Humic acids stability

Organic matter is also the essential component of natural soils and its association with microorganisms may influence the behavior and fate of toxic metals. A variety of batch complexation experiments were performed by Borrok et al. (2007) in single, binary and ternary systems for the three components natural organic matter (NOM), bacterium (B. subtilis) and metals (Pb, Cu, Cd, and Ni) to determine the significance of ternary complexation. They found that the formation of bacteria-metal-NOM complex is a rapid, fully-reversible chemical process. The stability of bacteria-metal-NOM complexes increases with the decrease of pH. All NOM fractions form ternary complexes to similar extents at circumneutral pH, but humic acid becomes the dominant NOM fraction in ternary complexes at low pH. The abundance of humic acid in ternary form is greatest with Ni or Cd systems and less with Pb and Cu systems. Their results suggest that... 

Conditional stability constants have been determined for cadmium binding to humic acid in freshwater, log Kk 6.5 [27], which may be comparable to binding to humic acid coated particles. The experiments demonstrated the importance of cadmium uptake from particles rather than from the dissolved phase. The authors recognised that the overall conclusion was similar to previous studies [28], but there remain inconsistencies in the uptake levels which may be related to the heterogeneity of the systems. Uptake from the intestine into the mucosal cells was not investigated. It was presumed that the material was digested extracellularly by hydrolytic enzymes and the released metal was taken up by facilitated diffusion. 

A major challenge that had to be met arose from the extreme bactericidal properties of phenols and catechols. Most microorganisms are poisoned at phenol or catechol concentrations in the 0.1-1 g/L range. The biocatalyst E. coli JMlOl [pHBP461] is no exception, and was inactivated by 200 mg/L of both 2-phenylphenol and 3-phenylcatechol." ° Furthermore, 3-substituted catechols are of limited stability in aerated aqueous solutions and form multimeric humic-acid-hke stmctures as unwanted side products. 3-Phenylcatechol, for instance, has a half-life time of only 14 h at pH 7.2. 

Because the mechanisms of 1-naphtol complexation with HA obtained by using these three techniques exhibit similar pathways, we present the results only from fluorescence spectroscopy. The ratio of fluorescence intensity in the absence (FJ and in the presence (F) of the quencher (HA) over time, as affected by pH and ionic strength, are illustrated in Fig. 16.20. The fluorescence intensity of a fluorophore in the absence of a quencher is directly proportional to its concentration in solution, and therefore time-dependent changes in E can be used to assess the stability of 1-naphtol under different pH and ionic strength. Quenching (FQ) of 1-naphtol fluorescence by humic acid increased with equilibration time from one to seven days. This time-dependent relationship was found to result from weak complexation of... 

Fiumic acid binds Fe " " predominantly or exclusively through carboxylate groups, though there may be a very small amount of ligation by phenolate. Some qualitative observations on stabilities and complex formation and dissociation reactivities are available for humic acid and for fulvic... 

Tables 7.6 and 7.7 compare the binding affinities of sites on the a-Al203 (0001), a-Al203 (1-102), and a-Fe203 (0001) surfaces and those in various types of bacteria, natural organic matter, and polyacrylic acid (PAA). The metal oxide substrates and humic acid are characterized by two binding sites, one strong and one weak, for Pb(II). The stability constant for the PAA-Pb(II) complex (i.e., ML2 species) is typically larger than those of higher affinity mineral surface binding sites (i.e., Mj).

Comparison of ( ) binding affinities (log stability constants) for various microorganisms, natural humic acid, and polyacrylic acid (PAA) (from [181]). 

The pH of the humic acid solutions behaved as expected for polymers containing weakly acidic groups. The pH of the 16-mg/mL stock, as detected by a combination glass-reference electrode, changed slowly and did not stabilize. A typical value for the pH after 30 min of stirring was 4.5. When the humic solution was supplemented with 0.5 M KC1 to aid the equilibrium of protons with titratable groups, the pH came to a stable value after a few minutes. A pH of 3.9 was then recorded. If the humic acid solution was progressively diluted to 20 /zg/mL in 0.5 M KC1, the equilibrium pH rose to 6.4. The pH of the humic acid diluted to 2.0 /zg/mL in the synthetic hard water was stable and did not differ detectably from the pH of the water before addition of the humate. 

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. 

Plutonium(III), (IV), and (VI) complex stability constants have been determined for some oxygen-donor (carboxylate) (114) and a few nitrogen-donor (115,116) ligands. Complexes of plutonium with natural complexants such as humic acids have also been studied extensively (89). 

Takamatsu, T. and Yoshida, T., 1978. Determination of stability constants of metal-humic-acid complexes by potentiometric titration and ion selective electrodes. Soil Sci., 125 377-386. 

Jekel, M. R. 1991. Particle stability in the presence of pre-ozonated humic acids. Aqua 40 18-24. 

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