REPORT#2

Mine-land Restoration: Phytoremediation of Heavy-Metal Contaminated Sites - a critical view

By EcoEng-Correspondent Astrid Kirchner, Austria, and Dr. Robert Nairn, USA

Introduction

After the industrial revolution and the demise of the traditional mining industry in the industrialized countries thereafter, the environmental legacy of this prosperous era became apparent. A heritage of deeply impacted and heavily altered ecosystems has been left behind. Heavy metal contaminations at these sites is of global concern, because it poses a major threat to human and environmental health. In many cases the industries responsible for the damage have abandoned their sites, and money in these regions has become scarce. Remediation efforts therefore need to be effective and affordable (Raskin et. al., 2000; Bradshaw, 1997).

Traditional engineering approaches rely on cost-intensive techniques such as 'excavation and land filling', 'soil washing', 'thermal treatment', and 'electro reclamation' (Vongronsveld et. al., 1998). Phytoremediation might present a viable economical option (Raskin et. al., 2000). However, as an ecological engineering approach, it is rather controversial.

Phytoremediation in general refers to "the use of plants to remediate contaminated soil or groundwater [1]." Many names are known to be used to describe this type of remediation such as 'phytorestoration', 'agronomic stabilization', 'in-place inactivation', 'phytostabilization', 'phytoextraction', 'phytovolatilization', 'phytomining', and 'rhizofiltration' (Vangronsveld et. al., 1998; Raskin et. al., 2000).

Table 1 summarizes semantics and their definitions.

Outcropping metalliferous soils present extreme soil conditions such as inhospitable physical situations, gross lack of nutrients, low pH, and toxicity, respectively depending on the metals present (Bradshaw, 1997). These factors consequently lead to a decline of indigenous flora and fauna, often leaving the terrain biologically barren (Vangronsveld et. al., 1998).

These aspects pose the additional risk of off-site migration of tainted soils due to aeolian and fluvial erosion and leaching of pollutants into the groundwater (Vangronsveld et. al., 1998). Such features must therefore be identified before remediation efforts are undertaken (Bradshaw, 1997).

What is phytoremediation?

Phytoremediation exploits the natural property of metal-tolerant plants (metallophytes) to accumulate metals in their tissue, thereby trans-locating the contaminant from the soils to the roots and above-ground shoots, such as stems and leaves (Ensley, 2000). After a plant cover is established, the contaminated soil is protected from erosion. Furthermore, the plant cover provides an accumulation of organic matter and of nutrients in a bioavailable form (Vangronsveld et. al., 1998; Bradshaw, 1997).

Plants suitable for phytoremediation must possess certain characteristics: tolerance to the prevailing contaminant, a high-biomass production (fast growth with large biomass), ease of handling and established cultural practices (phenotype suitable for easy harvest, treatment and disposal), and the species should preferably be indigenous to the region (Vangronsveld et. al., 1998; Ensley, 2000).

Natural adaptation is a very slow process with which to restore ecosystems, and without human intervention this procedure may take centuries or longer (Vangronsveld et. al., 1998). Natural selection, may encourage the establishments of metal-tolerant plants near or on contaminated soils, though immigration of these species is often slow. According to Bradshaw (1997), "there are genuine difficulties in appropriate species reaching a particular site, especially if they have heavy seeds, unless they already occur in the immediate vicinity." Using this knowledge, selected species are often sown or planted to ensure prompt treatment (Bradshaw, 1997).

Phytoremediation approaches may be enhanced with soil amendments to stabilize and reduce the bioavailability of the contaminants (Vangronsveld et. al., 1998). It is argued by Glass (2000) that phytoremediation is "limited in applicability to surface soils (i.e., perhaps the top 1m of the soil), and would be limited to the solubility or availability of the contaminant (which is especially a problem with metals)."

Metals are immutable, and therefore, plants may less commonly assist in altering their chemical form. Nevertheless, microorganisms, living in the rhizosphere of these plants may play a significant role (Vangronsveld, 1998). According to Daniel van der Lelie (1998) "many current strategies do not take into account a possible role of plant-associated bacteria in phytoremediation processes." These metabolic capacities or resistance of bacteria are presenting a new asset of phytoremediation and may help to develop new phytoremediation strategies (van der Lelie, 1998).

Phytoremediation may be a slow process compared to conventional engineering approaches, but it provides an ecologically and economically sound solution, preventing further alteration of already heavily damaged ecosystems. One of the fundamental premises of plant-based in situ remediation techniques is to reduce the relative bioavailability of heavy metals in soils and establish biological communities, which reduce the mobility of the contaminant (Vangronsveld et. al., 1998). The amount of hazardous materials is significantly reduced, and topsoils are preserved (Ensley, 2000).

Is phytoremediation ecological engineering?

As an ecological engineering application, however, phytoremediation, is rather dubious. The controversy starts with genetically altering hyperaccumulating plants to enhance their capacity for metal uptake. Further debate is triggered by three potential problems, which need to be addressed.

First, metal saturated plants may be eaten by animals and prospectively work their way up in the food chain, presenting the potential to cause harm. Some scientists argue against this, noting that metal contaminated plants are not eaten by insects that would normally be expected to eat the same plant. Still other researchers are cautious that insects might adapt to these conditions by developing a tolerance [3].

Secondly, one has to be aware that metal accumulating plants may release the stored contaminant during the natural process of decomposition if they are not harvested, consequently re-exposing the pollutant.

Thirdly, if the plants are harvested, one has to ask the question 'what is the ultimate fate of these plants?' Literature indicates that the harvested plants may be "composted, land-filled, incinerated, or extracted to recover economically important metals" (Vangronsveld et. al., 1998).

However, the author believes that composting and land-filling merely relocate the problem and would ultimately lead to the second issue addressed. Incineration, depending on the metals, may pose the risk of releasing contaminants into the atmosphere if no filtering techniques are employed. In the author's opinion, extracting economically lucrative metals presents the best solution. Unfortunately, such extraction processes themselves may not be reconcilable with fundamental ecological engineering principles.

Case-Study [4]

Uranium contamination poses a serious risk to human health and the environment. Remediation efforts using phytoremediation-techniques are presenting a viable option. According to Edenspace, "certain sunflower plants were found to have a high affinity for uranium," and are, therefore, commonly used.

In Ashtabula, Ohio, a pilot scale rhizofiltration system was built for final treatment of onsite contamination at a former uranium processing facility. The uranium concentration of the contaminated groundwater ranged from 21-874 m g/L. After the treatment the effluent contamination could be reduced to < 20 m g/L.

Literature indicates that the reduction was prompt: "within 10 hours after the groundwater flowed into phytofiltration system, the uranium concentration in the contaminated water was reduced below the discharge limit (20 m g/L) for uranium." For 8 weeks the pilot plant was operated for an average influent uranium concentration of 200 m g/L. During this time period the discharge limit was never exceeded.

Conclusion

Even though many questions about phytoremediation remain unsolved, it does present a lot of advantages over conventional techniques in restoring a heavy-metal contaminated site in an ecologically and economically sustainable manner. The question, whether phytoremediation may be considered an application of ecological engineering principles or not, is one the author doesn't feel competent enough to answer.

The following table briefly summarizes the advantages, disadvantages and concerns of phytoremediation.

Bibliography

• Bradshaw Anthony: "Restoration of mined lands -using natural processes" in 'Ecological Engineering' 8 (1997); 255-269; Elsevier Science
• Ensley Burt D: "Rationale for the Use of Phytoremediation" in 'Phytoremediation of Toxic Metals — Using Plants to Clean Up the Environment' (2000); John Wiley & Sons;
• Glass David J.: "Economic Potential of Phytoremediation" in 'Phytoremediation of Toxic Metals — Using Plants to Clean Up the Environment' (2000); John Wiley & Sons
• Raskin Ilya, Ensley Burt D.: "Preface" in 'Phytoremediation of Toxic Metals — Using Plants to Clean Up the Environment' (2000); John Wiley & Sons,
• Van der Lelie Daniel: "Biological Interactions: The Role of Soil Bacteria in the Bioremediation of Heavy Metal-Polluted Soils" in 'Metal-Contaminated Soils — In Situ Inactivation and Phytorestoration' (1998); Springer
• Vangronsveld Jons, Cunningham Scott D.: "Introduction to the Concepts" in 'Metal- Contaminated Soils — In Situ Inactivation and Phytorestoration' (1998); Springer

Footnotes

• [1] : http://members.tripod.com/~biremediation/index.html
• [2]: all definitions taken from the website: http://members.tripod.com/~biremediation/index.html
• [3]: http://ehpnet1.niehs.nih.gov/docs/1995/103-12/innovations.hml
• [4]: all information taken from the website: http://www.edenspace.com/CaseStudies.htm