Hydroxide minerals, reductive dissolution

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. 

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. 

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... 

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. 

Under reducing conditions, the dissolution of some oxide and hydroxide minerals of transition metals is greatly accelerated. This includes the environmentally important oxide/hydroxide minerals of Mn(III/IV), Fe(III), Co(in), and Pb(IV) (Stone, 1986). The lower valence ions produced by reduction, e.g., Fe and Mn, are much more mobile in aqueous systems than the ions of higher oxidation states. 

The rate of formation of dissolved iron(II), d[Fe2 + ]diss/df, depends, in addition, on the efficiency of detachment of reduced surface iron ions from the crystal lattice. The detachment step is a key step in the overall dissolution kinetics of slightly soluble minerals, since it is assumed to be the rate-determining step (Stumm and Furrer, 1987). The efficiency of detachment depends primarily on the crystallinity, and thus the stability of the iron(IIJ) hydroxide phase, and also on the coordinative surrounding of the reduced surface metal centers. It has been shown that a combination of a reductant and a ligand that forms stable surface complexes in the dark is especially efficient for the thermal reductive dissolution of hydrous iron(III) oxides (Banwart, et al. 1989). The role of a ligand as an electron donor and as a detacher in the photochemical dissolution of hydrous iron(III) oxides remains to be elucidated. 

Metal oxides, hydroxides and sulphides are represented in minerals whose properties are substantially defined by the nature of their central atom and its valence. The metal and sulphur valence depends on the redox environment. In this case all processes of dissolution and mineral-formation are viewed in conditions of stable redox environment, in which they do not change their charge. It is assumed that oxides are in oxidation medium and sulphide in the reduction medium, with solution Eh no greater than -0.2 v. 

Alkali hydroxide solutions of Am(vi) are yellow in color and, according to Cohen [239], can be prepared by ozone oxidation of a slurry of Am(OH)3 in any alkali hydroxide. Reduction of Am(vi) is observed on going from acid to alkaline pH and back again. A light-tan solid gradually precipitates from alkali hydroxide solutions of Am(vi). Dissolution of this substance in dilute mineral acids yields a solution of Am(v). It is claimed that Am(vi) disproportionates into Am(vii) and Am(v) in > 10 M NaOH [235]. 

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