Secondary mineralization pathways induced by dissimilatory iron reduction of ferrihydrite under advective flow

Abstract

Iron (hydr)oxides not only serve as potent sorbents and repositories for nutrients and contaminants but also provide a terminal electron acceptor for microbial respiration. The microbial reduction of Fe (hydr)oxides and the subsequent secondary solid-phase transformations will, therefore, have a profound influence on the biogeochemical cycling of Fe as well as associated metals. Here we elucidate the pathways and mechanisms of secondary mineralization during dissimilatory iron reduction by a common iron-reducing bacterium, Shewanella putrefaciens (strain CN32), of 2-line ferrihydrite under advective flow conditions. Secondary mineralization of ferrihydrite occurs via a coupled, biotic-abiotic pathway primarily resulting in the production of magnetite and goethite with minor amounts of green rust. Operating mineralization pathways are driven by competing abiotic reactions of bacterially generated ferrous iron with the ferrihydrite surface. Subsequent to the initial sorption of ferrous iron on ferrihydrite, goethite (via dissolution/reprecipitation) and/or magnetite (via solid-state conversion) precipitation ensues resulting in the spatial coupling of both goethite and magnetite with the ferrihydrite surface. The distribution of goethite and magnetite within the column is dictated, in large part, by flow-induced ferrous Fe profiles. While goethite precipitation occurs over a large Fe(II) concentration range, magnetite accumulation is only observed at concentrations exceeding 0.3 mmol/L (equivalent to 0.5 mmol Fe[II]/g ferrihydrite) following 16 d of reaction. Consequently, transportregulated ferrous Fe profiles result in a progression of magnetite levels downgradient within the column. Declining microbial reduction over time results in lower Fe(II) concentrations and a subsequent shift in magnetite precipitation mechanisms from nucleation to crystal growth. While the initial precipitation rate of goethite exceeds that of magnetite, continued growth is inhibited by magnetite formation, potentially a result of lower Fe(III) activity. Conversely, the presence of lower initial Fe(II) concentrations followed by higher concentrations promotes goethite accumulation and inhibits magnetite precipitation even when Fe(II) concentrations later increase, thus revealing the importance of both the rate of Fe(II) generation and flow-induced Fe(II) profiles. As such, the operating secondary mineralization pathways following reductive dissolution of ferrihydrite at a given pH are governed principally by flow-regulated Fe(II) concentration, which drives mineral precipitation kinetics and selection of competing mineral pathways.

Identification of iron-reducing microorganisms in anoxic rice paddy soil by 13C-acetate probing

“…Iron extraction using 0.5 M HCl facilitates detection of adsorbed iron(II) or siderite precipitates, for example in rice field soil (Ratering and Schnell, 2000), but not magnetite (Fe 3 O 4 ) (Raiswell et al, 1994), which could have potentially been formed in goethite amended slurries. Magnetite is a mixed Fe(II)-Fe(III) mineral, which may be observed in closed systems upon secondary mineral transformations of iron(III) phases, especially ferrihydrite (Hansel et al, 2003). Slightly higher levels of inorganic carbon (calculated from headspace CO 2 and Stable isotope probing of iron-reducing microorganisms T Hori et al slurry pH) were formed in goethite (2.9 mmol) compared with control treatments (2.2 mmol) and more acetate was degraded in goethite (65%) than in control treatments (44%).…”

Incorporation of Oxidized Uranium into Fe (Hydr)oxides during Fe(II) Catalyzed Remineralization

“…The solid Fe speciation was determined by linear combination fitting of Fe EXAFS data as used previously [30] and described in more detail in the Supporting Information. Uranium EXAFS scans were processed using the computer programs Athena [6,31,32], SixPACK [33], and Feff7 [34] (Supporting Information).…”

Stability of Uranium Incorporated into Fe (Hydr)oxides under Fluctuating Redox Conditions

“…Ferrihydrite was prepared according to the method described by Brooks et al (32), and then coated quartz sand as reported previously (32,33). Iron concentration on the sand was approximately 10 g Kg -1 (1% by weight).…”