Lead Hydr oxides

2 PbO from Thye Ming Industrial (Taiwan) Properties Average grain size 400 nm [123]. 

3 PbO from Ventron Properties 99.9% pure, specific surface area 0.44 m-/g, particle size 2 pm, litharge-massicot [1736]. 

4 Commercial, Origin Unknown Water-washed, and dried at 110°C. 

1 Precipitated from PbtNO,), Solution with 0.01 M NaOH at23°C, 

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The weathering of siderite (reaction (2)) and ankerite (reaction (5)) in the presence of 02 can sometimes lead to dissociation of dissolved HCO3", resulting in the release of C02 to the atmosphere a significant proportion of the atmospheric 02 can also be trapped in the solid form as secondary ferric (hydr)oxide precipitates (reactions (3) and (4)). 

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

Other metal sulfides, such as galena (PbS) and sphalerite (ZnS), may affect the mobility of arsenic in anoxic environments. However, immobilization depends on surface complexation rather than precipitation. In contrast to iron (oxy)(hydr)oxides (discussed later), As(III) adsorption on galena and sphalerite increases with pH (Bostick, Fendorf and Manning, 2003). Surface complexation does not occur by isomorphic substitution of lead or zinc, or by a ligand exchange mechanism. Instead, multinuclear, inner-surface arsenic-thiosulfide complexes probably form on galena or sphalerite surfaces (Bostick, Fendorf and Manning, 2003). 

Of the metal sorbents, amorphous to poorly crystalline iron (oxy)(hydr)oxides are most efficient at sorption because of their large surface areas (Chapters 2,3, and 7). However, as these compounds crystallize into hematite, magnetite, or other minerals, their surface areas decrease. Although the affinity of the iron (oxy)(hydr)oxides to sorb arsenic may not always change very much as a result of crystallization (Dixit and Hering, 2003), the reduction of surface area may lead to the release of surface-complexed arsenic (O Shea, 2006). Smedley and Kinniburgh (2002) provide a detailed list of sorption studies dealing with metal (oxy)(hydr)oxides (Table 6.1). 

Oxidation of arsenic-bearing pyrite with adsorption onto iron oxides and/or other metal (oxy)(hydr)oxides Nitrate reduction by pyrite oxidation (note that Appelo and Postma, 1999 referred to pure rather than arsenian pyrite) Manganese oxide reduction and release of sorbed arsenic Fe(lll) reduction on oxide surfaces changes net charge leading to arsenic desorption Iron oxide reductive dissolution and release of sorbed arsenic catalyzed by NOM degradation... 

The oxidation rate of As(III) in the presence of manganese and water may be substantially enhanced by manganese-oxidizing bacteria, such as Leptothrix ochracea (Katsoyiannis, Zouboulis and Jekel, 2004). Katsoyiannis, Zouboulis and Jekel (2004) found that the bacteria are important in oxidizing Mn(II) to Mn(IV), Fe(II) to Fe(III), and As(III) to As(V). The oxidation of Mn(II) leads to the precipitation of Mn(IV) (oxy)(hydr)oxides, which then abiotically oxidize additional As(III) and significantly sorb the As(V) that results from both abiotic and biotic oxidation. 

Hydrosilicate formation is also in evidence in the Cu(II)-Si02 system. Via precipitation from a homogeneous solution one can obtain highly dispersed copper oxide on silica (cf. above, Fig. 9.10, where it should be noted that the Cu case is more complicated than the Mn one in that intermediate precipitation of basic salts can occur). Reaction to copper hydrosilicate is evident from temperature-programmed reduction. As shown in Fig. 9.12 the freshly dried catalyst exhibits reduction in two peaks, one due to Cu(II) (hydr)oxide and the other, at higher temperature, to Cu(II) hydrosilicate. Reoxidation of the metallic copper particles leads to Cu(II) oxide, and subsequent reduction proceeds therefore in one step. The water resulting from the reduction of the oxide does not produce significant amounts of copper hydrosilicate, in contrast to what usually happens in the case of nickel. 

Nelson, Y.M. et al., Effect of oxide formation mechanisms on lead adsorption by biogenic manganese (hydr)oxides, iron (hydr)oxides, and their mix lures, Environ. Sci. Technol., 36, 421, 2002. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Upon dissolution of oxides and (hydr)oxides of manganese and iron under reducing conditions at depth, divalent cations of these elements can diffuse upwards through the pore waters to be oxidized and precipitated in near-surface oxic layers, leading to the characteristic near-surface enrichment of manganese and iron in the sediments of many well oxygenated lakes (see Section 3.2.4.3). Phosphorus and... 

To exemplify inhibition effects, we choose a few case studies with Fe(III)(hydr)oxides because these oxides are readily dissolved with protons, ligands, and reductants and are of great importance in the iron cycles in natural waters. The reductive dissolution of Fe(III) minerals by a reductant such as H2S is much faster than ligand- or proton-promoted dissolution. The dissolution reaction, as shown by Dos Santos-Afonso and Stumm (1992), is initiated by the formation of =FeS and =FeSH surface complexes the subsequent electron transfer within the complex leads to the formation of Fe(II) centers in the... 

In addition to sparingly soluble metal (hydr)oxides, salt type materials involving two such oxides or more, and clay minerals, whose crystallographic and thermochemical data are presented in Chapter 2, the zero points of zeolites, clays, and glasses are listed (in this order) after mixed oxides. Soils and other complex and ill-defined materials are on the end of the list. It should be emphasized that the terms soil , sediment", etc. have somewhat different meanings in different scientific and technical disciplines. This may lead to confusion, e.g. terms kaolin (clay) and kaolinite (clay mineral) are treated as synonyms in some publications. The zero points obtained for composite materials with a layer structure (core covered by coating) are listed separately from those in which the distribution of components is more uniform. 

Congruent dissolution of Fe(III) (hydr)oxides would be expected to release arsenic until the surface site concentration is depleted below the level of adsorbed arsenic (in combination with other high-affinity anions, such as phosphate). Secondary mineralization of iron, however, convolutes this view, leading to the potential for arsenic to be incorporated on or in the newly forming solids. 

The most widely used concept to describe the basic charging behaviour of metal (hydr)oxides is the two step protonation reaction which leads to a so called V o-pK model for the description of the basic charging ... 

It has been proposed that the extent to which mixed-cation hydroxide compounds actually do form in aquatic and terrestrial environments is limited more by slow rates of soil mineral dissolution, a necessary preliminary step, than by lack of thermodynamic favorability (57). Because the dissolution rates of clays and oxide minerals are fairly slow, the possibility of mixed-cation hydroxide formation as a plausible "sorption mode" in 24 hour-based sorption experiments (and also most long-term studies) containing divalent metal ions such as Mg, Ni, Co, Zn, and Mn and Al(III)-, Fe(III)-, and Cr(III)-(hydr)oxide or silicate minerals has been ignored in the literature 16,17). This study and others recently published (77), however, suggests that metal sorption onto mineral surfaces can significantly destabilize surface metal ions (A1 and Si) relative to the bulk solution, and therefore lead to an enhanced dissolution of the clay and oxide minerals. Thus, predictions on the rate and the extent of mixed-cation hydroxide formation in aquatic and terrestrial environments based on the dissolution rate of the mineral surface alone are not valid and underestimate the true values. 

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