Degradation Of Polycyclic Aromatic Hydrocarbons In Soil Research Articles

Importance of soil ecoenzyme stoichiometry for efficient polycyclic aromatic hydrocarbon biodegradation

Efficient remediation of soil contaminated by polycyclic aromatic hydrocarbons (PAHs) is challenging. To determine whether soil ecoenzyme stoichiometry influences PAH degradation under biostimulation and bioaugmentation, this study initially characterized soil ecoenzyme stoichiometry via a PAH degradation experiment and subsequently designed a validation experiment to answer this question. The results showed that inoculation of PAH degradation consortia ZY-PHE plus vanillate efficiently degraded phenanthrene with a K value of 0.471 (depending on first-order kinetics), followed by treatment with ZY-PHE and control. Ecoenzyme stoichiometry data revealed that the EEAC:N, vector length and angle increased before day five and decreased during the degradation process. In contrast, EEAN:P decreased and then increased. These results indicated that the rapid PAH degradation period induced more C limitation and organic P mineralization. Correlation analysis indicated that the degradation rate K was negatively correlated with vector length, EEAC:P, and EEAN:P, suggesting that C limitation and relatively less efficient P mineralization could inhibit biodegradation. Therefore, incorporating liable carbon and acid phosphatase or soluble P promoted PAH degradation in soils with ZY-PHE. This study provides novel insights into the relationship between soil ecoenzyme stoichiometry and PAH degradation. It is suggested that soil ecoenzyme stoichiometry be evaluated before designing bioremeiation stragtegies for PAH contanminated soils.

Remediation of PAHs contaminated soil enhanced by nano-zero-valent iron combined with white rot fungi Peniophora incarnata

Polycyclic aromatic hydrocarbons (PAHs) are hazardous organic contaminants commonly found in soils. This study investigated an integrated remediation approach combining white rot fungal augmentation and nano-zero-valent iron (nZVI) activated persulfate oxidation to enhance the degradation of PAHs in soil. The white rot fungus Peniophora incarnata demonstrated efficient degradation of lower molecular weight PAHs including phenanthrene (91% removal) and anthracene (71% removal) in liquid culture, but had limited degradation of high molecular weight benzo[a]pyrene (35% removal). Inoculation of P. incarnata in non-sterilized soil led to 54% phenanthrene and 46% anthracene removal after 42 days. Co-inoculation with Pseudomonas aeruginosa slightly improved the degradation by 15% and 13%, respectively. However, benzo[a]pyrene removal was negligible through bioremediation alone. Direct nZVI/persulfate oxidation removed 55–75% of lower molecular weight PAHs and 68% of benzo[a]pyrene in soil. Integrated treatment combining nZVI/persulfate oxidation followed by P. incarnata inoculation resulted in 92–96% degradation of phenanthrene, anthracene and benzo[a]pyrene after 42 days, which was significantly higher than either approach alone. The enhanced PAH removal was attributed to the coupled effects of chemical oxidation and microbial degradation. P. aeruginosa is capable of directly metabolizing and mineralizing lower molecular weight PAHs such as phenanthrene and anthracene. The presence of this additional PAH-degrading bacterium likely contributed to the increased degradation observed. Overall, the sequential nZVI/persulfate and fungal treatment provides an effective and sustainable option for remediating soils co-contaminated with PAHs and other persistent organics. Further research should optimize treatment parameters and evaluate long-term impacts on soil quality.

Surfactant-enhanced anoxic degradation of polycyclic aromatic hydrocarbons (PAHs) in aged subsurface soil at high temperature (60 °C)

Thermally enhanced anoxic biodegradation is emerging as a promising method for removing PAHs from subsurface soil. However, some PAHs still remain in soil following remediation with thermally enhanced anoxic degradation due to low bioavailability of these residual PAHs. The effects of five surfactants (Tween 80, TX 100, Brij 30, SDS, and SDBS) on the desorption of PAHs, anoxic degradation of PAHs, and native bacteria in soil at high temperature (60 °C) were evaluated in this study. The desorption of PAHs in soil increased as surfactant concentration increased. Low doses of surfactants (0.08%, w/w) enhanced the growth of potential PAHs degrading bacteria and promoted the anoxic degradation of PAHs, whereas high doses of surfactants (0.3%–0.8%, w/w) displayed the opposite effect, and the degree of inhibition increased with increasing surfactant concentration. The results also indicated that the inhibitory effect of anionic surfactants (SDS and SDBS) on microbial growth and PAHs degradation is stronger than that of nonionic surfactants (Tween 80, TX 100 and Brij 30) at the same concentration. These results suggest a feasible way of enhancing the anoxic degradation of PAHs in soil where heat cannot be effectively utilized when in situ thermal desorption (ISTD) technology is used.

Zeolite composites from fly ashes mixed with leonardite as a useful addition to fertilizer for accelerating the PAHs degradation in soil

Extensive soil degradation caused by polycyclic aromatic hydrocarbons (PAHs) leads to serious agriculture problems around the world. One of the most effective ways to restore soil functionality and fertility, limiting the possibility of PAHs uptaken by plants, is soil remediation. The remediation of PAHs in soils can be enhanced by adding mineral-organic additives rich in carbon. The aim of the work was the evaluate the effectiveness of the mineral-organic mixtures consisting of the zeolite-carbon (NaX-C) or zeolite-vermiculite (NaX-Ver) composites mixed with leonardite and mineral salts (NPK) in terms of their effect on the remediation of PAHs contaminated soils. The accumulation and translocation of PAHs in maize tissues, the fatty acids profile in grains and the BaP equivalent concentration (B(a)Peq) based on the ∑4 PAHs with the highest carcinogenic to humans were also investigated. The application of zeolite composites with leonardite increased the losses of PAHs content in soils with simultaneous reduction of their content in plants without a negative impact on the mass of maize grains. In soils, a significant correlation was found between 4, 5, and 6-rings PAHs and total organic carbon (TOC) content and between dehydrogenases activity and TOC content. The highest reduction of PAHs in plant tissues and the lowest B(a)Peq was observed after application of zeolite-carbon composites with the lover dose. The addition of the proposed amendments positively affected the unsaturated fatty acids/saturated fatty acids (UFA/SFA) ratio.

Degradation of PAHs in soil by activated persulfate system with activated carbon supported iron-based bimetal

We developed a material of activated carbon (AC)-supported highly active iron-based bimetal (iron–copper bimetal/AC, Fe-Cu/AC) with high efficiency for polycyclic aromatic hydrocarbons (PAHs) degradation in soil by activating persulfate, benefiting from the synergistic effect that the characteristics of AC with porous carbon backbone, multiple active functional groups, high loading capacity and the characteristics of FeCu bimetal with high activity. The addition of Cu to the Fe-based/AC activator not only improved the dispersibility of Fe particles but also maintained the stability of the metal in the Fe-Cu/AC. The thermal activation (50 °C) promoted the degradation of PAHs by the Fe-Cu/AC-activated S2O82− system. Of the various systems tested, the Fe-Cu/AC-activated S2O82− system had the best degradation efficiency for 19 PAHs, with the overall efficiency following the order of Fe-Cu/AC + S2O82− > Fe-Cu + S2O82− > Fe-Cu/AC > S2O82−. The degradation mechanism of the Fe-Cu/AC-activated S2O82− system on soil PAHs showed that OH, OOH, and SO4− were the main active groups involved in the degradation of target PAHs. The target pollutants and their degradation products in the Fe-Cu/AC-activated S2O82− system indicated specific exposure pathways, providing a theoretical basis for the remediation of PAH-contaminated soil.

Nitrogen transformation promotes the anaerobic degradation of PAHs in water level fluctuation zone of the Three Gorges Reservoir in Yangtze River, China: Evidences derived from in-situ experiment

Polycyclic aromatic hydrocarbons (PAHs) pose a great threat to human health and ecological system safety. The interception of nitrogen is common found in the riparian zone. However, there is no evidence on how nitrogen addition affects the anaerobic degradation of PAHs in soil of the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in Yangtze River, China. Here, we investigated the PAHs degradation rate, the variation of key functional genes and microbial communities after nitrogen addition in soil that experienced a flooding period of water-level-fluctuation. The results revealed that the ∑16PAHs were decreased 16.19 %–36.65 % and more 3–5-rings PAHs were biodegraded with nitrogen addition in WLFZ. The most genes involved in PAHs-anaerobic degradation and denitrification were up-regulated by nitrate addition, and phyla Firmicutes, Actinobacteria and Proteobacteria were more advantages in nitrogen addition groups. The Tax4Fun based genome function analysis revealed that the microbial activity of PAHs-degradation increased with nitrate addition. The co-occurrence network analysis indicated that nitrogen addition accelerated the metabolism of nitrogen and PAHs. It is the first time to provide the direct experimental evidences that nitrogen transformation in the WLFZ soil promotes anaerobic PAHs degradation. This work is of importance to understand the effect of nitrogen intercepted in the WLFZ soil of TGR in Yangtze River, China.

The combined enhancement of RL, nZVI and AQDS on the microbial anaerobic-aerobic degradation of PAHs in soil

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent organic pollutants in soil, which have carcinogenic, teratogenic and mutagenic hazards. The effects of rhamnolipid (RL), nano zero-valent iron (nZVI), and anthraquinone-2,6-disulfonic acid (AQDS) on the degradation of PAHs in soil were studied. It was found that the treatment of 5 mg·kg−1RL + 1% nZVI +0.2 mmol·kg−1AQDS had the highest degradation rate. The degradation rate of total PAHs and HMW-PAHs was 72.81% and 79.47% respectively after 90 days. High-throughput sequencing showed that in RL + nZVI + AQDS enhanced soil, Clostridium, Geobacter, Anaeromyxobacter and Sphingomonas were the dominant species for anaerobic degradation of PAHs. Rhodococcus, Nocardioides, and Microvirga are the dominant species for aerobic degradation of PAHs. The activities of methyltransferase, dehydrogenase and catechol 1,2-dioxygenase in the anaerobic-aerobic degradation process of PAHs were consistent with the degradation process of PAHs, indicating the role of these enzymes in the degradation of PAHs. RL, nZVI, and AQDS combined enhanced microbial anaerobic-aerobic degradation has great application potential in remediation of PAHs-contaminated soil.

UV Assisted Photocatalytic Remediation of Polycyclic Aromatic Hydrocarbons (Pahs) in Sewage Sludge Addition Soils Using Synthesized Nanometer Mixed-crystal TiO2: Experiment and Simulation

ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) entering into soils via sewage sludge could be posing arresting environmental risks. Photocatalytic remediation has been considered as a powerful tool of degrading PAHs in soils. In the present study, a synthesized nanometer mixed-crystal TiO2 was employed to investigate its effects on PAHs degradation in soils by Ultra Violet (UV).Two typical substances of PAHs widely detected in sewage sludge (phenanthrene and pyrene), were added into soils from farmland to provide polluted soil for a series of simulated experiments. These polluted soil samples were mixed with five different levels of the synthesized TiO2 (0, 1%, 2%, 3%, and 4%, wet weights) and the decreases of PAHs under different conditions were compared. The results showed that among the five treatments, the activities of the synthesized photocatalysts followed the order of 0<1%<2%<4%<3%, and the 3% of TiO2 addition was determined as the optimum condition. Moreover, it was found that the degradation of PAHs could increase with the increasements of H2O2, photon flux, and humic substances in treated soils. Our findings supports the perspective of using nanometer synthesized mixed-crystal TiO2 in the degradation of PAHs in contaminated soils by sewage sludge addition and provides hints on understanding the mechanism of developing photocatalytic remediation in PAHs-contaminated soils.

Analysis of Factors Influencing Plant–Microbe Combined Remediation of Soil Contaminated by Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are frequently detected in soil. Their biological toxicity and carcinogenic, teratogenic, and mutagenic effects pose a great threat to the ecological environment and human health. Firstly, the sources, physicochemical properties, and environmental hazards of PAHs are reviewed in this paper, and then their pollution status and different methods for their detection in soil are described in detail. The remediation technologies to treat pollution in the field and farmland are compared, and the technical status and factors influencing phytomicrobial remediation of PAHs in contaminated soil are evaluated in the most comprehensive way. The mechanisms of phytomicrobial remediation of PAHs-contaminated soil under different conditions are innovatively discussed. Additionally, the regulation mechanism of enzymes involved in plant and microbial degradation of PAHs in soils is studied. This is the first study on the regulation mechanism of degradation enzyme in a PAHs review. The aim of this paper is to review the pollution status, remediation technologies, mechanisms, and biodegradation actions of PAHs in soil. This review creatively provides reliable technical support for strengthening soil remediation and environmental management.

Earthworms and rice straw enhanced soil bacterial diversity and promoted the degradation of phenanthrene

BackgroundSince the industrial revolution, the contamination of agricultural soils by polycyclic aromatic hydrocarbons (PAHs) has increasingly become of serious global environmental concern and poses a huge threat to human beings and natural ecosystems. Microbial degradation is a proved technology mostly used to depollute polycyclic aromatic hydrocarbon (PAH) in the environment. However, very limited information is available regarding the interaction of earthworms with rice straw on the soil microbial community and the degradation of phenanthrene. This study was performed to enlighten the rice straw and earthworms’ interaction on soil bacterial abundance and structure and phenanthrene removal.ResultsResult about functional gene information revealed that both rice straw and earthworm enhanced phenanthrene degradation. Subsequently, both Shannon diversity index (r2 = − 0.8807, p < 0.001) and bacterial 16S rRNA genes (r2 = − 0.7795, p < 0.001) negatively correlated with the remaining phenanthrene concentration in soil. The application of both rice straw and earthworms in soil had the lowest ratio of soil remaining phenanthrene concentration (0.16 ± 0.02), the highest Shannon diversity index (6.45 ± 0.2) and the highest bacterial 16S rRNA genes. This implied that both earthworms and rice straw might improve the phenanthrene metabolism by increasing soil bacteria diversity. The abundance of genera Pseudomonas, Luteimonas, Rhodanobacter, Sphingomonas, Gemmatimonas, Flavobacterium, and Leifsonia was significantly increased in the presence of both earthworms and rice straw and was found to negatively correlate with the remaining phenanthrene concentration in soil.ConclusionBased on these results, this study offers clear and strong evidences that the positive interaction between earthworms and rice straw could promote phenanthrene degradation in soil. These finding will improve our understanding on the importance of the natural resources forsaken and how they can interact with the soil macro- and microorganisms to change soil structure and enhance PAH degradation in soil.

Effects of corn straw on dissipation of polycyclic aromatic hydrocarbons and potential application of backpropagation artificial neural network prediction model for PAHs bioremediation

In order to provide a viable option for remediation of PAHs-contaminated soils, a greenhouse experiment was conducted to assess the effect of corn straw amendment (1%, 2%, 4% or 6%, w/w) on dissipation of aged polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. Backpropagation artificial neural network (BP-ANN) was applied to model the relationships between soil properties and PAHs concentration in soils. The removal rate of PAHs, enzyme activity (catalase and dehydrogenase), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) in soils were investigated to evaluate the dissipation of PAHs under different ratio of corn straw amendment. The present study showed that corn straw amendment apparently accelerated the dissipation of PAHs after incubation of 112 days, especially under 4% and 6% treatments. Compared with non-amended soil, corn straw amendment significantly (p < 0.05) increased the removal rate of low molecular weight (LMW) PAHs and significantly (p < 0.05) enhanced the dissipation of high molecular weight (HMW) PAHs only under 6% treatment. Moreover, corn straw amendment increased activities of catalase and dehydrogenase, concentrations of DOC and MBC in soils, which are beneficial to the degradation of PAHs in soils. The performance of the BP-ANN model was assessed through the root mean square error (RMSE) and determination coefficient (R2). The results indicated that BP-ANN model could provide satisfactory prediction of PAHs concentration in soils during incubation period at R2 and RMSE values of 0.948, 187.4 μg kg−1, respectively. The results indicated that high amendment of corn straw was a potential option for remediation of PAHs-contaminated soils and that the BP-ANN model could successfully provide prompt prediction of PAHs concentration in soils.

Modelling PAH Degradation in Contaminated Soils in Canada using a Modified Process‐Based Model (DNDC)

Core Ideas A novel process‐based model for degradation of soil PAHs is proposed. Model is based on dynamic interactions between water, soil, vegetation and climate. Model effectively simulates biogeochemical processes of PAH degradation in soils. Simulated results agree well with data from remediation sites in Alberta. Polycyclic aromatic hydrocarbons (PAHs) are persistent pollutants of concern. A process‐based model of the PAH degradation can improve our understanding of ecological drivers and processes. In this paper, a process‐based biogeochemistry model, DeNitrification‐DeComposition (DNDC) is modified to simulate the dynamics of PAHs degradation in soils at abandoned oil and gas well sites. This new version of DNDC‐Organic Pollutants, called DNDC‐OP, coupled the rates of PAH degradation with dynamics of soil, vegetation and climate, such as soil moisture and temperature. The model was parameterized and validated against datasets of four soil PAHs: pyrene, fluorene, chrysene and anthracene, at three different abandoned oil and gas well site locations in Alberta, Canada. The sensitivity of the parameters was analyzed and tested. The simulated results were in good agreement with the measured data with a coefficient of determination (R2) of 70 to 97%, and the root mean square error (RMSE) of 4.5 to 9.1 at all three sites. We also evaluated the influence of environmental factors, such as soil temperature and moisture, on the degradation of PAHs. An increased degradation of all four PAHs occurred with increasing soil moisture content. An increase of soil temperature from 10 to 20°C and subsequently to 25°C resulted in a decreased appearance of all four PAHs from the three well sites. The result shows that this model can be used as a tool for evaluating PAH degradation for effective reclamation strategies.

Soil Particles and Phenanthrene Interact in Defining the Metabolic Profile of Pseudomonas putida G7: A Vibrational Spectroscopy Approach.

In soil, organic matter and mineral particles (soil particles; SPs) strongly influence the bio-available fraction of organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), and the metabolic activity of bacteria. However, the effect of SPs as well as comparative approaches to discriminate the metabolic responses to PAHs from those to simple carbon sources are seldom considered in mineralization experiments, limiting our knowledge concerning the dynamics of contaminants in soil. In this study, the metabolic profile of a model PAH-degrading bacterium, Pseudomonas putida G7, grown in the absence and presence of different SPs (i.e., sand, clays and humic acids), using either phenanthrene or glucose as the sole carbon and energy source, was characterized using vibrational spectroscopy (i.e., FT-Raman and FT-IR spectroscopy) and multivariate classification analysis (i.e., PLS-DA). The different type of SPs specifically altered the metabolic profile of P. putida, especially in combination with phenanthrene. In comparison to the cells grown in the absence of SPs, sand induced no remarkable change in the metabolic profile of the cells, whereas clays and humic acids affected it the most, as revealed by the higher discriminative accuracy (R2, RMSEP and sensitivity) of the PLS-DA for those conditions. With respect to the carbon-source (phenanthrene vs. glucose), no effect on the metabolic profile was evident in the absence of SPs or in the presence of sand. On the other hand, with clays and humic acids, more pronounced spectral clusters between cells grown on glucose or on phenanthrene were evident, suggesting that these SPs modify the way cells access and metabolize PAHs. The macromolecular changes regarded mainly protein secondary structures (a shift from α-helices to β-sheets), amino acid levels, nucleic acid conformation and cell wall carbohydrates. Our results provide new interesting evidences that SPs specifically interact with PAHs in defining bacteria metabolic profiles and further emphasize the importance of studying the interaction of bacteria with their surrounding matrix to deeply understand PAHs degradation in soils.

ENHANCED BIODEGRADATION OF HIGH MOLECULAR WEIGHT PAHS USING YEAST CONSORTIA IMMOBILIZED ON MODIFIED BIOWASTE MATERIAL

The degradation of high molecular weight (HMW) polycyclic aromatic hydrocarbons (PAHs) viz. benzo[a]pyrene, perylene and benzo[ghi]perylene in soil was studied using yeast consortia immobilized on various conventional matrices and biowaste materials. Enhanced biodegradation was noted using yeast consortia immobilized in sawdust powder (SDP) which was chemically modified (M-SDP). The chemical modification destroyed the benzene rings in the cellulose and improved the porosity of SDP by decreasing the crystallinity of the biomass. The physicochemical properties of M-SDP were characterized with SEM, EDAX analysis, FTIR analysis, X-ray diffraction and TGA analysis. Application of M-SDP showed enhanced degradation of benzo[a]pyrene (82.5%) followed by perylene (75.1%) and benzo[ghi]perylene (63.4%) after 40 days of incubation. The results of kinetic study demonstrated that HMW PAHs degradation in soil fitted the first order kinetic model. It can be concluded that chemically modified SDP can serve as potential immobilization matrix for supporting pollutant-degrading yeast consortia which can be employed for effective degradation of HMW PAHs.

Comparison of plants with C3 and C4 carbon fixation pathways for remediation of polycyclic aromatic hydrocarbon contaminated soils

The phytoremediation technique has been demonstrated to be a viable option for the remediation of polycyclic aromatic hydrocarbons (PAHs) contaminated sites. This study evaluated the potential applicability of plants with C3 and C4 carbon fixation pathways for the phytoremediation of recalcitrant high molecular weight (HMW) PAHs contaminated soil. A 60 and 120-day greenhouse study was conducted which showed higher degradation of HMW PAHs in soil grown with C4 plants when compared to C3 plants. Also, no PAHs were detected in the maize cobs, sunflower, wallaby, and Sudan grass seeds at the end of the experiment. The effect of plants in modifying the microbial community and dynamics in the rhizosphere was also examined by measuring soil biochemical properties such as dehydrogenase activity and water-soluble phenols. The results demonstrate a substantial difference in the microbial populations between planted and unplanted soils, which in turn facilitate the degradation of PAHs. To the best of our knowledge, this study for the first time evaluated the phytoremediation efficacy through the A. cepa cyto- and genotoxicity assay which should be considered as an integral part of all remediation experiments.

Assessment of PAH contaminated land: Implementing a risk-based approach

Polycyclic aromatic hydrocarbons (PAHs) are amongst the most common ubiquitous anthropogenic pollutants of terrestrial ecosystems. There are currently multiple sources of PAHs in Nigeria and land use activities have been shown to alter the composition of PAHs and in some cases increase the fractions of carcinogenic and recalcitrant components. This report considers the implementation of a more specific risk based corrective action to abate threats caused by carcinogenic PAHs in eroded and degraded soils for prospective risk assessment and realistic decision-making. Bioremediation is promoted for degradation of PAHs in soils, but faces several limitations that question the effectiveness of the approach. This review provides insights into bioaccessibility and chemical activity assessment of PAHs as a procedure of risk assessment and the potential use of specially produced biochar designed for specific risk mitigation remedial action was also considered.

Insight into the Modulation of Dissolved Organic Matter on Microbial Remediation of PAH-Contaminated Soils.

Microorganisms play a key role in degradation of polycyclic aromatic hydrocarbons (PAHs) in environments. Dissolved organic matter (DOM) can enhance microbial degradation of PAHs in soils. However, it is not clear how will the soil microbial community respond to addition of DOM during bioremediation of PAH-contaminated soils. In this study, DOMs derived from various agricultural wastes were applied to remediate the aging PAH-contaminated soils in a 90-day microcosm experiment. Results showed that the addition of DOMs offered a more efficient and persistent elimination of soil PAHs compared to the control which had no DOM addition. PAH removal effects were different among treatments with various DOMs; the addition of DOMs with high proportion of hydrophobic fraction could remove PAHs more efficiently from the soil. Low-molecular-weight (LMW) PAHs were more easily eliminated than that with high-molecular-weight (HMW). Addition of DOMs significantly increased abundance of 16S ribosomal RNA (rRNA), pdo1, nah, and C12O genes and obviously changed community compositions of nah and C12O genes in different ways in the PAH-contaminated soil. Phylogenetic analyses of clone libraries exhibited that all of nah sequences and most of C12O sequences were affiliated into Gammaproteobacteria and Betaproteobacteria. These results suggested that external stimuli produced by DOMs could enhance the microbial degradation of PAHs in soils through not only solubilizing PAHs but also altering abundance and composition of indigenous microbial degraders. Our results reinforce the understanding of role of DOMs in mediating degradation of PAHs by microorganims in soils.

Abundance and diversity of polycyclic aromatic hydrocarbon degradation bacteria in urban roadside soils in Shanghai.

Understanding the impact of polycyclic aromatic hydrocarbons (PAHs) on soil environments is of increasingly important concern. Therefore, the microbial degradation of PAHs in soils has drawn considerable attention, but little is known about the PAH degradation genes in urban soils. In this study, we examined the diversity and abundance of the PAH degradation bacteria and evaluated whether the specific bacteria can reflect PAH contents in the soils from urban roadsides directly receiving traffic emission. The results of phylogenetic analysis indicated that low PAH degradation bacterial diversity occurred in the urban roadside soils, only including Mycobacterium sp., Terrabacter sp., and one novel cluster. The community composition diversity of PAH degradation bacteria did not show a significant difference across the sampling sites. The abundance of PAH degradation genes ranged from 5.70 × 10(6) to 6.44 × 10(7) gene copies g(-1) dry soil, with an average abundance of 1.43 × 10(7) gene copies g(-1) dry soil, and their spatial variations were related significantly to PAH contents in the soils. The Mycobacterium sp. was the most widely detected and estimated to occupy 65.9-100 % of the total PAH degradation bacteria at most of the soil samples, implying that the Mycobacterium sp. might play a primary role in degrading PAHs in the contaminated urban soil environments.

Electrokinetics-enhanced biodegradation of heavy polycyclic aromatic hydrocarbons in soil around iron and steel industries

Bioremediation is a safe and cost-effective technology for the removal of polycyclic aromatic hydrocarbons (PAHs) contaminated soils, but its remediation rate is usually very slow at soils contaminated with heavy PAHs in high concentrations. This paper describes the feasibility of using electrokinetics to enhance the degradation of heavy PAHs in soil around iron and steel industries. Three bench-scale experiments were conducted for 90 days using historically polluted soil with a total PAHs content of 220.01mg/kg dry soil. All of the experiments were inoculated with PAHs degrading bacteria, but experiments II and III were performed using constant polarity and alternating polarity electrokinetic conditions, respectively. Results were compared with those from the control experiment (experiment I), which did not receive any electrokinetic treatment. The results demonstrated that the electrokinetic process could enhance the biodegradation extent of total PAHs and heavy PAHs in the soil. The final degradation extents of total PAHs were 9.5% and 13.5% higher in experiments II and III, respectively, as compared to experiment I. Under the electrokinetic and bacteria conditions, the relative enhancement in the degradation of four- to six-ring PAHs compared to the control experiment was much stronger and increased with increasing ring number. The final degradation extents of four- to six-ring PAHs increased by 7.9–8.6%, 11.0–18.4% and 17.2–25.6% in experiments II and III compared to experiment I, respectively. The results also showed that the electrokinetic operation mode could affect not only the degradation extent of total PAHs but also bacterial counts and soil moisture of different regions in soils. The use of alternating polarity electrokinetics was favorable to the bacterial growth and kept the soil properties uniform. In addition, there was a positive correlation between the degradation extent of PAHs, bacterial counts and moisture content by Pearson correlation analysis under electrokinetics. The results of this work demonstrate that the use of electrokinetics can significantly enhance the degradation of PAHs by influencing soil conditions. Therefore, the use of electrokinetic technology may provide a useful tool for enhancing the bioremediation of heavy PAHs in soil.

Influence of earthworm activity on microbial communities related with the degradation of persistent pollutants

Earthworms may promote the biodegradation of polycyclic aromatic hydrocarbons (PAHs) in soil, but the mechanism through which they exert such influence is still unknown. To determine if the stimulation of PAH degradation by earthworms is related to changes in microbial communities, a microcosm experiment was conducted consisting of columns with natural uncontaminated soil covered with PAH-contaminated dredge sediment. Columns without and with low and high Eisenia andrei densities were prepared. Organic matter and PAH content, microbial biomass, and dehydrogenase activity (DHA) were measured in soil and sediment over time. Biolog Ecoplate™ and polymerase chain reaction using denaturing gradient gel electrophoresis were used to evaluate changes in metabolic and structural diversity of the microbial community, respectively. Earthworm activity promoted PAH degradation in soil, which was significant for biphenyl, benzo[a]pyrene, and benzo[e]pyrene. Microbial biomass and DHA activity generally did not change over the experiment. Earthworm activity did change microbial community structure, but this did not affect its functioning in terms of carbon substrate consumption. Results suggest no relationship between changes in the microbial community by earthworm activity and increased PAH disappearance. The role of shifts in soil microbial community structure induced by earthworms in PAH removal needs further investigation.