Relative humic acids

Previous ultraviolet methods for determining nitrate have attempted to allow for humic acid interference [119-122], However, with the exception of Morries [122] these methods of allowance are inaccurate at humic acid concentrations above about 3.5 mg/1. Unfortunately, none of these methods have attempted to make any allowance for ultraviolet-absorbing pollutant organic compounds or interfering inorganic ions. Thus their application to water analyses other than for relatively unpolluted fresh waters is open to question. 

Brown and Bellinger [123] have proposed an ultraviolet technique that is applicable to both polluted and unpolluted fresh and some estuarine waters. Humic acid and other organics are removed on an ion exchange resin. Bromide interference in seawater samples can be minimised by suitable dilution of the sample but this raises the lower limit of detection such that only on relatively rich (0.5 mg/1 NO3N) estuarine and inshore waters could the method be used. Chloride at concentrations in excess of 10 000 mg/1 do not interfere. 

Kinetically, the adsorption of humic acids at a solid-water interface is controlled by convection or diffusion to the surface. Even at concentrations as low as 0.1 mg/e near-adsorption equilibrium is attained within 30 minutes. At high surface densities, a relatively slow rearrangement of the adsorbed molecules may cause a slow attainment of an ultimate equilibrium (Ochs, Cosovic and Stumm, in preparation). The humic acids adsorbed to the particles modify the chemical properties of their surfaces, especially their affinities for metal ions (Grauer, 1989). 

The enhancement of kerosene dissolution occurs even at low humic acid content in the aqueous solution. In view of the fact that humic substances are relatively high molecular weight species containing nonpolar organic moieties, Chiou et al. (1986) assumed that a partition-like interaction between a solute of very low solubility in aqueous solution and a microscopic organic environment of dissolved humic molecules can explain solute solubility enhancement. 

Fig. 16.34 Mineralization of [ C] toluene to (A) coupled to the reduction of AQDS or humus (B) by enriched Amsterdam petroleum harbor (APH) (A and B) sediments in anaerobic culture bottles containing phosphate-buffered basal medium supplemented with AQDS (5 mM) or with highly purified soil humic acids (12g/L). Uniformly C-labeled toluene was added at an initial concentration of 100p,M relative to the liquid volume. Unsupplemented controls were prepared in the same manner but without AQDS and humus. All data were corrected relative to the endogenous control (without C-labeled toluene addition). Data are means and standard deviations for triplicate incubations in each treatment d denotes days. (Cervantes et al. 2001). Reprinted with permission. Copyright American Society for Microbiology...

The aim of this study was to investigate the effect of high but environmentally realistic concentrations of humic acid on the relative long-term accumulation of Cd in the freshwater mussel D. polymorpha under controlled laboratory conditions. We assessed whether the long-term uptake of Cd is in agreement with the free-ion activity model and if Cd accumulation is related to the Cd -jon activity in the water. 

Large pores are desirable but not always possible when high surface area is also demanded. Therefore, a compromise is necessary. This compromise is evident in the choice of XAD-8, with its lower surface area, over XAD-7 when humic acids are accumulated from environmental waters (143, 167, 319-321). The same, but more subtle, considerations apply to XAD-2 and XAD-4. The 780 m2/g for XAD-4 will be available for the adsorption of intermediate weight organic solutes only if the water is relatively free of humic material that can block many of the pores. Therefore, a situation can exist for environmental waters for which the lower surface area XAD-2 is more effective than the XAD-4 polymer. 

That several model organic compounds were only partially or incompletely retained by the resins prompted us to investigate the use of Carbopack B as an alternative or complementary adsorbent. Test solutions without humic acids were used to verify the sorptive-desorptive behavior of several model compounds under the experimental conditions proposed by Bacaloni et al. (8), except that the compounds were desorbed with methylene chloride. The results of duplicate experiments are given in Table III. Isophorone and MIBK were not effectively retained by Carbopack B, whereas bis(2-ethylhexyl) phthalate was almost equally distributed between the aqueous phase and the carbon. The relatively poor recovery of 1-chlorododecane, 2,4 -dichlorobiphenyl, and 2,2, 5,5 -tetrachlorobiphenyl may be ascribed to sorptive losses onto reservoir glass wall, whereas furfural may be inefficiently... 

Humic acids and amino acids have been reported to produce mutagenic activity as a result of chlorination (4, 5). However, this work was performed by using relatively high substrate and chlorine concentrations... 

Measurements of alkali solubles in these coal samples—conventionally accepted as indices of humic acid concentrations—were initially performed by using Kreulen s method (7). However, even when the most stringent precautions were taken to exclude air, this method yielded markedly time-dependent results (presumably owing to oxidation of the coal by the relatively strong alkali solution), and a more satisfactory colorimetric technique (by J. F. Fryer) was therefore employed. This entailed extracting the coal sample with 0.1 N aqueous sodium hydroxide for 16-20 hours in an inert atmosphere and subsequent photoelectric scanning of the extract solutions. Actual humic acid concentrations were then obtained from specially constructed reference curves which related optical density (at an appropriate wavelength) to humic acid contents. The inherent error in this determination is estimated at less than 10%. 

Two minor points worthy of note are (a) formation of carbon dioxide and water at temperatures below ca. 300°C. appears, from the evidence of this study, to be caused mainly by Reaction 1—i.e., at such relatively low temperatures, Reaction 2 does not proceed significantly towards its ultimate conclusion (b) abstraction of humic acids by stripping of functional groups— i.e., by reactions characterized by fa, is likewise minimal at T < 300°C. (The fact that no observable decay of humic acids occurs during extended oxidation unless reaction temperatures approach 300°C. is, perhaps, a direct consequence of (a) and (b).)... 

No matter how the oxidation process is now formulated (or how it may ultimately come to be described), there can be no doubt that humic acids per se represent relatively unstable solids which are generally as susceptible to oxidation and decomposition as the parent coal and which will also undergo almost continuous changes in functionality. This is perhaps best demonstrated by one experiment in which the coal was oxidized for some 400 hours after... 

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