Abstract
We report a preliminary assessment of ferrate [Fe(VI)] for the treatment of acid mine drainage (AMD), focused on precipitation of metals (i.e., iron [Fe] and manganese [Mn]) and subsequent removal. Two dosing approaches were studied to simulate the two commercially viable forms of Fe(VI) production: Fe(VI) only, and Fe(VI) with sodium hydroxide (NaOH). Subsequent metal speciation was assessed via filter fractionation. When only Fe(VI) was added, the pH remained <3.6, and the precipitation of Mn and Fe was <30 and <70%, respectively, at the highest, stoichiometrically excessive Fe(VI) dose. When NaOH and Fe(VI) were added simultaneously, precipitation of Mn was much more complete, at doses near the predicted oxidation stoichiometric requirement. The optimal dosage of Fe(VI) for Mn treatment was 25 μM. The formation of Mn(VII) was noted at Fe(VI) dosages above the stoichiometric requirement, which would be problematic in full-scale AMD treatment systems. Precipitation of Fe was >99% when only NaOH was added, indicating that oxidation by Fe(VI) did not play a significant role when added. The Fe(III) and Al(III) particles were relatively large, suggesting probable success in subsequent removal through sedimentation. Resultant Mn-oxide particles were relatively small, indicating that additional particle destabilization may be required to meet Mn effluent goals. Ferrate seems viable for the treatment of AMD, especially when sourced through onsite generation due to the coexistence of NaOH in the product stream. More research on the use of Fe(VI) for AMD treatment is required to answer extant questions.
Highlights
Abandoned mine drainage (AMD) is a global environmental issue (Wolkersdorfer et al, 2004ac; Sun et al, 2013; Strosnider et al, 2014)
Fe(VI) seems viable for the treatment of AMD especially when sourced through on-site generation due to the coexistence of NaOH in the product stream
The abiotic oxidation of reduced Mn is a common concern in AMD treatment, both from passive and active perspectives
Summary
Abandoned mine drainage (AMD) is a global environmental issue (Wolkersdorfer et al, 2004ac; Sun et al, 2013; Strosnider et al, 2014). There are two common and effective approaches for AMD treatment: active and passive. Active treatment employs chemical dosing (e.g., NaOH, CaOH) and ongoing energy inputs (e.g., electricity for aeration/mixing) in traditional wastewater treatment unit processes (e.g., clarifiers, aeration basins). Hybrid systems exist that combine passive and active approaches within the same treatment train. Spontaneous removal of Fe and Al through oxidation (for Fe(II)) and precipitation is a relatively straightforward process. Rapid abiotic Mn removal requires pH > 9, which is not achievable in passive treatment systems and relatively expensive in active treatment systems (Younger et al, 2002; Skousen et al, 2017). The abiotic oxidation of reduced Mn is a common concern in AMD treatment, both from passive and active perspectives
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