Humic substance metal complexes

GEOCHEMISTRY OF HUMIC SUBSTANCES Metal Complexation Copper... 

Metals constitute an important source of pollution in soils. As discussed in Chapters 5 and 6, they can bind to soils through humic substances, surface complex-ation, or ion exchange. In some cases more than one type of interaction can occur, as in clay minerals (e.g., montmorillonite and vermiculite) that bind metals through ion exchange as well as surface com-... 

The origin of the colour can be related to one coloured product or pollutant, or to natural constituents (humic substances, metallic ions or complexes, for instance). The colour is well defined when only one coloured solute is present but, generally, several compounds are responsible for a mixture of colours giving a broad visible spectrum (Fig. 3). 

The measured values of the formation constant for Cu-NTA at intermediate pH will then be 10 which is significantly less than the value of 10 measured at high pH. The same is true for the humic substance-metal ion complex equilibrium constants measured at low pH values. Morgan estimates that at pH 2 to 3 the —COO groups, which are important in forming fulvic acid-metal ion complexes, are virtually all in the —COOH or protonated form. Thus to obtain the value of the formation constant at intermediate pH for metal ion fulvic acid complexes, when the carboxyl groups are ionized, we should divide the equilibrium constant values presented in Table 5-13 by the dissociation constant of the —COOH group, that is,... 

Any of these solution products can react with chemical species already present in the water, such as humic substances, metals, and metal complexes. As for the rest of the dissolved ammonia, while some will bind to sediments, suspended partieles, and organic matter available in the water, most will undergo nitrification by the Nitrosomonas and Nitrobacter bacteria species to yield nitrates that can be uptaken by aquatic plants and organisms. Many algae and phytoplankton have been found to utilize ammonia direetly as their source of nitrogen. Because of its low octanol/water partition coefficient, ammonia is not expected to adsorb strongly to sediments. 

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. 

Senesi N, Loffredo E (2005) Metal ion complexation by soil humic substances. In Tabatabai MA, Sparks DL (eds) Chemical processes in soils. Soil Sci Soc Am Book Series No 8, Madison, Wisconsin, pp 563-618... 

In addition to their complexing capability, humic substances can also reduce oxidized forms of metal cations and polyanions (Fe —> Fe MoO —> Mo " VO3 —> VO see Szilagyi, 1971). The reduced forms are then embodied in the structure of humic substances by functional-group fixation. 

Due to the various health risks of different element species, there are a multitude of applications for natural water samples in this field (e.g., Cr and Sb speciation or Br and I determination).19 The investigation of heavy metal complexes with humic substances by isotope dilution SEC-ICP-MS has been described, for example, by McSheehy and Mester.20 Copper, zinc and molybdenum were found to form complexes with similar size fractions of humic substances in seepage water samples from soils. Sturgeon s group proposed the use of solid phase microextraction (SPME)... 

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