Mercury is used in a variety of industries, including textile manufacturing and gold and silver mining. When released into the environment, this highly toxic element causes widespread contamination of soil. As mercury enters rivers, lakes and oceans, it is converted to methylmercury, a neurotoxin that moves into the food chain through fish and seafood, posing a serious risk to human health.
Conventional methods of remediating mercury-contaminated soil – such as adding activated carbon – can be quite expensive to apply on a large scale. However, recent research has found that biochar, a charcoal produced by superheating agriculture or forestry waste in the absence of oxygen, holds promise as a low cost, “green” alternative.
Most studies to date have focused on biochar’s ability to control the release of mercury and production of methylmercury from waterlogged soils. A team of researchers from the University of Waterloo recently set out to determine how effective biochar is at amending soils that see frequent drying and rewetting, such as floodplains.
PhD student Alana Wang and colleagues added two types of biochars – sulfurized wood and anaerobic digestate – to soil samples drawn from floodplains along Virginia’s South River. A chemical plant in Waynseboro, VA disposed of mercury waste in that river from 1929 to 1950.
Each soil sample was subjected to 10 cycles of wetting with river water and drying with repeated testing throughout. The research team was surprised to find that, early on in the study, the biochar-amended soil actually released higher levels of mercury.
“Our previous studies demonstrated that the addition of sulfurized hardwood biochar was very effective for removing mercury from aqueous solution under both stagnant and saturated-flowing conditions,” said Water Institute member Dr. Carol Ptacek, a member of the research team. As the wetting and drying cycles continued though, the biochar-treated soil released less mercury and had lower concentrations of methylmercury – more in keeping with what they had expected to find, based on other studies.
