Plants Clean Up Contaminated Sites

Abandoned mines, industrial dumps and other contaminated sites may get a new lease on life thanks to an innovative form of bioremediation that uses plants to remove toxic substances from soil, sediment and ground and surface water. Known as phytoremediation, this solar-driven technology is proving to be less invasive, less costly and less energy-intensive than existing physical, chemical and thermal remediation methods.

Phytoremediation works on the principle that plants naturally take up trace elements and other substances from soil and water and either store, accumulate, metabolize, or volatilize them to the atmosphere. Plant roots also foster the growth of micro-organisms that biodegrade organic contaminants, and many plants produce and release substances that help degrade organic pollutants in the root zone. Unlike traditional remediation methods, which often destroy organic material and leave soil inert, planting terrestrial and aquatic species can target specific contaminants and, in the process, restore the site to some level of ecosystem health and productivity.

Canada is taking the lead globally in generating a comprehensive database on the use of this technology to stabilize, remove and recover heavy metals such as lead, mercury, iron, manganese, zinc and copper from contaminated ecosystems. This March, two years of intensive study by scientists in Environment Canada's Environmental Technology Advancement Directorate culminated in the release of PHYTOREM--an interactive electronic database of more than 700 plants, lichens, algae, fungi and bryophytes that 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 will make it possible for 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.

Plants are effective at removing metals because, like humans, they require certain trace elements to survive. Some species, known as hyperaccumulators, can accumulate and tolerate very high concentrations of metals--some up to five per cent of their dry weight. This trait, combined with size and a speedy growth rate, determine whether or not a certain species is a cost-effective candidate for phytoremediation. Another advantage of phytoremediation is that once a crop has accumulated its optimum metal content it may be possible to harvest the crop and recover the valuable metal components from the contaminated biomass. The department's scientists are experimenting with a variety of techniques, including drying, composting, compacting, leaching, and high-temperature decomposition.

Graph showing removal of arsenic from contaminated surface water (1) as it passes through a constructed wetland with four "cells" containing various species of aquatic plants (2), trees (3), grasses (4) and cattails (5). The four lines represent tests carried out during early and late August, September and October 1998.

In response to the considerable interest industries and other stakeholders have expressed in this technology, Environment Canada has collaborated on a number of demonstration projects designed to test the viability of phytoremediation in controlled and experimental field plots. In British Columbia, scientists have created a series of hydroponic gardens to determine the effectiveness of various aquatic plants in removing zinc, cadmium, and arsenic at the COMINCO mine in Trail. Results from a pilot project at Île-aux-Corbeaux in the St. Lawrence River, where the shoreline was heavily contaminated with zinc and manganese from buried batteries, showed significant accumulations of these metals in American eelgrass and hornwort planted offshore after just over a month of growth.

In addition to testing the effectiveness of candidate species, Environment Canada scientists are also working with counterparts from the National Research Council and the United States Environmental Protection Agency to study biodiversity at metal-contaminated sites across Canada. Their work is aimed at determining whether seed banks can be established from these wild species, and whether they can be successfully propagated in greenhouses. This work could eventually lead to the creation of inventories of candidate cultivars that could be adapted for use at other metal-contaminated sites across Canada.

Scientists are hopeful that the next phase of the project--slated to begin this spring--will involve even more ambitious field trials. In the meantime, they continue to tackle other challenges, such as seeking further clarification on regulations that may apply to this non-traditional use of plants, improving understanding of how candidate species work and what makes contaminants accessible, developing protocols for determining efficiency and cost-effectiveness, and improving knowledge of the potential impacts of phytoremediation demonstration projects on wildlife.

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