Hematite-facilitated pyrolysis: An innovative method for remediating soils contaminated with heavy hydrocarbons

Journal of Hazardous Materials 383 (2020) 121165

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Hematite-facilitated pyrolysis: An innovative method for remediating soils contaminated with heavy hydrocarbons ⁎

Yuqin Liua,b, Qian Zhanga, Bin Wua, Xiaodong Lia,b, Fujun Maa, , Fasheng Lia, Qingbao Gua,b, a b

T

⁎

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China College of Water Sciences, Beijing Normal University, Beijing 100875, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

As a recalcitrant fraction of petroleum, heavy hydrocarbons (including aromatics, resins, and asphaltenes) can remain in contaminated soils even after decades of weathering, thereby causing serious harm to the soil ecosystem and human health. Pyrolysis is a promising technique for remediating petroleum-contaminated soil. However, this technique still presents some drawbacks, such as high energy consumption and damage to soil properties. Therefore, an innovative method using hematite (Fe2O3) for the catalytic pyrolysis of weathered petroleum-contaminated soil was developed in this study. Compared with soil pyrolyzed without Fe2O3 at 400 °C for 30 min, the residual concentrations of aromatics, resins, and asphaltenes in soil pyrolyzed with 5.0% Fe2O3 were reduced by 67.8%, 52.3%, and 67.9%, respectively. After pyrolysis with 5.0% Fe2O3, the water-holding capacity of soil was considerably increased and the soil became darker and rougher. Scanning electron microscopy analysis showed that many small holes occurred on the surface of the pyrolytic soil. X-ray photoelectron spectrometer analysis showed that a thin layer of graphitic C was formed and deposited on the surface of the pyrolytic soil. We also observed that the wheat germination percentage and biomass yield in the soil pyrolyzed with 5.0% Fe2O3 were even higher than those in clean soil.

Keywords: Heavy hydrocarbons Contaminated soil Pyrolysis Fe2O3

1. Introduction Crude oil is one of the world's main energy sources. In 2015, 0.97

million barrels of crude oil were consumed per day, and this demand is expected to increase to 1.20 million barrels per day in 2024 (IEA, 2019). Consequently, the accidental release of crude oil into the

⁎ Corresponding authors at: State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China. E-mail addresses: [email protected] (F. Ma), [email protected] (Q. Gu).

https://doi.org/10.1016/j.jhazmat.2019.121165 Received 22 May 2019; Received in revised form 4 September 2019; Accepted 4 September 2019 Available online 05 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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function after catalytic pyrolysis. To the best of our knowledge, this was the first study using Fe2O3 to promote the pyrolysis of soil contaminated with heavy hydrocarbons.

terrestrial environment has also significantly increased. As an important component of petroleum, heavy hydrocarbons, including aromatics, resins, and asphaltenes, are ultracomplex mixtures with various hetero-elements, chemical functional groups, and high aromaticity. They are difficult to degrade and can remain in contaminated soil even after decades of weathering (Leahy and Colwell, 1990; Das and Chandran, 2011; Atlas, 1991; Atlas and Hazen, 2011; Bento et al., 2005), thereby causing long-term ecotoxicological effects on the soil ecosystem and threatening human health (Obida et al., 2018; Li et al., 2016; Konieczko, 2005; Hentati et al., 2013). Therefore, there is a pressing need to remediate soil contaminated with heavy hydrocarbons. Crude oil mainly consists of saturates, aromatics, resins and asphaltenes (SARA) fractions. Generally, saturates lose their mass at 25–370 °C (low temperature), aromatics and resins lose their mass at 370–470 °C (moderate temperature) and the asphaltenes lose their mass at 470–580 °C (high temperature) (Kök et al., 1998). Thermal treatments, such as thermal desorption, pyrolysis, and incineration, are generally very efficient in the remediation of organic contaminated soils (Bucalá et al., 1994; Falciglia et al., 2011; Vidonish et al., 2016; Li et al., 2018). Among them, pyrolysis, as a promising and proven technology, has received increasing attention owing to its advantages in the removal of highly toxic persistent organic compounds (Vidonish et al., 2016; Li et al., 2018; Shi et al., 2018; Song et al., 2019a; Kim et al., 2019; Lee et al., 2018). Vidonish et al. (2016) employed pyrolysis to remediate soil artificially contaminated with crude oil at 420 °C for 3 h. Their results showed that total petroleum hydrocarbons (TPH) (including heavy hydrocarbons) were almost completely removed and the soil fertility was enhanced after treatment. A high removal efficiency of TPH in another artificial petroleum-contaminated soil was also obtained via pyrolysis at 500 °C for 30 min by Li et al. (2018). An enhanced thermolysis of artificial heavy petroleum-contaminated soil using CO2 for soil remediation and energy was achieved by Lee et al. (2018). Compared with artificial petroleum-contaminated soils, weathered petroleum-contaminated soils are dominated by the strong bound or recalcitrant fractions of hydrocarbons, which are more difficult to remediate (Trindade et al., 2005; Khan et al., 2018; Tang et al., 2012; Lemkau et al., 2014). To the best of our knowledge, the pyrolysis of weathered petroleum-contaminated soil has not been thoroughly explored. Therefore, there are many uncertainties related to the pyrolytic treatment of weathered petroleum-contaminated soil. Additionally, high treatment temperature and long residence time can result in high energy consumption and severe damage to the ecological function of soil. Therefore, with the growing need for sustainable treatment options including low energy consumption and soil ecosystem conservation (O’Brien et al., 2017), it is imperative to optimize the pyrolytic treatment of weathered petroleum-contaminated soil. To decrease the energy consumption and damage to the ecological function of soil, pyrolysis combined with additives or catalysts may be preferred to remediate weathered petroleum-contaminated soils. In catalytic aquathermolysis processes, hematite (Fe2O3) can transform heavy oil into useful light oil (Khalil et al., 2017; Galukhin et al., 2015). Day et al. reported that the degradation process of polypropylene was accelerated by over 100% when Fe2O3 was added during the thermal degradation of plastics (Day et al., 1995). As a promising additive, Fe2O3 has also been applied to upgrade petroleum residual oil and improve coal liquefaction (Sanjay et al., 1994; Fumoto et al., 2012). In addition, Fe2O3 is advantageous because it is a stable, innoxious, lowcost, and naturally abundant material. Therefore, we hypothesized that the Fe2O3 catalytic pyrolysis of soil contaminated with heavy hydrocarbons is an efficient and economical remediation alternative. In the present study, an innovative method using Fe2O3 for the catalytic pyrolysis of weathered soil contaminated with heavy hydrocarbons was conducted. The removal efficiencies of TPH in petroleumcontaminated soils were compared between pyrolysis with and without Fe2O3. In addition, soil properties and wheat growth experiments were investigated to better understand the enhancement of soil ecological

2. Materials and methods 2.1. Soil samples Petroleum-contaminated soil (composed of 94.8% sand, 4.6% silt and 0.6% clay) was obtained from the Shengli Oilfield in Shandong Province, China. The concentration of TPH in the soil was 119 ± 5 g/ kg. Clean soil was obtained from a farmland near the Shengli Oilfield. The collected soils were dried, homogenized, and sieved to remove large particles. 2.2. Soil pyrolysis For each soil sample, 15 g of sieved soil was placed in a valve bag (120 × 85 mm), and then corresponding percentages (0%, 0.2%, 0.5%, 1.0%, 2.0%, and 5.0%) of Fe2O3 powder (purchased from Sinopharm Chemical Reagent Co., Ltd.; purity of 99%) were added into the bag. The bag was sealed and then shaken up and down for 5 min to ensure adequate mixing before pyrolysis. The pyrolytic experiment was performed in a quartz boat heated by a pipe furnace under a continuous flow of N2 (purity of 99.999%) with a flow rate of 1 L/min. A thermoelectric couple of the pipe furnace recorded the temperature and guaranteed that the required pyrolytic temperature was achieved. Before the heating started, the air inside the pipe furnace device was exhausted by using a continuous N2 flow (about 0.5 L/min) for 10 min. Then, 15 g of soil was placed in the quartz boat reactor, transferred into the pipe furnace, and held there at the desired temperature for a certain time. After naturally cooling down to 25 ± 2 °C, the solid residue was collected for later analysis. The pyrolytic tail gas was absorbed by nhexane. 2.3. TPH measurement and SARA separation As an important index for the remediation of petroleum-contaminated soil, the TPH in petroleum-contaminated soil were extracted using ultrasonic extraction and then measured by ultraviolet spectroscopy (UV–vis) at 304 nm (Zhou et al., 2017). Detailed information about the determination of the TPH concentration in soil is provided in Text S1. The SARA fractions of extracted petroleum from the soil were separated according to the method described by Kök and Ozgen (Kök et al., 1998). All experiments were performed in triplicate. The determination of petroleum hydrocarbons (C10-C40) in soil samples was performed based on gas-chromatography-flame ionization detection (GC-FID) analysis according to the method of ISO 16703-2004, and the detailed information is provided in the Text S2. 2.4. Determination of soil properties Soil pH, hydrophobicity, particle size (e.g. clay, silt and sand) distribution, cation exchange capacity (CEC), and water-holding capacity were determined according to the method provided in Text S3. A PHI Quantera X-ray photoelectron spectrometer (XPS) was applied to measure the elemental composition and state of the soil surface (generally 1–10 nm), and the detailed measurement information is provided in Text S4. Scanning electron microscopy (SEM)-energy dispersive Xray (SEM-EDX) images of the soil samples were obtained by Hitachi S2700 SEM equipped with a PGT (Princeton Gamma-Tech) IMIX digital imaging system and a PGT PRISM IG (Intrinsic Germanium) detector for XRD analysis, and the measurement procedure is provided in Text S5. The elemental composition of the petroleum-contaminated soil was determined by X-ray fluorescence (XRF) (ARL PERFOEM X, Thermo Fisher, China) and the results are shown in Table S1. 2

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obtained in the weathered petroleum-contaminated soil in our study. Under the same pyrolysis conditions, removal efficiencies of only 2.0% and 28.5% were achieved, respectively. Such a significant difference in the TPH removal efficiency between the two types of soils suggested that weathered petroleum-contaminated soil was more difficult to remediate. The high proportion of heavy hydrocarbons in the soil and the strong interaction between the heavy hydrocarbons and soil caused by weathering may have contributed to this phenomenon (Lemkau et al., 2014). To save energy and reduce the damage to soil properties, 400 °C was selected as the pyrolysis temperature to investigate the effect of Fe2O3 on the TPH removal in the following study. As another key factor, batch pyrolysis experiments under residence time of 10, 20, 30, 60 and 120 min were conducted at 400 °C to investigate the effect of residence time on TPH removal in the petroleumcontaminated soil. As shown in Fig. 1(b), the removal efficiency of TPH increased with the increase in residence time during pyrolysis. When the petroleum-contaminated soil was treated for 10 min, the removal efficiency of TPH was only 11.2%, and it sharply increased to 42.7% and 70.3% when treated for 20 min and 30 min, respectively. When the residence time further increased, the removal efficiency of TPH was increased slowly and reached a maximum of 92.3% at 120 min. To investigate the catalytic effect of Fe2O3 on the removal efficiency of TPH in the petroleum-contaminated soil, a residence time of 30 min was selected in the following study. Fig. 1 (a) Effect of pyrolysis temperature on the removal efficiency of total petroleum hydrocarbons (TPH) in the petroleum-contaminated soil. Experimental conditions: residence time of 30 min and N2 flow rate of 1 L/min. (b) Effect of residence time on the removal efficiency of TPH in petroleum-contaminated soil. Experimental conditions: pyrolysis temperature of 400 °C and N2 flow rate of 1 L/min.

2.5. Plant germination and growth studies To investigate the reuse of the pyrolytic soil for vegetation growth, wheat germination and growth experiments were conducted using clean soil, untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with Fe2O3, respectively. In this study, wheat was selected as the model bioindicator owing to its several advantages, including worldwide availability, higher germination efficiency, and ability to grow in a wide range of soil types and environments (Banks and Schultz, 2005; Inckot et al., 2011). Wheat seed germination percentages were determined by placing 100 seeds in layers of moistened soil within a glass petri dish. The germination dishes were placed in a growth chamber under dark conditions until the seed germination percentage was constant. Then, the wheat germination rates in different soils were determined. For the wheat growth tests, wheat seedlings were grown for 21 d in plastic planters (7 cm × 7 cm × 7 cm) with the four different soils. Each planter contained 200 g (dry weight) of soil. A layer of gauze was placed in the bottom of each planter to prevent soil loss and to promote water transport from the external water reservoir into the soils. In each planter, 20 wheat seeds were placed beneath the surface of the soil. The planters were placed in the growth chamber and watered with deionized water every 12 h to keep the soil water content (65%) constant. The climate chamber was adjusted to 16 h of daylight, day/night temperature of 26/20 °C, and 70% relative humidity. On day 21, the wheat plants were harvested, and the root length, shoot length, wet weight and dry weight of each plant were measured. All variables were examined in triplicate.

3. Results and discussion 3.1. Pyrolysis of petroleum-contaminated soil without Fe2O3

3.2. Pyrolysis of petroleum-contaminated soil with Fe2O3

Temperature plays a key role in the process of pyrolytic remediation. In this study, pyrolysis experiments at 250–500 °C were investigated. As shown in Fig. 1(a), the removal efficiency of TPH in the untreated soil was 2.0% with pyrolysis at 250 °C for 30 min, and increased with the increase in pyrolysis temperature. When the temperature increased from 350 °C to 400 °C, the TPH removal efficiency sharply increased from 28.5% to 70.3%, respectively. It further increased to 86.8% when the pyrolysis temperature reached to 500 °C. Generally, thermal desorption is the main removal mechanism of TPH at lower pyrolysis temperatures (250–350 °C), and these removed contaminants are light and easily degraded compounds (Kök and Gul, 2013). These results indicated that heavy hydrocarbons started to be removed at temperatures above 400 °C. In the soil artificially contaminated with crude oil, the removal efficiencies of TPH at 250 °C and 300 °C for 30 min were approximately 70% and 90%, respectively (Li et al., 2018). In contrast, a very low TPH removal efficiency was

To investigate whether Fe2O3 can intensify the pyrolytic remediation of soil contaminated with heavy hydrocarbons, pyrolysis of weathered petroleum-contaminated soil with Fe2O3 dosages of 0.2%, 0.5%, 1.0%, 2.0%, and 5.0%, was conducted at 400 °C for 30 min. As shown in Fig. 2(a), compared with the soil pyrolyzed without Fe2O3, the TPH removal efficiency was increased by 11.5% when 0.2% Fe2O3 was added during the soil pyrolysis, and further increased with higher Fe2O3 contents. When 5% Fe2O3 was added, the removal efficiency of TPH reached to 95.8%. The residual TPH concentration in the contaminated soil decreased to 5 g/kg, which met the regulatory threshold of 10 000 mg/kg (Michelsen and Boyce, 1993). Moreover, such a residual TPH concentration was even lower than that in the soil pyrolyzed without Fe2O3 at 500 °C for 30 min (16 g/kg). It should be noted that, the initial weight percentage of Fe in the petroleum-contaminated soil was 4.3% though, while the removal efficiency of TPH clearly increased after 0.2% Fe2O3 added during pyrolysis. This phenomenon suggested Fig. 1. (a) Effect of pyrolysis temperature on the removal efficiency of total petroleum hydrocarbons (TPH) in the petroleum-contaminated soil. Experimental conditions: residence time of 30 min and N2 flow rate of 1 L/ min. (b) Effect of residence time on the removal efficiency of TPH in petroleum-contaminated soil. Experimental conditions: pyrolysis temperature of 400 °C and N2 flow rate of 1 L/min.

3

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Fig. 2. (a) Total petroleum hydrocarbons (TPH) removal efficiency of petroleum-contaminated soil by pyrolysis with different dosages of Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. (b) TPH removal efficiency of petroleum-contaminated soil with varying amounts of Fe2O3 in line with the kinetic measurements during pyrolysis. Experimental conditions: pyrolysis temperature of 400 °C and N2 flow rate of 1 L/min.

the soil after pyrolysis for t min at 400 °C; a and b (min−1) are constants modeled by the double constant model, qe is the TPH removal efficiency in the petroleum-contaminated soil at equilibrium, and k2 (min−1) is the rate constant of the pseudo-second-order kinetic model. The linear plot of ln(qt) versus ln(t) was used to calculate the constants a and b, and the determination coefficient R2. The rate constant k2 was calculated from the intercept and slope of the linear plot of t/qt versus t along with the value of the determination coefficient R2. As shown in Table S2, the R2 values obtained by the pseudo-second-order kinetic model were higher than 0.9 when the Fe2O3 addition amount was higher than 1%, suggesting the applicability of the pseudo-second-order kinetic model to describe the removal kinetics data of TPH. Fig. 2 (a) Total petroleum hydrocarbons (TPH) removal efficiency of petroleum-contaminated soil by pyrolysis with different dosages of Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. (b) TPH removal efficiency of petroleum-contaminated soil with varying amounts of Fe2O3 in line with the kinetic measurements during pyrolysis. Experimental conditions: pyrolysis temperature of 400 °C and N2 flow rate of 1 L/min.

that the compound forms of Fe in the initial soil did not have a catalytic effect on the TPH removal. Meanwhile, to verify whether Fe2O3 had the same high catalytic efficiency of TPH removal in other contaminated soils, a petroleum-contaminated soil (pH = 6.9, SOC = 181.5 mg/kg, CEC = 6.2 cmol/kg, composed of 0.3% clay, 3.2% silt and 96.5% sand) with a TPH concentration of 173 ± 8 g/kg collected from the Liaohe Oilfield in Liaoning Province, China was tested. The results showed that the removal efficiency of TPH was 61.0% after pyrolysis without Fe2O3, while it was increased to 79.7% as 5% Fe2O3 was added during pyrolysis. These results suggested that Fe2O3 could considerably increase the removal efficiency of TPH by pyrolysis at lower temperatures. It has been reported that Fe2O3 can lower the energetic barrier to commence the cracking reaction at a lower temperature, thereby accelerating the pyrolysis process. This ability has a more pronounced effect when pyrolysis temperatures are between 350–550 °C (Kök, 2011; Pu et al., 2015). Lotz et al. (2019) reported that iron oxide can decrease the temperature of devolatilization and oxidation by about 100 °C. Fig. 2(b) shows the kinetic measurement of the TPH removal efficiency of the petroleum-contaminated soil during pyrolysis with a varied amount of Fe2O3. The removal efficiency of TPH in the petroleum-contaminated soil increased with the increase in Fe2O3 addition amount and residence time. To further elucidate the TPH removal kinetics, two commonly used kinetic models, namely the double constant model and pseudo-second-order kinetic model, were employed. The linearized forms of the double constant model and pseudo-second-order kinetic model are given as follows: ln(qt) = a + bln(t)

(1)

t/qt = 1/(k2qe2) + t/qe

(2)

3.3. Variation in the SARA concentrations in soil pyrolysis Petroleum is a complex substance. Therefore, the extracted petroleum samples were divided into fewer complexes and more chemically representative components, namely the SARA fractions, which form a major portion of petroleum. The concentrations and separated solution image of the SARA fractions from untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3 are presented in Fig. 3. The percentages of SARA in the petroleum extracted from untreated soil were 67.0%, 12.3%, 8.3%, and 12.4%, respectively. Saturates, which

Where t (min) is the pyrolysis time; qt is the TPH removal efficiency in

Fig. 3. (a) The variation in concentrations of SARA fractions; (b) Images of the SARA solution extracted from the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. 4

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were the lightest fraction, showed a significant weight loss (92.4%) after pyrolysis without Fe2O3 at 400 °C for 30 min, and further decreased when 5% Fe2O3 was added during pyrolysis (98.4%) (Fig. 3(a)). Consistent with the saturates removal, the extracted solution became colorless after pyrolysis (Fig. 3(b)). The easy removal of saturates was attributed to their low activation energy in the SARA fractions (Kök et al., 1998). In contrast, the removal efficiencies of heavy hydrocarbons, including aromatics, resins, and asphaltenes, were comparatively low, and only reached to 21.6%, 37.2%, and 23.1%, respectively, in soil pyrolyzed without Fe2O3. However, when 5.0% Fe2O3 was added during pyrolysis, the removal efficiencies of aromatics, resins, and asphaltenes in the soil were increased by 66.8%, 52.3%, and 67.9%, respectively. The extracted solution of aromatics and resins also became more transparent corresponding to the contaminant removal (Fig. 3(b)), whereas no clear color change was observed in the solution of asphaltenes after pyrolysis, despite its high removal efficiency. This phenomenon could have been caused by the fact that asphaltenes are deeply colored. Based on the above results, we concluded that Fe2O3 could significantly enhance the pyrolysis efficiency of soil contaminated with heavy hydrocarbons by promoting the removal of aromatics, resins, and asphaltenes. It has been reported that Fe2O3 can form a strong interaction directly with N and S on the heteroatoms compounds (Khalil et al., 2017; Rosales et al., 2006). As a result, the bond energy (bond activation) of CeC, CeN, and CeS bonds on the heteroatoms molecules decreases, which gives rise to the activation of these bonds for scission and ultimately leads to the degradation of these heavy hydrocarbons. According to Kotantgama et al. Kotanigawa et al. (1997), Fe2O3 may react with S in petroleum-contaminated soil during pyrolysis, to form ferrous disulfide (FeS2), which is then oxidized to convert the surface of sulfides to sulfate species. Such a sulfate group inhibits the agglomeration of metal oxides and subsequently increases the surface area and catalyst dispersion (Pradhan et al., 1991), which also promoted heavy hydrocarbons removal during pyrolysis with 5% Fe2O3. Fig. 3 (a) The variation in concentrations of SARA fractions; (b) Images of the SARA solution extracted from the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min.

3.4. GC-FID analysis Fig. 4. GC-FID analysis of the (a) untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolyzed with 5% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/ min.

In order to investigate the catalytic effect of Fe2O3 on petroleum hydrocarbons with C10-C40, GC-FID analysis of the soil before and after pyrolysis with or without Fe2O3 was conducted. As shown in Fig. 4, the area under the chromatogram of the soil pyrolyzed without Fe2O3 showed an 87.5% reduction, while a 96.5% reduction was found in the soil pyrolyzed with 5% Fe2O3. Compared with the removal efficiency of TPH based on the GC-FID analysis, a comparatively lower removal efficiency (70.3%) of TPH in the soil pyrolyzed without Fe2O3 was found based on the UV–vis method, while almost the same removal efficiency of TPH was found in the soil pyrolyzed with 5% Fe2O3 (95.8%). This could be explained by the fact that some heavy hydrocarbons with macro molecules (such as aromatics, resins, and asphaltenes) in the weathered petroleum-contaminated soil could not be detected with GCFID, while they could be measured by the UV–vis method. However, because these heavy hydrocarbons (aromatics, resins and asphaltenes) were significantly removed by Fe2O3, the same removal efficiency of TPH in the soil pyrolyzed with Fe2O3 based on the UV–vis method compared with that of GC-FID analysis was obtained. Fig. 4 GC-FID analysis of the (a) untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolyzed with 5% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min.

3.5. Variation in polycyclic aromatic hydrocarbons (PAHs) concentrations in soil pyrolysis PAHs are contaminants of concern owing to their toxic and potentially carcinogenic characteristics. Therefore, this study measured the concentrations of 16 priority PAHs in untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. They were assessed against the risk-based screening levels (RBSLs) of soil contaminants in China (Table 1). In the untreated soil, benzo(a)pyrene B(a)P was the only PAH with a concentration higher than the RBSLs. After the pyrolysis with or without 5% Fe2O3, the concentrations of 16 PAHs in the two soils significantly decreased. It should be noted that, compared with the soil pyrolyzed without Fe2O3, the concentrations of some PAHs (such as benzo(k)pyrene, benzo(a)pyrene, and indeno(1,2,3-cd)pyrene) in the soil pyrolyzed with 5% Fe2O3 slightly increased. Dehydrogenation through polymerization and aromatization reactions during the pyrolysis with 5% Fe2O3 may have contributed to this phenomenon (Shie et al., 2002; Meng et al., 2003). However, the concentrations of PAHs in the soil pyrolyzed with 5% Fe2O3 were well below the RBSLs. 5

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Table 1 PAHs analysis of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3, and comparison with the risk-based screening levels (RBSLs) of soil contaminants in China. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. PAHs

Untreated soil

Soil pyrolyzed without Fe2O3

Soil pyrolyzed with 5.0% Fe2O3

(mg/kg) Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(a)pyrene Indeno(1,2,3-cd)pyrene Dibenz(a,h)anthracene Benzo(g,h,i)perylene

0.0231 0.0035 0.0037 0.0138 0.0362 0.0055 0.0112 0.0275 0.0583 0.2513 0.1338 0.5535 0.7410 0.0540 0.0620 0.1055

0.0132 0.0007 0.0002 0.0035 0.0146 0.0012 0.0034 0.0034 0.0003 0.0003 0.0009 0.0080 0.0017 ND 0.0005 0.0012

0.0138 0.0010 0.0019 0.0024 0.0088 0.0010 0.0036 0.0040 0.0018 0.009 0.0054 0.0024 0.0041 0.0049 0.0020 0.0025

RBSLs (mg/kg) Residential

Commercial/Industrial

Agricultural

25.00 – – – – – – – 5.50 490.00 5.50 55.00 0.55 5.50 0.55 –

70.00 – – – – – – – 15.00 1293.00 15.00 151.00 1.50 15.00 1.50 –

– – – – – – – – – – – – 0.55 – – –

“ND” represents below the detection limit, and the bold value indicates that concentration is higher than RBSLs.

concentrations of SOC and DOC in the untreated soil significantly decreased (Table 2), and were further reduced when Fe2O3 was added during pyrolysis (47 g/kg and 1.4 mg/L, respectively). With the toxicity of SOC and DOC eliminated after heavy hydrocarbons were removed (Song et al., 2019b), the residual concentrations of SOC and DOC in the soil pyrolyzed with 5.0% Fe2O3 may have been beneficial to plant growth (Roh et al., 2000). Owing to high viscosity of heavy hydrocarbons (especially the resins and asphaltenes), the major particle size of the untreated soil was sand accounted for 94.8%, while silt and clay were only contained 4.6% and 0.6%, respectively. With heavy hydrocarbons removed, the percentages of silt and clay in the soil pyrolyzed without Fe2O3 were sharply increased to 18% and 2.7%, respectively, while the corresponding percentage of sand was decreased to 79.3%. With high viscosity compounds (including aromatics, resins and asphaltenes) further removed significantly (Fig. 3), the percentages of silt and clay in the soil pyrolyzed with 5% Fe2O3 was further increased to 19.3% and 4.3%, respectively. There were no clear changes in CEC before and after pyrolysis without Fe2O3 at 400 °C for 30 min, while a slightly decrease in CEC was found in the soil pyrolyzed with 5% Fe2O3, which was consistent with the result obtained by Vidonish et al. (2016). High removal of aromatics, resins and asphaltenes during the pyrolysis with 5% Fe2O3 may have been contributed to this phenomenon (Costa et al., 2004; Asadu et al., 1997; Droge and Goss, 2013). Meanwhile, the increase of silt and clay content in the soil may have contributed to the increase of soil water-holding capacity of soil after pyrolysis. Table 2 Soil properties of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. In order to explore the changes in morphology and elemental distribution on the soil particles surface, SEM-EDX analysis was applied in our study and the results are shown in Fig. 5. As shown in the SEM images, the surfaces of untreated soil particles were smooth and bright.

Therefore, no risk was associated with PAHs formation in the soil pyrolyzed with 5% Fe2O3. Table 1 PAHs analysis of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3, and comparison with the risk-based screening levels (RBSLs) of soil contaminants in China. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. 3.6. Changes in soil properties caused by pyrolysis Thermal treatments are known to affect numerous soil properties, such as pH, water-holding capacity, and soil organic matter, to different extents. The variation in these properties may dictate land use after remediation. Therefore, the changes in soil properties before and after pyrolysis with or without Fe2O3 were studied. The obtained results are listed in Table 2. Pyrolysis led to an increase in soil pH. The higher soil pH found in the soil pyrolyzed with 5.0% Fe2O3 may have been ascribed to its higher carbonate decomposition, destruction of organic acids in heavy hydrocarbons, and release of cations from the soil organic matter (Vidonish et al., 2016). According to previous studies, such a change in soil pH may not have a significant effect on soil plantation (Vidonish et al., 2016; Roh et al., 2000). With the removal of heavy hydrocarbons removed during pyrolysis, both pyrolytic soils still presented high hydrophobicity, which may have been caused by the residual soil organic matter after pyrolysis. There were no clear changes in water-holding capacity between the untreated soil (0.39) and the non-catalytic pyrolytic soil (0.43), whereas the water-holding capacity of soil pyrolysis with 5.0% Fe2O3 was considerably increased (0.58) with significant removal of aromatics, resins and asphaltenes. The higher water-holding capacity of soil may be beneficial to healthy plant growth. Soil organic carbon (SOC), as an essential resource for heterotrophic life, is an important indicator of soil quality, while dissolved organic carbon (DOC) is an important constituent of SOC. After pyrolysis without Fe2O3, the

Table 2 Soil properties of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. Sample

pH

Hydrophobicity

Water-holding capacity (g water/ g soil)

SOC (g/kg)

DOC (mg/L)

CEC cmol/kg

Sand (%)

Silt (%)

Clay (%)

Untreated soil Soil pyrolyzed without Fe2O3 Soil pyrolyzed with 5.0% Fe2O3

8.0 ± 0.2 8.7 ± 0.1 9.5 ± 0.2

Class 7: very hydrophobic Class 7: very hydrophobic Class 7: very hydrophobic

0.39 ± 0.02 0.43 ± 0.02 0.58 ± 0.02

117.7 ± 2.4 62.4 ± 2.8 47.0 ± 0.3

25.0 ± 1.3 3.6 ± 0.3 1.4 ± 0.2

5.7 5.6 4.9

94.8 79.3 76.4

4.6 18.0 19.3

0.6 2.7 4.3

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Fig. 5. SEM-EDX images of the (a) untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolysis with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min.

soil particles before and after pyrolysis, XPS analysis was employed. The full energy spectra of the XPS provided the main elemental components of the soil surface, as shown in Fig. 6. The XPS results (Table S3) revealed that C was the dominant chemical element of the three soils in the top surface layer with percentages of 78.3%, 65.1%, and 61.0% in the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3, respectively. This decreasing trend in C was closely related to the removal of petroleum contaminants, which resulted in the increase in the amounts of O, Si, and Al elements in the top surface layer of the soil (Vidonish et al., 2018). The inset in Fig. 6 expands the XPS C1s spectra wavelength scale at 270–300 nm of (the major structural elements in the soil surface) (Fig. 6), and shows different levels of chemical shifts in the three soil samples (0.65 eV, 0.50 eV, and 0.15 eV for the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5% Fe2O3, respectively), thereby implying that the chemical bonding of the C atoms was changed, and many types of C were produced. Fig. 7 XPS C1s spectra of (a) the untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. Therefore, to corroborate the chemical state variances of C1s in the surface of the soil particles before and after pyrolysis, XPS-peak-differentiation-imitating analysis was conducted. Fig. 7 shows the potential effectiveness of this method, and the simulated results are listed in Table 3. Six peaks located at 284.8, 285.7, 286.9, 288.3, 289.6 and 291.9 eV belonging to CeC, CeN, CeO, C]O, O = CeO, and π-π (Liu

They became slightly rough and dark after the pyrolysis without Fe2O3 at 400 °C for 30 min, and darker and rougher when 5.0% Fe2O3 was added during pyrolysis. This phenomenon indicated that the components of the soil particle surface changed after pyrolysis. We also found many small holes on the surface of the soil pyrolyzed with 5.0% Fe2O3, which may have been why the soil water-holding capacity was significantly increased after pyrolysis (Table 2). The EDX results showed that the richest element in the petroleum-contaminated soil was C (38.5%), followed by O (28.1%), and the weight percentage of Fe was 3.1%. After pyrolysis without Fe2O3, the weight percentage of C decreased to 21.3% with the removal of heavy hydrocarbons, while the weight percentage of O increased to 37.1%. As 5% Fe2O3 was added during pyrolysis, the weight percentages of C and O decreased to 10.7% and 12.8%, respectively, with the significant removal of aromatics, resins and asphaltenes. As expected, owing to a thin layer of Fe2O3 coated on the soil surface, Fe showed the highest weight percentage of 63.1%. Fig. 5 SEM-EDX images of the (a) untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolysis with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. Fig. 6 XPS spectra of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. The inset shows the expanded C1s spectra wavelength scale at 270–300 nm. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. To further explore the variation in components in the surface of the

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Fig. 6. XPS spectra of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. The inset shows the expanded C1s spectra wavelength scale at 270–300 nm. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min.

et al., 2019; Vander Wal et al., 2011; Velasco et al., 2019; Qian et al., 2014; Zhu et al., 2019), respectively, were observed in the petroleumcontaminated soil surface. We found that the richest group was C]O, which accounted for 50.0% (Table 3). This high percentage of C bound to O may have been caused by biodegradation and photodegradation through weathering in the soil environment (Lemkau et al., 2014; Aeppli et al., 2012). After pyrolysis without Fe2O3, the groups of CeC and O = CeO in the soil surface were disappeared, and the percentages of C]O and π-π were slightly decreased with the removal of heavy hydrocarbons. However, the percentages of groups of CeO and CeN clearly increased. Generally, the heteroatoms of O and N in hydrocarbons are closely related to heavy hydrocarbons with large molecules, such as aromatics, resins, and asphaltenes. Therefore, a significantly high removal efficiency of saturates and comparatively low removal efficiency of aromatics, resins, and asphaltenes (Fig. 3) may have contributed to this phenomenon. Meanwhile, a group of graphitic C (Li et al., 2018; Qian et al., 2014) was found in the soil surface, which resulted in the slightly dark and rough surfaces of the soil particles (Fig. 5(b)). As 5% Fe2O3 was added during pyrolysis, the groups of CeO and π-π disappeared, and the percentages of groups of CeN and C]O also decreased significantly with the significant removal of aromatics, resins, and asphaltenes (Fig. 3). This result was consistent with the EDX result that the weight percentages of O and N were decreased significantly. Meanwhile, the percentage of graphic C was significantly increased to 72.0%, which resulted in the surface of the soil becoming darker and rougher (Fig. 5(c)). It has been reported that heavy hydrocarbons, including resins and asphaltenes, can be transformed into chars with a hollow structure during the pyrolysis process (Mahapatra et al., 2015; Gray, 2003). The deposition of such graphitic compounds (insoluble char) on pyrolytic soil is likely to improve the transportation of plant-available water and the hydraulic conductivity of soil

Table 3 XPS C1s spectra of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time = 30 min, and N2 flow rate of 1 L/min. Samples

Peak Position (eV)

Group

Area

Percentage (%)

Untreated soil

284.8 285.7 286.9 288.3 289.6 291.9 284.8 285.6 287.0 287.8 291.9 284.8 285.6 289.6

CeC CeN CeO C]O O]CeO π-π Graphitic C CeN CeO C]O π-π Graphitic C CeN C]O

18 175 16 275 6 375 50 075 8 675 675 20 594 31 985 1 685 34 715 425 50 774 18 681 1 041

18.1 16.2 6.4 50.0 8.7 0.6 23.0 35.8 18.9 38.8 0.5 72.0 26.5 1.5

Soil pyrolyzed without Fe2O3

Soil pyrolyzed with 5.0% Fe2O3

(Amonette and Joseph, 2009; Kinney et al., 2012). As shown in Table 4, the percentages of TPH in the petroleum-contaminated soil converted to GC-FID detectable (Text S2) and insoluble char were 47% and 23%, respectively, after pyrolysis without Fe2O3 at 400 °C for 30 min. As 5% Fe2O3 was added during the pyrolysis, the percentages of GC-FID detectable TPH and insoluble char were increased to 55% and 41%, respectively. Table 3 XPS C1s spectra of the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time = 30 min, and N2 flow rate of 1 L/min.

Fig. 7. XPS C1s spectra of (a) the untreated soil, (b) soil pyrolyzed without Fe2O3, and (c) soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. 8

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Table 4 The percentages of retained extractable TPH, GC-FID detectable TPH, and char in the soil pyrolyzed without Fe2O3 and soil pyrolyzed with 5% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. Samples

Extractable TPH (%)

GC-FID detectable (%)

Char (%)

Soil pyrolyzed without Fe2O3 Soil pyrolyzed with 5% Fe2O3

30 ± 1 4±1

47 ± 2 55 ± 1

23 ± 2 41 ± 3

Table 5 Wheat seedling germination rates and plant properties after 21 d of growth in the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min. Samples

TPH (mg/kg)

Germination rates (%)

Root length (cm)

Shoot length (cm)

21 d wet weight (mg)

21 d dry weight (mg)

Clean soil Untreated soil Soil pyrolyzed without Fe2O3 Soil pyrolyzed with 5.0% Fe2O3

– 119 ± 5 35 ± 2 5±1

68 28 61 76

6.0 5.6 6.7 7.5

19.4 16.7 19.5 21.2

213.5 176.0 195.8 235.1

42.8 32.3 39.8 46.1

± ± ± ±

5 2 7 6

± ± ± ±

1.7 1.2 1.7 1.6

± ± ± ±

4.0 2.5 5.0 4.2

± ± ± ±

6.0 2.5 8.1 7.9

± ± ± ±

4.7 4.6 7.5 7.6

thin film of graphitic C material deposited on the soil surface may have improved the water and nutrient availability in the soil, thereby improving the growth of wheat. Table 5 Wheat seedling germination rates and plant properties after 21 d of growth in the untreated soil, soil pyrolyzed without Fe2O3, and soil pyrolyzed with 5.0% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/min.

Table 4 The percentages of retained extractable TPH, GC-FID detectable TPH, and char in the soil pyrolyzed without Fe2O3 and soil pyrolyzed with 5% Fe2O3. Experimental conditions: pyrolysis temperature of 400 °C, residence time of 30 min, and N2 flow rate of 1 L/ min.

3.7. Variation in soil ecological function induced by catalytic pyrolysis In order to investigate the effect of the variation in soil physicochemical properties induced by Fe2O3 catalytic pyrolysis on soil ecological function, we conducted wheat germination and growth experiments in this study. As shown in Table 5, the germination rate of wheat in the untreated soil was significantly lower than that in the clean and pyrolyzed soils, thereby suggesting that the germination of wheat was highly impacted by petroleum contaminants. This result may have been caused by the dissolved toxic compounds and by the thin oil film formed on the top soil. These behaviors resulted in oxygen deficiency, inner dehydration, damaged biochemical function, and embryo death of wheat seeds (Tang et al., 2011). However, in our study, the germination rate of wheat in untreated soil was still higher than that of other studies in similar concentrations of petroleum-contaminated soil (Chaîneau et al., 1997). Lighter hydrocarbon contaminants with high phytotoxicity were degraded by the indigenous soil microorganisms in the weathered petroleum-contaminated soil, which may have contributed to the higher germination rate of wheat in our study (Khan et al., 2018; Mackinnon and Duncan, 2013; Siddiqui et al., 2001). With the removal of petroleum contaminants, the germination rate of wheat increased to 61% in the soil pyrolyzed without Fe2O3, thereby indicating that the inhibition of wheat germination was weakened. The germination rate of wheat in the soil pyrolyzed with 5.0% Fe2O3 was even higher than that in the clean soil. As expected, wheat plants grown in the untreated soil showed the lowest average values for root length, shoot length, wet weight, and dry weight at only 93%, 86%, 82%, and 76% of those in clean soil, respectively (Table 5); the wheat plants grown in different soils (2 d and 9 d, respectively) are shown in Fig. S2. It was found that wheat plants grew better when petroleum contaminants were removed by pyrolysis. However, compared with the soil pyrolyzed without Fe2O3, the average values of root length, shoot length, wet weight, and dry weight derived from the soil pyrolyzed with 5% Fe2O3 further increased to 123.5%, 109.1%, 110.2%, and 107.9% of those in the clean soil, respectively (Table 5). These results suggested that the changes in soil physicochemical properties induced by Fe2O3 were beneficial to wheat growth. Two possible reasons may been attributed to this result, namely 1) the concentrations of residual SOC and DOC in the soil pyrolyzed with 5.0% Fe2O3 provided necessary nutrients for wheat plant growth and 2) a

4. Conclusions This study investigated the feasibility of Fe2O3 facilitated catalytic pyrolysis of weathered petroleum-contaminated soil. Compared with the soil pyrolyzed without Fe2O3 at 400 °C for 30 min, the residual TPH concentration in soil pyrolyzed with 5.0% Fe2O3 was reduced by 25.5% (below the regulatory threshold of 10 000 mg/kg), and the residual concentrations of aromatics, resins, and asphaltenes were reduced by 66.8%, 52.3%, and 67.9%, respectively. After pyrolysis with 5.0% Fe2O3, the water-holding capacity of soil was considerably increased and the soil became darker and rougher. The SEM analysis showed that many small holes occurred on the surface of pyrolytic soil particles. The XPS analysis showed that a thin layer of graphitic C was formed and deposited on the surface of the pyrolytic soil. We also observed that the wheat germination percentage and biomass yield in the soil pyrolyzed with 5.0% Fe2O3 were even better than those in the clean soil. These results clearly suggested that Fe2O3 facilitated catalytic pyrolysis is a feasible and cost-effective method to remediate weathered petroleumcontaminated soil. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grant No.41807139) and the National Key Research and Development Program (Grant No. 2018YFC1802101). The authors would like to thank the anonymous reviewers for their constructive and valuable comments. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121165. References Aeppli, C., Carmichael, C.A., Nelson, R.K., Lemkau, K.L., Graham, W.M., Redmond, M.C., Valentine, D.L., Reddy, C.M., 2012. Oil weathering after the deepwater horizon disaster led to the formation of oxygenated residues. Environ. Sci. Technol. 46 (16),

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