Hydroxide minerals

The CAS registry Hsts 5,037 aluminum-containing compounds exclusive of alloys and intermetaUics. Some of these are Hsted in Table 1. Except for nepheline and alunite in the USSR and Poland, bauxite is the raw material for all manufactured aluminum compounds. The term bauxite is used for ores that contain economically recoverable quantities of the aluminum hydroxide mineral gibbsite or the oxide—hydroxide forms boehmite and diaspore. 

Mylanta Liquid—simethicone, sorbitol Nephrox Liquid—aluminum hydroxide, mineral oil Original Alka-Seltzer Effervescent Tablets—sodium bicarbonate, aspirin, citric acid, phenylalanine Riopan Plus—magaldrate, simethicone, sorbitol, sucrose Riopan Plus Suspension—magaldrate, simethicone, saccharin, sorbitol... 

The low solubility of Cu oxide and hydroxide minerals and relatively high solubility of its carbonate cause the preferred association of Cu with the oxide phases, such as CuFe204j that may determine the solubility of Cu2+ in soil solution (Lindsay, 1979). In soils with high pH, lead carbonate (PbC03 (cerussite)) is stable, but its solubility is still higher than that of Pb phosphates. 

The fluid contains arbitrarily small amounts of Ca++, Mg++, and HCO3, as is necessary in order for the program to be able to recognize dolomite. The initial magnesium content is set small to assure that the hydroxide mineral brucite is not supersaturated in the alkaline fluid. 

These acidic waters are toxic to plant and animal life, including fish and aquatic insects. Streams affected by acid drainage may be rendered nearly lifeless, their stream beds coated with unsightly yellow and red precipitates of oxy-hydroxide minerals. In some cases, the heavy metals in acid drainage threaten water supplies and irrigation projects. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Oxide/hydroxide minerals of Mn(III,IV), Fe(III), Co(III), and Pb(IV) are thermodynamically stable in oxygenated solutions at neutral pH, but are reduced to divalent metal ions under anoxic conditions in the presence of reducing agents. Changes in oxidation state dramatically alter their solubility. Reduction of Fe(III) to Fe(II), for example, increases iron solubility with respect to oxide/ hydroxide phases by as much as eight orders of magnitude (1). 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Effect of Oxide Mineralogy on Reductive Dissolution. Oxide/hydrox-ide surface structures and the coordinative environment of metal centers may change substantially throughout the course of a reductive dissolution reaction. Nonstoichiometric and mixed oxidation state surfaces produced during surface redox reactions may exhibit dissolution behavior that is quite different from that observed with more uniform oxide and hydroxide minerals. 

The role of transition metal oxide/hydroxide minerals such as Fe and Mn oxides in redox reactions in soils and aqueous sediments is pronounced (Stumm and Morgan, 1980 Oscarson et al., 1981a). These oxides occur widely as suspended particles in surface waters and as coatings on soils and sediments (Taylor and McKenzie, 1966). 

Reductive dissolution of transition metal oxide/hydroxide minerals can be enhanced by both organic and inorganic reductants (Stone, 1986). There are numerous examples of natural and xenobiotic organic compounds that are efficient reducers of oxides and hydroxides. Organic reductants associated with carboxyl, carbonyl, phenolic, and alcoholic functional groups of soil humic materials are one example. However, large... 

Aluminum, a silver-white, malleable, and ductile metal, is the most abundant metallic element in the lithosphere, comprising about 8% of the earth s crust. It is never found free in nature, but occurs combined with other elements, most commonly as aluminosilicates, oxides, and hydroxides in rock, minerals, clays, and soil. It is also present in air, water, and many foods. Bauxite, a weathered rock consisting primarily of aluminum hydroxide minerals, is the primary ore used in aluminum production. Aluminum enters environmental media naturally through the weathering of rocks and minerals. Anthropogenic releases are in the form of air emissions, waste water effluents, and solid waste primarily associated with industrial processes, such as aluminum production. Because of its prominence as a major constituent of the earth s crust, natural weathering processes far exceed the contribution of releases to air, water, and land associated with human activities. 

An octahedral sheet (0) is defined as a two-dimensional array of cations (usually, Al, Mg, Fe) octahedrally coordinated by oxygen and/or OH. The individual octahedra share edges, and the composition of the sheet can vary from M2(0,0H) (all trivalent cations and termed dioctahedral) to M3(0,0H)g (all divalent cations and termed trioctahedral), where M signifies the cation. The octahedral sheet forms the basis of the layer-hydroxide minerals, gibbsite (M = Al) and brucite (M = Mg). 

Hydrogen is the commonest element found in the Universe and is a major constituent of stars. It is relatively much less common on Earth but nevertheless it forms nearly 1 per cent by mass of the crust and oceans, mainly as water and in hydrates and hydroxide minerals of the crust. It is ubiquitous in biology. 

Foley and Ayuso (2008) suggest that typical processes that could explain the release of arsenic from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulfides, and these in general rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of arsenic-bearing iron—cobalt— nickel-sulfide minerals, dissolution (by reduction) of arsenic-bearing secondary arsenic and iron hydroxide and sulfate minerals, carbonation and/or oxidation of As-sulfide minerals, and desorption of arsenic from Fe-hydroxide mineral surfaces are all thought to be implicated. All of these processes contribute to the occurrence of arsenic in groundwaters in coastal Maine, as a result of the variability in composition and overlap in stability of the arsenic source minerals. Also, Lipfert et al. (2007) concluded that as sea level rose, environmental conditions favored reduction of bedrock minerals, and that under the current anaerobic conditions in the bedrock, bacteria reduction of the Fe-and Mn-oxyhydroxides are implicated with arsenic releases. 

Some experiments have shown efficient condensation of simple aldehyde phosphates in the hydroxide mineral (such as hydrotalcite, ([ Mg2 AKOI I)e ] [//H2OI) interlayer to form... 

Bish, D. L. (1980). Anion-exchange in takovite application to other hydroxide minerals. Bull. Mineral. 103, 170. 

When a salt is introduced to water (e.g., A1C13s), the charged metal (Al3+) has a strong tendency to react with H20 or OH" and forms various Al-hydroxy species. Metal-hydroxide reactions in solution exert two types of influences on metal-hydroxide solubility, depending on the quantity of hydroxyl supplied. They either decrease or increase metal solubility. The solubility of a particular metal-hydroxide mineral depends on its Ksp, quantity of available hydroxyl, and solution pH of zero net charge. For example, aluminum (Al3+) forms a number of hydroxy species in water as shown below ... 

Surprisingly, it was not until 10 years later that Huggins published a series of definitive papers on hydrogen bonding in organic molecules [18], in ice and liquid water [19], and Bernal and Megaw in hydroxide minerals [20]. It appears to have been these three authors who supplied the first descriptors. Huggins used the term... 

Addition of base to aqueous solutions of Fe(III) in the presence ofthe ligandN(CH2C00H)2(CH2CH20H) ( heidi ), produced 19-iron and 17-iron species, neither of which have a 3-D framework of Fe(ni) ions. These species contain close-packed iron hydroxide cores bound, via oxide and hydroxide bridges, to Fe(III) located on the inner surface of the heidi coat. The inner core, which is common to both Fen and Fei9 compounds and consists of an [Fe7(/U3-OH)6(/tr2-0H)4 (/u-3-0)Fe 2] + unit, derives from a portion of an infinite 2-D [Fe(OH)2+] framework. This suggests that the ligand shell, [Feio(heidi)io(H20)i2(/(r3-0)4(/U2-OH)4] , traps the iron in an unusual, for Fe(IIl), hydroxide mineral structure, and poses the question of whether the core of ferritin is a similarly trapped structure. ... 

Inasmuch as the number of natural bodies where primary or secondary iron hydroxide minerals are observed is very great, we will limit ourselves to recent sediments of water basins and to the Kerch iron ores as examples. 

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