Brett Robinson1*, Rainer Schulin1, Bernd Nowack1, Stéphanie Roulier1, Manoj Menon1, Brent Clothier2, Steve Green2 and Tessa Mills2
1 Institute of Terrestriale Ecology, Swiss Federal Institute of Technology (ETH), Universitätstrasse 16, CH-8092 Zürich, Switzerland. brett.robinson@env.ethz.ch, schulin@env.ethz.ch, nowack@env.ethz.ch, Stephanie.roulier@env.ethz.ch, menoneth@gmail.com
2 HortResearch, Private Bag 11030, Palmerston North, New Zealand. bclothier@hortresearch.co.nz, sgreen@hortresearch.co.nz, tmills@hortresearch.co.nz
* Corresponding author
Abstract摘要
Phytoremediation improves metal-contaminated sites by the extraction of contaminating metals (phytoextraction), or their immobilisation (phytostabilisation). Phytoextraction removes metals from the soil by repeated crops of plants that accumulate large amounts of one or more target metals in their above-ground biomass. The harvested plant material is removed from the site. Despite more than ten years of research, there are few examples of successful phytoextraction. This technology is limited by the long period required for cleanup, the restricted number of target metals that can be extracted, the limited depth that can be accessed by roots, and the difficulty of producing a high-biomass crop of the desired species. There is also concern about metal-accumulating plants providing an exposure pathway for toxic elements to enter the food chain. The addition of chelants to enhance plant-metal uptake, invariably increases the risk of metal leaching. Phytostabilisation exploits transpiration and root-growth to immobilise contaminants by reducing leaching, controlling erosion, creating an aerobic environment in the root-zone, and adding organic matter to the substrate that binds metals. Soil amendments can promote plant growth and enhance metal immobilisation. Phytostabilisation requires the establishment of tailored vegetation on the site that is left there in perpetuity. A succession of plant species may be used to establish the desired climax vegetation. Unlike phytoextraction, there are numerous examples of successful phytostabilisation on metal-contaminated sites.
植物修复通过提取污染金属（植物提取）或将其固定（植物稳定）来改善金属污染场地。植物提取是指反复种植能在地上部生物量中大量积累一种或多种目标金属的植物，从而将金属从土壤中去除，收获的植物材料随后被移出场地。尽管已有十余年的研究，成功的植物提取实例仍然寥寥无几。该技术受到以下因素的限制：修复所需周期长、可提取的目标金属种类有限、根系可达深度有限，以及难以使目标物种形成高生物量作物。此外，人们还担心金属积累植物会成为有毒元素进入食物链的暴露途径。添加螯合剂以促进植物对金属的吸收，无一例外地会增加金属淋溶的风险。植物稳定利用蒸腾作用和根系生长来固定污染物，其途径包括减少淋溶、控制侵蚀、在根区营造好氧环境，以及向基质中添加可结合金属的有机质。土壤改良剂可促进植物生长并增强金属固定。植物稳定需要在场地上建立适配的植被并永久保留。可利用一系列植物物种的演替来建立理想的顶极植被。与植物提取不同，植物稳定在金属污染场地上已有大量成功实例。
Phytoremediation technology is site specific due to the plethora of environmental variables that affect plant growth and metal mobility. Most contaminated sites contain a heterogeneous mixture of several elemental and organic contaminants. Plant-growth may be limited by other environmental variables, such as low pH, low nutrient availability, salinity, insufficient aeration or low water availability.The commercial success of phytoremediation is thus dependent on convincing decision makers that phytoremediation can satisfy environmental regulations. Obviously, field demonstrations at each site are not practical; therefore validated mechanistic models are required to calculate the effect of phytoremediation on metal fluxes. Central to such models is an understanding of root-metal interactions in these typically heterogeneous media.
由于影响植物生长和金属迁移性的环境变量众多，植物修复技术具有场地特异性。大多数污染场地含有多种元素污染物和有机污染物的非均质混合物。植物生长还可能受到其他环境变量的限制，如低pH、养分有效性低、盐度高、通气不足或水分有效性低。因此，植物修复的商业成功取决于能否使决策者确信植物修复可以满足环境法规的要求。显然，在每个场地都开展现场示范并不现实；因此，需要经过验证的机理模型来计算植物修复对金属通量的影响。此类模型的核心在于理解这些通常为非均质的介质中根系与金属的相互作用。
Keywords: hydraulic control, phytostabilisation, phytoextraction, bioenergy
关键词：水力控制，植物稳定，植物提取，生物能源
1Introduction引言
Phytoremediation is the use of plants to improve degraded environments. Plants affect metal fluxes via the extraction of contaminating metals into the above-ground biomass, or changing their mobility in the soil profile. Thus, there are two ways of achieving metal phytoremediation.
植物修复是利用植物改善退化环境的技术。植物通过将污染金属提取至地上部生物量中，或改变其在土壤剖面中的迁移性来影响金属通量。因此，实现金属植物修复有两种途径。
In phytoextraction, vegetation is engineered so that the site is eventually cleansed via metal removal, whereas in phytostabilisation metal mobility and metal toxicity are decreased, thus lowering the environmental risk. In both of these roles, plants function as bio-pumps (ROBINSON et al. 2003a) that use the sun’s energy to pump water and solutes from the soil to the above-ground portions, whereupon water returns to the atmosphere via transpiration. Plants also pump organic matter into the rhizosphere via root exudates and decaying tissue.This organic matter may reduce or promote metal flux.
在植物提取中，通过对植被进行工程化设计，最终经由金属去除使场地得到净化；而在植物稳定中，金属的迁移性和毒性降低，从而降低环境风险。在这两种作用中，植物均充当生物泵（ROBINSON et al. 2003a），利用太阳能将水分和溶质从土壤泵送至地上部，随后水分通过蒸腾作用返回大气。植物还通过根系分泌物和腐烂组织将有机质泵入根际。这些有机质可能减少或促进金属通量。
Water is the main vehicle for transport of metals through soil. Metal fluxes in soil are dependent on spatial concentration gradients driving diffusion and dispersion, and the mass flow of water (VOGELER et al. 2001). CLOTHIER and GREEN (1997) described roots as ‘‘the big movers of water and chemicals in soil. Of the global average rainfall of 720 mm that falls on soil each year (SELLERS 1965), some 410 mm is returned back to the atmosphere, either directly from the soil or, more often, by evapo-transpiration from vegetation growing therein (CLOTHIER and GREEN 1997). This represents a 57% reduction in the average water flux through soil, and a significant reduction in the volume of soil solution that exits the root-zone and enters receiving waters. In arid regions, evapotranspiration can eliminate drainage and hence render trace elements immobile.
水是金属在土壤中迁移的主要载体。土壤中的金属通量取决于驱动扩散和弥散的空间浓度梯度以及水的质量流（VOGELER et al. 2001）。CLOTHIER和GREEN（1997）将根系描述为“土壤中水分和化学物质的主要搬运者”。全球每年降落在土壤上的平均降雨量为720 mm（SELLERS 1965），其中约410 mm返回大气，或直接由土壤蒸发，或更常见的是经由生长其上的植被蒸散（CLOTHIER and GREEN 1997）。这意味着通过土壤的平均水通量减少了57%，离开根区并进入受纳水体的土壤溶液体积也显著减少。在干旱地区，蒸散可消除排水，从而使微量元素不发生迁移。
Phytoremediation can also affect metal fluxes via plant-induced physico-chemical changes in the soil.Vegetation affects the fate of trace elements in soil in the following ways:
植物修复还可以通过植物诱导的土壤物理化学变化来影响金属通量。植被通过以下方式影响土壤中微量元素的归趋：
- Changing the water flux改变水通量
- Adding organic matter to the soil向土壤中添加有机质
- Changing soil pH改变土壤pH
- Changing the metal speciation改变金属形态
- Promoting the growth of soil-borne organisms促进土壤生物的生长
- Changing the physical characteristics of the soil改变土壤的物理特性
- Bioaccumulation
Here we investigate these plant processes as they affect metal phytoremediation, the potential applications of metal phytoremediation and the actions required for the successful implementation of this technology. We divide metal phytoremediation into phytoextraction, which only has limited potential for the remediation of contaminated land, and phytostabilisation, which has a greater potential, but is constrained by the lacuna of knowledge on soilroot interactions.
本文探讨了影响金属植物修复的上述植物过程、金属植物修复的潜在应用，以及成功实施该技术所需采取的措施。我们将金属植物修复分为植物提取和植物稳定两类：前者对污染土地修复的潜力有限；后者潜力更大，但受制于对土壤–根系相互作用认识的不足。
2Phytoextraction: not ready for action植物提取：尚未具备实际应用条件
Phytoextraction describes the use of plants to remove metals and other contaminants from soils. Theoretically, metal-contaminated sites could be cleansed by the repeated cropping of plants, provided that harvested amounts of metals exceeds further inputs, until the soils’ metal concentrations have reached acceptable levels. Phytoextraction relies on plants that translocate large amounts of one or more target metals into the above ground biomass.
植物提取是指利用植物从土壤中去除金属及其他污染物。理论上，只要收获移除的金属量超过后续输入量，通过反复种植植物即可净化金属污染场地，直至土壤金属浓度达到可接受水平。植物提取依赖于能将大量一种或多种目标金属转运至地上部生物量中的植物。
After each cropping, the metal-rich biomass would be removed from the area and may be burned to reduce its volume, whereupon it could be stored in an appropriate area, such as a contained landfill, that does not pose a risk to the environment.
每次收获后，富含金属的生物量将被移出该区域，并可通过焚烧减小体积，随后贮存于不对环境构成风险的适当场所，如封闭式填埋场。
Successful phytoextraction requires that the soil be cleansed to a level that complies with environmental regulations, and from an economic viewpoint, this should be achieved at a lower cost than an alternate technology or the cost of inaction (ROBINSON et al. 2003a). Field trials or commercial operations that demonstrate successful phytoextraction are conspicuously absent.There is considerable scope to improve this technology by developing crops that remove greater amounts of metal, or combining phytoextraction with a profitmaking operation. Most phytoextraction studies focus on enhanced plant metal uptake, by discovering or engineering new plants, and by soil amendments to enhance metal uptake. However, even with these improvements, basic plant physiology renders phytoextraction ineffective for most contaminated sites. Here we discuss plant metal uptake as it relates to phytoextraction and explore the potential application of this technology.
成功的植物提取要求将土壤净化至符合环境法规的水平；从经济角度看，其成本还应低于替代技术或不采取行动的代价（ROBINSON et al. 2003a）。目前明显缺乏能够证明植物提取成功的田间试验或商业运营实例。通过培育可去除更多金属的作物，或将植物提取与营利性经营相结合，该技术仍有相当大的改进空间。大多数植物提取研究侧重于通过发现或改造新植物以及施用土壤改良剂来增强植物对金属的吸收。然而，即使有了这些改进，基本的植物生理学特性仍使植物提取对大多数污染场地无效。本文讨论与植物提取相关的植物金属吸收，并探讨该技术的潜在应用。
2.1 Plant metal uptake2.1 植物对金属的吸收
The factor determining the duration of phytoextraction is the mass of metal removed by the crop per unit of time (years) compared to the mass of metal in the soil. Theoretically, the number of years required to lower the soil’s metal concentration to acceptable levels can be calculated by:
决定植物提取持续时间的因素是作物单位时间（年）内移除的金属质量与土壤中金属质量之比。理论上，将土壤金属浓度降至可接受水平所需的年数可按下式计算：
where t is the time (years), Mi is the initial soil metal burden (g/ha), Mf is the target soil metal burden (g/ha), P is the crop metal concentration (g/t), and B is the crop biomass production (t/ha/yr). In the field situation, Eq. 1 gives the shortest possible time for phytoextraction, because it does not incorporate spatial or temporal heterogeneity. However, it is useful to determine which scenarios are not suitable for phytoextraction.
式中，t为时间（年），Mi为土壤初始金属负荷（g/ha），Mf为土壤目标金属负荷（g/ha），P为作物金属浓度（g/t），B为作物生物量产量（t/ha/yr）。在田间条件下，由于式1未考虑空间或时间非均质性，其给出的是植物提取的最短可能时间。然而，该式可用于判断哪些情形不适合采用植物提取。
The efficacy of phytoextraction is thus dependent on using plants with a high biomass and a high metal concentration in the above-ground portions. Some plants, known as hyperaccumulators (BROOKS et al. 1977) extract large amounts of trace elements from soil as part of their normal metabolic processes. BROOKS et al. (1977) used hyperaccumulation to describe plants that take up Ni to concentrations greater than 1000 mg/kg on a dry matter basis on Ni-rich ultramafic (serpentine) soil.This concentration is at least an order of magnitude greater than concentrations found in other plants growing in the same environment.At present, there are in total over 400 species of known hyperaccumulators for As, Cd, Mn, Na, Ni, Tl and Zn (BROOKS 1998). While hyperaccumulator plants can achieve a high metal concentration in their shoots, their biomass production is usually inferior to non-hyperaccumulator plants. Notable exceptions are Ni hyperaccumulators of the genera Alyssum (ROBINSON et al. 1997a) and Berkheya (ROBINSON et al. 1997b). These plants can achieve shoot Ni concentrations of >10000 mg/kg (1%) on a dry matter basis, while producing more than fifteen tonnes of dry matter per hectare per year.
因此，植物提取的效率取决于所用植物是否具有高生物量以及地上部高金属浓度。某些植物，即超富集植物（BROOKS et al. 1977），在其正常代谢过程中可从土壤中提取大量微量元素。BROOKS et al.（1977）使用“超富集”一词来描述在富镍超基性（蛇纹岩）土壤上吸收Ni、使其干物质浓度超过1000 mg/kg的植物。该浓度比生长在同一环境中的其他植物至少高出一个数量级。目前，已知的As、Cd、Mn、Na、Ni、Tl和Zn超富集植物共有400余种（BROOKS 1998）。虽然超富集植物的地上部可达到很高的金属浓度，但其生物量产量通常低于非超富集植物。值得注意的例外是Alyssum属（ROBINSON et al. 1997a）和Berkheya属（ROBINSON et al. 1997b）的Ni超富集植物。这些植物地上部Ni浓度按干物质计可达>10000 mg/kg（1%），同时每公顷每年可生产超过15吨干物质。
2.2 Induced hyperaccumulation2.2 诱导超富集
For some common metals, such as Pb, there are no reliable reports of any hyperaccumulator species. Induced hyperaccumulation is a possible solution. Induced hyperaccumulation requires that high concentrations of the target metal(s) be brought into soil solution and the disruption of the root-endodermis, allowing the metal in soil solution to pass directly into the root xylem via the apoplastic pathway. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are effective in enhancing the solubility of Pb, Cd, Cu, Zn and other trace element cations (HUANG and CUNNINGHAM 1996; BLAYLOCK et al. 1997; ROBINSON et al. 1999; THAYALAKUMARAN et al. 2003; TANDY et al. 2004).Addition of thiosulphate and thiocynate salts to mine spoil induced plants to accumulate Hg (MORENO et al. 2005) and, auspiciously,Au (ANDERSON et al. 1998). Chloride anions increased the Cd solubility in soils due to the formation of relatively stable chloride ion complexes [CdCl+ and CdCl2] (WEGGLER et al. 2004). Similarly, MCLAUGHLIN et al. (1994) demonstrated that the addition of chloride to soils enhanced plant Cd-uptake.
对于一些常见金属（如Pb），尚无任何超富集物种的可靠报道。诱导超富集是一种可能的解决方案。诱导超富集要求将高浓度的目标金属带入土壤溶液，并破坏根内皮层，使土壤溶液中的金属经由质外体途径直接进入根木质部。螯合剂如乙二胺四乙酸（EDTA）和次氮基三乙酸（NTA）可有效提高Pb、Cd、Cu、Zn及其他微量元素阳离子的溶解度（HUANG and CUNNINGHAM 1996; BLAYLOCK et al. 1997; ROBINSON et al. 1999; THAYALAKUMARAN et al. 2003; TANDY et al. 2004）。向矿山废土中添加硫代硫酸盐和硫氰酸盐可诱导植物积累Hg（MORENO et al. 2005），以及可喜地积累Au（ANDERSON et al. 1998）。由于形成了相对稳定的氯离子络合物[CdCl+和CdCl2]，氯阴离子提高了土壤中Cd的溶解度（WEGGLER et al. 2004）。类似地，MCLAUGHLIN et al.（1994）证明向土壤中添加氯化物可增强植物对Cd的吸收。
Solubilisation, however, does not necessarily induce bioaccumulation. For example, plants do not take up Cu that is solubilised by dissolved organic matter because the complex cannot penetrate the root endodermis and enter the xylem (BOLAN and DURAISAMY 2003). Chelant addition to the Ni hyperaccumulator Berkheya coddii caused a decrease in Ni uptake, despite enhancing the Ni solubility in the soil (ROBINSON et al. 1999). TANDY et al. (2005) demonstrated that chelants increased Pb uptake into the shoots but reduced Cu and Zn uptake from solution. Uptake of essential metals that are normally taken up via the symplastic pathway, is reduced by the addition of chelates by rendering the metal unavailable to the plant’s metal transporters into the symplast. Nevertheless, high concentrations of chelants and soluble metal in the substrate can induce plant uptake. Uptake of metals in the presence of chelates requires that the metal pass directly into the root xylem via the apoplastic pathway, in part helped by the disruption of the root endodermis caused by high concentrations of chelants in solution. A proposed strategy for chelate-enhanced phytoremediation is application of chelate to a mature crop growing on a contaminated soil. The application of selected pesticides can disrupt root-membranes allowing the complexed metal to pass directly into the root xylem via the apoplastic pathway (BLAYLOCK 2000).
然而，溶解并不一定诱导生物积累。例如，植物不吸收由溶解性有机质溶解的Cu，因为该络合物无法穿过根内皮层进入木质部（BOLAN and DURAISAMY 2003）。向Ni超富集植物Berkheya coddii添加螯合剂虽提高了土壤中Ni的溶解度，却导致Ni吸收量下降（ROBINSON et al. 1999）。TANDY et al.（2005）证明，螯合剂增加了地上部对Pb的吸收，但降低了从溶液中吸收Cu和Zn。通常经由共质体途径吸收的必需金属，在添加螯合剂后吸收量减少，这是因为螯合作用使金属无法被植物向共质体转运金属的转运蛋白所利用。尽管如此，基质中高浓度的螯合剂和可溶性金属仍可诱导植物吸收。在螯合剂存在下吸收金属，需要金属经由质外体途径直接进入根木质部，而溶液中高浓度螯合剂对根内皮层造成的破坏在一定程度上促进了这一过程。一种已提出的螯合剂强化植物修复策略是将螯合剂施用于生长在污染土壤上的成熟作物。施用特定农药可破坏根膜，使络合态金属经由质外体途径直接进入根木质部（BLAYLOCK 2000）。
There are environmental concerns about the use of induced bioaccumulation due to metal leaching through the soil profile, possibly entering groundwater (LOMBI et al. 2001). Processes such as preferential flow may exacerbate metal leaching (BUNDT et al. 2000). While EDTA is the most often studied compound for chelant-enhanced phytoextraction, its use in this role is unacceptable due to the severe risk of leaching high concentrations of mobile metal-complexes to groundwater (NOWACK 2002). EDTA is not readily degraded under natural conditions and thus persistent in the environment. Thayalakumaran et al. (2003) demonstrated that, in an undisturbed soil profile containing 300 mg/kg Cu, plants removed just 5% of the Cu solubilised by EDTA. The remaining 95% leached below the root zone.
由于金属可能沿土壤剖面淋溶并进入地下水，诱导生物积累的应用引发了环境方面的担忧（LOMBI et al. 2001）。优先流等过程可能加剧金属淋溶（BUNDT et al. 2000）。虽然EDTA是螯合剂强化植物提取中研究最多的化合物，但由于存在高浓度可移动金属络合物淋溶至地下水的严重风险，其在该方面的应用是不可接受的（NOWACK 2002）。EDTA在自然条件下不易降解，因而在环境中具有持久性。Thayalakumaran et al.（2003）证明，在含300 mg/kg Cu的原状土壤剖面中，植物仅移除了被EDTA溶解的Cu的5%，其余95%淋溶至根区以下。
To achieve adequate solubility, chelants are usually added as Na salts. The resultant Na concentrations in the soil can reduce plant growth. Sodium also causes the dispersion of clay minerals, possibly resulting in increased preferential flow. Most chelating agents also solubilise metals other than the target metals and these then may be phytotoxic. Examples include Al and Mn. Chelators can act as chemical ploughs, redistributing surface contamination down the soil profile.There is a reduction in the concentration near the soil surface, thereby reducing exposure pathways, but there is little effect on the total amount of contaminant within the profile.
为获得足够的溶解度，螯合剂通常以Na盐形式添加。由此导致的土壤Na浓度升高会抑制植物生长。钠还会引起黏土矿物分散，可能导致优先流增加。大多数螯合剂还会溶解目标金属以外的其他金属，这些金属可能具有植物毒性，例如Al和Mn。螯合剂可起到“化学犁”的作用，将表层污染物沿土壤剖面向下重新分布。这虽然降低了土壤表层附近的浓度，从而减少了暴露途径，但对剖面内污染物总量几乎没有影响。
Chelant-induced phytoextraction may therefore be limited to applications where the connection to receiving waters has been broken, or where leaching is unimportant. In the former case, phytoextraction could be conducted ex situ. Here, the contaminated material would be placed on a liner whereby any leachate could be collected and recycled (KOS and LESTAN 2003). Such systems are already used for soil washing and the recovery of Au from low-grade ore bodies. Plants would aid in metal recovery by concentrating the metal in their biomass. The economic feasibility of lixiviant-induced Au phytoextraction has been demonstrated (ANDERSON et al. 2005), although the focus here is on Au recovery, rather than the cleansing of contaminated soil.
因此，螯合剂诱导的植物提取可能仅限于与受纳水体的连通已被切断或淋溶问题不重要的情形。在前一种情形下，植物提取可以ex situ（异位）进行，即将污染物料置于衬垫上，收集并循环利用所有渗滤液（KOS and LESTAN 2003）。此类系统已用于土壤淋洗以及从低品位矿体中回收Au。植物可通过将金属富集于其生物量中来辅助金属回收。浸出剂诱导的Au植物提取的经济可行性已得到证实（ANDERSON et al. 2005），但其重点在于回收Au，而非净化污染土壤。
2.3 Phytovolatilisation2.3 植物挥发
Phytovolatilisation is a form of phytoextraction where plants transform soil contaminants into volatile compounds that disperse in the atmosphere. Plant-microbial systems have been discovered that volatilise Hg, As and Se (BROOKS 1998). One obvious drawback of phytovolatilisation is that there is no control on the destination of the volatilised elements. For essential trace elements such as Se, however, phytovolatilisation offers the possibility of redistributing this element from areas where Se toxicity exists to downwind areas where there is Se deficiency (ZAYED et al. 2000). Selenium volatilisation using genetically engineered Brassica juncea is one of the few examples of the successful field application of phytoextraction (BAÑUELOS et al. 2005; BAÑUELOS 2006).
植物挥发是植物提取的一种形式，即植物将土壤污染物转化为挥发性化合物并扩散至大气中。已发现可使Hg、As和Se挥发的植物–微生物系统（BROOKS 1998）。植物挥发的一个明显缺点是无法控制挥发元素的去向。然而，对于Se等必需微量元素，植物挥发为将该元素从Se毒害地区重新分配至下风向Se缺乏地区提供了可能（ZAYED et al. 2000）。利用基因工程Brassica juncea进行硒挥发，是植物提取成功应用于田间的少数实例之一（BAÑUELOS et al. 2005; BAÑUELOS 2006）。
2.4 Plant-metal uptake in the field: metal heterogeneity2.4 田间植物金属吸收：金属的非均质性
Most studies on plant-metal used homogeneous growth media in green-house environments. In the metal-contaminated sites where phytoextraction would be applied, the distribution of metals is typically highly heterogeneous both spatially and temporally. Eq. 1 does not account for such heterogeneity. Unlike pot trials, roots in the field may not be in intimate contact with the contaminated material, thus resulting in lower-than-expected metal uptake. Similarly, after successive croppings, the pool of metal that is available for plant uptake decreases – again reducing metal uptake. Areas of high metal contamination, so-called “hot spots”, may inhibit plant growth, rendering phytoextraction ineffective in these zones. Rewriting Eq 1 to incorporate site heterogeneity we get:
大多数植物金属研究在温室环境中使用均质生长介质。而在拟采用植物提取的金属污染场地中，金属分布通常在空间和时间上都高度不均。式1并未考虑这种非均质性。与盆栽试验不同，田间根系可能无法与污染物料紧密接触，从而导致金属吸收低于预期。同样，经过连续多次收获后，可供植物吸收的金属库减少，进一步降低金属吸收。高度金属污染区域，即所谓的“热点”，可能抑制植物生长，使植物提取在这些区域失效。将式1改写以纳入场地非均质性，得到：
where t is the time in years, x is the spatial position (latitude, longitude) Mi(x)max is the maximum initial metal burden (g/ha), Mf is the target metal soil burden (g/ha), P is the crop metal concentration (g/t), and B is the biomass production (t/ha/yr), both of which are a function of the root exposure to bio-available metal E (g/t). E can be calculated thus:
式中，t为时间（年），x为空间位置（纬度、经度），Mi(x)max为最大初始金属负荷（g/ha），Mf为土壤目标金属负荷（g/ha），P为作物金属浓度（g/t），B为生物量产量（t/ha/yr），后两者均为根系对生物有效金属暴露量E（g/t）的函数。E可按下式计算：
where z is depth (m), R is the root fraction (dimensionless) that is in contact with the bioavailable metal, C (g/t), which is a function of M.
式中，z为深度（m），R为与生物有效金属接触的根系比例（无量纲），C（g/t）为M的函数。
Eq. 3 requires a numerical solution because it is non-linear.The times calculated using Eq. 2 will always be longer than those using Eq. 1 because the maximum metal burden Mi(z)max is greater than the average metal burden, and because heterogeneity reduces plant uptake due to the above-mentioned reasons. Eq. 2 is applicable over a wide range of scales. Clearly, the degree of heterogeneity, and therefore cleanup time, will increase as the scale decreases. Setting the scale of heterogeneity is the domain of regulators.This presents difficulties, since it is an “unknown unknown” (RUMSFELD 2002).由于式3是非线性的，需要数值求解。使用式2计算的时间总是长于使用式1计算的时间，这是因为最大金属负荷Mi(z)max大于平均金属负荷，而且由于上述原因，非均质性会降低植物吸收。式2适用于很宽的尺度范围。显然，随着尺度减小，非均质程度以及修复时间都将增加。确定非均质性的尺度属于监管者的职责范围。这存在困难，因为它是一个“未知的未知”（RUMSFELD 2002）。
An important parameter influenciing the uptake of a metal is its bioavailability, C. Of the total metal concentration, only some fraction is available for plant uptake. Therefore, phytoextraction would be limited to this pool if there were no re-supply from less available pools. Such re-supply processes are, by definition, kinetically limited and may be very slow. HAMON and MCLAUGHLIN (1999) introduced the concept of “bioavailable contaminant stripping” for a phytoextraction procedure that aims to keep the bioavailable fraction of soil metals low enough to be harmless.
影响金属吸收的一个重要参数是其生物有效性C。在金属总浓度中，只有一部分可供植物吸收。因此，若没有来自有效性较低的库的补给，植物提取将仅限于这一部分。根据定义，此类补给过程受动力学限制，可能非常缓慢。HAMON和MCLAUGHLIN（1999）提出了“生物有效污染物剥离”的概念，指一种旨在使土壤金属生物有效部分保持在足够低、不致造成危害水平的植物提取方法。
Ploughing may decrease metal heterogeneity. It may also bring metal-contaminated soil at depth to the surface, thus increasing the volume of material that can be treated using phytoextraction. However, ploughing may increase metal mobility by creating dust and enhancing metal solubility due to an increase in organic ligands caused by oxidation of the soil’s organic matter.
翻耕可降低金属的非均质性，还可将深层的金属污染土壤翻至地表，从而增加可采用植物提取处理的物料体积。然而，翻耕可能会产生扬尘，并因土壤有机质氧化导致有机配体增加而提高金属溶解度，从而增加金属的迁移性。
2.5 Other challenges facing successful phytoextraction2.5 植物提取成功实施面临的其他挑战
Eq. 2 calculates the time to cleanse a site of only one metal. However, most contaminated sites contain more than one pollutant. Few plant species can extract high concentrations of more than one element. Consequently, specific crops may have to be grown sequentially to remove several contaminants. Additional contaminants may further reduce plant growth. Therefore, the time to cleanse sites with a suite of contaminants will be longer than that required for the removal of a single metal.式2计算的是去除场地中单一金属所需的时间。然而，大多数污染场地含有不止一种污染物。能够提取高浓度多种元素的植物物种很少。因此，可能需要依次种植特定作物以去除多种污染物。额外的污染物可能进一步抑制植物生长。因此，净化含多种污染物场地所需的时间将长于去除单一金属所需的时间。
The practical implementation of phytoextraction in the field presents additional challenges. At present, there are no commercial providers of seeds or seedlings of hyperaccumulator species. The growth of these crops may be reduced when they are cultivated outside their normal range. Plant-metal uptake may provide an additional exposure pathway into food chains if local herbivores consume these plants.
植物提取在田间的实际应用还面临其他挑战。目前，尚无超富集物种种子或幼苗的商业供应商。这些作物在其自然分布范围以外栽培时，生长可能会减弱。如果当地食草动物取食这些植物，植物对金属的吸收可能会成为进入食物链的额外暴露途径。
Phytoextraction requires ongoing site management and the processing and storage of the metal-rich biomass. Burning may reduce the volume of the biomass, however, specialised incineration facilities may be required to prevent metal-loss in the smoke (KELLER et al. 2005).
植物提取需要持续的场地管理以及对富含金属生物量的处理和贮存。焚烧可减小生物量体积，但可能需要专门的焚烧设施以防止金属随烟气散失（KELLER et al. 2005）。
Environmental regulator’s limited acceptance of phytoextraction as an effective land treatment option hinders its commercial application. Regulators may be more willing to accept a long-term cleanup operation if it were demonstrated that the environmental risk is minimal during the phytoextraction because of the stabilising action of plants on soil. Therefore, use of chelators that significantly increases the risk of contaminant leaching will do little to enhance the ability of phytoextraction to meet the demands of current environmental legislation.
环境监管者对植物提取作为有效土地治理方案的接受程度有限，阻碍了其商业应用。如果能够证明由于植物对土壤的稳定作用，植物提取期间的环境风险极小，监管者可能更愿意接受长期的修复作业。因此，使用会显著增加污染物淋溶风险的螯合剂，对于提升植物提取满足现行环境法规要求的能力几乎无益。
2.6 Outlook for metal phytoextraction2.6 金属植物提取的前景
High biomass plants can be genetically altered to extract larger amounts of metal from soils (RUGH et al. 1998). Similarly, the potential biomass of smaller varieties of hyperaccumulator plants is being improved (OW et al. 1998). DHANKHER et al. (2002) engineered Arabidopsis thaliana to accumulate As by inserting two bacterial genes that imparted tolerance and the ability to translocate As to the aerial portions. The soil’s microbiota plays a crucial role in plant – metal tolerance and uptake (WHITING et al. 2001). Engineering the rhizobiota could enhance plant uptake (NIE et al. 2002).
可对高生物量植物进行遗传改良，使其从土壤中提取更多金属（RUGH et al. 1998）。同样，较小型超富集植物品种的潜在生物量也在不断提高（OW et al. 1998）。DHANKHER et al.（2002）通过插入两个赋予耐受性及将As转运至地上部能力的细菌基因，改造Arabidopsis thaliana使其积累As。土壤微生物群在植物的金属耐受性和吸收中起着关键作用（WHITING et al. 2001）。对根际生物群进行工程改造可增强植物吸收（NIE et al. 2002）。
In its pure form, metal phytoextraction is only potentially applicable to a few sites because of the high costs associated with the length of time required for remediation, which may be several decades. This Achilles’ heel may be circumvented if phytoextraction is combined with a profit making operation that is unaffected by any elevated plant-metal loadings. NICKS and CHAMBERS (1994) demonstrated that hyperaccumulator plants could generate revenue by extracting saleable heavy metals from otherwise sub-economic ore bodies, a technology termed phytomining. An American company, Viridian Environmental, subsequently patented the phytomining process. (US patent Nos 5711784 and 5944872). Other such revenue-generating operations may include forestry (PULFORD et al. 1995) and bioenergy production. Recent concern over global warming due to CO2 emissions may provide economic incentives to produce plant-based fuels because such systems do not result in net CO2 production. As bio-fuels are not consumed, elevated metal concentrations in such fuel-crops are of lower concern than they would be in food crops. Bio-fuels production may be an effective way for the land to be cleansed while providing a positive economic return, thus rendering the clean up time less important.
就其单纯形式而言，由于修复所需时间较长（可能长达数十年）带来的高昂成本，金属植物提取仅可能适用于少数场地。如果将植物提取与不受植物金属含量升高影响的营利性经营相结合，则可规避这一致命弱点。NICKS和CHAMBERS（1994）证明，超富集植物可从原本不具经济开采价值的矿体中提取可销售的重金属而创造收益，这一技术被称为植物采矿。美国公司Viridian Environmental随后为植物采矿工艺申请了专利（美国专利号5711784和5944872）。其他此类创收经营可能包括林业（PULFORD et al. 1995）和生物能源生产。近来对CO2排放导致全球变暖的关注，可能为生产植物基燃料提供经济激励，因为此类系统不会产生CO2净排放。由于生物燃料不被食用，此类燃料作物中金属浓度升高的问题远不如粮食作物中那样令人担忧。生物燃料生产可能是一种既能净化土地又能带来正经济回报的有效途径，从而使修复时间变得不那么重要。
3Phytoremediation to limit metal fluxes利用植物修复限制金属通量
Immobile metals in soil pose little risk to humans unless they are consumed directly via soil ingestion. Soil organisms such as earthworms notwithstanding, immobile metals have limited negative effects on ecosystems. Regulators are now recognising the influence of metal solubility and mobility on environmental risk. Consequently, there is an increasing adoption of a risk-based approach when assessing soil quality (SWARTJES 1999; FERNÁNDEZ et al. 2005). Such risk-based regulatory systems are based on the effect of the contaminant, rather than on its total concentration in the soil. Here we discuss how phytostabilisation aims to limit metal fluxes and thus may result in site remediation under risk-based regulatory regimes.
除非通过摄入土壤而被直接摄取，否则土壤中不可移动的金属对人类的风险很小。除蚯蚓等土壤生物外，不可移动的金属对生态系统的负面影响有限。监管者目前已开始认识到金属溶解度和迁移性对环境风险的影响。因此，在评估土壤质量时越来越多地采用基于风险的方法（SWARTJES 1999; FERNÁNDEZ et al. 2005）。此类基于风险的监管体系依据的是污染物的效应，而非其在土壤中的总浓度。本文讨论植物稳定如何旨在限制金属通量，从而在基于风险的监管体制下实现场地修复。
In phytostabilisation transpiration and root growth immobilise contaminants by reducing leaching, controlling erosion, creating an aerobic environment in the root-zone, and adding organic matter to the substrate that binds the contaminant. Phytostabilisation involves the establishment of vegetation on the contaminated site that is left in perpetuity. Substrate amendments and a succession of plant species may be required to establish the desired climax vegetation. Establishing a healthy substrate microflora, especially mycorrhizal symbiots can greatly enhance phytostabilisation (VOSÁTKA 2001).
在植物稳定中，蒸腾作用和根系生长通过减少淋溶、控制侵蚀、在根区营造好氧环境以及向基质中添加可结合污染物的有机质来固定污染物。植物稳定需要在污染场地上建立植被并永久保留。可能需要基质改良以及一系列植物物种的演替来建立理想的顶极植被。建立健康的基质微生物区系，尤其是菌根共生体，可大大增强植物稳定效果（VOSÁTKA 2001）。
VANGRONSVELD et al. (1996) detailed how phytostabilisation could control erosion and leaching on metalliferous mine tailings. Establishing vegetation directly on the tailings reduces dust and leaching, enhances public appeal, and is often more cost effective than capping, which could require re-engineering of any tailings dam as well as a large earth-moving operation. DIX et al. (1997) described how phytostabilisation could prevent contaminants leaching to groundwater or local waterways, thus mitigating some of the negative environmental effects associated with tip-sites, land effluent disposal and intensive farming. Phreatophytic trees such as poplars and willows are particularly suited to this role (FERRO etal. 1997). Deep-rooting, high water-use, evergreen trees can be used to lower a saline water table thus reducing salt toxicity to crops, a technology that has been demonstrated to be effective on some Australian soils (BELL 1999).
VANGRONSVELD et al.（1996）详细阐述了植物稳定如何控制含金属矿山尾矿的侵蚀和淋溶。直接在尾矿上建立植被可减少扬尘和淋溶，提升公众认可度，且通常比覆盖封顶更具成本效益，后者可能需要对尾矿坝进行重新改造并开展大规模土方作业。DIX et al.（1997）阐述了植物稳定如何防止污染物淋溶至地下水或当地水道，从而减轻与垃圾场、废水土地处置及集约化农业相关的部分负面环境影响。杨树和柳树等潜水植物（地下水湿生植物）特别适合发挥这一作用（FERRO etal. 1997）。深根、高耗水的常绿树种可用于降低含盐地下水位，从而减轻盐分对作物的毒害，该技术已在一些澳大利亚土壤上被证明有效（BELL 1999）。
3.1 Metal phytostabilisation by hydraulic control3.1 通过水力控制实现金属植物稳定
Plants require water for growth.Transpiration cools the plant and translocates essential, and nonessential, elements to the aboveground portions. Solar radiation drives plant growth and water use, and the climate sets an upper limit on evapo-transpiration (ET), defined as the volume of water transpired plus rainfall that re-evaporates from the plant’s surface. Biological and soil variables determine the actual evapotranspiration of various vegetation types, which may be much less than the theoretical upper limit. In many climates, annual evapotranspiration is greater from fast-growing deep-rooted trees than from shallow rooted herbs or grasses (VOGELER et al. 2001). During periods of drought, deep-rooted species have greater access to water and continue to transpire after drought renders shallow-rooted species dormant. Tree canopies act as umbrellas. In addition to intercepting precipitation, canopies also reduce evaporation from below and thus keep the forest floor moist. Depending on climatic conditions, more than 15% of rainfall may evaporate before it reaches the ground (MCNAUGHTON and JARVIS, 1983).
植物生长需要水分。蒸腾作用可为植物降温，并将必需元素和非必需元素转运至地上部。太阳辐射驱动植物生长和水分利用，而气候设定了蒸散（ET）的上限。蒸散定义为蒸腾的水量加上从植物表面重新蒸发的降雨量。生物和土壤变量决定了各类植被的实际蒸散量，其可能远低于理论上限。在许多气候条件下，速生深根乔木的年蒸散量大于浅根草本或禾草（VOGELER et al. 2001）。在干旱期间，深根物种能获取更多水分，并在干旱使浅根物种进入休眠后继续蒸腾。树冠起着伞的作用：除截留降水外，树冠还能减少其下方的蒸发，从而保持林地地表湿润。视气候条件而定，超过15%的降雨可能在到达地面之前即已蒸发（MCNAUGHTON and JARVIS, 1983）。
3.2 Plant borne organic matter affecting metal flux3.2 植物来源有机质对金属通量的影响
Vegetation provides a continual source of organic matter in soil via plant exudates and the decomposition of litter. Metal mobility is affected by interactions between metals, mineral surfaces and ligands in solution.The effect of organic ligands on metal adsorption is dependent on the properties of both the ligand and the soil (BENJAMIN and LECKIE 1981). On one hand, organic ligands in solution compete with the surface functional groups for metal complexation and thus decrease the amount of metal adsorbed on surfaces. Dissolved Organic Carbon (DOC) is a main factor in mobilising metals, such as Cd, Cu, Pb and Zn in soils (TIPPING 2002). On the other hand, enhanced metal adsorption can occur in the presence of organic ligands by the formation of ternary surface complexes (surface-ligand-metal or surface-metal-ligand). Adding insoluble organic matter into soil can immobilise metals by increasing the amount of available binding sites.
植被通过植物分泌物和凋落物分解为土壤持续提供有机质来源。金属迁移性受金属、矿物表面和溶液中配体之间相互作用的影响。有机配体对金属吸附的影响取决于配体和土壤两者的性质（BENJAMIN and LECKIE 1981）。一方面，溶液中的有机配体与表面官能团竞争络合金属，从而减少吸附在表面上的金属量。溶解性有机碳（DOC）是土壤中Cd、Cu、Pb和Zn等金属活化的主要因素（TIPPING 2002）。另一方面，在有机配体存在时，可通过形成三元表面络合物（表面–配体–金属或表面–金属–配体）而增强金属吸附。向土壤中添加不溶性有机质可通过增加可用结合位点的数量来固定金属。
Only few studies have investigated the influence of plants on metal leaching from contaminated soils under controlled conditions. BANKS et al. (1994) found that the Zn leaching from a mine-tailing contaminated soil increased in the order no plants < plants with microbes < plants without microbes. In a further study using contaminated mine tailings and clean subsoil and topsoil covering, the presence of plants increased Cu and Cd leaching in all columns, while Pb was unaffected (ZHU et al. 1999). TURPEINEN et al. (2000) demonstrated that pine seedlings reduced Pb solubility by up to 93%. RÖMKENS et al. (1999) compared Cu speciation in soils with and without plants. Cu solubility was higher in planted pots, but the calculated free Cu2+ ion concentration was orders of magnitude lower than in soils without plants.
仅有少数研究在受控条件下考察了植物对污染土壤金属淋溶的影响。BANKS et al.（1994）发现，矿山尾矿污染土壤中Zn的淋溶量按无植物 < 有植物有微生物 < 有植物无微生物的顺序增加。在另一项使用污染矿山尾矿并覆盖清洁底土和表土的研究中，植物的存在增加了所有土柱中Cu和Cd的淋溶，而Pb未受影响（ZHU et al. 1999）。TURPEINEN et al.（2000）证明，松树幼苗可使Pb溶解度降低达93%。RÖMKENS et al.（1999）比较了有植物和无植物土壤中Cu的形态。种植植物的盆钵中Cu溶解度较高，但计算得到的游离Cu2+离子浓度比无植物土壤低数个数量级。
3.3 Rhizosphere acidification3.3 根际酸化
Plant roots excrete H+ ions that exchange with nutrient base cations. In addition, many plants exude organic acids that mobilise iron and perhaps other essential metals of low availability.These organic acids solubilise metals by competing for cation binding sites. Root exudations may acidify the rhizosphere by up to 2 pH units (SALISBURY and ROSS 1978). Such acidification invariably increases the solubility of non-essential metal cations such as Cd2+ (NAIDU et al. 1994).
植物根系分泌H+离子，与养分盐基阳离子发生交换。此外，许多植物分泌有机酸，以活化铁以及可能其他有效性较低的必需金属。这些有机酸通过竞争阳离子结合位点而溶解金属。根系分泌物可使根际pH降低多达2个单位（SALISBURY and ROSS 1978）。这种酸化无一例外地会提高Cd2+等非必需金属阳离子的溶解度（NAIDU et al. 1994）。
3.4 Vegetation promoting the growth of soil organisms3.4 植被促进土壤生物生长
The exudation of organic substances by plant roots into the rhizosphere generally promotes the growth of bacteria, fungi and soil-borne animals. Root exudates provide a substrate for growth and roots improve soil aeration by extracting soil moisture and forming continuous channels for drainage and air exchange. However, increased metabolic activity can result in anaerobic conditions if more oxygen is consumed than can be re-supplied. The stimulation of soil biological activity thus affects the speciation, and therefore mobility, of trace elements (PEDERSEN and ALBINSSON 1992). The influence of redox processes on toxic trace metal speciation in soil is particularly striking in the case of chromium. Some soil bacteria reduce Cr(IV) to Cr(III) (PAL and PAUL 2004). When present in the +6 oxidation state, Cr is more mobile, more readily bioaccumulated, and 100 to 1000 times more toxic than when present in the +3 oxidation state (KERNDORFF and SCHNITZER 1980). Motile soil animals such as worms and rotifers that feed on decaying plant matter can also affect the transport and distribution of trace elements in soils.
植物根系向根际分泌有机物质，通常会促进细菌、真菌和土栖动物的生长。根系分泌物为生长提供基质，而根系通过吸收土壤水分并形成连续的排水和气体交换通道来改善土壤通气。然而，如果耗氧量超过可补给量，代谢活动增强可能导致厌氧条件。因此，土壤生物活性的增强会影响微量元素的形态，进而影响其迁移性（PEDERSEN and ALBINSSON 1992）。氧化还原过程对土壤中有毒微量金属形态的影响在铬的情形中尤为显著。某些土壤细菌可将Cr(IV)还原为Cr(III)（PAL and PAUL 2004）。Cr以+6氧化态存在时，比以+3氧化态存在时迁移性更强、更易生物积累，且毒性高100至1000倍（KERNDORFF and SCHNITZER 1980）。以腐烂植物物质为食的蠕虫和轮虫等可移动土壤动物，也会影响土壤中微量元素的迁移和分布。
3.5 Root effects3.5 根系效应
The formation of root channels not only affects water flux via enhanced soil drainage and aeration, but also provides pathways for the rapid transport of solutes and suspended particles and colloids (LESTURGEZ et al. 2004). These transport pathways can exacerbate the risk of groundwater contamination by reducing the contact time of the soil solution with the soil matrix and soil organisms that could otherwise retard the movement by sorption and transformation processes (BUNDT et al. 2000).
根孔的形成不仅通过增强土壤排水和通气影响水通量，还为溶质以及悬浮颗粒和胶体的快速迁移提供了通道（LESTURGEZ et al. 2004）。这些迁移通道缩短了土壤溶液与土壤基质及土壤生物的接触时间（否则后者可通过吸附和转化过程阻滞其迁移），从而可能加剧地下水污染的风险（BUNDT et al. 2000）。
The roots of some tree species avoid metal-contaminated hotspots (DICKINSON et al. 1991; BRECKLE and KAHLE 1992). Conversely, roots of the zinc hyperaccumulator plant Thlaspi caerulescens actively forage zinc-rich hotspots in soils (SCHWARTZ et al. 1999; WHITING et al. 2000). Both types of growth response may profoundly affect metal uptake as well as leaching. However, there is a lacuna of knowledge on the underlying principles of how heterogeneously distributed trace elements influence root growth.
某些树种的根系会避开金属污染热点（DICKINSON et al. 1991; BRECKLE and KAHLE 1992）。相反，锌超富集植物Thlaspi caerulescens的根系会主动觅取土壤中的富锌热点（SCHWARTZ et al. 1999; WHITING et al. 2000）。这两类生长响应都可能深刻影响金属的吸收及淋溶。然而，对于非均匀分布的微量元素如何影响根系生长的基本原理，目前仍缺乏认识。
3.6 Implementing phytostabilisation3.6 植物稳定的实施
As discussed above, biological parameters are of prime importance for metal phytostabilisation. Species should necessarily tolerate local climatic and edaphic conditions. Shallow-rooted turf species control surface erosion and dust; however, most turf species do not remove water from deep within the soil profile. The shallow-rooted nature of many turf species results in greater contaminant leaching than would be the case if trees were planted (VOGELER et al. 2001). Populus spp. and Salix spp. are commonly-used tree species.They are effective because of their rapid establishment, high water-use, tolerance to a wide range of environmental conditions, ease of propagation, and their ability to take up high levels of some contaminants (QUIN et al. 2001).
如上所述，生物学参数对金属植物稳定至关重要。所选物种必须能够耐受当地的气候和土壤条件。浅根草坪物种可控制地表侵蚀和扬尘；但大多数草坪物种不能从土壤剖面深处吸收水分。许多草坪物种的浅根特性导致其污染物淋溶量大于种植乔木的情形（VOGELER et al. 2001）。Populus spp.和Salix spp.是常用树种。它们之所以有效，是因为其建植迅速、耗水量高、能耐受广泛的环境条件、易于繁殖，且能吸收高水平的某些污染物（QUIN et al. 2001）。
Phytoremediation systems that use several species or varieties overcome the risk that a new pest or climatic event destroys all the plants. Phytostabilisation may require plant trials to determine the optimal suite of species, particularly for non-soil media such as sewage sludge or mine tailings. Low-growing species may be combined with deciduous tree species to provide a transpiring green surface during the winter months. Legumes enhance fertility in nitrogen-deficient substrates.
采用多个物种或品种的植物修复系统，可以避免新的病虫害或气候事件毁灭全部植物的风险。植物稳定可能需要开展植物试验以确定最佳物种组合，尤其是对于污水污泥或矿山尾矿等非土壤介质。可将低矮物种与落叶树种相结合，以在冬季提供进行蒸腾的绿色地表。豆科植物可提高缺氮基质的肥力。
Before planting, capping contaminated sites with fertile soil provides a better substrate for plant growth and a buffer zone that stores water from heavy rainfall events. Although more expensive, such capping systems reduce leaching by providing a deeper root zone, thus providing more time for the vegetation to extract and transpire the infiltrated rain water. The cost of earthmoving and reengineering the site offsets the advantages of a soil cap.
在种植之前，用肥沃土壤覆盖污染场地可为植物生长提供更好的基质，并形成可储存强降雨水分的缓冲层。此类覆盖系统虽然成本较高，但通过提供更深的根区而减少淋溶，从而使植被有更多时间吸收并蒸腾入渗的雨水。然而，土方作业和场地改造的成本抵消了土壤覆盖层的优势。
Vegetative caps are porous and leaching will occur in humid climates. Similarly, surface runoff is likely after a bout of high intensity rainfall.Trapping any leachate and circulating it back onto the vegetation will further reduce the metal flux (NIXON et al. 2001). Leachate recirculation can occur ad infinitum: each pass through the root zone further modifies the leachate. An increase in the level of solutes, especially Na+ and Cl-, may be of concern during leachate reapplication. However, depending on the composition of the leachate, reapplication may have beneficial effects on plant growth compared to un-irrigated vegetation (NIXON et al., 2001). Leachate irrigation via overhead sprinklers increases total evaporation, but may negatively affect plant growth if the leachate contains high contaminant concentrations. Surface irrigation may avoid this problem. Drainage is inevitable when rainfall is greater than evapotranspiration. However, vegetative caps may eliminate drainage during low-rainfall periods. Depending on the metal, the high flow rates of receiving waters may dilute any leachate to the point that they do not pose an environmental risk.
植被覆盖层是多孔的，在湿润气候下会发生淋溶。同样，在一场高强度降雨之后很可能产生地表径流。收集渗滤液并将其回灌至植被上，可进一步降低金属通量（NIXON et al. 2001）。渗滤液回灌可以无限循环进行：每次通过根区都会进一步改变渗滤液的性质。在渗滤液回灌过程中，溶质水平（尤其是Na+和Cl-）的升高可能值得关注。然而，视渗滤液组成而定，与未灌溉的植被相比，回灌可能对植物生长产生有益影响（NIXON et al., 2001）。通过顶部喷灌进行渗滤液灌溉可增加总蒸发量，但若渗滤液含有高浓度污染物，则可能对植物生长产生不利影响。地面灌溉可避免这一问题。当降雨量大于蒸散量时，排水不可避免。然而，植被覆盖层可在少雨期消除排水。视金属种类而定，受纳水体的高流量可将渗滤液稀释至不构成环境风险的程度。
3.7 Limitations of phytostabilisation3.7 植物稳定的局限性
Like any remediation technology, phytostabilisation is not suitable for all metal-contaminated sites. Metal toxicity or adverse environmental conditions may prevent plant development. In high-rainfall regions, plant transpiration may not sufficiently reduce drainage from the site. The time required to implement phytostabilisation is dependent on the plant species. In general, it will take 2–4 years with perennial tree species. Most importantly, phytostabilisation requires that the site be permanently vegetated, thus limiting future land use options. Phytostabilisation is therefore more suitable for low value sites, where the land value is small compared to the cost of soil excavation and land-filling.As with phytoextraction, phytostabilisation requires that regulators be convinced of its efficacy. Unlike phytoextraction, providers of phytostabilisation technology can point to numerous examples of its successful application on areas as diverse as acidic mine tailings (BROWN et al. 2005), wood-waste piles (ROBINSON et al. 2003b) and disused sheep-dipping sites (ROBINSON and ANDERSON 2006).
与任何修复技术一样，植物稳定并不适用于所有金属污染场地。金属毒性或不利的环境条件可能阻碍植物发育。在高降雨地区，植物蒸腾可能不足以减少场地的排水。实施植物稳定所需的时间取决于植物物种，一般而言，采用多年生树种需要2–4年。最重要的是，植物稳定要求场地永久保持植被覆盖，从而限制了未来的土地利用选择。因此，植物稳定更适用于低价值场地，即土地价值相对于土壤挖掘和填埋成本较小的场地。与植物提取一样，植物稳定需要使监管者确信其有效性。与植物提取不同的是，植物稳定技术的提供者可以列举大量成功应用实例，涉及的场地类型多种多样，如酸性矿山尾矿（BROWN et al. 2005）、木材废料堆（ROBINSON et al. 2003b）以及废弃的羊只药浴场地（ROBINSON and ANDERSON 2006）。
4Conclusions结论
Unlike other remediation systems such as capping and soil removal, phytoremediation systems are site dependent. It is impractical to conduct long-term field trials to optimise the phytoremediation system for every site.Therefore, whole system models that calculate metal flux are an essential component of phytoremediation. Such models can eliminate unnecessary field trials by revealing where phytoremediation will not meet regulations under a risk-based regime. Conversely, validated models could be used to gain regulatory approval for phytoremediation without the need for lengthy demonstration trials. Site management can also be optimised via modelling.
与覆盖封顶和土壤移除等其他修复系统不同，植物修复系统具有场地依赖性。为每个场地开展长期田间试验以优化植物修复系统是不切实际的。因此，计算金属通量的全系统模型是植物修复的重要组成部分。此类模型可以揭示在基于风险的体制下植物修复在何处无法满足法规要求，从而省去不必要的田间试验。反之，经过验证的模型可用于在无需长期示范试验的情况下获得监管机构对植物修复的批准。场地管理也可以通过建模加以优化。
Although various aspects of vegetation-trace element interactions have been investigated in detail, there is, as yet, no quantitative model that integrates the aforementioned interactions thus calculating the environmental “fate” of trace elements in plant soil systems.This is illustrated by the current body of literature on the role of vegetation for the immobilisation of trace elements on contaminated sites. A wealth of information exists on plant metaltolerance and plant metal-accumulation, whereas the effects of phytostabilisation on the mobility of the trace elements have received only minor attention.
尽管植被与微量元素相互作用的各个方面已得到详细研究，但迄今为止尚无一个整合上述相互作用、从而计算植物–土壤系统中微量元素环境“归趋”的定量模型。关于植被在污染场地微量元素固定中作用的现有文献便说明了这一点：关于植物金属耐受性和植物金属积累的信息非常丰富，而植物稳定对微量元素迁移性的影响仅受到很少关注。
Existing “whole system” models such as Soil Plant Atmosphere System Model (SPASMO), Leaching Estimation and Chemistry Model (LEACHM), HYDRUS – 1D, Water and Agrochemicals in soil, crop and Vadose Environment (WAVE) are designed to calculate leaching of agrichemicals (SHARMAH et al. 2005). These models use the Penman-Monteith equation for evapotranspiration (ALLEN et al. 1998), Richards’ and the Convection – Dispersion Equations for water and solute transport, as well as simplified “tipping bucket” algorithms (GREEN et al. 1999). While such simulation models can be adapted to calculate trace element movement, they do not incorporate specific root-trace element interactions, nor do they calculate the long-term changes in soil induced by vegetation that affect trace element mobility. However, these models provide a frame work of water and solute-transport equations into which chemical and biological interactions could be incorporated.
现有的“全系统”模型，如土壤–植物–大气系统模型（SPASMO）、淋溶估算与化学模型（LEACHM）、HYDRUS – 1D、土壤–作物–包气带环境水分与农用化学品模型（WAVE），均用于计算农用化学品的淋溶（SHARMAH et al. 2005）。这些模型采用Penman-Monteith方程计算蒸散（ALLEN et al. 1998），采用Richards方程和对流–弥散方程计算水分和溶质运移，以及简化的“翻斗”算法（GREEN et al. 1999）。虽然此类模拟模型可经调整用于计算微量元素迁移，但它们既未纳入特定的根系–微量元素相互作用，也未计算植被引起的、影响微量元素迁移性的土壤长期变化。然而，这些模型提供了一个水分和溶质运移方程框架，可在其中纳入化学和生物相互作用。
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Revised version accepted August 10, 2006