Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions

Mineralogical transformations of 2-line ferrihydrite were studied under oxic and Fe-reducing conditions to establish the role, if any, of 6-line ferrihydrite (“well” organized ferrihydrite) in the reaction pathway and as a final product. In oxic experiments, concentrated suspensions (0.42 mol/L Fe in 0.1 mol/L NaClO4) of freshly synthesized 2-line ferrihydrite, with and without 3% Ni, were aged at an initial pH = 7.2 (unbuffered and unadjusted) and 25 C for more than three years. X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy measurements were performed on the solids after different aging periods. The primary mineralogical products observed were 6-line ferrihydrite and goethite, with minor hematite. Aggregation and crystallization of the 2line ferrihydrite liberated protons and depressed suspension pH, but coprecipitated Ni retarded this process. The joint, interrelated effects of Ni and pH influenced both the extent of conversion of 2line ferrihydrite and the identity of the major transformation products. Six-line ferrihydrite dominated in the Ni ferrihydrite suspension, whereas goethite dominated in the absence of Ni. Aggregation-induced crystallization of 2-line ferrihydrite particles seemed responsible for 6-line ferrihydrite formation. Mineralogical changes to Ni ferrihydrite under anaerobic conditions were investigated at circumneutral pH using the Fe-reducing bacterium Shewanella putrefaciens. Residual 6-line ferrihydrite dominated bioreduced samples that also contained goethite and magnetite. The conversion of 2-line ferrihydrite to 6-line ferrihydrite was considerably more rapid under anaerobic conditions. The sorption of biogenic Fe apparently induced intra-aggregate transformation of 2-line ferrihydrite to 6-line ferrihydrite. Collectively, abiotic and biotic studies indicated that 6-line ferrihydrite can be a transformation product of 2-line ferrihydrite, especially when 2-line ferrihydrite is undergoing transformation to more stable hematite or magnetite. Schwertmann et al. (1999) suggested that nano-particle aggregation was essential to induce crystallization of ferrihydrite to hematite. Ferrihydrite aggregation is maximal near the pH of zero net charge of the ferrihydrite “surface” (pH ~8), and crystallizing hematite nuclei are apparently supplied by a shortrange dissolution process involving precursor aggregates. Synthetic and natural ferrihydrites are poorly ordered, but both exhibit a continuum in structure from amorphous to partly crystalline (Carlson and Schwertmann 1981; Cornell and Schwertmann 1996). Ferrihydrite exhibits a range of XRD patterns; the least crystalline variety exhibits two broad peaks (2line ferrihydrite), and the more crystalline variety exhibits six broad peaks (6-line ferrihydrite). Several structural models of 6-line ferrihydrite have been proposed, with the defective hematite structural model proposed by Chukrov et al. (1973) and Towe and Bradley (1967) being the most widely accepted. In contrast, Drits et al. (1993) proposed on the basis of XRD simulations that all natural and synthetic ferrihydrites are multi-component phases comprised of defect-free and defective ferrihydrite mixed with ultradisperse hematite. The main difThe terms two-line ferrihydrite and HFO (hydrous ferric oxide) are commonly used synonymously (Cornell and Schwertmann 1996). The term HFO (e.g., Dzombak and Morel 1990) is applied to a material synthesized in the laboratory by rapid hydrolysis of a Fe salt solution, with approximately 4–8 h aging at pH 7. KUKKADAPU ET AL.: TRANSFORMATION OF 2-LINE FERRIHYDRITE 1904 ference between 2-line ferrihydrite and 6-line ferrihydrite is the size of their coherent scattering domains. Cornell and Schwertmann (1996) and Schwertmann et al. (1999) suggested that dilute suspensions of 2-line ferrihydrite do not transform to 6-line ferrihydrite with time, because these two forms of ferrihydrite precipitate under different conditions (e.g., Schwertmann and Cornell 1991; Schwertmann et al. 1999). In contrast, we recently observed the partial transformation of an aged 2-line Ni ferrihydrite (Ni/[Ni + Fe] mole fraction of ~0.03) to 6-line ferrihydrite in the presence of a dissimilatory iron-reducing bacterium (DIRB; Shewanella putrefaciens, strain CN32) at 25 C in bicarbonate-buffered solution at circumneutral pH (Fredrickson et al. 2001). Our finding of 6-line ferrihydrite as a biotransformation product was consistent with the recently observed conversion of 2-line arsenate (As) ferrihydrite to 6-line As ferrihydrite in sediment (Rancourt et al. 2001). Several authors have shown that 6-line ferrihydrite may convert to hematite with heat treatment (Johnston and Lewis 1983; Stanjek and Weidler 1992; Weidler 1995). Implied but not documented is that 6-line ferrihydrite may be an intermediate or metastable structure in the conversion of 2-line ferrihydrite to hematite. Recently, Schwertmann et al. (1999) attributed the transformation of an aqueous suspension of 6line ferrihydrite to hematite, without any transitional phases, to crystallization within ferrihydrite aggregates. In the present work, experiments were performed to evaluate whether 2-line ferrihydrite transforms to 6-line ferrihydrite during conversion to more crystalline Fe oxides such as hematite and magnetite. Our objectives were to define: (1) whether 2-line ferrihydrite gradually crystallizes to a more organized phase (e.g., 6-line ferrihydrite) in aerobic and anaerobic environments, (2) the effect of small amounts of coprecipitated Ni in ferrihydrite on such transformation, and (3) the commonality of 6-line ferrihydrite as an intermediate transformation product. These objectives were resolved by first performing aging experiments with concentrated suspensions (0.42 mol/L) of freshly precipitated 2-line and 2-line Ni ferrihydrites in 0.1 mol/ L NaClO4. These were aged at 25 C and pH 7.2 (initial) for 41 and 38 months, respectively. The aging conditions were chosen to promote the initial aggregation of ferrihydrite, and hence, hematite formation (Schwertmann and Fischer 1966; Fischer and Schwertmann 1975; Cornell and Giovanoli 1985; Schwertmann et al. 1999). The resulting mineral transformation products were characterized using a variety of techniques. Second, low-temperature Mössbauer measurements were performed on biotransformed Ni ferrihydrite from two incubations of Fredrickson et al. (2001) that produced different mineral associations with residual ferrihydrite. The combined results of these two experimental activities demonstrated that 6-line ferrihydrite can form as a transformation product of 2-line ferrihydrite under various conditions. MATERIALS AND METHODS