Dissipation of polycyclic aromatic hydrocarbons (PAHs) in the rhizosphere: Synthesis through meta-analysis

Polycyclic aromatic hydrocarbons (PAHs) are widespread and persistent organic pollutants with high carcinogenic effect and toxicity; their behavior and fate in the soil-plant system have been widely investigated. In the present paper, meta-analysis was used to explore the interaction between plant growth and dissipation of PAHs in soil based on the large body of published literature. Plants have a promoting effect on PAH dissipation in soils. There was no difference in PAH dissipation between soils contaminated with single and mixed PAHs. However, plants had a more obvious effect on PAH dissipation in freshly-spiked soils than in long-term field-polluted soils. Additionally, a positive effect of the number of microbial populations capable of degrading PAHs was observed in the rhizosphere compared with the bulk soil. Our meta-analysis established the importance of the rhizosphere effect on PAH dissipation in variety of the soil-plant systems.     

Plant–bacteria partnerships for the remediation of hydrocarbon contaminated soils

Plant–bacteria partnerships have been extensively studied and applied to improve crop yield. In addition to their application in agriculture, a promising field to exploit plant–bacteria partnerships is the remediation of soil and water polluted with hydrocarbons. Application of effective plant–bacteria partnerships for the remediation of hydrocarbons depend mainly on the presence and metabolic activities of plant associated rhizo- and endophytic bacteria possessing specific genes required for the degradation of hydrocarbon pollutants. Plants and their associated bacteria interact with each other whereby plant supplies the bacteria with a special carbon source that stimulates the bacteria to degrade organic contaminants in the soil. In return, plant associated-bacteria can support their host plant to overcome contaminated-induced stress responses, and improve plant growth and development. In addition, plants further get benefits from their associated-bacteria possessing hydrocarbon-degradation potential, leading to enhanced hydrocarbon mineralization and lowering of both phytotoxicity and evapotranspiration of volatile hydrocarbons. A better understanding of plant–bacteria partnerships could be exploited to enhance the remediation of hydrocarbon contaminated soils in conjunction with sustainable production of non-food crops for biomass and biofuel production.     

Uptake of selected PAHs from contaminated soils by rice seedlings (Oryza sativa) and influence of rhizosphere on PAH distribution

The uptake of selected polycyclic aromatic hydrocarbons (PAHs) by rice (Oryza sativa) seedlings from spiked aged soils was investigated. When applied to soils aged for 4 months, naphthalene, phenanthrene, and pyrene exhibited volatilization loss of 98, 95, and 30%, respectively, with the remaining fraction being fixed by soil organic matter and/or degraded by soil microbes. In general, concentrations of the three PAHs in rice roots were greater than those in the shoots. The concentrations of root associated PHN and PYR increased proportionally with both soil solution and rhizosphere concentrations. PAH concentrations in shoots were largely independent of those in soil solution, rice roots, or rhizosphere soil. The relative contributions of plant uptake and plant-promoted rhizosphere microbial biodegradation to the total mass balance were 0.24 and 14%, respectively, based on PYR concentrations in rhizosphere and non-rhizosphere soils, the biomass of rice roots, and the dry soil weight.     

Remediation mechanisms of mercapto-grafted palygorskite for cadmium pollutant in paddy soil

The immobilization agent was the key factor that determined the success of remediation of heavy metal polluted soil. In this study, mercapto-grafted palygorskite (MP) as a novel and efficient immobilization agent was utilized for the remediation of Cd-polluted paddy soil in pot trials, and the remediation mechanisms were investigated in the aspect of soil chemistry and plant physiology with different rice cultivars as model plants. Mercapto-grafted palygorskite at applied doses of 0.1–0.3% could reduce Cd contents of brown rice and straws of different cultivars significantly. Both reduced DTPA-extractable Cd contents in rhizosphere and non-rhizosphere soil and decreasing Cd contents in iron plaques on rice root surfaces confirmed that MP was an efficient immobilization agent for Cd pollutant in paddy soil. In the aspect of soil chemistry, the pH values of rhizosphere and non-rhizosphere soils had no statistical changes in the MP treatment groups, but their zeta potentials decreased obviously, indicating that MP could enhance the fixation or sorption of Cd on soil compositions. In the aspect of antioxidant system, MP could increase POD activity of rice roots significantly to alleviate the stress of Cd to roots, and resulted in the decrease of T-AOC, SOD, and CAT activities of rice roots of the selected cultivars. MP had no inhabitation or enhancement effects on TSH of rice roots but enhance the contents of MTs and NPT to binding Cd to complete detoxification process. MP as a novel and efficient immobilization agent could complete the remediation effects through soil chemistry and plant physiological mechanisms.

     

Utilising the Synergy between Plants and Rhizosphere Microorganisms to Enhance Breakdown of Organic Pollutants in the Environment (15 pp)

Background Phytoremediation is a promising technology for the cleanup of polluted environments. The technology has so far been used mainly to remove toxic heavy metals from contaminated soil, but there is a growing interest in broadening its applications to remove/degrade organic pollutants in the environment. Both plants and soil microorganisms have certain limitations with respect to their individual abilities to remove/breakdown organic compounds. A synergistic action by both rhizosphere microorganisms that leads to increased availability of hydrophobic compounds, and plants that leads to their removal and/or degradation, may overcome many of the limitations, and thus provide a useful basis for enhancing remediation of contaminated environments.Main Features The review of literature presented in this article provides an insight to the nature of plant-microbial interactions in the rhizosphere, with a focus on those processes that are relevant to the breakdown and/or removal of organic pollutants. Due consideration has been given to identify opportunities for utilising the plant-microbial synergy in the rhizosphere to enhance remediation of contaminated environments.Results and Discussion The literature review has highlighted the existence of a synergistic interaction between plants and microbial communities in the rhizosphere. This interaction benefits both microorganisms through provision of nutrients by root exudates, and plants through enhanced nutrient uptake and reduced toxicity of soil contaminants. The ability of the plant-microbial interaction to tackle some of the most recalcitrant organic chemicals is of particular interest with regard to enhancing and extending the scope of remediation technologies.Conclusions Plant-microbial interactions in the rhizosphere offer very useful means for remediating environments contaminated with recalcitrant organic compounds.Outlook A better knowledge of plant-microbial interactions will provide a basis for improving the efficacy of biological remediations. Further research is, however, needed to investigate different feedback mechanisms that select and regulate microbial activity in the rhizosphere.     

Accumulation of phenanthrene and pyrene in rhizosphere soil

A study was conducted to determine PAH concentrations in the rhizosphere of plants grown in soil containing phenanthrene or pyrene. The rhizosphere of tall fescue and wheat grown in sterile soil contained 4-5-fold higher pyrene concentrations than unplanted soil. The rhizosphere of several plant species grown in non-sterile soil temporarily contained appreciably more phenanthrene or pyrene than unplanted soil, but those PAHs were degraded with time. The data suggest that plants accumulate such hydrophobic compounds in the rhizosphere after facilitating their transport toward the roots.     

PAH dissipation in spiked soil: impacts of bioavailability, microbial activity, and trees

While trees have demonstrated potential in phytoremediation of several organic contaminants, little is known regarding their ability to impact the common soil contaminant PAHs. Several species of native North American trees were planted in soil artificially contaminated with three PAHs. Plant biomass, PAH dissipation, and microbial mineralization were monitored over the course of one year and environmental conditions were allowed to follow typical seasonal patterns. PAH dissipation and mineralization were not affected by planting. Extensive and rapid loss of PAHs was observed and attributed to high bioavailability and microbial activity in all treatments. The rate of this loss may have masked any significant planting effects. Anthracene was found to be more recalcitrant than pyrene or phenanthrene. Parallel soil aging studies indicated that sequestration to soil components was minimal. Contrary to common inferences in literature, amendment with decaying fine roots inhibited PAH degradation by the soil microbial community. Seasonal variation in environmental factors and rhizosphere dynamics may have also reduced or negated the effect of planting and should be taken into account in future phytoremediation trials. The unique root traits of trees may pose a challenge to traditional thought regarding PAH dissipation in the rhizosphere of plants.     

Phenol tolerance, changes of antioxidative enzymes and cellular damage in transgenic tobacco hairy roots colonized by arbuscular mycorrhizal fungi

Phytoremediation has been recognized as a cheap and eco-friendly technology which could be used for the remediation of organic pollutants, such as phenolic compounds. Besides, the extent to which plants react to environmental pollution might depend on rhizosphere processes such as mycorrhizal symbiosis. In the present work, phenol tolerance of transgenic tobacco hairy roots (HR), namely TPX1, colonized with an arbuscular mycorrhizal fungus (AMF) was studied. However, the question is whether AMF symbiosis can moderate adverse effects of phenol to the plant tissues. Thus, the antioxidative response as well as parameters of oxidative damage, like malondialdehyde (MDA) content, were determined. Antioxidative enzymes such as peroxidase, superoxide dismutase, ascorbate peroxidase were higher in TPX1 HR colonized with AMF, compared to wild type HR colonized by AMF, in the presence of increasing concentrations of the pollutant. Besides, MDA levels remained unaltered in TPX1 HR associated with AMF treated with the xenobiotic. These results, suggested that this culture could tolerate phenol and moreover, it has an efficient protective mechanism against phenol-induced oxidative damage, which is of great importance in the selection of species with remediation capacities. Thus, transgenic HR colonized with AMF could be considered as an interesting model system to study different processes which play a key role in the phytoremediation of organic pollutants.     

Effects of soil organic matter on the development of the microbial polycyclic aromatic hydrocarbons (PAHs) degradation potentials

The microbial activity in soils was a critical factor governing the degradation of organic micro-pollutants. The present study was conducted to analyze the effects of soil organic matter on the development of degradation potentials for polycyclic aromatic hydrocarbons (PAHs). Most of the degradation kinetics for PAHs by the indigenous microorganisms developed in soils can be fitted with the Logistic growth models. The microbial activities were relatively lower in the soils with the lowest and highest organic matter content, which were likely due to the nutrition limit and PAH sequestration. The microbial activities developed in humic acid (HA) were much higher than those developed in humin, which was demonstrated to be able to sequester organic pollutants stronger. The results suggested that the nutrition support and sequestration were the two major mechanisms, that soil organic matter influenced the development of microbial PAHs degradation potentials.     

Use of a lux-marked rhizobacterium as a biosensor to assess changes in rhizosphere C flow due to pollutant stress

The flow of carbon from plant roots into soil supports a range of microbial processes and is therefore critical to ecosystem function and health. Pollution-induced stress, which influences rhizosphere C flow is of considerable potential importance, and therefore needs to be evaluated. This paper reports on a method, based on reporter gene technology, for quantifying pollutant effects on rhizosphere C flow. The method uses the lux-marked rhizobacterium Pseudomonas fluorescens, where bioluminescence output of this biosensor is directly correlated with the metabolic activity and reports on C flow in root exudate. Plantago lanceolata was treated with paraquat (representing a model pollutant stress) in a simple microcosm system. The lux-biosensor response correlated closely with C concentrations in the exudate and demonstrated that the pollutant stress increased the C flow from the plantago roots, 24 h after application of the herbicide. The lux-reporter system therefore potentially offers a technique for use in assessing the impact of pollutant stress on rhizosphere C flow through the soil microbial biomass.     

Lability of polycyclic aromatic hydrocarbons in the rhizosphere

Remediation of soils containing high concentrations of polycyclic aromatic hydrocarbons (PAHs) seldom results in complete removal of contaminants, but residual toxicity often is reduced. In this study, soil from a former manufactured gas plant site was treated for 12 months by phytoremediation and then tested for total PAHs, Tenax-TA extractable ("labile") PAHs, aqueous soluble PAHs (PAH(wp)) , and biotoxicity assessed by earthworms survival, nematode mortality, emergence of lettuce seedlings, and microbial respiration. Prior to phytoremediation, the soil had toxic impacts on all bioassays (except the nematodes), and 12 months of remediation decreased this response. Change in labile PAHs was a predictor for change in total PAH for 3- and 4-ring compounds but not for the 5- and 6-ring. Decreases in labile PAHs were correlated (r(2)>or=0.80) with toxicity in the bioassays except microbial respiration. PAH(wp) was correlated only with nematode toxicity prior to remediation but with none of the tests after remediation. Total PAHs were not correlated with any of the bioassay tests. Tenax-TA appears to have potential for predicting residual toxicity in remediated soils and is superior to total concentrations for that application.     

Dissipation of PAHs in saturated, dredged sediments: a field trial

Sediments dredged from navigable rivers often contain elevated concentrations of recalcitrant, potentially toxic organic compounds such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). The presence of these compounds often requires that the sediments be stored in fully contained disposal facilities. A 3-year field study was conducted at the Jones Island disposal facility in Milwaukee, Wisconsin, to compare bioremediation of PAHs in contaminated dredged sediments in the absence of plants to phytoremediation with Salix nigra (black willow) (SX61), Spartina pectinata (prairie cord grass), Carex aquatalis (lake sedge), Lolium multiflorum (annual rye), and Scirpus fluviatilis (bulrush). Nine PAHs were detected initially in the sediments. Over the 3-year experiment, acenaphthene dissipation ranged from 94% to 100%, whereas anthracene, benzo[a]pyrene and indo[1,2,3-cd]pyrene generally had modest decreases in concentration (0-30% decrease). The remaining five PAHs ranged in degree of disappearance from 23% to 82%. Planted treatments did not enhance PAH dissipation relative to those without plants, but treatments with high biomass yield and high transpiration plant species had significantly less removal of PAHs than unplanted controls. Significant, negative correlations between nitrogen removal and decreases in PAH concentration suggest that competition for nutrients between plants and microorganisms may have impeded the microbial degradation of PAHs in the rhizosphere of the more rapidly growing plant species.     

Plant performance, dioxygenase-expressing rhizosphere bacteria, and biodegradation of weathered hydrocarbons in contaminated soil

Two greenhouse pot experiments were conducted to investigate the potential of 13 plant species (grasses, cruciferes, legumes, herbs) to thrive in a long-term contaminated soil from a former manufactured gas plant (MGP) site, to promote the proliferation of total and aromatic ring dioxygenase-expressing bacteria (ARDB) in the root zone, and to foster the biodegradation of petrol hydrocarbons (PHCs) and polycyclic aromatic hydrocarbons (PAHs). PHCs at 23200 mg kg(-1) and PAHs at 2194 mg kg(-1) reduced seed germination, plant survival, and shoot yields for most plants. Total bacteria and ARDB were generally more abundant in contaminated soil and were most numerous in the rhizosphere of mustard. During 68 d, the loss of total petrol hydrocarbons (TPHs) and total US EPA priority PAHs (TPAHs) was greatest in soil planted with hemp and mustard. Pea, cress, and pansy increased the amounts of PAHs extracted from soil, including an almost 60% increase for dibenzo(ah)anthracene. Plants may enhance the chemical extractability and perhaps biological availability of initially unextractable molecules.     

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.     

Evidence for bioaccumulation of PAHs within internal shoot tissues by a halophytic plant artificially exposed to petroleum-polluted sediments

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants of natural and anthropic origins. Despite their poor water solubility, they can be taken up and bioaccumulated by plants. This study was aimed at determining whether the PAHs present in sediments artificially polluted by heavy fuel oil are transferred to shoots of a coastal and edible plant, Salicornia fragilis Ball et Tutin. Bioaccumulation was quantified after a one-week exposure to sediments polluted with 0.2%, 2% and 20% fuel oil (w/w) and over a six-week monitoring at 0.2%. Quantification by GC-MS of PAH amounts in plants and sediments evidenced a bioaccumulation in the shoots by a soil-to-plant transfer through the root system. This bioaccumulation depended on the duration of exposure and on the substratum contamination. PAHs distributions in plants and sediments looked alike with a predominance of low- and medium-weight hydrocarbons. Moreover, high-weight PAHs were also detected in the upper part of plants.     

Studies in the Biodegradation of 5 PAHs (Phenanthrene,Pyrene, Fluoranthene,Chrysene und Benzo(a)pyrene) in the Presence of Rooted Poplar Cuttings

Cuttings of Populus nigra L. cv. Loenen were cultivated in sand treated with one of the following PAHs: phenanthrene (Phen), fluoranthene (Flt), pyrene (Pyr), chrysene (Chr) and benzo[a]pyrene (BaP). The PAHs were applied at varying levels of concentration to each test series. After 6 weeks the concentration and the distribution of the PAHs in the substrate of the various sets of tests were compared with the concentration in the substrate of the control. Additionally the substrate and the plant roots were tested for evidence of degradation products of PAHs. The results revealed that the levels of concentration of Phen and Pyr detected in the substrate surrounding the roots was in some cases significantly lower than in the corresponding section of substrate in the unplanted set (= control). This phenomenon did not occur for Flt and BaP and in the case of Chr only in those substrates, which had been treated with the highest levels of concentration. As the presence of lesser amounts of Phen and Pyr in the plant pots cannot only be attributed to their accumulation and metabolism in the roots, it is fair to assume that the chemical transformation of these three PAHs took place outside the roots. The set of tests treated with Phen revealed the presence of 2- or 3-hydroxy-Phen (main components), a hydroxy-methoxy-Phen, 9,10-Phenanthrenequinone and one unidentified compound in metabolite form. Altogether eleven metabolites of Pyr were identified in the root extracts, which can be divided into three groups: 1-Hydroxy-Pyr and derivatives, dihydroxy-Pyr and derivatives and ring fission products (4-Hydroxy-Pyr and a derivative of the 4-Phen-carbonic acid). However, the metabolite mass detected for Phen and Pyr represents only an insignificant percentage in comparison with the lesser amounts of PAHs observed in the planted set of tests. This indicates that the three PAHs were reduced to lower molecular compounds, which are methodically impossible to record, and subsequently translocated to other parts of the plant and integrated into the biomass. Although no lesser amount for Flt and BaP was found in the plant pots, 1-Hydroxy-Flt, an unidentified compound of Flt and 1-Methoxy-BaP were detected. These are presumably end products which were enhanced in the roots. It was not possible to identify any transformation products of Chr. It can be assumed that the majority of metabolites were not synthesised in the roots but are a result of microbial degradation in the rhizosphere. The test plants improved the conditions for the biotransformation of Phen and Pyr significantly and accumulated Flt, Pyr, Chr and BaP in their roots. It can therefore be concluded that the use of plants in the bioremediation of contaminated soils is a promising option.     

Assessment of Arthrobacter viscosus as reactive medium for forming permeable reactive biobarrier applied to PAHs remediation

Polycyclic aromatic hydrocarbons (PAHs) are significant environmental contaminants as they are present naturally as well as anthropogenically in soil, air and water. In spite of their low solubility, PAHs are spread to the environment, and they are present in surface water, industrial effluent or groundwater. Amongst all remediation technologies for treating groundwater contaminated with PAHs, the use of a permeable reactive biobarrier (PRBB) appears to be the most cost-effective, energy efficient, and environmentally sound approach. In this technology, the microorganisms are used as reactive medium to degrade or stabilize the contaminants. The main limits of this approach are that the microorganisms or consortium used for forming the PRBB should show adequate characteristics. They must be retained in the barrier-forming biofilm, and they should also have degradative ability for the target pollutants. The aim of the present work is to evaluate the viability of Arthrobacter viscosus as bioreactive medium for forming PRBB. Initially, the ability of A. viscosus to remove PAHs, benzo[a]anthracene 100 μM and phenanthrene 100 μM was evaluated operating in a batch bench-scale bioreactor. In both cases, total benzo[a]anthracene and phenanthrene removals were obtained after 7 and 3 days, respectively. Furthermore, the viability of the microorganisms was evaluated in the presence of chromium in a continuous mode. As a final point, the adhesion of A. viscosus to sepiolite forming a bioreactive material to build PRBB was demonstrated. In view of the attained results, it can be concluded that A. viscosus could be a suitable microorganism to form a bioreactive medium for PAHs remediation.     

Root establishment of perennial ryegrass (L. perenne) in diesel contaminated subsurface soil layers

The efficiency of rhizosphere biodegradation of petroleum hydrocarbons heterogeneously distributed in soils is dependent on the ability of plant roots to prospect into contaminated zones. Rhizobox experiments were conducted to study the influence of diesel contaminated layers on the spatial distribution and the development of the roots of perennial ryegrass. Root distribution and root and shoot development were monitored over time. The final root and above ground biomass and the final TPH concentration were determined. The spatial distribution of the contaminant as well as the irrigation method used affected root distribution, plant development and TPH degradation and therefore ryegrass remediation potential. The results show that roots colonise fully uncontaminated soil and grow preferentially between zones of contamination. Conversely, when no immediate uncontaminated soil is available, roots grow through contaminated zones in order to prospect for uncontaminated soil.     

Phytoremediation to increase the degradation of PCBs and PCDD/Fs

Phytoremediation is already regarded as an efficient technique to remove or degrade various pollutants in soils, water and sediments. However, hydrophobic organic molecules such as PAHs, PCBs and PCDD/Fs are much less responsive to bioremediation strategies than, for example, BTEX or LAS. PCDD/Fs and PCBs represent 3 prominent groups of persistent organic pollutants that share common chemical, toxicological and environmental properties. Their widespread presence in the environment may be explained by their chemical and biological stability. This review considers their fate and dissipation mechanisms. It is then possible to identify major sinks and to understand biological activities useful for remediation. Public health and economic priorities lead to the conclusion that alternative techniques to physical treatments are required. This review focuses on particular problems encountered in biodegradation and bioavailability of PCDD/Fs and PCBs. It highlights the potential and limitations of plants and micro-organisms as bioremediation agents and summarises how plants can be used to augment bacterial activity. Phytoremediation is shown to provide some new possibilities in reducing risks associated with dioxins and PCBs.