1,651, and 1,147 cm−1), aromatic C=C and C=O (1,615

and 1,513 cm−1), phenolic –OH (1,205 cm−1), C–O–C (1,068, 1, 038, and 770 cm−1). The bands at 1,521, 875, and 823 cm−1 are assigned to aromatic components. Wax extraction from tea leaves (T1 → T2) induced a severe reduction of the aliphatic – CH2 (2,925 and 2,854 cm−1) and a decrease of ester C=O and polysaccharide bands (1,651 cm−1; 1,068 and 770 cm−1). During the saponification treatment (T2 → T3), aliphatic – CH2 and ester C=O were further eliminated, which was corresponding to the removal of hydroxy-fatty acids. The T3 sample presented a significant increase in (O + N)/C atomic ratio compared with T1 and T2 (Table 1), indicating the relative abundance of polar functional groups. The T3 sample was dominated primarily by polysaccharides (1,068 cm−1). When the T3 sample was further hydrolyzed (T3 → T4), the polysaccharides were removed. Some functional groups like aromatic C=C and C=O (1,615, 1,513 cm−1) and phenolic –OH (1, 205 cm-1) were preserved in T4, indicating that aromatic bands were the major functional groups in T4 fraction. Peaks of –OH

Fig. 1 Fourier transform infrared spectra of tea leaf fractions

2925

1651 1705

1323 1615

2854

1205

1513

3366

1448

1147 823

628

Absorbance

T6

T5

T4

T3

T2

T1 1521

4000

3500

3000

2000

Wavenumber,cm-1

1377 1238 1068

1500

1038

1000

770

500

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(3,366 cm−1) and C–O–C (1,068, 1,038, and 770 cm−1) of carbohydrates were sharply reduced after acid hydrolysis. The mask of aliphatic moieties and aromatic core were exposed by exhausting of polar polysaccharides (Chen et al. 2008; Li et al. 2010ab). T5 and T6 samples were dominated by CH 2 bands and C=O stretching vibrations of ester groups after hydrolyzing the T2 and T1 samples, respectively. Compared with other fractions, T4 sample preserved more aromatic domains which were substituted by polar functional groups (e.g., –COOH and –OH). The surface features of different tea leaf fractions were studied by scanning electron microscope. The SEM micrographs of six tea leaf fractions are shown in Fig. 2. It can be seen that the surface morphology of bulk tea leaves (Fig. 2a) was abundant. After wax extraction treatment, the surface of T2 fraction became smoothly (Fig. 2b), with some grooves appearing on some parts of the region, suggesting that waxes Fig. 2 a–f The SEM images of tea leaf fractions including the bulk tea leaves (T1), the dewaxed tea leaves (T2), the nonsaponifiable tea leaves (T3), the dewaxed–non-saponifiable–desugared tea leaves (T4), the dewaxed–de-sugared tea leaves (T5), and the de-sugared tea leaves (T6)

were partly extracted. With saponification treatment (T2 → T3), the non-saponifiable fraction (T3) became thinner (Fig. 2c). Performing further acid hydrolysis treatment upon T3 sample, it can be seen that skeleton structure in T3 disappeared and changed into some fragmentary particles (Fig. 2f), which indirectly proved that aromatic components existed in T3 were surrounded by polysaccharides, supported by the CHN and FTIR data (Li et al. 2010a). In contrast, after acid hydrolysis treatment upon T1 and T2 samples, the skeleton structure still existed in the samples of T6 and T5 (Fig. 2d, e). For example, the surface morphology of T1 was similar to T6, presenting abundant wax configuration. The surface morphology of T2 and T5 presented similarity. These phenomena indicated that the distribution of polysaccharides in tea leaves with waxes and cutin were relatively independent. Therefore, the acid hydrolysis did not induce any obvious damage on the structure of waxes and cutin.

(d) T6

(a) T1

De-sugared

De-waxed

(e) T5

(b) T2

De-sugared

Saponification

(c) T3

(f) T4

De-sugared

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Sorption kinetics of tea leaf fractions with PAHs

Sorbed phenanthrene amount (mg/kg)

In order to investigate the sorption properties of tea leaf fractions with phenanthrene and pyrene, sorption kinetics were carried out and the kinetic curves are presented in Fig. 3. The sorption kinetics was fitted by Lagergren’s pseudo-first-order kinetic model and pseudo-second-order kinetic model (Ho and McKay 1998; Feng et al. 2009). The corresponding kinetic model parameters (q e, k 1, and k 2) and correlation coefficients for the two models are given in Table 3. The pseudo-second-order kinetic model presents a better fit than pseudo-first-order kinetic model and the calculated values of q e based on the pseudo-second-order kinetic model agreed fairly well with the experimental data, indicating that the sorption follows pseudo-second-order kinetic. The pseudo-second-order kinetic model for phenanthrene and pyrene sorption by tea fractions are shown in Fig. 3. As shown in Fig. 3, the sorption equilibration of phenanthrene to T1 was achieved within 24 h. In comparison with T1, the sorption equilibration of de-sugared T1 T2 T3 T4 T5 T6

a

6000 5000 4000 3000 2000 1000 0

0

20

40

60

80

100

120

140

Sorbed phenanthrene amount (mg/kg)

Time (h)

3500

T1 T2 T3 T4 T5 T6

b

3000 2500 2000 1500 1000 500 0 0

10

20

30

40

50

60

70

80

Time (h)

Fig. 3 Lagergren’s pseudo-second-order kinetic model for PAHs sorption kinetics by tea leaf fractions. a Phenanthrene, b pyrene. The bulk tea leaves, dewaxed tea leaves, non-saponifiable tea leaves, dewaxed–nonsaponifiable–de-sugared tea leaves, dewaxed–de-sugared tea leaves, and de-sugared tea leaves are termed T1, T2, T3, T4, T5, and T6, respectively

fraction (T6) was extended to 40 h after acid hydrolysis treatment. The equilibrium time of T6 became longer than that of T1, attributed to the increase of aromaticity of tea leaves as well as the decrease of polarity (Table 2). Similarly, the equilibrium time of phenanthrene to tea leaf fractions changed from 24 h (T2) to 40 h (T5) and from 15 h (T3) to 40 h (T4). For pyrene, the sorption equilibration of T1, T2, and T3 were achieved within 8 h, and extended to 40 h for T4, T5, and T6 samples. Among the tested samples, the T3 sample presented the largest sorption rate (Table 3), which was attributed to the highest polarity index (Table 2). Recent studies have shown that the condensed domain is attributed mainly to aromatic components and less to aliphatic components while the expanded domain appeared to be enriched mainly in aliphatic carbons (Johnson et al. 2001; Chen et al. 2005). With the presence of the polar moieties of cellulose and hemicellulose, T1–T3 samples may strongly interact with water to loosen the structure of organic region for the sorption of PAHs. Therefore, the fast sorption with the water-saturated organic matter of T1–T3 would dominate the PAH uptake, which is consistent with the generally fast equilibrium sorption of organic pollutants to plant biomass (Chen et al. 2012). After the removal of cellulose components (polar moieties) by acid hydrolysis, the sorption phases became condensed for T4–T6 samples, and then the sorption of T4–T6 samples displayed a relative slow rate. Similarly, the partial removal of polar-group contents in biochar (pyrolyzed biomass) increased the compactness of the partition medium, and then decreased the diffusion of the solute into the partition phase to result in a slow sorption rate (Chen et al. 2012). The effects of molecular size of sorbate on sorption kinetics of biomass fractions were sample dependent. The sorption rate (k 2) of pyrene to T1, T2, and T3 samples were relatively faster than that of phenanthrene, while the sorption rate (k 2) of pyrene to T4, T5, and T6 samples were relatively slower than that of phenanthrene. It is reasonable that the medium compactness affect the solute sorption rate more obviously on pyrene (large dimension) over phenanthrene. The magnitude of sorption capacity of different tea leaf fractions for phenanthrene (1 mg/L) followed the order of T6 (5,601.25 mg/kg) > T5 (5,403.24 mg/kg) > T4 (4,694.45 mg/ kg) > T1 (2,314.40 mg/kg) > T2 (1,941.88 mg/kg)>T3 (532.57 mg/kg). For pyrene (0.1 mg/L), the sorption capacity ranked in the order of T6 (2,995.94 mg/kg) > T5 (2,780.85 mg/ kg) > T4 (2,474.71 mg/kg) > T2 (341.04 mg/kg) > T1 (286.82 mg/kg) > T3 (135.83 mg/kg). The T6 fraction presented the maximum equilibrium sorption amount for both phenanthrene and pyrene, while the T3 fraction displayed the minimum sorption amount.

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Table 3 Kinetic parameters of the pseudo-first and pseudo-second-order model for PAHs sorption by tea leaf fractions Organic pollutants

Phen

Pyrene

a

Sorbenta

T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6

q e (exp.) (mg/kg)

2,493.34 2,148.43 585.22 4,942.77 5,684.14 5,889.71 301.92 358.41 148.74 2,714.33 3,081.95 3,322.32

First-order kinetic model

Second-order kinetic model

k 1 (h−1)

q e (cal.) (mg/kg)

R2

k 2 (kg mg−1 h−1)

q e (cal.) (mg/kg)

R2

0.025±0.004 0.052±0.008 0.067±0.009 0.074±0.013 0.071±0.009 0.060±0.005 0.036±0.003 0.039±0.004 0.056±0.006 0.024±0.003 0.029±0.005 0.038±0.004

2,241.60±39.63 1,901.65±31.34 526.48±6.48 4,613.50±64.16 5315.14±60.81 5,504.23±51.25 277.43±3.29 333.22±4.71 133.67±1.62 2,384.49±56.25 2,674.52±76.00 2,913.79±49.05

0.8209 0.8988 0.9381 0.9276 0.9548 0.9705 0.9308 0.9478 0.9694 0.8782 0.8333 0.9204

0.0013±0.0002 0.00369±0.0009 0.02489±0.0081 0.00259±0.0007 0.00217±0.0004 0.00171±0.0002 0.0142±0.001 0.01373±0.0022 0.07373±0.0169 0.000974±0.0001 0.00106±0.0002 0.0015±0.0002

2,314.40±34.08 1,941.88±31.17 532.57±7.12 4,694.45±61.01 5,403.24±50.72 5,601.25±39.91 286.82±2.42 341.04±4.77 135.83±1.87 2,474.71±53.45 2,780.85±61.98 2,995.94±45.89

0.9021 0.9204 0.9402 0.9489 0.9748 0.9859 0.9720 0.9566 0.9691 0.9161 0.9028 0.9456

The meanings for the biosorbents are presented in Table 1

Sorption isotherms of phenanthrene and pyrene by tea leaf fractions Sorbed phenanthrene amount(mg/kg)

a

T1 T2 T3 T4 T5 T6

5000

4000

3000

2000

1000

0 0.0

0.1

0.2

0.3

0.4

0.5

Equilibrium concentration (mg/L) 3000

Sorbed pyrene amount (mg/kg)

Sorption isotherms of phenanthrene and pyrene to the tea leaf fractions are presented in Fig. 4. Isotherms fit well to the Freundlich model, and the regression parameters are listed in Table 4. The sorption isotherm of phenanthrene and pyrene by bulk tea leaf (T1) and dewaxed fractions (T2) were practically linear because of the Freundlich exponent N ranging from 0.99 to 1.02 (equal to 1), which indicated that partitioning into organic matter dominated the sorption process of PAHs. Nevertheless, compared with precursory samples (T1– T3), the de-sugared samples (T4–T6) exhibited more non-linear isotherms due to the new formation of condensed domains (Wang and Xing 2007). Sorption coefficient (K d = Q /C e) is a good parameter to describe the partition efficiency with organic contaminants. The K d values of bulk tea leaf residue were 8, 580 and 22,716 L/kg for phenanthrene and pyrene, respectively. It was reported that wax has strong sorption capacity to organic contaminants (Zhu et al. 2007). However, the K d of tea leaf fractions just changed a little after dewaxed treatment, i.e., the K d ratio of T2/T1 = 0.65 (phen) and 1.17 (pyrene), and the K d ratio of T5/ T6 = 0.85 (phen) and 1.22 (pyrene). These phenomena demonstrated that wax was not the main sorption contributor for tea leaf sorption. Furthermore, as presented in Table 3, the K d dropped markedly after saponification, i.e., the K d ratio of T3/T2 = 0.19 for phenanthrene and 0.14 for pyrene, suggesting that the cutin component (polymeric lipid) dominated the overall sorption of bulk tea leaf residues.

6000

b

T1 T2 T3 T4 T5 T6

2500 2000 1500 1000 500 0 0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Equilibrium concentration (mg/L)

Fig. 4 Sorption isotherm of PAHs to tea leaf fractions. a Phenanthrene, b pyrene. The bulk tea leaves, dewaxed tea leaves, non-saponifiable tea leaves, dewaxed–non-saponifiable–de-sugared tea leaves, dewaxed–desugared tea leaves, and de-sugared tea leaves are termed T1, T2, T3, T4, T5, and T6, respectively

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Table 4 Sorption regression parameters of PAHs by tea leaf fractions Organic pollutants

Sorbenta

log K fb

Nb

R2

K d (L/kg)c

Linear R 2

K oc (L/kg)

K oc/K owcd

Phen

T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6

3.975±0.027 3.749±0.026 2.967±0.027 4.190±0.016 4.410±0.045 4.485±0.113 4.442±0.148 4.408±0.036 3.552±0.045 4.931±0.057 5.258±0.063 5.231±0.058

1.019±0.015 0.993±0.015 0.898±0.017 0.801±0.008 0.845±0.024 0.921±0.058 1.001±0.056 0.996±0.013 0.943±0.017 0.862±0.021 0.939±0.025 0.986±0.021

0.996 0.996 0.994 0.998 0.990 0.944 0.967 0.998 0.995 0.993 0.994 0.994

8,580±119 5,582±78 1,033±13 21,151±384 34,606±587 40,515±407 22,716±958 26,687±457 3,805±163 138,792±4100 227,703±3637 185,911±1795

0.996 0.996 0.997 0.994 0.995 0.998 0.967 0.995 0.966 0.985 0.996 0.998

17,991 12,454 2,458 37,062 56,918 61,026 47,633 59,543 9,053 243,196 374,512 280,029

0.47 0.33 0.06 0.98 1.50 1.61 0.44 0.55 0.08 2.24 3.46 2.58

Pyrene

a

The meanings for the biosorbents are presented in Table 1

The Freundlich parameters (K f and N) were calculated using the logarithmic form of the equation logQ = logK f + NlogC e, where Q is the amount sorbed per unit weight of sorbent (mg/kg), C e is the equilibrium concentration (mg/L), K f [(mg/kg)/(mg/L)N ] is the Freundlich capacity coefficient, and N (dimensionless) describes the isotherm curvature. R is regression coefficient

b

c

K d is the sorption coefficient (K d = Q/C e), calculated from the slope of linear equation

K oc is the carbon-normalized sorption coefficient (K oc = K d/f oc) and K owc is the carbon-normalized K ow (K owc = K ow/f oc, f oc 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

d

After acid hydrolysis, sorption coefficients of PAHs to the de-sugared fractions increased greatly. For phenanthrene, the de-sugared fraction (T6) presented the highest sorption capability (K d = 40,515 L/kg) among all modified fractions. While, for pyrene, the dewaxed–de-sugared fraction (T5) exhibited the highest sorption capability (K d = 227,703 L/ kg). According to Table 3, the dewaxed–non-saponifiable–desugared fraction (T4) presented obviously high sorption capacities (21,151 and 138,792 L/kg for phenanthrene and pyrene, respectively), which were about four times larger than that of bulk and dewaxed tea leaf fractions. The nonsaponifiable fraction (T3) was assumed to present high sorption capacity due to its relatively high lignin content. However, T3 sample presented the lowest sorption capacity among all tested fractions, i.e., K d = 1,033 L/kg (phen) and 3, 805 L/kg (pyrene), which were more than 20 times lower than those of T4. These phenomena indicated that the powerful sorption potential of lignin was seriously restricted by the coexisting polysaccharide component and could not make real contribution to the total sorption of T3 (Li et al. 2010a). Acid hydrolysis was an effective way to enhance sorption capacities of tea leaf fractions due to the liberation of the powerful sorption medium of lignin in tea leaves. As the T4 fraction (aromatic cores) presented more strong specific interaction with pyrene, sorption coefficients of pyrene increased more greatly than that of phenanthrene, i.e., the K d ratio of T4/T3 = 26.9 (pyrene) versus 15.1 (phenanthrene), the K d ratio of T5/T2 = 6.3 versus 4.6, and the K d ratio of T6/T1 = 5.9 versus 3.4.

The structure–function relationship in sorption of PAHs by tea leaf fractions The structure characteristics of tea leaf fractions (e.g., aromaticity and polarity) play an important role on the sorption properties of PAHs. From Table 4, the K d values were quite dependent on sorbent structural characteristics and sorbate properties. The relationship of the K oc values with the polarity index [i.e., (O + N)/C] of the sorbents for phenanthrene and pyrene are presented in Fig. 5. With increasing polarities of tea leaf fractions, sorption capacities decreased, presenting a negative role of polarity on the sorption of PAHs (Chen et al. 2005). Meanwhile, positive correlation of K d values with aromaticity (H/C) for tea leaf residues are obtained (Fig. 6). Some structural changes occurred after acid hydrolysis treatment to T1, T2, and T3. Firstly, the H/C atomic ratio and polarity index decreased (Table 2). Secondly, peaks of –OH (3,366 cm−1) and C–O–C (1,068, 1,038, and 770 cm−1) of carbohydrates were sharply reduced (Fig. 1). In addition, after acid hydrolysis to T3, it can be seen that the skeleton structure in T3 disappeared and changed into fragmentary particles (Fig. 2f). As a result, the removal of polar components (i.e., mainly sugar) and the exposure of aliphatic moieties and aromatic core significantly enhanced the sorption capacity. Based on Table 4, the T4 fraction exhibited the highest nonlinearity, which may be a result of the condensed domains in this sample (higher content of aromatic moieties and crystalline aliphatic carbons) as compared to the other fractions. The results were in line with the CHN and FTIR data mentioned

3326

4.6

(a) Phenanthrene

40000

T6

T4

4.4

log Koc

(a) Phenanthrene

T5

T6

T5 30000

y=-1.95x+5.52 R2=0.881

T1

Kd (L/kg)

4.8

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4.2 T2 4.0 3.8

20000

T4 y=-46829x+82889 R2=0.639 T1

10000

3.6

T2

T3

3.2 0.2

0

T3

3.4

1.0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.0

1.4

1.5

1.6

1.7

H/C

(N+O)/C 250000

5.8 5.6 T6

200000 T6

T4

5.4

150000

5.0

y=-2.27x+6.38 R2=0.866

Kd (L/kg)

5.2

log Koc

(b) Pyrene

T5

(b) Pyrene

T5

T2

4.8 4.6

T4 100000

y=-309512x+525998 R2=0.805

50000

T1

T2

4.4 0

4.2

1.0

T3

4.0 3.8 0.2

T1

T3

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.4

1.5

1.6

1.7

H/C 1.0

(N+O)/C

Fig. 6 Relationships between the K d of PAHs and the aromaticity of tea leaf fractions. a Phenanthrene, b pyrene

Fig. 5 Relationships between the log K oc of PAHs and the polarity index of tea leaf fractions. a Phenanthrene, b pyrene

above. The T6 fraction presented the highest sorption capacity for PAHs due to its lowest polarity. Li et al. (2010b) 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 non-polar aliphatic moieties; without the presence of amorphous cellulose, both aromatic and aliphatic moieties were effective in sorption. The availability of the chemical components of biomass to retain PAHs has been reported (Olivella et al. 2013a). The proposed PAH sorptive domains in different fractions of tea leaf residue as a model biomass are demonstrated in Fig. 7. Sorptive domains of biomass fractions evolve from non-polar aliphatic moieties under the presence of amorphous cellulose to both aromatic and aliphatic moieties after consumption of amorphous cellulose. For the sorption of PAHs by raw tea leaves, the powerful sorption capacity of aromatic components were suppressed, thus sorption was mainly contributed by non-polar aliphatic moieties (wax and cutin). With the removal of extractable lipids, the sorption was controlled by cutin. With the further removal of polymer lipids (i.e., cutin), the main sorption medium was the polysaccharides and

the sorption capacity of de-waxed–non-saponifiable tea leaf fraction decreased greatly, indicating that the cutin component dominated the overall sorption of bulk tea leaf residues. After acid hydrolysis, the polysaccharides which surrounded the aromatic core was removed, the polarity index decreased greatly as well as the increase of aromaticity, and the exposed aromatic medium greatly enhanced the sorption capacity of PAHs by tea leaf residues. The main sorption mediums of desugared tea leaf fraction were wax, cutin, and aromatic medium, while for de-waxed–de-sugared tea leaf fraction, the main sorption mediums were cutin and aromatic medium. Establishment of the relationship between the structures of tea leaf fractions and sorption capacities would provide an engineering gist to design novel biosorbent for environmental applications. Environmental application implication It is of great necessity to compare the sorption capacity of different kinds of sorbents, thus providing an evidence for further environmental application. Sorption coefficients of natural, modified, and synthetic sorbents are summarized in Table 5. Based on Table 5, sorption of phenanthrene to bulk

Environ Sci Pollut Res (2014) 21:3318–3330

3327

Fig. 7 a–f The proposed PAH sorptive domains of different fractions of tea leaf residue as a model biomass. The relative role of aromatic and aliphatic moieties is regulated by the amorphous cellulose component. Sorptive domains of biomass fractions evolve from non-polar aliphatic moieties under the presence of amorphous cellulose to both aromatic and aliphatic moieties after consumption of amorphous cellulose

De-sugared

(d) T6

(a) T1 De-waxed

De-sugared

(e) T5

(b) T2 Saponification

De-sugared

(f) T4 (c) T3

wax

tea leaves residue is much higher than that of many nature organic sorbents such as tender tea leaves (Lin et al. 2007), wood chip, ryegrass root, orange peel, bamboo leaf, pine needle (Chen et al. 2011), pine bark (Li et al. 2010a), cellulose (Salloum et al. 2002), aspen wood fiber (Huang et al. 2006), heat-killed fungal biomass (Chen et al. 2010), and pine needle cuticle (Li et al. 2010b), but lower than those of grape cuticle (Li and Chen 2009), algae, and lignin (Salloum et al. 2002). The high sorption affinity with PAHs by lignin and fruit cuticles may be attributed to their aliphatic-rich property (Chen et al. 2011). This phenomenon presents the important role of aliphatic carbon on PAHs sorption by nature organic sorbents. Sorption of pyrene to bulk tea leaf residues is higher than bamboo leaf (Chen et al. 2011), pine bark (Li et al. 2010a), sugar cane bagasse, green coconut shells (Crisafully et al. 2008), and aspen wood fiber (Huang et al. 2006), presenting a similar

cutin

polysaccharide

aromatic core

PAHs

phenomenon compared with phenanthrene, but lower than pine needle cuticle (Li et al. 2010b) and heatkilled fungal biomass (Chen et al. 2010). Lignin shows higher sorption capacity than cellulose (Table 5), indicating that lignin is a more effective storage medium for PAHs. Considering the ubiquity, environmental compatibility, and cost effectiveness, tea leaves could be a favorable biosorbent for PAHs wastewater treatment. Li et al. (2010a) pointed out that lignin is the main aromatic constitutes of natural sorbents and its strong sorption affinity with PAHs could be seriously restricted by the coexisting polysaccharide component. In order to enhance the sorption capability of tea leaf residues, we can remove polar components (i.e., mainly sugar) from raw biomass through acid hydrolysis. Hydrolyzed tea leaf fractions (T4, T5, and T6) were supposed to be the promising biosorbents for PAHs removal. Sorption capability of hydrolyzed tea leaves for

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Table 5 Sorption coefficients of phenanthrene and pyrene with selected natural and synthetic sorbents reported in the previous studies and the present work

Nature organic sorbent

Modified biosorbents

Synthetic sorbents

Sorbent

K d of phenanthrene (L/kg)

K d of pyrene (L/kg)

Source

Bulk tea leaves Tender tea leaves Mature tea leaves Wood chip

8,580 3,290–3,450 5,880–11,200 2,484

22,716

Present study Lin et al. (2007)

Ryegrass root Orange peel Bamboo leaf Pine needle Pine bark Sugar cane bagasse Green coconut shells Algae Cellulose Lignin Aspen wood fiber Heat-killed fungal biomass Sargassum hemiphyllum Cuticle of pine needle Cuticle of grape Hydrolyzed tea leaves Brewed tender tea leaves Brewed mature tea leaves

2,777 2,970 3,746 5,306 3,392

Fibric peat Surfactant modified peat Low-temperature hydrolyzed wood fibers High-temperature hydrolyzed wood fibers Hydrolyzed pine needle cuticle Hydrolyzed pine bark Natural chars Active carbon Black carbon (400 °C) Black carbon (700 °C)

PAHs is higher than some modified biosorbents such as brewed tea leaves (Lin et al. 2007), fibric peat, surfactant modified peat (Tang et al. 2010), and hydrolyzed pine bark (Li et al. 2010a), while are comparable to hydrolyzed wood fibers (Huang et al. 2006) and black carbon (Zhang et al. 2011), but lower than those of some synthetic sorbents, such as natural chars and active carbon (James et al. 2005) (Table 5). The hydrolyzed tea leaf fractions own several advantages to remove POPs from wastewater. On one hand, synthetic sorbents mainly rely on their huge surface area to sorb pollutants and sorption efficiency was easily affected by the coexistence of pollutants while plant biomass presented a noncompetitive partition process. On the other hand, the sorption

13,630 951.8 10,627 3,940–4,660 6,822 6,761 6,600 34,319 40,515 5,820–6,270 8,950–15,900 12,870 26,074 10,800–14,000 42,600–57,500 38,836 16,881 79,433–1,995,262 501,187–794,328

Chen et al. (2011)

15,430 21,250 21.5 38.3

Li et al. (2010a) Crisafully et al. (2008) Salloum et al. (2002)

12,000–14,440 24,140 40,720 185,911

88,590 178,388 25,400–31,075 170,000–214,000 263,274 90,159

111,299; 34,394 635,330; 96,161

Huang et al. (2006) Chen et al. (2010) Chung et al. (2007) Li et al. (2010b) Li et al. (2009) Present study Lin et al. (2007) Tang et al. (2010) Huang et al. (2006) Li et al. (2010b) Li et al. (2010a) James et al. (2005) Zhang et al. (2011)

capacity of hydrolyzed tea leaves increased 5–20-fold for phenanthrene and 8–36-fold for pyrene, presenting high sorption capacity. In addition, the hydrolyzed tea leaf fractions were much safer in wastewater treatment due to avoiding the release of dissolved organic matter during treatment (Li et al. 2010a). Moreover, the modified process of raw tea leaves was relatively simple and the processing cost was low. Therefore, hydrolyzed tea leaf fractions are expected to be effective biosorbents to remove persistent organic pollutants. Among all hydrolyzed tea leaf fractions (T4–T6), desugared fraction (T6) would be an ideal choice for environmental application due to its highest yield, less equilibrium time, highest sorption capability, and lowest modification cost. After a simple modification by HCl hydrolysis, the bulk

Environ Sci Pollut Res (2014) 21:3318–3330

tea leaves (T1) could be easily modified to the hydrolyzed tea leaves (T6). During the treatment, some powerful sorption mediums (i.e., cutin) were preserved and some sorption restrictors (i.e., sugar component) were removed, thus activating the potential sorption capability of lignin fraction.

Conclusions Tea leaf residue is of great potential as an effective low-cost biosorbent for PAHs removal in wastewater treatment. The amorphous cellulose components regulated the sorption kinetics, capacities, and mechanisms of biomass fractions. Negative correlation of partition coefficients with (O + N)/C for tea leaf fractions is obtained. Sorption capacities of desugared fractions were 5–20- and 8–36-fold higher than their original fractions for phenanthrene and pyrene, respectively. All de-sugared fractions exhibited nonlinear sorption due to strong specific interaction between PAHs and exposed aromatic domains while presenting a relative slow rate because of the formation of condensed aromatic domains. Biomasses like tea leaf residues can be tailored by acid hydrolysis to prepare a promising biosorbent for organic pollutant removal. Acknowledgments This project is supported by the National Natural Science Foundation of China (41071210), Zhejiang Provincial Natural Science Foundation of China (R5100105), the National High-Tech Research and Development Program of China (Grant 2012AA06A203), and the Doctoral Fund of Ministry of Education of China (J20091588).

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