Rhizospheric effects of alfalfa on biotransformation of polychlorinated biphenyls in a contaminated soil augmented with Sinorhizobium meliloti

A new dataset of PCB half-lives in soil: Effect of plant species and organic carbon addition on biodegradation rates in a weathered contaminated soil

This paper presents a new dataset of Polychlorinated Biphenyls (PCBs) half-lives in soil. Data were obtained from a greenhouse experiment performed with an aged contaminated soil under semi-field conditions, collected from a National Relevance Site (SIN) located in Northern Italy (SIN Brescia-Caffaro). Ten different treatments (combination of seven plant species and different soil conditions) were considered together with the respective controls (soil without plants). PCB concentration reduction in soil was measured over a period of 18 months to evaluate the ability of plants to stimulate the biodegradation of these compounds. Tall fescue, tall fescue cultivated together with pumpkin and tall fescue amended with compost reduced more than the 50% of the 79 measured PCB congeners, including the most chlorinated ones (octa to deca-PCBs). However, the data obtained showed that no plant species was uniquely responsible for the effective degradation of all isomeric classes and congeners. The obtained half-lives ranged from 1.3 to 5.6 years and were up to a factor of 8 lower compared to generic HL values reported in literature. This highlighted the importance of cultivation and plant-microbe interactions in speeding up the PCB biodegradation. This new dataset could contribute to substantially improve the predictions of soil remediation time, multimedia fate and the long-range transport of PCBs. Additionally, the half-lives obtained here can also be used in the evaluation of the food chain transfer of these chemicals, and finally the exposure and potential for effects on ecosystems.

Plants radically change the mobility of PCBs in soil: Role of different species and soil conditions

The mobility of Polychlorinated Biphenyls (PCBs) in soil cultivated with different plant species was evaluated by means of a column experiment to investigate the specific plant influence on PCB environmental fate and the potential for leaching. The soil was collected at a National Relevance Site for remediation located in Northern Italy (SIN Brescia-Caffaro) and underwent a rhizoremediation treatment for 18 months with different plant species (Festuca arundinacea, Cucurbita pepo ssp pepo and Medicago sativa). The same but unplanted soil was also considered as control for comparison. The columns were leached with tap water and PCB concentrations were measured in the leachate after 7 days of soil/water contact. Soil previously cultivated with different plant species exhibited statistically different behavior in terms of chemical leaching among the different fractions. Total PCB bulk concentrations ranged from 24 to 219 ng/L. Leachate samples were enriched in tetra- to hepta-PCBs. While PCB concentrations in the dissolved phases varied within a factor of 2 between controls and treatments, PCB associated to particulate organic carbon (POC) differed by more than one order of magnitude. More specifically, Medicago sativa enriched the soil with POC doubling PCB leaching with respect to the other plant species and the unplanted controls.

Rhizoremediation of weathered PCBs in a heavily contaminated agricultural soil: Results of a biostimulation trial in semi field conditions

This paper describes the results of a rhizoremediation greenhouse experiment planned to select the best plant species and soil management for the bioremediation of weathered polychlorinated biphenyls (PCBs). We evaluated the ability of different plant species to stimulate activity and diversity of the soil microbial community leading to the reduction of PCB concentrations in a heavily contaminated soil (at mg kg-1 dw level), of the national priority site for remediation (SIN) "Brescia-Caffaro" in Italy. Biostimulation was determined in large size (6kg) pots, to reflect semi-field conditions with a soil/root volume ratio larger than in most rhizoremediation experiments present in the literature. In total, 10 treatments were tested in triplicates comparing 7 plant species (grass and trees) and 5 soil/cultivation conditions (i.e., only one plant species, plant consociation, redox cycle, compost or ammonium thiosulfate addition) with the appropriate unplanted controls. After 18months of biostimulation the overall reduction of total PCBs varied between 14 and 20%. Microbial analysis revealed a shift in the microbial community structure over time and showed that all the planted treatments significantly enhanced microbial hydrolytic activity and the abundance of bacterial populations, including potential PCB degraders, in the soil surrounding plant roots. The plant species most effective in reducing the contaminant concentrations were Festuca arundinacea cultivated adding compost or in consociation with Cucurbita pepo ssp. pepo and Medicago sativa cultivated with Rhizobium spp. and mycorrhizal fungi; they reduced total PCB concentrations of about 20% and showed the significant depletion of a high number of PCB congeners (29, 37 and 23, respectively, out of the 79 measured). Our results suggest that these plant species are particularly efficient in increasing soil PCB bioavailability and in stimulating microbial degradation. They could be used in field rhizoremediation strategies to enhance the natural attenuation process and reduce PCB levels in historically contaminated sites.

Improving the SoilPlusVeg model to evaluate rhizoremediation and PCB fate in contaminated soils

Tools to predict environmental fate processes during remediation of persistent organic pollutants (POPs) in soil are desperately needed since they can elucidate the overall behavior of the chemical and help to improve the remediation process. A dynamic multimedia fate model (SoilPlusVeg) was further developed and improved to account for rhizoremediation processes. The resulting model was used to predict Polychlorinated Biphenyl (PCB) fate in a highly contaminated agricultural field (1089 ng/g d.w.) treated with tall fescue (Festuca arundinacea), a promising plant species for the remediation of contaminated soils. The model simulations allowed to calculate the rhizoremediation time (about 90 years), given the available rhizoremediation half-lives and the levels and fingerprints of the PCB congeners, to reach the legal threshold, to show the relevance of the loss processes from soil (in order of importance: degradation, infiltration, volatilization, etc.) and their dependence on meteorological and environmental dynamics (temperature, rainfall, DOC concentrations). The simulations showed that the effective persistence of PCBs in soil is deeply influenced by the seasonal variability. The model also allowed to evaluate the role of DOC as a possible enhancer of PCB degradation as a microorganism "spoon feeder" of PCBs in the soil solution. Additionally, we preliminary predicted how the contribution of PCB metabolites could modify the PCB fingerprint and their final total concentrations. This shows that the SoilPlusVeg model could be used in selecting the best choices for a sustainable rhizoremediation of a POP contaminated site.

Microbe and plant assisted-remediation of organic xenobiotics and its enhancement by genetically modified organisms and recombinant technology: A review

Environmental problems such as the deterioration of groundwater quality, soil degradation and various threats to human, animal and ecosystem health are closely related to the presence of high concentrations of organic xenobiotics in the environment. Employing appropriate technologies to remediate contaminated soils is crucial due to the site-specificity of most remediation methods. The limitations of conventional remediation technologies include poor environmental compatibility, high cost of implementation and poor public acceptability. This raises the call to employ biological methods for remediation. Bioremediation and microbe-assisted bioremediation (phytoremediation) offer many ecological and cost-associated benefits. The overall efficiency and performance of bio- and phytoremediation approaches can be enhanced by genetically modified microbes and plants. Moreover, phytoremediation can also be stimulated by suitable plant-microbe partnerships, i.e. plant-endophytic or plant-rhizospheric associations. Synergistic interactions between recombinant bacteria and genetically modified plants can further enhance the restoration of environments impacted by organic pollutants. Nevertheless, releasing genetically modified microbes and plants into the environment does pose potential risks. These can be minimized by adopting environmental biotechnological techniques and guidelines provided by environmental protection agencies and other regulatory frameworks. The current contribution provides a comprehensive overview on enhanced bioremediation and phytoremediation approaches using transgenic plants and microbes. It also sheds light on the mitigation of associated environmental risks.

Coupling between Nitrogen Fixation and Tetrachlorobiphenyl Dechlorination in a Rhizobium-Legume Symbiosis

Legume-rhizobium symbioses have the potential to remediate soils contaminated with chlorinated organic compounds. Here, the model symbiosis between Medicago sativa and Sinorhizobium meliloti was used to explore the relationships between symbiotic nitrogen fixation and transformation of tetrachlorobiphenyl PCB 77 within this association. 45-day-old seedlings in vermiculite were pretreated with 5 mg L^-1 PCB 77 for 5 days. In PCB-supplemented nodules, addition of the nitrogenase enhancer molybdate significantly stimulated dechlorination by 7.2-fold and reduced tissue accumulation of PCB 77 (roots by 96% and nodules by 93%). Conversely, dechlorination decreased in plants exposed to a nitrogenase inhibitor (nitrate) or harboring nitrogenase-deficient symbionts (nifA mutant) by 29% and 72%, respectively. A range of dechlorinated products (biphenyl, methylbiphenyls, hydroxylbiphenyls, and trichlorobiphenyl derivatives) were detected within nodules and roots under nitrogen-fixing conditions. Levels of nitrogenase-derived hydrogen and leghemoglobin expression correlated positively with nodular dechlorination rates, suggesting a more reducing environment promotes PCB dechlorination. Our findings demonstrate for the first time that symbiotic nitrogen fixation acts as a driving force for tetrachlorobiphenyl dechlorination. In turn, this opens new possibilities for using rhizobia to enhance phytoremediation of halogenated organic compounds.

Rhizoremediation half-lives of PCBs: Role of congener composition, organic carbon forms, bioavailability, microbial activity, plant species and soil conditions, on the prediction of fate and persistence in soil

Polychlorinated biphenyls (PCBs) are persistent organic pollutants widely produced and used in many countries until the increasing concern about their environmental risk lead to their ban in the 1980s. Although their emissions decreased, PCBs are nowadays still present in the environment and can be reemitted from reservoir compartments such as contaminated soils. In the last two decades, there has been a growing interest in bioremediation technologies that use plants and microorganisms (i.e. rhizoremediation) to degrade organic chemicals in contaminated sites. Different studies have been conducted to investigate the potential of plant-microbe interactions in the remediation of organic chemical contaminated soils. They range from short-term and laboratory/greenhouse experiments to long-term and field trials and, when correctly set up, they could provide useful data such as PCB rhizoremediation half-lives in soil. Such type of data are important input parameters for multimedia fate models that aim to estimate the time requested to achieve regulatory thresholds in a PCB contaminated site, allowing to draw up its remediation plan. This review focuses on the main factors influencing PCB fate, persistence and bioavailability in soil including PCB mixture congener composition, soil organic carbon forms, microorganism activity, plant species and soil conditions. Furthermore, it provides an estimate of rhizoremediation half-lives of the ten PCB families starting from the results of literature rhizoremediation experiments. Finally, guidance to perform appropriate experiments to obtain comparable, accurate and useful data for fate estimation is proposed.

Enhanced phytoremediation of soils contaminated with PAHs by arbuscular mycorrhiza and rhizobium

Greenhouse experiment was conducted to evaluate the potential effectiveness of a legume (Sesbania cannabina), arbuscular mycorrhizal fungi (AMF) (Glomus mosseae), and rhizobia (Ensifer sp.) symbiosis for remediation of Polycyclic aromatic hydrocarbons (PAHs) in spiked soil. AMF and rhizobia had a beneficial impact on each other in the triple symbiosis. AMF and/or rhizobia significantly increased plant biomass and PAHs accumulation in plants. The highest PAHs dissipation was observed in plant + AMF + rhizobia treated soil, in which >97 and 85-87% of phenanthrene and pyrene, respectively, had been degraded, whereas 81-85 and 72-75% had been degraded in plant-treated soil. During the experiment, a relatively large amount of water-soluble phenolic compounds was detected in soils of AMF and/or rhizobia treatment. It matches well with the high microbial activity and soil enzymes activity. These results suggest that the mutual interactions in the triple symbiosis enhanced PAHs degradation via stimulating both microbial development and soil enzyme activity. The mutual interactions between rhizobia and AMF help to improve phytoremediation efficiency of PAHs by S. cannabina.

Polychlorinated biphenyls in alfalfa: Accumulation, sorption and speciation in different plant parts

The accumulation, chemical speciation and distribution of polychlorinated biphenyls (PCBs) were investigated in various parts of alfalfa. Moreover, the adsorption characteristics for PCB 28 by alfalfa and the influencing factors of the adsorption characteristics were studied. There were different degrees of PCB accumulation in alfalfa roots, root nodules and shoots. The decreasing order of the accumulation of PCBs in plant tissues was root nodules > roots > shoots, and the decreasing order of the total PCB contents was roots > shoots > root nodules, indicating that the roots were the main sink for PCB accumulation. There were three modes of PCB speciation in alfalfa roots and root nodules, comprising strong sorption (78%) and weak sorption (19%) on tissue surfaces and absorption within tissues (2%). The adsorption isotherms of PCB 28 indicate that the adsorption capacities of root nodules and shoots were both significantly higher than that of the roots. Both lipids and carbohydrates, and especially lipids, affected the PCB adsorption capacities of the tissues. These results may help in the elucidation of the mechanisms of sorption and accumulation of PCBs in the plants and their main influencing factors and thus contribute to the development of phytoremediation technologies for PCB-contaminated soils.

Plant-bacteria partnerships for the remediation of persistent organic pollutants

High toxicity, bioaccumulation factor and widespread dispersal of persistent organic pollutants (POPs) cause environmental and human health hazards. The combined use of plants and bacteria is a promising approach for the remediation of soil and water contaminated with POPs. Plants provide residency and nutrients to their associated rhizosphere and endophytic bacteria. In return, the bacteria support plant growth by the degradation and detoxification of POPs. Moreover, they improve plant growth and health due to their innate plant growth-promoting mechanisms. This review provides a critical view of factors that affect absorption and translocation of POPs in plants and the limitations that plant have to deal with during the remediation of POPs. Moreover, the synergistic effects of plant-bacteria interactions in the phytoremediation of organic pollutants with special reference to POPs are discussed.

Isolation of the PCB-degrading bacteria Mesorhizobium sp. ZY1 and its combined remediation with Astragalus sinicus L. for contaminated soil

A bacterial strain ZY1 capable of utilizing PCBs as its carbon source was isolated from the root nodules of Chinese milk vetch (Astragalus sinicus L.). The strain was identified as Mesorhizobium sp. according to its physiological-biochemical properties and the analysis of its 16S rRNA gene sequence. When the initial OD600 was 0.15, 62.7% of 15 mg L(-1) 3,3',4,4'-TCB in a liquid culture was degraded by Mesorhizobium sp. ZY1 within 10 days. Mesorhizobium sp. ZY1 also greatly increased the biotransformation of soil PCBs. Pot experiments indicated that the soil PCB concentrations of a single incubation of strain ZY1 (R) and a single planting of A. sinicus (P) decreased by 20.5% and 23.0%, respectively, and the concentration of PCBs in soil treated with A. sinicus and strain ZY1 decreased by 53.1%. We also observed that A. sinicus-Mesorhizobium sp. ZY1 treatment (PR) improved plant biomass and the concentration of PCBs in plants compared with a single A. sinicus planting treatment (P). The results suggest that the synergistic association between A. sinicus and PCBs-degrading Mesorhizobium sp. ZY1 can stimulate the phytoextraction of PCBs and the rhizosphere microflora to degrade PCBs, and might be a promising bioremediation strategy for PCB-contaminated soil.

Rhizobia and their bio-partners as novel drivers for functional remediation in contaminated soils

Environmental pollutants have received considerable attention due to their serious effects on human health. There are physical, chemical, and biological means to remediate pollution; among them, bioremediation has become increasingly popular. The nitrogen-fixing rhizobia are widely distributed in the soil and root ecosystems and can increase legume growth and production by supplying nitrogen, resulting in the reduced need for fertilizer applications. Rhizobia also possess the biochemical and ecological capacity to degrade organic pollutants and are resistant to heavy metals, making them useful for rehabilitating contaminated soils. Moreover, rhizobia stimulate the survival and action of other biodegrading bacteria, thereby lowering the concentration of pollutants. The synergistic action of multiple rhizobial strains enhances both plant growth and the availability of pollutants ranging from heavy metals to persistent organic pollutants. Because phytoremediation has some restrictions, the beneficial interaction between plants and rhizobia provides a promising option for remediation. This review describes recent advances in the exploitation of rhizobia for the rehabilitation of contaminated soil and the biochemical and molecular mechanisms involved, thereby promoting further development of this novel bioremediation strategy into a widely accepted technique.

Bacterial degradation of Aroclor 1242 in the mycorrhizosphere soils of zucchini (Cucurbita pepo L.) inoculated with arbuscular mycorrhizal fungi

A greenhouse experiment was conducted to investigate the effects of zucchini (Cucurbita pepo L.), inoculated with the arbuscular mycorrhizal (AM) species Acaulospora laevis, Glomus caledonium, and Glomus mosseae, on the soil bacterial community responsible for Aroclor 1242 dissipation. The dissipation rates of Aroclor 1242 and soil bacteria abundance were much higher with the A. laevis and G. mosseae treatments compared to the non-mycorrhizal control. The biphenyl dioxygenase (bphA) and Rhodococcus-like 2,3-dihydroxybiphenyl dioxygenase (bphC) genes were more abundant in AM inoculated soils, suggesting that the bphA and Rhodococcus-like bphC pathways play an important role in Aroclor 1242 dissipation in the mycorrhizosphere. The soil bacterial communities were dominated by classes Betaproteobacteria and Actinobacteria, while the relative proportion of Actinobacteria was significantly (F=2.288, P<0.05) correlated with the PCB congener profile in bulk soil. Our results showed that AM fungi could enhance PCB dissipation by stimulating bph gene abundance and the growth of specific bacterial groups.

Cucurbita spp. and Cucumis sativus enhance the dissipation of polychlorinated biphenyl congeners by stimulating soil microbial community development

A number of Cucurbita species have the potential to extract polychlorinated biphenyls (PCBs) from soil, but their impact on the soil microbial communities responsible for PCB degradation remains unclear. A greenhouse experiment was conducted to investigate the effect of three Cucurbita and one Cucumis species on PCB dissipation and soil microbial community structure. Compared to the unplanted control, enhanced losses of PCBs (19.5%-42.7%) were observed in all planted soils. Cucurbita pepo and Cucurbita moschata treatments were more efficient in PCB dissipation, and have similar patterns of soil phospholipid fatty acids (PLFAs) and PCB congener profiles. Cucurbita treatments tend to have higher soil microbial biomass than Cucumis. Gram-negative (G(-)) bacteria were significantly correlated with PCB degradation rates (R(2) = 0.719, p < 0.001), while fungi and G(-) bacteria were correlated with dissipation of the penta homologue group (R(2) = 0.590, p < 0.01). Therefore, Cucurbita related soil microorganisms could play an important role in remediation of PCB contaminated soils.

Comparing anthracene and fluorene degradation in anthracene and fluorene-contaminated soil by single and mixed plant cultivation

The ability of three plant species (sweet corn, cucumber, and winged bean) to remediate soil spiked with 138.9 and 95.9 mg of anthracene and fluorene per kg of dry soil, respectively, by single and double plant co-cultivation was investigated. After 15 and 30 days of transplantation, plant elongation, plant weight, chlorophyll content, and the content of each PAH in soil and plant tissues were determined. Based on PAH removal and plant health, winged bean was the most effective plant for phytoremediation when grown alone; percentage of fluorene and anthracene remaining in the rhizospheric soil after 30 days were 7.8% and 24.2%, respectively. The most effective combination of plants for phytoremediation was corn and winged bean; on day 30, amounts of fluorene and anthracene remaining in the winged bean rhizospheric soil were 3.4% and 14.3%, respectively; amounts of fluorene and anthracene remaining in the sweet corn rhizospheric soil were 4.1% and 8.8%, respectively. Co-cultivation of sweet corn and cucumber could remove fluorene to a higher extent than anthracene from soil within 15 days, but these plants did not survive and died before day 30. The amounts of fluorene remaining in the rhizospheric soil of corn and cucumber were only 14% and 17.3%, respectively, on day 15. No PAHs were detected in plant tissues. This suggests that phytostimulation of microbial degradation in the rhizosphere was most likely the mechanism by which the PAHs were removed from the spiked soil. The results show that co-cultivation of plants has merit in the phytoremediation of PAH-spiked soil.

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.

Overview of in situ and ex situ remediation technologies for PCB-contaminated soils and sediments and obstacles for full-scale application

Polychlorinated biphenyls (PCB) are persistent organic pollutants used worldwide between the 1930s and 1980s. Although their use has been heavily restricted, PCB can be found in contaminated soils and sediments. The most frequent remediation solutions adopted are "dig and dump" and "dig and incinerate", but there are currently new methods that could be more sustainable alternatives. This paper takes a look into the remediation options available for PCB-contaminated soils and sediments, differentiating between biological, chemical, physical and thermal methods. The use of combined technologies was also reviewed. Most of them are still in an initial development stage and further research in different implementation issues is needed. There is no single technology that is the solution for PCB contamination problem. The successful remediation of a site will depend on proper selection, design and adjustment of the technology or combined technologies to the site characteristics.

Bioaugmentation with Petroleum-Degrading Consortia Has a Selective Growth-Promoting Impact on Crop Plants Germinated in Diesel Oil-Contaminated Soil

Rhizoremediation is a complex type of green clean-up technology that involves both plants and the rhizosphere-associated microorganisms to decompose hazardous compounds. The success of the strategy strongly depends on plant tolerance towards the pollutant, as well as plant's interactions with the rhizospheric microbes. The microorganisms may be stimulated by the secreted root exudates, which results in an increased breakdown of contaminants in the rhizosphere. The main goal of this study was to establish a potential rhizoremediation combination for a diesel-polluted site. Inoculation of plant roots or seeds with indigenous rhizospheric populations is a common approach in the rhizoremediation. However, we introduced hydrocarbon-degrading consortia (M10, R3, and K52) that were previously isolated from crude oil-contaminated soil instead of indigenous microbes. Bioaugmentation with these petroleum degraders was applied to screen four high biomass crop species (Indian mustard, alfalfa, high erucic acid rapeseed, HEAR, and low erucic acid rapeseed, LEAR) for their tolerance towards diesel oil. At no pollution, a promoting effect of M10 bacteria could be observed on germination and root elongation of all plant species. Moreover, M10 consortiums increased the germination index at 6,000 mg diesel oil per kilogram dry soil in the case of Indian mustard, alfalfa, and HEAR. The latter species was found to increment its dry weight upon bioaugmentation with M10 bacteria and all diesel oil treatments (6,000 and 24,000 mg diesel oil per kilogram dry soil). The initial results indicate HEAR and the M10 bacterial consortium as a promising plant-microbe tandem for a long-term rhizoremediation process.

Spacial characteristics of pyrene degradation and soil microbial activity with the distance from the ryegrass (Lolium perenne L.) root surface in a multi-interlayer rhizobox

To investigate rhizosphere effects on the biodegradation of pyrene with the distance away from root surface in the rhizosphere of ryegrass (Lolium perenne L.), a glasshouse experiment was conducted using a multi-interlayer rhizobox where ryegrass were grown in a soil spiked with pyrene. The largest and most rapid dissipation of pyrene in planted soil appeared at 2 mm zone from the root zone. The pyrene degradation gradient followed the order: near-rhizosphere>root compartment>far-rhizosphere soil zones. In contrast, there was no difference in pyrene concentration with distance in the unplanted soil. Dynamic changes of soil microbial biomass carbon (C(mic)) and the activities of both soil polyphenol oxidase and dehydrogenase were to some extent coincident with the degradation of pyrene with distance away from the root compartment in planted soils, which indicated the changes of soil microorganisms in different soil zones of rhizosphere were mainly responsible for the observed pyrene degradation. The largest C(mic) and activities of both soil polyphenol oxidase and dehydrogenase also occurred in near-rhizosphere, especially in 2mm zone from the root surface. The above results suggest that the effect of root proximity is important in the degradation of pyrene in ryegrass growing soil.

Bioremediation of benzene-, MTBE- and ammonia-contaminated groundwater with pilot-scale constructed wetlands

In this pilot-scale constructed wetland (CW) study for treating groundwater contaminated with benzene, MTBE, and ammonia-N, the performance of two types of CWs (a wetland with gravel matrix and a plant root mat) was investigated. Hypothesized stimulative effects of filter material additives (charcoal, iron(III)) on pollutant removal were also tested. Increased contaminant loss was found during summer; the best treatment performance was achieved by the plant root mat. Concentration decrease in the planted gravel filter/plant root mat, respectively, amounted to 81/99% for benzene, 17/82% for MTBE, and 54/41% for ammonia-N at calculated inflow loads of 525/603 mg/m(2)/d, 97/112 mg/m(2)/d, and 1167/1342 mg/m(2)/d for benzene, MTBE, and ammonia-N. Filter additives did not improve contaminant depletion, although sorption processes were observed and elevated iron(II) formation indicated iron reduction. Bacterial and stable isotope analysis provided evidence for microbial benzene degradation in the CW, emphasizing the promising potential of this treatment technique.

Dissipation of pentachlorophenol in the aerobic-anaerobic interfaces established by the rhizosphere of rice ( Oryza sativa L.) root

Phytoremediation is an emerging technology for the detoxification and remediation of organic pollutants such as pentachlorophenol (PCP). To investigate the dissipation behavior of PCP in the aerobic-anaerobic interfaces established by the rhizosphere of rice ( L.) root, a glasshouse experiment was conducted using a specially designed rhizobox. The possible biogeochemical mechanisms were also studied through illustration of the dynamic behavior of important electron acceptors and donors that are potentially involved in the reductive dechlorination and aerobic catabolism processes of PCP. The soil was spiked with 20 ± 0.25 and 45 ± 0.25 mg of PCP kg soil. Soil in the rhizobox was divided into five different compartments at various distances from the root surface. Maximum dissipation of PCP in planted soil was observed at 3-mm distance from the root zone as well as rapid changes in concentrations of sulfate, chloride, nitrate, and ammonium at the same distance from the root. In contrast, in the unplanted soil, no difference was observed in the PCP concentration with increasing distance. After 45 d, a significantly higher concentration of PCP was degraded in planted soil compared with unplanted soil. In the unplanted microcosms, about 45% of the initial PCP was lost at both low and high added rates, respectively. This was, proportionately, a significantly smaller percentage compared with the planted rhizosphere (an average of 66 and 64.5%, respectively). Moreover, the correlations of PCP dissipation with SO, NO, and Fe were significantly negative, while the correlations of PCP dissipation with NH, Fe, and Cl were significantly positive. This suggested the oxidization of soil constituents can inhibit aerobic catabolism of PCP by consuming O, and the reduction of soil constituents can inhibit anaerobic reductive dechlorination of PCP. Therefore, the significance of the rhizosphere in phytoremediation of chlorinated compounds such as PCP differs significantly between wetland and rainfed systems.

Influence of Rhizobium meliloti on phytoremediation of polycyclic aromatic hydrocarbons by alfalfa in an aged contaminated soil

Microbe-assisted phytoremediation is emerging as one of the most effective means by which plants and their associated rhizosphere microbes degrade organic contaminants in soils. A pot study was conducted to examine the effects of inoculation with Rhizobium meliloti on phytoremediation by alfalfa grown for 90 days in an agricultural soil contaminated with weathered polycyclic aromatic hydrocarbons (PAHs). Planting with uninoculated alfalfa (P) and alfalfa inoculated with R. meliloti (PR) significantly lowered the initial soil PAH concentrations by 37.2 and 51.4% respectively compared with unplanted control soil. Inoculation with R. meliloti significantly increased the counts of culturable PAH-degrading bacteria, soil microbial activity and the carbon utilization ability of the soil microbial community. The results suggest that the symbiotic association between alfalfa and Rhizobium can stimulate the rhizosphere microflora to degrade PAHs and its application may be a promising bioremediation strategy for aged PAH-contaminated soils.

Potential for biodegradation of polychlorinated biphenyls (PCBs) by Sinorhizobium meliloti

Resting cell assay and soil microcosms were set up to investigate the biodegradation capability and metabolic intermediate of polychlorinated biphenyls (PCBs) by a rhizobial strain Sinorhizobium meliloti. Biodegradation was observed immediately after 2,4,4'-TCB was supplied as a sole source of carbon and energy in liquid cultures. After 6 days, the percent biodegradation of 2,4,4'-TCB was 77.4% compared with the control. The main intermediate was identified as 2-hydroxy-6-oxo-6-phenylhex-2,4-dienoic acid (HOPDA) for 2,4,4'-TCB as determined by gas chromatography-mass spectrometry (GC-MS). Inoculation with S. meliloti greatly enhanced the degradation of target PCB mixtures in the soil. Moreover, soil culturable bacteria, fungi and biphenyl degrading bacteria counts showed significant increase after inoculation of S. meliloti. This study suggests that S. meliloti is promising in PCB bioremediation.

Influence of arbuscular mycorrhiza and Rhizobium on phytoremediation by alfalfa of an agricultural soil contaminated with weathered PCBs: a field study

A field experiment was conducted to study the effects of inoculation with the arbuscular mycorrhizal fungus Glomus caledonium and/or Rhizobium meliloti on phytoremediation of an agricultural soil contaminated with weathered PCBs by alfalfa grown for 180 days. Planting alfalfa (P), alfalfa inoculated with G. caledonium (P + AM), alfalfa inoculated with R. meliloti (P + R), and alfalfa co-inoculated with R. meliloti and G. caledonium (P+AM+R) decreased significantly initial soil PCB concentrations by 8.1, 12.0, 33.8, and 43.5%, respectively. Inoculation with R. meliloti and/or G. caledonium (P+AM+R) increased the yield of alfalfa, and the accumulation of PCBs in the shoots. Soil microbial counts and the carbon utilization ability of the soil microbial community increased when alfalfa was inoculated with R. meliloti and/or G. caledonium. Results of this field study suggest that synergistic interactions between AMF and Rhizobium may have great potential to enhance phytoremediation by alfalfa of an agricultural soil contaminated with weathered PCBs.

Enhanced removal of polychlorinated biphenyls from alfalfa rhizosphere soil in a field study: the impact of a rhizobial inoculum

Polychlorinated biphenyls (PCB) are persistent pollutants in soil environments where they continue to present considerable human health risks. Successful strategies to remediate contaminated soils are needed that are effective and of low cost. Bioremediation approaches that include the use of plants and microbial communities to promote degradation of PCB have significant potential but need further assessment under field conditions. The effects of growth of alfalfa (Medicago sativa L.) and inoculation with a symbiotic nitrogen fixing bacterium (Rhizobium meliloti) on the removal of polychlorinated biphenyls (PCB) from rhizosphere soil were evaluated in a field experiment. The initial PCB content of the soil ranged from 414 to 498 microg kg(-)(1). PCB removal for the rhizosphere soil was enhanced in the planted treatments, an average of 36% decrease in PCB levels compared to a 5.4% decrease in the unplanted soil, and further enhanced when plants were inoculated with the symbiotic Rhizobium (an average of 43% decrease) when evaluated at 90 days after planting. Plant biomass production was higher in the inoculated treatment. The total PCB content was increased from 3.30 microg kg(-)(1) to 26.72 microg kg(-)(1) in plant shoots, and from 115.07 microg kg(-)(1) to 142.23 microg kg(-)(1) in roots in the inoculated treatment compared to the planted treatment. Increased colony forming units (cfu) of total heterotrophic bacteria, biphenyl-degrading bacteria and fungi were observed in the rhizosphere of inoculated plants. PCB removal from the rhizosphere soil was not significantly correlated with the direct PCB uptake by the plants in any of the treatments but was significantly correlated with the stimulation of rhizosphere microflora. Changes in the soil microbial community structure in the planted and inoculated treatment were observed by profiling of bacterial ribosomal sequences. Some bacteria, such as Flavobacterium sp., may have contributed to the effective degradation of PCB and deserve further investigation.

Enhanced phytoremediation potential of polychlorinated biphenyl contaminated soil from e-waste recycling area in the presence of randomly methylated-beta-cyclodextrins

The crude recycling of electronic and electric waste (e-waste) is now creating soil pollution problems with organic compounds such as polychlorinated biphenyls (PCBs). The present study aimed to compare the phytoremediation potential of four plant species (rice, alfalfa, ryegrass and tall fescue) for PCBs contaminated soil from Taizhou city, one of the largest e-waste recycling centers in China. In addition, the enhanced effects of randomly methylated-beta-cyclodextrins (RAMEB) on PCBs phytoremediation potential were evaluated. Higher PCBs removal percentages of 25.6-28.5% in rhizosphere soil were observed after 120 days, compared with those of the non-rhizosphere (10.4-16.9%) and unplanted controls (7.3%). The average PCBs removal percentages of four plant species increased from 26.9% to 37.1% in the rhizosphere soil with addition of RAMEB. Meanwhile, relatively high microbial counts and dehydrogenase activity were detected in planted soils and a stimulatory effect by RAMEB addition was found. The present study indicated that all the plant candidates were feasible for phytoremediation of PCBs contaminated soil from the e-waste recycling area, and tall fescue with RAMEB amendment seemed as a promising remediation strategy. High PCBs removal percentage was due to the increased PCBs bioavailability as well as biostimulation of microbial communities after plantation and RAMEB addition.

Phytoremediation of contaminated soils and groundwater: lessons from the field

BACKGROUND, AIM, AND SCOPE

The use of plants and associated microorganisms to remove, contain, inactivate, or degrade harmful environmental contaminants (generally termed phytoremediation) and to revitalize contaminated sites is gaining more and more attention. In this review, prerequisites for a successful remediation will be discussed. The performance of phytoremediation as an environmental remediation technology indeed depends on several factors including the extent of soil contamination, the availability and accessibility of contaminants for rhizosphere microorganisms and uptake into roots (bioavailability), and the ability of the plant and its associated microorganisms to intercept, absorb, accumulate, and/or degrade the contaminants. The main aim is to provide an overview of existing field experience in Europe concerning the use of plants and their associated microorganisms whether or not combined with amendments for the revitalization or remediation of contaminated soils and undeep groundwater. Contaminations with trace elements (except radionuclides) and organics will be considered. Because remediation with transgenic organisms is largely untested in the field, this topic is not covered in this review. Brief attention will be paid to the economical aspects, use, and processing of the biomass.

CONCLUSIONS AND PERSPECTIVES

It is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).

Technical note phytoremediation of triazophos by Canna indica Linn. in a hydroponic system

The phytoremediation of triazophos (O, O-diethyl-O-(1-phenyl-1, 2, 4-triazole-3-base) sulfur phosphate, TAP) by Canna indica Linn. in a hydroponic system was studied. After 21 d of exposure, the removal kinetic constant (K) of TAP was 0.0229-0.0339 d(-1) and the removal percentage of TAP was 41-55% in the plant system and the K and removal percentage of TAP were about 0.002 d(-1) and 1%, respectively, in darkness and disinfected control. However, the K and removal percentage of TAP were 0.006 d(-1) and approximately 11%, respectively, in the treatment with eluate from the media of constructed wetland. The contribution of plant to the remediation of TAP was 74% and C. indica played the most important role in the hydroponic system. Under the stress of TAP and without inorganic phosphorus nutrient, the activity of phosphatase in the plant system increased and phytodegradation was observed. The production and release of phosphatase is seen as the key mechanism for C. indica to degrade TAP. C. indica, which showed the potential of phytoremediation of TAP, and is commonly used in constructed wetland, so the technique of phytoremediation of TAP from contaminated water can be developed with the combination of constructed wetland.

Influence of citric acid amendments on the availability of weathered PCBs to plant and earthworm species

A series of small and large pot trials were conducted to assess the phytoextraction potential of several plant species for weathered polychlorinated biphenyls (PCBs) in soil (105 microg/g Arochlor 1268). In addition, the effect of citric acid on PCB bioavailability to both plants and earthworms was assessed. Under small pot conditions (one plant, 400 g soil), three cucurbits (Cucurbita pepo ssp pepo [zucchini] and ssp ovifera [nonzucchini summer squash], Cucumis sativus, cucumber) accumulated up to 270 microg PCB/g in the roots and 14 microg/g in the stems, resulting in 0.10% contaminant removal from soil. Periodic 1 mM subsurface amendments of citric acid increased the stem and leaf PCB concentration by 330 and 600%, respectively, and resulted in up to a 65% increase in the total amount of contaminant removed from soil. Although citric acid at 10 mM more than doubled the amount of PCB desorbed in abiotic batch slurries, contaminant accumulation by two earthworm species (Eisenia foetida and Lumbricus terrestris) was unaffected by citric acid at 1 and 10 mM and ranged from 11-15 microg/g. Two large pot trials were conducted in which cucurbits (C. pepo ssp pepo and ssp ovifera, C. sativus) and white lupin (Lupinus albus) were grown in 70 kg of PCB-contaminated soil White lupin was the poorest accumulator of PCBs, with approximately 20 microg/g in the roots and 1 microg/g in the stems. Both C. pepo ssp ovifera (summer squash) and C. sativus (cucumber) accumulated approximately 65-100 microg/g in the roots and 6-10 microg/g in the stems. C. pepo ssp pepo (zucchini) accumulated significantly greater levels of PCB than all other species, with 430 microg/g in the roots and 22 microg/g in the stems. The mechanism by which C. pepo spp pepo extracts and translocates weathered PCBs is unknown, but confirms earlier findings on the phytoextraction of other weathered persistent organic pollutants such as chlordane, p,p'-DDE, and polycyclic aromatic hydrocarbons.