Closing the loop on passive treatment of mining-influenced waters: Reuse of Ferruginous Mine drainage residuals as phosphate sorbents

Mine drainage (MD) passive treatment systems (PTS) have been demonstrated to be effective applications of ecological engineering that improve water quality. Recently, there has been a global push for the sustainable management of accumulated treatment residuals known as mine drainage residuals (MDRs). One proposed management strategy is the reuse of MDRs as phosphorus (P) sorbents to address eutrophication due to elevated anthropogenic P. This study evaluated the reuse of three MDRs from an untreated net-acidic coal MD discharge (Gowen), a net-alkaline coal MD PTS (Rock Island #7 Passive Treatment System, RI7PTS), and a net-alkaline hard-rock MD PTS (Mayer Ranch Passive Treatment System, MRPTS) in Oklahoma, USA as P sorbents. Sorption experiments found that the MRPTS MDRs sorbed 244 ± 86 mg g⁻¹ P while the MDRs from Gowen and RI7PTS sorbed 61 ± 48 and 88 ± 45 mg g⁻¹ P, respectively, at an initial concentration of 4000 mg L⁻¹ P. Similarly, sorption studies found that all MDRs removed over 88 % of P (4.41–4.95 g⁻¹ P) within 24 h, with an initial concentration of 50 mg L⁻¹ P and a dose of 10 g MDR L⁻¹. A limiting factor with reusing MDRs is the potential release of metals. In these experiments, the release of metals at ecotoxic concentrations typically occurred at unnatural concentrations of P (>1000 mg L⁻¹ P) due to the increased acidity of the solutions, overestimating the metal leachability of the MDRs. These results indicate that MDRs from these different MD chemistries can be sustainably reused as P sorbents, closing the resource recovery loop of PTS.