Effects of depth on mineralogy, chemistry and phosphorus sorption capacity of mine drainage residuals from two passive treatment systems

Mine drainage residuals (MDRs) formed in mine drainage passive treatment systems are predominantly iron oxide minerals which have been shown to be effective phosphorus (P) sorbents. As iron oxides transform over time into more crystalline mineral forms, their sorption capacity for P and metals may be limited. This study investigated how MDRs from two passive treatment systems transformed in-situ over time as inferred bt depth and how this affects their P sorption capacity and potential to release metals, impacting their potential to be beneficially reused as P sorbents. It was found that initially formed MDRs found near the surface of the sludge column are primarily ferrihydrite and poorly crystalline goethite with a great specific surface area that transforms into more crystalline goethite with a lesser specific surface area found at depth. In laboratory sorption studies, the fresher MDRs sorbed more P than older MDRs found deeper in the sludge column, yet all MDRs removed over 75% of P within 24 hours with a dose of 10 g L-1 and an initial P concentration of 50 mg L -1 P. Despite the elevated metal concentrations of the MDRs, desorption of metals was minimal and did not exceed chronic or acute aquatic life criteria for freshwater systems.