New Technologies Turn Out Cleaner Dirt

Fuel spills, mine tailings, smelting, and leaks from decaying batteries and other chemical-containing equipment have contaminated thousands of sites across Canada—lacing once-clean soil with toxic organic compounds and heavy metals. Scientists and engineers with Environment Canada's Environmental Technology Centre (ETC) and its alternative service contractor, SAIC Canada, are helping to develop speedier, more efficient ways to clean this soil and restore it to useable condition.

After more than four years of research and development, three new soil remediation techniques are moving toward full-scale field testing, and may reach the commercial market in a few years. These mobile technologies, some developed in collaboration with universities, are not only faster and more thorough than conventional methods for removing organic chemicals and/or heavy metals, but also have the potential to be less expensive.

The Membrane-Assisted Soil Leaching (MASL) process is designed to remove heavy metals, such as mercury, lead, arsenic, chromium, copper and cadmium, from sites contaminated by mining, smelting, old batteries, leaded fuels and other spills. The conventional way to extract these contaminants from soil is to leach them out with acid—a process that can slow down as the concentration of metals in the solution increases. MASL uses this same leaching method, but adds another step—membrane filtration to separate the heavy metal ions from the leachate. This enables the process to continue at acceptable rates, while unspent acid can be recovered for re-use in the process. The volume of liquid waste is reduced by 80-90 per cent and the metal-laden stream concentrated so that it is more economical to treat or dispose of, thereby resulting in cleaner soil than conventional leaching.

The CHELASOL process was originally developed to speed the preparation of lab samples for analysis by simultaneously separating the heavy metal and organic contaminants from soils. This new technology, which combines methods for removing the two types of contaminants in a single process, has also been evaluated for the remediation of contaminated soil. Potential applications are many, since nearly 40 per cent of contaminated sites in Canada are estimated to contain both heavy metal and organic contamination. With the CHELASOL process, acid is used to leach heavy metals out of the soil before a "chelant" (pronounced key-lant) is added that binds with the metal ions to form chelates. Solvent is then added to dissolve both the organic contaminants and the chelates, which are more soluble in the solvent than in water.

Tests show that CHELASOL performs as well as acid leaching in removing heavy metals from soil. CHELASOL was also able to extract nearly 80 per cent of polychlorinated biphenyls compared to only 55 per cent achieved through conventional solvent extraction. Removal levels for other organics, such as polycyclic aromatic hydrocarbons, were similar to those of the conventional extraction methods. Ninety-five to 100 per cent of the solvent used in the CHELASOL process can be recovered from the contaminated liquid through distillation, and heavy metals can be recovered by releasing them from the chelants and precipitating them out.

Scientists at SAIC Canada are also helping to take the Two-Phase Partitioning Bioreactor process, the brainchild of chemical engineers at Queen's University in Kingston, from the bench to the field, and then to the commercial market. Although bioreactors containing microbes are a fairly common way of destroying organic contaminants in soils, problems have occurred because concentrations of these contaminants must be high enough to sustain microbial growth but not so high that they kill the microbes. The new two-phase process gets around the problem by using a solvent to extract the organic contaminants, and then transferring the contaminated solvent to the bioreactor, where it forms an immiscible layer on the surface of a water/microbe mixture. Some of the contaminants in the solvent dissolve in the water until equilibrium occurs, and are broken down by the microbes. As this occurs, the equilibrium shifts and more contaminants transfer from the solvent phase, creating a self-regulating system that provides the microbes with only as much contamination as they can handle. Researchers at Queen's are using sophisticated software to determine the most effective combinations of microbes and solvents for the system.

Major steps for the future include moving all three technologies from batch to continuous processes that will take in contaminated soil at one end and produce uncontaminated soil at the other. SAIC Canada is currently looking for industrial partners interested in extending the techniques to field-scale, on-site demonstrations to further evaluate the systems before they can become commercially available.

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