Enhanced sorption of polycyclic aromatic hydrocarbons from aqueous solution by modified pine bark

Bioresource Technology 101 (2010) 7307–7313

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Enhanced sorption of polycyclic aromatic hydrocarbons from aqueous solution by modified pine bark Yungui Li, Baoliang Chen *, Lizhong Zhu Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Zhejiang University, Hangzhou, Zhejiang 310028, China

a r t i c l e

i n f o

Article history: Received 5 February 2010 Received in revised form 27 April 2010 Accepted 30 April 2010 Available online 23 May 2010 Keywords: Modification Pine bark Polycyclic aromatic hydrocarbon Enhanced-sorption Removal efficiency

a b s t r a c t To enhance removal efficiency of natural sorbent with polycyclic aromatic hydrocarbons (PAHs), singlesolute and bi-solute sorption of phenanthrene and pyrene onto raw and modified pine bark were investigated. Pine bark was modified using Soxhlet extraction, saponification and acid hydrolysis, yielding six bark fractions with different chemical compositions. Raw pine bark exhibited high affinities with PAHs, and sorption was dominated by partitioning. The relatively nonlinear sorption isotherms of modified bark were attributed to the specific interaction between sorbate and aromatic core of sorbent. Comparison with lipid and suberin, lignin was the most powerful sorption medium, but which was almost completely suppressed by coexisting polysaccharide. After consuming polysaccharide by acid hydrolysis, sorption of pine bark fractions was notably increased (4–17 folds); and sorption of pyrene just decreased 16–34% with phenanthrene as a competitor. These observations suggest that pine bark is of great potential for PAHs removal and can be significantly promoted by acid hydrolysis for environmental application. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Persistent organic pollutants, such as polycyclic aromatic hydrocarbon (PAHs), have been widely detected at elevated concentrations in storm water run off and wastewater streams (Busetti et al., 2006; Huang et al., 2006). Due to their low solubility and resistance to biodegradation, the typically wastewater treatment technologies including both aerobic and anaerobic biological process are not effective in the removal of PAHs. Sorption is assumed to be a useful and economical treatment method for removing PAHs, and the most effective conventional sorbent is assumed to be activated carbon (Ma and Zhu, 2006). However, its notably high operating cost prohibits the treatment of large amounts of wastewater and storm water. Thus, innovative alternatives have been developed by the use of low-cost natural organic solid residue from agricultural and wood industrial activities (Delval et al., 2006; Sciban et al., 2007; Crisafully et al., 2008). Pine bark, one by-product in the timber industry, has received extensive interest to environmental scientists and engineers for its high sorption capacity toward heavy metals (Acemioglu, 2004; Argun and Dursun, 2008; Nehrenheim and Gustafsson, 2008; Hernandez-Apaolaza and Guerrero, 2008). Recently a few scientists also discovered that pine bark was effective in retaining hydropho* Corresponding author at: Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China. Tel./fax: +86 571 8827 3901. E-mail address: [email protected] (B. Chen). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.04.088

bic organic compounds such as organochlorine pesticides, pentachlorophenol and PAHs (Bras et al., 1999; Ratola et al., 2003; Seo et al., 2007). Bras et al. (2005) found that pentachlorophenol (PCP) could be strongly bonded to pine bark, achieving an average removal efficiency of 92%, with 0.1 mg PCP/g pine bark. Seo et al. (2007) used pine bark as an organic mulch biowall for PAHs contaminated groundwater remediation, which showed high PAHs removal efficiency. The high affinity with hydrophobic compounds of pine bark was attributed to its organic composition (Bras et al., 1999). The main chemical compositions of pine bark were lignin including polyphenolics (44 wt%), polysaccharides (39 wt%), water and organic solvent extractives (17%), as well as ashes (1 wt%) (Fradinho et al., 2002). Lignin is anticipated to be the main sorption medium in pine bark with organic pollutants for its relatively hydrophobic nature (Dizhbite et al., 1999; Huang et al., 2006; Wang et al., 2007). It demonstrated that polysaccharides play a negative effect in PAHs binding with plant cuticle (Li and Chen, 2009). To tailor natural organic materials into engineered sorbent for environmental application, however, the interaction mechanisms of organic pollutant with natural material fractions are wanted. The main objective of the current study is to elucidate the relative role of the different chemical components in PAHs removal by natural organic material. To this end, pine bark was selected and modified via three chemical treatment including Soxhlet extraction, alkaline saponification and acid hydrolysis. The raw and modified sorbents were characterized by elemental analysis and Fourier transform infrared spectroscopy. Phenanthrene and pyrene

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were chosen as the model PAHs because they are widely spread in the wastewater treatment plant and are not efficiently removed by the conventional physicochemical methods (Busetti et al., 2006; Crisafully et al., 2008). Single-solute and bi-solutes sorption onto raw and modified sorbents were conducted. 2. Methods 2.1. Modification of pine barks Pine barks were collected from the campus woods in Zhejiang University, China on April 2008, and then modified by a similar method to that used in earlier studies (Vazquez et al., 2000; Chen et al., 2008). Briefly, raw pine bark (PB1) was oven-dried at 60 °C, ground, and sieved (<0.18 mm) before the sequential modification procedure. Firstly, extractable lipids (main component, waxes) were removed from PB1 by Soxhlet extraction with chloroform/ methanol (1:1) at 70 °C for 24 h, yielding dewaxed-sorbent (PB2). Secondly, PB2 was saponified with 1% potassium hydroxide in methanol for 3 h at 70 °C under refluxing and stirrer-spinning conditions, producing the nonsaponifiable sorbent (PB3). In this process, the depolymerizable lipid fractions of (i.e., suberin) pine bark were removed. 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 nonsaponifiable-, dewaxed-, and raw pine bark (i.e., PB3, PB2 and PB1), yielding lignin material (PB4), dewaxed-desugared fraction (PB5), and desugared sorbent (PB6), respectively. All sorbents 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) to adjust these materials to neutral conditions and to remove dissolved organic matter sorbed by these residue. The yields of bark sorbents (PB2-PB5) were all calculated to the percentage contents of PB1. 2.2. Characterization of pine bark sorbents

concentrations were always <0.1% of the total solution volume to avoid cosolvent effects. The initial concentrations were ranging from 5 to 950 lg/L for phenanthrene, and from 0.5 to 95 lg/L for pyrene. Each isotherm contained ten concentration points with a constant solid–solution ratio, and each point including the control (without sorbent) and calibration (with sorbent), was run in duplicate. Certain amount (1–5 mg) sorbent was placed into the 8 or 40 mL screw cap vials and then filled with sorbate solution to minimize evaporation and ensure 30–80% removal rate of sorbate. After 3 days equilibration, the solution was separated by centrifugation at 4000 rpm for 15 min, and 0.5 mL supernatant was mixed with 0.5 mL acetonitrile for HPLC analysis. Phenanthrene and pyrene concentrations were measured by an Agilent 1200 High Performance Liquid Chromatograph (HPLC) fitted with G1321A fluorescence detector and Agilent Eclipse XDB-C 18 column (4.6 mm  250 mm  5 lm). Injection volumes of 15 lL, 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-amount was calculated by the aqueous concentration difference of sorbate between the control and calibration. Competitive sorption experiments were also performed with a similar procedure to single-solute sorption experiments at 25 ± 0.5 °C. Pyrene was regarded as the primary solute with various concentrations ranging from 0.5 to 95 lg/L, and phenanthrene as a competitor was fixed at one initial concentration (i.e., 0.5 mg/L). Sorption isotherms of pyrene were gained with the presence of the competitor. 2.4. Data analysis All sorption data were fitted to the logarithmic form of the Freundlich equation

log Q ¼ log K f þ N log C e

ð1Þ

Elemental (C, H, N) analyses were conducted via an EA 112 CHN elemental analyzer (Thermo Finnigan), while the oxygen content was calculated by the mass difference. To assess the polarity and aliphatic characteristics of the sorbents, (O + N)/C and H/C atomic ratios were calculated. FTIR spectra were recorded in the 4000– 400 cm1 region for a KBr-pellet by a Bruker Vector 22 FTIR spectrophotometer with a resolution of 4.0 cm1. The specific surface areas were measured with N2 (0.162 nm2) adsorption at liquid nitrogen temperature using a NOVA-2000E surface area analyzer. Four data points, with relative pressures of 0.05–0.3, were used to construct the monolayer adsorption capacity.

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 (log Kf and N) were calculated using the logarithmic form of the equation. Since the Kf value depends on the N value, it is not possible to compare the K values for isotherms with different N values. Thus, 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 each sorbent.

2.3. Batch sorption studies

3. Results and discussion

The selected physicochemical properties for phenanthrene and pyrene are as follows: molecular weight of 178.2 and 202.3 g/mol; aqueous solubility of 1.1 and 0.13 mg/L (25 °C); and octanol–water partition coefficient (Kow) of 28,840 and 80,000, respectively. Both chemicals were of analytical grade. Since removal efficiency and sorbed-amount are related with the selected solid-to-solution ratios and concentration of pollutants, sorption isotherms of pine bark sorbents (PB1–PB6) were conducted by a batch equilibration technique at 25 ± 0.5 °C to evaluate their sorption capacities (Li and Chen, 2009). In brief, the selected sorbate stock solution was dissolved in a solution containing 0.01 mol/L CaCl2 to keep a constant ionic strength, and 200 mg/L NaN3 to prevent biological degradation. Because of the low water solubility, stock phenanthrene and pyrene solutions were made at high concentrations in MeOH before being added to the background solution. Methanol

3.1. Characterization of pine bark sorbents The yields and elemental composition of pine bark sorbents (PB1–PB6) are listed in Table 1. As a lignocellulosic material, pine bark is usually characterized in terms of its main constituents like cellulose, hemicellulose and lignin (Fradinho et al., 2002; Bras et al., 2004). The organic carbon content of PB1 was 52.13%, similar to the result (55%) reported by Bras et al. (2004). The yield of dewaxed-sorbent (PB2) was relatively high, amounted to 88.9%, indicating that the percentage content of the extractable lipids in pine bark was about 11.1%, which was closed to the content extracted with organic solvents (13.4%, Fradinho et al., 2002). However, no observed mass loss occurred when dewaxed pine bark fraction (PB2) was saponified in methanolic KOMe, due to the low content of suberin in pine bark (Vazquez et al., 2000). After Soxhlet

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Y. Li et al. / Bioresource Technology 101 (2010) 7307–7313 Table 1 Relative yields, elemental analysis, atomic ratios and BET-N2 specific surface area (SA) of the different pine bark fractions. Samplea

Yield, wt%b

C, wt%

H, wt%

N, wt%

O, wt%c

(N + O)/C

H/C

SA, m2/g

PB1 PB2 PB3 PB4 (lignin) PB5 PB6 Waxd Sugard

100 88.9 88.9 53.71 61.15 70.56 11.1 35.19

52.13 51.15 43.87 57.17 57.30 57.47 59.91 33.49

5.82 5.68 4.80 4.64 4.75 5.12 6.98 6.76

0.32 0.32 0.17 0.16 0.17 0.17 0.26 0.51

41.74 42.85 51.17 38.03 37.78 37.24 32.86 59.25

0.61 0.63 0.88 0.50 0.50 0.49 0.42 1.34

1.33 1.32 1.30 0.97 0.99 1.06 1.39 2.41

0.98 1.15 0.76 1.79 2.65 2.17 – –

a PB: pine bark. The number in the name of each sample was identified as follows: ‘‘1’’ for raw bark, ‘‘2’’ for dewaxed, ‘‘3’’ for nonsaponifiable, ‘‘4’’ for lignin, ‘‘5’’ for dewaxed-hydrolyzed, ‘‘6’’ for desugared. b The yields of each fraction were calculated to the percentage contents of raw sample (PB1). c Oxygen content was calculated by the mass difference. d Calculated results based on mass balance.

extraction and alkaline saponification, the main components of nonsaponifiable residue (PB3) were deemed to be polysaccharides and lignin. The amount of non-structural and hemicellulosic carbohydrates removed by HCl hydrolysis was up to 35.2% of pine bark. The final residue PB4 mainly composed of lignin including polyphenolics materials, which was termed lignin as a whole. PB4 amounted to 53.7% of pine bark, which was a little higher than the reported data (44%, Fradinho et al., 2002), suggesting that polysaccharide like crystalline cellulose was not removed completely from PB3 by HCl hydrolysis. The yield of desugared fraction (PB6) was 70.6%, similar to the acid insoluble residue of pine bark amount (70.7%) reported by Vazquez et al. (2000). With the removal of lipids by extraction process (i.e., from PB1 to PB3), organic carbon content decreased from 52.13% to 43.87%, whereas oxygen content increased from 41.74% to 51.17%. On the contrast, C content rose with O content dropped markedly with hemicelluloses removal, resulting lower polarity of desugared-fractions [(O + N)/ C = 0.49–0.50]. Nonsaponifiable sorbent PB3 presented the highest polarity [(O + N)/C = 0.88] among all modified pine bark fractions for its highest polysaccharides content (39.6%). Plotting H/C atomic ratios (y) with lignin contents (x) of raw and modified pine bark sorbents, a negative linear relationship was observed [y = 0.0091x + 1.828 (n = 6, R2 = 0.958), see Fig. 1], suggesting lignin was the main aromatic constitutes of sorbents. According to the FTIR spectra between 4000 and 400 cm1 (figure not shown), pine bark was dominated by the large band at

3600–3200 cm1 which is assigned to –OH stretching vibrations of polymeric compounds especially polysaccharides (cellulose) (Argun and Dursun, 2008; Argun et al., 2009). Peaks of ester C = O (1736 cm1) in pine bark were unconspicuous, consistent with its comparable low suberin content. Band at 1650– 1500 cm1 was assigned to the bend vibration of C@C (aromatic skeletal mode of lignin). 1450 cm1 was assigned to phenolic OH groups. Peak at 1150–1000 cm1 was vibration of C–O–C and OH of polysaccharides. FTIR spectra showed that pine bark sorbents was mainly composite of polymeric OH groups, phenolic OH and carboxylate groups, C@C of lignin, and OH groups of polysaccharides, which is in line with its yield and elemental composition of different factions. As expected, the BET-N2 specific areas of all pine bark sorbents were quite small, ranging from 0.76 to 2.65 m2/g (see Table 1), which were consistent with the reported result 0.74 m2/g (Bras et al., 2004). After acid hydrolysis treatment, the surface area was enlarged about 1 m2/g, i.e., 0.76 (PB3) ? 1.79 m2/g (PB4), 1.15 (PB2) ? 2.65 m2/g (PB5), and 0.98 (PB1) ? 2.17 m2/g (PB6). 3.2. Sorption isotherm Sorption isotherms of phenanthrene and pyrene to raw and modified pine bark sorbents were presented in Fig. 2 and Fig. 3. Isotherms fit well to the Freundlich model, and the regression

1.4

6000

Sorbed amount (mg/kg)

1.3

H/C atomic ratio

1.2

y=-0.0091x+1.828 R2= 0.958 1.1

1.0

PB1 PB2 PB3 PB4 PB5 PB6

4500

3000

1500

0.9

0.8 50

60

70

80

90

100

Suberan content(%) Fig. 1. Correlation of H/C atomic ratios with lignin content of raw and modified pine bark fractions.

0 0.0

0.1

0.2

0.3

0.4

0.5

Equilibrium concentration (mg/L) Fig. 2. Single-solute sorption isotherms of phenanthrene to raw and modified pine bark sorbents.

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parameters are listed in Table 2. Sorption coefficients (Kd and Koc) and Koc/Kowc ratios were also calculated in Table 2, where the Kowc is the carbon-normalized Kow (Kowc = Kow/foc, foc is the percentage of carbon content of octanol, i.e., 73.8%). For a given sorbent, interestingly, the Koc/Kowc value of phenanthrene (three-ring) was less than that of pyrene (four-ring), which may be attributed to the amount of aromatic rings favoring sorption. Sorption isotherms of phenanthrene and pyrene on raw pine bark (PB1) were weak nonlinear with Freundlich N value of 0.94, suggesting the interaction mechanism between PAHs with pine bark was dominated by partition process. Since the surface area of PB1 was relatively low (0.98 m2/g, Table 1), the weak nonlinear sorption isotherms were likely to be the result of specific interaction between the PAH molecules and aromatic cores of pine bark, i.e., p–p electron interaction. After chemical modification, more aromatic cores exposed (H/C value decreased, Table 1), resulting in stronger specific p–p interaction. Therefore, the nonlinearity of

250

Sorbed pyrene amount (mg/kg)

200

150

PB1 without phen PB2 without phen PB3 without phen PB1 with phen PB2 with phen PB3 with phen

100

50

0 0.000

0.002

0.004

0.006

0.008

0.010

Equilibrium concentration (mg/L)

Sorbed pyrene amount (mg/kg)

1200

900

600

PB4 without phen PB5 without phen PB6 without phen PB4 with phen PB5 with phen PB6 with phen

300

0 0.000

0.004

0.008

0.012

0.016

0.020

Equilibrium concentration (mg/L) Fig. 3. Single-solute and bi-solute sorption isotherms of pyrene to raw and modified pine bark sorbents with the absence and presence of phenanthrene (0.5 mg/L) as a competitor.

isotherms increased more or less (e.g., Freundlich N index decreased, Table 2). Sorption coefficient (Kd) is a good indicator to evaluate the partition efficiency with organic contaminants. The Kd values were 3392 and 21,250 L/kg for phenanthrene and pyrene, respectively, calculated from the slope of the linear sorption isotherms. Sorption coefficients of selected natural and synthetic sorbents were cited in Table 3. Based on Table 3, sorption of raw bark is much higher than that of many natural sorbents, such as sugar cane bagasse and green coconut shells, but lower than those of algae and aspen wood fiber, as well as manufactured organoclay, traditional active carbon and novel carbon nanomaterials. The soluble lipids extracted by organic solvents (i.e., waxes) were well-known for their strong affinities with organic contaminants (Chiou et al., 2001; Zhu et al., 2007). Therefore, wax was assumed to be the most significant sorption medium of pine bark for PAHs removal. This is, however, not the case in the current study. There was no distinctive difference in sorption coefficients (Koc) after waxes removal, i.e., PB2/PB1 = 0.98 (phen) and 1.01 (pyrene), PB5/PB6 = 1.04 (phen) and 1.01 (pyrene), indicating that wax was not the key sorption medium in pine bark material. As presented in Table 4, Kd dropped markedly after alkaline saponification, i.e., PB3/PB2 = 0.40 for phenanthrene and 0.29 for pyrene, suggesting that suberin fractions in PB2 showed significant contribution on PAHs uptake despite of its considerable low mass content (<0.1%). It is hard to believe that such a little matter could obviously affect the affinity of pine barks, but the mechanism is still unclear. The PB4 (lignin) exhibited the highest sorption capacity, i.e., Kd = 20,631 (phen) and 105,566 L/kg (pyrene) among all modified bark fractions. Interestingly, the main component of the nonsaponifiable residue (PB3) was lignin (61.4%), but PB3 presented the lowest sorption coefficient, i.e., Kd = 1294 L/kg (phen) and 6181 L/kg (pyrene), which are close to those of cellulose material reported by Jonker (2008). These phenomena indicated that the powerful sorption potential of lignin was seriously restricted by the coexisting polysaccharide component (39.6%), and it couldn’t make real contribution to the total sorption of PB3. Chen and Schnoor (2009) reported that the sorption capacity of suberan components (i.e., lignin and cellulose) in the root tissue of switchgrass was completely masked by coexisting hemicellulose materials. Therefore, acid hydrolysis was a necessary modification process to liberate the powerful sorption capability of lignin in pine bark. As a result, the dramatically enhancement of sorption capacities for three hydrolyzed sorbents (4–17) were observed. Huang et al. (2006) also found that acid hydrolysis of the wood matrix can effectively increase its sorption efficiency for PAHs. As presented in Fig. 4, sorption capacities decreased with increasing polarities of raw and modified pine bark sorbents, indicating the negative role of polarity on the sorption of aromatic organic contaminants (Chen et al., 2005). Ratios of sorption coefficient between companion sorbents (KdA/KdB) was used to describe the impact of chemical modification (from sorbent A to sorbent B) on the removal efficiency of PAHs (Table 4). According to Table 4, a negative trend between Kd ratio and (O + N)/C ratio was observed (Fig. 5), which confirmed the negative effect of polarity on PAHs removal. Therefore, it is necessary to remove these polar components in pine bark (i.e., mainly sugar) to gain more powerful sorbents for organic pollutants removal. Hydrolyzed pine barks (PB4, PB5 and PB6) were assumed to be the favorable choice. Sorption efficiency of all hydrolyzed sorbents was of the same order of magnitude as that for hydrolyzed wood fibers and organoclay (Table 3). Considering different interaction mechanism, it is difficult to directly compare the sorption efficiency of pine bark with that of traditional active carbon and novel carbon nanomaterials, which approved to be superior sorbents for organic pollutants by their dramatically large surface area. Hence, single-point

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Y. Li et al. / Bioresource Technology 101 (2010) 7307–7313 Table 2 Sorption regression parameters of phenanthrene and pyrene to modified pine bark sorbents, and their sorption coefficients (Kd and Koc). Organic pollutants

Sorbenta

log Kfb

Nb

Freundlich R2

Kd (L/kg)c

Linear R2

Koc (L/kg)

Koc/Kowcd

Kd,0.05Sw

Kd,0.5Sw

Phenanthrene

PB1 PB2 PB3 PB4 PB5 PB6 PB1 PB2 PB3 PB4 PB5 PB6 PB1 PB2 PB3 PB4 PB5 PB6

3.529 ± 0.016 3.494 ± 0.017 3.081 ± 0.026 4.204 ± 0.015 4.114 ± 0.010 4.115 ± 0.011 4.189 ± 0.058 4.177 ± 0.048 3.621 ± 0.037 4.681 ± 0.069 4.728 ± 0.085 4.652 ± 0.047 4.202 ± 0.061 4.368 ± 0.093 3.640 ± 0.058 4.951 ± 0.082 4.736 ± 0.096 4.786 ± 0.061

0.941 ± 0.011 0.920 ± 0.013 0.878 ± 0.018 0.774 ± 0.009 0.755 ± 0.006 0.800 ± 0.007 0.937 ± 0.019 0.921 ± 0.016 0.905 ± 0.013 0.825 ± 0.022 0.855 ± 0.028 0.837 ± 0.016 0.965 ± 0.023 1.050 ± 0.036 0.934 ± 0.024 1.020 ± 0.030 0.980 ± 0.038 0.966 ± 0.024

0.999 0.999 0.997 0.999 1.000 0.999 0.996 0.998 0.998 0.995 0.991 0.997 0.996 0.992 0.995 0.993 0.988 0.995

3392 ± 66 3273 ± 72 1294 ± 30 20,631 ± 593 17,507 ± 506 16,881 ± 399 21,250 ± 602 21,092 ± 681 6181 ± 152 105,566 ± 3246 90,186 ± 2689 90,159 ± 1813 17,036 ± 409 17,627 ± 326 5200 ± 118 72,231 ± 1695 59,503 ± 1060 62,305 ± 1991

0.997 0.999 0.999 0.993 0.993 0.995 0.992 0.991 0.994 0.992 0.991 0.997 0.995 0.997 0.996 0.995 0.997 0.991

6507 6399 2950 36,087 30,553 29,374 40,763 41,236 14,089 184,653 157,393 156,880 32,680 34,461 11,853 126,344 103,845 108,413

0.17 0.17 0.08 0.95 0.81 0.78 0.48 0.49 0.17 2.19 1.87 1.86 0.39 0.41 0.14 1.50 1.23 1.29

4012 3933 1717 30,809 26,461 23,277 21,222 19,489 6742 115,808 110,950 93,138 18,991 18,140 6086 80,772 60,220 72,504

3502 3272 1296 18,310 15,053 14,687 18,356 16,247 5417 77,400 79,456 66,700 17,521 20,354 5228 84,578 57,510 67,044

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 = Kf C N 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 = Q/Ce), calculated from the slope of linear equation. d Koc is the carbon-normalized sorption coefficient (Koc = Kd/foc), and Kowc is the carbon-normalized Kow (Kowc = 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

Table 3 Sorption coefficient of selected natural and synthetic sorbents reported in the previous studies and present wok.

Nature organic sorbent

Modified Nature organic sorbent

synthetic sorbents

Sorbent

Kd of phenanthrene (L/kg)

Kd of pyrene (L/kg)

Source

Pine bark Sugar cane bagasse Green coconut shells Chitin Chitosan Cellulose Cellulose Algae Lignin Aspen wood fiber Hydrolyzed pine bark Low-temperature Hydrolyzed wood fibers High-temperature hydrolyzed wood fibers Char MWNT40 Organobentonites Organobentonites Activated carbon

3392

21,250 21.5 38.3 19.6 10.1 245

Present study Crisafully et al. (2008) Crisafully et al. (2008) Crisafully et al. (2008) Crisafully et al. (2008) Jonker (2008) Salloum et al. (2002) Salloum et al. (2002) Salloum et al. (2002) Huang et al. (2006) Present study Huang et al. (2006) Huang et al. (2006)

170 951.8 13,630 10,627 4660–3940 16,881 14,000–10,800 57,500–42,600

14,440–12,000 90,159 31,075–25,400 214,000–17,000

50,000–199,52,623 20,600;36,900 11,500–43,400 426–36,184 630,957–125,89,254

James et al., 2005 Wang et al., 2008 Chen and Zhu, 2001 Changchaivong and Khaodhiar, 2009 James et al., 2005 and therein

Table 4 Ratios of polarity index (O + N)/C and sorption coefficients (Kd) between different modified pine bark fractions. Different coexisting component

Samples

(O + N)/C ratio

Kd,phen ratio

Kd,pyr ratio

Kd,pyr ratio with competitor

Same coexisting component

Sugar

PB6/PB1 PB5/PB2 PB4/PB3 PB2/PB1 PB5/PB6 PB3/PB2 PB4/PB5

0.80 0.79 0.57 1.03 1.02 1.40 1.00

4.98 5.35 15.94 0.96 1.04 0.40 1.18

4.24 4.28 17.08 0.99 1.00 0.29 1.17

3.66 3.38 13.89 1.03 0.96 0.30 1.21

Wax, suberin, lignin Suberin, lignin Lignin Sugar, suberin, lignin Suberin, lignin Sugar, lignin Lignin

Wax Suberin

sorption coefficient (Kd) of carbon nanotubes (CNT) were used to draw comparisons. As presented in Table 2, Kd of phenanthrene at 0.05 Sw by hydrolyzed pine barks (PB4–PB6) were 23,277– 30,809 L/kg, a little higher than that of multi-walled nanotube (MWNT40) (20,600 L/kg, Wang et al., 2008).

3.3. Competitive sorption Knowledge of competitive sorption characteristics is critical for their environmental application of these modified sorbents in wastewater treatment because lots of organic pollutants always

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2.8

phenanthrene pyrene pyrene with 0.5mg/L phenanthrene PB4 2.1

PB6

Koc/Kowc

PB5

1.4

0.7

PB1

PB2

0.63

0.61

PB3 0.0 0.88

0.50

050

0.49

(O+N)/C Fig. 4. Relationship between sorption coefficient ratio (Koc/Kowc) of PAH and polarity index [(O + N)/C] of modified pine bark fractions.

3.4. Sorption advantages of hydrolyzed barks Since the main sorption mechanism is partition, the sorption coefficients can be used to predict the removal efficiency (R) and maximum sorption amount (Qmax) by the following equations:

18

15

Ratio of Sorption Coefficient Kd

In comparison with the single-solute system, the ratios of Kd on hydrolyzed sorbents to their precursor fractions (i.e., PB6/PB1, PB5/ PB2, PB4/PB3) lowered in the bi-solute system (Table 4), suggesting that the restriction effect of sugar fractions weakened with the presence of competitor. On the contrary, sorption behavior of waxes and suberin was not distinctively influenced by the coexisting competitor, supported by the similar Kd ratios of dewaxed-sorbents to their precursor fractions [i.e., wax effect (PB2/PB1, PB5/ PB6) and suberin effect (PB3/PB2, PB4/PB5)] in both single-solute and bi-solute systems. Sorption isotherms of pyrene became linear with the presence of phenanthrene as a competitor, indicating that the selected pollutant (i.e., pyrene) removal mechanism was partition in this bisolute sorption system (Shechter et al., 2006). Since partition is a non-competitive process, sorption capacity of pine bark fractions would keep the same order of magnitude even if more kinds of pollutants were added in the system. But for the synthetic carbon nanotube (MWNT15), the magnitude of sorption capacity was significantly decreased with the presence of competitive solute (Yang et al., 2006).

phenanthrene pyrene pyrene with 0.5mg/Lphenanthrene

12

9

6

3

0 0.6

0.8

1.0

1.2

1.4

Ratio of Polarity Index (O+N)/C Fig. 5. Correlation of sorption coefficient (Kd) ratio with polarity index [(O + N)/C] ratio between different modified pine bark fractions (data from Table 4).

mixed together. Competitive sorption behavior was observed in the bi-solute system for all selected sorbents (PB1–PB6, see Fig. 3) due to the specific p–p electron interaction between the sorbates and the aromatic cores of sorbents. As the main sorption mechanism was partition, and the contribution of specific p–p interaction on the total sorption behavior was significant only at lower concentration. With the presence of large amount of phenanthrene as a competitor (i.e., 0.5 mg/L), the specific p–p electron interaction between bark fractions and pyrene was seriously inhibited, which were more obviously for PB4–PB6 fractions than those of PB1–PB3 fractions. Therefore, the sorption isotherms of pyrene became linear (Freundlich N  1) and sorption efficiency of pyrene with all sorbents decreased in the bi-solute system compared with single-solute system. In detail, sorption coefficients (Koc) of PB1– PB3 decreased 16–19%, whereas Koc values of desugared-fractions (PB4–PB6) declined as much as 31–34%.

K d ¼ Q =C e

ð2Þ

Q ¼ ðC 0  C e Þ  m=V

ð3Þ

R ¼ ðC 0  C e Þ=C 0 ¼ K d  m=ðK d  m þ VÞ

ð4Þ

Q max ¼ K d  Sw

ð5Þ

where C0 is the initial concentration before treatment; Ce is the equilibrium concentration after treatment, mg/L; m is the dose of sorbent, kg; and V is the volume of wastewater solution; Sw is aqueous solubility of sorbate. As described in Table 5, removal efficiency of phenanthrene by raw pine bark was only 62.91% with sorbent dose of 0.5 g/L, which reached 89.41–91.16% after acid hydrolysis. Due to the more hydrophobic nature, pine bark sorbents presented higher sorption efficiency of pyrene. With the sorbent dose of 0.2 g/L, the removal efficiency of pyrene by hydrolyzed barks were more than 90% even in the existence of competitor pollutant. Maximum sorption amount of hydrolyzed barks were up to 18.57–22.69 mg/g for phenanthrene and 7.74–9.39 mg/g for pyrene. Table 5 Removal efficiency of phenanthrene and pyrene from aqueous solution by raw and modified pine barks. Sorbentsa Removal efficiency (%)b

PB1 PB2 PB3 PB4 PB5 PB6 a

Maximum sorption capacity (mg/g)c

Phenanthrene Pyrene Pyrene with Phenanthrene 0.5 mg/L phen

Pyrene

62.91 62.07 39.28 91.16 89.75 89.41

2.21 2.29 0.68 9.39 7.74 8.10

80.95 80.84 55.28 95.48 94.75 94.75

77.31 77.90 50.98 93.53 92.25 92.57

3.73 3.60 1.42 22.69 19.26 18.57

The meaning for the sorbent name was presented in Table 1. Removal efficiency were calculated by the equation R = Kd  m/(Kd  m + V), where Kd is the sorption coefficient of sorbent presented in Table 2, L/kg; m is the sorbent dose, kg; V is the solution volume, L. The given sorbent dose of phenanthrene and pyrene were 0.5 and 0.2 g/L. c Maximum sorption capacity Qmax was obtained by Kd  Sw. Aqueous solubility (Sw) is 1.1 and 0.13 mg/L for phenanthrene and pyrene, respectively. b

Y. Li et al. / Bioresource Technology 101 (2010) 7307–7313

Besides the increase of sorption capacity (4–5 folds) in comparison with raw pine bark, another advantage is that hydrolyzed bark present less risk for toxicity and much safer in wastewater treatment. Ribe et al. (2009) identified the potential risks and limitations of using pine bark as a filter material, and found that release of organic acids and desorbed metals occurred. However, these dissolved organic matter and metals can be effectively desorbed during the acid hydrolysis process. Furthermore, heavy metals were proved to be sorbed on lignin rather than cellulose and hemicelluloses materials (Dizhbite et al., 1999; Wu et al., 2008; Argun and Dursun, 2008). Therefore, hydrolyzed pine bark material is also expected to an effective sorbent for both organic pollutants and heavy metals removal. Comparing with the so-called supersorbent such as active carbon (AC) and carbon nanotubes (CNT), the hydrolyzed barks showed high sorption potential in the competitive sorption systems. AC and CNT attracted organic pollutants via the huge surface area, thereby their sorption efficiency would quickly drop with the presence of lots of pollutants. However, hydrolyzed barks retained organic pollutants mainly by the non-competitive partition process. Among all hydrolyzed sorbents, desugared-sorbent PB6 is deemed to be an ideal choice for environmental application because of its highest yield (70.6%) and lowest modification cost. PB6 was produced after a single modification using HCl hydrolysis, which reserved the natural powerful sorption medium (i.e., suberin and waxes) and liberated the potential sorption capability of lignin materials by removing their sorption restrictors (i.e., sugar component). 4. Conclusions Pine bark is of great potential as natural sorbent material for wastewater treatment, and aromatic lignin is the most powerful sorption medium for PAHs. However, its huge sorption potential was suppressed by the coexisting sugar component, which would be liberated after the consumption of the sugar by acid hydrolysis. Hydrolyzed sorbents demonstrated notably removal efficiency, and their sorption coefficients were 4–17 higher than their original fractions. Considering the producing yield and modification cost, the desugared bark (PB6) is assumed to be the ideal choice for organic pollutants removal among all modified pine bark sorbents. Acknowledgements This study was supported by National Natural Science Foundation of China(20977081, 20737002, 40671168), the Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 200765), and the Doctoral Fund of Ministry of Education of China (No. J20091588). References Acemioglu, B., 2004. Removal of Fe(II) ions from aqueous solution by Calabrian pine bark wastes. Bioresour. Technol. 93, 99–102. Argun, M.E., Dursun, S., 2008. A new approach to modification of natural adsorbent for heavy metal adsorption. Bioresour. Technol. 99, 2516–2527. Argun, M.E., Dursun, S., Karatas, M., 2009. Removal of Cd(II), Pb(II), Cu(II) and Ni(II) from water using modified pine bark. Desalination 249, 519–527. Bras, I., Ciasantos, L., Alves, A., 1999. Organochlorine pesticides removal by pinus bark sorption. Environ. Sci. Technol. 33, 631–634. Bras, I., Lemos, L.T., Alves, A., Pereirac, M.F.R., 2004. Application of pine bark as a sorbent for organic pollutants in effluents. Manage. Environ. Qual. 15, 491–501. Bras, I., Lemos, L.T., Alves, A., Pereirac, M.F.R., 2005. Sorption of pentachlorophenol on pine bark. Chemosphere 60, 1095–1102.

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