Oxide mineral reductive capacity

Fendorf SE, Jardine PM, Taylor DL, Brooks SC, Rochette EA (1998) Auto-inhibition of oxide mineral reductive capacity toward Co(II)EDTA. ACS Symp Ser 715 (Mineral-Water Interfacial Reactions), American Chemical Society, Washington, DC, p 358-371... 

Auto-Inhibition of Oxide Mineral Reductive Capacity Toward Co(II)EDTA... 

Non-enzymatic attack In non-enzymatic attack of minerals by microbes, reactive products of microbial metabolism come into play. The microbial enzymes responsible for metabolic product formation are located below the cell envelope, in the cytoplasm of prokaryotes (Bacteria and Archaea) and in cell organelles and/or the cytoplasm of eukaryotes (e.g. fungi, algae, lichens). In these instances of microbial attack, physical contact of the microbial cells with the surface of a mineral being attacked is not essential. The reactive metabolic products are formed intracellularly and are then excreted into the bulk phase where they are able to interact chemically, i.e. non-enzymatically, with a susceptible mineral. Depending on the type of metabolic product and mineral, the interaction with the mineral may result in mineral dissolution or mineral diagenesis by oxidation or reduction or acid or base attack. Mineral dissolution or diagenesis may also be the result of complexation by a microbial metabolic product with that capacity. In some instances mineral attack may involve a combination of some of these reactions. 

Not all iron oxides are available for reduction. Some iron minerals are solid crystals or even entire iron grains, which makes them resistant to microbial reduction (Lovley, 1991 Postma, 1993 Heron et al., 1994b). Other iron oxides or hydroxides are amorphous and readily reducible. Over time, even some crystalline minerals such as goethite and hematite may be reduced in the complex environment in leachate (Heron and Christensen, 1995). This indicates that the importance of iron as a redox buffer controlling the size of plumes is not given just by the amount of iron oxides present. The composition and microbial availability of iron for reduction are key parameters. Methods for the actual quantification of the microbial iron reduction capacity have, however, not been developed. 

Reactive fractions have been addressed by mild chemical extractions (hydrochloric acid or ascorbic acid), but this is only an operationally defined quantity of easily dissolved, oxidized minerals. The actual pool of microbial available iron reduction capacity may be better determined using microbial assays. 

Several studies have shown that the more crystalline the Fe(III) and Mn(IV) oxides, the slower the rate of their reduction (reviewed by Lovley, 1991, 2004). In laboratory experiments, Ottow and Klopotek (1969, cited by Lovley, 1987) reported that the reduction capacity of select iron minerals was found to be in the order of FeP04 > Fe(OH)j > FeOOH > Fe203. Amorphous and poorly crystalline forms of Fe(III) and Mn(IV) oxides dominate wetlands that undergo frequent wet and dry cycles. These systems are dynamic, and repeated oxidation-reduction reactions involving iron and manganese will not allow time for stable crystalline forms of Fe(III) and Mn(IV) oxides to form in these systems. Strong relationships are observed between Fe(III) reduction rates and poorly crystallined forms of Fe(III) oxides (as determined by hydroxylamine extraction) in fresh and brackish water sediments (Lovley and Philips, 1987). 

The preceding results were conducted under laboratory conditions using freshly prepared natural mineral surfaces that were reacted for relatively short time periods. In order to assess the effectiveness of natural Fe(II) oxides in reducing and immobilizing transition metal under natural aquifer conditions, several important parameters need to be assessed. These include the reductive capacity of the oxide minerals, the impact of surface passivation and the effects of competition and poisoning by of other aqueous species. 

Soil-leaching studies indicate that some silica is released from soil rather rapidly. McKeague and Cline (20) have shown that in soil—water mixtures at 100% water saturation, the silica in solution after 5 minutes was approximately half as great as that after 10 days. After the first day or two the silica concentration increased very slowly. They also demonstrated that pH has a marked effect on silica concentrations in soil solutions (21). These authors attributed the control of silica concentration to pH-dependent adsorption and indicated that, of the common soil minerals, iron and aluminum oxides have appreciable adsorption capacity. Jones and Handreck (22, 23) studied the effects of iron and aluminum oxides on silica concentrations in soil solutions and concluded that both caused a significant reduction in dissolved silica, with aluminum oxides being most effective. Minimum silica concentrations occurred at pH 9-10 in solutions in contact with iron and aluminum oxides. Harder and Flehmig (24) reported that the hydroxides of iron, aluminum, and other elements could remove silica from solutions containing as little as 0.5 mg/liter SiOo. 

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