Removal of Mn via coprecipitation and sorption by Fe(II), Fe(III), and Al in mine drainage

In mine drainage, Fe and Mn are the two most abundant elements exceeding the discharge criteria. Although Mn removal generally requires a pH exceeding 9.5-10.0, its coprecipitation and sorption by Fe and/or Al can significantly reduce the required pH. In this study, Mn removal efficiencies, mechanisms, and required pH were investigated by experiments involving varying concentrations of Mn, Fe, and Al at different pH, and X-ray photoelectron spectroscopy (XPS) analyses. At pH > 7.9, precipitation as Mn (hydr)oxides was the principal Mn removal process, as indicated by the Mn removal plots, geochemical modeling, and XPS results. The precipitation was highly promoted by the heterogeneous oxidation of Fe and Al hydroxides. Coprecipitation-sorption experiments showed 65-80% lower Mn concentrations than those of sorption experiments at similar dosages and pH near 7.5. Fe(III) exhibited higher coprecipitation efficiency than Fe(II), possibly due to the prior oxidation of Fe(II). Fe(III) also displayed a coprecipitation-sorption efficiency five times more than Al. To decrease the Mn concentrations from 17-25 mg L^-1 to <2 mg L^-1 by coprecipitation-sorption, Fe(III)/Mn and Fe(II)/Mn ratios of ∼10 and ∼20, respectively, at pH 9.0 were required. Similarly, an Al/Mn ratio of ∼7 at pH 9.0 was required to reduce the Mn concentration to <5 mg L^-1. Furthermore, the required Fe/Mn ratio decreased significantly when the initial Mn concentration decreased to 8-11 mg L^-1. Utilizing the deduced relationships, required pH for Mn removal could be estimated and the design of Mn treatment facilities can be more efficient.