Humic substances reactivity

Bailey G.W., Akim L.G., Shevchenko S.M. Predicting chemical reactivity of humic substances for minerals and xenobiotics use of computational chemistry, scanning probe microscopy, and virtual reality. In Humic Substances and Chemical Contaminants, C.E. Clapp, M.H.B. Hayes, N. Senesi, P.R. Bloom, P.M. Jardine, eds. Madison, WI Soil Science Society of America, Inc., 2001. 

Phenolic compounds have also been oxidatively polymerized to humic substances by clay minerals (29) and by the mineral fraction of a latasol (66). After a 10-day equilibration period, montmoril-lonite and illite clay minerals yielded 44 to 47% of the total added phenolic acids as humic substances whereas quartz gave only 9%. Samples of a latasol yielded over 63% of the total amount, from mixtures in varied proportion, of mono-, di- and trihydroxy phenolic compounds as humic substances (66). Extractions of the reaction products yielded humic, fulvic, and humin fractions that resembled soil natural fractions in color, in acid-base solubility, and in infrared absorption spectra. Wang and co-workers (67) further showed that the catalytic polymerization of catechol to humic substances was, enhanced by the presence of A1 oxide and increased with pH in the 5.0 to 7.0 range. Thus the normally very reactive products of Itgnin degradation can be linked into very stable humic acid polymers which will maintain a pool of potentially reactive phytotoxins in the soil. 

FIGURE 6 Potential interactive pathways and processes of humic substances emanating from decomposition products of higher plant tissues with extracellular and surface-bound enzymes and photolytic reactions, particularly with UV irradiance. Humic acid-enzyme complexes can be stable for long periods (weeks and months) and subsequently reactivated upon exposure to weak UV light. Further photolysis can cleave simple compounds from the macromolecules for subsequent utilization by microbes. 

Boavida, M.-J., and R. G. Wetzel. 1998. Inhibition of phosphatase activity by dissolved humic substances and hydrolytic reactivation by natural UV. Freshwater Biology 40 285—293. 

Piccolo, A. (1989). Reactivity of added humic substances towards plant-available heavy metals in soils. Sci. Total Environ. 81-82, 607-614. 

REACTIVITY OF HUMIC SUBSTANCES IN AMENDED SOILS 4.5.1. Interaction with Metal Ions... 

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). 

The situation is different for aqueous species of humic substances, the organic matter in soil that is not identifiable as unaltered or partially altered biomass or as conventional biomolecules.21 Humic substances comprise organic compounds that are not synthesized directly to sustain the life cycles of the soil biomass. More specifically, they comprise polymeric molecules produced through microbial action that differ from biopolymers because of their molecular structure and their long-term persistence in soil. This definition of humic substances implies no particular set of organic compounds, range of relative molecular mass, or mode of chemical reactivity. What is essential is dissimilarity to conventional biomolecular structures and biologically refractory behavior. 

The adsorption-desorption reaction in Eq. 4.3 has been applied to soils in an average sense in a spirit very similar to that of the complexation reactions for humic substances, discussed in Section 2.3.11 Although no assumption of uniformity is made, the use of Eq. 4.3 to describe adsorption or desorption processes in chemically heterogeneous porous media such as soils does entail the hypothesis that effective or average equilibrium (or rate) constants provide a useful representation of a system that in reality exhibits a broad spectrum of surface reactivity. This hypothesis will be an adequate approximation so long as this spectrum is unimodal and not too broad. If the spectrum of reactivity is instead multimodal, discrete sets of average equilibrium or rate constants—each connected with its own version of Eq. 4.3—must be invoked and if the spectrum is very broad, the sets of these parameters will blend into a continuum (cf. the affinity spectrum in Eq. 2.38). 

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