Iron oxide, precipitation ferrihydrite

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

The majority of the As in mine water from the internal shaft occurs as As3+, with only a minor component of As5+ (Fig. 5). Iron and As are predominantly in solution in these waters. Water from the internal shaft oxidises as it flows from the sump to the storage dam, and the amount of Fe and As in solution drops sharply as As is absorbed onto precipitating ferrihydrite (Fig. 6). The dominant As species in the sump and dam is As5+. Natural attenuation through passive oxidation removes 98% of the As in solution from the mine waters. 

Can this model published in 2003 (Marion et al. 2003a) explain all the geochemical findings of the 2004 Mars Exploration Rover (MER) missions Not exactly In our model we predicted that ferrous iron would precipitate as siderite (FeCOo) early in the temporal sequence, and siderite would ultimately be oxidized to ferric minerals such as ferrihydrite [Fe(OH)3] and hematite (FeoOo) (Fig. 5.10). There is no place in this conceptual model for the precipitation of ferrous or ferric sulfate minerals as suggested by the MER missions (Squyres et al. 2004 Lane, 2004). This problem could be simply rectified by drawing an arrow from siderite through the surface acidification... 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

Ferrihydrite is generally the initial precipitate that results from rapid hydrolysis of Fe solutions. Its crystallinity, i.e. crystal size and order, is usually lower than that of any of the other Fe oxides described except feroxyhyte and schwertmannite. It is usually named according to the number of its XRD peaks, with 6-8 broad peaks for well crystalline (6-line-) ferrihydrite and only two very broad ones for the most poorly crystalline form (2-line-ferrihydrite). The 2-line ferrihydrite is commonly but incorrectly called hydrous ferric oxide (HFO) or, amorphous iron oxide . In natural environments all forms of ferrihydrite are widespread usually as yoimg Fe oxides and they play an important role as an active sorbent due to their very high surface area. 

Goethite is synthesised by a wide variety of methods that are described in Cornell and Schwertmarm (1996). The main pigment variety is Mars yellow, which was produced from oxidation of iron(II) sulfate ( green vitriol or copperas, q.v.) mixed with alum and precipitated by means of an alkah. Other methods include the oxidation of iron chloride, and the conversion of the iron oxide hydroxides lepidocrocite and ferrihydrite (qq.v.) in alkaline solutions. 

About 35% of the iron and 75% of the manganese in soils and sediments is in the form of free oxides (Canfield, 1997 Cornell and Schwertmann, 1996 Thamdrup, 2000). The remainder occurs as a minor constituent of silicate minerals. The lattice stmcture of Fe(III) oxide minerals varies widely. Freshly oxidized Fe(III) precipitates rapidly as ferrihydrite (Fe(OH)3), a reddish-brown, amorphous, poorly crystalline mineral. Ferrihydrite is the dominant product of Fe(II) oxidation whether it occurs by abiotic oxidation, aerobic microbial oxidation, or anaerobic microbial oxidation (Straub et al., 1998). Over a period of weeks to months, amorphous ferrihydrite crystals undergo diagenesis to yield well-ordered, strongly crystalline, stable minerals such as hematite(a-Fe203) and goethite (a-FeOOH) (Cornell and Schwertmann, 1996). 

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