An overview of phytoremediation as a potentially promising technology for environmental pollution control

Phenol Removal Capacity of the Common Duckweed (Lemna minor L.) and Six Phenol-Resistant Bacterial Strains From Its Rhizosphere: In Vitro Evaluation at High Phenol Concentrations

The main topic of this study is the bioremediation potential of the common duckweed, Lemna minor L., and selected rhizospheric bacterial strains in removing phenol from aqueous environments at extremely high initial phenol concentrations. To that end, fluorescence microscopy, MIC tests, biofilm formation, the phenol removal test (4-AAP method), the Salkowski essay, and studies of multiplication rates of sterile and inoculated duckweed in MS medium with phenol (200, 500, 750, and 1000 mg L⁻¹) were conducted. Out of seven bacterial strains, six were identified as epiphytes or endophytes that efficiently removed phenol. The phenol removal experiment showed that the bacteria/duckweed system was more efficient during the first 24 h compared to the sterile duckweed control group. At the end of this experiment, almost 90% of the initial phenol concentration was removed by both groups, respectively. The bacteria stimulated the duckweed multiplication even at a high bacterial population density (>10⁵ CFU mL⁻¹) over a prolonged period of time (14 days). All bacterial strains were sensitive to all the applied antibiotics and formed biofilms in vitro. The dual bacteria/duckweed system, especially the one containing strain 43-Hafnia paralvei C32-106/3, Accession No. MF526939, had a number of characteristics that are advantageous in bioremediation, such as high phenol removal efficiency, biofilm formation, safety (antibiotic sensitivity), and stimulation of duckweed multiplication.

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.

Evaluation of the phytoremediation potential of dominant plant species growing in a chromium salt-producing factory wasteland, China

The metal contents of the soil and plant tissues in a large chromium salt-producing factory wasteland were determined to assess the properties of soil contamination and to identify plant species accumulating a range of heavy metals. Total metal contents in the factory soils presented a high heterogeneity, and the principal contaminants were Cd and Cr. All plant species examined were metal-tolerant, but to different extents. Especially, the maximum accumulation of Cd (15.61 mg kg^-1) and Cr (925.07 mg kg^-1) was found in Melia azedarach L. Subsequently, the Cd and Cr bioaccumulation and diverse physiological properties of M. azedarach seedlings exposed to different concentrations of Cd(II), Cr(VI), or Cd(II) + Cr(VI) in nutrient solutions were further investigated. All treated seedlings were able to survive under heavy metal stress, and the accumulation of both metals in plant tissues increased with elevation of metal exposure strength. M. azedarach showed a BCF greater than 147.56 for Cd and 36.76 for Cr. Meanwhile, the TF was lower than 0.25 for Cd and 0.32 for Cr. The highest bioaccumulation in root tissues was 2708.03 mg kg^-1 Cd and 824.65 mg kg^-1 Cr for seedlings cultured with 20 mg L^-1 Cd(II) or 20 mg L^-1 Cr(VI). Cd and Cr increased each other's uptake in seedlings although a reduced accumulation in roots occurred when exposed to the highest concentration of Cd(II) + Cr(VI) treatment (20 mg L^-1). At either level of concentration, the degree of plant growth inhibition and oxidative damage caused by heavy metals was Cd(II) + Cr(VI) > Cr(VI) > Cd(II). Superoxide dismutase and peroxidase exhibited positive and effective responses to low-Cd(II) or Cr(VI) concentration stress, but their activities decreased with increasing metal exposure strength. The behavior of the non-enzymatic antioxidants (GSH, soluble protein, and proline) in plant involved in the detoxification of ROS induced by metal exposure was correlated well with higher Cd and Cr accumulations. Here, the potentiality of M. azedarach with the capacity to accumulate and stabilize Cd/Cr in metal-contaminated soil by phytoremediation process has been explored.

SaHsfA4c From Sedum alfredii Hance Enhances Cadmium Tolerance by Regulating ROS-Scavenger Activities and Heat Shock Proteins Expression

The heat shock transcription factor (Hsf) family, an important member in plant stress response, affects cadmium (Cd) tolerance in plants. In this study, we identified and functionally characterized a transcript of the Hsf A4 subgroup from Sedum alfredii. Designated as SaHsfA4c, the open reading frame was 1,302 bp long and encoded a putative protein of 433 amino acids containing a complete DNA-binding domain (DBD). Heterologous expression of SaHsfA4c in yeast enhanced Cd stress tolerance and accumulation, whereas expression of the alternatively spliced transcript InSaHsfA4c which contained an intron and harbored an incomplete DBD, resulted in relatively poor Cd stress tolerance and low Cd accumulation in transgenic yeast. The function of SaHsfA4c under Cd stress was characterized in transgenic Arabidopsis and non-hyperaccumulation ecotype S. alfredii. SaHsfA4c was able to rescue the Cd sensitivity of the Arabidopsis athsfa4c mutant. SaHsfA4c reduced reactive oxygen species (ROS) accumulation and increased the expression of ROS-scavenging enzyme genes and Hsps in transgenic lines. The present results suggest that SaHsfA4c increases plant resistance to stress by up-regulating the activities of ROS-scavenging enzyme and the expression of Hsps.

Ethanol-blended petroleum fuels: implications of co-solvency for phytotechnologies

In recent decades, there has been increasing interest in the use of ethanol-blended fuels as alternatives to unblended fossil fuels. These initiatives are targeted at combating CO2 and particulate matter emissions, as these oxygenates leave behind a lesser carbon footprint. Noble as it may appear, this innovation is not without attendant ugly consequences. One major implication is the effect of co-solvency on the applicability of various forms of phytotechnologies for contaminant removal. By means of gas chromatography-mass spectrometry, this research investigated the effect of diesel fuel ethanol addition on the leaching potentials of petroleum hydrocarbons. Since phytoremediation of hydrocarbons depends largely on rhizodegradation of contaminants by the root-associated microbiome, the leaching of petroleum hydrocarbons beyond the rooting zones of plants may limit the effectiveness of this process as a reclamation strategy for ethanol-blended fuel spills. The analyses presented in this paper highlight the need for energy scientists to carefully consider the environmental impacts of ethanol-blended innovations holistically.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

Unravelling the role of vegetation in the attenuation of contaminants of emerging concern from wetland systems: Preliminary results from column studies

Water pollution with contaminants of emerging concern (CECs) is widespread in water bodies due to the low effectiveness of industrial and urban wastewater treatment systems. In recent decades, the implementation of vegetation-based wastewater treatment systems such as wetlands has been observed to help solve this issue. However, there is a lack of knowledge regarding the removal percentage attributable to plants and how plants affect this removal improvement. In this study, we monitored planted (Phragmites australis) and unplanted sand columns to assess the effect of vegetation on the attenuation of 5 well-known CECs (benzotriazole, sulfamethoxazole, carbamazepine, bisphenol A, and diclofenac) and link it to the presence of different root exudates. The columns were operated in a continuous feeding mode for more than 6 months at 3 hydraulic loading rates (HLRs) (70, 140, and 280 mm d^-1). We found that the presence of vegetation increased CEC attenuation from no effect to more than 200% compared to the unplanted columns. The highest effect was observed for carbamazepine (94-200%), followed by diclofenac (22-171%), benzotriazole (48-127%), and sulfamethoxazole (no effect to 43%), depending on the tested HLR. Furthermore, the greater CEC attenuation in planted columns was linked to the release of certain root exudates that may shape the root microbiome. We expect our assay to be a starting point for exploring the role of root exudates in enhancing CEC removal efficiency in wastewater.

Enhancement of phytoextraction by Taiwanese chenopod and Napier grass by soapnut saponin and EDDS additions

Employment of biosurfactants and biodegradable chelants could further promote sustainability of soil and groundwater remediation tasks. Biosurfactant (soapnut saponin) and biodegrading chelants (ethylenediamine-N,N'-disuccinic acid (EDDS)) were employed to enhance the phytoextraction by native Taiwanese chenopod (Chenopodium formosanum Koidz.), Napier grass (Pennisetum purpureum) cultivar Taishi No. 4, and soapwort (Saponaria officinalis). Ethylene diamine tetraacetic acid (EDTA) was also employed as the control. Contaminated soils as silty clay loam texture was collected from a defunct rice paddy, containing chromium (Cr), cadium (Cd), and copper (Cu). Addition of both soapnut saponin and EDDS proportionally increased bioaccumulation factors (BCFs) of aboveground biomass for all three plants. Taiwanese chenopod demonstrated the best BCF values among three plants, with BCF increased from 0.76 to 2.6 and 1.3 for Cu under the presence of the highest dosages of EDDS and saponin. Plant aboveground biomass did exhibit negative correlation toward biomass metal concentrations. Presence of saponin did exhibit the least negative slopes among the correlations of all three additives for three plants. Taiwanese chenopod did exhibit the least negative slopes among the correlations of all three additives for three plants. Above observations suggested that saponin may have some protection for plants, especially for Napier grass. Taiwanese chenopod could possess more tolerance toward heavy metals than Napier grass does.

Temporal and spatial variations of macrofouling organisms on ecological floating beds in Yundang Lagoon, China

Spatial-temporal variations of macrofouling organisms that attach to ecological floating beds (EFBs) in the Yundang Lagoon were investigated to identify factors that influence the appearance of macrofouling organisms. Results show that the composition, abundance, biomass, and dominance of macrofouling organisms on EFBs exhibited significant seasonal variation. Pearson correlation analysis indicates that the abundance and biomass of the bivalve Mytilopsis sallei showed negative correlation with root biomass (p < 0.05) and particulate matter (p < 0.05). Environmental (temperature and salinity, p < 0.05) and biological (bottom-up control) factors are the main drivers of population turnover. There were significant species differences of macrofouling organisms within the different parts of the lagoon, which were attributed to environmental characteristics such as hydrodynamics, dissolved oxygen, and the degree of eutrophication. Results of this study provide a basis for controlling macrofouling organisms, while improving the stability of EFBs and the efficiency of ecological restoration.

Phytoremediation potential and control of Phragmites australis as a green phytomass: an overview

Phragmites australis (common reed) is one of the most extensively distributed emergent plant species in the world. This plant has been used for phytoremediation of different types of wastewater, soil, and sediments since the 1970s. Published research confirms that P. australis is a great accumulator for different types of nutrients and heavy metals than other aquatic plants. Therefore, a comprehensive review is needed to have a better understanding of the suitability of this plant for removal of different types of nutrients and heavy metals. This review investigates the existing literature on the removal of nutrients and heavy metals from wastewater, soil, and sediment using P. australis. In addition, after phytoremediation, P. australis has the potential to be used for additional benefits such as the production of bioenergy and animal feedstock due to its specific characteristics. Determination of adaptive strategies is vital to reduce the invasive growth of P. australis in the environment and its economic effects. Future research is suggested to better understand the plant's physiology and biochemistry for increasing its pollutant removal efficiency.

Pilot-scale demonstration of phytoremediation of PAH-contaminated sediments by Hydrilla verticillata and Vallisneria spiralis

The results of phytoremediation of sediments contaminated with polycyclic aromatic hydrocarbons (PAHs) by two submerged aquatic plants (Vallisneria spiralis and Hydrilla verticillata) at pilot scale were reported for the first time in this study. During a 108-day period, the plants grew well, and more PAHs were dissipated in planted sediments than in unplanted sediments. At the end, dissipation ratios of phenanthrene and pyrene were 85.9% and 79.1% in sediments planted with V. spiralis, 76.3% and 64.6% in sediments planted with H. verticillata, but only 64.8% and 55.8% in unplanted sediments. V. spiralis exhibited higher phytoremediation ability, which was significantly related to its root oxygenation as indicated by the redox potential in sediments. The remediation results at pilot scale were also compared with those previously obtained in our laboratory. The ratio of root weight to sediment weight showed a similar trend to PAH dissipation enhancement. Bioconcentration factors of PAHs in the two plants were larger in the pilot experiment than in the laboratory tests as a result of quicker increase of plant weight in the pilot experiment.

FC. Soil Bioremediation Techniques

Soil pollution is rising rapidly due to the existence of pollutants or natural alterations in the soil. It makes the drinking water ineffective and unusable by the human beings. The major cause of the soil contamination is agricultural activities, industrial activities, and inadmissible disposal of waste in the soil. The most common pollutants to accumulate in the soil are petroleum hydrocarbons, solvents, pesticides, lead, and other heavy metals. The important technology to remediate the pollutants or contaminants in the soil is bioremediation. The utilization of bioremediation in the contaminated soil is increasing rapidly due to the presence of toxic pollutants. It is the most advanced technologies which make use of organisms to deteriorate the harmful compounds in order to prevent the soil pollution. The aim of the chapter is to describe the available bioremediation technologies and their application in removing the pollutants exist in the soil.

Remediation of soil polluted with HMW-PAHs by alfalfa or brome in combination with fungi and starch

High-molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) are common pollutants in soil of coal mining areas that affect the safety of crops and the environment. In a pot experiment, we compared the remediation potential of alfalfa (Medicago sativa Linn) and brome (Bromus inermis Leyss.) either alone or in combination with starch or Fusarium sp. strain ZH-H2 for a farmland soil contaminated with 4-6-ring PAHs from a coal mine area. The alfalfa and brome alone treatments reduced the concentrations of most HMW-PAHs. However, when starch was added, the removal rates of indeno(1,2,3-cd)pyrene and benzo(ghi)perylene were significantly higher for brome than for alfalfa. When ZH-H2 was combined with brome, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and benzo(ghi)perylene degradation rates were significantly enhanced compared with brome alone. In contrast, an antagonistic effect was observed between alfalfa and Fusarium. The brome, starch and ZH-H2 combination resulted in far better removal rates than the alfalfa combination. Maximum removal rates were obtained with the brome + starch + ZH-H2 combination for benzo(k)fluoranthene (42.64%), benzo(a)pyrene (51.01%), indeno(1,2,3-cd)pyrene (62.29%), and benzo(ghi)perylene (74.85%). These removal rates were 829.78%, 182.34%, 46.13%, and 70.94% higher than the equivalent alfalfa combination treatments. The lignin peroxidase activity was significantly increased in the presence of starch, ZH-H2 and brome, consistent with the increased removal rates of HMW-PAHs.

Potential of four aquatic plant species to remove 60Co from contaminated water under changing experimental conditions

This study aimed to compare the potential of Lemna minor, Spirodela sp., Eichhornia crassipes and Pistia stratiotes to remove ⁶⁰Co from a realistic aquatic environment. Although all four plant species performed similarly well after 3 days of exposure to 50 kBq L^{-1 60}Co, Lemna minor and Spirodela sp. came forward as having higher ⁶⁰Co removal potential. This conclusion is, in first instance, based on the high ⁶⁰Co removal percentage obtained after a short contact time (e.g. more than 95% could be removed after 6 h by Spirodela sp.). Additionally, Lemna minor and Spirodela sp. accumulated a high amount of ⁶⁰Co per gram of biomass. For example, Lemna minor accumulated over three times more ⁶⁰Co per gram of biomass compared to Pistia stratiotes and Eichhornia crassipes. Both plants also performed well in the pH range 5-9. We used Lemna minor to test the influence of the initial ⁶⁰Co concentration (10, 50, 100 and 200 kBq L^{-1 60}Co) on its phytoremediation capacity but no differences could be observed in removal percentage. In addition, it was shown that by optimising the initial amount of biomass, radioactive waste production can be minimised whilst maintaining high ⁶⁰Co removal rates. Our study shows that these aquatic plants can be used for phytoremediation of ⁶⁰Co from contaminated water and can be considered as a "green" addition or alternative for conventional remediation techniques.

Potential of higher plants, algae, and cyanobacteria for remediation of radioactively contaminated waters

The potential of photosynthetic organisms to remediate radioactively contaminated water was evaluated for scenarios related to nuclear installations and included the following radionuclides: ¹³⁷Cs, ¹³⁴Cs, ¹³⁶Cs, ⁹⁰Sr, ¹³¹I, ²³⁹Pu, ²⁴¹Am, ¹³²Te/¹³²I, ⁵⁸Co, ⁶⁰Co, ⁵¹Cr, ^110mAg, ⁵⁴Mn, ¹²⁴Sb, ⁵⁹Fe, ⁶⁵Zn, ⁹⁵Zr, and ⁹⁵Nb. An extensive literature review was undertaken leading to the creation of a database including more than 20,000 entries from over 100 references in which terrestrial and aquatic plants, macro- and microalgae, cyanobacteria and biosorbents derived from these organisms were used to clean water from these specific radionuclides or their stable isotopes. In a first phase, the remediation potential of the organisms and biosorbents was evaluated for the individual elements based on parameters such as plant uptake, removal percentage, and bioconcentration factor, and for two radionuclide mixtures based on the ability of the organisms/biosorbents to work under mixture conditions. As the experimental and environmental conditions will influence the performance of the organisms and biosorbents, a literature-based evaluation of the most influencing or restricting parameters was made and water pH, competing ions, and the chemical modification of biosorbents showed to be of major importance. Finally, the most promising organisms and biosorbents were identified using a specifically developed selection procedure taking into account their performance and robustness. Ranking was done based on clear criteria with a distinct weight and scoring scheme. As such, 20 organisms/biosorbents were identified that showed high potential to clean waters contaminated with (mixtures of) radionuclides related to nuclear installations and which can be used for further experimental investigations.

Tea saponins: effective natural surfactants beneficial for soil remediation, from preparation to application

Tea saponins, found in Camellia plants, are natural non-ionic surfactants that offer obvious beneficial effects in soil remediation. Most tea saponins are extracted from the Camellia oleifera seed meal, with the leaves and flowers of Camellia sinensis as potential sources. Water extraction and ultrasound-assisted water extraction combined with acetone precipitation are recommended for the industrial extraction and purification of tea saponins, considering multiple factors. The detailed physical, chemical and biochemical properties of tea saponins need to be clarified, especially whether tea saponins with slightly different structures from distinct sources have different soil remediation properties. Applied in leaching remediation, phytoremediation and microbial remediation, tea saponins desorb heavy metals from contaminated soil as well as enhancing their bioavailability. Tea saponins improve the accumulation of pollutants by hyperaccumulators as well as the degradation of organic pollutants by microorganisms. Currently the mechanisms of tea saponins are not clear, although they are proven to be effective natural surfactants for the remediation of contaminated soils. This review enriches our understanding of tea saponins from various aspects and encourages further studies of industrial extraction and purification, and the field remediation mechanisms of tea saponins, making better use of Camellia plants and contributing to environmental protection.

Two mulberry phytochelatin synthase genes confer zinc/cadmium tolerance and accumulation in transgenic Arabidopsis and tobacco

Phytochelatin synthase (PCS) is an enzyme involved in the synthesis of phytochelatins, cysteine-rich peptides which play a key role in heavy metal (HM) detoxification of plants. Mulberry (Morus L.), one of the most ecologically and economically important tree genera, has the potential to remediate HM-contaminated soils. However, genes involved in HM detoxification in Morus, such as the PCS genes, have not been identified and characterized. In this study, we identified two Morus notabilis PCS genes based on a genome-wide analysis of the Morus genome database. Full-length MnPCS1 and MnPCS2 cDNAs were 1509 and 1491bp long, respectively. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that, under 200μM Zn2+ or either 30 or 100μM Cd2+ stress, the relative expression of each of the two MaPCSs (from Morus alba) was induced in root, stem and leaf tissues within 24h of exposure to the metals, with Cd2+ inducing expression more strongly than did Zn2+. Based on the analysis of total root length and fresh weight of seedlings, overexpression of MnPCS1 and MnPCS2 in Arabidopsis and tobacco enhanced Zn2+/Cd2+ tolerance in most transgenic individuals. The results of transgenic Arabidopsis lines overexpressing MnPCS1and MnPCS2 suggest that MnPCS1 play a more important role in Cd detoxification than MnPCS2. Zn2+/Cd2+ concentrations in both shoots and roots of the transgenic Arabidopsis seedlings were higher than in wild type (WT) seedlings at two Zn2+/Cd2+ concentrations. In addition, there was a positive correlation between Zn accumulation and the expression level of MnPCS1 or MnPCS2. Our results indicated that the Morus PCS1 and PCS2 genes play important roles in HM stress tolerance and accumulation, providing a useful genetic resource for enhancing tolerance to HMs and for increasing the HM phytoremediation potential of these plants.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

Bioethanol production from recovered napier grass with heavy metals

Using plants to absorb and accumulate heavy metals from polluted soil, followed by the recycling of explants containing heavy metals, can help achieve the goal of reverting contaminated soil to low heavy-metal content soil. However, the re-use of recovered explants can also be problematic. Meanwhile, bioethanol has become a popular energy source. In this study, napier grass was used for the remediation of soil contaminated with heavy metals (artificially contaminated soil). The influence of bioethanol production from napier grass after phytoremediation was also investigated. The concentration of Zn, Cd, and Cr in the contaminated soil was 1000, 100, and 250 mg/kg, respectively. After napier grass phytoremediation, the concentration (dry biomass) of Zn, Cd, and Cr in the explants was 2701.97 ± 173.49, 6.1 ± 2.3, and 74.24 ± 1.42 mg/kg, respectively. Biomass production in the unpolluted soil was 861.13 ± 4.23 g. The biomass production ratio in high Zn-polluted soil was only 3.89%, while it was 4.68% for Cd and 21.4% for Cr. The biomass obtained after napier grass phytoremediation was pretreated using the steam explosion conditions of 180 °C, for 10 min, with 1.5% H₂SO₂, followed by enzymatic hydrolysis. The efficiency of enzymatic hydrolysis for Zn-polluted biomass was 90% of the unpolluted biomass, while it was 77% for Cd, and approximately the same for Cr. The fermentation efficiency of the heavy-metal-containing biomass was higher than the control biomass. The fermentation ethanol concentration obtained was 8.69-12.68, 13.03-15.50, and 18.48-19.31 g/L in Zn, Cd, and Cr environments, respectively. Results show that the heavy metals had a positive effect on bacteria fermentation. However, the fermentation efficiency was lower for biomass with severe heavy metal pollution. Thus, the utilization of napier grass phytoremediation for bioethanol production has a positive effect on the sustainability of environmental resources.

Opinion: Taking phytoremediation from proven technology to accepted practice

Phytoremediation is the use of plants to extract, immobilize, contain and/or degrade contaminants from soil, water or air. It can be an effective strategy for on site and/or in situ removal of various contaminants from soils, including petroleum hydrocarbons (PHC), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), solvents (e.g., trichloroethylene [TCE]), munitions waste (e.g., 2,4,6-trinitrotoluene [TNT]), metal(loid)s, salt (NaCl) and radioisotopes. Commercial phytoremediation technologies appear to be underutilized globally. The primary objective of this opinion piece is to discuss how to take phytoremediation from a proven technology to an accepted practice. An overview of phytoremediation of soil is provided, with the focus on field applications, to provide a frame of reference for the subsequent discussion on better utilization of phytoremediation. We consider reasons why phytoremediation is underutilized, despite clear evidence that, under many conditions, it can be applied quite successfully in the field. We offer suggestions on how to gain greater acceptance for phytoremediation by industry and government. A new paradigm of phytomanagement, with a specific focus on using phytoremediation as a "gentle remediation option" (GRO) within a broader, long-term management strategy, is also discussed.

Bioremediation techniques-classification based on site of application: principles, advantages, limitations and prospects

Environmental pollution has been on the rise in the past few decades owing to increased human activities on energy reservoirs, unsafe agricultural practices and rapid industrialization. Amongst the pollutants that are of environmental and public health concerns due to their toxicities are: heavy metals, nuclear wastes, pesticides, green house gases, and hydrocarbons. Remediation of polluted sites using microbial process (bioremediation) has proven effective and reliable due to its eco-friendly features. Bioremediation can either be carried out ex situ or in situ, depending on several factors, which include but not limited to cost, site characteristics, type and concentration of pollutants. Generally, ex situ techniques apparently are more expensive compared to in situ techniques as a result of additional cost attributable to excavation. However, cost of on-site installation of equipment, and inability to effectively visualize and control the subsurface of polluted sites are of major concerns when carrying out in situ bioremediation. Therefore, choosing appropriate bioremediation technique, which will effectively reduce pollutant concentrations to an innocuous state, is crucial for a successful bioremediation project. Furthermore, the two major approaches to enhance bioremediation are biostimulation and bioaugmentation provided that environmental factors, which determine the success of bioremediation, are maintained at optimal range. This review provides more insight into the two major bioremediation techniques, their principles, advantages, limitations and prospects.

Remediation of radiocesium-contaminated liquid waste, soil, and ash: a mini review since the Fukushima Daiichi Nuclear Power Plant accident

The radiation contamination after the Fukushima Daiichi Nuclear Power Plant accident attracts considerable concern all over the world. Many countries, areas, and oceans are greatly affected by the emergency situation other than Japan. An effective remediation strategy is in a highly urgent demand. Though plenty of works have been carried out, progressive achievements have not yet been well summarized. Here, we review the recent advances on the remediation of radiocesium-contaminated liquid waste, soil, and ash. The overview of the radiation contamination is firstly given. Afterwards, the current remediation strategies are critically reviewed in terms of the environmental medium. Special attentions are paid on the adsorption/ion exchange and electrically switched ion exchange methods. Finally, the present review outlines the possible works to do for the large-scale application of the novel remediation strategies.

Can the use of natural biostimulants be a potential means of phytoremediating contaminated soils from goldmines in South Africa?

Biostimulants offer great potential in improving phytoremediation of contaminated soils. In the current greenhouse-based study, Brassica juncea seedlings grown on soils collected from Krugersdorp Goldmine and the adjourning areas (a Game Reserve and private farmland) were supplemented with different biostimulants (Kelpak® = KEL, vermicompost leachate = VCL, smoke-water = SW). Indole-3-butyric acid (IBA) was included in the study for comparative purposes because these biostimulants are known to enhance rooting. Prior to the pot trial, concentrations of elements in the three soil types were determined using Inductively Coupled Plasma-Optical Emission Spectroscopy. Plants were harvested after 105 days and the growth and concentrations of elements in the various plant organs were determined. TheB. juncea seedlings with and without biostimulants did not survive when growing in soil from the Krugersdorp Goldmine. The Game Reserve and private farmland soils supplemented with KEL produced the highest plant biomass and the lowest accumulation of metals in the organs of B. juncea. High concentrations (>13 000 mg kg(-1)) of zinc and aluminium were quantified in the roots of IBA-supplemented soils from the Game Reserve. Generally, IBA and SW enhanced the phytoremediation of B. juncea due to elevated levels of elements that accumulated in their different organs.

Relative importance of plant uptake and plant associated denitrification for removal of nitrogen from mine drainage in sub-arctic wetlands

Reactive nitrogen (N) species released from undetonated ammonium-nitrate based explosives used in mining or other blasting operations are an emerging environmental problem. Wetlands are frequently used to treat N-contaminated water in temperate climate, but knowledge on plant-microbial interactions and treatment potential in sub-arctic wetlands is limited. Here, we compare the relative importance of plant uptake and denitrification among five plant species commonly occurring in sub-arctic wetlands for removal of N in nitrate-rich mine drainage in northern Sweden. Nitrogen uptake and plant associated potential denitrification activity and genetic potential for denitrification based on quantitative PCR of the denitrification genes nirS, nirK, nosZI and nosZII were determined in plants growing both in situ and cultivated in a growth chamber. The growth chamber and in situ studies generated similar results, suggesting high relevance and applicability of results from growth chamber experiments. We identified denitrification as the dominating pathway for N-removal and abundances of denitrification genes were strong indicators of plant associated denitrification activity. The magnitude and direction of the effect differed among the plant species, with the aquatic moss Drepanocladus fluitans showing exceptionally high ratios between denitrification and uptake rates, compared to the other species. However, to acquire realistic estimates of N-removal potential of specific wetlands and their associated plant species, the total plant biomass needs to be considered. The species-specific plant N-uptake and abundance of denitrification genes on the root or plant surfaces were affected by the presence of other plant species, which show that both multi- and inter-trophic interactions are occurring. Future studies on N-removal potential of wetland plant species should consider how to best exploit these interactions in sub-arctic wetlands.

Distribution and chemical forms of copper in the root cells of castor seedlings and their tolerance to copper phytotoxicity in hydroponic culture

The subcellular localization and chemical forms of copper in castor (Ricinus communis L.) seedlings grown in hydroponic nutrient solution were identified by chemical extraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. The wild castor seeds were harvested from an abandoned copper mine in Tonglu Mountain, Daye City of Hubei Province, China. The results revealed that (1) the seedlings grew naturally in MS liquid medium with 40.00 mg kg(-1) CuSO4, in which the seedling growth rate and biomass index were 0.14 and 1.23, respectively, which were the highest values among all the treatments. The copper content in castor seedlings increased along with elevated CuSO4 concentration in the medium, reaching a maximum value of 16 570.12 mg kg(-1)(DW) when exposed to 60.00 mg L(-1) CuSO4, where 91.31% of the copper was accumulated in roots. (2) The copper existed in various chemical forms in the roots of the castor seedlings. Copper of 67.66% was extracted from the components of cell walls, such as exchangeable acidic polar compounds, cellulose and lignin, protein and pectin, and less concentrated in cell cytoplasm and nuclei. (3) Furthermore, the root cell walls were thickened when the castor seedlings exposed to CuSO4, with a large amount of high-density electron bodies, attached to the thickened cell walls. In the cell walls, most copper was bound to the carboxyl (-COOH) and hydroxyl (-OH) groups of acidic polar compounds, cellulose, hemicellulose, and polysaccharides. The conclusion showed that castor exhibited a strong tolerance to copper, the copper were accumulated mainly in the root cell, the root cell walls of castor were the major location of patience and detoxification in copper stress.

Effects of Potamogeton crispus L.-bacteria interactions on the removal of phthalate acid esters from surface water

To investigate the mechanism of submerged macrophyte-bacteria interactions on the removal of phthalic acid esters from surface water, experiments with and without Potamogeton crispus L. were performed. A two-compartment (i.e., water and plant) kinetic model was developed. The model adequately described the variation of dibutyl phthalate (DBP) and di-2-ethylhexyl phthalate (DEHP) in the plant-water system by providing the first-order rate constants of plant uptake (k1) and release (k2), microbial degradation in water (k3) and plant degradation (k4). During 10-d incubation, the presence of P. crispus enhanced the removal of DBP and DEHP from water by 6.3% and 22.4%. Compared with the experiment without P. crispus, biodegradation of DBP in water with P. crispus decreased by 8.3% because of plant uptake even though k3 increased by 30%. 21.4% of DBP transferred from water to plants, of which only small amount (5.1%) retained in the plant and the rest (94.9%) was degraded. Different from DBP, biodegradation of DEHP in water with P. crispus was a slightly higher than that without P. crispus. 25.5% of DEHP transferred from water to plants, of which a large portion (73.3%) retained in the plant and the rest (26.7%) was degraded. This finding reveals that the enhancement of DBP removal from surface water is mainly related to faster degradation in the plant, whereas it is mainly related to higher plant accumulation for DEHP.

Developing and validating a practical decision support tool (DST) for biomass selection on marginal land

Marginal, often contaminated, sites exist in large areas across the world as a result of historic activities such as industry, transportation and mineral extraction. Remediation, or other improvements, of these sites is typically only considered for sites with high exploitation pressure and those posing the highest risks to human health or the environment. At the same time there is increasing competition for land resources for different needs such as biofuel production. Potentially some of this land requirement could be met by production of biomass on brownfield or other marginal land, thereby improving the land while applying the crop cultivation as part of an integrated management strategy. The design and decision making for such a strategy will be site specific. A decision support framework, the Rejuvenate DST (decision support tool) has been developed with the aim of supporting such site specific decision making. This tool is presented here, and has been tested by applying it to a number of case study sites. The consequent SWOT (strength, weakness, opportunities and threats) analysis is discussed and evaluated. The DST was found to be systematic, transparent, and applicable for diverse sites in France, Romania and Sweden, in addition to the sites to which it was applied through its development. The DST is regarded as especially useful if applied as a checklist in an iterative way throughout the decision process, from identifying potential crops to identifying knowledge gaps, working/non-working management strategies and potential risks. The DST also provides a structure promoting effective stakeholder engagement.