Techniques

Soil contamination occurs when either a solid or liquid substance mixes with the soil and becomes physically or chemically attached to soil particles or trapped in the spaces between them. This can result in an actual or calculated threat to human health or the environment. In sufficient quantities, chemicals become soil contaminants. Thus, they can harm humans, plants and animals, and they can leach contaminants into the ground water at unacceptable levels, and cause unacceptable degradation of the soil resources. In remediation practice, however, decision makers often face an intriguing question: "How clean is clean?". This frequently turns out to be difficult to answer (Belluck and al. 2006).

From a regulatory or clean-up perspective, a contamination is often defined as a concentration exceeding a particular value to which a level of risk has been assigned. Because risk varies with each metal and the associated exposure pathways, the definition of the degree of contamination is specific for each contaminant. Regulatory limits also differ depending on site-specific factors and specific land-use restrictions. In addition, the regulatory limits for metal concentrations in soil vary considerably according to State and even according to site (Raskin and Ensley 2000) and they frequently do not address categories that would be of a comparable level, although the limiting concentrations may be of the same order of magnitude. For example, there are criteria that have been defined for residential direct-contact soil clean-up according to the New Jersey Department of Environmental Protection and Risk. These are in the range of critical concentrations that are defined by the government of Slovenia as those at which the damaging effects on human health and the environment makes the polluted soil not suitable for the raising of plants intended for human or animal consumption (Table 1). These definitions have however largely differing consequences for phytoremediation management practices and local residents.

Once it has been determined that a soil is unacceptably contaminated, the necessary clean-up can be achieved through: (i) soil excavation, treatment and/or disposal; (ii) in-situ soil treatment; (iii) soil containment to prevent soil contaminant movement; or (iv) limiting exposure to the contaminated soil. In the past, soil remediation was primarily carried out by the physical removal of soils from contaminated sites, which went for landfilling, incineration or chemical stripping of the contaminants from soil that results in soils sufficiently clean to leave on site.

Table 1. Soil clean-up criteria established by the New Jersey Department of Environmental Protection, with risk concentrations for the metal limits for dangerous substances in the soil defined in R. Slovenia (NJDEP, 1996, from Raskin andEnsley 2000; OJ. RS, No. 68/96)

Clean-up criteria Risk concentrations

Clean-up criteria Risk concentrations

Residential

Non-residential

:Warning

2Critical

Cd

1

100

2

12

Cu

600

600

100

300

Ni

250

2.400

70

210

Pb

400

600

100

530

Zn

1.500

1.500

300

720

Cr

n.d.

n.d

150

380

Hg

n.d.

n.d

2

10

Co

n.d

n.d

50

240

Mo

n.d

n.d

40

200

As

20

20

30

55

1 The concentration that indicates the probability of damaging effects on human health or the environment for certain types of soil use.

The concentration at which due to damaging effects on human health or the environment the polluted soil is not suitable for the raising of plants intended for human or animal consumption or for the retaining or filtering of water. nd: no data.

1 The concentration that indicates the probability of damaging effects on human health or the environment for certain types of soil use.

The concentration at which due to damaging effects on human health or the environment the polluted soil is not suitable for the raising of plants intended for human or animal consumption or for the retaining or filtering of water. nd: no data.

However, due to significant damage having frequently been caused to a site, an evaluation of alternative methods that were less invasive but that provided similar clean-up results was required. With the development of phytoremediation technologies, site clean-up managers have new options that can allow for site clean-up without necessarily disrupting soil profiles and function (Belluck and al. 2006). Phytoremediation is defined as the use of vegetation for the in-situ treatment of contaminated soils, sediments and water. It is best applied to sites with a shallow contamination of metals, nutrients or organic pollutants that are amenable to one of its applications: rhizofiltration, bioremediation of the rhizosphere, phytotransformation, phytoextraction, and in extreme cases phytomining or phytostabilisation (Figure 3). In addition to providing very competitive remediation technologies when it comes to costs, these are aesthetically pleasing and have a high public acceptability (GWRTAC 1997; Salt al. and 1998; Raskin and Ensley 2000; Brooks 2000; Khan and al. 2004).

Rhizofiltration refers to the use of plant roots to absorb, concentrate and precipitate the metal contaminants in surface or groundwater (Table 2). The roots of these plants can absorb large quantities of lead and chromium from soil water or water that is passed through the root zone of the densely growing vegetation. It can also be applied to radionuclide contamination, nutrients and organic pollutants (GWRTAC 1997; Raskin and Ensley 2000; Brooks 2000).

Rhizosphere bioremediation increases the soil organic carbon, bacteria and mycorrhizal fungi, all of which are factors that encourage the degradation of organic chemicals in the soil (Table 2).

2. Phytoremedial technologies and their applications for remediation of different types of pollution (adopted from GWRTAC 1997; Salt al. and 1998; Raskin and Ensley 2000; Brooks 2000)

Phyto technology

Rhizofiltration

Rhizo sphere bioremediation

Phytotransformation

Phytoextraction

Phytomining

Phytostabilisation

Remediation of

water

V

n.a.

V

n.a.

n.a.

soil

n.a.

V

V

V

V

V

sediment

V

V

V

V

V

Pollutants

metals

V

V

V

V

V

V

organics

V

V

V

V

radionuclids

V

V

V

Plants

herbs

V

V

V

V

V

grasses

V

V

V

V

V

woody plants

V

V

V

V

aquatic plants

V

V

V

n.a.

n.a.

hyperaccumulating

V

V

V

V

V

n.a.: not applicable.

n.a.: not applicable.

Figure 3. A schematic representation of phytoremedial technologies and their applications in relation to the levels of pollution.

pollution

Figure 3. A schematic representation of phytoremedial technologies and their applications in relation to the levels of pollution.

The role of plants is either direct, with their exudates helping, stimulating or degrading the soil contaminants enzymatically, or indirectly through sustaining the growth of other soil microbes via the leaking of up to 20% of photosynthates and/or the aerating of the rhizosphere (GWRTAC 1997; Salt al. and 1998; Khan 2005).

Phytotransformation refers to the uptake of organic and nutrient contaminants from the soil and groundwater and their subsequent transformation by the plants (Table 2). For its environmental application, it is vitally important that the transformed metabolites that accumulate in the vegetation are non-toxic, or at least significantly less toxic than the parent compound. This also includes phytovolatilization, whereby volatile chemicals or their metabolic products are released into the atmosphere through plant transpiration (GWRTAC 1997; Salt al. and 1998; Brooks 2000).

Phytoextraction refers to the use of metal-accumulating plants that translocate and concentrate the metals from the soil into their roots and above-ground parts (Table 2). Chelate-assisted phytoextraction, in particular, is more developed than continuous phytoextraction, and it is being implemented commercially. This can be applied to metal and radionuclide contamination, including sites with mixed wastes. An important issue in phytoextraction is whether the metals can be economically recovered from the plant tissue, thus becoming the process of phytomining (Table 2), or whether disposal of the waste is required. As a general rule, the readily bioavailable metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium and copper, with those moderately bioavailable being cobalt, manganese and iron; lead, chromium and uranium are not very bioavailable (GWRTAC 1997; Salt al. and 1998; Raskin and Ensley 2000; Brooks 2000; Ernst 2005).

Phytostabilization aims to immobilize the toxic contaminants in soils and sediments by vegetation, therefore preventing the spread of contamination by windblown dust, which represents an important pathway for human exposure to several hazardous contaminants (Table 2). This also contributes to the hydraulic control of water due to transpiration, thus preventing the leaching of contaminants into the groundwater. It can also be applied to low-level radionuclide contaminants, and especially where the half-lives are not too long. In the case of contamination with pollutants that are easily translocated to the shoots, like cadmium, care needs to be taken in the assessment of the risk to the food chain. Phytostabilization is especially applicable to sites where the contaminants are below the levels for regulatory action, and to severe metal contamination of sites, where the removal or treatment are not practical (GWRTAC 1997; Raskin and Ensley 2000; Brooks 2000) and where the potential of other phytoremediation technologies appears less promising.

The main constraints of phytoremediation include the difficulties involved with treating wastes greater than three metres in depth, the long periods of time required to remediate below action levels, the difficulties in establishing plant cover due to high toxicity at sites, and the possible migration of contaminants off-site due to their spread with the plant material or water flow. In addition, as in any other treatment systems, the maintenance including irrigation and agronomic inputs are required. The technology is currently being tested at several sites and it remains to be seen if it is effective at full scale and whether it will become a major new technology in the future (GWRTAC 1997; Raskin and Ensley 2000; Brooks 2000).

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