Evaluation of oil removal efficiency and enzymatic activity in some fungal strains for bioremediation of petroleum-polluted soils
© Mohsenzadeh et al.; licensee BioMed Central Ltd. 2012
Received: 24 July 2012
Accepted: 8 December 2012
Published: 15 December 2012
Petroleum pollution is a global disaster and there are several soil cleaning methods including bioremediation.
In a field study, fugal strains were isolated from oil-contaminated sites of Arak refinery (Iran) and their growth ability was checked in potato dextrose agar (PDA) media containing 0-10% v/v crude oil, the activity of three enzymes (Catalase, Peroxidase and Phenol Oxidase) was evaluated in the fungal colonies and bioremediation ability of the fungi was checked in the experimental pots containing 3 kg sterilized soil and different concentrations of petroleum (0-10% w/w).
Four fungal strains, Acromonium sp., Alternaria sp., Aspergillus terreus and Penicillium sp., were selected as the most resistant ones. They were able to growth in the subjected concentrations and Alternaria sp. showed the highest growth ability in the petroleum containing media. The enzyme assay showed that the enzymatic activity was increased in the oil-contaminated media. Bioremediation results showed that the studied fungi were able to decrease petroleum pollution. The highest petroleum removing efficiency of Aspergillus terreus, Penicillium sp., Alternaria sp. and Acromonium sp. was evaluated in the 10%, 8%, 8% and 2% petroleum pollution respectively.
Fungi are important microorganisms in decreasing of petroleum pollution. They have bioremediation potency that is related to their enzymatic activities.
KeywordsEnzymatic activity Fungi Petroleum removing Soil pollution
Petroleum pollution is a global disaster that is a common phenomenon in the oil-bearing and industrial regions . Petroleum pollution of environments is dangerous for plants, animals and people [2, 3]. Iran, as an oiled country, contained a lot of petroleum polluted-environments and the pollutions are increasing in recent years .
There are several soil cleaning methods including burning, washing, chemical applying and bioremediation . Bioremediation is using of plants and microorganisms to remove or detoxify environmental contaminants. Bioremediation has been intensively studied over the past two decades, driven by the need for a low-cost, sustainable with natural environment, and in-situ alternative to more expensive engineering-based remediation technologies [1, 6, 7]. Bioremediation has been applied to remove crude oil [8–11], motor oil , and diesel fuel  from soil but the removal efficiency is highly variable .
Bioremediation of petroleum-polluted media were done using plants or plant-associated micro flora [15, 16]. There are different economically and environmentally important uses for microorganisms, such as remediation and rehabilitation of petroleum contaminated soils [11, 17–22].
Bioremediation of petroleum-contaminated soils is mainly based on biodegradation by the fungal strains that are present in the associated with plants or in the soils of petroleum polluted sites . Some prior researchers reported that some fungal species are resistant to petroleum-pollution and they are capable to remove soil pollution. The results of Ulfig et al.  indicated that keratinolytic fungi, especially Trichophyton ajelloi, is a potential tool for assessment of soil petroleum hydrocarbon contamination and associated bioremediation progress. Fungal strains namely Alternaria alternate, Aspergillus flavus, Curvularia lunata, Fusarium solani, Mucor racemosum, Penicillium notatum and Ulocladium atrum were isolated from the soils in the petroleum polluted areas in Saudi Arabi . Eggen and Majcherczykb  showed that white rot fungus, Pleurotus ostreatus, was able to remove polycyclic aromatic hydrocarbons (PAH) from contaminated soil. Little attention has been paid to the role of fungal species in the environmental biotechnology and bioremediation of petroleum pollution, specially in Middle Eastern region [18, 25]. Some fungal strains including Alternaria alternate, Aspergillus flavus, Curvularia lunata, Fusarium solani, Mucor racemosum, Penicillium notatum and Ulocladium atrum were isolated from the soils in the petroleum-polluted areas in Iran . The aim of this research was to collect fungal strains from petroleum-polluted soils of Arak refinery, evaluation of their ability in removing of petroleum pollution in experimental conditions and determination of their enzymatic activity during petroleum removing.
The studied area
The Arak oil refinery, located near the Arak city in the center of Iran was selected in this study. The city is located in the central part of Iran (34° 5' 8" North, 49° 41' 2" East) with elevation average about 1723 meters above sea level. The population of the city is 503673. Arak is the capital city of Markazi province and is mostly arid or semiarid, subtropical along Caspian coast. It rains most in winter and is moderately warm in summer. Its annual precipitation is 317.7 mm, mean annual temperature is 11.8°C and 46% humidity.
Arak oil refinery is located at 25 km far away from Arak city. Arak refinery is a relative new refinery with the production capacity of 22434 barrel in day that funded in 1992. Soil characters of the area was evaluated as sandy loam containing 80% sand, 12% loam, 6% sludge and 2% organic material with pH 6.8. Chemical composition of the used crude oil in the refinery is 13.4% saturated hydrocarbons, 40% aromatic hydrocarbons, 46.6% polar compounds (Refinery office data). Due the oil refining activities in this region, a high degree of petroleum pollution (5-10%) was reported in the refinery areas . The identification of soil contamination was also possible based on a visual examination of the soil.
Selection of fungal strains
Since the amounts of microorganisms in the around of plant roots are up to 200 times more than soil , root samples were harvested from the plants growing in the polluted area of Arak refinery, and sliced into segments with 1 cm length, washed and then dried. The samples were kept in Sodium hypo chloride 1% (30 sec) and then ethanol 70% (30 sec), for removing the peripherally attached microorganisms, and dried after washing with distilled water . The samples were kept in potato dextrose agar (PDA) media containing lactic acid. The Petri dishes were incubated in 25 ± 2°C for 4 days. Then, different fungal colony were isolated and cultured separately in PDA . Fungal specimens were examined under light microscope after preparations and identified using morphological characters and taxonomical keys provided in the mycological keys [26–28]. The specimens were also sent to the department of mycology in our university for confirmation of their scientific names.
Determination of the fungal growth ability under petroleum pollution
The growth assay was used to find the resistant fungal species to petroleum contamination of the soil. The assays were conducted by comparing the growth rates of fungal strains, as colony diameter, on the oil contaminated and control Petri dishes. Test dishes were prepared by adding crude oil to warm PDA solution. In order to have a uniform concentration of oil in all plates, the solution was thoroughly mixed with a magnetic stirrer, right before it was added to the plates. Different concentrations of oil/PDA mixture (2, 4, 6, 8 and 10% v/v) were prepared. Pure PDA was used in control plates. All dishes were incubated with 2 mm plugs of fungal mycelia taken from agar inoculums plate. The dishes were incubated at 25±2°C in an incubator. Fungal mycelia extension on the plates (colony diameter) was measured using with measuring tape after 7 days and compared with the control plates.
Evaluation of petroleum removing
The four fungal strains that showed the highest resistant and growth ability in the prior stage, were chosen for this study. They are common and native fungi that isolated from the studied petroleum polluted area. Ninety-six pots were selected for this study and divided in to four groups; each group containing 24 pots and used for each fungal strain. Each pot was filled with 3kg of sterile agricultural soil and mixed with 3g of the studied fungi. The experimental groups were as groups A, B, C, and D. Each group including one of the above-mentioned fungal strains and sub-groups are growing in the pots added different concentrations (0, 2, 4, 6, 8 and 10% w/w) of crude oil.
The pots were incubated in a greenhouse in the temperature of 25±2°C for three months. The soil of experimental and control pots were homogenized separately and were kept in 4°C at refrigerator until future study. Concentrations of crude oil (TOG%) were determined and compared in the soil of experimental and control pots.
Determination of Total Oil and Grease (TOG)
The soil samples from experimental and control pots were collected separately. Each sample, without fungal segments, was homogenized and stored at 4°C until further processing. TOG was analyzed according to the EPA method 9071 A and EPA Method 3540 B . Five gram of the soils in two replicates were acidified with hydrochloric acid to pH=2 and dehydrated with magnesium sulphate monohydrate. After 15 min, samples were transferred into paper extraction thimbles and placed into a Soxhlet type apparatus. TOG was extracted with dichloromethane for 8 h. The extract was filtered through filter paper (Whatman No. 4) with 1g sodium sulphate. The solvent was evaporated with a rotary evaporator and the weight of dry extract was determined. Percentage of TOG decreasing was calculated based on soil weight and compared in the experimental and control pots.
Determination of enzymatic activity
For the assays of Catalase (CAT) and Peroxidase (POX) enzymes, mycelia (200 mg) was homogenized in an ice-cooled mortar, grounded in 1 ml of 100 mM potassium phosphate buffer (pH 7.4) and centrifuged at 10,000 rpm for 10 min under cooling and the supernatant was used for enzyme assay. The activities of CAT and POX were determined according to Aebi . CAT activity was determined by measuring the decomposition of H2O2 and the decline in absorbance at 240 nm was followed for 3 min. The reaction mixture contained 50 mM phosphate buffer (pH 7.0), 15 mM H2O2, and 0.1 ml of enzyme extract, was used which started the reaction in 3 ml. The activity of POX evaluated by measuring the oxidation of guaiacol and the increase in absorbance at 470 nm was recorded for 3 min. The reaction mixture contained 50 μl of 20 mM guaiacol, 2.8 ml of 10 mM phosphate buffer (pH 7.0), and 0.1 ml enzyme extract. The reaction was started with 20 μl of 40 mM H2O2. The activity was defined as differences of optical density per min, for each mg of fresh weight of samples (Δ OD/ min/ mg FW).
The Phenol Oxidase (POD) activity was studied in the extracts of fungi growing in PDA media with different concentrations. The procedure adopted by Tate  and Theorell  was followed. Pure standard horseradish POD (SIGMA, USA) of RZ value 3.04 was used as standard. The POD activity was measured using guaiacol (1.11 mg/ml density) as chromogenic on spectrophotometer. The extract free of all cellular components was heated at 65°C for three minutes in a water bath and then cooled promptly by placing in ice bucket for inactivation of catalase activity.
In order to detect a significant difference between the experimental groups and control ones analysis of variance (ANOVA) followed by the least significant difference test (LSD) that was performed between studied groups . Each data was represented as the means ± SD of 5 samples for experimental groups and also 5 for control.
The fungi growing in the petroleum-polluted areas of Arak refinery were isolated and their growth ability was checked under petroleum pollution. Four fungal strains that showed the highest abundance in the polluted area and also the highest growth ability were chosen and identified by morphological characters and taxonomical keys. The results of the taxonomic determination for the fungi showed that the selected fungal species that present in the petroleum polluted soils are: Acromonium sp., Alternaria sp., Aspergillus terreus and Penicillium sp.
Fungal growth ability under petroleum pollution
For Alternaria sp. petroleum removing in the pots with 8% crude oil is the highest (55% decreasing) and the lowest decrease (50%) was in the pots with 2%. Finally, for Penicillium sp., the highest decreasing of petroleum was evaluated in the pots containing 8% petroleum (54% decreasing) and the lowest decrease was in the pots containing 2% petroleum (25%) (Figure 2). Based on the results, the all fungi are effective in petroleum removing from soil of the pots.
Different Enzyme activity (Unit/mg) in fungal strains under different petroleum concentrations
Results showed that the activity of peroxidase (POX) in the fungal strains growing in the petroleum-polluted media, was different with control ones. In Acromonium sp. (Table 1), POX activity was decreased with increasing of petroleum pollution and the highest activity was in non-polluted media. In Alternaria sp., POX activity was increased with increasing of petroleum pollution. The highest activity was observed in the group with 8% pollution but then decreased in the group containing 10% pollution. The lowest activity was evaluated in control group. In Aspergillus terreus, POX activity was increased with petroleum pollution straightly (Table 1). So, the highest activity was in 10% pollution and the lowest one was in non-polluted group. In Penicillium sp. the highest activity was in the group containing 8% petroleum pollution and the activity of POX in the groups with 6 and 10% was also higher than non-polluted group but in the groups with 2 and 4% pollution are near to control ones (Table 1).
Phenol oxidase (POD) activity was compared in the fungi growing in petroleum-polluted and control media. Results showed that in Acromonium sp. the highest activity was in the groups treated whit 2 and 4% petroleum. In other experimental groups its activity was similar with control ones (Table 1). In Alternaria sp., POD activity was increased with increasing the petroleum pollution until 8%, but it was decreased slightly in the group growing in media with 10% petroleum pollution (Table 1). In Aspergilus terreus, POD activity was increased with increasing of petroleum pollution and the highest activity was determined in the group growing in media with 10% petroleum pollution. Finally, for Penicillium sp., the highest POD activity was evaluated in the group growing in the media containing 8% petroleum pollution and the lowest activity was in the group growing in the non-polluted media (Table 1).
Study on the fungal species showed that Acromonium sp., Alternaria sp., Aspergillus terreus and Penicillium sp. were the common fungi, with high frequency in the petroleum polluted areas. It seems that petroleum pollution could not inhibit the growth and variation of fungal strains in petroleum polluted areas. It seems that the fungal species used oil compounds as nutrients and petroleum pollution cause to increase fungal growth. The similar results were reported by some researchers [11, 16–21]. Penicillium oxalicum was also isolated from petroleum-polluted soils and reported as degradability potential microorganism for bioremediation of crude oil .
The In vitro growth test of the isolated fungi showed a species-specific response. All of the studied fungal strains were able to growth in 2% v/v oil pollution and therefore could be useful for the remediation of light soil pollution. Although the growth of fungal species were reduced by increasing oil concentrations (more than 4% v/v), but all of them were still able to growth in the high concentrations of petroleum. They were produced sufficient colonies in the high-polluted media but with a lagging time. It seems that they could be used also for oil degradation in the soils with high pollution effectively. Our results are accordance with the some finding of other researchers about other different fungal species [11, 16–21].
Results of this research showed that the amounts of petroleum pollution were decreased in the presence of the studied fungal strains considerably. It means that the fungal strains were able to degrade crude oil and consumption of its components. Although there are several reports about the fungal ability in removing of petroleum and its derivers from the polluted soils [11, 16, 18, 20, 21], but this is the first report about the petroleum removing ability of the studied fungal strains. The results of our study proposed the above-mentioned fungi for using in remediation of petroleum-polluted environments in a field study. It means that the data of this study indicated that isolated fungi Acromonium sp., Alternaria sp., Aspergillus terreus and Penicillium sp. may have the potential for bioremediation of soil in highly polluted conditions especially in semi-dry regions.
Enzymatic assay indicated that activities of the all studied enzymes were increased with the increasing of petroleum pollution. In Acromonium sp., only the activity of catalase was decreased with the increasing of petroleum pollution. In other fungal strains, there is a sharp rise of enzymatic activity in the petroleum-polluted media. Penicillium sp. showed the highest catalase activity and the lowest one is observed in Aspergillus sp. For peroxidase and phenol oxidase, the highest activity was evaluated in Penicillium sp. and the lowest one was in Acromonium sp. Kotik et al.  reported that enzymatic activity of microorganisms were increased in the petroleum polluted soils and they were able to find hydrolase epoxide as a new enzyme that has major role in the degradation of crude oil. High activity of catalase and peroxidase was also reported in the soil microorganisms in petroleum-polluted soils [36, 37] that is accordance with our results. Tang et al.  applied ryegrass and effective microorganisms for bioremediation of petroleum polluted soils and increasing of enzymatic activity was observed in the soil microorganisms including Edwardsiella tarda, Bacterium aliphaticum, Bacillus megaterium, Bacillus cereus, Pseudomonas maltiphilia, Fusarium vertiaculloide, Botryodiphodia thiobroma, Fusiarum oxysporum, Cryptococcus neofomas, Aspergillus niger and Candida tropicalis. Ugochukwu et al.  reported that biochemical analysis revealed that except B. aliphaticum, which had high lipase activity, fungal isolates generally recorded higher lipase activities than bacterial isolates.
Based on the results of this study, it seems that fungal strains are the most effective organisms that are abundant in petroleum polluted soils and they were able to digest and remove petroleum compounds enzymatically. Based on our results they were able to grow in petroleum-polluted media effectively and Alternaria sp. is the most resistant fungal strain to high degree of petroleum pollution than others. Bioremediation tests with the fungal strains showed that they are effective in decreasing of petroleum pollution from environment and based on the results Alternaria sp. and Penicillium sp. were the most effective ones. This means that the fungal strains had bioremediation potency for petroleum-polluted media and their enzymatic activity have a major role in degradation of petroleum.
Our results showed that the studied fungi were able to growth in the subjected petroleum concentrations and Alternaria sp. showed the highest growth ability in the petroleum containing media. Results of bioremediation tests showed that the studied fungi were able to decrease petroleum pollution. The highest petroleum removing efficiency of Aspergillus terreus, Penicillium sp., Alternaria sp. and Acromonium sp. was evaluated in the 10%, 8%, 8% and 2% petroleum pollution respectively. Enzymatic activities were increased in the fungal colonies growing in the oil-polluted media; this means that the fungal enzymes have a critical role for petroleum degradation.
This research was done using a financial support provided by Islamic Azad University (Broujerd Branch) and also Bu-Ali Sina University partly.
- Merkel N, Schultez-Kraft R, Infante C: Phytoremediation of petroleum-contaminated soils in the tropics- preselection of plant species from eastern Venezuela. J Applied Bot Food Quality. 2004, 78: 185-192.Google Scholar
- Santodonato J, Howard P, Basu D: Health and ecological assessment of polynuclear aromatic hydrocarbons. J Environ Pathology Toxicol. 1981, 5: 351-364.Google Scholar
- Prasad MS, Kumari K: Toxicity of crude Oil to the survival of the fresh water fish puntius sophore (HAM.). Acta hydrochimica et hydrobiologica. 2006, 15: 26-36.Google Scholar
- Petroleum Ministry Data Center: Petroleum accidents in Iran. 2012, Available at: http://www.khabaronline.ir,Google Scholar
- Gallegos Martinez MG, Gomez Santos AG, Gonzalez Cruz LG, Montes De Oca Garcia MA, Yanez Trujillo LY, Zermeno Eguia Liz JA, Gutierrez-Rojas M: Diagnostic and resulting approaches to restore petroleum-contaminated soil in a Mexican tropical swamp. Water Sci Technol. 2000, 42: 377-384.Google Scholar
- Chehregani A, Malayeri B: Removal of heavy metals by native accumulator plants. Inter J Agri Biol Sci. 2007, 9: 462-465.Google Scholar
- Chehregani A, Noori M, Lari Yazdi H: Phytoremediation of heavy metal polluted soils: screening for new accumulator plants and evaluation of removal ability. Ecotoxicol Environ Safety. 2009, 72: 1349-1353.View ArticleGoogle Scholar
- Wiltse CC, Rooney WL, Chen Z, Schwab AP, Banks MK: Greenhouse evaluation of agronomic and crude oil-phytoremediation potential among alfalfa genotypes. J Environ Quality. 1998, 27: 169-173.View ArticleGoogle Scholar
- Radwan SS, Al-Awadhi H, Sorkhoh NA, El-Nemer IM: Rhizospheric hydrocarbon-utilizing microorganisms as potential contributors to phytoremediation for the oily Kuwait desert. Microbiol Res. 1998, 153: 247-251.View ArticleGoogle Scholar
- Merkel N, Schultez-Kraft R, Infante C: Assessment of tropical grasses and legumes for phytoremediation of petroleum-contaminated soils. Water Air Soil Pollut. 2005, 165: 235-242.Google Scholar
- Mohsenzadeh F, Nasseri S, Mesdaghinia A, Nabizadeh R, Zafari D, Khodakaramian G, Chehregani A: Phytoremediation of petroleum-polluted soils: application of polygonum aviculare and its root-associated (penetrated) fungal strains for bioremediation of petroleum-polluted soils. Ecotox Environ Saf. 2010, 73: 613-619.View ArticleGoogle Scholar
- Dominguez-Rosado E, Pichtel J: Phytoremediation of soil contaminated with used motor oil: II. Greenhouse studies. Environ Engine Sci. 2004, 21: 169-180.View ArticleGoogle Scholar
- Chaineau CH, More JL, Oudot J: Biodegradation of fuel oil hydrocarbons in the rhizosphere of maize. J Environ Quality. 2000, 29: 568-578.View ArticleGoogle Scholar
- Angehrn D, Gälli R, Zeyer J: Physicochemical characterization of residual mineral oil contaminants in bioremediated soil. Toxicol Environ Chem. 1998, 17: 268-276.View ArticleGoogle Scholar
- Cunningham SD, Anderson TA, Schwab PA, Hsu FC: Phytoremediation of soils contaminated with organic pollutants. Advanced in Agronomy. 1996, 56: 44-114.Google Scholar
- Mohsenzadeh F, Nasseri S, Mesdaghinia A, Nabizadeh R, Chehregani A, Zafari D: Identification of petroleum resistant plants and rhizospheral fungi for phytoremediation petroleum contaminated soils. J Japan Petrol Inst. 2009, 52: 198-204.View ArticleGoogle Scholar
- Eggen T, Majcherczykb A: Removal of polycyclic aromatic hydrocarbons (PAH) in contaminated soil by white rot fungus pleurotus ostreatus. Inter Biodeter Biodeg. 1998, 4: 111-l 17.View ArticleGoogle Scholar
- Yateem A, Balba MT, AI-Awadhi N: White rot fungi and their role in remediating oil-contaminated soil. Environ Inter. 1997, 24: 181-187.View ArticleGoogle Scholar
- Nicolotti G, Egli S: Soil contamination by crude oil: impact on the mycorhizosphere and on the revegetation potential of forest trees. Environ Pollution. 1998, 99: 37-43.View ArticleGoogle Scholar
- Obuekwe CO, Badrudeen AM, Al-Saleh E, Mulder JL: Growth and hydrocarbon degradation by three desert fungi under conditions of simultaneous temperature and salt stress. Intern. Biodeter Biodeg. 2005, 56: 197-206.View ArticleGoogle Scholar
- Dritsa V, Rigas F, Natsis K, Marchant R: Characterization of a fungal strain isolated from a polyphenol polluted site. Biores Technol. 2007, 98: 1741-1747.View ArticleGoogle Scholar
- Friedrich J, Zalar P, Mohorcic M, Klun U, Krzan A: Ability of fungi to degrade synthetic polymer nylon-6. Chemosphere. 2007, 67: 2089-2095.View ArticleGoogle Scholar
- Frick CM, Farrell RE, Germida JJ: Assessment of phytoremediation as an In-situ technique for cleaning oil-contaminated sites. 1999, Calgary: Petroleum Technology Alliance CanadaGoogle Scholar
- Ulfig K, Płaza G, Worsztynowicz A, Manko T, Tien AJ, Brigmon RL: Keratinolytic fungi as indicators of hydrocarbon contamination and bioremediation progress in a petroleum refinery. Polish J Environ Studies. 2003, 12: 245-250.Google Scholar
- Hashem AR: Bioremediation of petroleum contaminated soils in the Persian gulf region: a review. J Kuwait Sci. 2007, 19: 81-91.View ArticleGoogle Scholar
- Nelson PE, Tousooun TA, Marasas WFO: Fusarium species: an illustrated manual for identification. 1983, Pennsylvania, USA: The Pennsylvania State University PressGoogle Scholar
- Gilman JC: A manual of soil fungi. 1998, India: Daya publishing houseGoogle Scholar
- Watanabe T: Pictorial atlas of soil and seed fungi: morphology and key to species. 2002, India: CRC Press, 2View ArticleGoogle Scholar
- U.S. EPA: United States Environmental Protection Agency Quality Assurance Management Staff. 1994, Available at: http://www.epa.gov,Google Scholar
- Aebi H: Methods of enzymatic analysis. 1973, Germany: Verlag ChemieGoogle Scholar
- Tate RL: Soil Microbiology. 1995, New York: John Wiley and SonsGoogle Scholar
- Theorell H: Crystalline peroxidase. Enzymologia. 1942, 10: 250-252.Google Scholar
- Chehregani A, Malayeri B, Golmohammadi R: Effect of heavy metals on the developmental stages of ovules and embryonic sac in euphorbia cheirandenia. Pakistan J Biol Sci. 2005, 8: 622-625.View ArticleGoogle Scholar
- Opasols AO, Adewoye SO: Assessment of degradability potential of penicillium oxalicum on crude oil. Advances in Applied Sci Res. 2010, 1: 182-188.Google Scholar
- Kotik M, Brichac J, Kyslik P: Novel microbial epoxide hydrolases for biohydrolysis of glycidyl derivatives. J Biotech. 2005, 12: 364-375.View ArticleGoogle Scholar
- Ugochukwu KC, Agha NC, Ogbulie JN: Lipase activities of microbial isolates from soil contaminated with crude oil after bioremediation. African J Biotech. 2008, 7: 2881-2884.Google Scholar
- Akubugwo EI, Ogbuji GC, Chinyere CG, Ugbogu EA: Physicochemical properties and enzymes activity studies in a refined oil contaminated soil in isiukwuato, abia state. Nigeria. Biokemisrti. 2009, 21: 79-84.Google Scholar
- Tang L, Niu X, Sun Q, Wang R: Bioremediation of petroleum polluted soil by combination of ryegrass with effective microorganisms. J Environ Technol Engin. 2010, 3: 80-86.Google Scholar
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