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.