Showing papers on "Microbial biodegradation published in 2006"

Exocellular electron transfer in anaerobic microbial communities.

Abstract: Exocellular electron transfer plays an important role in anaerobic microbial communities that degrade organic matter. Interspecies hydrogen transfer between microorganisms is the driving force for complete biodegradation in methanogenic environments. Many organic compounds are degraded by obligatory syntrophic consortia of proton-reducing acetogenic bacteria and hydrogen-consuming methanogenic archaea. Anaerobic microorganisms that use insoluble electron acceptors for growth, such as iron- and manganese-oxide as well as inert graphite electrodes in microbial fuel cells, also transfer electrons exocellularly. Soluble compounds, like humic substances, quinones, phenazines and riboflavin, can function as exocellular electron mediators enhancing this type of anaerobic respiration. However, direct electron transfer by cell-cell contact is important as well. This review addresses the mechanisms of exocellular electron transfer in anaerobic microbial communities. There are fundamental differences but also similarities between electron transfer to another microorganism or to an insoluble electron acceptor. The physical separation of the electron donor and electron acceptor metabolism allows energy conservation in compounds as methane and hydrogen or as electricity. Furthermore, this separation is essential in the donation or acceptance of electrons in some environmental technological processes, e.g. soil remediation, wastewater purification and corrosion.

Microbial growth and substrate utilization kinetics

Abstract: Microbial growth on and utilization of environmental contaminants as substrates have been studied by many researchers. Most times, substrate utilization results in removal of chemical contaminant, increase in microbial biomass and subsequent biodegradation of the contaminant. These are all aimed at detoxification of the environmental pollutants. Several microbial growth and biodegradation kinetic models have been developed, proposed and used in bioremediation schemes. Some of these models include Monod’s, Andrews, Bungay’s weighted model, general substrate inhibition models (GSIM) and sum kinetic models. Most research on microbial potentials to degrade chemical pollutants has been performed on a laboratory scale. There is a need to extend such studies to pilot scale as well as to full-scale field applications. Key words: Microbial growth, substrate utilization, biodegradation, kinetics, detoxification, organic contaminants, models, environmental pollutants.

Microbial degradation of organophosphorus xenobiotics: metabolic pathways and molecular basis

Abstract: Organophosphorus (OP) xenobiotics are used worldwide as pesticides and petroleum additives. OP compounds share the major portion of the pesticide market globally. Owing to large-scale use of OP compounds, contaminations of soil and water systems have been reported from all parts of the world. OP compounds possess very high mammalian toxicity and therefore early detection and subsequent decontamination and detoxification of the polluted environment is essential. Additionally, about 200,000 tons of extremely toxic OP chemical warfare agents are required to be destroyed by 2007 under Chemical Warfare Convention (1993). Chemical and physical methods of decontamination are not only expensive and time-consuming, but also in most cases they do not provide a complete solution. These approaches convert compounds from toxic into less toxic states, which in some cases can accumulate in the environment and still be toxic to a range of organisms. Bioremediation provides a suitable way to remove contaminants from the environment as, in most of the cases, OP compounds are totally mineralized by the microorganisms. Most OP compounds are degraded by microorganisms in the environment as a source of phosphorus or carbon or both. Several soil bacteria have been isolated and characterized, which can degrade OP compounds in laboratory cultures and in the field. The biochemical and genetic basis of microbial degradation has received considerable attention. Several genes/enzymes, which provide microorganisms with the ability to degrade OP compounds, have been identified and characterized. Some of these genes and enzymes have been engineered for better efficacy. Bacteria capable of complete mineralization are constructed by transferring the complete degradation pathway for specific compounds to one bacterium. In the present article, we review microbial degradation and metabolic pathways for some OP compounds. The biochemical and molecular basis of OP degradation by microbes and the evolution and distribution of genes/enzymes are also reviewed. This article also examines applications and future use of OP-degrading microbes and enzymes for bioremediation, treatment of OP poisoning, and as biosensors.

Biodegradation of endosulfan by a soil bacterium.

Abstract: A bacterium capable of metabolizing endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine3-oxide) was isolated from cotton-growing soil and effectively shown to degrade endosulfan into endosulfan sulfate. The bacterium degraded 50% of the compound within 3 days of incubation. Endosulfan sulfate was the only terminal product and no other metabolites were formed during the incubation. Endosulfan and its metabolites were analyzed by gas chromatography. The metabolites formed indicated that the organism follows an oxidative pathway for metabolism of this pesticide. Therefore, the present study, microbial degradation of endosulfan by a soil bacterium, may provide a basis for the development of bioremediation strategies to remediate the pollutants in the environment.

Environmental pollution promotes selection of microbial degradation pathways

Abstract: Astonishing as it may seem, one organism's waste is often ideal food for another. Many waste products generated by human activities are routinely degraded by microorganisms under controlled conditions during waste-water treatment. Toxic pollutants resulting from inadvertent releases, such as oil spills, are also consumed by bacteria, the simplest organisms on Earth. Biodegradation of toxic or particularly persistent compounds, however, remains problematic. What has escaped the attention of many is that bacteria exposed to pollutants can adapt to them by mutating or acquiring degradative genes. These bacteria can proliferate in the environment as a result of the selection pressures created by pollutants. The positive outcome of selection pressure is that harmful compounds may eventually be broken down completely through biodegradation. The downside is that biodegradation may require extremely long periods of time. Although the adaptation process has been shown to be reproducible, it remains very difficult ...

Effects of surface sorption on microbial degradation of glyphosate

Abstract: Sorption may affect the bioavailability and biodegradation of pesticides in soils. The aim of this study was to test the effect of surface sorption on microbial utilization of the herbicide glyphosate as a source of phosphorus, nitrogen, or carbon. We added goethite to a humus soil to manipulate the soil's glyphosate sorption capacity. The addition of glyphosate generally either decreased microbial CO2 production or produced no effect. Additions of glyphosate, in combination with glucose and N, did not change the respiration rate in comparison with the same treatment but without glyphosate. In contrast, glyphosate additions combined with glucose and P decreased microbial growth, whereas the combination with goethite counteracted the negative effect. The different treatments were examined using attenuated total reflectance Fourier transform (ATR−FTIR) spectroscopy; the results suggest that glyphosate was de-carboxylated in the sorbed state. Stimulating microbial growth by the addition of glucose and nitrog...

Microbial degradation of street dust polycyclic aromatic hydrocarbons in microcosms simulating diffuse pollution of urban soil

Abstract: Summary Diffuse pollution with polycyclic aromatic hydrocarbons (PAHs) of topsoil in urban regions has caused increasing concerns in recent years. We simulated diffuse pollution of soil in microcosms by spiking sandy topsoil (A-horizon) and coarse, mineral subsoil (C-horizon) with street dust (PM63) isolated from municipal street sweepings from central Copenhagen. The microbial communities adapted to PAH degradation in microcosms spiked with street dust in both A-horizon and C-horizon soils, in spite of low PAH-concentrations. The increased potential for PAH degradation was demonstrated on several levels: by slowly diminishing PAH-concentrations, increased mineralization of 14C-PAHs, increasing numbers of PAH degraders and increased prevalence of nah and pdo1 PAH degradation genes, i.e. the microbial communities quickly adapted to PAH degradation. Three- and 4-ring PAHs from the street dust were biodegraded to some extent (10–20%), but 5- and 6-ring PAHs were not biodegraded in spite of frequent soil mixing and high PAH degradation potentials. In addition to biodegradation, leaching of 2-, 3- and 4-ring PAHs from the A-horizon to the C-horizon seems to reduce PAH-levels in surface soil. Over time, levels of 2-, 3- and 4-ring PAHs in surface soil may reach equilibrium between input and the combination of biodegradation and leaching. However, levels of the environmentally critical 5- and 6-ring PAHs will probably continue to rise. We presume that sorption to black carbon particles is responsible for the persistence and low bioaccessibility of 5- and 6-ring PAHs in diffusely polluted soil.

Biodegradation of hexachlorocyclohexane-isomers in contaminated soils

Abstract: Several sites that are contaminated with isomers of the chlorinated insecticide hexachlorocyclohexane (HCH) are present across the globe and cause toxicity. For their bioremediation, we studied the degradation of HCH-isomers in contaminated soils by an isolate Pseudomonas aeruginosa ITRC-5. The degradation is optimal at 2 mg technical-HCH (t-HCH)/g soil, 15% water content, pH 8.0, temperature 28 °C and inoculum density 106 colony forming unit/g soil. Under these conditions, from 5 kg soil, >98% α- and γ-HCH, 17% β-HCH and 76% δ-HCH are degraded after 15 days of incubation, which is accompanied with the release of 600 μg chloride/mg t-HCH. Concomitant to the degradation, a four-fold reduction in the toxicity of HCH-isomers to earthworm, Eisenia foetida, is also observed. Addition of ITRC-5 enhanced the degradation of soil-applied HCH-isomers in ‘open field’ conditions as well, and 97%, 43%, 94% and 77% of α-, β-, γ- and δ-HCH, respectively, are degraded after 12 weeks of incubation. Thus, the bacterium causes microbial degradation and detoxification of HCH-isomers, and can be used for the bioremediation of contaminated soils.

The microbial degradation of azimsulfuron and its effect on the soil bacterial community

Abstract: Aims: Azimsulfuron is a recently introduced sulfonylurea herbicide useful in controlling weeds in paddy fields. To date very little information is available on the biodegradation of this pesticide and on its effect on the soil microbial community. The aim of this work was to study its biodegradation both in slurry soil microcosms and in batch tests with mixed and pure cultures. Methods and Results: Azimsulfuron was applied to forest bulk soil in order to study its effect on the structure of the bacterial soil community, as detectable by denaturant gradient gel electrophoresis (DGGE) analyses. Biodegradation and abiotic processes were investigated by HPLC analyses. In addition, a microbial consortium was selected, that was able to use azimsulfuron as the sole energy and carbon source. One of the metabolites produced by the consortium was isolated and identified through LC-MS analyses. Cultivable bacteria of the consortium were isolated and identified by 16S rDNA sequencing (1400 bp). Conclusions: Azimsulfuron treatment seems to have the ability to cause changes in the bacterial community structure that are detectable by DGGE analyses. It is easily biodegraded both in microcosms and in batch tests, with the formation of an intermediate that was identified as 2-methyl-4-(2-methyl-2H-tetrazol-5-yl)-2H-pyrazole-3-sulfonamide. Significance and Impact of the Study: The study increases the knowledge on the biodegradation of azimsulfuron and its effects on the soil microbiota.

Effects of nutrient source and supply on crude oil biodegradation in continuous-flow beach microcosms

Abstract: Ammonium and nitrate were used as nitrogen sources to support microbial biodegradation of crude oil in continuous-flow beach microcosms to determine whether either nutrient was more effective in open systems, such as intertidal shorelines. No differences in the rate or extent of oil biodegradation were observed, regardless of whether these nutrients were provided continuously or intermittently. Nutrients were provided once every two weeks to intermittent-input microcosms and washed out within four to five days. In continuous-input microcosms, ammonium and nitrate were assimilated as quickly as they were provided during the first week, but both accumulated to greater than 10 mg N/L thereafter. The sensitivity of the oil mineralization rate to nutrient input decreased rapidly as the extent of oil degradation increased, and after about two weeks the rate of oil-mineralization appeared to be independent of nutrient input. Therefore, there may be little value in maintaining a long-term supply of nutrients in contact with oil-contaminated sediments. The rates of microbial assimilation of ammonium and nitrate followed similar trends. Both compounds were assimilated more slowly as the extent of oil biodegradation increased, and the nitrate uptake rates approached zero after about two weeks. Ammonium assimilation continued at a low rate throughout the six-week experiment, but this did not appear to affect the rate of oil mineralization. Assimilation of ammonium resulted in a sharp decrease in the pH of the synthetic seawater that was pumped continuously through the microcosms, but nitrate had a much smaller effect on pH. The magnitude of the ammonium-associated pH change was never as large as was observed in previous studies involving oil biodegradation in batch reactors, however, and did not affect the oil-biodegradation rate.

Degradation of Polycyclic Aromatic Hydrocarbons in the Rhizosphere of Festuca arundinacea and Associated Microbial Community Changes

Abstract: A phytoremediation growth chamber study was conducted to evaluate the contribution of soil microbial diversity to the contaminant degradation. Target contaminant removal from soil was assessed by monitoring concentrations of polycyclic aromatic hydrocarbons (PAHs), along with changes in the bacterial community structure over a time period of 10 months in the presence of tall fescue (Festuca arundinacea). Enhanced degradation of PAHs was observed in rhizosphere soil, with a maximum reduction in pyrene at a rate 36% higher than that noted for the unvegetated control. The dissipation of 4-ring PAHs in unvegetated soil was 70%, 54%, and 49% respectively, whereas a higher dissipation rate was observed in tall fescue treated soil of 78%, 68%, and 61% at the end of the study. Microbial enumeration results showed greater total bacterial numbers and PAH-degrading bacteria in rhizosphere soil when compared to unvegetated soil. The results from the terminal restriction frag...

Szlaki tlenowej biodegradacji wêglowodorów ropy naftowej

Abstract: Summary The paper reviews the aspects of physiology and biochemistry of microbial biodegradation of the main components of petroleum oil: alkanes (larger than methane), alkenes, alkines, cyclic, polycyclic, aromatic and polyaromatic hydrocarbons. It focuses on aerobic degradation pathways. The review provides key information prepared on the basis of more than 30 years of research on microbial degradation of hydrocarbons. The areas discussed include new pathways of biodegradation of branched-chain alkanes, cyclic and polyaromatic hydrocarbons which have been discovered during the past few years. Special attention was paid to oxygenases – enzymes initiating aerobic metabolism of hydrocarbons and phenomenon of co-oxidation which enables assimilation of most recalcitrant components of crude oil.

Effect of organic nutrient on microbial utilization of hydrocarbons on crude oil contaminated soil

Abstract: The effect of organic nutrient (poultry manure) on biodegradation of soil (5 kg) contaminated with crude oil (50 g) was investigated for seven weeks. Four different test options were prepared namely; (i) 100 g of contaminated soil + 30 g of poultry manure; (ii) 100 g of contaminated soil + 60 g of poultry manure; (iii) 100 g of contaminated soil + 90 g of poultry manure; (iv) 100 g of contaminated soil only (control). The microbial degradation was monitored by the measurement of total heterotrophic count (THC), hydrocarbon utilizing bacterial count (HUB) and gravimetric loss of the crude oil with time. The cumulative THC of 6.9x107, 9.0x107, 1.03x108 and 3.1x107 cfu/g were recorded for test options (i), (ii), (iii) and (iv), respectively. The hydrocarbon utilizing bacterial counts (HUB) were 1.68x105, 1.63x105, 1.9x105 and 4.8x104 cfu/g for tests options (i), (ii), (iii) and (iv), respectively. There was a corresponding gravimetric hydrocarbon loss of 40.0, 45.26, 49.47 and 29.47% for test conditions (i), (ii), (iii), and (iv), respectively. The results of the study suggest that addition of organic nutrient (especially 90 g poultry manure) will further enhance microbial utilization of hydrocarbons. Key words: Biodegradation, crude oil, poultry manure.

Multiple microbial activities for volatile organic compounds reduction by biofiltration.

Abstract: In the northeast of Italy, high volatile organic carbon (VOC) emissions originate from small-medium companies producing furniture. In these conditions it is difficult to propose a single, efficient, and economic system to reduce pollution. Among the various choices, the biofiltration method could be a good solution, because microbial populations possess multiple VOC degradation potentials used to oxidize these compounds to CO2. Starting from the air emissions of a typical industrial wood-painting plant, a series of experiments studied in vitro microbial degradation of each individual VOC. Isolated strains were then added to a laboratory-scale biofiltration apparatus filled with an organic matrix, and the different VOC behavior demonstrated the potential of single and/or synergic microbial removal actions. When a single substrate was fed, the removal efficiency of a Pseudomonas aeruginosa inoculated reactor was 1.1, 1.17, and 0.33 g m−3hr−1, respectively, for xylene, toluene, and ethoxy propyl ace...

Sorption of a Xenobiotic Contaminant in Clean and Petroleum-Contaminated Soil: Roles of Water and Xenobiotic Size

Abstract: Environmental Context.Soil uptake of xenobiotics (e.g. pesticides) can be a complex phenomenon where it is useful to distinguish readily reversible sorption from longer-term retention. A scheme for doing this using fluorescence detection is presented here, along with application to uptake of a model compound in clean and oil-contaminated soils. Both the wetting of the soil and the size of the xenobiotic seem to be important. The present data concern uptake. Desorption is expected to exhibit dependencies on similar factors. The data have implications for understanding persistence. Abstract.Description of sorption of xenobiotics (e.g. pesticides) into soils requires identification of at least two kinetic components. In the present work, the distinction between ‘labile’ (readily reversible) and ‘non-labile’ (not reversible) uptake was extended, introducing a fluorescence-based method using 9-anthracenepropionic acid as a probe molecule. Study of clean, oil-contaminated wettable, and water-repellent oil-contaminated soils has given new perspectives into the role that water plays in xenobiotic uptake. Non-labile uptake is unimportant in the water-repellent soils; however, non-labile components are observed in both clean and wettable oil-contaminated samples, supporting earlier suggestions that water plays a role in non-labile uptake processes. A soil pre-exposed to water exhibited different labile sorption behavior from one where xenobiotic was added simultaneously with water to an air-dried soil. The comparatively rapid non-labile component of uptake (3 days) of 9-anthracenepropanoic acid by a clean soil contrasted with much longer times in earlier studies of 2,4-D and atrazine. This pointed to another factor influencing the sorption phenomenon. Literature data supports a suggestion that the non-labile component of xenobiotic sorption may be more strongly influenced by the size of the xenobiotic than by the structure (e.g. polarity) of the xenobiotic or soil composition.

Biodegradation of Organic Pollutants

Abstract: Biodegradation refers to the microbial decomposition of compounds, typically of those that negatively impact human health, in the environment. The genes, enzymes and pathways have been elucidated to understand environmental processes, engineer remediation of polluted environments and to predict the fate of chemicals in the environment. Keywords: biodegradation; bioremediation; bacteria; pollutants; evolution

Bioremediation of Oil-Contaminated Soil Using Rhizobacteria and Plants

Abstract: Phytoremediation is an economical and environmentally friendly bioremediation technique using plants which can increase the microbial population in soil. Unlike other pollutants such as heavy metals, poly-chlorinated biphenyl, trichloroethylene, perchloroethylene and so on, petroleum hydrocarbons are relatively easily degradable by soil microbes. For successful phytoremediation of soil contaminated with petroleum hydrocarbons, it is important to select plants with high removal efficiency through microbial degradation. In this study, we clarified the roles of plants and rhizobacteria and identified their species effective on phytore-mediation by reviewing the papers previously reported. Plants and rhizobacteria can degrade and remove the petroleum hydrocarbons directly and indirectly by stimulating each other's degradation activity. The preferred plant species are alfalfa, ryegrass, tall fescue, poplar, corn, etc. The microorganisms with a potential to degrade hydrocarbons mostly belong to Pseudomonas spp., Bacillus spp., and Alcaligenes spp. It has been reported that the elimination efficiency of hydrocarbons by soil microorganisms can be improved when plants were simultaneously applied. For more efficient restoration, it's necessary to understand the plant-rhizobacteria interaction and to select the suitable plant and microorganism species.

Advances in Research of Microbial Degradation of High Molecular Weight Polycyclic Aromatic Hydrocarbons-Benzo(a)pyrene

Abstract: Polycyclic aromatic hydrocarbons (PAHs), a kind of organic chemical pollutants consisting of two or more fused benzene rings, are widely distributed in environment. PAHs, particularly the types with higher molecular weight, cause great environmental concern because of their mutagenic, teratogenic and carcinogenic properties. Benzo(a)pyrene (BaP) is a five-ring polycyclic aromatic compound with acute carcinogenicity. It is classified as a priority pollutant in environment assessment. This paper reviews the occurrence and distribution of BaP in environment, as well as the ability and pathway for BaP degradation by microorganisms. In addition, approaches for improving microbial degradation of BaP are discussed. Fig 4, Tab 3, Ref 51

Kinetic study of biodegradation of organic matter extracted from black shale ore

Abstract: Microbial degradation of organic matter extracted from black shale ores was investigated. The kinetic models of biodegradation of organic compounds were discussed. The effect of chloroform on the rate of biomass growth was described. Our results suggested that the process of biodegradation of organic matter is rather slow. The Haldane kinetic model could be available to describe the kinetic of biodegradation of organic matter.

Enhanced Microbial Remediation of Long-term Polycyclic Aromatic Hydrocarbons (PAHs) Polluted Soils

Abstract: An incubation experiment was carried out with farmland soils long-term polluted by PAHs under different carbon sources, aeration condition, and soil moisture contents. The results showed that aeration treatment enhanced degradation rates of Phe and B[a]P in soils to 59.44% and 26.14%, while submerging treatment enhanced them to 46.48% and 13.27%, respectively. Glucose and starch carbon sources were beneficial to the growth of soil microorganisms, thus accelerated the degradation of PAHs in polluted soils. Soil PAHs degradation rate was not only correlated with carbon sources, but also closely correlated with their application contents. The results also indicated soil PAHs-degrading microorganisms were positively correlated with total microorganism numbers in long-term PAHs polluted soils.

Limitations and possibilities for microbial degradation of organic contaminants in aquifers

Abstract: Many factors can influence the rate of microbial degradation of organic contaminants in aquifers. Some of these factors, e.g. sorption, the presence of dissolved organic matter (DOM), biomass, redox conditions, cometabolism, microbial community composition, and activity of degrading microorganisms, are addressed in the thesis, with the ambition to gain insights for bioremediation in aquifers, and to prevent contaminants from spreading to drinking water supplies. The influence of sorption of organic contaminants and bacteria on biodegradation was investigated under both diffusion limited and advective flow conditions. The results demonstrated that degradation by suspended bacteria of dissolved aniline and 2,4-dichlorophenol (2,4-DCP) was faster than the degradation of the same compounds sorbed to the solids. Nevertheless, sorption to the solids was not sufficient to exclude trace contaminants from degradation. In another investigation, the main effect of DOM on microbial phenanthrene degradation in groundwater was stimulation of microbial growth and activity of degrading populations, with hydrophilic DOM having a better effect than hydrophobic. In another study, glyphosate degradation was different in a mixture of sediment and groundwater from two aquifers (Vejen, Denmark and Vomb, Sweden). Laboratory experiments excluded sorption and organic carbon limitation as major sources of the observed differences. Glyphosate degradation was positively correlated to the density of bacteria, but the difference in density between the two sites was too small to account for the degradation difference. Instead, it was found that the differences in metabolic activity of the degrading strains and the microbial community composition of the aquifers were large and coincided with differences in rates of biodegradation. Glyphosate sorption was lower and biodegradation was slower under anaerobic conditions compared with aerobic, and most of the degraded glyphosate was not mineralized but cometabolized to AMPA. The thesis pin-points to the possibilities to increase contaminant biodegradation in aquifers by the addition of i) oxygen to stimulate aerobic degradation, ii) a primary growth substrate, e.g. DOM, to increase biomass and thereby stimulate metabolic or cometabolic degradation, and iii) microorganisms to increase the degrading population and/or genetic capacity.

Screening and the Degradation Conditions of Pyrene-degrading Bacterium

Abstract: Elevated concentrations of polycyclic aromatic hydrocarbons(PAHs) have been found in soils due to petroleum pollution,and microbial degradation has been suggested as the best way to remove PAHs from contaminated soils.One pyrene-degrading bacterium(B4) was isolated from petroleum contaminated soil.The bacterial strain was identified as Pseudomonas sp.based on its morphological and biochemical characteristics.The degradation of pyrene by bacterial strain B4 and the effects of salinity,pH,heavy metals and the co-metabolism substrates such as glucose and salicylic acid on biodegradation potential were also investigated.The results showed that the bacterial strain B4 had a potential of pyrene degradation,with the degradation rate of pyrene 91.70% when the pyrene concentration was 50 mg·L-1 in solution culture.A significantly positive correlation was found between bacterial growth and pyrene degradation.The bacterial growth was inhibited when pH was 4 or salinity was 8%.Cd2+(100 mg·L-1) was toxic to the bacterial strain,Cu2+(50 mg·L-1) could partially inhibit the growth of strain B4,Zn2+(100 mg·L-1) and Pb2+(200 mg·L-1) had no significant influence on the bacterial growth.The degradation rate of pyrene could reach 95.55% under salicylic acid(50 mg·L-1) co-existing with pyrene.The addition of glucose(50 mg·L-1) had a little inhibition influence on growth of bacterial strain B4 and biodegradation of pyrene.These results may provide scientific theories and effective PAHs-degrading bacteria for bioremediation of PAHs contaminated soils.