Characteristics, sources, and in situ phytoremediation of polycyclic aromatic hydrocarbon in rural dumpsites
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Without precaution to deal with gas emissions and leachate generation, dumpsites have become a severe environmental problem in many developing countries. The objectives of this study were to investigate the pollution status of polycyclic aromatic hydrocarbons (PAHs) in dumpsite soil in rural areas of China and to verify phytoremediation effectiveness with Sedum alfredii Hance and alfalfa (Medicago sativa L.) under complex pollution conditions in PAH-contaminated soil.
Materials and methods
In this study, we collected soil cores from four dumpsites in rural areas of North China (Hebei Province) for analysis, and correspondingly conducted an in situ phytoremediation experiment using Sedum alfredii Hance and alfalfa (Medicago sativa L.) at one of these sites, monitoring the total PAH concentration in soil.
Results and discussion
Results showed generally moderate pollution by PAHs in soil samples from dumpsites with pockets of heavy pollution. PAH concentrations in dumpsite soil ranged from 827 to 1101 ng/g (dry weight). High-molecular-weight PAHs were present in higher proportions at oldest dumpsite in operation. Certain molecular ratios of PAHs can be used to diagnose the source of PAHs in soil, and it indicated that the main sources were combustion of domestic coal and biomass, as well as the automobile exhaust and kitchen exhaust. A 17-month in situ phytoremediation experiment resulted in the effective removal of PAHs in the Sedum alfredii and alfalfa plots, with total PAH concentrations decreasing by 82.4% and 81.3%, respectively. Furthermore, PAH concentrations in plants correlated to plant growth conditions.
This study indicated that the soils of the dumpsites were generally moderately polluted by PAHs, and some parts of the area were heavily polluted. Both Sedum alfredii and alfalfa absorbed PAHs from soil, and PAH concentrations in these two plants correlated to the growth conditions of the plants. Phytoremediation can effectively be used for PAH removal in open dumpsites.
KeywordsContaminated soil In situ phytoremediation PAH sources Polycyclic aromatic hydrocarbon Rural dumpsite
Dumpsites are open lands where solid waste is dumped without precaution to deal with gas emissions and leachate generation, which have become a serious environmental problem in many developing countries (Nagendran et al. 2006; Zarate et al. 2008; Chakraborty et al. 2018a; Padilla and Trujillo 2018; Peter et al. 2018). In China, with the ever-accelerating commercialization and the continuous improvement in the living standards of rural residents, the quantity of solid waste continues to increase in rural areas (Zeng et al. 2016; Wang et al. 2018). Solid waste generation from villages and towns in China has reached 210 million tons per year, and the annual growth rate of per capita of rural residents is 8–10% (Wang et al. 2018). However, in China’s suburban areas and rural areas, a well-defined system for the sorting, collection, and disposal of solid waste does not yet established, and only less than 30% of rural solid waste was harmlessly disposed (He et al. 2014; Zeng et al. 2015). Consequently, a large amount of waste is randomly dumped, resulting in a large number of rural dumpsites (Li’ao et al. 2009; Huang et al. 2013). Over 80% of the dumpsites were within 100 m of residential areas (Wang et al. 2016). With poor precaution and occasional open burning of waste solid, it would be a significant threat to the surrounding environment (Prechthai et al. 2008; Chakraborty et al. 2018a; Chakraborty et al. 2018b). This is not only a source of visual pollution; it could also result in severe soil contamination through the seepage of leachates into soil and the surrounding environment by means of rainfall and surface runoff.
Leachate poses a serious threat to the surrounding ecosystem, it contains a large number of hazardous compounds, such as polycyclic aromatic hydrocarbons (PAH) and heavy metals, as well as high ammonia nitrogen (NH3–N) concentrations, and it often percolates down and contaminates groundwater and soil and even runs as a stream into the surrounding water bodies (Motorykin et al. 2013; He et al. 2015; Xu et al. 2015; Yang et al. 2017). The PAHs are ubiquitous in waste and leachate originating from various household materials, such as batteries, used paints, polyvinyl chloride (PVC) products, oils, solvents, pesticides, treated wood scraps, discarded electronic equipment, and construction and demolition debris (Chrysikou et al. 2008), and they could be formed during incomplete combustion of organic material, vehicle emissions, and industrial processes (Wei et al. 2014; Wang et al. 2015). Due to their resistance to biodegradation, bioaccumulation potential, and carcinogenicity, PAHs have caused significant environmental concern.
Under conditions of dispersed rural solid waste (RSW) sources, flawed legislation, and poor infrastructure in rural areas, the aim of this study was to propose an economically viable and manageable method to remediate soil pollution caused by the accumulation of PAHs in rural dumpsites in China.
Phytoremediation is a convenient and economic operation, which could remediate the polluted soil in situ (Nagendran et al. 2006; Luo et al. 2017). It does not interfere with the ecosystem but confers to the treated land an added esthetic value through the plant cover (Cristaldi et al. 2017). It can also reduce secondary pollution, restore degraded land to its original state, and be easily scaled up to include larger areas. There have been studies on phytoremediation of soil with heavy metal and organic pollutants, such as soil in smelting, mining areas, sewage irrigation areas, and soil with petroleum, as well as soil in e-waste disassembly areas (Cook and Hesterberg 2013; Liao et al. 2016; Petrová et al. 2017). Compared with the previous studies, soil in the dumpsites contains more complex potential pollutants, such as PAHs and heavy metals, as well as high ammonia nitrogen (NH3–N) concentrations. In the process of phytoremediation, the toxic effects of various pollutants on plants could be superimposed, synergistically or antagonistically, which makes the phytoremediation process ambiguous.
In this study, we investigated PAH compounds and concentrations in rural dumpsites in North China, while also conducting an in situ phytoremediation experiment (Hebei Province, North China). This study was conducted (1) to investigate the concentration and composition of PAHs in rural dumpsites; (2) to evaluate the degree of pollution and potential local sources of PAHs; (3) to conduct a phytoremediation experiment at a local dumpsite using Sedum alfredii Hance and alfalfa (Medicago sativa L.) to verify their remediation effectiveness under complex pollution conditions in PAH contaminated soil.
2 Materials and methods
2.1 Study areas
The study area is located in Hebei Province, North China. The climate is characteristic of a temperate continental monsoon climate, and the soil type is sandy loam.
Four typical dumpsites (designated S1, S2, S3, S4) of different size, age, and position and one background site (CK) were chosen to examine and describe pollution characteristics. The surrounding villagers have been dumping household waste on these dumpsites for decades and have burned waste aperiodically and illegally. No safeguards or measures were taken to protect the soil from contaminants produced by waste or combustion. Samples collected from a poplar forest, where was less affected by vehicle emissions and had no waste there, could therefore be regarded as CK. S1 is a relatively new dumpsite that has only been in use for approximately 1 year, while S2 and S3 have been in use for approximately 3–4 years. S4 is an abandoned dry river channel that has functioned as a dumpsite for greater than 7 years, and it is covered by numerous types of waste material. S2 was chosen as the site to conduct the in situ phytoremediation experiment.
2.2 Soil and plant sampling and preparation
Physicochemical properties of test soil
Value of S
Value of CK
Total organic carbon (g/kg)
30.63 ± 4.21
4.47 ± 0.54
Total nitrogen (g/kg)
2.52 ± 0.27
0.66 ± 0.15
Total phosphorus (g/kg)
0.91 ± 0.07
0.40 ± 0.02
17.92 ± 4.34
8.25 ± 0.37
87.16 ± 22.69
41.78 ± 0.37
0.74 ± 0.23
0.11 ± 0.04
121.83 ± 25.39
16.56 ± 0.81
198.62 ± 57.02
35.51 ± 8.82
85.72 ± 16.03
26.51 ± 11.79
For the in situ phytoremediation site (S2), samples were collected once every other month or two in order to trace changes in pollutants. The soil samples were freeze-dried, grinded in a mortar, and subsequently sieved through a 2-mm mesh. Each plant sample (roots and shoots) was a composite of three plants from the same plot. Roots were washed with distilled water to remove any residual soil particles. Plant samples were separately cut into small pieces, homogenized, dried at 35 °C in an oven for 7 days, and then weighed and pulverized into fine powder using a cutting mill (Retsch SM100, Germany). Each soil and plant sample was divided into two technical replicates for PAH analysis.
2.3 Chemicals and reagents
The standard PAH solution consisted of a mixture of 16 priority PAHs at a concentration of 100 μg/ml (AccuStandard Inc., USA). This mixture was prepared in methanol/dichloromethane (v/v = 1:1). In addition, a mixture of six perdeuterated PAHs (i.e., naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, perylene-d12, and benzo[g,h,i]perylene-d12) was used as a surrogate standard, and 2-fluorobiphenyl was used as an internal PAH standard. Silica gel (100–200 mesh, Sinopharm Chemical Reagent Co., Ltd., China) and anhydrous sodium sulfate (Xilong Chemical Co., Ltd., China) were activated at 450 °C for 4 h. Dichloromethane, n-hexane, and acetone were all of HPLC grade (J.T. Baker Inc., USA).
2.4 Polycyclic aromatic hydrocarbon sample extraction and analysis
The samples were freeze-dried, and after the removal of stones, wastes, and residual roots, they were homogenized using a mortar and pestle prior to being sieved (0.149 mm).
The PAHs in soil were extracted in duplicate using an accelerated solvent extractor (ASE 300, Dionex Corporation, USA), and a mixture of dichloromethane/n-hexane (v/v = 1:1) was used as the extracting solvent. Briefly, 10 g of soil was extracted twice under the following conditions: oven temperature, 100 °C; pressure, 1500 psi; pre-heating period, 5 min; static time, 5 min; flush volume, 60% of the extraction cell volume; nitrogen purge, 100 s.
The extracts were concentrated to 1 ml by rotary vacuum evaporation at 25 °C. Concentrated extracts were cleaned (adsorbed) using the silica gel column chromatography method (25 cm × 1 cm internal diameter). The glass chromatography column, fitted with a Teflon stopcock, was packed from the bottom with absorbent cotton, 5 g of silica gel, 6 cm of alumina, and 1 cm of anhydrous sodium sulfate. The fraction containing PAHs was eluted with 15 ml n-hexane and then 30 ml n-hexane/dichloromethane (v/v = 7:3). Following this step, the fraction was concentrated to 2 ml using a rotary evaporator and further concentrated to 1 ml under a gentle stream of nitrogen. This sample was then transferred to a GC vial. Finally, 100 μl of the internal standard was added to the extract prior to quantitative analysis.
The PAHs were analyzed using a Varian 3800 gas chromatography-4000 ion trap mass spectrometry (GC/MS) system fitted with a DB-5 capillary column (30-m length, 0.25-mm id, and 0.25-μm film thickness) and an 8400 autosampler injector (1 μl). The injection port temperature was maintained at 280 °C (Song et al. 2015). The GC oven temperature was kept at 80 °C for 3 min, increased to 230 °C at a rate of 12 °C/min, and kept at 230 °C for 2 min. The temperature was then increased to 280 °C at 2 °C/min and kept for 5 min. PAH identification was based on the selected ions and the relative retention time between the samples and the standard solution containing the individual PAHs.
The correlation coefficients for PAH calibration curves using GC/MS were all higher than 0.99. The recovery of 2-fluorobiphenyl, which was used as the recovery standard, ranged from 72 to 94% in the real soil samples.
2.5 In situ phytoremediation
The Sedum alfredii plot was denoted Plot-A, and the alfalfa plot was denoted Plot-B. The duration of the in situ phytoremediation experiment was 17 months, from November 2013 to April 2015. The plants were grown in natural agro-climatic conditions, with neither fertilization nor optimum irrigation. Soil samples surrounding plants were collected to determine PAH concentrations at the start of plot construction (November 2013) and then collected semimonthly, plant samples were collected at the beginning of plant growth (June 2014) and after harvest. Sampling and detection methods used are provided in Sections 2.2, 2.3, and 2.4.
3 Results and discussion
3.1 Pollution status of polycyclic aromatic hydrocarbons in dumpsite soil
The following sections describe PAH concentration and composition as well as identifying their sources.
3.1.1 Concentration of polycyclic aromatic hydrocarbons
According to the classification criteria for PAH pollution of soil in the Netherlands recommended by MaliszewskaKordybach, soil PAH concentrations less than 200 ng/g are not considered to be contaminated, Ʃ16PAHs between 200 and 600 ng/g indicates weak contamination, a concentration between 600 and 1000 ng/g indicates moderate contamination, and a concentration over 1000 ng/g indicates heavy contamination (MaliszewskaKordybach 1996). According to this classification, CK is considered uncontaminated, and the S-series dumpsites are considered contaminated. In this study, soil PAHs in dumpsites generally reached moderate pollution levels (S1~S4) with some pockets of heavy pollution (S1, S2).
3.1.2 Composition and source identification of polycyclic aromatic hydrocarbons in dumpsites
Characteristic values of selected molecular ratio for pyrolytic and petrogenic origins of PAHs
Ant/Ant + Phe
Fla/Fla + Pyr
BaA/BaA + Chr
IcdP/(IcdP + BghiP)
Nearby residents typically discard household coal ash, kitchen waste, and corn straw at these dumpsites, and the waste is frequently burned, which could make a contribution to PAHs in dumpsites. Moreover, by being close to residential areas and roads, automobile exhaust and kitchen exhaust can also be the sources of PAHs in dumpsites. Compared to LMW PAHs, HMW PAHs, which are more toxic and carcinogenic, tend to accumulate in soil through dry and wet deposition of ambient particles, and will remain in the soil under absorption, adsorption, and sequestration processes. Compared to the other plots, S4 was older and therefore experienced more cycles of incomplete combustion and vehicle emissions. Therefore, S4 had a greater proportion of HMW PAHs, and the total concentration of HMW PAHs was approximately greater by a factor of two compared to LMW PAHs.
3.2 In situ phytoremediation effects of Sedum alfredii and alfalfa
3.2.1 Changes in the total concentration of the 16 PAHs in dumpsite soil
Concentrations of two–three-ring PAHs generally exhibited a decreasing trend, but an increase was observed in initial concentrations throughout remediation. This was because HMW PAHs during the initial stage of the phytoremediation process degraded to LMW aromatic hydrocarbons, after which monocyclic benzene rings degraded into other substances. During the latter stage of remediation, LMW PAH concentrations decreased, and their proportion decreased in the total PAH concentration, indicating that the degradation of HMW PAHs in phytoremediation was more difficult.
In November, four–six-ring PAH concentrations significantly increased in both Plot-A and Plot-B. Taking into account that residents near the testing sites burn coal and other fuels in winter, these pyrolytic sources released a certain amount of PAHs. After migration and atmospheric sedimentation, PAHs were deposited back into the soil. However, lower temperatures in winter would make it more difficult for PAHs to evaporate.
3.2.2 Effects of plant absorption
PAH concentrations in Sedum alfredii and alfalfa decreased gradually from June 2014 to November 2014 (Fig. 7), while a same tendency occurred in soil, suggesting that concentration of pollutants in plants had positive correlation on concentration in soil (Sun et al. 2011). In April 2015, with warmer temperatures and sufficient and stable hydrothermal conditions, biomass of plants increased quickly. Meanwhile, there were sharp increases in both plans, suggesting that PAH concentrations in plants were correlated to the biomass.
Comparing the two remediation plants, removal effects of PAHs were high and very close. Meanwhile, the variation trend of PAHs concentration in two plants was similar. The close values and similar trends indicated that both Sedum alfredii and alfalfa could effectively be used for PAH removal in open dumpsites.
The initial content of the 16 PAHs in rural dumpsite soil was 827–1101 ng/g, the soil of rural dumpsites in North China were generally moderately polluted by PAHs, while pockets of soil were heavily polluted. The average PAH concentration in contaminated soil was greater by a factor of 33.8 compared to the background site.
The PAHs in dumpsite soil are mainly derived from pyrolytic sources. The main sources of PAHs are residential coal combustion, biomass burning (especially corn straw), automobile exhaust, and kitchen exhaust. There was a higher proportion of HMW PAHs in the oldest dumpsite in operation.
The 17-month in situ phytoremediation experiment effectively removed PAHs in both the Sedum alfredii and alfalfa plots (Plot-A and Plot-B), and total PAH concentrations decreased by 82.4% and 81.3%, respectively, which is much higher than natural degradation processes. Both Sedum alfredii and alfalfa absorbed PAHs from soil, and PAH concentrations in these two plants correlated to the growth conditions of the plants.
In conclusion, Sedum alfredii and alfalfa proved to be both effective in removing PAHs from soil of rural dumpsites in North China and cost effective, as well as easily deployable and convenient to manage; therefore, phytoremediation should be promoted as a means to improve the worldwide problem of PAHs pollution over vast rural areas.
This work was supported by the National Key R&D Program of China (2017YFA0605003), the Key Laboratory for Solid Waste Management and Environment Safety (Tsinghua University, SWMES2015-12), and the National Science and Technology Support Program (2012BAJ21B03-01).
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