Optimization of Ex-Situ Washing Removal of Polycyclic Aromatic Hydrocarbons from a Contaminated Soil Using Nano-Sulfonated Graphene

Optimization of Ex-Situ Washing Removal of Polycyclic Aromatic Hydrocarbons from a Contaminated Soil Using Nano-Sulfonated Graphene

Pedosphere 27(3): 527–536, 2017 doi:10.1016/S1002-0160(17)60348-5 ISSN 1002-0160/CN 32-1315/P c 2017 Soil Science Society of China ⃝ Published by Else...

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Pedosphere 27(3): 527–536, 2017 doi:10.1016/S1002-0160(17)60348-5 ISSN 1002-0160/CN 32-1315/P c 2017 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press

Optimization of Ex-Situ Washing Removal of Polycyclic Aromatic Hydrocarbons from a Contaminated Soil Using Nano-Sulfonated Graphene GAN Xinhong1,2 , TENG Ying1,2,∗ , REN Wenjie1 , MA Jun3 , Peter CHRISTIE1 and LUO Yongming1,2 1 Key

Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 2 University of Chinese Academy of Sciences, Beijing 100049 (China) 3 College of Materials and Chemistry, Tongren University, Tongren 554300 (China) (Received January 17, 2017; revised April 12, 2017)

ABSTRACT Ex-situ soil washing technology offers advantages such as speed and efficiency of remediation and range of application and has been developed to conform with legal requirements and best management practices in USA and some European countries. In this study, nano-sulfonated graphene (SGE) was used as a washing agent to evaluate different processing (washing) parameters for the ectopic leaching removal of polycyclic aromatic hydrocarbons (PAHs) from a coking plant soil. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) were used to analyze the characteristics of the SGE tested. The results showed that SGE had a strong adsorption capacity for PAHs through the role of π-π, H-π, and anion-π interactions. The washing parameters, an SGE concentration of 2 000 mg L−1 , a liquid/soil (L/S) ratio of 10:1, and 4 cycles of successive washing, were set to arrive to the optimum condition for achieving more than 80% of PAH removal. Under the optimum condition, the PAH removal rate decreased with increasing ring numbers. After one washing cycle at SGE concentration of 2 000 mg L−1 and L/S ratio of 10:1, the PAH removal rate of the SGE tested was much higher than that of Tween 80 (TW80) or methyl-β-cyclodextrin (MCD) (P < 0.01). Therefore, SGE is a promising material for the nanoremediation of PAH-contaminated soils. Key Words:

adsorption capacity, leaching mechanism, nanoremediation, optimum condition, washing agent, washing parameter

Citation: Gan X H, Teng Y, Ren W J, Ma J, Christie P, Luo Y M. 2017. Optimization of ex-situ washing removal of polycyclic aromatic hydrocarbons from a contaminated soil using nano-sulfonated graphene. Pedosphere. 27(3): 527–536.

INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are common persistent organic pollutants in industrially contaminated soils including soils polluted by the coking industry. Polycyclic aromatic hydrocarbons have high thermal stability and hydrophobicity and are therefore difficult to degrade (Feng et al., 2016). These compounds can exist and accumulate in soils for a long time, thereafter posing negative impacts on soil sustainability, soil organisms, and soil function (Maliszewska-Kordybach et al., 2007). Polycyclic aromatic hydrocarbons can also be magnified through food chain and then become potential carcinogens to humans (Wilcke, 2007). In recent years, urbanization has accelerated in China, where many coking plants located in urban areas have been closed down or relocated due to pollution problems. The re-use of the abandoned sites has become increasingly prominent. ∗ Corresponding

author. E-mail: [email protected]

The key to solving these problems is to develop rapid and efficient methods for the remediation of contaminated sites (Ding et al., 2008). Ex-situ soil washing technology offers advantages of remediation speed and efficiency and application range. The technology has been developed in compliance with legal requirements and best management practices in USA and some European countries (Rivero-Huguet and Marshall, 2011). A key aspect of the technology is to discover appropriate washing agents for the soils in specific contaminated sites. Various chemical agents have been studied including surfactants, biosurfactants, microemulsions, natural surfactants, cyclodextrins, vegetable oils, and solutions with solid phase particles (Lau et al., 2014). Peng et al. (2011) selected two nonionic surfactants of Tween80 (TW80) and Triton X-100 (TX100) to investigate different factors influencing the leaching removal rates of soil PAH. They suggested that the average removal

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rates under optimum conditions reached 79% and 83% for TW80 and TX100, respectively. Mousset et al. (2016) selected hydroxypropyl-β-cyclodextrin and TW80 as extracting agents and use electro-Fenton process to elute waste liquid. Six PAHs with different numbers of ring were then determined and the pollutants were completely degraded (> 99%) after 4 or 8 h. Elgh-Dalgren et al. (2009) investigated the leaching efficiency of different surfactants and their mixtures in removing PAHs from a long-term wood preservation site. Their results indicated that a combination of the chelating agent methylglycine diacetic acid and the biodegradable nonionic surfactant alkyl polyglucoside gave a very good removal of PAHs after 10 min of washing at 50 ◦ C. Sun et al. (2013) suggested that successive washing of three cycles using a combined treatment of elevated temperature and ultrasonication at 100 g L−1 methyl-β-cyclodextrin (MCD) was effective in removing PAHs. This study differed from these approaches by adopting a new type of washing nano-agent, nano-sulfonated graphene (SGE), to remove PAHs from a contaminated soil. Graphene is a fascinating two-dimensional carbonbased material possessing atomic thickness and has attracted considerable worldwide attention since its discovery. It has broad application prospects in various fields (Yang et al., 2010), such as energy, electronics, and medicine because of its high specific surface area (about 2 630 m2 g−1 ), high thermal conductivity (3 000 W m−1 K−1 ), high electrical conductivity, and changeable surface chemical properties. Hydroxyl, carboxyl, carbonyl, epoxy, and other chemical groups have been embedded in the graphene surface to transform the material into functional graphene and to expand its range of applications (Ramesha et al., 2011). The sulfonated treatment of graphene can increase its dispersion in water and retain its original excellent properties (Si and Samulski, 2008). Zhao et al. (2011) suggested that highly dispersed SGE sheets could adsorb persistent organic pollutants (POPs) effectively from aqueous solutions. Zhang et al. (2012) utilized SGE sheets as a sorbent in micro-solid-phase extraction for the determination of PAHs in water. Shen and Chen (2015) found that SGE could strongly adsorb PAHs and rapidly remove them from aqueous solutions. To date there have been no reports of removal of contaminants from soils by SGE. However, SGE has some properties that might be useful in a soil washing agent, such as high dispersibility in water, strong adsorption of pollutants, high specific surface area, and no deposition with soil during centrifugation. This leads to the question of whether or not SGE has a strong ability to remove

PAHs from soils with optimization of the washing conditions. Currently, most studies have focused on the potential environmental impact of graphene. Liao et al. (2011) demonstrated that particle size, particulate state, and oxygen content/surface charge of graphene had a strong impact on biological/toxicological responses to human red blood cells. Ren et al. (2015) supposed that adding less than 1 000 mg kg−1 graphene to the red soil had little effect on soil microbial community structure. Ren et al. (2016) suggested that low concentration (50 mg kg−1 ) of SGE could scavenge reactive oxygen species (ROS) in roots and improve maize health, whereas a high concentration of SGE (500 mg kg−1 ) promoted the generation of ROS and led to cell death in roots. In addition, Li (2015) studied the effect of SGE on the growth of maize and Eisenia fetidai and demonstrated that at the SGE concentration of less than 1 000 mg kg−1 , the growth was not affected by SGE. Therefore, the aim of this study was to investigate the adsorption capacity of SGE to elute PAHs from a coking plant soil and the rational use of the material for further development of its application range. MATERIALS AND METHODS Reagents A mixture standard of 16 US Environmental Protection Agency (EPA) priority PAHs (200 mg L−1 ), pyrene (≥ 98%), and hexane (high performance liquid chromatography (HPLC) grade) were purchased from Supelco Co. (Bellefonte, USA), Sigma-Aldrich Co. (St Louis, USA), and Tedia Company (Fairfield, USA), respectively. Nano-sulfonated graphene (powder, purity > 90%) was provided by Graphene-Tech Inc. (Suzhou, China). Acetone, cyclohexane, and dichloromethane (analytical grade) (Tianjin Kermel Chemical Reagent Co. Ltd., Tianjin, China) were distilled prior to use. Anhydrous sodium sulfate (Na2 SO4 ) was heated at 200 ◦ C for 4 h. Silica gel (0.15 mm) was activated at 130 ◦ C for 2 h and deactivated by adding deionized water (3% of the silica gel weight), further homogenized, and equilibrated for 6 h before use. Distilled deionized water was obtained using a Millipore-Q purification system (Millipore Co., Bedford, USA). All other chemical reagents used were of analytical grade. Soil preparation Soil samples were taken from an abandoned coking plant located in Beijing, China. This plant was in operation between the 1960s and 2000s. The samples were homogenized, air-dried for 7 d at room tempera-

WASHING REMOVAL OF PAHs USE SULFONATED GRAPHENE

ture, and then passed through a 0.25-mm sieve. Analysis of physico-chemical properties showed that the soil had a pH of 7.3 (water:soil ratio, 2.5:1) and contained 22 g kg−1 organic matter on a dry weight basis. Total amount of the 16 US EPA priority PAHs in the original soil was 31.99 ± 4.147 mg kg−1 on a dry weight basis and more details of their contents are shown in Table I. TABLE I Contents of 16 priority polycyclic aromatic hydrocarbon (PAH) in the original soil PAH

Abbreviation

Sum of 2(+3)-ring PAHs Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Sum of 4-ring PAHs Fluoranthene Pyrene Benz(a)anthracene Chrysene Sum of 5-ring PAHs Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Sum of 6-ring PAHs Indeno(1,2,3-cd)pyrene Dibenz(a, h)anthracene Benzo(ghi)perylene Total

NAP APY APE FLE PHE ANT FLT PYR BaA CRY BbF BkF BaP I123P DaA BghiP

Content mg kg−1 1.25 ± 0.676a) NFb) 0.14 ± 0.078 NF 0.03 ± 0.017 0.56 ± 0.297 0.50 ± 0.410 9.88 ± 4.095 2.17 ± 1.172 2.26 ± 0.486 3.01 ± 1.438 2.43 ± 1.285 11.16 ± 0.850 3.25 ± 1.637 2.83 ± 0.375 5.09 ± 1.558 9.70 ± 1.523 3.11 ± 1.010 2.92 ± 1.596 3.66 ± 1.424 31.99 ± 4.147

± standard deviation. found.

a) Mean b) Not

Characterization of the SGE tested The structure and surface morphology of SGE were characterized by the Brunauer-Emmett-Teller (BET) -N2 specific surface area (V-Sorb 2800P, Gold APP Instruments Co., Beijing, China) and scanning electron microscopy (SEM) (Quanta FEG 250, FEI, Hillsboro, USA). Analyses of fourier transform infrared spectroscopy (FTIR) (Nicolet 380, Thermo Fisher Scientific, Waltham, USA) and X-ray photoelectron spectroscopy (XPS) (5000 Versa Probe, UlVAC-PHI, Kanagawa, Japan) were carried out to validate the mechanism for the tested SGE adsorbing PAHs. Batch adsorption experiments of pyrene (PYR, as a PAH model compound), which was used to characterize the adsorption capacity of SGE, were conducted as described by Wang et al. (2014). Given that PYR is almost insoluble in water and SGE is difficult to separate from water, polyoxymethylene (POM) (Vink Kunst-

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stoffen BV, Didam, the Netherlands) as a solid-phase extraction (Jonker and Koelmans, 2001) was applied to the adsorption test. In brief, 10 mg SGE was transferred into to a 50-mL conical flask sealed with aluminum foil at 25 ± 1 ◦ C. Background solution (pH 7.0), containing 0.01 mol L−1 KNO3 in deionized water with 200 mg L−1 NaN3 as a biological inhibitor, was used. The 0.10 g POM strips were added, and then a three-phase system (POM-SGE-water) was spiked with 2–200 µL PYR solution (at 1 000 mg L−1 ) in acetone. Each concentration treatment was in triplicate. The total volume of the liquid system was 20 mL. To reach equilibration the conical flasks were placed on a shaker and agitated in the dark at 150 r min−1 for 96 h to ensure complete reaction of the initial oversaturated PYR solution with the adsorbent (Wang et al., 2014). To avoid the effect of acetone on adsorption the volume fraction of the organic phase was < 1% (Ball and Roberts, 1991). After adsorption with high mass of PYR, the SGE was filtered through a 0.1-µm membrane and the material on the filter was freeze-dried for XPS and FTIR analyses. Following these procedures the POM strips were removed from the flasks, cleaned with moist tissues, and soxhlet-extracted with dichloromethane for 4 h. The extracts were cleaned up and analyzed for PYR by HPLC (LC-20AT, Shimadzu Corporation, Kyoto, Japan) equipped with a fluorescence detector (Huang et al., 2013). The mass balance of the three-phase system is given by: Qtot = Cp Mp + qs Ms + Cw Vw

(1)

where Qtot is the total amount of PYR in the threephase system (µg); qs , Cp , and Cw are the equilibrium concentrations of PYR on SGE (µg kg−1 ), on POM (µg kg−1 ) and in water (µg L−1 ), respectively; Ms and Mp are the masses of SGE and POM (kg), respectively; and Vw is the volume of water (L). The relationships for the POM-water distribution coefficient (KPOM ) (Jonke and Koelmans, 2002) is given by: KPOM = Cp /Cw

(2)

and rewriting gives: qs = (Qtot − Cp Mp − Cp Vw /KPOM )/Ms

(3)

Except for the addition of the test SGE, the determination of KPOM for PYR in a two-phase system (POM-water) was consistent with batch adsorption experiments. The mass balance of PYR in the two-phase system (POM-water) was given by: Qtot = Cp Mp + Cw Vw

(4)

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Substitution of Eq. 2 into Eq. 4 gives: KPOM = Cp Vw /(Qtot − Cp Mp )

Statistical differences between mean values were analyzed at P < 0.01 using a student t-test. (5)

According to KPOM and Cp , the Cw and the corresponding qs were calculated based on the above equations, and then these data could be pooled to obtain an adsorption isotherm of PYR. Soil washing A series of soil washing experiments were conducted to examine the effects of different washing parameters on PAH removal with SGE. The four key parameters were tested in the following order: Test 1, eluent concentration (0, 50, 100, 500, 1 000, 2 000, 4 000, and 8 000 mg L−1 SGE); Test 2, liquid/soil (L/S) ratio (volume (mL):weight (g), 5:1, 10:1, and 20:1); Test 3, successive washing cycle (1, 2, 3, and 4); and Test 4, washing agent (SGE, TW80, and MCD). When each of the parameters was tested at different values, the others were kept constant. The value generating the best leaching removal was adapted in the following test. The initial soil washing parameters in Test 1 were set to room temperature 25 ◦ C, pH 7, L/S ratio 10:1, and one washing cycle (6 h) at different SGE concentrations. Each soil sample (4 g) was placed in a 100-mL glass centrifuge tube with a Teflon-lined cap, to which an appropriate amount of different washing solutions was added, followed by ultrasound (80 kHz, 600 W) washing for 30 min and shock washing (150 r min−1 , 25 ◦ C) for 24 h. The mixtures were then centrifuged (716 × g, 10 min), and the sediments were frozen and dried for analysis of PAHs. Analysis of PAHs Polycyclic aromatic hydrocarbons in soil samples were extracted using the soxhlet-extraction procedure (EPA method 3540c) (Ping et al., 2007; Mao et al., 2012). The PAHs were analyzed by gas chromatography-mass spectrometry (GC-MS) (7890 GC-5975 MSD, Agilent Technologies, Santa Clara, USA) fitted with a DB-5 ms capillary column (30 m × 0.25 mm × 0.25 µm) (Restek Co., Bellefonte, USA) following the method of Hou et al. (2015). The carrier gas was He at 1.4 mL min−1 ; the injection temperature was 280 ◦ C; the temperature program was 50 ◦ C (held for 3 min) to 45 ◦ C at 2 ◦ C min−1 and then to 300 ◦ C (held for 5 min) at 25 ◦ C min−1 . Statistical analysis All experiments were performed in triplicate. The data are presented as means ± standard deviations.

RESULTS AND DISCUSSION Characterization of the SGE tested The SGE tested was in powder form and showed good dispersion in water (Fig. 1a). The BET specific surface area of SGE was 287.6 m2 g−1 , much lower than the theoretical value (2 630 m2 g−1 ). This may be related to incomplete exfoliation and aggregation during the redox process. The SEM images of SGE are presented in Fig. 1b. The SGE sheet was distributed around the copper net clearly. The surface of SGE was smooth and flat with a small amount of folding. The surface functional groups of SGE were analyzed by XPS (Fig. 1c). Deconvolution of the C1s peak of SGE resolved to a main peak at 284.3 eV, which may be of sp2 C atoms (Pei et al., 2013). The peaks at 285.4 and 287.5 eV correlated to the C atoms in C=O/C–O and C–S, respectively. The C, O, and S atomic percentages of SGE were 64.57%, 35.12%, and 4.3%, respectively. Carbon to O ratio (C/O) of SGE was about 1.84. This showed that there was a large amount of –SO3 H on the surface of SGE. The FTIR spectra of SGE and the SGE after PYR adsorption are shown in Fig. 1d. In the case of the SGE, the strong peak at 3 392 cm−1 resulted from the stretching vibration of O–H. The peaks at 1 616, 1 385, 1 189, and 1 041 cm−1 were attributed to the stretching vibrations of benzene rings C=C, carboxyl O=C–O, sulfonyl S=O, and ether C–O bonds, respectively. The adsorption isotherms of PYR on SGE are given in Fig. 2. The sorption data were nonlinear and fitted the Langmuir model well: qs = qm kCw /(1 + kCw )

(6)

where qs is the equilibrium absorbed concentration in mg g−1 ; Cw is the equilibrium solution phase concentration in mg L−1 ; k is the Langmuir constant (L g−1 ); and qm represents the maximum adsorption capacity of the adsorbent (mg g−1 ). The regression parameters of the isotherms by the Langmuir model were as follows: R2 = 0.97, qm = 50.3 mg g−1 , and k = 6.33 L g−1 . In the case of the SGE, the theoretically adsorbed amount via monolayer coverage was 50.3 mg g−1 for PYR. Zhao et al. (2011) investigated that the maximum adsorption capacity of naphthalene of SGO could be up to 2.4 mmol g−1 . Compared to activated C (0.805 mmol g−1 ) (Anbia and Moradi, 2009), ordered mesoporous silica (0.136 mmol g−1 ), and oxidized multiwalled carbon nanotubes (0.212 mmol g−1 ) (Sheng et al., 2010), SGE had a strong capacity to adsorb PAHs. Here, the

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Fig. 1 Characterization of the nano-sulfonated graphene (SGE) tested: different mass concentrations of SGE in water (from left to right, SGE mass concentrations of 1.0, 5.0, 50.0, and 100.0 g L−1 , respectively.) (a), scanning electron microscopy image of SGE, where circular hole is the copper net (b), X-ray photoelectron spectroscopy spectra of SGE (c), and fourier transform infrared spectroscopy spectra of SGE and the SGE after PYR adsorption (SGE + PYR) (d).

Eluent concentration

Fig. 2 Equilibrium adsorption isotherm of pyrene (PYR) on nano-sulfonated graphene (SGE) in batch adsorption experiments. qs = equilibrium absorbed concentration; Cw = equilibrium solution phase concentration.

absorption results indicate a great potential of SGE in environmental pollution management.

Test 1 was carried out with one washing cycle at different concentrations of SGE under the condition: stirring speed 150 r min−1 , L/S ratio 10:1, washing time 6 h, and pH 7. It could be observed that removal rates of PAHs increased sharply with increasing SGE concentrations up to 2 000 mg L−1 (Fig. 3a). The increment in SGE concentrations above 4 000 mg L−1 did not result in appreciable enhancement and might even lead to a decrease in PAH removal. This showed that increasing SGE concentrations allowed PAHs to migrate from soil particles into the liquid phase. However, further increase in SGE concentration might cause the SGE mixed with PAHs to precipitate into the solid phase, where the mixture was centrifuged. The removal rate was then reduced (Neilson et al., 2003). The PAH removal rate was maximum at an SGE concentration of about 4 000 mg L−1 , and the rate could be up to 30.9%. However, the removal rate at 2 000 mg

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Fig. 3 Removal rates of polycyclic aromatic hydrocarbons (PAHs) from a coking plant soil washed by nano-sulfonated graphene (SGE) at different washing parameters, SGE concentrations (a), liquid/soil (L/S) ratios (b), and successive washing cycles (c) at L/S ratios of 10:1 and 20:1; the relationship between removal rate and washing cycle using a fitting soil washing model (L/S ratio 10:1, 4 washing cycles, and 2 000 mg L−1 SGE) (d); and changes in removal rate of different ring-number PAHs at optimum washing parameters (L/S ratio 10:1; 4 washing cycles, SGE concentration 2 000 mg L−1 ). Vertical bars indicate standard deviations of means.

L−1 was 29.8%, which was not significantly different from that at 4 000 mg L−1 (P > 0.05). Therefore, 2 000 mg L−1 could be taken as the optimum SGE concentration in this case. Liquid/soil ratio Liquid/soil ratio is a critical parameter in the process of soil washing. Peng et al. (2011) proposed that the highest solubilization percentage (SP, a parameter to measure leaching efficiency of eluent) occurred at the L/S ratio of 10:1 for TX100 and TW80, and the L/S ratio of 10:1 was taken as the optimum in the study. Test 2 was carried on at the optimal SGE concentration of 2 000 mg L−1 determined in Test 1. The other parameters were the same as those in Test 1. The results showed that increasing L/S ratios had a positive effect on the removal rate of PAHs (Fig. 3b). At an L/S ratio of 10:1 or 20:1, the removal rate of PAHs was 29.8% or 35.2%, respectively. Both values were much higher than that (14.8%) at an L/S ratio of 5:1 (P < 0.01). Although the removal rate of PAHs at an L/S ratio of 20:1 was higher than that at the ratio of 10:1 (not higher than 20%), a higher L/S ratio denoted more wastewater output and a higher requirement for equipment and

energy to handle it (Zou et al., 2008). At the same eluent concentration, a lower L/S ratio could reduce the impact on soil physico-chemical properties (Kedziorek et al., 1998). Consequently, in Test 3, the L/S ratios of 10:1 and 20:1 were selected to study the influence of single vs. successive washings. Successive washings Although SGE could remove PAHs from the soil, only one washing cycle was not adequate to completely decontaminate the soil. Successive washing cycles (1, 2, 3, and 4) with 2 000 mg L−1 SGE solution (L/S ratio, 20:1 and 10:1) were performed to reach a higher removal rate and the other parameters were the same as those in Test 2. The results are illustrated in Fig. 3c. The removal rates increased with increasing washing cycles from 1 to 4. The removal rates at L/S ratios of 10:1 and 20:1 from the soil were 30.9% and 36.0% after the 1st cycle and 80.9% and 84.4% after the 4th cycle. The increased removal can be explained by: (1) the initial step(s) mainly aim(s) at removing a major portion of the water-soluble and organic acidsoluble fractions of PAHs in the soil (Sabat´e et al., 2006; Wang et al., 2015; Mousset et al., 2016), and

WASHING REMOVAL OF PAHs USE SULFONATED GRAPHENE

there is less competitive adsorption of PAHs between SGE and the soil; (2) as the washing cycles increase they can remove some organically bound fractions and residual fractions of PAHs which may be trapped in the pores of the soil or re-adsorbed onto the sediment particles during the sequential washing procedure (Zou et al., 2008). According to the leaching model of Mousset et al. (2016), a similar fitting model was obtained as shown in Fig. 3d. It was established under the washing condition of L/S ratio 10:1, 4 washing cycles, and SGE concentration 2 000 mg L−1 . On the basis of the fitting model, the theoretical removal rate of PAHs could be up to 100% after 7 successive washing cycles. However, at an L/S ratio of 20:1, the removal rate of PAHs after the 4th cycle was not significantly different from that after the 3rd cycle (P > 0.05); it also did not differ significantly from that after the 3rd cycle at an L/S ratio of 10:1 (P > 0.05). These phenomena could be attributed to the following reasons. After the previous extractions where the mobile forms are extracted, the PAHs are released from the less mobile forms, i.e., bound and residual fractions (Northcott and Jones, 2001; Zou et al., 2008). Less than 100% removal rates indicate that sorption of PAHs and/or residual incrustation or soil-trapped PAHs exist, especially in the case of coking plant soils (Peng et al., 2011). The results also showed that with increasing successive washing cycles, the advantages of a high L/S ratio decreased. Therefore, an L/S ratio of 10:1, 4 successive washing cycles, and an SGE concentration of 2 000 mg L−1 were taken as the optimum washing parameters. The removal rates of 2(+3)-, 4-, 5-, 6-ring PAHs and the total PAHs were 96.0%, 90.1%, 80.7%, 69.7% and 80.9%, respectively, under the washing condition characterized by the above optimum parameters (Fig. 3e). The PAH removal rates decreased with increasing ring number. This could be explained by the soil organic carbon-water partition coefficient (Koc ) values of the PAHs. The higher the Koc value, the stronger the bind between the PAH and soil (Zheng et al., 2007; Mousset et al., 2014). With the knowledge that the Koc values increase when the number of PAH rings increases, it is more difficult to extract PAHs with higher numbers of rings (Mousset et al., 2016). Different types of washing agents Recent studies have shown that TW80 (Paterson et al., 1999; Chang et al., 2000; Alc´antara et al., 2009) and cyclodextrins (CDs) (G´omez et al., 2010; Sun et al., 2013; S´anchez-Trujillo et al., 2013) can effectively remove PAHs from soils. In order to compare the leach-

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ing efficiency of SGE with that of other efficient washing agents, TW-80 and MCD were used while maintaining a mass concentration of 2 000 mg L−1 and the other washing parameters were the same as those in Test 1. The results are shown in Table II. The PAH removal rates of TW-80, MCD, and SGE were 15.4%, 19.39%, and 30.9%, respectively, under the same conditions. At the same mass concentration, the PAH removal of SGE was the highest (P < 0.01). The PAH removal rate of SGE was twice as high as that of TW80 and 1.5 times higher than that of MCD. The unit price and cost of disposing one ton soil by the three types of soil washing agents are shown in Table II. The SGE and MCD methods cost more than TW80 at the present stage. However, with the improvement of production process, SGE would be produced in large scale and its cost would be greatly reduced. Besides, compared with TW80, SGE and MCD have minor impacts on the environment (Sun et al., 2013). Thus, it can be seen that SGE is more effective than the other soil washing agents previously studied for leaching removal of PAHs, and SGE also has certain application prospects in the leaching remediation of PAH-contaminated soils. TABLE II Costs and polycyclic aromatic hydrocarbon (PAH) removal rates of the three types of soil washing agents Agenta)

Unit price kg−1

SGE MCD TW80

$ 142.8 114.3 1.64

Cost t−1

$ soil 5 714 4 571 65.7

PAH removal rate % 30.9 ± 4.02b) 19.4 ± 1.50 15.4 ± 4.01

a) SGE

= nano-sulfonated graphene; MCD = methyl-β-cyclodextrin; TW80 = Tween-80. b) Mean ± standard deviation.

Possible leaching mechanisms The FTIR spectra of SGE after adsorption with PYR are shown in Fig. 1d. No bias shift was observed at 1 616 cm−1 corresponding to the peak of the C=C stretching vibration after adsorption with PYR. It is known that the sp2 hybridized structure is damaged during the oxidation process, with only a few πelectrons valid (Stankovich et al., 2007). So π-π interaction between PAHs and SGE should still work because of the residual π-electrons on SGE (Ramesha et al., 2011). However, the peak of the sulfonyl S=O stretching vibration shifted from 1 189 to 1 177 cm−1 , and the peak of the stretching vibration of O–H shifted from 3 392 to 3 385 cm−1 after adsorption with PYR. Clear changes reflected by FTIR spectra after adsorption confirmed that interactions between the oxygen-

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Fig. 4 Schematic diagram of leaching mechanisms of polycyclic aromatic hydrocarbons (PAHs) from a coking plant soil washed with nano-sulfonated graphene (SGE).

containing functional groups of SGE and PYR molecules occurred, especially for –SO3 H (Wang et al., 2013, 2014). In order to clarify the possible mechanisms of soil washing directly, a schematic diagram is shown in Fig. 4. The SGE tested was characterized by unfolding and good dispersion in water and it could compete with the soil particles for adsorption of PAHs through the role of π-π, H-π, and anion-π interactions. The PAH removal by leaching usually involves the water/organic acid-soluble fraction and most of the bound fraction. Then the well-dispensed and unfolded SGE carries PAHs into the liquid phase. However, a portion of the PAHs (residual fraction) will be “locked” into the organic matter of the soil particles and become a part of the soil skeleton. This fraction can not be removed from the soil under natural conditions, especially in the case of coking plant soils, and is unavailable to plants and microorganisms (Wang et al., 2015). Therefore, the removal rate of PAHs can not be close to 100%. The FTIR spectra of the original and treated soils (after 4 successive washing cycles) are shown in Fig. 5. The peak of the stretching vibration of O–H shifted from 3 420 to 3 432 cm−1 and no bias shift was observed at 1 426 cm−1 corresponding to the peak of the benzene C=C stretching vibration after washing treatment. It is known that the soil benzene content decreases and some groups (such as –SO3 H) containing hydroxyl play a role in the removal of PAHs. The concentrations of PAHs in different soil particle-size fractions are significantly different (Wilcke, 2000; Wei et al., 2014). The soil particle sizes of the original and treated soils were analyzed with a laser particle size analyzer (LS13320). The results are shown in Fig. 6. Compared to the original soil, the sand

and clay fractions of the treated soil were reduced by 11.4% and 43.6%, respectively. The surface morphology of the original and treated soils was observed under the SEM (Fig. 7). Compared with the original soil, the treated soil had more pores on the surface and smaller particles disappeared in the washing process. This phenomenon supported the results of the particle-size analysis. Wei et al. (2014) suggested that PAHs were mainly concentrated in the sand and clay fractions of aging soils. Therefore, differences in soil particle size may also contribute to the removal of PAHs from soils.

Fig. 5 Fourier transform infrared spectroscopy spectra of original and treated (after 4 successive washing cycles) soils.

CONCLUSIONS The SGE tested can strongly adsorb PAHs from the coking plant soil by the ex-situ soil washing technology. Three key soil washing parameters, an L/S ratio of 10:1, 4 successive washing cycles, and an SGE concentration of 2 000 mg L−1 , are taken as the optimum

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clarify the specific washing mechanism, in order to better study and utilize SGE in the washing remediation of soil PAHs. ACKNOWLEDGEMENTS This study was supported by the Distinguished Young Scholar Programe of Jiangsu Province of China (No. BK20150049) and the National Natural Science Foundation of China (No. 41401565). REFERENCES

Fig. 6 Particle-size distribution of original and treated (after 4 successive washing cycles) soils. Vertical bars indicate standard deviations of means.

Fig. 7 Scanning electron microscopy images of original (a) and treated (after 4 successive washing cycles) (b) soils.

condition for the treatment, giving a PAH removal rate of more than 80%. In addition to the washing parameters, different fractions of PAHs in soil and PAH ring numbers might also influence the removal of PAHs. After one washing cycle at SGE concentration of 2 000 mg L−1 and L/S ratio of 10:1, the SGE tested was more effective than the current hot washing agents (TW80 and MCD) used for PAH removal. The SGE tested may play a useful role in the washing remediation of the coking plant soil. However, further studies are needed to

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