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Humic acids chemical properties

Kinetically, the adsorption of humic acids at a solid-water interface is controlled by convection or diffusion to the surface. Even at concentrations as low as 0.1 mg/e near-adsorption equilibrium is attained within 30 minutes. At high surface densities, a relatively slow rearrangement of the adsorbed molecules may cause a slow attainment of an ultimate equilibrium (Ochs, Cosovic and Stumm, in preparation). The humic acids adsorbed to the particles modify the chemical properties of their surfaces, especially their affinities for metal ions (Grauer, 1989). [Pg.114]

Oik DC, Cassman KG, Mahieu N, Randall EW. 1998. Conserved chemical properties of young humic acid fractions in tropical lowland soil under intensive irrigated rice cropping. European Journal of Soil Science 49 337-349. [Pg.273]

Piret, E., White, R., Walther, H., and Madden, A., Jr. (1960). Some physico-chemical properties of peat humic acids. Sci. Proc. R. Dublin Soc. Al, 69-79. [Pg.141]

Senesi, N., Sposito, G., Holtzclaw, K. M., and Bradford G. R. (1989). Chemical properties of metal-humic acid fractions of a sewage sludge-amended aridisol. I. Environ. Qual. 18, 186-194. [Pg.180]

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). [Pg.354]

The purposes of this paper are then (a) to refer on some additional results obtained, in order to confirm the previously suggested [9, 10 ] involvement of humic free radicals in the cross coupling reactions of humic substances with xenobiotic phenoxy radicals and (b) to discuss comparatively the different behaviours of humic acids in these reactions, as a function of their different origin and chemical properties. [Pg.184]

In conclusion, toxic chlorinated phenol intermediates formed during the chemical, photochemical and/or enzymatic degradation of chlorophenoxyalkanoic compounds would temporarily be detoxified when they are incorporated into the humic acid, since their bioavailability and movement into terrestrial and aquatic ecosystems would be greatly reduced. However, the knowledge of the potential toxicity problems which these bound-residues could give rise to in the environment is still very limited. Xenobiotic chemicals incorporated into humic polymers are not really removed from the ecosystem and they may maintain their identity and toxic properties for unknown time spans, eventually causing time-delayed pollution problems, if and when they will be released from humic substances. [Pg.195]

Research on the chemical properties of humic substances was extended by the Swedish investigator Berzelius (1839). One of his main contributions was the isolation of two light-yellow-colored humic substances from mineral waters and a slimy mud rich in iron oxides. They were obtained from the mud by extraction with base (KOH), which was then treated with acetic acid containing copper acetate. A brown precipitate was obtained ctilled copper apocrenate. When the extract was neutralized, another precipitate was obtained, called copper crenate. The free acids, apocrenic and crenic acids, were then brought into solution by decomposition of the copper complexes with alkali. These newly described humic substances were examined in considerable detail, including isolation, elementary composition, and properties of their metal complexes (Al, Fe, Cu, Pb, Mn, etc). [Pg.15]

General Chemical Properties of Humic and Fulvic Acids... [Pg.21]

The formation of humic substances in a peatland environment is a complex humification process which is principally due to certain enzymatic and microbial activities. These organic matter transformation processes are influenced by the nature of the peat-forming plants and certain physical and chemical properties within a particular peatland. In very acidic or low nutrient peatlands a very different microflora may exist than in a more eutrophic and less wet situation. In the former situation humification may be retarded and the peatland plants will be preserved and thus accumulate. On the other hand, in less acid environments with moderate amounts of nutrients and periodic water-table fluctuations humification proceeds relatively rapidly and leads to decomposed organic soils such is the case in drained and cultivated organic soils. [Pg.84]

Elemental composition is an important chemical property which can be used to establish the nature and source of humic substances. When the percent composition data are displayed as the atom ratios H C, 01C, and NIC, some general characteristics become visible. Soil, coal, marine, and aquatic humates may be distinguished, one from the other. Structural trends may be identified in specific environments, such as lake sediments and soil profiles. Nonhumate contaminants can be detected. Atom ratios may also aid the investigator in proposing hypothetical structures for humic and fulvic acids and serve as a guide in the synthesis of artificial humates. [Pg.457]


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