The aim of this research was to select plant species that could be effective in the phytoremediation ofa former oil-sludge pit. Seven crop plants (Triticum aestivum L., Secale cereale L., Avena sativa L., Hordeum vulgare, Sorghum bicolor L Moench, Panicum miliaceum L, and Zea mays L.),five wild grasses (Lolium perenne L., Bromopsis inermis, Agropyron cristatum L., Agropyrum tenerum L., and Festuca pratensis Huds.), and three legumes (Medicago sativa L., Trifolium pratense L., and Onobrychis antasiatica Khin.) were screened for phytotoxicity, including the assessment of germination, shoot biomass, and root biomass, in a pot experiment. The estimation of oil-sludge degradation in the root zone of the tested plants showed that rye accelerated cleanup most effectively, degrading all of the main contaminant fractions in the oil sludge by a total of 52%. Although alfalfa had a lower phytoremediation potential than did rye, it maintained large numbers of soil microorganisms, including polycyclic aromatic hydrocarbon degraders, in its rhizosphere. Rye and alfalfa were chosen for a large-scale study to remediate an oil-sludge pit on the grounds of a petroleum refinery. Remediation monitoring confirmed the effectiveness of rye: the oil-sludge content decreased consistently for 3 years and remained low in comparison with the results from other plant species.
Thirteen-year monitoring of the vegetation growing in the industrial and adjacent areas of an oil refinery showed the prevalence of yellow medick (Medicago falcata L.) over other plant species, including alfalfa (Medicago sativa L.). A comparative field study of the two Medicago species established that yellow medick and alfalfa exhibited similar resistance to soil petroleum hydrocarbons and that the pollutant concentration in their rhizosphere was 30% lower than that in the surrounding bulk soil. In laboratory pot experiments, yellow medick reduced the contaminant content by 18% owing to the degradation of the major heavy oil fractions, such as paraffins, naphthenes, and alcohol and benzene tars; and it was more successful than alfalfa. Both species were equally effective in stimulating the total number of soil microorganisms, but the number of hydrocarbon-oxidizing microorganisms, including polycyclic aromatic hydrocarbon degraders, was larger in the root zone of alfalfa. In turn, yellow medick provided a favorable balance of available nitrogen. Both Medicago species equally stimulated the dehydrogenase and peroxidase activities of the soil, and yellow medick increased the activity of soil polyphenol oxidase but reduced the activity of catalase. The root tissue activity of catalase, ascorbate oxidase, and tyrosinase was grater in alfalfa than in yellow medick. The peroxidase activity of plant roots was similar in both species, but nondenaturing polyacrylamide gel electrophoresis showed some differences in the peroxidase profiles of the root extracts of alfalfa and yellow medick. Overall, this study suggests that the phytoremediation potentials of yellow medick and alfalfa are similar, with some differences.
The result of monitoring of natural vegetation growing on oil-contaminated (2.0-75.6 g/kg) and uncontaminated (0.04-2.0 g/kg) soils of a petroleum refinery for a period of 13 years is presented. Floristic studies showed that the families Poaceae, Asteraceae, Fabaceae, and eventually Brassicaceae were predominant in the vegetation cover of both types of soils. Over time, the projective vegetation cover of the contaminated sites increased from 46 to 90%; the species diversity increased twofold: in the ecological-cenotic structure of the flora, the number of ruderal plant species decreased; and the number of steppe, i.e., zonal, plant species increased. Using 62 dominant plant species, we conducted a field study of plant characteristics such as resistance to oil pollution, the ability to enrich the rhizosphere soil with microorganisms and bioavailable mineral nitrogen, and reduction of the concentration of petroleum hydrocarbons. The results enable us to characterize the phytoremediation potential (PRP) of the native plants and identify species that, probably, played a key role in the natural restoration of oil-contaminated soils. Statistical analysis showed correlations between the PRP constituents, and the leading role of rhizosphere microorganisms in the rhizodegradation of petroleum hydrocarbons was proven. A conditional value of PRP was proposed which allowed the investigated plants to be ranked in 11 classes. The study of a large sample of plant species showed that some plants held promise for the use in reclamation of soils in arid steppe zone, and that other species can be used for the rehabilitation of saline soils and semideserts.
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