Humate anions

Metal ions, M, react with humate anions, A, to form complexes of the general type MAj, taking only mononuclear species into account ... 

Since the metal ions interact with the humate anions to form the complexes the dissociation of the humic acid should be taken into account ... 

As [a] in the present context is the concentration of humate anions, which may possibly participate in the complex formation has to be regarded as an apparent ionization constant, not of nes-cessity equal to that obtained by acid-base titration. 

Based on previously reported results, it is expected that Eu3+ ions will interact strongly with the humate anions, leading to complexes exhibiting rather high stability constants. Bertha and Choppin (18) reported simultaneous formation of 1 1 and 1 2 complexes, the corresponding stability constants being log 81 ... 

T = 30°C). Additionally some results on Co-FA complexes are available (11, 15). In the cases of strontium and cesium no reports on the actual interaction with humate anions in solution have appeared. However, it has been reported that both types of ions sorb onto solid humic acid (24, 25). 

Stabilization of a radical anion of humic acid may be caused by an adsorption effect. Bijl (3) observed that solid barium hydroxide octahydrate turned blue when placed in a solution of quinhydrone the blue solid was highly paramagnetic. Under the conditions we used for preparing these salts, insoluble sodium humate (with a large surface area) could have stabilized the anion radical by adsorption from the basic solution. Weiss and McNeil (18) observed a similar phenomenon with base soluble xanthenes, and proposed that biradicals may be formed in such a system. His compounds, however, do not appear to have the structural requirements to satisfy such a stabilized system. The recent report by Weber (29) on the spin content increase associated with the basification of a naphthoquinone-naphthohydroquinone system seems to parallel our observations quite closely. 

Finally, use of square (Lee et al., 2002b) or half (Lee et al., 2003) wave-pulsed electric fields of given frequency was found to be effective to mitigate anionic membrane fouling in presence of sodium humate. 

Lee, H.-J., Choi, J.-H., Cho, J., and Moon, S.-H. 2002a. Characterization of anion exchange membranes fouled with humate during electrodialysis. J. Membr. Sci. 203, 115-126. 

SOAP = soluble organic anions and polyanions (e.g., low FW carboxylates such as acetate, uronides, and phenolates as well as high FW fulvates and humates). 

The diversity of reactions which actinides can undergo in natural waters is pres ted schematically in Figure 22.9. Complexation by anions such as hydroxide, carbonate, phosphate, humates, etc. determine the species in solution. Sorption to colloids and suspended material increases the actinide concentration in the water while precipitation of hydroxides, phosphates, carbonates, and/or sorption to mineral and biological material limit the amount in the solution phase. 

Among the various sources of cadmium contamination are the plating operations and the disposal of cadmium-containing wastes. In these cases, the forms of cadmium also depend on the treatment of the waste prior to disposal. The most common forms include Cd ", cadmium-cyanide complexes, or Cd(OH)2 solid sludge. Hydroxide (Cd(OH)2) and carbonate (CdCOs) solids dominate at high pH, whereas Cd " and aqueous sulfate species are the dominant cadmium forms at pH <8. Under reducing conditions, when sulfide is present, the stable solid CdS(s> is formed. Cadmium also precipitates in the presence of phosphate, arsenate, chromate, and other anions, although solubility will vary with pH and other chemical factors [77]. The free Cd " appears to be the form readily taken up by plants, whereas CdCU is taken up more slowly, while Cd-humate is not adsorbed. 

The rate of adsorption of humified plant residues on clay has been studied by Inoue and Wada [1968], who have found that allophane absorbs considerably more organic matter than layer silicate clays. Their adsorption curves may be explained by the reaction of humate groups on the anion exchange sites of allophane. It has also been observed that adsorbed organic matter tends to reduce the anion exchange capacity of allophane. 

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