Humic acids redox potential

Humic substances not only contribute to increase Fe bioavailability through their Fe chelating properties, but are also known to be redox reactive and capable of chemically reducing metals, including Fe3+ (Skogerboe and Wilson, 1981 Struyk and Sposito, 2001). Standard redox potentials for fulvic and humic acids have been evaluated to be around 0.5 and 0.7 V, respectively. It has been shown that reduction of Fe3+ occurs significantly at pH values lower than 4 at higher pH values, reduction is decreased by formation of complexes between Fe3+ and humic molecules (Chen et al., 2003). 

Considering the pH range 4—10 as the geochemical range, the redox potential due to the breakdown of water due to redox processes, the rare earths are predominantly present as Ln(III). Since the anions OH- and CO2 are present in natural environments, rare earths combine with these anions to form insoluble hydroxides and carbonates and hence immobilized. At lower pH, rare earth ions are adsorbed on clays, which are natural ion exchangers. The interactions of rare earth ions with humic and fulvic acids in soils, and Fe/Mn oxides are so strong, that they become immobile. 

Metal-complexing ligands, including humic and fulvic acids, by preferentially com-plexing with Fe or Fe " actually shift the redox potential of soil solutions. This fact can be illustrated simply by considering the Nernst equation for the Fe /Fe redox couple (see Table 7.1) ... 

Donors of appropriate redox potential to react with holes at the anatase surface include organic acids, carbohydrates, fats, CN, and halides 2 ). (The cyanide reaction has been studied for its utility in treatment of the waste streams from Hold mininK operations in the Canadian Northwest Territories.) More immediately releyant to natural water is the observation that an anatase slurry could effect the decoloration of a chlorinated bleach plant effluent. A sample of amber colour, pH = 1.8, and low residual chlorine was irradiated in the presence of 0.5% (wt) anatase with li((ht of 350 nm for periods up to 18 hr. The optical absorbance decreased by half in 1080 min. Small amounts of chloride and formaldehyde were detected ( ). This reaction may provide a precedent for observation of a relation between photobleachinK of humics in water and metal ions. If so, we are brouj ht to the question of the reactivity of colloidal iron oxides. 

The oxidation of pyrite is suppressed by coating it with oxalic acid because this treatment lowers the standard redox potential of Fe(III)/Fe(II) and forms a complex (Sasaki et al. 1996). Belzile et al. (1997) studied the possible passivation of pyrite by oxalic acid as compared with other agents such as acetyl acetone, humic acid and sodium silicate on samples from Kidd Creek mine in Timmins, Ontario. They concluded that oxalic acid was more efficient than the other agents tested in reducing oxidation of pre-oxidized samples. However, it was noted that the effectiveness of oxalic acid requires high temperature (65 °C), which restricts its field application it is also toxic (Belzile et al. 1997). 

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

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