Humic complexes

G. Cacco and G. Dell Agnola, Plant growth regulator activity of soluble humic complexes. Can. J. Soil Sci, 64 225 (1984). 

In fresh water, silver may form complex ions with chlorides, ammonium (in areas of maximum biological activity), and sulfates form soluble organic compounds such as the acetate and the tartrate become adsorbed onto humic complexes and suspended particulates and become incorporated into, or adsorbed onto, aquatic biota (Boyle 1968). Where decaying animal and plant material are abundant, silver strongly precipitates as the sulfide or combines with humic materials (Smith and Carson 1977). 

A major experimental issue to be addressed is the rate and means by which particles are hydrolyzed and solubilized to provide substrates for heterotrophic bacteria, and the role of free enzymes in this process. Burns (1982) reviewed the possible locations and origin of enzyme activities in soils, and particularly underscored the potential importance of enzyme-humic complexes in microbial catalysis of substrates. As Burns (1982) discussed, enzymes associated with soil particles or humic substances are not subject to the same biochemical and physical restraints as are enzymes newly produced by microbial cells. Soil-held (or sediment-held) enzymes may therefore play a catalytic trigger role in substrate degradation, providing critical signals about substrate availability to the local microbial community. The conceptual model presented by Vetter et al. (1998) suggested that release of free enzymes into the environment may in fact represent... 

FIGURE 3 Diagenetic versus trophic perspectives on the relationship between molecular size and reactivity. Perceptions of size-reactivity relationships depend on whether the focus is on what is consumed or what remains. From the trophic perspective, small molecules turn over more rapidly than polymers, which turn over more rapidly than humic complexes. From the diagenetic perspective, large molecules are progressively stripped of reactive moieties, leaving behind small refractory residuals. 

Laird, D. (2001). Nature of clay-humic complexes in an agricultural soil II. Scanning electron microscopy analysis. Soil Sci. Soc. Am. I. 65,1419-1425. 

Andisols Modified from Ando From volcanic ejecta, dominated by alophane or humic complexes... 

Commonly, humic substances form a strong complex with clay. The most likely mechanisms of clay-humic complex formation are ... 

The relative contribution of the various mechanisms to forming clay-humic complexes are different for different clay minerals. 

A component can undergo considerable physico-chemical speciation alterations in an estuary. With respect to dissolved constituents, the composition and concentration of available ligands changes. Depending upon the initial pH of the riverine water, OH may become markedly more important down the estuary. Similarly, chlorocomplexes for metals such as Cd, Hg and Zn become more prevalent as the salinity increases. Conversely, the competitive influence of seawater derived Ca and Mg for organic material decreases the relative importance of humic complexation for Mn and Zn. 

The sediments are very organic and anoxic conditions have been formed. It is known that plutonium forms organic humic complexes. In watersheds from the catchment areas the plutonium concentrations are as high as up to 300-400 pBq and Cs, 30 mBq 1 In the major lake from where the river starts the concentrations are respectively 100 and 10 times lower. Our data suggests that concentrations downstream is due to bunding and transport by organic sediments which are transported by the river and trapped in the dam. 

There is presently no unique and completely satisfactory way of quantifying the metal-humic complexation reactions. The approach used depends on the kinds of questions one seeks to answer. 

Values for the thermodynamic stability constants of inoiganic complexes (with HCO, CO3 , S0 , Cl and 0H ) and organic complexes (except for Cu) of trace metals and major ions were chosen principally from the compilation by Zirino and Yamamoto [ 10] Tor Cd, Whitfield and Turner [11 ]for Pb, Van den Berg [9 ]for Cu, and Truesdell and Jones [12] for major ions. The formation of polynuclear complexes has generally been discounted (except for Pb). The activity coefficients of all species have been calculated using the Davies equation [l3 ]. The stability constants of humic complexes of Cu ( Cu-L been calculated in this work. 

Naturally-occurring humic-metal complexes have been isolated from estuarine systems and seawater using solid phase extraction (SPE) onto a Cig HPLC column to preconcentrate the sample (JO-12). Samples were subsequently eluted from the SPE colunm at a much higher concentration and injected onto another HPLC column and detected by UV absorbance and a metal-sensitive detector, such as atomic fluorescence spectroscopy. The concentration of metal-humic complexes in natural aquatic environments was then calculated. However, there was some evidence of competitive binding of the metal ion between the organic matter and free silanol groups in the stationary phase resulting in a loss of metal in the column and erroneously low metal values (10). 

In a study utilizing SEC and fluorescence, retention time increased with increased concentration of Cu " which in this mode of HPLC would indicate that there had been a decrease in molecular size (3). It was also hypothesized that ionic interactions between the humic-metal complex and charged surfaces such as free silanol groups in the column packing may have also contributed to the increased retention time. In addition to changes in retention time, there was a decrease in peak area with increased copper concentration for both the UV and fluorescence chromatograms. This may have been due to an irreversible binding (in the time-scale of the separation) of copper-humic complexes to the stationary phase in the presence of increased Cu. ... 

An incomplete list of some of the factors that influence metal-humic complexation includes concentration of humic substances and metal ions, competing ligands and metal ions, source of humic substances, type and... 

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