Humic substances complexation capacity

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

Clay minerals, oxides, and humic substances are the major natural subsurface adsorbents of contaminants. Under natural conditions, when humic substances are present, humate-mineral complexes are formed with surface properties different from those of their constituents. Natural clays may serve also as a basic material for engineering novel organo-clay products with an increased adsorption capacity, which can be used for various reclamation purposes. 

The capacity of complexing of humic substances is ascribed to their oxygen-based functional group (table 8.19). 

Frimmel, F. H., Immerz, A., and Niedermann, H. (1984). Complexation capacities of humic substances isolated from freshwater with respect to copper(II), mercury(II) and iron(II,III). In Complexation of Trace Metals in Natural Waters, Kramer, C. J. M., and Diunker, J. C., eds., Martinus Nijhoff/Dr. W. Junk Publishers, The Hague, pp. 329-343. 

Due to the ubiquitous occurrence of humic substances and colloids (defined as entities of Inm-lpm size) in natural waters, their specific properties, in particular their scavenging capacities towards metallic cations and also their well-established mobility (1-6), these organic and inorganic species could have important effects on the fate and mobility of these cations in natural systems. On one hand, the formation of organic complexes or pseudocolloidal species will modify the speciation (distribution of chemical species) of the cation of interest and its solubility. On the other hand, they can retard cation migration... 

Trace elements in cationic form are probably not dominantly sorbed on 001 faces of phyllosilicates because they are always vastly outnumbered by other cations with which they compete (Jackson, 1998). They may be strongly sorbed only on the edges of the phyllosilicates. However, clay minerals also have an important role as carriers of associated oxides and humic substances forming organomineral complexes, which present peculiar sorption capacities different from those of each single soil constituent (Jackson, 1998 Violante and Gianfreda, 2000 Violante et al., 2002c). 

Thus, aquatic humic substances are interesting because of their capacity to complex trace elements (Thurman and Malcolm, 1981 Huljev, 1986 Thanabalasingam and Pickering, 1986 Perdue, 1989 Dellis and Moulin, 1989 Chin et ai, 1994). 

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