Iron oxides bioavailability

Sharpley, A.N. (1993) An innovative approach to estimate bioavailable orthophosphate in agricultural runoff using iron oxide-impregnated paper. J. Environ. Qual. 22 597-601... 

N. Sequi, P. (1994) Interlaboratory comparison of iron oxide-impregnated paper to estimate bioavailable phosphorus. J. Environ. 

The iron oxide and clay content of soils and sediments can affect the bioavailability of selenium markedly. The strong pH dependence of adsorption is an important control. Maximum adsorption occurs between pH 3 and pH 5 and decreases as the pH rises. Organic matter also removes selenium from soil solution, possibly as a result of the formation of organometallic complexes. Addition of PO4 to soils increases selenium uptake by plants, because the PO ion displaces selenite from soil particles making it more bioavailable. Conversely, increasing the concentrations of PO4 in soils can... 

Hacherl, E.L. et al.. Measurement of iron (III) bioavailability in pure iron oxide minerals and soils using anthraquione-2,6-disulfonate oxidation. Emiron. Sci. Technoi, 35. 4886, 2001. 

In sulfate-dominated wetlands, production of sulfide (through biological reduction of sulfate) and formation of ferrous sulfides may preclude phosphorus retention by ferrous iron in regulating phosphorus bioavailability (Caraco et al., 1991). In iron- and calcium-dominated systems, Moore and Reddy (1994) observed that iron oxides likely control the behavior of inorganic phosphorus under aerobic conditions, whereas calcium phosphate mineral precipitation governs the solubility under anaerobic conditions. This difference is in part due to a decrease in pH under aerobic conditions as a result of oxidation of ferrous iron compounds, whereas an increase in pH occurs under anaerobic conditions as a result of reduction of ferric iron compounds. The juxtaposition of aerobic and anaerobic interfaces promotes oxidation-reduction of iron and its regulation of phosphorus solubility. 

Reduction of Fe(HI) oxides (at pH = 7) occurs in an Eh range of 0-100 mV (Figure 10.13). Typically, iron oxides are not reduced in the presence of nitrate, whereas they can be reduced in the presence of manganese oxides. However, if manganese oxides are present in bioavailable form, it is likely that facnltative bacteria wonld prefer Mn(IV) over Fe(HI). The redaction of Fe(III) and Mn(IV) can also occnr in different redox zones of the soil profile. In permanently waterlogged soils, most of the bioavailable Fe(III) and Mn(IV) are already present in rednced forms. Under these... 

The forms and amounts of Fe(lII) and Mn(lV) oxides are important factors in controlling the extent of organic matter decomposition with these metals as electron acceptors. The most available forms of these electron acceptors for bacterial reduction are dissolved Fe(lll) and Mn(lV) forms, which include Fe(III) and Mn(lV) in solution under acidic pH conditions Fe(IIl) and Mn(IV) complexes and Fe(III) and Mn(lV) complexes (chelates) with DOM. Insoluble forms include amorphous and a range of crystalline forms of Fe(III) and Mn(IV) oxides. In addition, particle size and available surface area may also influence the bioavailability of Fe(III) and Mn(lV) oxides. These oxides also occur as a complex mixture of each other and as coatings on clay, silt, and sand particles. Iron oxides can also be present as occluded coprecipitates on soil particles. Thus, the bioavailability of Fe(III) and Mn(IV) oxides is in the order of dissolved Fe and Mn metals complexed with DOM amorphous forms > crystalline forms. 

The geochemical phases of phosphorus are the important parameters that reflect the bioavailability and ecological effects of phosphorus. As is well known, aqueous phosphate shares with nitrate the characteristic of being a major growth-limiting nutrient in the global biosphere, and apatite and iron oxide weathering are the major aquatic supplies. 

Elevated lead contents were recorded in various species of plants from the vicinity of metal smelters, roadsides, soils heavily contaminated with lead, natural ore deposits, and lead recycling factories. Bioavailability of lead in soils to plants is limited, but is enhanced by reduced soil pH, reduced content of organic matter and inorganic colloids, reduced iron oxide and phosphorus content, and increased amounts of lead in soils. Lead, when available, becomes associated with plants by way of active transport through roots and by absorption of lead that adheres to foliage. Lead concentrations were always higher in the older parts of plants than in shoots or flowers. 

Photolysis in ice has a role also for inorganic compounds. As an example, it was found that the photoreductive dissolution of iron oxide particles to form bioavailable iron (Fe(II)aq) was slow in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice. This occurred independently on the irradiation wavelength and on the type of oxides [hematite. 

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