Abstract
Demand for aluminum is expected to increase by more than 50% by 2040. However, every ton of raw bauxite ore refined and smelted into aluminum produces two tons of residue, which in turn produces leachate that is alkaline and could leach contaminants. Some promising nature-based solutions exist including the inherent capacities of certain plants to modify their environment. Constructed wetlands using such plants are an affordable, passive system for the remediation of bauxite tailings. Previous research into constructed wetlands for the treatment of bauxite residues has investigated their effectiveness in warm or tropical climates, using species such as Phragmites australis that are considered invasive plants in North America. Little research has been done in cold climates, where subsurface flow systems are more effective than the more commonly used surface flow systems. Operating a continuous plant-based passive treatment system in a cold climate area presents its own set of challenges, but following a proof-of-concept conducted in 2022 at the bauxite residue site of Rio Tinto in Saguenay primarily for caustic pH neutralization and aqueous fluoride fixation, it was shown that certain native wetland plants from eastern Canada can adapt to hyperalkaline conditions. As a result, a passive treatment pilot combining substrate and plants has been designed and operated in 2024 in dynamic mode directly on-site. The device is designed to lower pH and fix fluorides in two consecutive steps. Ultimately, the results suggest that Typha latifolia can adapt to hyperalkaline conditions and help treat large volumes of alkaline contaminated water to levels that fall below local regulatory guidelines for pH. However, further testing will be required to achieve the fluoride treatment objectives.