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PHYTOREMEDIATION OF SOIL CONTAINING HEAVY METALS AND HYDROCARBONS

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Table of Content Abstract Main features Project objectives / phases Background Technology Results Potential and limitations Information

ABSTRACT

To facilitate site rehabilitation of soils contaminated with heavy metals and hydrocarbons, the Inspec-Sol company worked on an innovative technology: phytoremediation in a northern climate. This technique uses plants to extract soil contaminants from the ground. Once extracted, the metals are concentrated in the plants' roots, stems and leaves. Following greenhouse and on-site trials, Inspec-Sol proved that phytoremediation was possible even in the harsh climates of Quebec and northern regions. Lead, copper and zinc were drawn out of the soil by three species of plants: willow, Indian mustard and fescue. Controlled experiments were carried out to optimize metal extraction by these three plants. Also, the addition of a demineralising agent proved to be beneficial. Experiments showed that the microorganisms in and around the plants' roots also degraded organic compounds. In sum, the results of the study showed that phytoremediation is a promising solution to restore urban brownfields contaminated by both organic and inorganic pollutants.

MAIN FEATURES

Technology

• Use of plants to absorb heavy metals (lead, copper and zinc) and to biodegrade organic compounds present in the soil;
• Use of herbaceous plants (Indian mustard and fescue) and shrubs (willow);
• Optimized extraction with the addition of a demineralising agent (EDTA).

Environment

• Metal concentration levels in the leaves reaching 1 500 to 2 300 ppm;
• Extraction of between 2 and 13 kilograms of metal per hectare, per growth period;
• Decrease in the levels of contaminant migration towards the air, water or the ground;
• Minimal volume of waste needing to be buried at authorized landfills or incinerated.

Economy

• Low decontamination costs;
• Depreciation of rehabilitation costs over several years.

PROJECT OBJECTIVES/ PHASES

The project's aim was to test the effectiveness of phytoremediation in Quebec's climate, on a site contaminated by both heavy metals and hydrocarbons. The project also intended to measure the effect of a demineralising agent (EDTA) on the process' performance. The project consisted of the following phases, which took place over a two-year period:

• Identification and characterization of the experimental site;
• Selection of the most contaminant-absorbent plant species (in the laboratory);
• Assessment of the effectiveness of the phytoremediation process (on-site);
• Performance comparisons of experimental carried out in the presence of a demineralising agent.

BACKGROUND

In 2003, the National Round Table on the Environment and the Economy revealed that there are over 30 000 contaminated sites in Canada that could be rehabilitated. Many of these sites are contaminated by heavy metals. Mining sediment, foundry operations, electroplating, energy generation, production-line cleaning and intensive agriculture may have lead to toxic metals being released into the environment. Today, the number of technologies available to rehabilitate sites contaminated by heavy metals is limited. Among these technologies, those centred on contaminant fixation or those that involve washing or leaching the soil are costly. Furthermore, excavation and burial are not a sustainable solution to the problem.

TECHNOLOGY

To increase the number of options offered to landowners of brownfields contaminated with heavy metals, Inspec-Sol tested a new method: phytoremediation. This approach uses plants to draw out heavy metals from the soil. The plants retain the contaminants in their leaves, stems and roots. To verify that phytoremediation was possible in Quebec's climate, the experts at Inspec-Sol conducted on-site trials on a brownfield in a former railway yard belonging to the city of Montréal. The site was contaminated with both heavy metals and polycyclic aromatic hydrocarbons (PAH). Exhaustive analyses helped to define the mineralogical, granulometric, microbiological and ecotoxicological characteristics of the soil. Laboratory trials were also conducted to identify high-performance plants, taking into consideration the levels of contamination as well as the typical climatic conditions of northern regions. Three species were chosen:

• Salix viminalis (willow): a shrub with important root development, capable of storing a large quantity of metals;
• Brassica juncea (Indian mustard): an herbaceous annual known for its ability to sequester many metals including lead, as well as for its rapid growth (2 to 3 harvests per year);
• Festuca arundinacea (fescue): an herbaceous perennial with a highly developed root system capable of supporting the microbial flora responsible for biodegrading PAH.

RESULTS

Both laboratory and on-site trials demonstrated excellent plant growth. Although the plants were initially chosen for their ability to grow even in Quebec's soil and climatic conditions, their performance in the presence of contaminants was uncertain. In this respect, results were eloquent. Trials showed an important accumulation of metals in the plants' leaves, stems and roots, with a more important accumulation in the roots. In fact, it is difficult for the plant to transport metals to its top. Only zinc proved to be mobile enough to easily reach the stems and the leaves. Further experiments showed that by adding a demineralising agent (EDTA) to the soil, plants could accumulate a larger quantity of metals. Testing also demonstrated that the demineralising agent facilitated the migration of metals towards the top of the plant (stems and leaves).Brassica juncea proved to be the most effective plant in transporting metals to its shoots. Festuca arundenecea was the most efficient plant in holding metals in its roots. In trials, concentrations between 1 500 and 2 300 ppm were measured in the leaves. With these results, between 2 and 13 kilograms of metals would be extracted per hectare of soil, per growth period. Trials also showed that plant roots did not increase the speed at which PAH's were degraded, or the quantity. This can be explained by the presence of an important bioflora in the soil. Therefore, in this particular case, the rhyzosphere did not have a significant influence. After the experiments, plants were disposed of in an authorized landfill.

POTENTIAL AND LIMITATIONS

Potential

Entirely natural, phytoremediation is in direct accordance with principles of sustainable development. This approach encourages reforestation and the growth of local flora that will, in turn, absorb greenhouse gases. The low cost of decontamination is another clear advantage of the method. Phytoremediation offers a worthwhile option for sites that cannot be rehabilitated because of the high cost of the technologies currently available. In addition, phytoremediation is ideal for brownfields that do not require rehabilitation in the short or medium term. Also, the quantity of waste generated by phytoremediation is very small as compared to the quantities produced by other technologies, thus reducing the volume of matter needing to be incinerated or buried. Finally, the use of plants reduces contaminant migration towards the air, water or the ground. Root growth stabilizes the soil and limits the formation of dust clouds, reducing the risk of inhalation or of direct exposure to the contaminant.

Limitations

Like other soil decontamination technologies, phytoremediation has certain limits. Notably, the effectiveness of the process depends on the type of soil as well as on the contaminant itself, its concentration, distribution and bioavailability. Phytoremediation cannot be used for deep contamination. Also, treatment time is long and entails follow-ups. It also requires that plant selection be made according to the types of contaminants present in the soil. In some cases, a demineralising agent may be necessary for metal extraction to occur in plants. We should also pay attention to plant disposal. The concentration of contaminants, the quantity of plants and the cost determine the final mode of elimination to privilege.

INFORMATION

This technology data sheet is based on the results of studies conducted by Inspec-Sol in collaboration with the Montréal Centre for Excellence in Brownfields Rehabilitation (MCEBR). The project received the support of the Institut de recherche en biologie végétale, COREM, and the NRC's Biotechnology Research Institute. Financial support was granted by FPGST (E) of the ministère de l'Environnement du Québec and the MCEBR.

For additional information, contact:

Inspec-Sol Inc.
René Leblanc
4600, chemin Côte-Vertu, Bureau 200
Ville Saint-Laurent (Québec)
H4S 1C7
Telephone: (514)333-5151
Fax: (514) 333-4674
Email: rleblanc@inspecsol.com

Montréal Centre for Excellence in Brownfields Rehabilitation
3705, St. Patrick Street
Montréal, Quebec
H4E 1A1
Phone: (514) 872-4323
Fax: (514) 872-0189
Email: cemrs@bellnet.ca