Phytoremediation of Organic Contaminants: A Review of Phytoremediation Research at the University of Washington

Crotalaria juncea L. enhances the bioremediation of sulfentrazone-contaminated soil and promotes changes in the soil bacterial community

Sulfentrazone (STZ) is an efficient tool for the pre- and post-emergence control of monocotyledonous and dicotyledonous weeds in fields of crops such as pineapple, coffee, sugarcane, citrus, eucalyptus, tobacco, and soybean. However, this herbicide persists in the soil, causing phytotoxicity in the subsequent crop. Therefore, it is important to use efficient strategies for the remediation of STZ-contaminated areas. The aim of this study was to evaluate the effects of Crotalaria juncea L. on the remediation of STZ-contaminated soil and on the microbial activity and bacterial community structure therein. The study was conducted in three stages: (i) cultivation of C. juncea in soil contaminated with 200, 400, and 800 g ha-1 STZ; (ii) determination of the soil microbial activity (basal respiration, microbial biomass carbon, and bacterial community structure); and (iii) cultivation of a bioindicator species and determination of the residual fraction of STZ. The soil microbial activity was impacted by the soil type and STZ dose. Soil previously cultivated with C. juncea (rhizospheric soil) displayed higher CO2 and lower qCO2 values than non-rhizospheric soil (no previous C. juncea cultivation). Increasing doses of STZ reduced the activity and lowered the diversity indices of the soil microorganisms. The bacterial community structure was segregated between the rhizospheric and non-rhizospheric soils. Regardless of soil type, the bioindicator of remediation (Pennisetum glaucum R.Br.) grew only at the STZ dose of 200 g ha-1, and the plant intoxication level was also lower in rhizospheric soil treated with this herbicide dose. All P. glaucum plants died in the soils treated with 400 and 800 g ha-1 STZ. Previous cultivation of C. juncea in soils contaminated with 200, 400, and 800 g ha-1 STZ reduced the residual fraction of the herbicide by 4.8%, 12.5%, and 17.4%, respectively, compared with that in the non-rhizospheric soils. In conclusion, previous cultivation with C. juncea promoted increases in the soil bacterial activity and diversity indices, mitigated the deleterious effects of STZ on the bioindicator crop, and reduced the residual fraction of the herbicide in the soil.

Contemporary enzyme based technologies for bioremediation: A review

The persistent disposal of xenobiotic compounds like insecticides, pesticides, fertilizers, plastics and other hydrocarbon containing substances is the major source of environmental pollution which needs to be eliminated. Many contemporary remediation methods such as physical, chemical and biological are currently being used, but they are not sufficient to clean the environment. The enzyme based bioremediation is an easy, quick, eco-friendly and socially acceptable approach used for the bioremediation of these recalcitrant xenobiotic compounds from the natural environment. Several microbial enzymes with bioremediation capability have been isolated and characterized from different natural sources, but less production of such enzymes is a limiting their further exploitation. The genetic engineering approach has the potential to get large amount of recombinant enzymes. Along with this, enzyme immobilization techniques can boost the half-life, stability and activity of enzymes at a significant level. Recently, nanozymes may offer the potential bioremediation ability towards a broad range of pollutants. In the present review, we have described a brief overview of the microbial enzymes, different enzymes techniques (genetic engineering and immobilization of enzymes) and nanozymes involved in bioremediation of toxic, carcinogenic and hazardous environmental pollutants.

Use of Zea mays L. in phytoremediation of trichloroethylene

Trichloroethylene (TCE) is a chlorinated aliphatic organic compound often detected as pollutant in soils and ground water. "Green technologies" based on phytoremediation were proven to be effective to reclaim organic pollutants (e.g. TCE) and heavy metals from different environmental matrices. In this work, we use Zea mays L. for the removal of high TCE concentrations from medium cultures. In particular, we investigated a sealed bioreactor where the growth medium was contaminated with an increasing amount of TCE, in the range 55-280 mg/L; the removal capability of the maize plants was assessed by means of GC-MS and LC-MS analyses. An accurate mass balance of the system revealed that the plants were able to remove and metabolise TCE with an efficiency up to 20 %, depending on the total amount of TCE delivered in the bioreactor. Morphometric data showed that the growth of Z. mays is not significantly affected by the presence of the pollutant up to a concentration of 280 mg/L, while plants show significant alterations at higher TCE concentrations until the growth is completely inhibited for [TCE] ≃ 2000 mg/L. Finally, the presence of several TCE metabolites, including dichloroacetic and trichloroacetic acids, was detected in the roots and in the aerial part of the plants, revealing that Z. mays follows the green liver metabolic model. These results encourage further studies for the employment of this plant species in phytoremediation processes of soils and waters contaminated by TCE and, potentially, by many other chlorinated solvents.

The fate of the herbicide propanil in plants of the littoral zone of the Three Gorges Reservoir (TGR), China

The anti-seasonal hydrology with 30m water fluctuations in the Three Gorges Reservoir (TGR) of China attracts growing environmental and ecological concerns. We investigated the biotransformation of the herbicide propanil in plants dominating in the littoral zone of the TGR by applying the ¹⁴C-ring-labeled herbicide into non-aseptic hydroponic plant systems (Cynodon dactylon, Nelumbo nucifera and Bidens pilosa), aseptic plants (Lemna minor and Lemna gibba) and cell suspension cultures (C. dactylon and L. minor). (1) Propanil absorbed in plants of the hydroponic systems was (12.46±1.63)% of applied radioactivity (AR) (C. dactylon), (52.36±6.38)% (N. nucifera) and (76.55±6.07)% (B. pilosa), respectively. The ¹⁴C-residues in the plant extractable fractions and the corresponding media were confirmed by radio-Thin Layer Chromatography (TLC), radio-High Performance Liquid Chromatography (HPLC) and Gas Chromatography-Electron Ionization Mass Spectrometry (GC-EIMS) as propanil, 3,4-dichloroaniline (DCA) and N-(3,4-dichlorophenyl)-β-d-glucopyranosylamine (Glu-DCA). (2) About 8% of AR was taken up by both aseptic plants, from which 7.0% of AR was extracted and identified also as propanil, DCA and Glu-DCA. (3) Concerning cell suspension cultures, (39.22±9.39)% of AR was absorbed by C. dactylon after 72hr, whereas the accumulated ¹⁴C-propanil by L. minor cell suspension culture amounted to (65.04±1.72)% after 7days. The identified compounds in cell cultures are consistent with those in the tested plants. Most of the pesticide residues in the intact plants were un-extractable, which are recognized as the end of the detoxification process. We therefore consider these plants as suitable for the phytoremediation of the herbicide propanil in the TGR region.

The validation of an analytical method for sulfentrazone residue determination in soil using liquid chromatography and a comparison of chromatographic sensitivity to millet as a bioindicator species

Commonly used herbicides, such as sulfentrazone, pose the risk of soil contamination due to their persistence, bioaccumulation and toxicity. Phytoremediation by green manure species has been tested using biomarkers, but analytical data are now required to confirm the extraction of sulfentrazone from soil. Thus, the present work was carried out to analyze sulfentrazone residues in soil based on liquid chromatography with a comparison of these values to the sensitivity of the bioindicator Pennisetum glaucum. The soil samples were obtained after cultivation of Crotalaria juncea and Canavalia ensiformis at four seeding densities and with three doses of sulfentrazone. The seedlings were collected into pots, at two different depths, after 75 days of phytoremediator sowing and then were used to determine the herbicide persistence in the soil. A bioassay with P. glaucum was carried out in the same pot. High-performance liquid chromatography (HPLC), using UV-diode array detection (HPLC/UV-DAD), was used to determine the herbicide residues. The HPLC determination was optimized and validated according to the parameters of precision, accuracy, linearity, limit of detection and quantification, robustness and specificity. The bioindicator P. glaucum was more sensitive to sulfentrazone than residue determination by HPLC. Changes in sulfentrazone concentration caused by green manure phytoremediation were accurately identified by the bioindicator. However, a true correlation between the size of the species and the analyte content was not identified.

Successive development of soil ecosystems at abandoned coal-ash landfills

The main goal of the present study was to determine the effect of the native vegetation on the successive development of the soil ecosystem at abandoned coal-ash landfills of the Angren coal-fired power plant in Uzbekistan. Two different landfills (one not in use for 3 years, termed newer, and the other not in use for 10 years, termed older) with different degrees of vegetation cover were chosen to assess the time and vegetation effects on soil biota and habitat development. The soil biotic structure, including soil microorganisms and soil free-living nematode communities, was investigated both at open plots and under different native plants at the coal-ash landfill area. The observed soil microorganisms were found to be the most important component of the observed ecosystems. Total abundance, biomass, species, trophic and sexual diversity of soil free-living nematodes, along with fungi and organic-matter content, were found to be correlated with trace metals. The nematode trophic and species abundance and diversity increased from the newer toward the older coal-ash landfills. The sex ratio of the nematode communities was found to be dependent on the environmental conditions of the study area, with the males being the most sensitive nematode group. All applied ecological indices confirmed that open landfill plots distant from plants are the most unfavorable areas for soil biota. In that respect, the native plants Alhagi maurorum Desv. and Tamarix sp. were found to be important environmental components for the natural remediation of a soil ecosystem in the coal-ash landfill area.

Eichhornia crassipes capability to remove naphthalene from wastewater in the absence of bacteria

The aim of the current study was to investigate the potential of an aquatic plant, the water hyacinth (Eichhornia crassipes) devoid rhizospheric bacteria, to reduce naphthalene (a polyaromatic hydrocarbon) present in wastewater and wetlands. The capability of sterile water hyacinth plants to remove naphthalene from water and wastewater was studied in batch systems. Water hyacinths enhance the removal of pollutants through their consumption as nutrients and also through microbial activity of their rhizospheric bacteria. Experimental kinetics of naphthalene removal by water hyacinth coupled with natural rhizospheric bacteria was 100% after 9 d. Plants, decoupled of rhizospheric bacteria, reduced naphthalene concentration up to 45% during 7 d. Additionally, naphthalene uptake by water hyacinth revealed a biphasic behavior: a rapid first phase completed after 2.5 h, and a second, considerably slower rate, phase (2.5-225 h). In conclusion, water hyacinth devoid rhizospheric bacteria reduced significantly naphthalene concentration in water, revealing a considerable plant contribution in the biodegradation process of this pollutant.

Role of microbial enzymes in the bioremediation of pollutants: a review

A large number of enzymes from bacteria, fungi, and plants have been reported to be involved in the biodegradation of toxic organic pollutants. Bioremediation is a cost effective and nature friendly biotechnology that is powered by microbial enzymes. The research activity in this area would contribute towards developing advanced bioprocess technology to reduce the toxicity of the pollutants and also to obtain novel useful substances. The information on the mechanisms of bioremediation-related enzymes such as oxido-reductases and hydrolases have been extensively studied. This review attempts to provide descriptive information on the enzymes from various microorganisms involved in the biodegradation of wide range of pollutants, applications, and suggestions required to overcome the limitations of their efficient use.

Growth response and tissue accumulation trends of herbaceous wetland plant species exposed to elevated aqueous mercury levels

The impacts of elevated aqueous mercury levels (0, 2, and 4 ppm) on the growth status and mercury tissue concentrations of Eleocharis parvula, Saururus cernuus, Juncus effuses, Typha latifolia, and Panicum hemitomon were determined. Both short-term (net CO2 assimilation) and long-term (biomass) indicators of plant growth status suggest that Eleocharis parvula, Saururus cernuus, and Juncus effuses were relatively unimpacted by elevated mercury levels, whereas Typha latifolia and Panicum hemitomon were somewhat impacted at elevated mercury levels. Eleocharis parvula, Panicum hemitomon, and Typha latifolia generally had the greatest overall belowground tissue concentrations of mercury (2 ppm treatment: 7.21, 7.32, and 9.64 ppm respectively; 4 ppm treatment: 16.23, 18.23, and 13.98 ppm, respectively) and aboveground tissue concentrations of mercury (2 ppm treatment: 0.01, 0.04, 0.02; 4 ppm treatment: 0.26; 0.11; 0.17 ppm, respectively). However, the species investigated in this study demonstrated lower levels of mercury accumulation into tissues when compared with similar investigations of other aquatic plants, suggesting that the above species are not optimal for phytoremediation efforts.

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).

Phylogenetic variation in the tolerance and uptake of organic contaminants

An investigation into the phylogenetic variation of plant tolerance and the root and shoot uptake of organic contaminants was undertaken. The aim was to determine if particular families or genera were tolerant of or accumulated organic pollutants. Data were collected from sixty-nine studies. The variation between experiments was accounted for using a residual maximum likelihood analysis to approximate means for individual taxa. A nested ANOVA was subsequently used to determine differences at a number of differing phylogenetic levels. Significant differences were observed at a number of phylogenetic levels for the tolerance to TPH, the root concentration factor and the shoot concentration factor. There was no correlation between the uptake of organic pollutants and that of heavy metals. The data indicate that plant phylogeny is an important influence on both the plant tolerance and uptake of organic pollutants. If this study can be expanded, such information can be used when designing plantings for phytoremediation or risk reduction during the restoration of contaminated sites.

Monitoring and assessing processes of organic chemicals removal in constructed wetlands

Physical, chemical and biological processes interact and work in concert during attenuation of organic chemicals in wetland systems. This review summarizes the recent progress made towards understanding how the various mechanisms attributed to organic chemicals removal interact to form a functioning wetland. We also discuss the main degradation pathways for different groups of contaminants and examine some of the key characteristics of constructed wetlands that control the removal of organic chemicals. Furthermore, we address possible comprehensive approaches and recent techniques to follow up in situ processes within the system, especially those involved in the biodegradation processes.

"Phytoscreening": the use of trees for discovering subsurface contamination by VOCs

We tested the possibility of using tree cores to detect unknown subsurface contamination by chlorinated volatile organic compounds (Cl-VOCs) and petroleum hydrocarbons, a method we term "phytoscreening". The scope and limitations of the method include the following: (i) a number of widespread Cl-VOC contaminants are readily found in tree cores, although those with very high vapor pressures or low boiling points may be absent; (ii) volatile petroleum hydrocarbons were notwell-expressed in tree cores; (iii) trees should be sampled during active evapotranspiration and from directions that are well exposed to sunlight; (iv) there is not necessarily a direct correlation between concentrations measured in tree cores and those in the subsurface; (v) detection of a contaminant in a tree core indicates that the subsurface is contaminated with the pollutant; (vi) many possible causes of false negatives may be predicted and avoided. We sampled trees at 13 random locations in the Tel Aviv metropolitan area and identified Cl-VOCs in tree coresfromthree locations. Subsequently, subsurface contamination at all three sites was confirmed. Phytoscreening is a simple, fast, noninvasive, and inexpensive screening method for detecting subsurface contamination, and is particularly useful in urban settings where conventional methods are difficult and expensive to employ.

Stimulating natural defenses in poplar clones (OP-367) increases plant metabolism of carbon tetrachloride

Groundwater contamination by carbon tetrachloride (CCl4) presents a health risk as a potential carcinogen and pollutant that is capable of depleting the ozone layer. Although use of poplar trees in a phytoremediation capacity has proven to be cost effective for cleaning contaminated sites, minimizing leaf emission of volatile contaminants remains a pressing issue. We hypothesized that recently fixed carbon plays a key role in CCl4 metabolism in planta yielding nonvolatile trichloroacetic acid (TCA) and that the extent of this metabolism can be altered by heightening plant defenses. Labeling intact leaves with (11)CO2 (t 1/2 20.4 m) can test this hypothesis, because the extremely short half-life of the tracer reflects only those processes involving recently fixed carbon. Using radio-HPLC analysis, we observed [(11)C]TCA from leaf extract from poplar clones (OP-367) whose roots were exposed to a saturated solution of CCl4 (520 ppm). Autoradiography of [(11)C]photosynthate showed increased leaf export and partitioning to the apex within 24 h of CCl4 exposure, suggesting that changes in plant metabolism and partitioning of recently fixed carbon occur rapidly. Additionally, leaf CCl4 emissions were highest in the morning, when carbon pools are low, suggesting a link between contaminant metabolism and leaf carbon utilization. Further, treatment with methyljasmonate, a plant hormone implicated in defense signal transduction, reduced leaf CCl4 emissions two-fold due to the increased formation of TCA.

Phytoremediation of MTBE with hybrid poplar trees

This research investigates the fate and transport of methyl tert-butyl ether (MTBE) in phytoremediation, particularly the uptake and volatilization of MTBE in lab-scale hydroponic systems. The research reveals that MTBE was taken up by hybrid poplar cuttings and volatilized to the atmosphere. Volatilization of MTBE occurred through both stems and leaves. The concentration of MTBE in the transpiration stream declined exponentially with height, indicating that the uptake and volatilization along the stems are an important removal mechanism of MTBE in phytoremediation. Volatilization, via diffusion from the stems, has not been directly measured previously. No volatile MTBE metabolites were detected; however, mass balance closure and metabolite detection were not primary objectives of this study. The greatest amount of MTBE in plant biomass was associated with the woody stems from the previous year's growth, owing in part to the large biomass of stems. MTBE in the plant tissues appears to reach a steady state concentration and there does not appear to be an accumulation process that could lead to highly elevated concentrations relative to the groundwater source.

Phytoremediation of organic contaminants in soils

Soil pollution, a very important environmental problem, has been attracting considerable public attention over the last decades. Unfortunately, the enormous costs associated with the removal of pollutants from soils by means of traditional physicochemical methods have been encouraging companies to ignore the problem. Phytoremediation is an emerging technology that uses plants to clean up pollutants in the environment. As overwhelmingly positive results have become available regarding the ability of plants to degrade certain organic compounds, more and more people are getting involved in the phytoremediation of organic contaminants. Phytoremediation of organics appears a very promising technology for the removal of these contaminants from polluted sites.