Using plants like willow to remove soil contaminants

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By Tom Woods
Master Gardener
I came across this article in a recent issue of Soil Science and found it interesting. There are many closed and razed industrial sites that could be reclaimed as useful ground using this technology.
Cleaning up soils contaminated with heavy and toxic metals is a difficult task, but a technique called phytoextraction can ease the challenge by using metal-tolerant plants to rid soils of metals and contaminants.
Many specific plants have been tested for their ability to extract contaminants from soil. Often, plants capable of phytoextraction can only take up specific heavy metals. A team of researchers from the University of Melbourne has investigated the potential of using plants that can absorb a multitude of metals at the same time, such as willows.
Willows are trees known to accumulate and tolerate elevated amounts of cadmium and zinc, but they may take up other metals as well. These fast-growing trees are easily propagated, produce high annual biomass and re-sprout vigorously after harvesting the aboveground biomass. A willow-based phytoextraction system would involve growing these woody plants as a crop with annual or biennial harvests of metal-rich aboveground biomass.
The University of Melbourne study, led by W.S. Laidlaw, investigated the potential of using willows as a sustainable phytoextraction system to reduce contaminant metals in biosolids. Also known as sludge, biosolids are a byproduct of sewage treatment facilities that can retain significant amounts of heavy metals and other environmental contaminants. However, this sewage sludge is rich in elements and organic matter that is valuable to nutrient-poor soils, making it a useful resource if metals can be removed.
Using phytoextraction to reduce the metals and contaminants in biosolids can convert sewage sludge into a low-cost, valuable resource.
In the study, seven cultivars of willow were planted directly in to a stockpile of biosolids and drip-irrigated with fresh water. The trials were specifically aimed to test the suitability of each willow cultivar to grow and extract metals under field conditions for three years.
Each year, willow biomass was harvested and weighed. While the majority of cultivars produced less biomass when growing in biosolids compared to soil, two willows, Salix matsudana and Salix x reichardtii, produced similar biomass in both soil and biosolids.
After harvest, stems and leaves were analyzed for heavy metals and metalloids including arsenic, cadmium, copper, chromium, mercury, nickel, lead, and zinc. The magnitude of metal extraction revealed Salix matsudana and Salix x reichardtii as the most efficient cultivars. Although these willows had the lowest tissue concentration of heavy metals, they produced the greatest biomass and ultimately extracted the most metals and contaminants from the biosolids.
The willows were effective in extracting cadmium, nickel and zinc, the most readily available metals in the biosolids. Copper and chromium were extracted to a lesser degree, and the low-bioavailable elements arsenic, mercury and lead were not detected in the harvested biomass.
This finding shows that assessing the bioavailablity of metals in soils can give an accurate indication of which metals can be extracted by metal-tolerant plants.
This field study demonstrated willows can be grown directly in biosolids contaminated with several heavy metals. It also showed that extracting certain metals is dependent on their bioavailability. For biosolids containing multiple metal contaminants, a willow phytoextraction system may be insufficient to remove all metals. Integration of other plant species and metal mobilization techniques with the willows will be required to produce a phytoextraction system capable of extracting contaminants of interest in the shortest time possible.
Disposing of biomass after phytoextraction remains a challenge. Several methods of contaminated crop disposal have been researched, including ashing, incineration and liquid extraction. Currently, incineration is proposed as the most feasible and environmentally sound disposal method.
This project was funded by the Australian Research Council Linkage Program in partnership with the Melbourne Water Corporation and the University of Melbourne. The complete results from this study can be found in the January issue of the Journal of Environmental Quality.
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