| Polycyclic Aromatic Hydrocarbons(PAHs) belong to one class of organic pollutants which are persistent and prevalent in the environment.They are sparingly water-soluble, chemically stable,and are mainly produced due to incomplete combustions of organic materials during the industrial productions.This type of chemicals has the strong "Carcinogenicity-Mutagenicity-Teratogenicity" effect.With the development of industries and agricultures,there is a tendency of accumulation of PAHs in the soils.PAHs in soil can be absorbed by plant roots into the plant,and through the food chain being transferred to human beings and subsequently accumulated in the body.Therefore,they pose serious dangers on the safety of the soil ecological system as well as the quality of agricultural products.Remediation of the PAHs-contaminated soil has become a hot,as well as challenging topic in the field of environmental engineering.Among the different methods of remediation,the use of Phytoremediation to remove PAHs from the soil safely and effectively has brought many scholars into attention.Currently,there are numerous studies reported, discussing the roles of the tolerant plants in the degradation of PAHs.However,the method of combining those plants with soil fauna to enhance the degradation of PAHs in the soil-plant system is rarely reported.Earthworms,as a typical soil fauna,are known to be "masters in ameliorating the soil". Researches have shown that on one hand,they are able to improve soil structure and fertility, and to promote the growth of plant roots.On the other hand,they can also increase the number,as well as the activity of the microorganisms in the soil due to their secretion products and their faeces,thereby promoting the degradation of the pollutants in the soil. Earthworms have already been used successfully to treat domestic daily wastes,nutrient salts and heavy metals pollution.In order to understand the role of earthworms in promoting the degradation of PAHs on soil-plant system and investigate the remediating effect of earthworms on PAHs polluted soils,a series of pot culture experiments have been conducted, using phenanthrene and pyrene as the representatives of polycyclic aromatic hydrocarbons. Simulation has been deployed,to artificially imitate the different levels of pollutions in the soil.The main focuses of this research are(1) the differences in the degradation effects of PAHs in the soil when different test plant species were used,therefore selecting the species with the most efficient remediating effect;(2) the adaptability of earthworms in the PAHs-contaminated soil and;(3) the enhancement by the earthworms on the degradation rate of PAHs in the soil-plant system.The main results are summarized below:1.Ryegrass(Loliunmultiflorum.L.),wild mustard(Brassica oleracea L.),Sudan grass (Sorghum vulgate L.),white clover(Trrifolium repens L),alfalfa(Medicago sativa L.) and field mustard(Brassica campestris L.) can all grow normally in the 80 mg.kg-1 phenanthrene and pyrene contaminated soil.Different species showed different levels of tolerance towards phenanthrene and pyrene.60 days after the plants were grown,phenanthrene was found to promote the growth of most of the plant species.The total biomass(dry weight) of a single plant of ryegrass,wild mustard,Sudan grass,white clover,and alfalfa increased by 31%, 41.82%,36.02%,13.86%,and 25.83%respectively,comparing with the control in which no phenanthrene was added.Only the total biomass of the field mustard plant fell by 6.43%. Pyrene was found to inhibit the growth of most of the plant species.The total biomass(dry weight) of a single plant of wild mustard,Sudan grass,white clover,alfalfa and field mustard decreased by 4.83%,13.94%,6.44%,38.39%and 51.46%respectively,comparing with the control in which no pyrene was added.Only the total biomass of the ryegrass plant increased by 62.21%.Phenanthrene and pyrene did not result in a significant change on the distribution of growth of all the 6 test plant species,(n=25,P＞0.05).Among them,the root-to-cap ratio of the Sudan grass plant is the largest,which is approximately 0.3. 2.In PAHs-contaminated soils with phenanthrene and pyrene concentrations of 80 mg.kg-1, all the six test plant species showed significant effect on the degradation of the phenantlene and pyrene.After 60 days of experiment,the percentages of degradation of phenanthrene and pyrene were merely 23.01%and 17.78%if none of the plants were cultivated.However,the percentages of degradation of phenanthrene in the PAHs contaminated soils reached as high as 62.7%,45.72%,76.25%,76.56%,57.8%and 26.48%,if ryegrass,wild mustard,Sudan grass,white clover,alfalfa and field mustard was cultivated respectively.Comparing with the control in which no plants were cultivated,the percentage increased by 39.69%,22.71%, 53.24%,53.55%,34.79%and 3.47%,which showed that Sudan grass and white clover had the best degradation effect on phenanthrene among them.In the case of pyrene,the percentage degradations were 47.8%,42.66%,65.26%,48.74%,46.55%and 23.56%,which demonstrated an increase of 30.06%,24.88%,47.48%,30.96%,28.77%and 5.78%, comparing with the control without plant cultivation.This showed that Sudan grass had the best degradation effect on pyrene.Under a constant pollution level,the percentage degradation of phenanthrene is higher than that of pyrene,showing that pyrene is more difficult to be degraded than phenanthrene.3.Pheretima(Pheretima sp.) had a certain level of tolerance towards PAHs polluted soils.In contaminated soils with phenanthrene and pyrene concentrations between 0mg.kg-1 and 320mg.kg-1,the survival rate of the earthwonns were 100%.Under different pollution levels of phenanthrene,the bioconcentration factors(BCFs) of the earthworms were 0.339,0.245, 0.200,0.218,and 0.214 respectively.For pyrene,the values were 0.671,0.442,0.357,0.266, and 0.220.The higher the concentration of PAHs,the lower the BCF of the earthworms towards is.Phenanthrene and pyrene were found to inhibit the growth of Pheretima to a certain extent.Comparing with the case that when no phenanthrene and pyrene were added, the weight of the earthworms decreased by 7.09%-25.19%and 6.79%-27.83%respectively. The higher the concentration of PAHs in the soil,the more the decrease was.There is a negative correlation between them.In this experiment,the earthworms were found to have a certain role in promoting the degradation of phenanthrene and pyrene in the soil,but not to a significant extent,(n=15,p＞0.05).In polluted soils with varying concentrations of phenanthrene and pyrene in between 0mg.kg-1 and 320mg.kg-1,the percentages of degradation of the phenanthrene and pyrene were higher by 0.76%-3.32%and 0.45%-2.24%respectively, with the incorporation of earthworms into the soil system comparing soils without earthworms.It can be seen that the enhancement effect of the earthworms on phenanthrene and pyrene degradation decreased with increasing concentrations of pollutants.4.In the soil-Sudan grass systems,under a constant pollution level,the biomass of the Sudan grass was higher with the incorporation of earthworms into the system by 14.41%-25.91% and 14.39%-23.9%in phenanthrene and pyrene contaminated soils respectively.Moreover, under the same pollution level,the phenanthrene and pyrene residues concentration was lower in an earthworm-inoculated contaminated soil system than a non-earthworm inoculated system.There was an increase of 4.20%-9.76%and 3.69%-9.38%in the percentage degradation of phenanthrene and pyrene respectively with the inoculation of earthworms.In soil systems without plants,the percentages degradation of phenanthrene and pyrene were 0.76%-3.12%and 0.45%-2.24%higher respectively with the inoculation of earthworms than if no earthworms were introduced.This is a positive correlation between the activity of the earthworms on the degradation of phenanthrene and pyrene,and the growth of Sudan grass. Under various pollution levels of PAHs,the urease and polyphenol oxidase activities were higher in the earthworm inoculated system than in the the control system without earthworms. After the incorporation of the earthworms,the activity of the urease increased by 3.88% -11.59%in the treatment of phenanthrene contaminated soil,and by 2.15%-6.90%in the treatment of pyrene contaminated soil.The percentage increases in the activity of polyphenol oxidase were 1.91%-7.61%and 2.29%-3.41%,in the treatment of phenanthrene and pyrene contaminated soils respectively.In the treatment of earthworm-inoculated as well as non-earthworm inoculated polluted soils,it was found that the relationship between the activities of the urease and polyphenol oxidase,and the initial concentration of PAHs could be modeled with a reduced power function. |
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