Photo: SGI
Contaminated soils are often excavated and disposed of in landfill. This requires financial resources, energy and raw materials. Researchers at the Swedish Geotechnical Institute (SGI) have investigated whether contaminants can instead be immobilised in situ, reducing their mobility and environmental risk. This approach has the potential to reduce the need for soil excavation while allowing continued land use.
In the study, the researchers evaluated the effects of biochar and peat amendment on the solubility and bioavailability of heavy metals in contaminated soil.
“The results show that stabilisation with biochar is a promising method for sustainable remediation of moderately contaminated soils. Biochar reduced the solubility of several metals and thereby their potential dispersal in the environment,” says Charlotta Tiberg, researcher in environmental chemistry at the Swedish Geotechnical Institute (SGI).
The results showed that application of 3% biochar reduced the solubility of cationic metals, including copper, mercury, zinc, cadmium and nickel, by 60-80%. In addition, metal uptake in both earthworms and grass were reduced. Increasing the biochar application rate to 6% did not result in substantial additional benefits, indicating that moderate amendment levels may be sufficient to achieve effective remediation.
The reduction in metal solubility was not primarily driven by changes in soil pH, which is often cited as the main immobilisation mechanism in previous studies. Instead, the findings suggest that biochar reduced concentrations of dissolved organic carbon in the soil solution which, in turn, reduced metal mobility because several metals bind strongly to organic matter.
However, not all the effects were positive. The solubility of certain substances, such as arsenic and chromium, tended to increase. The evident increase in chromium could largely be mitigated through peat amendment.
Previous studies of biochar and heavy metals have generally focused on one or two contaminants, most often under laboratory conditions. In contrast, the present study examined a multi-contaminated soil and in addition to laboratory analyses, the researchers conducted a field trial outside Helsingborg, using cultivation beds.
The biochar used in the study was produced from wood through gasification at 750°C, resulting in a stable carbon-rich material with a high pore volume and a high degree of aromaticity. The biochar was mixed with the contaminated soil, in some treatments together with peat. Effects were assessed through analyses of soil solutions collected from field-installed lysimeters, standardised batch leaching tests (ISO 21268-2:2019), and measurements of metal uptake in grass (Lolium perenne L.) and earthworms (Eisenia fetida).
Within the study, a comparison between batch leaching tests and soil solution collected with lysimeters was conducted to evaluate the feasibility of batch tests to evaluate remediation efficiency. The batch test generally showed the same trends as the soil solution, with biochar decreasing solubility. However, mobilization of colloids during shaking in the batch test induced artefacts, leading to an overestimation of the solubility of some metals, especially Pb and Hg.
“Biochar can contribute both to circular resource use and carbon sequestration. At the same time, site-specific evaluations and long-term studies are needed to ensure that remediation effects persist and that any unintended side effects are identified at an early stage,” says Charlotta Tiberg.
The published study covers one growing season. Researchers are currently compiling five years of follow-up data to determine whether the observed effects are sustained over time. The study also highlights practical challenges that need to be addressed before wider implementation, including cost-effective methods for incorporating biochar directly at contaminated sites.
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Shorelines along fairways are exposed to erosion caused by vessel traffic. Various types of erosion protection are used to mitigate this erosion. Traditionally, these have consisted of different types of hard protection, often rock revetments, which can have negative impacts on the environment. In new guidance, the Swedish Geotechnical Institute (SGI) presents nature-based erosion protection as an alternative and provides guidance on how such measures can be designed and constructed.
Contaminated soils are often excavated and disposed of in landfill. This requires financial resources, energy and raw materials. Researchers at the Swedish Geotechnical Institute (SGI) have investigated whether contaminants can instead be immobilised in situ, reducing their mobility and environmental risk. This approach has the potential to reduce the need for soil excavation while allowing continued land use.
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