Phytoremediation Of Contaminated Soils With Polycyclic Aromatic Hydrocarbons And Its Ecologically Enhanced Techniques

Polycyclic aromatic hydrocarbons（PAHs） consist of two or more fused aromatic rings and are formed during the incomplete combustion of fossil fuel,wood,and other organic substances.They are discharged into the environment as industrial by-products of fossil fuel combustion,asphalt production,wood preservation,and coal-processing.PAHs are persistent due to their recalcitrance to microbial degradation,high hydrophobicity,and ability to be absorbed strongly to soil particles.The partial breakdown products of PAH degradation have toxic,mutagenic,and carcinogenic properties, and thus remediation is of great importance.Phytoremediation is a promising alternative approaches to soil remediation due to its convenience,cost-effectiveness and environmental acceptability.Plants can enhance the remediation of soils containing PAHs by various processes,such as influence on the microbial community structure and functional diversity,improvement of soil physicochemical conditions release of the root exudation,and improvement of oxygen diffusion by their roots through providing channels for air flow.However,the impact of each process and basic mechanisms involved are up-to-date not clearly elucidated,its efficacy varies greatly among plant species and is closely related to properties of contaminants and environmental characteristics.In this dissertation,the potentials of twelve plant species on the degradation of phenanthrene（Phe） or pyrene（Pyr） in soil were investigated by pots experiments in a greenhouse,and the mechanisms and efficiencies of the removal and remediation of PAH in soils by different planting patterns were also compared.The effects of earthworm activity on plant uptake and accumulation of PAH in spiked soils were evaluated.Based on these experimental results,a multi-process phytoremediation system composed of plants,soilfauna（earthworm） and aboriginal microbe was developed.The main original conclusions are shown as follows.1.Screening of twelve plant species for phytoremediation of phenanthrene or pyrene in soilsTwelve plant species were screened for their ability for the cleanup of PAH-spiked soil at their initial concentrations of ranging from 20 to 322 mg·kg^-1 by pots experiments in a greenhouse.The results of the screening experiment indicated that,at the end of the experiment（55d）,the remaining respective concentrations of Phe or Pyr in spiked vegetated soils,with initial Phe of 161.44 mg·kg^-1 and Pyr of 161.44 mg·kg^-1,were 36.33～122.39 mg·kg^-1 and 45.73～124.82 mg·kg^1-,lower than their concentrations in the nonvegetated soils.The loss of Phe or Pyr in the vegetated soils was 24.18%～77.49%and 22.29%～71.53%of the added amounts of these contaminants,respectively. Among plants investigated,Ophiopogon japnicus,Sorghum vulgare,Pogonatherum paniceum, Lolium multiforum and Festuca arundinacea caused a more significant decrease in the PAHs concentration in the planted soil than in the unplanted soil,and would be effective in the phytoremediation of PAH-spiked soils.After 55 days plantation,about 75.83%of Phe or 75.83%of Pyr was removed from the spiked soils with initial Phe of 40.88 mg·kg^-1 or initial Pyr of 39.58 mg·kg^-1（i.e.,T₂ contamination level）,and over 60.03%of Phe or 53.04%of Pyr was removed when their initial concentrations were at 161.44 or 160.64 mg·kg^-1（i.e.,T₄ contamination level）, respectively.Meanwhile,Brassica campestris,Lactuca sativa,Brassica oleracea and Iris japonica Thunb exhibited relatively poor potential on the remediation of soil PAHs pollution during the experiment,and only 31.94%～46.45%of Phe or 28.17%～40.79%of Pyr was removed from the spiked soils with T₂ contamination level,and 24.18%～34.45%of Phe or 22.29%～30.16%of Pyr was removed when their initial concentrations were at T₄ contamination level,respectively.Results also suggested that with the increment of their initial concentrations,the accumulations concentrations of Phe or Pyr in plant tissues monotonically increased,but RCFs（root concentration factors;defined as the ratio of PAH concentrations in roots and in the soils on a dry weight basis） of these compounds gradually decreased.Under the same contamination level,PAHs concentrations in shoot and SCFs （shoot concentration factors;defined as the ratio of PAH concentrations in shoots and in the soils on a dry weight basis） were always significantly lower than those concentrations in root and RCFs,and the concentrations of Phe in plants are always lower than those of Pyr.Plant uptake and accumulation of these compounds were evident,RCFs of these compounds significantly positively correlated to root lipid contents,but similar correlations to root water contents could not be found. Results from this study suggested a feasibility of the establishment of phytoremediation for soil PAH contamination.2.Removal and remediation mechanisms of PAHs in soil by Pogonatherum PaniceumPot experiments were carried out to investigate the accumulation and removal mechanisms of PAHs by rock plant P.paniceum.The results showed that the growth of P.paniceum was not effected by Phe or Pyr at their initial concentrations of 20 to 322 mg·kg^-1 in soils,P.paniceum significantly removed Phe or Pyr from soils.After 70 days plantation ofP.paniceum,the extractable PAHs were lower in planted soils than in non-planted soils.About 50.97%～86.77%of Phe and 46.45%～76.7%of Pyr were removed from the soils,respectively.P.paniceum removed averagely 63.56%of Phe and 58.6%of Pyr from the soils as compared to CK₁（with addition of 0.1%NAN₃）, and 46.09%of Phe and 42.92%of Pyr as compared to CK₂（without NAN₃）.P.paniceum did show ability to accumulate PAHs from the soils,contents of PAHs in its root and shoot increased with the increase of PAHs concentrations in the soils.The bioconcentration factors（BCFs） for PAHs tended to decrease with increasing concentrations of these contaminants in soil,BCFs for Pyr were higher than those for Phe,and BCFs for Phe（0.12～1.63） and Pyr（1.03～5.61） in shoots were much lower than those in roots（0.21～3.08 and 1.31～10.11） at same treatment.Despite plantation of P.paniceum evidently enhanced the remediation of PAHs in soils,contributions of biotic and abiotic factors to phytoremediation process displayed distinct diversity.Abiotic loss,plant accumulation, phytodegredation and microbial degradation accounted for 5.10%,0.32%,4.22%and 17.47%of the total removal of Phe,and 2.56%,2.01%,4.27%and 15.68%of Pyr from soils,respectively.In contrast,41.56%of the total removal of Phe and 36.64%of Pyr were attributed to the contributions of plant-microbial interactions.Thus plant-microbial interactions are the main mechanisms for the remediation of soil PAHs pollution.3.Effects of mnltispecies phytoremediation on the fate of phenanthrene or pyrene in soilThe potentials of three plant species,rape,alfalfa and white clover,separately or jointly on the degradation of PAHs in soil were estimated by pots experiments in a greenhouse.The results showed that the presence of vegetation apparently enhanced the dissipation of PAHs in soils at their initial concentrations ranging from 20.05 to 322.06 mg·kg^-1,but the efficacy enhanced varied greatly among plant species and cropping patterns.Within the 70 d experimental period,alfalfa and white clover showed higher efficicncies for the removal of PAHs than those of rape,and mixed cropping greatly enhanced the dissipation of PAHs as compared to single cropping.On average 74.87%of Phe or 62.81%of Pyr were removed from soils with mixed cropping of rape and alfalfa,and 72.01%of Phe or 68.44%of Pyr removed by mixed cropping of rape and white clover.Under similar pollution level,PAH accumulated in plant tissues,irrespective of root and shoot,was much lower in mutli-plantcd treatments than in mono-planted treatments.Despite the presence of vegetation evidently enhanced the rcmediation of PAHs in soils,contributions of abiotic loss,plant uptake, accumulation and microbial degradation was much lower than those of plant-microbial interactions in the process of phytoremediation,predominant pathway responsible for the degradation of soil Phe or Pyr under mixed cropping pattern was the plant-microbial interactions,and the plant uptake and accumulation of these compounds was negligible.Results suggested a feasibility of the establishment of multispecies remediation for the improvement of remediation efficiencies of PAHs, which may also decrease the accumulations of PAHs in plants and thus reduce their risks to humans.4.Dynamic changes of contributions of removal factors to PAH degradation and soil enzyme activities in the phytoremediation processIn this study,the changes of contributions of biotic & abiotic factors to degradation of PAHs in soil-plant（S.vulgare） system with initial Phe of 81.05 mg·kg^-1 or Pyr of 79.86 mg·kg^-1 and soil enzyme（polyphenoloxidase,dehydrogenase and urease） activities were monitored every 12d in pot experiments.The results showed that contribution rate of removal factors to dissipation of PAHs and soil enzyme activities varied greatly at different monitoring stages during the 72d experiments.At early monitoring time（0～36d）,the predominant pathway responsible for the degradation of PAHs was the microbial degradation among all pathways,but at late monitoring time（36～72d）,the plant-microbial interactions was the main mechanisms for the remediation of soil PAHs pollution. Although enzyme activities in soil were inhibited within 24 days for polyphenoloxidase and dehydrogenase or within 36 days for urease respectively,and they were gradually reactivated or Stimulated along with the stretch of phytoremediation;during the whole phytoremediation process, dehydrogenase,urease and polyphenoloxidase activities significantly positively correlated to contribution rates of plant-microbial interactions on dissipation of PAHs in soils,and their correlation coefficients were up to 0.991,0.996 and 0.968 in Phe-spiked soils while in Pyr-spiked soils were up to 0.996,0.993 and 0.968,respectively.This suggests that in soil-plant system, plant-microbial interactions may facilitate the degradation of PAHs in soils by means of reactivating or activating soil enzyme activities.5.Earthworm’s tolerance and its ecological effects to/on PAH-contaminated soilsThe interaction between earthworm（Pheretima sp.） and PAH-spiked soils were investigated, and earthworm’s ability to tolerate PAHs in soils and its influence on physicochemical properties of spiked soils were elucidated.The results showed that earthworms were able to survive in medium spiked soils with initial Phe of 20.05～322.06 mg·kg^-1 or initial Pyr 20.24～321.42 mg·kg^-1,but the growth of earthworms was inhibited significantly under heavily spiked conditions where the initial concentrations of Phe or Pyr were at 502.82 mg·kg^-1 or 509.33 mg·kg^-1,respectively.Compared with nonearthworm,the pH of medium spiked soils decreased remarkably by 0.16～0.21 units for Phe-spiked soils or by 0.13～43.19 units for Pyr-spiked soils in the presence of earthworm,but did it only by 0.07,0.05 units while in heavily spiked soils.Although available N,P and K content in soil were improved to a certain extent,the increment of available K was lower than available N or P. Within the 35-d experimental period,about 15.03%of Phe and 12.30%of Pyre was removed from the soils inoculated earthworms,which were slightly higher than those in control soil samples with degradation of 13.20%Phe or 10.58%Pyr added.The accumulation of PAHs by earthworm was evident,and with the increment of their initial concentrations,accumulations concentrations of PAHs in earthworm tissues monotonically increased,but BCFs（bioconcentration factors;defined as the ratio of PAH concentrations in earthworm tissues and in soils on a dry weight basis） of these compounds gradually decreased.This study indicated that earthworms were able to survive in medium spiked soils and their activities were also beneficial to the improvements of soil properties.6.Enhanced dissipation of PAHs in planted soils by earthworm activitiesThe potentials of Sudan grass,with or without inoculating earthworms（Pheretima sp.）,on the degradation of PAHs in soils were estimated by pot experiments in a greenhouse.Results showed that plantation of vegetation apparently enhanced the dissipation of PAHs in soils at their initial concentrations ranging from 20.05 to 322.06 mg·kg^-1.During the 70 d experiment,about 88.43%～60.42%of Phe and 80.18%～53.99%of Pyr was removed from planted soils,only 30.08%～14.73% degradation of Phe and 24.23%～12.28%of Pyr occurred in non-planted soils.After inoculating earthworms,the dissipation ratios of Phe and Pyr in planted soils were increased up to 93.42%～66.40%and 86.39%～60.96%,which were 5.63%～9.93%and 7.73%～12.91%（denoted as the dissipation promotion of PAHs in with vs.without earthworms soils）higher than those in corresponding soils without earthworms.Among all possible pathways,the plant-microbial interactions was the primary means of contaminant degradation,either with or without inoculating earthworms;and under similar pollution level,the enhanced extent of Pyr was always greater than those of Phe.Results suggested a feasible way for the establishment of high efficiency phytoremediation of PAHs with inoculating earthworms,which might be especially beneficial for promoting dissipation of PAHs compounds containing more benzene rings.7.plant-earthworm-aboriginal microbe system for removal of PAHs in soilsThe orthogonal design with L₁₆（4⁵） in pot-tests was used to investigate the effect of three kinds of environmental factors（soil moisture,humus content and earthworms activity） on degradation of PAHs in soils with their initial concentrations ranging from 20.05 to 322.06 mg·kg^-1.The results showed that growth status of plant root system played an important role in the phytoremediation process,and the removal rate of PAHs was significantly positively correlated to root weight per plant.In addition to imposing a direct impact on soil properties,environmental conditions（moisture content,humus content,earthworm activity） also further promoted the extent of dissipation of PAHs by improving the growth status of plant root system in soil;among environmental factors monitored, the moisture content was the dominant factor influencing degradation of PAHs in soils,followed by addition of earthworms,humus content is relatively small.During the 70 d experiment,about 92.63%,87.54%and 83.73%of Phe and 82.79%,77.86%and 79.47%of Pyr was removed from the orthogonal portfolio S₁₀(i.e.,with a density of 6 earthworm·pot^-1,humus content of 13.44 g·kg^-1 and 45%WHC),S₉(i.e.,with a density of 6 earthworm.pot^-1,humus of 5.94 g·kg^-1 and 30%WHC) and S₁₄(i.e.,with a density of 9 earthworm.pot^-1,humus of 3.44 g·kg^-1 and 45%WHC) compared with their initial concentrations of 40.88（39.58） mg.kg^-1,and about 77.59%,72.84%and 72.13%of Phe and 71.13%,66.33%and 67.77%of Pyr was removed from S₁₀,S₉ and S₁₄ compared with their initial concentrations of 161.44（160.64） mg·kg^-1,respectively,implying that soil-plant（S.vulgare） system with environmental conditions S₁₀,S₉ and S₁₄ was the more efficient in promoting degradation of PAHs in soils.Results from this study suggest that in the appropriate environmental conditions,utilizing the synergy between plant-soil fauna（earthworms）-indigenous microbe to achieve ecological remediation of PAH-contaminated environments is feasible.