Bioresource Technology 163 (2014) 193–198
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Selective removal of polycyclic aromatic hydrocarbons (PAHs) from soil washing effluents using biochars produced at different pyrolytic temperatures Helian Li a,b,⇑, Ronghui Qu a, Chao Li a, Weilin Guo a, Xuemei Han a, Fang He a, Yibing Ma a,c, Baoshan Xing b a
School of Resources and Environment, University of Jinan, Jinan 250022, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA National Soil Fertility and Fertilizer Effects Long-term Monitoring Network, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China b c
h i g h l i g h t s Pyrolytic temperature influenced wheat straw-derived biochar properties. Biochars selectively removed PAHs from effluents and recovered Triton X-100. PAH removal and Triton X-100 loss increased with increasing pyrolytic temperature. Selective adsorption was PAH property dependent.
a r t i c l e
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Article history: Received 26 February 2014 Received in revised form 10 April 2014 Accepted 11 April 2014 Available online 19 April 2014 Keywords: Biochar Selective adsorption PAHs Soil washing effluents Surfactant recovery
a b s t r a c t Wheat straw biochars produced at 400, 600 and 800 °C (BC400, BC600 and BC800) were used to selectively adsorb PAHs from soil washing effluents. For soil washing effluents contained Phenanthrene (PHE), Fluoranthene (FLU), Pyrene (PYR) and Triton X-100 (TX100), biochars at 2 (for BC800) or 6 g L1 (for BC400 and BC600) can remove 71.8–98.6% of PAHs while recover more than 87% of TX100. PAH removals increase with increasing biochar dose. However, excess biochar is detrimental to the recovery of surfactant. For a specific biochar dose, PAH removal and TX100 loss increase with increasing pyrolytic temperature. For BC400 and BC600, PAH removal follows the order of PHE > FLU > PYR, while the order is reversed with PYR > FLU > PHE for BC800. Biochars have much higher sorption affinity for PAHs than for TX100. It is therefore suggested that biochar is a good alternative for selective adsorption of PAHs and recovery of TX100 in soil washing process. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants that are ubiquitous in the environment. Coal processing is one of the most important sources of PAHs. Coke oven plants, manufactured gas plants and areas of coal tar spillage are of high contamination of PAHs (Paria and Yuet, 2006; Viglianti et al., 2006). In developing countries, with rapid urbanization and subsequent industry transfer, many PAH contaminated sites were left to be remediated. Due to their known or potential genotoxicity and carcinogenicity, remediation of these PAH-contaminated sites is of particular environmental concern (Woo et al., 2001; Ahn et al., 2008). ⇑ Corresponding author at: School of Resources and Environment, University of Jinan, Jinan 250022, China. Tel.: +86 0531 82769233. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.biortech.2014.04.042 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Soil washing has been proven as an effective method to remove PAHs from contaminated soils. Surfactant is the most widely used detergent in soil washing process, which can significantly remove the PAHs through micellar solubilization at levels higher than the critical micelle concentration. Although the addition of surfactants can greatly improve the removal efficiencies of PAHs, the surfactant cost may account for 50% of the total operating expenses (Mann and Peterson, 1993). Therefore, if we can take an effective method to remove PAHs from the washing wastewater and recycle the surfactant solution, the cost of soil washing remediation could be greatly reduced. Many techniques have been used to treat soil washing effluents, such as photocatalysis (Fabbri et al., 2009), electrochemical treatment (Gómez et al., 2010), advanced oxidation process (Bandala et al., 2008) and selective adsorption by activated carbons (Ahn et al., 2008; Wan et al., 2011). Compared with other aforementioned
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treatment processes, activated carbon has advantage in recovering surfactant from the soil washing effluents. Using batch experiments or fixed column process, researchers (Ahn et al., 2007; Liu et al., 2013) demonstrated that activated carbon can remove most of the PAHs and recover more than 90% surfactants from the soil washing effluents. Biochar is a C rich product produced during the pyrolysis of common biomass such as wood, animal wastes, crop residues, municipal waste and biosolids (Lehmann and Joseph, 2009). Biochars are characterized by high affinity for organic contaminants (Sun et al., 2011), thus these materials may potentially be an alternative for activated carbon. Since biochar is a greener and more cost effective material compared with activated carbon (Denyes et al., 2012), its use in environmental remediation would be more preferable. There is a huge amount of literature on the adsorption of PAHs by biochar from water or soil. Previous research has revealed that sorption of aromatic compounds by biochar was driven by not only p–p interactions but also their affinity for chars as affected by hydrophobicity, pore distribution, and surface chemistry (Wang et al., 2006). To our knowledge, it still remains unclear how these factors would influence the removal of PAHs in the presence of surfactant. In this study, biochars produced at different pyrolytic temperatures were used to selectively adsorb PHE, FLU and PYR from soil washing effluents. PHE, FLU and PYR were chosen to represent PAHs with different hydrophobicity and molecular size. Moreover, they are usually present at high concentrations in soil washing effluents. TX100 was employed because it is widely used to clean up organic pollutants like PAHs from soils. The aim of this paper was to investigate (1) the applicability of biochars in the selective removal of PAHs from TX100 solutions, and (2) the effect of pyrolysis temperature for making biochar and PAH properties on the selective adsorption performance. Since biochar is similar to activated carbon in adsorption of PAHs, we can hypothesize that biochar can be used as an alternative for surfactant recovery in soil washing process. Our second hypothesis is that high-temperature biochar would be more efficient in selectively remove PAHs from surfactant solutions.
2. Methods 2.1. Chemicals PHE, FLU and PYR with purities of over 98% were purchased from Aldrich (USA). TX-100 was obtained from Amresco (USA), with a purity of 99%. Selected physicochemical properties of the compounds are presented in Table S1. Methanol and acetonitrile were HPLC grade purchased from J.T. Baker (Phillipsburg, NJ, USA). Deionized water was produced by a Milli-Q system (Millipore Co., USA).
2.2. Biochar sample preparation and characterization Biochars BC400, BC600 and BC800 were produced in a muffle furnace (SX2-14-12, Longkou City Factory Furnace, China). Wheat straw powder was placed in a ceramic pot, covered with a fitting lid, and pyrolyzed under the oxygen-limited conditions at 400, 600 and 800 °C for 6 h (Chun et al., 2004a). The samples were treated in 200 mL of 1 M HCl solution for 12 h to remove minerals. Then, they were thoroughly washed with distilled water till neutral pH. Finally, the biochar samples were oven-dried overnight at 80 °C, gently ground to pass through a 100-mesh sieve, and stored for subsequent use.
Elemental (C, N, H and S) analyses were conducted using a Vario EI III elemental analyzer (Elementar, Germany), and oxygen content was determined by difference as follows: O (%) = 100 (C + H + N + S + Ash) (Pereira et al., 2011). Ash content of biochar samples were determined by dry combustion at 760 °C for 6 h (Novak et al., 2009). The surface area and pore size of the biochars were determined by N2 adsorption at 77 K using an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics, USA). The BET (Brunauer–Emmett–Teller) surface areas were calculated from the linear fit of the N2 adsorption data by applying the BET equation in the relative pressure (P/P0) range between 0.05 and 0.35, and taking the average area occupied by a molecule of N2 in the complete monolayer to be equal to 0.1620 nm2. The micropore volumes were obtained by the t-plot method, and the total pore volumes were estimated from the adsorbed amount of N2 at a relative pressure of 0.99. Scanning electron microscope (SEM) analysis was carried out using a FEI QUANTA FEG250 scanning electron microscope (FEI, USA). For FTIR measurements, 1 mg of the slightly ground biochar was gently mixed with 200 mg of oven-dried (at 105–110 °C) KBr and then the mixture was pressed into a pellet. FTIR spectra were obtained using a Nicolet 380 FTIR spectrometer (Thermo Electron, USA) with a resolution of 2 cm1 between wave numbers of 800 and 4000 cm1. 2.3. Preparation of soil washing effluents An uncontaminated soil was collected from the suburb of Jinan city, China. The soil was air dried and passed through a 2 mm sieve. The soil contained 53.2% sand, 24.4% silt and 22.4% clay. The soil had a pH of 6.8, a total organic content of 0.8%, and a cation exchange capacity of 6.8 cmol kg1. The contaminated soil was prepared as described by Zhou et al. (2007). In brief, an appropriate quantity of PHE, FLU and PYR was dissolved in methanol and a known weight of soil was added slowly, with continuous mixing. This slurry was mixed thoroughly and the solvent was allowed to evaporate slowly. The dry contaminated soil was transferred into a bottle and tumbled for about a week before the experiments. The ultimate concentration of PHE, FLU and PYR in the contaminated soil was 200 mg kg1 for each. Fifty grams of contaminated soils were added to 1 L Erlenmeyer flasks which were then filled with 500 mL of surfactant solution at 5 g L1. The flasks were equilibrated on a reciprocating shaker at 160 rpm and 25 ± 1 °C for 48 h. Then the soil suspensions were centrifuged at 5000 rpm for 30 min. The supernatant was used for a subsequent selective adsorption step by adding biochars. 2.4. Selective adsorption of PAHs by biochars The selective adsorption experiments were conducted with soil washing effluents containing PHE, FLU, PYR and TX100. Biochars at a concentration of 0, 1, 2, 4, 6 and 8 g L1 were added into 100 mL Erlenmeyer flasks and then filled with 50 mL of soil washing effluents. These samples were then equilibrated on a reciprocating shaker for 48 h at 25 ± 1 °C and subsequently centrifuged at 5000 rpm for 30 min. An appropriate aliquot of the supernatant was removed and analyzed for PHE, FLU, PYR and TX100 by HPLC. The sorption amounts of PHE, FLU, PYR and TX100 on biochar were computed simply from the difference of the initial and final concentrations. PHE, FLU, PYR and TX100 were analyzed by a DIONEX U3000 high-performance liquid chromatography (Dionex, USA) using an ultraviolet detector at 254 nm for PHE, FLA, PYR and 230 nm for TX100. The analytical column was a reversed-phase SUPELCOSIL LC-PAH column (150 4.6 mm, 5 lm). The mobile phase (70% acetonitrile and 30% de-ionized water) was eluted at a flow rate of 1.2 ml min1. Approximately 1.5 ml of liquid sample was withdrawn with a disposable glass pipette and filtered by a pre-condi-
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tioned 0.22 lm PTFE filter (Tianjin Jinteng Experiment Co., Ltd., China). 2.5. Single-point partition coefficient and selectivity Single-point adsorption coefficient Kd can be used to compare the affinity of biochars for PAHs.
K d ¼ C s =C e
ð1Þ
where Cs and Ce are the concentrations of adsorbates sorbed to biochars and the aqueous concentrations after adsorption, respectively. The efficiency of the selective adsorption process can be evaluated by selectivity (S) as that described by Ahn et al. (2008) and Wan et al. (2011)
S¼
K Pd
ð2Þ
K Sd
where K Pd and K Sd are the single-point adsorption coefficients of PAH and TX100 between biochars and solutions, defined by Eq. (1).
quantity and size distribution of pores for different biochars. Abundant micropores occurred on the cross section and longitudinal section of the biochar produced at higher temperature. The biochar surface properties as affected by the pyrolytic temperature are further illustrated by the FTIR data (Fig. S2). With an increase of charring temperature, the carbonized components lead to the increasing upward drift in baseline at high wave number which is assigned to low-energy electron excitations of condensed aromatic structures (Sharma et al., 2004). The spectra of biochar samples are characterized by four principal bands at wave numbers of 1105, 1385, 1620, and 3430 cm1. Specifically, the band at 1105 cm1 is assigned to the aliphatic ether (CAO) stretching (Chun et al., 2004b). The band at 1385 cm1 is attributed to the bending stretching of –CH3. The band at 1620 cm1 represents contribution from aromatic C@O ring stretching related to both acidic and basic groups (Chun et al., 2004a). The band at 3430 cm1 is assigned to phenolic hydroxyl stretching (Qiu and Ling, 2006). With increasing pyrolytic temperature, the band intensities gradually decreased as a result of the acceleration of dehydration reaction (Chen et al., 2012).
3. Results and discussion 3.2. Removal efficiency of PAHs and surfactant loss 3.1. Biochar characteristics Elemental composition and atomic ratio of biochars are listed in Table 1. Biochar characteristics are highly pyrolytic temperaturedependent. With an increase of temperature, the C content increased while the O, H and S contents decreased, and the N content changed slightly. The increase of C content stems from progressive loss of O-containing functional groups by dehydration and decarboxylation reactions (Wang et al., 2013). Losses in H and O content at high pyrolytic temperature were attributed to the cleavage and cracking of weak bonds within the biochar structure (Kim et al., 2012). The atomic ratios can be used to explain the degree of aromaticity (H/C) and polarity (O/C and (O + N)/C) of the biochar samples (Zhao et al., 2013). H/C, O/C and (O + N)/C ratios decreased with increasing pyrolytic temperature, indicating that high-temperature biochars contained less amount of original organic residues and exhibits lower polarity and higher aromaticity. Table 2 presents the surface and pore structure characterization of the biochars. The biochar surface area and total pore volume increase whereas the pore width decreases with increasing charring temperature. This is mainly due to the condensation of amorphous carbon to crystalline carbon. The number of micropores increases with the removal of aliphatic and volatile components, increasing the pore volume and surface area (Meng et al., 2013). The high surface areas of biochars suggest that they possess some fine-pore structures. This is furthered validated by their SEM images (Fig. S1). Biochars produced at different temperature all presented honey comb porous structure due to the tubular structures originally formed from plant cells (Wang et al., 2013). Similar observations are reported for corn and rice straw biochars (Silber et al., 2010). In general, micropores in biochars are the residual cellular structure after pyrolysis of plant tissues, so the pore morphology is very similar. However, there are some differences in the
The concentrations of PHE, FLU, PYR and TX100 in the soil washing effluents are 9.07 ± 0.08 mg L1, 10.05 ± 0.04 mg L1, 10.57 ± 0.07 mg L1, and 4.55 ± 0.12 g L1, respectively. It was observed that PAH removal from the spiked soil follows an order of PYR > FLU > PHE. This is because that the more hydrophobic compound (Table S1) is favored during competitive solubilization from the PAH mixtures (Prak and Pritchard, 2002). Fig. 1 presents the removal efficiencies of PAHs by biochars and TX100 loss. Biochars at concentrations higher than 2 (BC800) or 6 g L1 (BC400 and BC600) can significantly remove PAHs (95.8–98.6% for BC800, 71.8–88.1% for BC400, and 82.4–93.4% for BC600) from the soil washing effluents while recover more than 87% of TX100. This is attributed to the difference in physicochemical properties of PAHs and TX100. The interaction mechanisms between organic pollutants and biochars include: (a) electrostatic attraction or repulsion, (b) hydrogen bonding, (c) hydrophobic and p–p interaction and (d) micropore filling. The monomer and the micelle size of TX100 are 2.7 nm (Robson and Dennis, 1977) and 11.6 nm (Levitz et al., 1984), respectively. It is observed that the average pore widths and the micropore widths of the biochars are 3.89–4.92 nm and 0.539–0.593 nm (Table 2). This means that the biochar micropores are not accessible for TX100. Molecular dimensions of PHE, FLU and PYR were small enough to enter the biochar micropores (Table S1). Moreover, PAHs have more benzene rings than TX100, indicating a stronger p–p interaction. Therefore, the biochars would be used as an alternative adsorbent for selective adsorption of PAHs from surfactant solution and the recovery of TX100. It is shown that the removal efficiencies of PAHs and TX100 loss increase with increasing biochar dose. For example, the removal of PHE, FLU, PYR and loss of TX100 by BC600 between 1 and 8 g L1 are 16.1–97.4%, 6.74–93.7%, 4.12–91.5% and 5.54–14.1%, respectively. At a specific biochar dose, the removal of PAHs and TX100 loss increase with increasing pyrolytic temperature. This may be
Table 1 Elemental composition and atomic ratios of biochars. Sample
BC400 BC600 BC800
Elemental composition
Atomic ratios
Ash (%)
C%
H%
N%
S%
O%
(N + O)/C
O/C
H/C
58.51 67.72 80.08
3.23 2.41 1.71
0.99 0.94 1.13
0.88 0.52 0.44
35.84 26.19 13.23
0.47 0.30 0.14
0.46 0.29 0.12
0.66 0.43 0.26
2.07 4.63 5.12
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Table 2 Surface and pore structure characterization of biochars. Sample
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore width (nm)
Micropore volume (cm3/g)
Micropore width (nm)
BC400 BC600 BC800
427 537 652
0.526 0.574 0.634
4.92 4.27 3.89
0.063 0.075 0.082
0.593 0.541 0.539
100
Removal of FLU (%)
Removal of PHE (%)
100 80 60
BC400 BC600 BC800
40 20
80 60
BC400 BC600 BC800
40 20 0
0 0
2
4
6
8
0
10
2
40
Loss of TX100 (%)
Removal of PYR (%)
100 80
BC400 BC600 BC800
40 20
6
8
10
8
10
Biochar dose (g L )
Biochar dose (g L )
60
4
-1
-1
0
BC400 BC600 BC800
30 20 10 0
0
2
4
6
8
10
0
2
Biochar dose (g L-1)
4
6
Biochar dose (g L-1)
Fig. 1. Removal efficiencies of PAHs by biochars and TX100 loss.
2000 1500
(b) 5000
PHE FLU PYR TX100
1000 500
2
4
6
8
10
-1
Kd (L kg-1)
3000 2000
0
2
4
6
8 -1
Biochar dose (g L )
30000
4000
0 0
40000
PHE FLU PYR TX100
1000
0
(c)
removal efficiencies of FLU by BC400, BC600 and BC800 at 2 g L1 are 24.4%, 30.3% and 98.5%, respectively. Although BC600 has significant higher surface area than BC400 (Table 2), they are compar-
Kd (L kg-1)
(a) Kd (L kg-1)
attributed to the increased aromaticity and hydrophobicity of the biochars which enhanced p–p and hydrophobic interactions between contaminants and biochar surface. For example, the
Biochar dose (g L )
PHE FLU PYR TX100
20000 10000 0 0
2
4
6
8
10
Biochar dose (g L-1) Fig. 2. Single-point adsorption coefficients of PAHs and TX100 by (a) BC400, (b) BC600 and (c) BC800.
10
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H. Li et al. / Bioresource Technology 163 (2014) 193–198 Table 3 Selectivity and surfactant recovery. Sample
Biochar dose (g L1)
Selectivity
BC400
1 2 4 6 8
4.8 ± 0.5 19.8 ± 0.5 36.0 ± 3.9 98.0 ± 0.1 151.6 ± 2.9
1.2 ± 0.2 9.7 ± 0.2 18.5 ± 3.0 46.5 ± 2.6 70.3 ± 5.6
0.8 ± 0.1 7.0 ± 0.2 14.0 ± 2.2 33.8 ± 1.9 51.4 ± 4.8
96.5 ± 0.3 96.8 ± 0.4 92.4 ± 2.8 93.0 ± 0.8 91.0 ± 1.5
BC600
1 2 4 6 8
24.1 ± 3.3 45.3 ± 3.5 81.9 ± 1.0 204.5 ± 36.9 355.6 ± 61.4
15.4 ± 1.7 22.1 ± 1.6 39.5 ± 0.1 87.6 ± 10.0 140.2 ± 2.6
7.9 ± 1.5 16.6 ± 1.0 30.3 ± 0.2 67.4 ± 7.5 102.8 ± 5.1
99.4 ± 0.6 98.1 ± 0.1 95.5 ± 0.4 93.5 ± 0.5 90.4 ± 0.7
BC800
1 2 4 6 8
27.7 ± 0.6 155.3 ± 8.7 – – –
76.8 ± 0.7 431.3 ± 19.4 – – –
82.1 ± 1.1 463.7 ± 6.9 – – –
91.3 ± 0.2 87.0 ± 0.2 79.4 ± 0.01 71.7 ± 0.3 64.7 ± 1.4
PHE
Surfactant recovery (%) FLU
ative in PAH adsorption (Fig. 1), suggesting that BET surface area alone might not govern the adsorption of PAHs. Other structural or chemical properties of adsorbents may also contribute to the sorption of PAHs (Park et al., 2013). Among the three biochars, BC800 had the best ability for PAH adsorption, with removal efficiencies of more than 95% at a concentration of 2 g L1. This should be ascribed to its high specific area (Table 2), high surface hydrophobicity (Table 1) and fine-pore structures (Fig. S1). It is interesting to note that PAH removal follows the order of PHE > FLU > PYR for BC400 and BC600, while the order was reversed with PYR > FLU > PHE for BC800. It is demonstrated that pore filling was not the dominant adsorption mechanism for BC800, while hydrophobic and p–p interactions may account for the highest removal of PYR. When BC800 was added in the soil washing effluents at concentrations higher than 2 g L1, no PAH was detected. However, more TX100 was adsorbed on the biochar under these circumstances, which is detrimental to the recovery of surfactant. The loss of TX100 by BC800 at 8 g L1 reached 35.2%. While in the case of BC400 and BC600, less than 10% of TX100 was absorbed on the biochars. It is suggested that biochar dose is an important parameter in the selective adsorption process for recovery of TX100. An appropriate dose should not only effectively remove the pollutants but also guarantee the recovery of surfactant.
PYR
used as an alternative for selective adsorption of PAHs and to recover surfactant in soil washing process. 4. Conclusions PAH removals and TX100 recovery from soil washing effluents were affected by pyrolytic temperature, biochar dose and PAH properties. Among the three biochars, BC800 had the best ability for PAH adsorption. It is demonstrated that biochars can remove 71.8–98.6% of PAHs while recover more than 87% of the TX100, which is comparable to the performance of activated carbons. This result has important implications in soil remediation; the cost for treating soil washing effluents along with surfactant recovery will be greatly reduced with appropriate biochars as a substitute for activated carbon. Acknowledgements Financial supports are from the National Natural Science Foundation of China (21207049, 21107031, 41301567), Shandong Provincial Higher Educational Science and Technology Program (J12LC02), China Scholarship Council, International Cooperation and Training Program for Outstanding Youth Teachers at Colleges and Universities in Shandong Province, and USDA ARFI (MAS #: 000982).
3.3. Biochar sorption affinity and selectivity Fig. 2 and Table 3 are the single-point adsorption coefficients and selectivity results. The sorption affinity is pollutant and biochar property dependent. The sorption affinity of the biochars follows an order of BC800 > BC600 > BC400. In terms of different adsorbates, the sorption affinity of BC400 and BC600 for PAHs and TX100 follows the order of PHE > FLU > PYR TX100. In the case of BC800, the order is PYR FLU > PHE TX100. At the range studied in this research, the Kd increased with increasing biochar dose. The adsorption coefficients of PAHs were much higher than TX100. Except the case of BC400 at 1 g L1 for FLU and PYR, the selectivity for PAHs over TX100 was much higher than 1 over a wide range of biochar doses (Table 3). The performance of BC800 at 1 g L1 was comparable or superior to that of activated carbon. For example, Ahn et al. (2007) reported selectivity values of 38.3–74.9 for PHE over TX100, and there was only one PAH (PHE, 10 mg L1) used in their study. When the initial concentration of PHE increased to 30 mg L1 or 100 mg L1, they got selectivity values of 21.5–55.3 and 6.0–26.5, respectively. Thus, biochar can be
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Selective removal of polycyclic aromatic hydrocarbons (PAHs) from soil washing effluents using biochars produced at different pyrolytic temperatures Helian Li a,b,⇑, Ronghui Qu a, Chao Li a, Weilin Guo a, Xuemei Han a, Fang He a, Yibing Ma a,c, Baoshan Xing b a
School of Resources and Environment, University of Jinan, Jinan 250022, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA National Soil Fertility and Fertilizer Effects Long-term Monitoring Network, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China b c
h i g h l i g h t s Pyrolytic temperature influenced wheat straw-derived biochar properties. Biochars selectively removed PAHs from effluents and recovered Triton X-100. PAH removal and Triton X-100 loss increased with increasing pyrolytic temperature. Selective adsorption was PAH property dependent.
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 10 April 2014 Accepted 11 April 2014 Available online 19 April 2014 Keywords: Biochar Selective adsorption PAHs Soil washing effluents Surfactant recovery
a b s t r a c t Wheat straw biochars produced at 400, 600 and 800 °C (BC400, BC600 and BC800) were used to selectively adsorb PAHs from soil washing effluents. For soil washing effluents contained Phenanthrene (PHE), Fluoranthene (FLU), Pyrene (PYR) and Triton X-100 (TX100), biochars at 2 (for BC800) or 6 g L1 (for BC400 and BC600) can remove 71.8–98.6% of PAHs while recover more than 87% of TX100. PAH removals increase with increasing biochar dose. However, excess biochar is detrimental to the recovery of surfactant. For a specific biochar dose, PAH removal and TX100 loss increase with increasing pyrolytic temperature. For BC400 and BC600, PAH removal follows the order of PHE > FLU > PYR, while the order is reversed with PYR > FLU > PHE for BC800. Biochars have much higher sorption affinity for PAHs than for TX100. It is therefore suggested that biochar is a good alternative for selective adsorption of PAHs and recovery of TX100 in soil washing process. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants that are ubiquitous in the environment. Coal processing is one of the most important sources of PAHs. Coke oven plants, manufactured gas plants and areas of coal tar spillage are of high contamination of PAHs (Paria and Yuet, 2006; Viglianti et al., 2006). In developing countries, with rapid urbanization and subsequent industry transfer, many PAH contaminated sites were left to be remediated. Due to their known or potential genotoxicity and carcinogenicity, remediation of these PAH-contaminated sites is of particular environmental concern (Woo et al., 2001; Ahn et al., 2008). ⇑ Corresponding author at: School of Resources and Environment, University of Jinan, Jinan 250022, China. Tel.: +86 0531 82769233. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.biortech.2014.04.042 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Soil washing has been proven as an effective method to remove PAHs from contaminated soils. Surfactant is the most widely used detergent in soil washing process, which can significantly remove the PAHs through micellar solubilization at levels higher than the critical micelle concentration. Although the addition of surfactants can greatly improve the removal efficiencies of PAHs, the surfactant cost may account for 50% of the total operating expenses (Mann and Peterson, 1993). Therefore, if we can take an effective method to remove PAHs from the washing wastewater and recycle the surfactant solution, the cost of soil washing remediation could be greatly reduced. Many techniques have been used to treat soil washing effluents, such as photocatalysis (Fabbri et al., 2009), electrochemical treatment (Gómez et al., 2010), advanced oxidation process (Bandala et al., 2008) and selective adsorption by activated carbons (Ahn et al., 2008; Wan et al., 2011). Compared with other aforementioned
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treatment processes, activated carbon has advantage in recovering surfactant from the soil washing effluents. Using batch experiments or fixed column process, researchers (Ahn et al., 2007; Liu et al., 2013) demonstrated that activated carbon can remove most of the PAHs and recover more than 90% surfactants from the soil washing effluents. Biochar is a C rich product produced during the pyrolysis of common biomass such as wood, animal wastes, crop residues, municipal waste and biosolids (Lehmann and Joseph, 2009). Biochars are characterized by high affinity for organic contaminants (Sun et al., 2011), thus these materials may potentially be an alternative for activated carbon. Since biochar is a greener and more cost effective material compared with activated carbon (Denyes et al., 2012), its use in environmental remediation would be more preferable. There is a huge amount of literature on the adsorption of PAHs by biochar from water or soil. Previous research has revealed that sorption of aromatic compounds by biochar was driven by not only p–p interactions but also their affinity for chars as affected by hydrophobicity, pore distribution, and surface chemistry (Wang et al., 2006). To our knowledge, it still remains unclear how these factors would influence the removal of PAHs in the presence of surfactant. In this study, biochars produced at different pyrolytic temperatures were used to selectively adsorb PHE, FLU and PYR from soil washing effluents. PHE, FLU and PYR were chosen to represent PAHs with different hydrophobicity and molecular size. Moreover, they are usually present at high concentrations in soil washing effluents. TX100 was employed because it is widely used to clean up organic pollutants like PAHs from soils. The aim of this paper was to investigate (1) the applicability of biochars in the selective removal of PAHs from TX100 solutions, and (2) the effect of pyrolysis temperature for making biochar and PAH properties on the selective adsorption performance. Since biochar is similar to activated carbon in adsorption of PAHs, we can hypothesize that biochar can be used as an alternative for surfactant recovery in soil washing process. Our second hypothesis is that high-temperature biochar would be more efficient in selectively remove PAHs from surfactant solutions.
2. Methods 2.1. Chemicals PHE, FLU and PYR with purities of over 98% were purchased from Aldrich (USA). TX-100 was obtained from Amresco (USA), with a purity of 99%. Selected physicochemical properties of the compounds are presented in Table S1. Methanol and acetonitrile were HPLC grade purchased from J.T. Baker (Phillipsburg, NJ, USA). Deionized water was produced by a Milli-Q system (Millipore Co., USA).
2.2. Biochar sample preparation and characterization Biochars BC400, BC600 and BC800 were produced in a muffle furnace (SX2-14-12, Longkou City Factory Furnace, China). Wheat straw powder was placed in a ceramic pot, covered with a fitting lid, and pyrolyzed under the oxygen-limited conditions at 400, 600 and 800 °C for 6 h (Chun et al., 2004a). The samples were treated in 200 mL of 1 M HCl solution for 12 h to remove minerals. Then, they were thoroughly washed with distilled water till neutral pH. Finally, the biochar samples were oven-dried overnight at 80 °C, gently ground to pass through a 100-mesh sieve, and stored for subsequent use.
Elemental (C, N, H and S) analyses were conducted using a Vario EI III elemental analyzer (Elementar, Germany), and oxygen content was determined by difference as follows: O (%) = 100 (C + H + N + S + Ash) (Pereira et al., 2011). Ash content of biochar samples were determined by dry combustion at 760 °C for 6 h (Novak et al., 2009). The surface area and pore size of the biochars were determined by N2 adsorption at 77 K using an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics, USA). The BET (Brunauer–Emmett–Teller) surface areas were calculated from the linear fit of the N2 adsorption data by applying the BET equation in the relative pressure (P/P0) range between 0.05 and 0.35, and taking the average area occupied by a molecule of N2 in the complete monolayer to be equal to 0.1620 nm2. The micropore volumes were obtained by the t-plot method, and the total pore volumes were estimated from the adsorbed amount of N2 at a relative pressure of 0.99. Scanning electron microscope (SEM) analysis was carried out using a FEI QUANTA FEG250 scanning electron microscope (FEI, USA). For FTIR measurements, 1 mg of the slightly ground biochar was gently mixed with 200 mg of oven-dried (at 105–110 °C) KBr and then the mixture was pressed into a pellet. FTIR spectra were obtained using a Nicolet 380 FTIR spectrometer (Thermo Electron, USA) with a resolution of 2 cm1 between wave numbers of 800 and 4000 cm1. 2.3. Preparation of soil washing effluents An uncontaminated soil was collected from the suburb of Jinan city, China. The soil was air dried and passed through a 2 mm sieve. The soil contained 53.2% sand, 24.4% silt and 22.4% clay. The soil had a pH of 6.8, a total organic content of 0.8%, and a cation exchange capacity of 6.8 cmol kg1. The contaminated soil was prepared as described by Zhou et al. (2007). In brief, an appropriate quantity of PHE, FLU and PYR was dissolved in methanol and a known weight of soil was added slowly, with continuous mixing. This slurry was mixed thoroughly and the solvent was allowed to evaporate slowly. The dry contaminated soil was transferred into a bottle and tumbled for about a week before the experiments. The ultimate concentration of PHE, FLU and PYR in the contaminated soil was 200 mg kg1 for each. Fifty grams of contaminated soils were added to 1 L Erlenmeyer flasks which were then filled with 500 mL of surfactant solution at 5 g L1. The flasks were equilibrated on a reciprocating shaker at 160 rpm and 25 ± 1 °C for 48 h. Then the soil suspensions were centrifuged at 5000 rpm for 30 min. The supernatant was used for a subsequent selective adsorption step by adding biochars. 2.4. Selective adsorption of PAHs by biochars The selective adsorption experiments were conducted with soil washing effluents containing PHE, FLU, PYR and TX100. Biochars at a concentration of 0, 1, 2, 4, 6 and 8 g L1 were added into 100 mL Erlenmeyer flasks and then filled with 50 mL of soil washing effluents. These samples were then equilibrated on a reciprocating shaker for 48 h at 25 ± 1 °C and subsequently centrifuged at 5000 rpm for 30 min. An appropriate aliquot of the supernatant was removed and analyzed for PHE, FLU, PYR and TX100 by HPLC. The sorption amounts of PHE, FLU, PYR and TX100 on biochar were computed simply from the difference of the initial and final concentrations. PHE, FLU, PYR and TX100 were analyzed by a DIONEX U3000 high-performance liquid chromatography (Dionex, USA) using an ultraviolet detector at 254 nm for PHE, FLA, PYR and 230 nm for TX100. The analytical column was a reversed-phase SUPELCOSIL LC-PAH column (150 4.6 mm, 5 lm). The mobile phase (70% acetonitrile and 30% de-ionized water) was eluted at a flow rate of 1.2 ml min1. Approximately 1.5 ml of liquid sample was withdrawn with a disposable glass pipette and filtered by a pre-condi-
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tioned 0.22 lm PTFE filter (Tianjin Jinteng Experiment Co., Ltd., China). 2.5. Single-point partition coefficient and selectivity Single-point adsorption coefficient Kd can be used to compare the affinity of biochars for PAHs.
K d ¼ C s =C e
ð1Þ
where Cs and Ce are the concentrations of adsorbates sorbed to biochars and the aqueous concentrations after adsorption, respectively. The efficiency of the selective adsorption process can be evaluated by selectivity (S) as that described by Ahn et al. (2008) and Wan et al. (2011)
S¼
K Pd
ð2Þ
K Sd
where K Pd and K Sd are the single-point adsorption coefficients of PAH and TX100 between biochars and solutions, defined by Eq. (1).
quantity and size distribution of pores for different biochars. Abundant micropores occurred on the cross section and longitudinal section of the biochar produced at higher temperature. The biochar surface properties as affected by the pyrolytic temperature are further illustrated by the FTIR data (Fig. S2). With an increase of charring temperature, the carbonized components lead to the increasing upward drift in baseline at high wave number which is assigned to low-energy electron excitations of condensed aromatic structures (Sharma et al., 2004). The spectra of biochar samples are characterized by four principal bands at wave numbers of 1105, 1385, 1620, and 3430 cm1. Specifically, the band at 1105 cm1 is assigned to the aliphatic ether (CAO) stretching (Chun et al., 2004b). The band at 1385 cm1 is attributed to the bending stretching of –CH3. The band at 1620 cm1 represents contribution from aromatic C@O ring stretching related to both acidic and basic groups (Chun et al., 2004a). The band at 3430 cm1 is assigned to phenolic hydroxyl stretching (Qiu and Ling, 2006). With increasing pyrolytic temperature, the band intensities gradually decreased as a result of the acceleration of dehydration reaction (Chen et al., 2012).
3. Results and discussion 3.2. Removal efficiency of PAHs and surfactant loss 3.1. Biochar characteristics Elemental composition and atomic ratio of biochars are listed in Table 1. Biochar characteristics are highly pyrolytic temperaturedependent. With an increase of temperature, the C content increased while the O, H and S contents decreased, and the N content changed slightly. The increase of C content stems from progressive loss of O-containing functional groups by dehydration and decarboxylation reactions (Wang et al., 2013). Losses in H and O content at high pyrolytic temperature were attributed to the cleavage and cracking of weak bonds within the biochar structure (Kim et al., 2012). The atomic ratios can be used to explain the degree of aromaticity (H/C) and polarity (O/C and (O + N)/C) of the biochar samples (Zhao et al., 2013). H/C, O/C and (O + N)/C ratios decreased with increasing pyrolytic temperature, indicating that high-temperature biochars contained less amount of original organic residues and exhibits lower polarity and higher aromaticity. Table 2 presents the surface and pore structure characterization of the biochars. The biochar surface area and total pore volume increase whereas the pore width decreases with increasing charring temperature. This is mainly due to the condensation of amorphous carbon to crystalline carbon. The number of micropores increases with the removal of aliphatic and volatile components, increasing the pore volume and surface area (Meng et al., 2013). The high surface areas of biochars suggest that they possess some fine-pore structures. This is furthered validated by their SEM images (Fig. S1). Biochars produced at different temperature all presented honey comb porous structure due to the tubular structures originally formed from plant cells (Wang et al., 2013). Similar observations are reported for corn and rice straw biochars (Silber et al., 2010). In general, micropores in biochars are the residual cellular structure after pyrolysis of plant tissues, so the pore morphology is very similar. However, there are some differences in the
The concentrations of PHE, FLU, PYR and TX100 in the soil washing effluents are 9.07 ± 0.08 mg L1, 10.05 ± 0.04 mg L1, 10.57 ± 0.07 mg L1, and 4.55 ± 0.12 g L1, respectively. It was observed that PAH removal from the spiked soil follows an order of PYR > FLU > PHE. This is because that the more hydrophobic compound (Table S1) is favored during competitive solubilization from the PAH mixtures (Prak and Pritchard, 2002). Fig. 1 presents the removal efficiencies of PAHs by biochars and TX100 loss. Biochars at concentrations higher than 2 (BC800) or 6 g L1 (BC400 and BC600) can significantly remove PAHs (95.8–98.6% for BC800, 71.8–88.1% for BC400, and 82.4–93.4% for BC600) from the soil washing effluents while recover more than 87% of TX100. This is attributed to the difference in physicochemical properties of PAHs and TX100. The interaction mechanisms between organic pollutants and biochars include: (a) electrostatic attraction or repulsion, (b) hydrogen bonding, (c) hydrophobic and p–p interaction and (d) micropore filling. The monomer and the micelle size of TX100 are 2.7 nm (Robson and Dennis, 1977) and 11.6 nm (Levitz et al., 1984), respectively. It is observed that the average pore widths and the micropore widths of the biochars are 3.89–4.92 nm and 0.539–0.593 nm (Table 2). This means that the biochar micropores are not accessible for TX100. Molecular dimensions of PHE, FLU and PYR were small enough to enter the biochar micropores (Table S1). Moreover, PAHs have more benzene rings than TX100, indicating a stronger p–p interaction. Therefore, the biochars would be used as an alternative adsorbent for selective adsorption of PAHs from surfactant solution and the recovery of TX100. It is shown that the removal efficiencies of PAHs and TX100 loss increase with increasing biochar dose. For example, the removal of PHE, FLU, PYR and loss of TX100 by BC600 between 1 and 8 g L1 are 16.1–97.4%, 6.74–93.7%, 4.12–91.5% and 5.54–14.1%, respectively. At a specific biochar dose, the removal of PAHs and TX100 loss increase with increasing pyrolytic temperature. This may be
Table 1 Elemental composition and atomic ratios of biochars. Sample
BC400 BC600 BC800
Elemental composition
Atomic ratios
Ash (%)
C%
H%
N%
S%
O%
(N + O)/C
O/C
H/C
58.51 67.72 80.08
3.23 2.41 1.71
0.99 0.94 1.13
0.88 0.52 0.44
35.84 26.19 13.23
0.47 0.30 0.14
0.46 0.29 0.12
0.66 0.43 0.26
2.07 4.63 5.12
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Table 2 Surface and pore structure characterization of biochars. Sample
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore width (nm)
Micropore volume (cm3/g)
Micropore width (nm)
BC400 BC600 BC800
427 537 652
0.526 0.574 0.634
4.92 4.27 3.89
0.063 0.075 0.082
0.593 0.541 0.539
100
Removal of FLU (%)
Removal of PHE (%)
100 80 60
BC400 BC600 BC800
40 20
80 60
BC400 BC600 BC800
40 20 0
0 0
2
4
6
8
0
10
2
40
Loss of TX100 (%)
Removal of PYR (%)
100 80
BC400 BC600 BC800
40 20
6
8
10
8
10
Biochar dose (g L )
Biochar dose (g L )
60
4
-1
-1
0
BC400 BC600 BC800
30 20 10 0
0
2
4
6
8
10
0
2
Biochar dose (g L-1)
4
6
Biochar dose (g L-1)
Fig. 1. Removal efficiencies of PAHs by biochars and TX100 loss.
2000 1500
(b) 5000
PHE FLU PYR TX100
1000 500
2
4
6
8
10
-1
Kd (L kg-1)
3000 2000
0
2
4
6
8 -1
Biochar dose (g L )
30000
4000
0 0
40000
PHE FLU PYR TX100
1000
0
(c)
removal efficiencies of FLU by BC400, BC600 and BC800 at 2 g L1 are 24.4%, 30.3% and 98.5%, respectively. Although BC600 has significant higher surface area than BC400 (Table 2), they are compar-
Kd (L kg-1)
(a) Kd (L kg-1)
attributed to the increased aromaticity and hydrophobicity of the biochars which enhanced p–p and hydrophobic interactions between contaminants and biochar surface. For example, the
Biochar dose (g L )
PHE FLU PYR TX100
20000 10000 0 0
2
4
6
8
10
Biochar dose (g L-1) Fig. 2. Single-point adsorption coefficients of PAHs and TX100 by (a) BC400, (b) BC600 and (c) BC800.
10
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H. Li et al. / Bioresource Technology 163 (2014) 193–198 Table 3 Selectivity and surfactant recovery. Sample
Biochar dose (g L1)
Selectivity
BC400
1 2 4 6 8
4.8 ± 0.5 19.8 ± 0.5 36.0 ± 3.9 98.0 ± 0.1 151.6 ± 2.9
1.2 ± 0.2 9.7 ± 0.2 18.5 ± 3.0 46.5 ± 2.6 70.3 ± 5.6
0.8 ± 0.1 7.0 ± 0.2 14.0 ± 2.2 33.8 ± 1.9 51.4 ± 4.8
96.5 ± 0.3 96.8 ± 0.4 92.4 ± 2.8 93.0 ± 0.8 91.0 ± 1.5
BC600
1 2 4 6 8
24.1 ± 3.3 45.3 ± 3.5 81.9 ± 1.0 204.5 ± 36.9 355.6 ± 61.4
15.4 ± 1.7 22.1 ± 1.6 39.5 ± 0.1 87.6 ± 10.0 140.2 ± 2.6
7.9 ± 1.5 16.6 ± 1.0 30.3 ± 0.2 67.4 ± 7.5 102.8 ± 5.1
99.4 ± 0.6 98.1 ± 0.1 95.5 ± 0.4 93.5 ± 0.5 90.4 ± 0.7
BC800
1 2 4 6 8
27.7 ± 0.6 155.3 ± 8.7 – – –
76.8 ± 0.7 431.3 ± 19.4 – – –
82.1 ± 1.1 463.7 ± 6.9 – – –
91.3 ± 0.2 87.0 ± 0.2 79.4 ± 0.01 71.7 ± 0.3 64.7 ± 1.4
PHE
Surfactant recovery (%) FLU
ative in PAH adsorption (Fig. 1), suggesting that BET surface area alone might not govern the adsorption of PAHs. Other structural or chemical properties of adsorbents may also contribute to the sorption of PAHs (Park et al., 2013). Among the three biochars, BC800 had the best ability for PAH adsorption, with removal efficiencies of more than 95% at a concentration of 2 g L1. This should be ascribed to its high specific area (Table 2), high surface hydrophobicity (Table 1) and fine-pore structures (Fig. S1). It is interesting to note that PAH removal follows the order of PHE > FLU > PYR for BC400 and BC600, while the order was reversed with PYR > FLU > PHE for BC800. It is demonstrated that pore filling was not the dominant adsorption mechanism for BC800, while hydrophobic and p–p interactions may account for the highest removal of PYR. When BC800 was added in the soil washing effluents at concentrations higher than 2 g L1, no PAH was detected. However, more TX100 was adsorbed on the biochar under these circumstances, which is detrimental to the recovery of surfactant. The loss of TX100 by BC800 at 8 g L1 reached 35.2%. While in the case of BC400 and BC600, less than 10% of TX100 was absorbed on the biochars. It is suggested that biochar dose is an important parameter in the selective adsorption process for recovery of TX100. An appropriate dose should not only effectively remove the pollutants but also guarantee the recovery of surfactant.
PYR
used as an alternative for selective adsorption of PAHs and to recover surfactant in soil washing process. 4. Conclusions PAH removals and TX100 recovery from soil washing effluents were affected by pyrolytic temperature, biochar dose and PAH properties. Among the three biochars, BC800 had the best ability for PAH adsorption. It is demonstrated that biochars can remove 71.8–98.6% of PAHs while recover more than 87% of the TX100, which is comparable to the performance of activated carbons. This result has important implications in soil remediation; the cost for treating soil washing effluents along with surfactant recovery will be greatly reduced with appropriate biochars as a substitute for activated carbon. Acknowledgements Financial supports are from the National Natural Science Foundation of China (21207049, 21107031, 41301567), Shandong Provincial Higher Educational Science and Technology Program (J12LC02), China Scholarship Council, International Cooperation and Training Program for Outstanding Youth Teachers at Colleges and Universities in Shandong Province, and USDA ARFI (MAS #: 000982).
3.3. Biochar sorption affinity and selectivity Fig. 2 and Table 3 are the single-point adsorption coefficients and selectivity results. The sorption affinity is pollutant and biochar property dependent. The sorption affinity of the biochars follows an order of BC800 > BC600 > BC400. In terms of different adsorbates, the sorption affinity of BC400 and BC600 for PAHs and TX100 follows the order of PHE > FLU > PYR TX100. In the case of BC800, the order is PYR FLU > PHE TX100. At the range studied in this research, the Kd increased with increasing biochar dose. The adsorption coefficients of PAHs were much higher than TX100. Except the case of BC400 at 1 g L1 for FLU and PYR, the selectivity for PAHs over TX100 was much higher than 1 over a wide range of biochar doses (Table 3). The performance of BC800 at 1 g L1 was comparable or superior to that of activated carbon. For example, Ahn et al. (2007) reported selectivity values of 38.3–74.9 for PHE over TX100, and there was only one PAH (PHE, 10 mg L1) used in their study. When the initial concentration of PHE increased to 30 mg L1 or 100 mg L1, they got selectivity values of 21.5–55.3 and 6.0–26.5, respectively. Thus, biochar can be
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