Effects of agronomic practices on phytoremediation of an aged PAH-contaminated soil

Biochar application strategies for polycyclic aromatic hydrocarbons removal from soils

Polycyclic aromatic hydrocarbons (PAHs) are known as a hazardous group of pollutants in the soil which causes many challenges to the environment. In this study, the potential of biochar (BC), as a carbonaceous material, is evaluated for the immobilization of PAHs in soils. For this purpose, various bonding mechanisms of BC and PAHs, and the strength of bonds are firstly described. Also, the effect of impressive criteria including BC physicochemical properties (such as surface area, porosity, particle size, polarity, aromaticity, functional group, etc., which are mostly the function of pyrolysis temperature), number of rings in PAHs, incubation time, and soil properties, on the extent and rate of PAHs immobilization by BC are explained. Then, the utilization of BC in collaboration with biological tools which simplifies further dissipation of PAHs in the soil is described considering detailed interactions among BC, microbes, and plants in the soil matrix. The co-effect of BC and biological remediation has been authenticated by previous studies. Moreover, recent technologies and challenges related to the application of BC in soil remediation are explained. The implementation of a combined BC-biological remediation method would provide excellent prospects for PAHs-contaminated soils.

Changes in the pattern of polycyclic aromatic hydrocarbons in soil treated with biochar from a multiyear field experiment

The influence of biochar added to an agricultural soil on polycyclic aromatic hydrocarbon (PAH) levels, PAH diagnostic ratios and soil properties was investigated in a five-year field experiment. The experiment was carried out in an Italian vineyard and included two biochar treatments: 16.5 t ha^-1 of biochar applied in 2009 (soil B); 16.5 t ha^-1 in 2009 and further 16.5 t ha^-1 in 2010 (soil BB). A set of 75 samples that included five replicates and a control soil (untreated) was characterized in terms of organic carbon, pH, cation exchange capacity (CEC), bulk density and concentration of PAHs. Biochar addition to soil caused an increase in organic carbon, pH and CEC, and a decrease of bulk density. After almost two years the first application of biochar, PAH concentrations were higher in soil B (56 ng g^-1) and BB (153 ng g^-1) in comparison to control soil (24 ng g^-1). Thereafter, PAH concentrations decreased significantly, but the original PAHs levels were reached only in soil B after five years. The naphthalene/(naphthalene + phenanthrene) ratios were higher in the treated soils in accordance to the dominance of naphthalene in the original biochar. The cross plots naphthalene/(naphthalene + phenanthrene) vs. fluoranthene/(fluoranthene + pyrene) enabled to trace the signature of biochar PAHs up to five years after its first application. Diagnostic ratios can be a useful tool to study the persistence of PAHs introduced in soil by biochar when the pattern of these contaminants in biochar and original soil are different.

Reduced bioavailability and plant uptake of polycyclic aromatic hydrocarbons from soil slurry amended with biochars pyrolyzed under various temperatures

Biochar has high potential for organic pollutant immobilization due to its powerful sorption capacity. Nevertheless, potential risks may exist when biochar-sorbed organic pollutants are bioavailable. A direct plant exposure assay in combination with an organic solvent extraction experiment was carried out in this study to investigate the bioavailability of polycyclic aromatic hydrocarbons (PAHs) with the application of pine needle biochars pyrolyzed under different temperatures (100, 300, 400, and 700 °C; referred as P100-P700 accordingly). Biochar reduced solvent extractability and plant uptake of PAHs including naphthalene (Naph), acenaphthene (Acen), phenanthrene (Phen), and pyrene (Pyr), especially for three- and four-ring PAHs (Phen and Pyr) with high-temperature biochar. Plant uptake assay validates with organic solvent extraction for bioavailability assessment. Sorption of PAHs to biochars reduced plant uptake of PAHs in roots and shoots by lowering freely dissolved PAHs. Aging process reduced the bioavailability of PAHs that were bound to biochar. High pyrolysis temperature can be recommended for biochar preparation for purpose of effectively immobilizing PAHs, whereas application of moderate-temperature biochar for PAH immobilization should concern the potential risks of desorption and bioavailability of PAHs.

Phytoremediation: role of terrestrial plants and aquatic macrophytes in the remediation of radionuclides and heavy metal contaminated soil and water

Nuclear power reactors are operating in 31 countries around the world. Along with reactor operations, activities like mining, fuel fabrication, fuel reprocessing and military operations are the major contributors to the nuclear waste. The presence of a large number of fission products along with multiple oxidation state long-lived radionuclides such as neptunium ((237)Np), plutonium ((239)Pu), americium ((241/243)Am) and curium ((245)Cm) make the waste streams a potential radiological threat to the environment. Commonly high concentrations of cesium ((137)Cs) and strontium ((90)Sr) are found in a nuclear waste. These radionuclides are capable enough to produce potential health threat due to their long half-lives and effortless translocation into the human body. Besides the radionuclides, heavy metal contamination is also a serious issue. Heavy metals occur naturally in the earth crust and in low concentration, are also essential for the metabolism of living beings. Bioaccumulation of these heavy metals causes hazardous effects. These pollutants enter the human body directly via contaminated drinking water or through the food chain. This issue has drawn the attention of scientists throughout the world to device eco-friendly treatments to remediate the soil and water resources. Various physical and chemical treatments are being applied to clean the waste, but these techniques are quite expensive, complicated and comprise various side effects. One of the promising techniques, which has been pursued vigorously to overcome these demerits, is phytoremediation. The process is very effective, eco-friendly, easy and affordable. This technique utilizes the plants and its associated microbes to decontaminate the low and moderately contaminated sites efficiently. Many plant species are successfully used for remediation of contaminated soil and water systems. Remediation of these systems turns into a serious problem due to various anthropogenic activities that have significantly raised the amount of heavy metals and radionuclides in it. Also, these activities are continuously increasing the area of the contaminated sites. In this context, an attempt has been made to review different modes of the phytoremediation and various terrestrial and aquatic plants which are being used to remediate the heavy metals and radionuclide-contaminated soil and aquatic systems. Natural and synthetic enhancers, those hasten the process of metal adsorption/absorption by plants, are also discussed. The article includes 216 references.

Phytoremediation potential of Brassica juncea in Cu-pyrene co-contaminated soil: comparing freshly spiked soil with aged soil

A comparison was made between the dissipation of pyrene as well as the uptake of copper (Cu) in soil freshly spiked with Cu, pyrene or Cu + pyrene and in aged soil. The potential of B juncea for phytoremediation was also investigated. The biomass of Brassica juncea significantly decreased (>50% reduction) in freshly spiked soil when compared to aged soil in all treatments. However, the accumulation of Cu in shoot was significantly reduced (60-88%) in aged soil after 60 days of planting. The total removal of Cu from co-contaminated soil was always higher (>2-3 fold) in aged soil than in freshly spiked soil when lower Cu concentration (50 mg kg(-1)) was co-contaminated with 250 or 500 mg kg(-1) of pyrene while in other co-contaminated treatments, the total removal of Cu from aged soil were significantly lower. The level of pyrene in both planted and un-planted freshly spiked soil decreased significantly (>67%) over the 60 days of plant trial. In aged soils, there were no significant differences in residual pyrene concentration between planted and unplanted soil. This suggests that the presence of B. juncea in aged soil did not enhance the dissipation of pyrene and that the prediction of pyrene dissipation in laboratory prepared soil may not have reflected the true situation in the fields.

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.

The inoculation method affects colonization and performance of bacterial inoculant strains in the phytoremediation of soil contaminated with diesel oil

Plants in combination with microorganisms can remediate soils, which are contaminated with organic pollutants such as petroleum hydrocarbons. Inoculation of plants with degrading bacteria is one approach to improve remediation processes, but is often not successful due to the competition with resident microorganisms. It is therefore of high importance to address the persistence and colonization behavior of inoculant strains. The objective of this study was to determine whether the inoculation method (seed imbibement and soil inoculation) influences bacterial colonization, plant growth promotion and hydrocarbon degradation. Italian ryegrass was grown in non-sterilized soil polluted with diesel and inoculated with different alkane-degrading strains Pantoea sp. ITSI10, Pantoea sp. BTRH79 and Pseudomonas sp. MixRI75 individually as well as in combination. Inoculation generally had a beneficial effect on plant biomass production and hydrocarbon degradation, however, strains inoculated in soil performed better than applied by seed imbibement. Performance correlated with the colonization efficiency of the inoculated strains. The highest hydrocarbon degradation was observed in the treatment, in which all three strains were inoculated in combination into soil. Our study revealed that besides the degradation potential and competitive ability of inoculant strains the inoculation method plays an important role in determining the success of microbial inoculation.

Phenanthrene and pyrene uptake by arbuscular mycorrhizal maize and their dissipation in soil

Polycyclic aromatic hydrocarbons (PAHs) commonly found in soils can be degraded in rhizosphere, but may also be taken up by plants. The effects of arbuscular mycorrhizal (AM) fungi on uptake of phenanthrene (PHE) and pyrene (PYR) in maize and on their dissipation in soil were investigated using the three-compartmentalized rhizoboxes. Inoculation of Glomus mosseae significantly (p<0.01) increased PHE and PYR concentrations in maize roots and significantly (p<0.05) enhanced PYR translocation from roots to stems in the soil treatments of the PHE+PYR spiked-soils added into the central compartment of the rhizoboxes. There was a significant (p<0.05) dissipation gradient of PHE and PYR observed away from the maize roots, with the highest dissipation rates recorded in rhizosphere zone in the central compartments of the rhizoboxes, followed by near rhizosphere zone and bulk soil zone in the outer compartments. However, G. mosseae only exerted minimal impacts on dissipation of PHE and PYR in the rhizosphere. The present study suggested that the hyphae and extraradical mycelium of AM fungi could play important roles in the uptake and translocation of PHE and PYR in plants. The present results indicated that there is a potential for the use of AM fungi and plant for remediating PAHs contaminated soils.

Soil type affects plant colonization, activity and catabolic gene expression of inoculated bacterial strains during phytoremediation of diesel

The combined use of plants and associated microorganisms has great potential for cleaning up soils contaminated with petroleum hydrocarbons. Apart from environmental conditions the physicochemical properties of the soil are the main factors influencing the survival and activity of an inoculated strain as well as the growth of plants. This study examined the effect of different soil types (sandy, loamy sand and loam) on the survival, gene abundance and catabolic gene expression of two inoculated strains (Pseudomonas sp. strain ITRI53 and Pantoea sp. strain BTRH79) in the rhizosphere and shoot interior of Italian ryegrass vegetated in diesel contaminated soils. High colonization, gene abundance and expression in loamy soils were observed. By contrast, low colonization, gene abundance and absence of gene expression in sandy soil were found. The highest levels of genes expression and hydrocarbon degradation were seen in loamy soil that had been inoculated with BTRH79 and were significantly higher compared to those in other soils. A positive correlation was observed between gene expression and hydrocarbon degradation indicating that catabolic gene expression is necessary for contaminant degradation. These results suggest that soil type influences the bacterial colonization and microbial activities and subsequently the efficiency of contaminant degradation.

Enhancement of pyrene removed from contaminated soils by Bidens maximowicziana

The research utilized Bidens maximowicziana along with pyrene-degrading bacteria to evaluate their potential in cleaning up pyrene contamination. The removal of pyrene from the planted soil was obviously higher than that from the unplanted soils. After 50 d of B. maximowicziana growth, the average removal ratio of pyrene in planted soil was 79%, which was 28% higher than that of pyrene in unplanted soil. In contrast to other plants, both roots and shoots of B. maximowicziana could accumulate a large amount of pyrene from the soil and pyrene uptake increased with the soil pyrene concentration. Through analysis of pathways of pyrene removal, this enhanced removal of pyrene by plant-microbial association might be mainly the result of B. maximowicziana-promoted microbial degradation. Both the catalase and polyphenol oxidase activities in soil were higher in planted soil than those in unplanted soil. And the bacteria populations in soil, especially in rhizosphere, were also inspired by the growth of B. maximowicziana. These could be explained by the rhizosphere effect. Therefore, bio-removal of pyrene in the contaminated soils was feasible using B. maximowicziana.

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.