Effect of root death and decay on dissipation of polycyclic aromatic hydrocarbons in the rhizosphere of yellow sweet clover and tall fescue

Amide herbicides: Analysis of their environmental fate, combined plant-microorganism soil remediation scheme, and risk prevention and control strategies for sensitive populations

In this study, we predicted the environmental fate of amide herbicides (AHs) using the EQC (EQuilibrium Criterion) model. We found that the soil phase is the main reservoir of AHs in the environment. Second, a toxicokinetic prediction indicated that butachlor have a low human health risk, while the alachlor, acetochlor, metolachlor, napropamide, and propanil are all uncertain. To address the environmental and human-health-related threats posed by AHs, 27 new proteins/enzymes that easily absorb, degrade, and mineralize AHs were designed. Compared with the target protein/enzyme, the comprehensive evaluation value of the new proteins/enzymes increased significantly: the absorption protein increased by 20.29-113.49%; the degradation enzyme increased by 151.26-425.22%; and the mineralization enzyme increased by 23.70-52.16%. Further experiments revealed that the remediating effect of 13 new proteins/enzymes could be significantly enhanced to facilitate their applicability under real environmental conditions. The hydrophobic interactions, van der Waals forces, and polar solvation are the key factors influencing plant-microorganism remediation. Finally, the simulations revealed that appropriate consumption of kiwifruit or simultaneous consumption of ginseng, carrot, and spinach, and avoiding the simultaneous consumption of maize and carrot/spinach are the most effective means reduce the risk of exhibiting AH-linked toxicity.

Assessment of the Suitability of Melilotus officinalis for Phytoremediation of Soil Contaminated with Petroleum Hydrocarbons (TPH and PAH), Zn, Pb and Cd Based on Toxicological Tests

The article presents issues related to the possibility of using toxicological tests as a tool to monitor the progress of soil treatment contaminated with petroleum substances (TPH, PAH), Zn, Pb and Cd in bio-phytoremediation processes. In order to reduce the high content of petroleum pollutants (TPH = 56,371 mg kg⁻¹ dry mass, PAH = 139.3 mg kg⁻¹ dry mass), the technology of stepwise soil treatment was applied, including basic bioremediation and inoculation with biopreparations based of indigenous non-pathogenic species of bacteria, fungi and yeasts. As a result of basic bioremediation in laboratory conditions (ex-situ method), the reduction of petroleum pollutants TPH by 33.9% and PAH by 9.5% was achieved. The introduction of inoculation with biopraparation-1 prepared on the basis of non-pathogenic species of indigenous bacteria made it possible to reduce the TPH content by 86.3%, PAH by 40.3%. The use of a biopreparation-1 enriched with indigenous non-pathogenic species of fungi and yeasts in the third series of inoculation increased to an increase in the degree of biodegradation of aliphatic hydrocarbons with long carbon chains and PAH by a further 28.9%. In the next stage of soil treatment after biodegradation processes, which was characterized by an increased content of heavy metals (Zn, Pb, Cd) and naphthalene, chrysene, benzo(a)anthracene and benzo(ghi)perylene belonging to polycyclic aromatic hydrocarbons, phytoremediation with the use of Melilotus officinalis was applied. After the six-month phytoremediation process, the following was achieved: Zn content by 25.1%, Pb by 27.9%, Cd by 23.2% and TPH by 42.2% and PAH by 49.9%. The rate of removal of individual groups of hydrocarbons was in the decreasing order: C₁₂–C₁₈ > C₆–C₁₂ > C₁₈–C₂₅ > C₂₅–C₃₆. PAHs tended to be removed in the following order: chrysene > naphthalene > benzo(a)anthracene > benzo(ghi)perylene. The TF and BCF coefficients were calculated to assess the capacity of M. officinalis to accumulate metal in tissues, uptake from soil and transfer from roots to shoots. The values of TF translocation coefficients were, respectively, for Zn (0.44), Pb (0.12), Cd (0.40). The calculated BCF concentration factors (BCF_roots > BCF_shoots) show that heavy metals taken up by M. officinalis are mainly accumulated in the root tissues in the following order Zn > Pb > Cd, revealing a poor metal translocation from the root to the shoots. This process was carried out in laboratory conditions for a period of 6 months. The process of phytoremediation of contaminated soil using M. officinalis assisted with fertilization was monitored by means of toxicological tests: Microtox, Ostracodtoxkit F^TM, MARA and Phytotoxkit^TM. The performed phytotoxicity tests have indicated variable sensitivity of the tested plants on contaminants occurring in the studied soils, following the sequence: Lepidium sativum < Sorghum saccharatum < Sinapis alba. The sensitivity of toxicological tests was comparable and increased in the order: MARA < Ostracodtoxkit F^TM < Microtox. The results of the toxicological monitoring as a function of the time of soil treatment, together with chemical analyses determining the content of toxicants in soil and biomass M. officinalis, clearly confirmed the effectiveness of the applied concept of bioremediation of soils contaminated with zinc, lead and cadmium in the presence of petroleum hydrocarbons.

Rhizosphere effect on removal and bioavailability of PAHs in contaminated agricultural soil

The rhizosphere effect of ryegrass (Lolium perenne L.) on polycyclic aromatic hydrocarbons (PAHs) dissipation, bioavailability and the structure change of microbial community was investigated using a compartmented device-rhizobox. The PAHs removal efficiency, bioavailability and the change in structure of the microbial community were ascertained using HPLC, Tenax-TA extraction and PCR-DGGE, respectively. The results showed that in the root area (R1) and bulk soil (CK), the removal of 3-ring PAHs were 97.72 ± 0.34% and 95.51 ± 0.75%, 4-ring PAHs were 89.01 ± 1.61% and 78.65 ± 0.47%, 5-ring PAHs were 77.64 ± 4.05% and 48.63 ± 3.19%, 6-ring PAHs were 68.69 ± 3.68% and 36.09 ± 1.78%, respectively. The average removal efficiency of the total PAHs after 80 days followed the order: R1M (91.1%) > CKM (84.9%) > CK (77.6%), indicating that planted soil with inoculation of Mycobacterium sp. as well as non-planted soil inoculated with Mycobacterium sp. could both significantly accelerate the removal of PAHs compared to control soil. The bioavailability ratio of PAHs with 3 and 4 rings tended to decrease (from 59.9% to 14.8% for 3-ring and 7.61% to 5.08% for 4-ring, respectively in R1) while those with 5 rings increased significantly (from 2.41% to 33.78% in R1) during the last 40 days, indicating that bioavailability alteration varies with the number of rings in the PAHs. In addition, PAH bioavailability generally did not show a significant difference between treated soil and control soil. These results suggest that ryegrass rhizosphere effect as well as inoculation of Mycobacterium sp. can accelerate PAH removal in polluted soil. The bacteria community structure demonstrated a complex interplay of soil, bacteria and ryegrass root, and potential PAH degraders were present in abundance. This study provides the exploring data of rhizosphere and bioaugmentation effect on PAH dissipation in agricultural soil, as well as the change of bioavailability and microbial composition thereof.

Combined remediation of polychlorinated naphthalene-contaminated soil under multiple scenarios: An integrated method of genetic engineering and environmental remediation technology

This study explored the types of polychlorinated naphthalene (PCN)-contaminated soil and determined the practicable scheme of combined remediation using an integrated method of genetic engineering and environmental remediation technology. A multi-scenario comprehensive evaluation system of a plant-microbial combined bioremediation of PCN-contaminated soil was established using the intelligent integration of analytic hierarchy process and formula evaluation methods based on the current situation of PCN contamination in China, which showed the bioremediation of PCN-contaminated soil by the plant-microbial system could be divided into four scenarios. QSAR models were constructed to quantify the remediation mechanism that electronic parameter ∆E was the key factor changing the efficiency of combined bioremediation. Moreover, the macro-control scheme of PCN-contaminated soil was established, which indicated that four new multifunctional proteins promoted the absorption, degradation, and mineralization of PCNs in specific soil pollution types significantly, were obtained through cross gene recombination. The molecular dynamics (MD) simulation results showed the efficiency of the plant-microbial combined bioremediation were increased by 15.45% (Scenario 1, 2, 3) and 20.02% (Scenario 4) under the optimal regulation scheme. The findings will be helpful to realize the regional control of PCN-contaminated soil.

Plant-microorganism combined remediation of polychlorinated naphthalenes contaminated soils based on molecular directed transformation and Taguchi experimental design-assisted dynamics simulation

The molecular directed transformation procedure was adopted by combining molecular docking and homology modeling to reconstruct the proteins, which are involved in the absorption, degradation, and mineralization of polychlorinated naphthalenes (PCNs). A comprehensive evaluation system for developing new proteins that are responsible for the absorption (aquaporin: 1Z98), degradation (peroxidase: 1ATJ), and mineralization (lignin peroxidase: 1B85) of PCNs was established using the Rank Sum Ratio (RSR) and weighted average methods. The Taguchi experimental design-assisted dynamics simulation was used to determine the optimal external stimulus conditions of plant-microorganism combined remediation system to absorb, degrade, and mineralize PCNs. Results showed that a total of 60 amino acid sequences were designed, and 19 new proteins (increasing amplitude: 66.67%-500.00%) were significantly higher than those of target proteins through the screening of comprehensive evaluation system. Additionally, 10 new proteins improved the efficiency of absorption, degradation, and mineralization of PCNs in a real environment which were simulated under the optimal external stimulus conditions. Moreover, remediation efficiency was significantly enhanced when the template proteins was replaced with a combination of 1Z98-9, 1ATJ-7, and 1B85-20 in plant-microorganism systems, and the van der Waals force and polar solvation were the main factors affecting the absorption, degradation, and mineralization of PCNs.

The effect of different levels of leachate on phytoremediation of pyrene-contaminated soil and simultaneous extraction of lead and cadmium

Pyrene is one of the 16 group combinations of polyaromatic hydrocarbons, which are known as primary pollutants in the U.S. Environmental Protection Agency (USEPA) list. This study aimed to investigate the cross effect of different levels of landfill leachate on phytoremediation of pyrene-contaminated soil using the sorghum bicolor plant. The study parameters included the presence or absence of the plant, different concentrations of pyrene (150, 300, 500, 750, and 1000 mg kg^-1), time (30, 60, and 90 days), and different levels of irrigation with leachate (0, 30, 50, 70, and 100%). Soil pyrene was measured every 30 days, and heavy metals (lead and cadmium) added to the soil by irrigation with leachate were measured in the soil and the plant at the end of 90 days. According to the results, pyrene removal efficiency after 90 days was 96% in irrigation treatments with 30% leachate in the presence of the plant and 67% in irrigation treatments with tap water in the presence of the plant. In addition, 95% of lead and 49% of cadmium added to the soil by irrigation with 30% leachate were extracted from the soil by the sorghum bicolor. According to the results, by increasing nutrients and number of soil bacteria during the cross treatment, landfill leachate increased the pyrene removal efficiency significantly during phytoremediation (p < 0.006) and the sorghum bicolor plant extracted the lead and cadmium of the leachate. In non-planting treatments, although adding high levels of leachate to the soil significantly improved the pyrene removal, it caused the levels of heavy metals, such as lead and cadmium, to exceed the allowable limit (p < 0.001).

Application of Festuca arundinacea in phytoremediation of soils contaminated with Pb, Ni, Cd and petroleum hydrocarbons

Phytoremediation is a promising "green technique" used to purify contaminated soils. The performed phytoremediation experiments assisted by the fertilization process involving pots of F.arundinacea grown on soils with diverse concentrations and types of contaminations produced the following decreased percentages after 6 months: Pb (25.4-34.1%), Ni (18.7-23.8%), Cd (26.3-46.7%), TPH (49.4-60.1%). Primarily, TPH biodegradation was occurring as a result of basic bioremediation stimulated by adding optimal volumes of biogenic substances and corrections in the soil reaction, while phytoremediation improved this process by 17.4 - 23.1%. The highest drop in a range of 45.6 - 55.5% was recorded for the group of C₁₂-C₁₈ hydrocarbons, with the lowest one for C₂₅-C₃₆, amounting to 9.1-17.4%. Translocation factor values were: TF<1 and ranged, respectively, for: Pb (0.46-0.53), Ni (0.29-0.33), and Cd (0.21-0.25), which indicate that heavy metals absorbed by Festuca arundinacea they mainly accumulated in the root of the tissue in descending order: Cd <Ni <Pb, showing poor metal translocation from roots to shoots. Co-occurrence of petroleum pollutants (TPH) in contaminated (Pb, Cd, Ni) soils results in reducing their contents in Festuca arundinacea roots. The process of phytoremediation of contaminated soil using F.arundinacea assisted with fertilization was monitored by means of toxicological tests: Microtox SPT (inhibition of the luminescence of V. fischeri), Ostracodtoxkit F (mortality, growth inhibition Heterocypris incongruens) and MARA (growth of 11 microorganisms) and Phytotoxkit F (germination assessment, inhibition root growth: Sorghum saccharatum, Lepidium sativum and Sinapis alba). The sensitivity of toxicological tests used was comparable and increased in the order: MARA<Ostracodtoxkit<Microtox. The performed phytotoxicity tests have indicated variable sensitivity of the tested plants on contaminants occurring in the studied soils, following the sequence: L. Sativum<S. saccharatum<S. alba. The obtained results indicate a decrease in soil toxicity during phytoremediation assisted by the fertilization process using Festuca arundinacea, which correlates with a decrease in the amount of harmful impurities contained in soils subjected to phytoremediation.

Enhanced phytoremediation of PAHs-contaminated soil from an industrial relocation site by Ochrobactrum sp

Nowadays, the remediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil has received wide attention. In this work, Ochrobactrum sp. (PW) was isolated through selective enrichment from PAHs-contaminated soil in coking plant of Beijing, and the effects of PW on phytoremediation of that soil by alfalfa (Medicago sativa L.) and ryegrass (Lolium multiflorum Lam.) were investigated through pot experiments. Plant biomass, peroxidase (POD) activity, malondialdehyde (MDA) contents, soil enzyme activity (polyphenol oxidase and dehydrogenase activity), and residual concentration of PAHs in soils were determined to illustrate the ability of PW for enhancing the degradation of PAHs by plants. The results showed that the fresh weight of ryegrass and alfalfa inoculated with PW was significantly (p < 0.05) increased while the activity of POD and MDA contents were notably (p < 0.05) reduced than that without inoculation. Additionally, PW enhanced the activity of polyphenol oxidase and dehydrogenase in soil significantly (p < 0.05), and further enhanced the degradability of the system to PAHs. Different treatment methods could be ranked by the following order according to the degradability: SP (alfalfa + PW) > RP (ryegrass + PW) > PW (PW) > S (alfalfa) > R (ryegrass). The combined action of PW and alfalfa/ryegrass could accelerate the degradability of PAHs from soil contaminated by coking plants. PW could be used as potential bacteria to promote phytoremediation of the soil contaminated by PAHs.

Effect of salicylic acid and mycorrhizal symbiosis on improvement of fluoranthene phytoremediation using tall fescue (Festuca arundinacea Schreb)

Polycyclic aromatic hydrocarbons are an important group of pollutants that are widely distributed in the environment. The present study aimed to investigate the effect of salicylic acid (a phenolic phytohormone) and mycorrhizal fungi on the growth and phytoremediation ability of tall fescue in the soil contaminated by fluoranthene. The initial concentrations of fluoranthene in this study were 100, 200, and 300 mg kg^-1. The experimental treatments were included: T0 uncultivated soil; T1 cultivated soil with tall fescue; T2 cultivated soil with tall fescue + salicylic acid application; T3 cultivated soil with tall fescue + application of mycorrhizal fungi; T4 cultivated soil with tall fescue + salicylic acid and mycorrhizal fungi application; and P planting tall fescue in uncontaminated soil. The removal of fluoranthene was measured after 90 days. Furthermore, at the end of the experiment, the amount of shoot and root biomass, soil bacteria, and dehydrogenase activity were measured. According to the results, in all levels of contamination, removal of fluoranthene in cultivated treatments significantly was higher than uncultivated treatments. Increasing the concentration of fluoranthene had a negative effect on the shoot and root biomass in different treatments. Salicylic acid and mycorrhizal fungi significantly increased the shoot and root biomass and also the number of soil bacteria, dehydrogenase activity, and fluoranthene removal in T2, T3, and T4 treatments compared to T1. At the highest concentration of fluoranthene, as a result of simultaneous application of salicylic acid and mycorrhizal fungi (T4), the fluoranthene removal increased by 63, 21, 13, and 16% in comparison with T0, T1, T2, and T3, respectively. Based on the results, salicylic acid and mycorrhizal fungi, either alone or in combination, have a significant effect on the improvement of phytoremediation potential in tall fescue.

A review of germination and early growth as a proxy for plant fitness under petrogenic contamination - knowledge gaps and recommendations

Germination-an important stage in the life cycle of plants-is susceptible to the presence of soil contaminants. Since the early 1990s, the use of germination tests to screen multiple plant species to select candidates for phytoremediation has received much attention. This is due to its inexpensive methodology and fast assessment relative to greenhouse or field growth studies. Surprisingly, no comprehensive synthesis is available of these studies in the scientific literature. As more plant species are added to phytoremediation databases, it is important to encapsulate the knowledge thus far and revise protocols. In this review, we have summarised previously-documented effects of petroleum hydrocarbons on germination and seedling growth. The methods and materials of previous studies are presented in tabulated form. Common practice includes the use of cellulose acetate filter paper, plastic Petri dishes, and low numbers of seeds and replicates. A general bias was observed for the screening of cultivated crops as opposed to native species, even though the latter may be better suited to site conditions. The relevance of germination studies as important ecotoxicological tools is highlighted with the proposed use of root imaging software. Screening of novel plant species, particularly natives, is recommended with selection focussed on (i) species phylogeny, (ii) plant morphological and functional traits, and (iii) tolerance towards harsh environmental stresses. Recommendations for standardised protocols for germination and early growth monitoring are made in order to improve the robustness of statistical modelling and species selection in future phytoremediation evaluations and field programs.

Variation in soil aggregate-size distribution affects the dissipation of polycyclic aromatic hydrocarbons in long-term field-contaminated soils

Soil organic matter (SOM) is the main adsorbent for polycyclic aromatic hydrocarbons (PAHs) and the principal aggregating agent for soil aggregation that can affect PAH bioavailability and bioaccessibility in soils. The objective of this study was to analyze the relationship between PAH dissipation and variation in soil aggregate-size distribution in two field-contaminated soils with different soil organic C (SOC) content (Anthrosols, 1.41% SOC; Phaeozems, 8.51% SOC) in phytoremediation with alfalfa. The results showed that there were significant reductions of 10.2 and 15.4% of the total PAHs in unplanted and planted treatments, respectively, for Anthrosols. However, there was no significant reduction of total PAHs in either unplanted or planted treatment for Phaeozems. For Anthrosols, mass percentages of coarse sand and fine sand were significantly reduced while coarse silt and fine silt were significantly increased for the planted soil compared to the initial soil (p < 0.05). For Phaeozems, there was no significant variation in aggregate-size distribution among different treatments except that coarse silt in planted and unplanted soil was slightly reduced. The main reason for the dissipation of PAHs in Anthrosols could be that macroaggregates were broken into microaggregates, which made some trapped PAHs become bioaccessible to soil microorganisms.

Dissipation and phytoremediation of polycyclic aromatic hydrocarbons in freshly spiked and long-term field-contaminated soils

Pot experiments were used to compare the dissipation and phytoremediation effect of alfalfa (Medicago sativa L.) for polycyclic aromatic hydrocarbons (PAHs) in a freshly spiked soil and two field-contaminated soils with different soil organic carbon (SOC) contents (Anthrosols, 1.41% SOC; Phaeozems, 8.51% SOC). In spiked soils, the dissipation rates of phenanthrene and pyrene were greater than 99.5 and 94.3%, respectively, in planted treatments and 95.0 and 84.5%, respectively, in unplanted treatments. In field-contaminated Anthrosols, there were limited but significant reductions of 10.2 and 15.4% of total PAHs in unplanted and planted treatments, respectively. In field-contaminated Phaeozems, there were no significant reductions of total PAHs in either unplanted or planted treatments. A phytoremediation effect was observed for the spiked soils and the Anthrosols, but not for the Phaeozems. The results indicated that laboratory tests with spiked soils cannot reflect the real state of field-contaminated soils. Phytoremediation efficiency of PAHs in field-contaminated soils was mainly determined by the content of SOC. Phytoremediation alone has no effect on the removal of PAHs in field-contaminated soils with high SOC content.

Mineralization of pyrene induced by interaction between Ochrobactrum sp. PW and ryegrass in spiked soil

This study was conducted to investigate the capability of pyrene-degrading bacterium Ochrobactrum sp. PW and ryegrass (Lolium multiflorum) grown alone and in combination on the degradation of pyrene in soil. After 60 days of ryegrass growth, plant biomass, pyrene-degrading microbial mass, soil enzyme activity (catalase activity and polyphenol oxidase activity) and residual concentration of pyrene in soils were determined. Higher dissipation rates were observed in PW inoculation treatments: ryegrass+PW rhizosphere soil (RP-r) and ryegrass+PW non-rhizosphere soil (RP-nr), than planting of ryegrass alone, rhizosphere (R-r) or non-rhizosphere (R-nr). The inoculation with PW significantly (p<0.05) increased the dry weight of ryegrass root and shoot, nearly 2.8 and 3.3 times higher than ryegrass treatment. The pyrene-degrading microbial mass indicated that a much larger mass of bacteria, actinobacteria were present in RP treatment. The catalase activity in all different treatments were significantly (p<0.05) higher than in with treatment R-nr, and the polyphenol oxidase activity was also significantly (p<0.05) increased by inoculation with PW, leading to enhanced mineralization of pyrene from soil. Our results suggest that adding of PAHs-degrading bacteria to soil can enhance remediation of PAHs contaminated soil, while improving plant growth.

The effects of simultaneous application of plant growth regulators and bioaugmentation on improvement of phytoremediation of pyrene contaminated soils

Polycyclic aromatic hydrocarbons (PAHs) refer to a wide group of soil contaminants whose presence in the environment is a cause of concern. The present study aimed to evaluate the effects of Indole Acetic Acid (IAA) and Pseudomonas aeruginosa on the phytoremediation activities of sorghum bicolor and increase in pyrene removal efficiency in the soil. The initial concentration of pyrene was 150 and 300 mg kg(-1) in this experiment. The treatments included unplanted soil (T0), planted soil with sorghum (T1), planted soil with application of IAA (T2), planted soil with application of Pseudomonas sp. (T3), and planted soil with simultaneous application of IAA and Pseudomonas sp. (T4). The pyrene removal rate in the soil was measured every 30 days. Moreover, plant biomass and soil bacteria were measured after 90 days. The results showed that pyrene removal rate significantly increased in the planted treatments compared to the unplanted ones. After 90 days, at the initial concentration of 150-300 mg kg(-1), pyrene removal efficiency was 52-92% in T1-T4 and 35-47% in the unplanted treatment (T0). Application of IAA and Pseudomonas sp. significantly increased plant biomass, soil bacteria, and pyrene removal rate in T2, T3, and T4 compared to T1. Therefore, application of IAA in the planted treatments with sorghum could have a significant effect on increasing the removal efficiency of pyrene.

Rhizosphere effects of PAH-contaminated soil phytoremediation using a special plant named Fire Phoenix

The rhizosphere effect of a special phytoremediating species known as Fire Phoenix on the degradation of polycyclic aromatic hydrocarbons (PAHs) was investigated, including changes of the enzymatic activity and microbial communities in rhizosphere soil. The study showed that the degradation rate of Σ8PAHs by Fire Phoenix was up to 99.40% after a 150-day culture. The activity of dehydrogenase (DHO), peroxidase (POD) and catalase (CAT) increased greatly, especially after a 60-day culture, followed by a gradual reduction with an increase in the planting time. The activity of these enzymes was strongly correlated to the higher degradation performance of Fire Phoenix growing in PAH-contaminated soils, although it was also affected by the basic characteristics of the plant species itself, such as the excessive, fibrous root systems, strong disease resistance, drought resistance, heat resistance, and resistance to barren soil. The activity of polyphenoloxidase (PPO) decreased during the whole growing period in this study, and the degradation rate of Σ8PAHs in the rhizosphere soil after having planted Fire Phoenix plants had a significant (R(2)=0.947) negative correlation with the change in the activity of PPO. Using an analysis of the microbial communities, the results indicated that the structure of microorganisms in the rhizosphere soil could be changed by planting Fire Phoenix plants, namely, there was an increase in microbial diversity compared with the unplanted soil. In addition, the primary advantage of Fire Phoenix was to promote the growth of flora genus Gordonia sp. as the major bacteria that can effectively degrade PAHs.

Phytoremediation efficiency of a pcp-contaminated soil using four plant species as mono- and mixed cultures

Bioremediation of soil polluted by pentachlorophenol (PCP) is of great importance due to the persistence and carcinogenic properties of PCP. Phytoremediation has long been recognized as a promising approach for removal of PCP from soil. The present study was conducted to investigate the capability of four plant species; white clover, ryegrass, alfalfa, and rapeseed grown alone and in combination to remediate pentachlorophenol contaminated soil. After 60 days cultivation, white clover, raygrass, alfalfa, and rapeseed all significantly enhanced the degradation of PCP in soils. Alfalfa showed highest efficiency for the removal of PCP in single cropping flowed by rapeseed and ryegrass. Mixed cropping significantly enhanced the remediation efficiencies as compared to single cropping; about 89.84% of PCP was removed by mixed cropping of rapeseed and alfalfa, and 72.01% of PCP by mixed cropping of rape and white clover. Mixed cropping of rapeseed with alfalfa was however far better for the remediation of soil PCP than single cropping. An evaluation of soil biological activities as a monitoring mechanism for the bioremediation process of a PCP-contaminated soil was made using measurements of microbial counts and dehydrogenase activity.

Is it possible to increase bioavailability but not environmental risk of PAHs in bioremediation?

The current poor predictability of end points associated with the bioremediation of polycyclic aromatic hydrocarbons (PAHs) is a large limitation when evaluating its viability for treating contaminated soils and sediments. However, we have seen a wide range of innovations in recent years, such as an the improved use of surfactants, the chemotactic mobilization of bacterial inoculants, the selective biostimulation at pollutant interfaces, rhizoremediation and electrobioremediation, which increase the bioavailability of PAHs but do not necessarily increase the risk to the environment. The integration of these strategies into practical remediation protocols would be beneficial to the bioremediation industry, as well as improve the quality of the environment.

Native Michigan plants stimulate soil microbial species changes and PAH remediation at a legacy steel mill

A 1.3-acre phytoremediation site was constructed to mitigate polyaromatic hydrocarbon (PAH) contamination from a former steel mill in Michigan. Soil was amended with 10% (v/v) compost and 5% (v/v) poultry litter. The site was divided into twelve 11.89 m X 27.13 m plots, planted with approximately 35,000 native Michigan perennials, and soils sampled for three seasons. Soil microbial density generally increased in subplots of Eupatorium perfoliatum (boneset), Aster novae-angliae (New England aster), Andropogon gerardii (big bluestem), and Scirpus atrovirens (green bulrush) versus unplanted subplots. Using enumeration assays with root exudates, PAH degrading bacteria were greatest in soils beneath plants. Initially predominant, Arthrobacter were found capable of degrading a PAH cocktail in vitro, especially upon the addition of root exudate. Growth of some Arthrobacter isolates was stimulated by root exudate. The frequency of Arthrobacter declined in planted subplots with a concurrent increase in other species, including secondary PAH degraders Bacillus and Nocardioides. In subplots supporting only weeds, an increase in Pseudomonas density and little PAH removal were observed. This study supports the notion that a dynamic interplay between the soil, bacteria, and native plant root secretions likely contributes to in situ PAH phytoremediation.

Treatment and Remediation of Petroleum-Contaminated Soils Using Selective Ornamental Plants

Pot-culture experiments were carried out to assess the phytoremediation potential of 14 ornamental plants in weathered petroleum-contaminated soil, which was collected in the Shengli Oil Field, one of the biggest oil fields in China, by examining their impact on the degradation potential of total petroleum hydrocarbons (TPHs) and its composition. Results showed Gaillardia aristata, Echinacea purpurea, Fawn (Festuca arundinacea Schreb), Fire Phoenix (a combined F. arundinacea), and Medicago sativa L. could effectively reduce TPHs and its composition in 10,000 mg kg(-1) TPH-contaminated soil. After a 30-day pot-culture experiment, the removal rates were 37.16%, 46.74%, 49.42%, 41.00%, and 37.93%, respectively, significantly higher than that in the control (only 12.93%). Removal rates of TPH composition including saturated hydrocarbon, aromatic hydrocarbon, asphaltene, and polar compound reached 39.41%, 38.47%, 45.11%, 42.92%, and 37.52%, respectively, also higher than that in the control (only 6.90%). Further, the total biomass did not significantly decrease for all plants tested in 10,000 mg kg(-1) TPH-contaminated soil. Fourier transform infrared spectroscopy confirmed the presence of oil in the plant tissues. These results suggested that the typical ornamental species including G. aristata, E. purpurea, Fawn, Fire Phoenix, and M. sativa can be adopted in phytoremediation of oil-contaminated soil.

Study of PAH dissipation and phytoremediation in soils: comparing freshly spiked with weathered soil from a former coking works

A comparison was made between the dissipation of polycyclic aromatic hydrocarbons (PAHs) in soil freshly spiked with pure PAHs, soil spiked with a coal tar mixture and a contaminated soil from a former coking works where the PAHs had been present for more than a century. The potential of five selected plant species for phytoremediation was investigated. The levels of all 7 PAHs in chemically amended soil, both planted and unplanted, fell significantly (>80% reduction) over the 12 weeks of the growing trial. In the coal tar treated soils all PAHs were significantly reduced. In both the planted and unplanted soils the 2-3 ringed compounds demonstrated much greater loss (>77%) than the 4-6 ringed (16-39%). The 3-4 ringed compounds demonstrated strong evidence of phytoremediation but not the 5-6 ringed. The coking soil showed limited reduction (7-24%) of all 12 PAHs present. There was little difference in dissipation between the PAHs and little evidence of a phytoremediation effect in coking soil. The results demonstrated that the form in which PAHs were added to soil influenced their susceptibility to dissipation. Therefore, predictions of PAH dissipation from laboratory amended soil do not reflect the true situation in the field.

Hydrocarbon phytoremediation in the family Fabaceae--a review

Currently, studies often focus on the use of Poaceae species (grasses) for phytoremediation of hydrocarbon-contaminated soils. Research into the use of Fabaceae species (legumes) to remediate hydrocarbons in soils has been conducted, but these plants are commonly overlooked due to slower recorded rates of degradation compared with many grass species. Evidence in the literature suggests that in some cases Fabaceae species may increase total degradation of hydrocarbons and stimulate degradative capacity of the soil microbial community, particularly for contaminants which are normally more recalcitrant to degradation. As many recalcitrant hydrocarbons have negative impacts on human and ecosystem health, development of remediation options is crucial. Reconsideration of Fabaceae species for removal of such contaminants may lead to environmentally and economically sustainable technologies for remediation of contaminated sites.

Isolation and characterization of pyrene metabolizing microbial consortia from the plant rhizoplane

Most research on the ecology of PAH degrading bacteria in the rhizosphere has focused on individual strains that grow on specific PAHs. Thus, there are fundamental questions as to importance of microbial consortia for PAH degradation in the plant rhizosphere. The study reported here characterized cultivable pyrene degrading rhizoplane microbial communities from two different plant species using a root printing technique on agar plates. Colonies were revealed by formation of clearing zones on medium containing a thin film of pyrene on the surface of a mineral nutrient agar. Prints of the rhizoplane colonies were obtained from roots of Melilotus officinalis (sweet yellow clover) and Andropogon gerardii (big bluestem) plants. Phylogenetic characterizations of selected pyrene degrading colonies were assessed by PCR-DGGE and DNA sequencing. Results showed that different populations of cultivable pyrene degraders were obtained from representative consortia that were examined. Many of the PAH degrading consortia consisted of mixtures of bacterial species that were unable to degrade pyrene by themselves. While this study focused on culturable PAH degraders, the results suggest that pyrene degradation in the rhizosphere commonly involves the activity of bacterial consortia in which various species of bacteria interact to achieve PAH degradation.

Degradation of phenanthrene and pyrene in spiked soils by single and combined plants cultivation

The present study was conducted to investigate the capability of four plant species (tall fescue, ryegrass, alfalfa, and rape seed) grown alone and in combination to the degradation of phenanthrene and pyrene (polycyclic aromatic hydrocarbons, PAHs) in spiked soil. After 65 days of plant growth, plant biomass, dehydrogenase activity, water-soluble phenolic (WSP) compounds, plant uptake and accumulation and residual concentrations of phenanthrene and pyrene were determined. Our results showed that presence of vegetation significantly enhanced the dissipation of phenanthrene and pyrene from contaminated soils. Higher degradation rates of PAHs were observed in the combined plant cultivation (98.3-99.2% phenanthrene and 88.1-95.7% pyrene) compared to the single plant cultivation (97.0-98.0% phenanthrene and 79.8-86.0% pyrene). Contribution of direct plant uptake and accumulation of phenanthrene and pyrene was very low compared to the plant enhanced dissipation. By contrast, plant-promoted biodegradation was the predominant contribution to the remediation enhancement. The correlation analysis indicates a negative relation between biological activities (dehydrogenase activity and WSP compounds) and residual concentrations of phenanthrene and pyrene in planted soils. Our results suggest that phytoremediation could be a feasible choice for PAHs contaminated soil. Moreover, the combined plant cultivation has potential to enhance the process.

Advances in the field of high-molecular-weight polycyclic aromatic hydrocarbon biodegradation by bacteria

Interest in understanding prokaryotic biotransformation of high-molecular-weight polycyclic aromatic hydrocarbons (HMW PAHs) has continued to grow and the scientific literature shows that studies in this field are originating from research groups from many different locations throughout the world. In the last 10 years, research in regard to HMW PAH biodegradation by bacteria has been further advanced through the documentation of new isolates that represent diverse bacterial types that have been isolated from different environments and that possess different metabolic capabilities. This has occurred in addition to the continuation of in-depth comprehensive characterizations of previously isolated organisms, such as Mycobacterium vanbaalenii PYR-1. New metabolites derived from prokaryotic biodegradation of four- and five-ring PAHs have been characterized, our knowledge of the enzymes involved in these transformations has been advanced and HMW PAH biodegradation pathways have been further developed, expanded upon and refined. At the same time, investigation of prokaryotic consortia has furthered our understanding of the capabilities of microorganisms functioning as communities during HMW PAH biodegradation.

Pyrene effects on rhizoplane bacterial communities

Certain plant species promote biodegradation of polycyclic aromatic hydrocarbons (PAHs), but few studies have examined the microbial populations that are associated with the rhizoplane of these plants. In this study, the bacterial composition of the rhizoplane were characterized for four plant species during in soils with different histories of exposure to PAH and in the presence or absence of a pyrene spike at 100 mg kg(-1) pyrene. Three of the plant species including Andropogon gerrardii, Panicum coloratum and Melilotus officinalis were known to stimulate PAH degradation. Wheat (Triticum aestivum) was used as a reference species. Results showed that after 90 days, approximately 45% of the pyrene spike disappeared from soil without plants. In contrast, cultivation of plants resulted in 95% disappearance of pyrene. There were no significant differences in the extent of pyrene disappearance for different plants. In all cases, 16S rRNA gene profiles of the rhizoplane were less complex in the pyrene-spiked soils, suggesting that richness and evenness of the predominant bacteria were reduced. Our results show that pyrene contamination results in significant shifts in the composition of rhizosphere bacterial communities that are still further influenced by the plant species and prior exposure history to PAH contamination.

Impact of phenanthrene on organic acids secretion and accumulation by perennial ryegrass, Lolium perenne L., root

A solution culture experiment was performed to investigate the impact of phenanthrene (PHE) on organic acids secretion and accumulation by Lolium perenne L. root. Data showed that, oxalic acid was the dominant composition of organic acids in root and root exudates. In root exudates, increased levels of PHE resulted in higher oxalic acid and its secrete proportion; oxalic acid arranged from 3.00 to 4.72 mg/g FW under spiked PHE treatments, in control, it was 2.33 mg/g FW. In root, oxalic acid rose to 25.61 mg/g FW at 1 mg/L PHE treatment, while the PHE concentration was continuously increasing, organic acids in root decreased.

Evaluation of plant-microorganism synergy for the remediation of diesel fuel contaminated soil

The remediation of diesel fuel contaminated soil over a 2-year period by the plant-microorganism synergy was evaluated. Results indicated that the growth of Astragalus adsurgens was affected significantly, when the diesel fuel concentration was higher than 10 g kg(-1) dry soil. After a 2-year period, the removal of diesel fuel was >67%, and about 58-70% removal of aromatic hydrocarbons was obtained in these treatments. The removal of diesel fuel and its components was 13-30% higher than that of plant alone. These results show that an appropriate plant-microorganism synergy may serve as a low-cost, effective remedial technology for diesel-contaminated soil.

Petroleum-degrading microbial numbers in rhizosphere and non-rhizosphere crude oil-contaminated soil

Phytoremediation can be a cost-effective and environmentally acceptable method to clean up crude oil-contaminated soils in situ. Our research objective was to determine the effects of nitrogen (N) additions and plant growth on the number of total hydrocarbon (TH)-, alkane-, and polycyclic aromatic hydrocarbon (PAH)-degrading microorganisms in weathered crude oil-contaminated soil. A warm-season grass, sudangrass (Sorghum sudanense (Piper) Stapf), was grown for 7 wk in soil with a total petroleum hydrocarbon (TPH) level of 16.6 g TPH/kg soil. Nitrogen was added based upon TPH-C:added total N (TPH-C:TN) ratios ranging from 44:1 to 11:1. Unvegetated and unamended controls were also evaluated. The TH-, alkane-, and PAH-degrading microbial numbers per gram of dry soil were enumerated from rhizosphere and non-rhizosphere soil for vegetated pots and non-rhizosphere soil populations were enumerated from non-vegetated pots. Total petroleum-degrading microbial numbers were also calculated for each pot. The TH-, alkane-, and PAH-degrading microbial numbers per gram of dry soil in the sudangrass rhizosphere were 3.4, 2.6, and 4.8 times larger, respectively, than those in non-rhizosphere soil across all N rates. The presence of sudangrass resulted in significantly more TH-degrading microorganisms per pot when grown in soil with a TPH-C:TN ratio of 11:1 as compared to the control. Increased plant root growth in a crude oil-contaminated soil and a concomitant increase in petroleum-degrading microbial numbers in the rhizosphere have the potential to enhance phytoremediation.

Promotion of pyrene degradation in rhizosphere of alfalfa (Medicago sativa L.)

Pot experiment was conducted to evaluate the phytoremediation of pyrene-contaminated soil using alfalfa (Medicago sativa L.). Alfalfa biomasses, microbial viable counts, dehydrogenase activity, residual pyrene concentration and pyrene removal percentage were determined after 60 days of alfalfa growth. The results indicated that pyrene had an inhibitive effect on alfalfa growth, and higher pyrene concentration seriously affected alfalfa growth. In addition, the inhibitive effect on the root was more severe than that on the shoot. When pyrene concentration reached 492 mg kg(-1) in soil, the shoot and root biomasses were only 34% and 22% of those of alfalfa growing in non-spiked soil, respectively. The rhizospheric bacterial and fungi counts were 5.0-7.5 and 1.8-2.3 times higher than those in non-rhizosphere soil, respectively. The residual concentrations of pyrene in the rhizosphere soil were lower than those in the non-rhizosphere soil. After 60 days, 69-85% and 59-80% of spiked pyrene disappeared from the rhizosphere and non-rhizosphere soils, respectively. The removal percentage decreased with increasing pyrene concentration. However, the average removal of pyrene in the rhizosphere soil was 6% higher than that in the non-rhizosphere soil. Therefore, the presence of alfalfa roots was effective in promoting the phytoremediation of freshly added pyrene into the soil.

Phytoremediation of polycyclic aromatic hydrocarbons in soil: part I. Dissipation of target contaminants

Phytoremediation has been demonstrated to be a viable cleanup alternative for soils contaminated with petroleum products. This study evaluated the application of phytoremediation to soil from a manufactured gas plant (MGP) site with high concentrations of recalcitrant, polycyclic aromatic hydrocarbons (PAHs). Two greenhouse studies investigated the potential dissipation and plant translocation of PAHs by fescue (Festuca arundinacea) and switchgrass (Panicum virgatum) in the first experiment and zucchini (Curcubita pepo Raven) in the second. The MGP soil was highly hydrophobic and initially inhibited plant growth. Two unplanted controls were established with and without fertilization. In the first experiment, concentrations of PAHs decreased significantly in all treatments after 12 mo. Plant biomass and microbial numbers were statistically equivalent among plant species. PAH concentrations in plant biomass were negligible for fescue and switchgrass. In the second experiment, zucchini enhanced the dissipation of several PAHs after 90 d of treatment when compared to the unvegetated soil. Plant tissue concentrations of PAHs were not elevated in the zucchini roots and shoots, and PAHs were not detectable in the fruit.

Comparison of plant families in a greenhouse phytoremediation study on an aged polycyclic aromatic hydrocarbon-contaminated soil

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, recalcitrant, and potentially carcinogenic pollutants. Plants and their associated rhizosphere microbes can promote PAH dissipation, offering an economic and ecologically attractive remediation technique. This study focused on the effects of different types of vegetation on PAH removal and on the interaction between the plants and their associated microorganisms. Aged PAH-polluted soil with a total PAH level of 753 mg kg(-1) soil dry weight was planted with 18 plant species representing eight families. The levels of 17 soil PAHs were monitored over 14 mo. The size of soil microbial populations of PAH degraders was also monitored. Planting significantly enhanced the dissipation rates of all PAHs within the first 7 mo, but this effect was not significant after 14 mo. Although the extent of removal of lower-molecular-weight PAHs was similar for planted and unplanted control soils after 14 mo, the total mass of five- and six-ring PAHs removed was significantly greater in planted soils at the 7- and 14-mo sampling points. Poaceae (grasses) were the most effective of the families tested, and perennial ryegrass was the most effective species; after 14 mo, soils planted with perennial ryegrass contained 30% of the initial total PAH concentration (compared with 51% of the initial concentrations in unplanted control soil). Although the presence of some plant species led to higher populations of PAH degraders, there was no correlation across plant species between PAH dissipation and the size of the PAH-degrading population. Research is needed to understand differences among plant families for stimulating PAH dissipation.

Effects of tree root-derived substrates and inorganic nutrients on pyrene mineralization in rhizosphere and bulk soil

This study investigated the effects of organic and inorganic nutrients on the microbial degradation of the common soil contaminant pyrene. The material used in this investigation was collected from potted trees that had been growing for over a year in a soil artificially contaminated with polycyclic aromatic hydrocarbons. Soil was removed from the nonroot (bulk) and root (rhizosphere) zones of these pots and used in mineralization studies that tracked microbial degradation of 14C-pyrene. The factors influencing degradation in these zones were then tested by amendment with essential inorganic nutrients or with root-derived materials. As expected, pyrene mineralization was greater in soil removed from the rhizosphere than in bulk soil. The rate of mineralization in rhizosphere soil was inhibited by inorganic nutrient amendment, whereas nutrients stimulated mineralization in the bulk soil. Pyrene mineralization in bulk soil was also increased by the addition of root extracts intended to mimic exudation by living roots. However, amendment with excised fine roots that were allowed to decay over time in soil initially inhibited mineralization. With time, the rate of mineralization increased, eventually exceeding that of unamended bulk soil. Combined, the initial inhibition and subsequent stimulation produced a zero net impact of decaying fine roots on bulk soil mineralization. Our results, in conjunction with known temporal patterns of fine root dynamics in natural systems, support the idea that seasonal variations in nutrient and substrate availability may influence the long-term effect of plants on organic degradation in soil, possibly reducing or negating the beneficial effects of vegetation that are often observed in short-term studies.

Assessment of contaminant lability during phytoremediation of polycyclic aromatic hydrocarbon impacted soil

Polycyclic aromatic hydrocarbons (PAHs) are recalcitrant compounds, some of which are known carcinogens, often found in high residual soil concentrations at industrial sites. Recent research has confirmed that phytoremediation holds promise as a low-cost treatment method for PAH contaminated soil. In this study, the lability of soil bound PAHs in the rhizosphere was estimated using solid phase extraction resin. An extraction time of 14 days was determined to be appropriate for this study. Resin-extractable PAHs, which are assumed to be more bioavailable, decreased during plant treatments. Significant reductions in the labile concentrations of several PAH compounds occurred over 12 months of plant growth. The differences in concentration between the unplanted and the planted soil indicate that the presence of plant roots, in addition to the passage of time, contributes to reduction in the bioavailability of target PAHs.