Rates of low-pH biological Fe(II) oxidation in the Appalachian Bituminous Coal Basin and the Iberian Pyrite Belt

Low-pH Fe(II) oxidation can be exploited for the treatment of acid mine drainage (AMD). However, natural or engineered terraced iron formations (TIFs) are underutilized for AMD treatment because of uncertainties with respect to treatment performance. To address this problem we measured the rates of Fe(II) oxidation multiple times at eight sites in the Appalachian Bituminous Coal Basin and at three sites in the Iberian Pyrite Belt (IPB). Longitudinal geochemical transects were measured downstream of emergent anoxic AMD sources. Water velocities were measured at each sampling location and used to transform concentration versus distance profiles into concentration versus travel time for kinetic analysis of field data. Zero-order Fe(II) oxidation rates ranged from 8.60 to 81.3 × 10−7 mol L−1 s−1 at the Appalachian sites and 13.1 to 67.9 × 10−7 mol L−1 s−1 at the IPB sites. First-order Fe(II) oxidation rate constants ranged from 0.035 to 0.399 min−1 at the Appalachian sites and 0.003 to 0.010 min−1 at the IPB sites. Faster rates of Fe(II) oxidation were measured at two sites (one in Appalachia and one in IPB) where the emergent pH values were the lowest and little to no oxidative precipitation of Fe(III) occurred. Laboratory-based rates of Fe(II) oxidation were measured with TIF sediments and emergent AMD collected from seven Appalachian sites. First-order laboratory rate constants normalized to sediment biomass concentrations (measured by phospholipid fatty acids; PLFA) were positively correlated to first-order field rate constants. Biomass composition was relatively similar between all sites, and predominately comprised of proteobacteria and general PLFAs. A zero-order lab-based removal rate for dissolved Fe(T) was used to calculate area-based design criteria of 2.6–8.7 g Fe day−1 m−2 (GDM) for both natural and engineered TIFs.