Oxyhydroxides

In view of the behaviour of the OH in simple hydroxides we may expect layer structures to occur frequently. Orderly replacement of two-thirds of the Mg atoms in the Mg(OH)2 layer by K and of the remainder by gives the structure of K2Sn(OH)6 and the isostructural Pb and Pt compounds. Like NaSb(OH)6 these crystals contain discrete M(OH)6 groups. 

Binuclear hydroxy-ions (a) occur in Ba2Al2(OH)io, consisting of a pair of edge-sharing octahedra. There is a small difference between the lengths of the bridging (1-98 A) and terminal (1 89 A) Al—OH bonds. 

The largest class of compounds of which the structures are known are of the type MO. OH, formed by Al, Sc, Y, V, Cr, Mn, Fe, Co, Ga, and In. A number of these compounds, like the trihydroxides and sesquioxides of Al and Fe, exist in a and 7 forms. (The so-called d-FsO. OH is not a pure oxyhydroxide it has the a-Mn02 structure, and is stable only if certain interstitial ions such as Cl are enclosed within the framework. We refer later to types of oxyhydroxide other than MO. OH. As in the case of hydroxides we may distinguish between structures in which there is hydrogen bonding and those in which this does not occur it is convenient to deal first with a structure of the latter type. 

---

X-ray photoelectron spectroscopic study of the spontaneously passive amorphous Fe-10Cr-13P-7C alloy in 1 N HCl revealed that the passive film consists of Cr, 0 , OH" and HjO, and hence the passive film has been called a passive hydrated chromium oxyhydroxide film (CrO (OH)j Subsequent investigations have revealed that... 

The passive films formed by the addition of sufficient amounts of valve metals to amorphous nickel-valve-metal alloys are exclusively composed of valve-metal oxyhydroxides or oxides such as TaOjCOH) , Nb02(OH) or TajO,. Consequently, amorphous alloys containing strongly passivating elements, such as chromium, niobium and tantalum, have a very high ability... 

The effect of metalloids on the corrosion resistance of alloys also varies with the stability of polyoxyanions contained in their films. Phosphorus and carbon contained in iron-chromium-melalloid alloys do not produce passive films of phosphate and carbonate in strong acids, and so do not interfere with the formation of the passive hydrated chromium oxyhydroxide... 

In acidic solutions the film has been reported to be hydrated nickel oxyhydroxide, NiOy(OH)2 2y. MH2O in which y is greater in the passive film than in the pre-passive film formed in the active region . In neutral solutions films consisting of NiO and Ni(OH)2 possibly with some NiO have been described. In alkaline solutions NifOHlj has been reported . 

Delmas and his co-workers have done extensive work on pyroaurite-type materials which has recently been reviewed [73], In addition to precipitation methods, they have prepared the materials by mild oxidative hydrolysis of nickelates that were prepared by thermal methods similar to those used for the preparation of LiNiOz [74]. A cobalt-substituted material NaCoA ( Ni( A02) was prepared by the reaction of Na20, Co304 and NiO at 800 °C under a stream of oxygen. The material was then treated with a 10 molL-1 NaCIO +4 molL 1 KOH solution for 15h to form the oxidized y -oxyhydroxide. The pyroau-... 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

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. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

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). 

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Leckie, J. O., Appleton, A. R., Ball, N. B., Hayes, K. F. and Honeyman, B. D. (1986). Adsorptive removal of trace elements from fly-ash pond effluents onto iron oxyhydroxide. Final Report EPRI-RP-910-1, Electric Power Research Institute, Palo Alto, CA. 

Powell AK (1997) Polyiron Oxides, Oxyhydroxides and Hydroxides as Models for Biomineralisation Processes. 88 1-38... 

The synthesis of WOx-Zr02 solids with high isomerization activity requires oxyhydroxide precursors, calcination at 1000-1100 K, and W loadings of 10-12% wt., as also reported... 

Northeastern Lau Basin Papatua expedition site (15°17 S, 174"45 W) 2100 Axial region of northeasterly trending active spreading ridge of the northern Lau Basin. Dredged black smoker chimney samples. Wurtzite, pyrite, chalcopyrite, barite, and amorphous silica. Thin film of Mn-oxyhydroxide. 

Western Woodlaik Basin Franklin Seamount (9°55 S, 151L50 W) 2143-2366 Westernmost propagating tip of spreading center. Basaltic andesite and inferred sodic rhyolite. Spires aud mounds of Fe-Mn-Si oxide up to several meters thick and 200 m in extent. Venting 20-30 C clear solution. Inactive barite silica chimneys contain up to 21 ppm Au. vSi-bearing Fe oxyhydroxide. 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

Initial °Th and Pa are generally considered to be associated with a detrital component that becomes cemented, or occluded, within the speleothem. This component may be composed of clays, alumino-silicates or Fe-oxyhydroxides (Fig. 3) with strongly adsorbed and Pa. Th and Pa incorporated in speleothems and similar deposits may also have been transported in colloidal phases (Short et al. 1998 Dearlove et al. 1991), attached to organic molecules (Langmuir and Herman 1980 Gaffney et al. 1992) or as carbonate complexes in solution (Dervin and Faucherre 1973a, b Joao et al. 1987). 

Ames LL, McGarrah JE, Walker BA, Salter PF (1983d) Uranium and radium sorption on amorphous ferric oxyhydroxide. Chem Geol 40 135-148... 

Hsi C, Langmuir D (1985) Adsorphon of uranyl onto ferric oxyhydroxides applications of the surface complexation site-binding model. Geochim Cosmochim Acta 49 1931-1941 Ingri J, Widerlund A, Land M, Gustafsson O, Anderson P, Ohlander B (2000) Temporal variation in the fractionation of the rare earth elements in a boreal river the role of colloidal particles. Chem Geol 166 23-45... 

The operational separation of colloids does not directly identify the nature of the colloid fraction, which may consist of clays, Fe and Mn oxyhydroxides, or... 

Davis, J.A. and Leckie, J.O., Surface ionization and complexation at the oxide/water interface, II surface properties of amorphous iron oxyhydroxide and adsorption of metal ions, J. Colloid Interface Sci. 67, 90-107, 1978. 

---