By Tereza Dan, Ph.D.

Environmental pollution with metals has accelerated dramatically since the beginning of the industrial revolution. The build-up of metals in the soil, aqueous waste streams and ground water renders polluted lands unfit for agriculture and human inhabitation. Heavy metals are primarily a concern because they cannot be destroyed by degradation. There are a number of conventional remediation technologies, employed to remediate environmental contamination with heavy metals such as solidification/stabilization, soil flushing, soil washing, excavation, retrieval and off-site disposal. A majority of these technologies, however, are costly to implement and cause further disturbance to the already damaged environment.

     In Canada, the federal government estimates there are more than: 5,000 contaminated sites (owned or managed by the federal government), approximately 10,000 abandoned mines, 90,000 active/abandoned drilling sites in the Prairie provinces, 6,000 abandoned tailing sites, approximately 875 millions of tonnes of radioactive tailings from uranium mines and 29,000 Provincial Brownfield sites. It was estimated that the potential cost to remediate contaminated soils in Canada is $6 to $20 billion (Environment Canada, 1999), while in the United States (U.S.), the projected cost is $187 billion (U.S. Environmental Protection Agency (EPA), 1999). Metals of concern include lead, cadmium, selenium, mercury, arsenic, iron, manganese, nickel, zinc and copper originating from industrial activities and mining. In Ontario, the Ministry of Environment estimates that municipal sewage treatment plants alone release nearly 18 tonnes of organic compounds and 1,100 tonnes of heavy metals in Ontario waterways annually. Often, these polluted lands contain a mixture of numerous organic and inorganic contaminants (for example, biosludge disposal sites of petroleum industries), which makes it impractical to employ any simple remediation approach.

     An alternative “green” solution to this problem is phytoremediation. Phytoremediation can be defined as the process of utilizing plants to absorb, accumulate, detoxify and/or render harmless, contaminants in the growth substrate (soil, water and air) through physical, chemical or biological processes. The concept of using plants to clean contaminated environments is not new. About 300 years ago, plants were first proposed for use in the treatment of wastewater. The idea of using plants to extract metals from contaminated soil was reintroduced in the early 80s, and the first field trial on zinc and cadmium phytoextraction was done in early 90s. Recently, Environment Canada’s Environmental Technology Advancement Directorate released the PHYTOREM data base, an interactive electronic database of more than 700 plants, lichens, algae, fungi and bryophytes, which have demonstrated an ability to tolerate, accumulate or hyperaccumulate a range of 19 different metals. Species that show considerable potential to-date include sunflowers, ragweed, cabbage, Indian mustard, geranium and jack pine. Accompanying this database are 35 different search fields containing additional geographical, regulatory and eco-physiological data on each species. This allows the owners and managers of contaminated sites to choose the species that suit their site conditions and take the steps necessary to secure regulatory approval for their use. An additional database (Phytopet, available in CD format) was developed to describe plant species with a demonstrated ability for tolerance to petroleum hydrocarbons and the capacity to reduce contaminant levels in terrestrial or wetland environments. These plants grow in the Canadian prairies and boreal plains, but may have useful applications in other environments as well. The rigorous seasonal changes of Canada’s climate have forced Canadian species to be highly adaptive. These plants may possess genetic material that supports the development of agricultural crops that can withstand greater temperature ranges. The database also contains botanical surveys of a number of “historical” (weathered) hydrocarbon contaminated (adjacent and non-contaminated) sites in Alberta and Saskatchewan.

     The use of plants for remediating contaminated soils has multifold advantages:

• large scale application
• provide an aesthetic value to the landscape
• plants concentrate the contaminants within their tissues, thereby reducing the amount of hazardous waste; concentrated hazardous waste would require smaller reclamation facilities for extracting the heavy metals
• increased aeration of the soil, which, in turn, enables microbial degradation of organic contaminants and microbe-assisted uptake of metal contaminants.

     It is important to mention that phyto­remediation has several disadvantages:

• growth rate and seasonality will prolong the phytoremediation time of site as compared with other more traditional cleanup technologies
• root system — it is required that the contaminants be in contact with the root zone of the plants so either the plants must be able to extend the roots to the contaminants, or the contaminated media must be moved to within range of the plants’ continuous performance
• regulatory acceptance is currently being developed.

     Although the basic concept of utilizing plants to remediate contaminated sites remains the same, phytoremediation technology can be subdivided into several distinct subtechnologies:

phytoextraction, involves specific plant species that can absorb and accumulate metal contaminants and/or excess nutrients in harvestable root and shoot tissue, from the growth substrate (soil). This approach is suitable to remove most metals (such as lead, cadmium, nickel, copper, chromium and vanadium) and excess nutrients from contaminated soils. Examples of plants species used: the Brassicaceae family of plants: Thlaspi sp., Brassica sp. and Alyssum sp.

rhizofiltration, utilizes plant roots to take up and sequester metal contaminants and or excess nutrients from aqueous growth substrates (waste water streams, nutrient-recycling systems). This approach is suitable for remediating most metals (such as lead, cadmium, nickel, copper, chromium and vanadium), excess nutrients, and radio­nuclides (such as uranium, caesium and strontium) contaminated water. Examples of plant species used are: Helianthus sp., Brassica sp., Populus sp., Lemna sp., and Thlaspi sp.

phytostabilization, involves use of plants, especially roots and/or plant exudates, to stabilize, demobilize and bind the contaminants in the soil matrix, thereby reducing their bioavailability. This approach is suitable for both organic and metal contaminated soils

phytovolatilization, uses the plants’ ability to absorb and subsequently volatilize the contaminant into the atmosphere. This approach is suitable for remediating metals such as mercury and selenium from contaminated soils.

phytodegradation/phytostimulation, utilizes the rhizospheric associations between plants and soil microorganisms to degrade complex organic-metal contaminant mixtures. This approach is suitable for remediating TNT, PAH and petroleum hydrocarbons from contaminated soils. Examples of plant species used are: Medicago sp. and several grasses.

     To date, several success stories have been reported showing the results of phytoremediation in actual field trials. These include sites contam­inated with heavy metals such as lead, zinc and cadmium and nickel (single or in combin­ation) and organic contaminants. One trial saw the use of Brassica juncea by Phytotech, Inc. to remove Pb from the soil on a site in Trenton, NJ, formerly occupied by a battery manufacturer. At project initiation, 40 per cent of the plot exceeded the regulatory limit of 400 mg/kg and seven per cent was higher than 1000 mg/kg of soil. After three harvests, only 28 per cent of the plot exceeded 400 mg/kg and no portion exceeded 1000 mg/kg. The Brassica plants harvested at the site had taken up and accumulated 0.3 per cent lead per weight in their tissues, removing approximately 20 to 30 parts per million of lead from the top 18 inches of soil.

     Phytotech also used Brassica juncea plants to remediate a lead-contaminated site in Bayonne, NJ and a residential site in Dorchester, MA. At the Dorchester site, the soil initially contained hot spots of over 1000 mg/kg. Three six-week growth periods of B. juncea was enough to substantially reduce the lead concentration at these hotspots.

     The project conducted at Pig’s Eye Landfill site in St. Paul, MN, under the supervision of USDA showed that Alpine pennycress (Thlaspi caerulescens) was the best plant species for taking in cadmium, zinc and lead from the contaminated soil. Pennycress accumulated up to 30,000 ppm of zinc in its leaves without any sign of damage to the plant or yield reduction. Most plants show signs of zinc toxicity when they accumulate more than 500 ppm zinc in their tissues. Pennycress was shown to take in zinc at the rate of 125 kg per hectare (kg/ha) per year (108 pounds/acre) and cadmium at the rate of 2 kg/ha per year (1.7 lbs./acre) when the plants were provided with optimum growth conditions.

     Initial field studies on the ability of plants to remove Cs-137 from contaminated soils are presently under way at Brookhaven National Lab, NJ and in Ashtabula, OH. Preliminary studies in pots revealed that certain species of the mustard (Brassicaceae) and amaranth (Amaranthaceae) families were able to accumulate three times more cesium-137 in leaf and stem biomass than the concentration in the soil. In field trials, redroot pig­weed (A. retroflexus L.) plants were able to accumulate over 900 pCi/gm, which was well over the goal of 300 pCi/gm. In these trials, pigweed showed better performance than B. juncea and Phaseolus acutifolius. Field trials on cesium-137 and strontium-90 contaminated soils are currently under way at the Idaho National Engineering Laboratory.

     In a field trial on surface water from the Cher­nobyl nuclear disaster in the Ukraine, contaminated with cesium-137 and strontium-90, Phytotech used sunflower plants grown on rafts in a pond contaminated with the radionuclides. The plants dramatically reduced the levels of radionuclides in the water during the four- to eight-week exper­imental period. After the first 12 days, sunflower roots had accumulated cesium and strontium at concentrations 8,000 and 2,000 times the concentration of cesium and strontium present in the ground­water. As a growing technology, the major requirement for the success of phytoremediation technology, is the need to understand that phyto­remediation requires a multidisciplinary approach, that relies on the involvement of various specialists (agronomists, environmental scientists and engineers) who understand the site-specific problems of contamination. This will allow the design of a distinct phytoremediation system that will answer the site-specific requirements and will assure an effective remediation. The emerging field of phytoremediation offers us a low-cost alternative to conventional remediation technologies for controlling the persistent global problem of environmental pollution.

Tereza Dan, Ph.D. is an Environmental Scientist with Jacques Whitford Environment Limited. Her expertise is in Ecological Engineering.