Passive Treatment of Mine Drainage Waters: The Use of Biochars and Wood Products to Enhance Metal Removal Efficiency

Passive biological treatments have been proposed as a possible efficient and cost effective treatment method for metal bearing water discharged from mine sites after closure. Several biofilters are under study in Yukon and have produced variable, but promising results up to now. However, concerns are typically expressed around biological treatments and their suitability in northern, colder climates. Biofilters allow for metal removal using a variety of chemical, physical and biological mechanisms. If biological processes are affected by a cold climate to some extent, chemical processes are typically not affected by the temperature the same way and can be reliable in cold waters. This study focused on metal sorption and metal removal by chemical mechanisms and assessed the sorption capacity of biochar and wood products which could be later introduced in bioreactors to help with metal removal from mine- impacted cold waters.
Biochars allowed for more than 90% removal of Cd, Cu and Zn from a metal-bearing effluent along with 35 to 69% removal of arsenic. Wood products displayed good removal capacity as well, in the range of 51 to 94% for Cd, Cu and Zn. However, arsenic and selenium removal by wood products was limited; Se also showed minimal sorption on biochars and was in one case released during sorption testing. Metal leaching from the materials was observed to some extent, including Cu and Zn from poplar and spruce products. Amongst spruce products, the chips from the trunk proved to be slightly more efficient than the needles. Overall, biochars and wood products showed potential for use in water treatment for metal sequestration in combination with other mechanisms such as sulfide precipitation in sulfate-reducing actors. Such materials could be collected or produced on remote mine sites and could help with mine remediation.