Complexes humic substances

Plant materials decompose to more simple products from which the complex humic substances are synthesized. 

As stated by Van der Sloot (1998), several factors can influence the release of contaminants from both granular and monolithic materials. These include major element chemistry, pH, redox status of the system, presence of complexants, humic substances or other dissolved organic compounds, liquid to solid ratio, and biological activity. 

Figure 5. Cu(II)-humic complex on the goethite surface, showing a Type A complex (Cu(II) forms bridge between humic substance and the goethite surface) and a Type complex (humic substance forms a bridge between Cu(II) and the goethite surface). Also shown is an inner-sphere bidentate Cu(II) complex on the goethite surface. XAFS data consistent with this model were collected on NSLS beam line X-l 1 A. [Reprinted from Alcacio et al. (2001), Fig. 1, with permission from Elsevier Science]...

Similarly, polyphenols are synthesized by microorganisms from nonhumic substances, which are subsequently converted to quinines through enzymatic reactions. The conversion of polyphenols formed from both sources to quinines is catalyzed by oxygen-requiring phenol oxidase enzymes. Quinones polymerize with or without amino compounds to from complex humic substances (Figure 5.15b). 

Humic substances can form complexes with metals, including cationic micronutrients (36), thanks to the presence of electron-donor functional groups in these molecules. It therefore appears evident that due to these properties, humic substances can contribute to the regulation of the chemical balances of metals, thus influencing their solubility (5). With regard to plant availability, the molecular dimension and solubility of humic substances are very important. 

J. Gerke, Aluminium and iron (III) species in the soil solution including organic complexes with citrate and humic substances. Z. Pflanzenemarhr. Bodenk. 160 421 (1997). 

Complexation Partly Humic substances are very significant factors in near-surface complexation processes, probably less so in die deep-well environment. Data on complexation in saline waters are probably most relevant. 

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]. 

In natural waters, dissolved zinc speciates into the toxic aquo ion [Zn(H20)6]2+, other dissolved chemical species, and various inorganic and organic complexes zinc complexes are readily transported. Aquo ions and other toxic species are most harmful to aquatic life under conditions of low pH, low alkalinity, low dissolved oxygen, and elevated temperatures. Most of the zinc introduced into aquatic environments is eventually partitioned into the sediments. Zinc bioavailability from sediments is enhanced under conditions of high dissolved oxygen, low salinity, low pH, and high levels of inorganic oxides and humic substances. 

Florence, T. M., Powell, H. K. J., Stauber, J. L. and Town, R. M. (1992). Toxicity of lipid-soluble copper(II) complexes to the marine diatom Nitzschia Closterium -amelioration by humic substances, Water Res., 26, 1187-1193. 

Filella, M. and Town, R. M. (2001). Heterogeneity and lability of Pb(II) complexation by humic substances practical interpretation tools, Fresenius J. Anal. Chem., 370, 413 118. 

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