Organic Matter Complexation

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

A. S. Boyd and M. M. Mortland, Enzyme interactions with clays and clay-organic matter complexes. Soil Biochemistry, Vol. 6 (J.-M. Bollag and G. Stotzky, eds.), Marcel Dekker, New York, 1990, p. I. 

Under aerated conditions at neutral to alkaline pH, inorganic iron is extremely insoluble (8), such that plants and microorganisms rely absolutely on iron uptake from organic matter complexes or iron that has been solubilized by siderophores and organic compounds contained in root exudate. Low-molecular-weight root exudates that dissolve iron include organic acids that are secreted by plant roots as a specific response to iron deficiency (9) or that are released constitutively at... 

Delhi soils by studying its speciation in the soil profile and to assess if there was any spatial variability. Soils representing the Aravali Ridge and the alluvial floodplains of river Yamuna were collected as a single, undisturbed core up to a depth of lm and the profile differentiated into four layers- 0-17 cm, 17-37 cm, 37-57 cm, and 57-86 cm. Pseudo total Aluminum and Iron in the soils were speciated into the operationally defined species (weakly exchangeable, organic matter complexes, amorphous oxides and hydroxides, and crystalline or free oxides) by widely recommended selective extraction procedures. Both A1 and Fe in these soils are bound predominantly as Fe oxides and silicates and have only very low percentages in the easily mobilizable pools. 

Important selectivity enhancements are observed upon complexing with neutral ligands. The stability constants of the adsorbed complexes exceed the values in aqueous solution by two to three orders of magniture. Such observation may be relevant to the behaviour of transition elements in the environment in that stability constants of adsorbed organic matter complexes may differ from the values found for solution phase equilibria. Such effects are indeed observed for Cu (139) and Ca (132). 

Ardakani, M.S., and Stevenson, F.J. A modified ion exchange technique for the determination of metal-soil organic matter complexes. Soil Sci. Soc. Amer. Proc. 36, 884-890 (1972). 

The organic-matter (giving inert Cu-Organic-matter complexes) concentration was determined by a chelation study of copper in the collected samples (see text). 

In their study of the Weddell Sea, Westerlund and Ohman tried to measure speciation by comparing the total dissolved Cd concentration and the recoverable metal concentration using an imminodiacetic resin on samples at natural pH (127). The results showed that, especially for samples collected over the Filchner shelf, the Cd recovered by Chelex resin in the upper 500 m of the water column was appreciably lower than the total concentration. They concluded that this layer probably contains organic matter complexing this element and they related this to the particle composition along the water column. 

Zunino, H., Galindo, G., Peirano, P., and Aguilera, M. (1972). Use of the resin exchange method for the deteimination of stability constants of the metal-soil organic matter complexes. Soil Sci. 114, 229-233. 

Fitch, A., and Stevenson, F. J. (1983). Stability constants of metal-organic matter complexes theoretical aspects and mathematical models. In The Significance of Trace Elements in Solving Paragenetic Problems and Controversies, ed. Augustithis, S. S., Theoprastus Publications, Athens, Greece, 645-649. 

Dumat, C., Quiquampoix, H., and Staunton, S. (2000). Adsorption of caesium by synthetic clay-organic matter complexes effect of the nature of organic polymers.Environ. Sci. Technol. 34, 2985-2989. 

A further contribution to stability in metal-organic matter complexes is the chelate effect, that is, the entropy created by the complexation reaction. This is only a consideration if the metal bonds to two or more functional groups, for example ... 

This pattern of complexation and precipitation behavior is qualitatively similar for all metal ions that complex readily with humus and hydrolyze to form insoluble hydroxides or oxides. For example, Fe complexed by humus at very low pH is probably bonded by polyphenols but as the pH is raised, precipitation of Fe(OH)3 reduces iron solubility and strips Fe from organic matter complexes. 

Organic Matter Complexation ModeL Soil humus contains a variety of acidic functional groups, some of which have high specificity for Al complexation or chelation (see section 4.3b). Al-organic complex formation requires that the acidic group, RH , dissociate ... 

Cobalt occurs in two oxidation states in soil, +2 and H-3, but Co is the dominant form in soil solution. This metal associates preferentially with Fe and Mn oxides because of chemisorption and co-precipitation. There is evidence that, on Mn oxides, Co is oxidized and strongly bound as Co. Consequently, strongly oxidizing conditions in the soil are likely to favor the adsorption of cobalt. As the soil pH is raised, Co solubility decreases because of increased chemisorption on oxides and silicate clays, complexation by organic matter, and possibly precipitation of Co(OH)2. Organic matter complexes with Co are fairly labile, so that organically bound Co ... 

The speciation and chemical form of Pu in this sediment system has not been determined, but a fraction of the sediment-plutonium inventory may be in a chemical form (i.e., chelated, associated with organic matter, complexed with inorganic substances, or soluble) that is more mobile in the system than the balance of the inventory. While ingestion of sediment appears responsible for the highest levels of Pu (body burden) in fish, this mechanism apparently has not enhanced availability of Pu to biota because concentration factors for biota in WOL were relatively low compared to those observed at other study sites. Concentration factors for biota of WOL were low even though 12% of the plutonium in the water column was a soluble form (Table VI). 

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