Manganese (Mn) desorption and re-adsorption in filter media (i.e., anthracite coal, silica sand, hematite, and garnet) from a water treatment plant undergoing conversion to biofiltration was evaluated. Mn contamination of the filter media was a by-product of multiple years of pre-chlorination while using Mn-containing ferric chloride (FeCl3) for coagulation. During pilot- and full-scale testing at pH 6.5, FeCl3 coagulation (without chlorine) released soluble Mn in the filter effluent at nearly four times the level compared to alum (105 ug/L for FeCl3 and 27 ug/L for alum). Pilot-scale experiments led to the conclusion that Mn was present in the filter column in three valence states [Mn(II), Mn(III), and Mn(IV)]. Mn from the FeCl3 was Mn(II). Mn on the anthracite media was hypothesized to be Mn(III)/Mn(IV) and first physically displaced by ferric iron, aluminum, and hydrogen, followed by re-adsorption in the filter column through catalytic oxidization to Mn(IV) by the silica sand. The resulting solid-phase MnO2 migrated via gravity towards the lower sand, hematite or garnet layers. This study was unable to evaluate the shift in Mn mineralogy during conversion from anthracite adsorbed Mn to solid-phase MnO2 in the hematite or garnet layers. An effective control measure to prevent Mn desorption was operating the filter column at pH 8.0.

Sequential manganese desorption–Adsorption in anthracite coal and silica sand filter media

253rd National Meeting (2017)

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