Phytoremediation: History
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Sang-Hwan Lee
,
Hyun Park
,
Jeong-Gyu Kim

Phytoremediation is defined as the use of plant species as a means of purifying polluted soil, water, and air. The term derives from the Greek word phyto (“plant”) and the Latin word remedium (“to correct or remove an evil”). Phytoremediation is a more economically feasible and efficient remediation option than other techniques, such as washing, flushing, or solidification.

  • phytoremediation
  • reclamation
  • mining area

1. Introduction

Mining activity worldwide has led to a large number of polluted and physically damaged sites, many of which have subsequently been successfully reclaimed, restored, and/or remediated to overcome or ameliorate their negative environmental and ecological impacts. Mining adversely impacts vegetation, soil, and microbial communities and has long-term implications for both natural vegetation and future land use. In particular, mining and related activities require the removal of the vegetation cover, which results in a loss of soil, soil microbes, and seed banks and also leads to soil compaction, soil acidification, and a reduction in the soil water-holding capacity. As a result, estimates of the cost of the ecological restoration of abandoned mines must account for the degradation of water and soil quality, the loss of biodiversity, and the risks to human health [1]. This is also true for Korea, where there were 5544 licensed mining sites in 2016, with 701 of these in operation. The closed mines may be long-term sources of environmental pollution, especially when the mining and refining facilities are left to decay, and the mine tailings and waste runoff are uncontrolled [2].
As public awareness of the adverse effects of pollution on ecosystems within mining areas has increased, so too has interest in developing guidelines and techniques to restore ecosystem health [3]. As such, the ecological restoration, reclamation, and/or remediation of mining sites have become important components of sustainable development strategies in many countries. Environmentally friendly management plans and environmental management minimize the impact of mining on the environment and preserve the biodiversity. In Korea, most abandoned mines are located in remote areas, where land prices are very low; thus, innovative, low-cost, and low-input technologies that are accepted by local communities are needed for their restoration and/or reclamation [3].
A number of physiochemical strategies have been developed for the remediation of mining sites. Physical treatment, such as dumping, covering, and solidification, is the simplest and most effective method of ecological restoration for almost all mining areas. It can lead to a rapid improvement in the soil conditions, prevent the leaching of contaminants, and promote plant growth. Some of the most common chemical treatments include leaching/acid extraction and washing to remove contaminants. However, these physicochemical remediation methods are limited by their excessive cost and the fact that the treatment itself may damage the ecosystem quality [4]. For these reasons, the revegetation of mining areas has been identified as an efficient way to reduce environmental risks by stabilizing mining soil. The greening of disturbed ecosystems not only improves their aesthetics but can also restore the site. Indeed, phytoremediation has been demonstrated to improve ecosystem quality by increasing the levels of organic matter, nutrients, and biological activity [5].
Compared to physicochemical processes, plant-based ecological restoration is cost-effective and environmentally friendly [6]. Wan et al. [7] estimated that the cost of plant remediation of soil contaminated with As, Cd, and Pb was USD 75,375 hm−2 (USD 37.7 m−3), which is significantly lower than physicochemical approaches. In addition, while physiochemical techniques irreversibly alter ecological properties, phytoremediation can improve the physical, chemical, and biological quality of contaminated soil in mining areas [8]. Phytoremediation techniques that have been employed in mining areas include plant extraction, plant stabilization, plant evaporation, and root filtration [9][10][11][12].

2. Overview of Phytoremediation

Phytoremediation is defined as the use of plant species as a means of purifying polluted soil, water, and air [13]. The term derives from the Greek word phyto (“plant”) and the Latin word remedium (“to correct or remove an evil”). Phytoremediation is a more economically feasible and efficient remediation option than other techniques, such as washing, flushing, or solidification [13][14].

2.1. Phytoremediation in Mining Areas

The removal of contaminants such as toxic trace elements (TTEs) from polluted sites involves phytoextraction, phytostabilization, and phytovolatilization (Figure 1) [15]. Phytoextraction, in which contaminants are absorbed through the roots, plays a key role in the removal of metals and metalloids from contaminated soils, water, and sediments [13][16]. The absorbed and extracted contaminants are transported from the roots to the aboveground, harvestable parts of the plant, which are then disposed of as hazardous waste or incinerated.
Sustainability 15 02761 g001
Figure 1. Phytoremediation in a mining area.
Plant stabilization (or plant immobilization) refers to the use of plants to reduce the mobility and/or bioavailability of contaminants, thus preventing their entry into the surrounding environment or food chain. Contaminants in the root zone are fixed within the plant rhizosphere by adsorption or precipitation. The low mobility of the contaminants trapped in the plant rhizosphere means that they cannot easily leach into groundwater or spread to agricultural land, preventing their entry into the food chain [13][17][18]. However, the introduction and initial growth of plants can be limited by the physicochemical characteristics of mine waste, including extreme pH levels, high salinity, low water retention, high pollutant concentrations, and a lack of soil organic matter and fertility. Therefore, improvements in the physicochemical and biological properties of mine waste-contaminated soil [19][20], such as organic and/or inorganic amendments [21][22][23], are often required before plants can be introduced. In this aided phytostabilization, the main purpose is not to remove contaminants but to reduce the risk that they pose by reducing their mobility and biological effectiveness [21][23]. In phytovolatilization, soluble pollutants are taken up together with water and released into the atmosphere via stomatal diffusion, which is accompanied in some cases by evaporation into the atmosphere via the leaves [24][25]. TTEs, however, are not completely removed from the atmosphere; rather, they move from one system to another. Phytovolatilization has been applied to sites contaminated with metalloids, such as Hg and Se [13][17][18][26].

2.2. Suitable Plants for Phytoremediation in Mining Areas

Plant screening is required for successful phytoremediation. In general, species native to the region are preferred for ecosystem restoration or remediation due to concerns about the potential harm caused by the invasion of alien species, including a reduction in local plant diversity (and thus biodiversity), which can disturb the ecosystem [25]. High remediation efficiency and the successful establishment of stable vegetation cover also require plants that are well adapted to the local environment and require little management (e.g., with fertilizers). Since the availability of nutrients in polluted soil is generally low [27], the use of nitrogen-fixing legumes has been widely examined [28][29][30]. Other factors that need to be considered in plant screening include the root structure, the properties of the contaminants and soil, and the climatic conditions [31]. Plant root length increases from grasses to shrubs and from shrubs to trees, with deeper roots allowing for the removal of metal complexes bound deep in the soil. However, grass is the most widely used plant in soil remediation because of its high biomass, rapid growth, high tolerance, and versatility [32]. Plants with well-developed roots reduce soil erosion, generally remain well-confined, and reduce air pollution by creating vegetation cover [33][34]. In the field, a multi-faceted approach using grasses, shrubs, legumes, perennial grasses, and other long-living trees is likely to be more effective than employing a monoculture.
Hyperaccumulators are plants that are able to accumulate relatively large amounts of pollutants, such as 100 mg cadmium kg–1; 1000 mg Cu, Pb, or As kg–1; and 10,000 mg Zn kg–1 [35][36]. The most effective species for phytostabilization are those that prevent contaminants from entering the food chain or other environmental media by accumulating them underground. Plant species that have been frequently used in phytostabilization include Agrostis spp. and Festuca spp. [23][37]. Since plant growth can actually promote metal leaching due to soil acidification and the production of dissolved organic matter, plant species that cause low soil acidification and that do not translocate metals to their leaves in high quantities are most suitable for phytostabilization [25].

This entry is adapted from the peer-reviewed paper 10.3390/su15032761

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