Other oxyhydroxides

For other oxyhydroxides of V see p. 471. Uranyl hydroxide, U02(OH)2, is not a compound of the type we have been considering but is the dihydroxide of the UO2 ion. The structures of two of its forms are described in Chapter 28. 

The reduction of M0O3, by a variety of methods, gives compounds Mo03 c(OH) c (0 5 x 2) of which the first member is Mo205(OH). This compound has essentially the same (layer) structure as M0O3. The H atoms have not been located but some 0-0 separations of 2 80 A between the layers presumably indicate hydrogen bonds. l In M0O3 the metal atoms are displaced 

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In most cases, the first oxyhydroxide precipitate is amorphous, having been formed under conditions of substantial supersaturation. Early in precipitation, poorly crystalline goethite and poorly crystalline lepido-crocite may also form. Other oxyhydroxides than these three are rarely precipitated from natural waters. 

Phosphate is also ubiquitous as a minor component within the crystal lattices of other minerals or adsorbed onto the surface of particles such as clays, calcium carbonate, or ferric oxyhydroxides (Ruttenberg, 1992). Therefore, in general, transport of these other particulate phases represents an important transport pathway of P as well. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

Observations of the same clay sample in a very finer scale (500 nm) by TEM, may help to identify the potential Fe-oxyhydroxide surfaces attached on a sediment grain (Fig.6). Moreover, abundances of wide spread oxides that may have formed oxide minerals after binding with other elements such as Si, Fe and Al can easily be recognized from the right part of the TEM image (Fig. 7). 

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

Why mammalian ferritin cores contain ferrihydrite-like structures rather than some other mineral phase is less easy to understand, and presumably reflects the way in which the biomineral is built up within the interior of the protein shell together with the geometry of the presumed nucleation sites. The phosphate content in the intracellular milieu can readily be invoked to explain the amorphous nature of the iron core of bacterioferritins and plants. Indeed, when the iron cores of bacterioferritins are reconstituted in the absence of phosphate, they are found to be more highly ordered than their native counterparts, and give electron diffraction lines typical of the ferrihydrite structure. Recently it has been reported that the 12 subunit ferritin-like Dps protein (Figure 19.6), discussed in Chapter 8, forms a ferrihydrite-like mineral core, which would seem to imply that deposition of ferric oxyhydroxides within a hollow protein cavity (albeit smaller) leads to the production of this particular mineral form (Su et al., 2005 Kauko et al., 2006). 

Examples of electroactive NP materials discussed in the review include Ti02, Mn02, iron oxides, other metal oxides, hydroxides and oxyhydroxides and Prussian Blue. We use the term electroactive N Ps to refer to the faradaic electroactivity in such materials and to distinguish them from NPs comprised of metals (such as Au, Ag, Pt, Co, etc.) or semiconductors (such as CdS, CdSe, etc.). This distinction is based on the ability of many electroactive NPs to undergo faradaic oxidation or reduction of all of the metal (redox) centers in the NP. This is in contrast to the behavior of many metal and semiconductor NPs for which oxidation or reduction is fundamentally an interfacial, double-layer process. This deflnition is somewhat arbitrary, since the smallest metal and semiconductor NPs behave molecularly, blurring the distinction... 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

Due to very high GHSV (>l(/h ), an extremely low limit of vapor pressure (<10 Pa) has been fixed as a rough criterion to match the 8000 h constraint of catalyst life in GT combustors [95]. Estimates made considering all the relevant species (metals, oxides, hydroxides, oxyhydroxides) under the oxidizing and water-containing atmosphere of GT combustors showed that Pd is able to match such a constraint up to about 1000 °C, whereas most other components (including Pt) fail [95]. 

Because of the elevated concentrations of potentially ecotoxic metals and acid in mine water, and because of the potential for precipitation of benthos-smothering Fe, Al, Mn-oxyhydroxides (and other phases) in downstream recipient watercourses, water draining from mines is often regarded as a major environmental problem. Considerable effort and expense are incurred by treating... 

Direct interactions of aqueous metal salt solutions with bases failed, as a rule, to yield uniform particles thus, it was necessary to design some other approaches to achieve such dispersions. The fundamental ideas were based on the fact that seldom, if ever, can a metal ion in an aqueous solution be directly converted to an oxide. Instead, the initial steps involve the formation of metal hydroxides or oxyhydroxides. Either these products remain stable as precipitated, or they convert to oxides on aging by processes that may take different lengths of time but that can be accelerated by various dehydration methods. Consequently, metal ion hydrolysis... 

The amount of charges on particle surfaces depends on the mineralogy of the solid and the nature of the aqueous solution in which it occurs. Several important kinds of surfaces are common in the environment (Table 11.3). Here we especially consider (1) oxides or oxyhydroxides, (2) alumino-silicates or clay minerals, and (3) natural organic matter and other solids like carbonates. 

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