Impacts of charcoal characteristics on sorption of polycyclic aromatic hydrocarbons

Available online at www.sciencedirect.com

Chemosphere 71 (2008) 2113–2120 www.elsevier.com/locate/chemosphere

Impacts of charcoal characteristics on sorption of polycyclic aromatic hydrocarbons Hongwen Sun *, Zunlong Zhou Key Laboratory of Environmental Pollution Process and Standard (Nankai University), Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China Received 24 July 2007; received in revised form 9 January 2008; accepted 9 January 2008 Available online 10 March 2008

Abstract Sorption of three polycyclic aromatic hydrocarbons (PAHs, phenanthrene, anthracene and pyrene) on three charcoals and their precursor substance (sawdust) was studied. The charcoals obtained by heating at 400 °C for different periods were different in chemical composition and structure. Sorption characteristics were described by a Polanyi–Dubinin–Manes model combined with poly-parameter linear free energy relationships. The results revealed that though partition could not be neglected for sawdust and charcoal containing large sawdust residue, adsorption controlled the sorption of PAHs on matured charcoals, where p–p electron donor–acceptor (EDA) exerted as the main molecular-scale interactions. Charring elevated partition coefficients (Koc) of the three PAHs more than one order of magnitude, which ranged from 105.74 to 106.58 on charcoals (for PAHs at equilibrium concentration Ce = 0.005Sw). Adsorption increased with the aromaticity of the charcoals, however, polar aromatic structure may stimulate sorption of PAHs due to the presence of p–p EDA interactions. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Charcols; PAHs; Sorption; LFER; Pore filling; p–p EDA

1. Introduction Sorption is a major process determining the fate and bioavailability of organic pollutants, especially hydrophobic organic chemicals in soils and sediments. Charcoal is the residue of incomplete biomass burning (Goldberg, 1985) which is present in soil, water, sediment with varied fractions (10–70%) in organic carbons, where it acts as an important sorbent for organic pollutants. Charcoal particles tend to sorb organic pollutants more strongly than the macromolecular forms of organic matters (OMs) (Cornelissen and Gustafsson, 2005; Koelmans et al., 2006; Morelis et al., 2007). Hence, they influence the transfer and bioavailability of organic pollutants in the environment even though their levels are not high.

*

Corresponding author. Tel./fax: +86 22 23509241. E-mail address: [email protected] (H. Sun).

0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.01.016

Sorption of organic pollutants onto charcoals tends to be strong and nonlinear (Yang and Sheng, 2003; Cornelissen and Gustafsson, 2005), where adsorption or pore-filling part instead of partition is the dominant process. For example, Chun et al. (2004) demonstrated that organic pollutants’ sorption on charcoals obtained by heating at high temperatures (500 and 700 °C) occurred almost exclusively by adsorption, which includes surface adsorption and pore-filling processes. In comparison, sorption onto low-temperature (300 and 400 °C) charcoals resulted from adsorption and concurrent smaller partition into the residual OM phase. Attempts have been made to examine the molecularscale mechanisms determining the sorption of organic pollutants on charcoals and other natural geosorbents. For example, it has been showed that sorption on soil/sediment organic matter (SOM) is usually negatively related to their O/C atomic ratio or polarity index (Chen et al., 1996; Huang and Weber, 1997). A studies by Yang et al. (2004) and Chun et al. (2004) showed an opposite trend for more

2114

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

matured sorbents, such as charcoals. Recently, Sander and Pignatello (2005) found an enhanced sorption of nitrobenzene on charcoals compared to benzene and toluene. 1H NMR results, in combination with the result of Zhu and Pignatello (2005), were used to support the presence of p–p electron donor–acceptor (EDA) interaction between nitrobenzene and polycyclic aromatic charcoal surface. These results showed that multi-processes control the sorption of organic pollutants on charcoals. Hence, further research is needed to quantitatively elucidate the molecular-scale structural factors of charcoals controlling their sorption of organic pollutants. Poly-parameter linear free energy relationship (ppLFER) is a powerful tool to reveal the molecular-scale interactions in sorption processes of organic pollutants. Nguyen et al. (2005) applied a pp-LFER for estimating Koc of SOM. However, for black carbon (BC), sorption isotherms are highly nonlinear, and Koc is not identical along the isotherm. This disfavors relating pp-LFER with concentration-dependent Koc. Polanyi-based adsorption isotherms have been widely applied in hard carbon sorption (Xia and Ball, 2000; Kleineidam et al., 2002; Gavin et al., 2005; Nguyen and Ball, 2006; Pikaar et al., 2006) to describe its nonlinear sorption behavior; however, little attempt has been made to combine it with pp-LFER. Crittenden et al. (1999) used a combination of Polanyi potential theory and linear solvation energy relationship (LSER) to estimate the adsorption capacity of activated carbons in water treatment. The combination of Polanyi potential theory with pp-LFER is thought to be a good approach for investigating the contributions of different sorption forces and estimating sorption capacity of charcoal since it takes the nonlinearity of sorption into consideration. To better understand charcoal sorption of PAHs and the molecular-scale mechanisms, the sorption of phenanthrene, anthracene and pyrene onto three synthesized pine-wood charcoals and their precursor substances was investigated. Batch experiments were conducted and the isotherms were fitted with both Polanyi-based model combined with pp-LFER analysis and Freundlich model. 2. Experimental section 2.1. Sorbent preparation and characterization Charcoals were produced by pyrolyzing pine (Pinus massoniana Lamb.) sawdust. About 15 g of air-dried and ground sawdust was put into porcelain crucibles. They were covered to protect the sawdust from direct contact with air, and heated isothermally at 400 °C in a preheated muffle furnace. The sawdust was heated for 0.5, 4 and 8 h, respectively, to obtain three pine-wood charcoal (PC) samples, which were referred to as PC1 (0.5 h), PC2 (4 h) and PC3 (8 h), respectively. C, H, N contents of the finished charcoals and the sawdust were measured by an element analyzer

(Elementar Vario EL, Elementar Analysensysteme, German). The amount of ashes was determined by mass loss after heating the samples at 750 °C for 4 h (Xing, 2001). Functionalities of the charcoals and sawdust were determined by solid-state CP/MAS 13C NMR at 100 MHz carbon frequency using a Varian Infinityplus-400 MHz NMR spectrometer (Varian, USA). The instrument was run under the following conditions: contact time, 1 ms; spinning speed, 4 KHz; 90° 1H pulse, 5 ls; acquisition delay, 2.2 s; line broadening, 50 Hz. Specific surface area (SSA) was obtained by nitrogen adsorption data at 77 K using a high-resolution gas adsorption analyzer (AutoSorb-1MP, Quantachrome, USA). Data in the range of 0.06– 0.2 P/Po (relative pressure) were modeled separately using the BET method for SSA estimation. 2.2. Chemicals Phenanthrene (PHE), anthracene (ANT) and pyrene (PYR) (purity > 98%) were purchased from Acros Corporation (New Jersey, USA). Stock solutions of the three PAHs were prepared in HPLC-grade methanol. All other chemicals and solvents used were analytical grade or better. 2.3. Sorption experiments Batch equilibrium sorption experiments were conducted in 40 ml screw-capped glass vials (Agilent, USA). 5.0 mg charcoal and 0.1 g sawdust were added to each vial. 40 ml test solution containing 5 mM CaCl2 (to maintain a constant ionic strength) and 200 mg l1 NaN3 (to prevent bacterial activity) were added to the vials. However, it should be noted that the concentration of used NaN3 may interact with the sorption of PAHs to charcoals according to Chefetz et al. (2006). Then, specific amounts of PAHs stock solutions were added to the suspensions to attain 30–80% PAHs uptake by charcoals at equilibrium. The methanol content in the test solution was controlled less than 0.1% (v/v), which is not expected to cause co-solvent effect. All vials were immediately sealed tightly using screw caps with Teflon liners. The vials were first hand-shaken for a few minutes and then horizontally shaken in a constant temperature (20 ± 0.5 °C) shaker oscillating at 150 rpm for 20 d (sawdust for 7 d). Our preliminary test showed that apparent sorption equilibrium was reached in less than 20 d. After equilibrium, the vials were centrifuged at 1500g for 20 min to separate solid and aqueous phases. Concentrations of PAHs in the supernatants were determined by HPLC. All experiments were conducted in triplicate. 2.4. Analysis method The HPLC system was SCL-10AVP (Shimadzu, Kyoto, Japan) equipped with a programmable fluorescence detector. A reverse-phase column (VP-ODS Kromasil C18, 150 mm  4.6 mm  5 lm) was used under an ambient temperature. The flow rate of the mobile phase containing 80%

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

2115

acetonitrile and 20% water (Milli-Q) was 1.0 ml min1, and the injection volume was 20 ll. The fluorescence detection excitation/emission wavelengths for PHE, ANT and PYR were 250/364 nm, 250/400 nm, and 260/373 nm, respectively. The amount of PAHs was determined with external PAHs standards. Blank samples indicated that PAHs loss due to sorption onto the reactor walls and volatilization was less than 2%. Thus, sorbed mass was calculated by subtracting the residue mass in aqueous phase from the initial solute mass.

of SOM, which includes both partition and pore-filling process:

2.5. Models for sorption description

3. Results and discussion

Isotherm models. Two isotherm models were used, i.e., Polanyi–Dubinin–Manes (PDM) model as described by Kleineidam et al. (2002) and Freundlich equation

3.1. Characterization of sawdust and synthesized charcoals

Freundlich equation : log Qe ¼ log K F þ n log C e

ð1Þ

Ce (lg l1) and Qe (lg kg1) are the aqueous and sorbed chemical concentrations at equilibrium, respectively, n is an index for non-linearity, and KF ((lg kg1)/(lg l1)n) is Freundlich coefficient. PDM model : Qe ¼ V 0  q  exp½RT  ð lnðC e =S w ÞÞ=E2

ð2Þ

V0, q, R, and T are the maximum volume of sorbed chemical per unit mass of the sorbent (cm3 g1), compound density (g cm3), ideal gas constant, and temperature (K), respectively. Sw (lg l1) is water solubility of the compound. Pure liquid of a chemical is usually used as the reference state in studies of partition processes (Schwarzenbach et al., 2003). E (kJ mol1) is the characteristic adsorption free energy of a compound compared to that of a reference compound (Crittenden et al., 1999). V0 and E can be obtained by applying the data to the model. Combining the PDM model with LFERs, the correlation can be expressed as following equation (Crittenden et al., 1999): E ¼ v  V  102 þ p  p þ a  a þ b  b þ c

ð3Þ

where V is the molecular volume, and p*, a, and b are indices for polarity/polarizability, electron donor and acceptor interactions, respectively. A Polanyi-based Dual Model (Xia and Ball, 2000) was proposed on consideration of the heterogeneous nature

Qe ¼ K p  C e þ V 0  q  exp½RT  ð lnðC e =S w ÞÞ=E2

ð4Þ

where Kp (l kg1) is the partitioning coefficient. Kp can also be obtained by applying the data to the model. The E in Eq. (4) cannot be simply used in LFERs as partitioning also contributes to the free energy of sorption.

Elemental composition. The elemental composition of the finished charcoals and original sawdust, their atomic ratios of H/C and O/C, and polarity index ((N + O)/C) are presented in Table 1. The contents of H and O of the three charcoals decreased dramatically compared to those of the sawdust, and correspondingly, their H/C and O/C decreased. Among the three charcoals, H/C of PC1 (0.80) is the highest. Since H is mainly associated with plant organic carbon (Kuhlbusch, 1995), the high H/C implies a high fraction of pine-wood residue in PC1. This is consistent with the fact that PC1 experienced the shortest heating time. The thermal treatment decreased atomic O/C ratio during the first 4.0 h, as indicated by the reduction of O/ C ratio from 0.67 for sawdust to 0.19 for PC2. However, after another 4 h of heating, O/C ratio of PC3 (0.25) became higher than that of PC2. Accordingly, the polarity index ((N + O)/C) of PC1 was the highest, followed by PC3 and PC2. Solid state CP/MAS 13C NMR. The results of NMR analysis for the samples are presented in Fig. S1 and Table S1 (Supplementary Material). Within the 0–220 ppm chemical shift range, C atoms are assigned to alkyl carbon (0– 50 ppm), alcohols, amines, carbohydrates, ethers, methoxyl and acetal carbon (50–108 ppm), aromatic carbon (108– 145 ppm), phenolic groups (145–162 ppm), carboxyl carbon (168–190 ppm), and carbonyl carbon (190–220 ppm) (Cook and Langford, 1998). There are strong peaks at 73 ppm and 105 ppm only in PC1 and sawdust. The two peaks indicated substantial amount of wood residue in PC1 (Gunasekara et al., 2003). Small peaks around

Table 1 Structural parameters of three charcoals and the original sawdust Elemental composition (wt%)

Sawdust PC1a PC2 PC3 a b c

Ash (wt%)

C

H

N

O

48.66 65.33 73.03 68.69

6.89 4.36 3.70 3.79

NDb 0.85 0.83 0.99

43.20 29.46 18.12 23.04

1.25 3.02 4.32 3.49

Atomic ratios H/C

O/C

(N + O)/C

1.70 0.80 0.61 0.66

0.67 0.34 0.19 0.25

0.67 0.35 0.19 0.26

SSA (m2 g1)

Aromaticityc (%)

NDb 15.6 25.9 26.8

21.2 39.7 48.8 56.7

PC1, PC2 and PC3 were charcoals acquired by heating pine sawdust at 400 °C for 0.5, 4 and 8 h. Not detected. The aromaticity of the samples was determined by integrating the peak area in the 108–162 ppm chemical shift region.

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

PAHs sorption on PC1

7

10

Qe / µg kg -1

29 ppm (Fig. S1B in Supplementary Material), which are assigned to amorphous aliphatic carbons, were observed in the spectra of PC1. While crystalline aliphatic carbons (33 ppm) were observed in PC2 and PC3. These demonstrated that the rigidity of OM structure significantly increased in PC2 and PC3, compared to that in PC1. The total aliphatic C region (0–108 ppm, Table S1 in Supplementary Material) decreased from 54.5% for PC1 to 49.0% for PC2, and to 34.6% for PC3, whereas the contents of aromatic (108–145 ppm) increased. Moreover, it should be noted that carbohydrates, alcohols, amines, ethers, methoxyl and acetal carbons like moieties (50– 108 ppm) were highly reduced while the charring time was increased from 4 to 8 h. However, substantial phenolic and carboxylic groups were produced in PC3 as compared to PC2. It could be that a portion of hydroxylic groups were oxidized to carbonyl or carboxylic groups while the charring time was increased from 4 to 8 h. Hence, PC2 had the least content of polar aromatic carbon, which are mainly assigned to O-substitued aromatic carbon region (145–162 ppm), whereas PC3 had the greatest content of polar aromatic carbon. Surface area. The SSA values of the charcoals are presented in Table 1, and pore volume distributions of PC2 and PC3 are presented in Fig. S2 (Supplementary Material). The specific surface area and pore structure of PC2 (25.9 m2 g1) was similar to those of PC3 (26.8 m2 g1), and PC1 had the smallest SSA (15.8 m2 g1).

PHE ANT PYR

5 -2

-1

10

0

10

10

Ce /Sw PAHs sorption on PC2

7

10

6

10

PHE ANT PYR 5

10

-2

10

-1

0

10

10

Ce /Sw PAHs sorption on PC3

3.2. Polanyi–Dubinin–Manes model

7

10

Qe / µg kg -1

Sorption isotherms (Qe versus Ce/Sw, which means hydrophobicity-normalized sorption) of the three PAHs on the three charcoals are plotted in Fig. 1. Sorption varied among different charcoals and PAHs due to the differences in their structural parameters and molecular properties. Generally, sorption was greater on PC3 than those on PC1 and PC2, which is determined by their structure, and will be discussed later. The hydrophobicity-normalized adsorption of the three PAHs varied greatly, which indicates hydrophobic partition is not the only driving force for adsorption of PAHs on charcoals. The hydrophobicity-normalized adsorption increased in the order of PHE > ANT P PYR, with the largest PYR being the least adsorbed. Moreover, the hydrophobiciy-normalized adsorption of PHE was significantly higher than that of ANT, though its size is similar with ANT. The results can be explained by steric effect and solidphase condensation mechanism. First, the sorption isotherms were checked by PDM model (Eq. (2)). PDM model has been successfully applied in describing the sorption of organic compounds in highly meso- and microporous solids, which is applicable to both micropore-filling and surface-adsorption mechanisms (Kleineidam et al., 2002; Wang and Xing, 2007). The sorption data on PC2 and PC3 fitted to the PDM model well (Fig. 2). In pore-filling and surface-adsorption mechanisms, the smaller PHE

6

10

10

Qe / µg kg -1

2116

6

10

PHE ANT PYR 5

10

-2

10

-1

10

0

10

Ce /Sw Fig. 1. PAHs sorption isotherms on three charcoals. PC1, PC2 and PC3 were charcoals acquired by heating pine sawdust at 400 °C for 0.5, 4 and 8 h.

˚ 3) can penetrate into the meso- and micropores of (170 A the charcoals, and acquired quite greater adsorption than ˚ 3). The adsorption of PHE on the charlarger PYR (188 A coals was larger than ANT though they possess similar molecular volume, which cannot be explained only by steric effect (Wang and Xing, 2006). Xia and Ball (1999) have invoked solid-phase condensation mechanism to explain the differences in adsorption of liquid and solid chemicals to activated carbons and natural adsorbents. They proposed that those chemicals which exist as solid (crystalline) phases in the adsorbed state cannot pack as efficiently as those which exist as liquid. The melting point of PHE (101 °C) is quite smaller than that of ANT (217.5 °C).

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

PAHs sorption on PC1-experiment, dual model 7

1.5x10

PHE-experiment PHE-model ANT-experiment ANT-model PYR-experiment PYR-model

7

Qe / µg kg -1

1.2x10

6

9.0x10

6

6.0x10

6

3.0x10

0.0 10

0

10

1

Ce / µg L

10

2

-1

PHE sorption on PC1-experiment,dual model and its adsorption part 7

1.5x10

PHE-experiment PHE-dual model PHE-adsorption part of dual model

Qe / µg kg -1

7

1.2x10

6

9.0x10

6

6.0x10

Hence, it was concluded that the sorption of PAHs on the charcoals was controlled by adsorption or pore-filling mechanism on PC2 and PC3, whereas partition could not be neglected on PC1, especially at high concentrations. This effect of partition agrees well with the previous research (Gavin et al., 2005). Parameters of PDM model are listed in Table S2 (Supplementary Material). For the tested charcoals, V0 exceeded the estimate of cumulative micropore volume (Fig. S2, Supplementary Material), which implies substantial adsorption in mesopore regions. For PHE sorption on PC3, V0 even exceeded the cumulative mesopore volume, which shows an shift from the pore-filling mechanism to surface-adsorption mechanism (Wang and Xing, 2007). Moreover, the maximum adsorption capacity, M0, was in the order of PC3 > PC2 > PC1. As it was shown in Table 1, the SSA of the three charcoals increased, and their aromaticity increased from 39.7% for PC1, to 48.8% for PC2, and to 56.7% for PC3. These structural characteristics imply that, with the extending of heating time, the structure of charcoal became condensed and complex (larger SSA) in PC2 and PC3, which favors the pore-filling process of PAHs. 3.3. PDM model combined with LFERs

6

3.0x10

10

2117

0

10

1

Ce / µg L

10

2

-1

Fig. 2. Isotherm data of PAHs on PC1 that fits to Polanyi-based dual Model. PC1 was charcoals acquired by heating pine sawdust at 400 °C for 0.5 h.

Hence, it is reasonable to deduce that the physical state of the sorbed PHE and ANT might be different, which led to greater adsorption of PHE compared to ANT. For PC1, the predicted data by PDM model are smaller than the experimental data (Fig. S3 in Supplementary Material), whereas the Dual Model (Eq. (4)) fitted the experimental sorption data well (Fig. 2). This was because the structure of PC1, with the greatest wood residue and carbohydrates amount (Table S1 in Supplementary Material), was different from those of PC2 and PC3. Partition to the residual amorphous aliphatic components might be an important contributor for the sorption of PAHs onto PC1. Moreover, the percentage contributions of hydrophobic partition and adsorption (pore-filling) to the entire sorption on PC1 changed with pollutant concentrations. Take PHE as an example (Fig. 2), at a low concentration (Ce = 0.005Sw), adsorption accounted for almost 90% of the sorption; however, at a high concentration (Ce = 0.5Sw), adsorption accounted for only 25% of the sorption. Due to the contribution of hydrophobic partition, the sorption of the more hydrophobic PYR approaches to that of ANT on PC1, while the sorption of ANT was quite greater than that of PYR on PC2 and PC3.

The pp-LFERs were used to normalize the PDM model (Eq. (3)) for PC2 and PC3. The values of V  102, p*, a, b in Eq. (3) are 1.02, 0.81, 0, 0.20 for PHE and 1.28, 0.92, 0, 0.25 for PYR, respectively, which were cited from or calculated according to James and Passino-Reader (1991). Substituting these values into Eq. (3), the following four equations can be obtained: PHE on PC2 : 1:02  v2 þ 0:81  p2 þ 0:20  b2 þ c2 ¼ 10:9 ð5Þ PYR on PC2: 1:28  v2 þ 0:92  p2 þ 0:25  b2 þ c2 ¼ 9:92 ð6Þ PHE on PC3: 1:02  v3 þ 0:81  p3 þ 0:20  b3 þ c3 ¼ 10:7 ð7Þ PYR on PC3: 1:28  v3 þ 0:92  p3 þ 0:25  b3 þ c3 ¼ 9:11 ð8Þ v2, p2, b2, c2 and v3, p3, b3 and c3 are corresponding coefficients for PC2 and PC3, respectively. Subtracting Eq. (5) from Eq. (6) and (7) from Eq. (8), negative DE2 (1.01 kJ mol1) and DE3 (1.60 kJ mol1) were obtained for PC2 and PC3, respectively. PC2 : 0:26  v2 þ 0:11  p2 þ 0:05  b2 ¼ 1:01

ð9Þ

PC3 : 0:26  v3 þ 0:11  p3 þ 0:05  b3 ¼ 1:60

ð10Þ

These negative DE values indicate that one of the three coefficients (v, p, and b) must be negative. The v is the formation energy for a cavity. As water molecules interact more strongly with each other through hydrogen bonding than the molecules of charcoal do, v is positive. p* term

2118

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

accounts for nonspecific interactions that involve interactions of induced dipole–induced dipole (ID–ID), induced dipole–dipole (ID–D), and dipole–dipole (D–D). Hunter et al. (2001) reported that there is little evidence to suggest that D–D is of major importance in the interactions in aromatic systems. As ID–ID, ID–D between PAHs and charcoal was larger than those between PAHs and water, p should be positive. Thus, for a given sorbent, those negative DE’s should result from the contribution of a negative b, which is the coefficient for hydrogen bond or p–p EDA. When Crittenden et al. (1999) examined the sorption of aromatic and halogenated aromatic compounds on activated carbons, they also obtained a negative b. For the sorption of PAHs on charcoals, hydrogen bond was excluded from the underlying mechanisms (Sander and Pignatello, 2005), hence p–p EDA was thought to attribute to the negative b*b term. Thus, it can be concluded that p–p EDA played an important role for the adsorption of aromatic chemicals on our synthesized charcoals, especially on PC3, b value of which was more negative. The calculated E values of PHE were larger than those of PYR. This implies that p–p EDA interaction contributes more in the sorption of PHE than that of PYR due to sites accessibility, for PYR has a larger molecular volume ˚ 3) than PHE (170 A ˚ 3). The inversed relationship of (188 A maximum adsorption capacity and sorbent-sorbate contact area was also proved by Van Noort et al. (2004). PDM model was also applied to the sorption data of ANT. If the sorbed ANT and PHE are in the liquid state, the values of V  102, p*, a, b in Eq. (3) for ANT should be the same with PHE as they are isomeric compounds. However, E value of ANT deviated significantly from PHE (p < 0.05), which implies a difference in their condensation state. Cornelissen et al. (2004) mentioned ‘packing’

effect of ANT on BC surface. It is proposed that part of sorbed ANT might not exist as liquid phase as PHE does due to its higher Tm (217.5 °C). This proposition means that the values of V  102 of ANT in Eq. (3) deviate from 1.02. Therefore, the deviated E values must result from the deviated V  102 value of ANT, which indicates the different condensation state of sorbed PHE and ANT. 3.4. Freundlich isotherm The data were fitted with Freundlich equation (Table 2). Freundlich nonlinearity exponents (n) are far from 1.0 (n = 0.418–0.559). Compared to the linear partition of the PAHs to sawdust, charring made the solids more heterogeneous and the sorption isotherms more nonlinear. Freundlich coefficients [KF((lg kg1)/(lg l1)n)] ranged from 6.360 for PYR on PC2 to 6.71 on PC3. However, precise comparison cannot be made among KF values because of their different units as a result of the nonlinear isotherms. Therefore, the concentration-dependent OC sorption coefficients, Koc (Koc = Kd/foc, l kg1) at three selected concentrations (Ce = 0.005, 0.05, and 0.5Sw) were employed to compare PAHs sorption on charcoals. Log Koc (Table 2) varied greatly with the concentrations. At Ce/Sw = 0.005, Koc (105.7–106.8) on the charcoals are about 1–2 orders of magnitude higher than the reported data for SOMs (Yang and Sheng, 2003). The values of charcoals’ Koc are more than 1 order of magnitude higher than the data of their precursor substance (sawdust, Table 2). These results emphasize the importance of charcoals with respect to the sorption of organic pollutants in soils or sediments. The Koc sequence for the three PAHs in Table 2 (which is ANT > PHE > PYR) is different from the changing tendency shown in Fig. 1 because Ce of X-axis in Fig. 1 has been normalized by aqueous solubility of the compounds.

Table 2 Freundlich Model fits to sorption isotherms of PHE, ANT and PYR on charcoals and linear model fits to sorption isotherms on sawdust Parameters

Phenanthrene (PHE) Sawdust PC1c PC2 PC3 Anthracene (ANT) Sawdust PC1 PC2 PC3 Pyrene (PYR) Sawdust PC1 PC2 PC3 a b c d

nd

KFa,d

R2

log Kocb,d Ce = 0.005Sw

Ce = 0.05Sw

Ce = 0.5Sw

/ 0.44 ± 0.02 0.42 ± 0.02 0.47 ± 0.03

/ 7.20 ± 0.06 7.12 ± 0.06 7.96 ± 0.07

3.80 ± 0.02 5.87 5.74 6.05

5.36 5.20 5.54

4.85 4.57 4.98

0.997 0.989 0.983 0.975

/ 0.54 ± 0.02 0.50 ± 0.01 0.55 ± 0.01

/ 6.78 ± 0.05 6.76 ± 0.05 6.60 ± 0.05

3.89 ± 0.04 6.30 6.26 6.58

5.84 5.76 6.13

5.38 5.26 5.67

0.991 0.991 0.996 0.995

/ 0.63 ± 0.02 0.48 ± 0.01 0.56 ± 0.01

/ 6.71 ± 0.06 6.36 ± 0.03 6.71 ± 0.05

4.54 ± 0.04 5.91 5.78 6.09

5.54 5.26 5.65

5.18 4.74 5.21

0.992 0.989 0.999 0.995

The unit of KF is (lg kg1)/(lg l1)n. The unit of Koc is (l kg1). PC1, PC2 and PC3 were charcoals acquired by heating pine sawdust at 400 °C for 0.5, 4 and 8 h. Average ± standard error (P < 0.01).

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

The structural differences of the three charcoals impacted their sorption ability. The Koc varied greatly among the three charcoals, and PC3 had the largest Koc, followed by PC1 and PC2. There was no correlation between log Koc (Table 2) with either aliphatic or aromatic components of the charcoals (Table S1 in Supplementary Material). Literature is inconsistent on whether aromatic or aliphatic groups are dominantly responsible for the sorption of organic pollutants on OMs. For example, old OM containing relatively a larger amount of aromatic moieties found in shale or coal has higher Koc values than younger OM obtained from a surface soil (Grathwohl, 1990; Chen et al., 1999). In contrast, a positive trend between aliphaticity of OM and their log Koc value for PHE in many studies (Chefetz et al., 2000; Chefetz, 2003) supports the importance of aliphatic moieties. Different sorption processes (partitioning and pore-filling or surface adsorption) control sorption on different sorbates. This may explain the previous conflicting research conclusions about the relationships between the sorption and aliphatic/aromatic carbons. In most cases, O- or N-containing polar functional groups disfavor the sorption of organic pollutants (Huang and Weber, 1997). But, when p–p EDA interaction plays a role, the polar groups linked to aromatic sheets (polar aromatic structure) might promote the interaction through providing p electron-deficient sites. Correspondingly, these interactions stimulate the sorption of PAHs. As it was shown in previous analysis, p–p EDA contributed more for PC3 than PC2, for PC2 had less polar aromatic carbon (145–162 ppm) than PC3. This was also noted by Yang et al. (2004), who showed that log Koc of PHE had a positive relationship with the content of polar aromatic carbon in seven kerogens. Hence, the content of polar aromatic carbon might be one of important structural parameters of charcoals governing PAHs sorption. But, the role of polar functionalities might vary at aromatic pores with different size. Polar atoms could strongly bond water molecules, which might crowd out PAHs at micropores (Zhu et al., 2005). However, this ‘crowding out’ effect might not be significant for polar atoms at mesoporous regions, as this study showed. In the context of similar SSA and pore structure, the extra sorption potential sourced from p–p EDA interactions makes the sorption on PC3 extend to more regions, so much as to macropores. 4. Conclusions In this paper, the sorption of three PAHs onto three charcoals was studied. The data were described by a combined model of Polanyi–Dubinin–Manes (PDM) model and pp-LFERs, which has not been applied in charcoal sorption before. The results showed that adsorption or pore-filling controlled PAH sorption on the charcoals, whereas partition may play a role when the content of wood residue was high (in the case of PC1), especially at high concentrations. PHE has the

2119

lowest melting point and a small molecular volume, which favor PHE most in the pore-filling process. The combined model revealed that p–p electron donor–acceptor (EDA) interaction was important in the sorption of PAHs on PC2 and PC3. The sorption data of charcoals were also examined by Freundlich equation. Charring effect makes all isotherms highly nonlinear, with Freundlich nonlinearity index (n) being 0.418–0.633. Koc of the three charcoals had no correlation with either their aromatic or aliphatic moieties. It is supposed that polar atoms exert opposite effect on the sorption to charcoals through different mechanisms. When considering the presence of p–p EDA, polar aromatic carbons may be an important structural factor affecting the sorption of PAHs on those highly aromatic geosorbents like charcoals. Acknowledgements We acknowledge financial support from the National Natural Science Foundation of China (20737002) and Tianjin Municipal Science and Technology Commission (06TXTJJC14000). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere. 2008.01.016. References Chefetz, B., 2003. Sorption of phenanthrene and atrazine by plant cuticular fractions. Environ. Toxicol. Chem. 22, 2492–2498. Chefetz, B., Deshmukh, A.P., Hatcher, P.G., Guthrie, E.A., 2000. Pyrene sorption by natural organic matter. Environ. Sci. Technol. 34, 2925– 2930. Chefetz, B., Stimler, K., Shechter, M., Drori, Y., 2006. Interactions of sodium azide with triazine herbicides: effect on sorption to soils. Chemosphere 65, 352–357. Chen, Z., Xing, B., McGill, W.B., 1999. A unified sorption variable for environmental applications of the Freundlich equation. J. Environ. Qual. 28, 1422–1428. Chen, Z., Xing, B., McGill, W.B., Dudas, M.J.R., 1996. a-Naphthol sorption as regulated by structure and composition of organic substances in soils and sediments. Can. J. Soil Sci. 76, 513–522. Chun, Y., Sheng, G.Y., Chiou, C.T., Xing, B.S., 2004. Compositions and sorptive properties of crop residue-derived chars. Environ. Sci. Technol. 38, 4649–4655. Cook, R.L., Langford, C.H., 1998. Structural characterization of a fulvic acid and a humic acid using solid-state ramp-CP-MAS13 C nuclear magnetic resonance. Environ. Sci. Technol. 32, 719–725. Cornelissen, G., Gustafsson, O., 2005. Importance of unburned coal carbon, black carbon, and amorphous organic carbon to phenanthrene sorption in sediments. Environ. Sci. Technol. 39, 764–769. Cornelissen, G., Elmquist, M., Groth, I., Gustafsson, O., 2004. Effect of sorbate planarity on environmental black carbon sorption. Environ. Sci. Technol. 38, 3574–3580. Crittenden, J.C., Sanongraj, S., Bulloch, J.L., Hand, D.W., Rogers, T.N., Speth, T.F., Ulmer, M., 1999. Correlation of aqueous-phase adsorption isotherms. Environ. Sci. Technol. 33, 2926–2933.

2120

H. Sun, Z. Zhou / Chemosphere 71 (2008) 2113–2120

Gavin, J., Sabatini, D.A., Chiou, C.T., Rutherford, D., Scott, A.C., Karapanagioti, H.K., 2005. Evaluating phenanthrene sorption on various wood chars. Water Res. 39, 549–558. Goldberg, E.D., 1985. Black Carbon in the Environment. J. Wiley & Sons, New York. Grathwohl, P., 1990. Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: implications on Koc correlations. Environ. Sci. Technol. 24, 1687–1693. Gunasekara, A.S., Simpson, M.J., Xing, B., 2003. Identification and characterization of sorption domains in soil organic matter using structurally modified humic acids. Environ. Sci. Technol. 37, 852– 858. Huang, W., Weber Jr., W.J., 1997. A distributed reactivity model for sorption by soils and sediments. 10. Relationships between desorption, hysteresis, and the chemical characteristics of organic domains. Environ. Sci. Technol. 31, 2562–2569. Hunter, C.A., Lawson, K.R., Perkins, J., Urch, C.J., 2001. Aromatic interactions. J. Chem. Soc. Perk. T. 2., 651–669. James, P.H., Passino-Reader, D.R., 1991. Linear solvation energy relationships: ‘‘rule of thumb” for estimation of variable values. Environ. Sci. Technol. 25, 1753–1760. Kleineidam, S., Schuth, C., Grathwohl, P., 2002. Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants. Environ. Sci. Technol. 36, 4689–4697. Koelmans, A.A., Jonker, M.T.O., Cornelissen, G., Bucheli, T.D., van Noort, P.C.M., Gustafsson, O., 2006. Black carbon: the reverse of its dark side. Chemosphere 63, 365–377. Kuhlbusch, T.A.J., 1995. Method for determining black carbon in residues of vegetation fires. Environ. Sci. Technol. 29, 2695–2702. Morelis, S., van den Heuvel, H., van Noort, P.C.M., 2007. Competition between phenanthrene, chrysene, and 2, 5-dichlorobiphenyl for highenergy adsorption sites in a sediment. Chemosphere 68, 2028–2032. Nguyen, T.H., Ball, W.P., 2006. Absorption and adsorption of hydrophobic organic contaminants to diesel and hexane soot. Environ. Sci. Technol. 40, 2958–2964. Nguyen, T.H., Goss, K.-U., Ball, W.P., 2005. Poly-parameter linear free energy relationships for estimating the equilibrium partition of organic

compounds between water and the natural organic matter in soils and sediments. Environ. Sci. Technol. 39, 913–924. Pikaar, I., Koelmans, A.A., van Noort, P.C.M., 2006. Sorption of organic compounds to activated carbons. Evaluation of isotherm models. Chemosphere 65, 2343–2351. Sander, M., Pignatello, J.J., 2005. An isotope exchange technique to assess mechanisms of sorption hysteresis applied to naphthalene in kerogenous organic matter. Environ. Sci. Technol 39, 7476–7484. Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M., 2003. Environmental Organic Chemistry. John Wiley & Sons, New York. Van Noort, P.C.M., Jonker, M.T.O., Koelmans, A.A., 2004. Modeling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs. Environ. Sci. Technol. 38, 3305–3309. Wang, X., Xing, B., 2006. Competitive sorption of pyrene on wood chars. Environ. Sci. Technol. 40, 3267–3272. Wang, X., Xing, B., 2007. Sorption of organic contaminants by biopolymer-derived chars. Environ. Sci. Technol. 41, 8342–8348. Xia, G., Ball, W.P., 1999. Adsorption-partitioning uptake of nine low polarity organic chemicals on a natural sorbent. Environ. Sci. Technol. 33, 262–269. Xia, G., Ball, W.P., 2000. Polanyi-based models for the competitive sorption of low-polarity organic contaminants on a natural sorbent. Environ. Sci. Technol. 34, 1246–1253. Xing, B., 2001. Sorption of naphthalene and phenanthrene by soil humic acids. Environ. Pollut. 111, 303–309. Yang, Y., Sheng, G., 2003. Enhanced pesticide sorption by soils containing particulate matter from crop residue burns. Environ. Sci. Technol. 37, 3635–3639. Yang, C., Huang, W., Xiao, B., Yu, Z., Peng, P., Fu, J., Sheng, G., 2004. Intercorrelations among degree of geochemical alterations, physicochemical properties, and organic sorption equilibria of kerogen. Environ. Sci. Technol. 38, 4396–4408. Zhu, D., Pignatello, J.J., 2005. Characterization of aromatic compound sorptive interactions with black carbon (charcoal) assisted by graphite as a model. Environ. Sci. Technol. 39, 2033–2041. Zhu, D., Kwon, S., Pignatello, J.J., 2005. Adsorption of single-ring organic compounds to wood charcoals prepared under different thermochemical conditions. Environ. Sci. Technol. 39, 3990–3998.