Metal-humate complex

Commonly, the amount of metal ions adsorbed by the solid surface increases with increasing pH for humic substances, clays, or clay—humic acid mixtures. Metal-ions adsorbed in acid media increase with pH until the threshold value required for partial dissolution of solid and formation of soluble metal-humate complexes is exceeded (Fig. 3.18). Metal-organic complexes experience three types of interactions, which... 

Manunza, B., S. Deiana, V. Maddau, C. Gessa, and R. Seeber. 1995. Stability constants of metal-humate complexes Titration data analyzed by bimodal Gaussian distribntion. Soil Science Society of America Journal 59, no. 6 1570-1574. 

Metal/ligand complexes (such as Al[OH]2+, Cu-humate) and organic/ligand complexes tend to be mobile in groundwater. 

Takahashi, Y., Minai, Y., Ambe, S., Makide, Y., Ambe, F. and Tominaga, T. (1997) Simultaneous determination of stability constants of humate complexes with various metal ions using multitracer technique. Sci. Total Environ., 198, 61-71. 

Humic substances, although a major fraction of soil DOM, do not facilitate oxide dissolution in laboratory studies. The mobilization of metals in soils may, however, be indirectly affected by humic substances. Solubilization of metals may be inhibited by competitive adsorption of humic substances and LMW organic ligands. The opposite effect—an enhancement of metal mobility—might result from the stabilization of dissolved metals as humate complexes. 

Finedly, it should also be pointed out that the alkali and alkali earth metal ions also participate in important non-specific interactions. These include (1) acting as counter ions in the electrical double layer stabilisation of ma-cromolecular and colloidal organo-metallic complexes (Shapiro, 1964 Parks, 1975 Eckert and Sholkovitz, 1976) and (2) in regulating the electrolyte properties and ionic strength (I) of natural waters (Whitfield, 1975) which, in turn, will affect the activity coefficients and conditional stability constants (K ) of organo-metallic complexes. For example, log values for Cu -, Pb - and Cd -humate complexes were found to decrease linearly with -v/l (Stevenson, 1976) or simply with I (Schnitzer and Khan, 1972). 

Humates complexes formed between metals and humic acid. 

Materials such as humates, fulvates, and melanins are related to, or contain, peptide or protein molecules or moieties. Kinetic patterns for their interactions with metal ions may be complicated by the probability that more than one complex species will be involved. Thus it has been demonstrated that Ni2+-fulvate solutions contain at least four kinetically distinct species, i.e. Ni2+aq and three complexes, as indicated in reactions of such solutions with for example par (514). Moreover in a metal-exchange reaction the metal ions M and M may complex at some... 

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