Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant.

J. Resour. Ecol. 2011 2(2) 97-105

DOI:10.3969/j.issn.1674-764x.2011.02.001

https://www.sodocs.net/doc/ec689931.html,

Received: 2010-11-05 Accepted: 2011-01-24

Foundation: National Natural Science Foundation of China (40930739; 21037002). * Corresponding author:ZHOU Qixing. Email: zhouqx@https://www.sodocs.net/doc/ec689931.html,.

1 Introduction

Petroleum hydrocarbons (PHCs) usually consist of alkanes (linear or branched), cycloalkanes, aromatic hydrocarbons or more complex chemicals like asphaltenes (Zhou et al. 2004; Pernar et al. 2006). As a major pillar of energy sources in the world, availability of petroleum is a critical concern for many nations. By the end of 2007, the output of global petroleum had reached 2.7 billion barrels (1970–2007). During the processes of exploration, refining, transporting and marketing petroleum products, an increasing number of sites have been polluted by PHCs. The Deepwater Horizon oil spill in April 2010, the largest marine oil spill in the history of the petroleum industry, particularly caused extensive damage to marine and wildlife habitats in China, where oil fields are widely exploited, PHC contamination has become a critical environmental issue (Wang et al. 2010; Zhu et al. 2010). The accumulation of PHCs is now seriously affecting the safety of ecosystems and human health (Harvey et al. 2002; Liste et al. 2006; Meagher 2000). Thus, the remediation of PHC contaminated soils has attracted world wide attention (Chaudhry et al. 2005; Euliss et al. 2008; Vaajasaari and Joutti 2006).

In comparison with conventional ex situ methods, such as incineration, off-site storage, soil washing and in situ capping for stabilization, in situ phytoremediation as a polishing green technology that uses higher plants to degrade, transform, assimilate, metabolize, or detoxify hazardous pollutants from environments has a lot of advantages (Chaudhry et al. 2005; Schr?der 2003; Susarla et al. 2002; Trapp and Karlson 2001; Zhou et al. 2004). It has been well documented that the estimated cost to phytoremedy a ton of soil is significantly lower than the cost of other alternative remediation technologies, such as soil washing and incineration (Schnoor 1997; Smits et al.

Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant

ZHOU Qixing*, CAI Zhang, ZHANG Zhineng and LIU Weitao

Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), College of Environmental Science and Engineering,

Nankai University, Tianjin 300071, China

Abstract: Considerable efforts have been undertaken to accelerate the breakdown of existing anthropogenic petroleum hydrocarbons (PHCs) by appropriate in situ remediation technologies. In situ phytoremediation, using higher plants to remove, stabilize, degrade, and/or metabolize hazardous contaminants, has emerged as a promising green technology for cleaning up environments contaminated with PHCs. Weed plants are generally considered to have great potential for use in phytoremediation due to their extensive fibrous root systems and relatively robust characteristics, thus helping establish a strong rhizosphere through contaminated soils. In this review, some important mechanisms involved in phytoremedation of PHC contaminated soils, including phytoaccumulation, phytostabilization, phytodegradation, phytovolatilization, and rhizodegradation, were summarized and discussed. In recent years, a large number of laboratory approaches have been developed to further enhance the phytoremediation efficiency of PHC contaminated soils. The success of these laboratory studies has encouraged researchers to attempt phytoremediation of PHC contaminated soils in the field. However, many limitations still exist in order to successfully apply laboratory experiments to trials in the field.

Key words: Petroleum hydrocarbons (PHCs); weed plants; phytoremediation; contaminated soil; review

2006) (Table 1). The cost to maintain phytoremediation is also minimal after completing initial soil planting and preparation. This plant-based green technology does not contribute to deterioration of soil quality and there is no secondary pollution when compared with traditional techniques (Cai et al . 2010). There is no size limitation for sites using in situ phytoremediation, thus this technology can be employed in any geographic region as long as it can support the growth of a plant. Nutrients and oxygen are also added into the soil through plant growth and microbial metabolic processes, which can improve the overall quality and texture of the soil during phytoremediation. Furthermore, phytoremediation is easily accepted by the public as it offers aesthetically pleasing results and is an environmentally friendly approach for the cleanup of polluted sites (Zhou et al . 2004).

In many restoration sites, in situ phytoremediation is generally considered to be a terminal process following the initial physico-chemical treatments of high and extremely high-polluted sites. In moderately polluted sites, phytoremediation may be the most cost-effective technology (Jones 1991). However, phytoremediation applied elsewhere cannot be readily transferred to local habitats due to significant differences in different natural environments, soil types and environmental regulation performance criteria (Michael et al . 2007; Zhou et al . 2004). In recent years, native plant species were screened for phytoremediation of PHCs in local contaminated sites. Identification of native plant species used for phytoremediation has been carried out in Brachiaria decumbens (Australian native grass) (Gaskin et al . 2008) and Vetiveria zizanioides (L.) (Venezuelan native grass) (Brandt et al . 2006; Merkl et al . 2005). Among the selection of candidate plants for phytoremediaiton of PHCs, weed plants (Wei et al . 2005; Wei et al . 2006) are generally considered to be the best potential plant species due to their extensive fibrous root system offering a large root surface to establish a strong rhizosphere through the contaminated soil. A variety of common weed species used in the cleanup of PHC compounds include tall fescue (Festuca arundinacea ) (Besalatpour et al . 2008; Liu et al .

2010; Schwab et al . 1998; Siciliano et al . 2003), ryegrass (Lolium perenne ) (Meng et al . in press; Cheema et al . 2010; Hutchinson et al . 2001; Rezek et al . 2008; White et al . 2006), alfalfa (Brassica campestris ) (Wei and Pan 2010; Kirk et al . 2005; Muratova et al . 2008; Schwab et al . 2006; Wiltse et al . 1998), smooth meadowgrass (poapretensis) (Palmroth et al . 2006), crabgrass (Digitaria sanguinalis (L.) Scop.) (Klomjek and Nitisoravut 2005), bermudagrass (Cynodon dactylon (L.) Pers) (White et al . 2003), and switchgrass (Panicum virgatum L.) (Chen et al . 2003).

Although there are a number of review papers on phytoremediation, a review of the latest research concerning phytoremediation of PHC contaminated soil with weed plants is lacking. This paper reviews recent approaches and mechanisms involved in phytoremediation of PHC contaminated soils, including phytoaccumulation, phytostabilization, phytodegradation, phytovolatilization, and rhizodegradation. Laboratory-scale and field-scale enhancement for the phytoremediation of PHC contaminated soils are emphatically summarized and discussed.

2 Mechanisms of phytoremediation of PHC contaminated soils

2.1 Phytoaccumulation

Phytoaccumulation is the process whereby plant roots directly uptake contaminants from the soil and translocate them to aboveground tissues (Fig. 1) (Wei et al . 2006). In comparison with phytoextraction of heavy metals, direct uptake of organic pollutants relies mainly on the physicochemical characteristics of the target compounds, such as the log K ow factor (compounds with an optimal uptake between log K ow 0.5 and 3 are easily transported to the xylem and translocated to the shoot) and the octanol-water partition coefficient, among others (Alkorta et al . 2001). These factors contribute to the bioavailability of the pollutants for uptake and translocation in plant tissues.

In field-scale phytoremediation of PHCs with grasses, both the toxicity and hydrophobic nature of PHCs prevent their bioavailability and extractability in soils. Palmroth et al . (2002) found that PHCs accumulate in grass roots. In the rhizosphere, plant-microbe association may play an important role in making PHCs more available for uptake by grasses, thus accumulating more PHCs, specifically long-chain PHCs called polycyclic aromatic hydrocarbons (PAHs), in grass tissues (Euliss et al . 2008; Liste et al . 2006). Radwan et al . (2000) have confirmed that long-chain PHCs accumulate in Vicia faba (L.). GC-MS analysis of plant tissues indicated that a low amount of PAHs (25.50 mg kg -1 dry biomass) were detected in goose grass roots growing in the contaminated soil (Lu et al . 2010). PAH accumulation in the roots was most likely the result of high sorption of PAHs to the roots or uptake

Alternative remediation technologies Cost ($ ton -1 soil)Phytoremediation 10–35In situ bioremediation 50–150Soil washing 80–200Soil venting 20–220Stabilization

240–340Solvent extraction 360–440Incineration

200–1500

Table 1 Cost comparison of alternative remediation

technologies for soil contaminated with organic compounds (Schnoor 1997).

ZHOU Qixing, et al.: Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant99

into plant tissues. Plants have been shown to accumulate diesel-range compounds in the range of 10 g kg-1 dry plant matter in the roots of fescue, and ryegrass (Palmroth et al. 2002). If phytoaccumulation of PHCs remains in plant roots, harvesting biomass for subsequent treatment may be difficult. Moreover, contaminants may still remain in the soil. It is most desirable for the tolerant grasses to translocate PHCs to the aboveground tissues (Nellessen et al. 1993) (Fig. 1).

2.2 Phytostabilization

Phytostabilization is a process where certain plants are used to mechanically immobilize pollutants and reduce pollutant transfer to other ecosystems and food webs (Cunningham et al. 1995). Pollutants are absorbed and accumulated by the roots, adsorbed onto the root surface or precipitated in the rhizosphere. This process will prevent migration of the target pollutants into groundwater, thus preventing adverse effects on the ecosystem. When applied to the remediation of organic compounds however, this process is of minor significance when compared with rhizodegradation.

2.3 Phytodegradation

Phytodegradation is the process where the partial or complete degradation of contaminants takes place inside the plant or within the rhizosphere and is driven by plant enzymes. This form of phytoremediation has been demonstrated numerous times (Chen et al. 2003; Corseuil et al. 2001). In the process of phytodegradation, plant enzymes act on organic compounds and mineralize them either completely into inorganic compounds, such as CO2 and water, or into stable molecules which can be stored in plant tissues, such as lignin (Cunningham et al. 1995). For example, Chen et al. (2003) using 14C-labeled pyrene as target pollutant found that the mineralization of pyrene was 37.7% and 30.4% in tall fescue (Festuca arundinacea) and switchgrass (Panicum virgatum L.), respectively, compared with 4.3% in unplanted soil. Plant enzymes involved in phytodegradation include Cytochrome P450s, peroxidases, peroxygenases, laccases, phosphatases, nitroreductases and dehalogenases (Schnoor et al. 1995). However, further processes regarding phytodegradation still need to be studied, especially involving tolerant weed species.

2.4 Phytovolatilization

In the process of phytovolatilization the metabolic activity of plants and their associated microbes are employed to transform pollutants into volatile compounds and then release them into the atmosphere (Wenzel 2009). As phytovolatilization of pollutants occurs, it simultaneously dilutes and disperses the soil pollutants, thus assisting the plant and its associated microbes to degrade target pollutants in a reduced stress environment. Agamuthu et al. (2010) suggested that the mechanism of hydrocarbon removal by the Jatropha plants may be via phytovolatilizaion or rhizodegradation. However, Günther et al. (1996) studied the phytoremediation of hydrocarbons using ryegrass and found that abiotic loss of hydrocarbons by evaporation was of minor significance, and elimination of pollutants was accompanied by an increase in microbial numbers and activities. In other words, biodegradation of hydrocarbons in the rhizosphere is stimulated by plant roots.

2.5 Rhizodegradation

In polluted sites many of the restrictions to the remediation of organic contaminants can be overcome by the dynamic synergism that exists between plant roots and microorganisms in the rhizosphere. It is well documented that rhizodegradation is responsible for the enhanced removal of total petroleum hydrocarbons (TPHs) from soil by annual and perennial species such as ryegrass (Lolium perenne), switchgrass (Panicum virgatum), sedge (Carex stricta), arrowhead (Sagitaria latifolia), eastern gamagrass (Tripsacum dactyloides), willow (Salix exigua) and poplar (Populus spp.) (Thygesen and Trapp 2002; Euliss et al. 2008; Rezek et al. 2008). Uptake and bioavailability of petroleum hydrocarbons may be restricted by both their hydrophobic nature and toxicity. The synergy existing between plant roots and soil microorganisms plays an

Fig. 1 Overview of phytoremediation mechanisms.

Journal of Resources and Ecology V ol.2 No.2, 2011 100

important role in the phytoremediation of petroleum contaminants (Lafrance et al. 1998). This stimulation of microbial transformations is driven by the abundant energy offered by root exudates and oxygen from the roots (El-Shatnawi et al. 2001). The molecules exuded by plant roots include carbohydrates, amino acids, fatty acids, nucleotides, organic acids, phenolics, plant growth regulators, putrescines, sterols and vitamins (Kang et al. 2010). Evidence suggests that the activities of soil microorganisms in the rhizosphere may be controlled by plants in return for the provision of root exudates and oxygen. In addition, microorganisms benefit the plant by supplying vitamins, amino acids, auxins and cytokinins that stimulate plant growth (Atlas et al. 1998) and lead to enhanced TPH degradation. Convincing evidence for this argument comes from consistent findings that microbial numbers in a rhizosphere are generally several orders of magnitude greater than those in a non-vegetated soil (Cai et al. 2010).

3 Laboratory scale enhancement of phytoremedying PHC contaminated soils Despite the fact that remediation of PHC contaminated soil with weed plants has shown significant potential, phytoremediation is still in its infancy. The use of living weeds alone is generally considered to be a restrictive factor for phytoremediation. A large number of the latest studies have paid more attention to relative technologies used to enhance phytoremediation efficacy at the laboratory scale.

3.1 Soil amendment for enhancing phytoremediation The application of soil amendment appears to be a valuable option for the phytoremediation of PHC contaminated soil. It enables great vegetative coverage and increases the rate of PHC removal in soil. For example, the addition of compost to soil helps reduce the negative effects of PHCs on ryegrass growth and increases PHC removal from the soil (Vouillamoz et al. 2001). Palmroth et al. (2006) confirmed that in soil amended with NPK fertilizer 65% of hydrocarbons were removed and the addition of municipal biowaste compost removed 60% of hydrocarbons over 39 months; hydrocarbons did not significantly decline in non-amended soil. Adding Jatropha curcas amended with organic wastes (BSG) to soil greatly increases the removal of waste lubricating oil to 89.6% and 96.6% in soil contaminated with 2.5% and 1.0% oil, respectively. A loss of 56.6% and 67.3% was recorded in the corresponding planted soils without organic amendment over 180 days (Agamuthu et al. 2010).

Though conventional amendments such as NPK fertilizer have contributed to plant productivity and effective degradation of PHC pollutants, when overused the soil-remaining fertilizers not taken up by the plants usually “burn” the plants and can even cause environmental problems (Kang et al. 2010). Naturally-produced biosurfactants (rhamnolipids), which have no phytotoxicity to plants and can increase PHC bioavailability, are proven to enhance PHC degradation (Zhang et al. 1997) and may be a better application for the remediation of contaminated soil. Previous studies have shown that rhamnolipids can enhance the uptake of PAHs by ryegrass roots and the degradation of PAHs by alfalfa (Zhang et al. 2010; Zhu et al. 2008). The advantages of using biosurfactants indicate that biosurfactant-enhanced phytoremediation has the potential to become promising technology for remediation of contaminated soil.

3.2 Plant growth-promoting rhizobacteria for

enhancing phytoremediation

Plant growth-promoting rhizobacteria (PGPR) are bacteria capable of promoting plant growth by colonizing the plant root surface and the closely adhering soil interface (Kloepper et al. 1980, Kloepper et al. 1981). PGPR strains can produce indoleacetic acid (an auxin), siderosphores and enzyme 1-amino-cyclopropane-1-carboxylic acid (ACC) deaminase. Soil contamination generally stimulates ethylene production in plants, leading to plant growth retardation. Enzyme ACC deaminase can consume ACC, the precursor of ethylene into 2-oxobutanoate and ammonia (Glick 2005). Decreased ethylene levels allow plants used in phytoremediation to grow and survive better in heavily contaminated soils. Moreover, PGPR strains can act as bio-control agents, protecting the rhizosphere from pathogenic microbes (Compant et al. 2005; Whipps 2001). The introduction of PGPR strains in phytoremediation can provide better plant growth and thereby increase plant resistance to contaminants in the soil than using plants alone (Huang et al. 2004b; Kang et al. 2010; Koo et al. 2010). As a result, PGPR can help accelerate degradation of contaminants.

According to Huang et al. (2004a; 2004b) during a greenhouse experiment the germination frequency for wild rye increased by 61% with PGPR at 0.5 g kg-1 of creosote. For tall fescue, plant germination frequency increased by 40% with PGPR at 3 g kg-1 of creosote. Moreover, the introduction of PGPR greatly enhanced the PHC (polycyclic aromatic hydrocarbons) and creosote removal when compared with phytoremediation alone. PGPR strains can enhance the grass germination frequency and stimulate grasses to grow better in heavily contaminated soils, thus promoting decontamination of PHCs. Table 2 shows recent studies on phytoremediation of PHCs by tolerant grasses with the assistance of PGPR.

3.3 Inoculation of plants with microbes for enhancing

phytoremediation

Weeds with a fibrous root system such as grasses are preferred for phytoremediation due to their large root

ZHOU Qixing, et al.: Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant101

surface area, which can help establish active microbial activity and populations (Aprill et al. 1990). Euliss et al. (2008) suggested that different plants may enhance rhizosphere degradation by selecting for a variety of microbial communities. Fang et al. (2001) found that microbes isolated from grass planted soils were more effective at degrading PHCs than those from non-vegetated soils. Thus, inoculation of plants with microbes in rhizosphere may not only protect plant roots from pollutant toxicity (Robert et al. 2008), but also enhance phytoremediation efficacy. Recently, more studies have been devoted to enhancing rhizodegradation efficiency by inoculating microbes, particularly indigenous microbes isolated from contaminated habitats. Autochthonous microbes are more compatible with local contaminated sites than allochthonous microbes, which do not occupy a functional niche (Atlas et al. 1998). For example, the indigenous microbial population present in Hong Kong soil degraded diesel oil more efficiently than the microbial consortium introduced from Long Beach soil (Bento et al. 2005). Cyperus laxus Lam., a native plant growing in swamps, inoculated with autochthonous microbial strains isolated from C. laxus rhizosphere degraded PHCs two times higher than non-inoculated plants after 60 days in culture. Furthermore, the root biomass of C. laxus was 1.6 times greater than non-inoculated plants (Escalante-Espinosa et al. 2005). Efficient hydrocarbon-degrading bacterial strains that can compete with the native habitat and are closely linked to plants are also promising candidates for phytoremediation. For instance, Italian ryegrass (Lolium multiflorum var. Taurus) in combination with an alkane-degrading strain (BTRH79) showed higher hydrocarbon degradation than that in other treatments (Yousaf et al. 2010).

Previous literature has focused on inoculation using hydrocarbon-degrading bacteria to enhance phytoremediation. However, plants with added fungal strains are also more effective at increasing PHC decontamination than phytoremediation alone (Hashem 2007). The addition of fungal strains (Fusarium acuminatum, F. equiseti, F. oxysporum, F. solani, and F. reticulatum) to Polygonum avicular L., a plant native to Iran, provided a greater removal of PHCs than using P. avicular alone (Mohsenzadeh et al. 2010). Mutual benefits between plants and inoculated hydrocarbon-degrading microbes greatly promote phytoremediation of PHC. Plant characteristics and phytoremediation efficiency can be greatly improved with the addition of these special microbes. The inoculation of plants with special microbial strains may be a promising alternative for the bioremediation of PHC contaminated soils.

3.4 Genetic engineering technology for enhancing

phytoremediation

The utilization of plants for the cleanup of toxic compound contaminated soils is limited by the slow growth rate of the plants, meaning several years is often required for the restoration of contaminated sites. The efficiency of using plants can be substantially improved through genetic engineering technologies (Bennett et al. 2003; Kawahigashi 2009). The first transgenic plants for phytoremediation were developed for remedying heavy metal contaminated sites. Transgenic Arabidopsis thaliana seeds expressing merApe9, a mercuric ion reductase, evolved considerable amounts of inert metallic mercury (Hg°) relative to control plants (Rugh et al. 1996). In recent years, researchers have

Huang et al.

2005

Gurska et al.

2009

PGPR strain Latin name Common name Contaminants Result Reference

Pseudomonas putida UW3 Azospirillum brasilense Cd Enterobacter cloacae CAL 2Festuca arundinacea,

Elymus triticoides,

Poa pratensis

Tall fescue,

Wild rye,

Kentucky

bluegrass

Polycyclic

aromatic

hydrocarbons

(PAHs),

Creosote

Increased

plant tolerance

to PAHs and creosote

Enhanced PAH and

creosote removal

Huang et al.

2004a; b

Enterobactor cloaca e UW4 Enterobacter cloacae CAL 2Festuca

arundinacea

Tall fescue Total petroleum

hydrocarbons

(TPHs)

Promoted plant growth

and increased plant

tolerance to TPHs

Pseudomonas sp. UW3 Pseudomonas putida UW4Lolium perenne,

Festuca arundinacea,

Secale cereale,

Hordeum vulgare

Annual

Ryegrass,

Tall fescue,

Fall rye,

Barley

Total petroleum

hydrocarbons

(TPHs)

Increased

plant biomass

via alleviation

of plant stress

Table 2 Examples of bioremediation of PHCs by weeds with the assistance of plant growth-promoting rhizobacteria (PGPR).

Journal of Resources and Ecology V ol.2 No.2, 2011 102

devoted more effort to developing transgenic plants for phytoremediation of organic contaminates. For example, the expression of human cytochrome P450 genes in rice became more tolerant toward herbicides than non-transgenic ones (Kawahigashi et al. 2006). Transgenic poplar (Populus spp.) showed increased removal rates of trichloroethylene, chloroform and benzene from hydroponic solution (Doty et al. 2007). Other organic compounds including explosives, carbon tetrachloride and halogenated hydrocarbons have been widely remedied by transgenic plants (Doty et al. 2000; Van Aken 2008, 2009). Despite these findings, little research has focused on using transgenic weeds for phytoremediation of PHC contaminated soils. The utilization of transgenic plants, especially transgenic weeds, requires further study in order to increase the efficiency of phytoremediation.

3.5 Combined approaches for enhancing

phytoremediation

In many cases, remediation technology using plants and one enhancement approach and plants may still be inefficient. For a phytoremediation system to be more effective plant tolerance and TPH degradation needs to be improved by use of a combination of the approaches outlined above. A multi-process phytoremediation system (MPPS) has been suggested to combine agronomic treatment, inoculation with contaminant degrading bacteria, and the growth of the contaminant-tolerant plants such as tall fescue (Festuca arundinacea) with plant growth-promoting rhizobacteria (PGPR). Huang et al. (2004a, 2005) showed that during the first four months in culture, the removal of TPHs and 16 priority PAHs by MPSS was twice that of agronomic treatment, 50% more than inoculation with microbes, and 45% more than phytoremediation alone. A combined approach consisting of phytoremediation, surfactant flushing and microbial degradation effectively dissipates oil pollutants from loess soil and is recommended for restoration of PHC contaminated sites (Zhu et al. 2010). Zhang et al. (2010) have introduced a multi-technique phytoremediation system consisting of mycorrhizal fungi, aromatic hydrocarbon degrading bacteria (ARDB) and rhamnolipids for the bioremediation of PAHs. After 90 days, the total PAH removal by the multi-technique phytoremediation system was 251.83% greater than that of phytoremediation alone. These studies show that applying one approach alone is not very efficient, but combining multiple processes can remedy defects. Therefore, phytoremediation in conjunction with multiple approaches may be an optimal solution for enhancing PHC removal.

4 Field scale phytoremediation of PHC contaminated soils

Initial phytoremediation results have shown great promise for cost-effective remedial technology, prompting international efforts to focus on transitioning experiments from the laboratory to the field (Liste et al. 2006; Palmroth et al. 2006). For example, a two-year field trial was conducted at a weathered hydrocarbon flare-pit site in southeastern Saskatchewan, Canada. Significant differences were observed in degradation trends for the first growing season, with Altai wild rye (Elymus angustus Trin.) promoting greater than 50% TPH degradation (Phillips et al. 2009). The phytoremediation treatment decreased TPH by 30%, twice that of non-planted soils, after a two year field trial (Siciliano et al. 2003). Gurska et al. (2009) used plant growth-promoting rhizobacteria enhanced phytoremediation to successfully lower TPH from 130 g/kg to approximately 50 g/kg over a three year period. However, a three year field study conducted at the Jones Island disposal facility in Milwaukee, USA showed planted treatments including black willow (Salix nigra) (SX61), prairie cord grass (Spartina pectinata), lake sedge (Carex aquatalis), annual rye (Lolium multiflorum), and bulrush (Scirpus fluviatilis) did not enhance PAH dissipation relative to those without plants (Smith et al. 2008). Differences between laboratory and field experiments include precipitation, temperature, plant nutrients and plant pathogens and may affect seed germination and plant growth and thus negatively impacting phytoremediation efficiency. Moreover, the accumulation of pollutants in plants will likely be released into the environment anew in field scale studies. Management options, such as fencing, could help minimize pollutant entry into food webs. Potential problems surrounding the widespread application of phytoremediation in field trials needs to be further explored.

5 Conclusions and future prospects

Although numerous studies on phytoremediation have been conducted at the laboratory scale under short-term controlled conditions, more research is still needed to gain a better understanding of the performance and potential for phytoremediation with weed plants over a longer-term and in the field. To reduce the potential ecological risk on local ecosystems posed by non-native weed species (transgenic weed species included), more effective native weeds that are compatible with local habitats are preferred and need to be tested for use in phytoremediation. Successful phytoremediation is dependent on a high production of root biomass and high translocation of pollutants from the roots to aboveground tissues. Environmentally friendly enhancement approaches in conjunction with phytoremediation are proposed to promote healthy plant growth by overcoming plant stress. In addition, the mechanisms of PHC phytoremediation by weeds should be further investigated, especially the complex interactions involving roots, root exudates, rhizosphere soils and

ZHOU Qixing, et al.: Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant103

microbes. Standard protocols, such as those developed by the Remediation Technologies Development Forum (https://www.sodocs.net/doc/ec689931.html,/download/rtdf/542f06005.pdf), will be necessary to assess the efficiency of phytoremediation sites. New protocols are also needed in order to appropriately interpret data from remediation sites. While there is still much to be investigated, phytoremediation associated with environmentally friendly processes has emerged as a cost-effective technology for PHC remediation.

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ZHOU Qixing, et al.: Ecological Remediation of Hydrocarbon Contaminated Soils with Weed Plant105基于杂草植物的石油烃污染土壤生态修复

周启星, 蔡 章, 张志能, 刘维涛

南开大学环境科学与工程学院，环境污染过程与基准教育部重点实验室，天津 300071

摘要: 在过去的几十年内，人们在致力于采用合适的原位修复技术以加速对现有人为污染的石油烃的降解与净化方面做了大量工作。至今，原位植物修复，即采用高等植物去除、稳定、降解和/或代谢有毒有害污染物的方法，已经成为对石油烃污染环境治理修复的具有前途的新兴技术。杂草植物由于具有大量的纤维根系和极为强壮的特征，因其有助于穿透污染土壤而建立一个强大的根际圈，进而在植物修复方面一般表现为巨大的潜力。本综述首先对石油烃污染土壤实施植物修复所涉及的一些重要机制，包括植物积累、植物稳定、植物降解、植物挥发和根际降解作用，进行了概述和探讨。近年来，在进一步促进石油烃污染土壤的植物修复效率方面，改进并研制了相当数量的实验方法。此外，来自实验研究结果的成功给予研究人员以极大的鼓励，尤其促使人们在田间尺度上实施石油烃污染土壤的植物修复方面做了一些有益的尝试。然而，所有这些工作均存在着诸多局限和困难，有待我们去克服，从而使我们的研究从实验室的尺度发展到田间的应用上。

关键词: 石油烃(PHCs); 杂草植物; 植物修复; 污染土壤; 综述