Acids fulvic

In this work a novel five-step leaching scheme for HM has been developed addressing exchangeable, acid soluble (carbonate), easily reducible (bound to Mn oxides), easily oxidizable (bound to humic and fulvic acids), and moderately reducible (bound to amorphous Ee oxides) fractions extractable by 0.05 M Ca(N03), 0.43 M CH3COOH, 0.1 M NH,OH-HCl (non-acidified), 0.1 M K/,03 (pH 11), and 0.1 M (NH4),C,04 (pH 3), respectively. The sequence of extractants was chosen according to recent studies on the selectivity of leachants toward dissolved phases of soils. 

Prepai ative isolation of nonvolatile and semivolatile organic compounds fractions (hydrophobic weak acids, hydrophobic weak bases, hydrophobic neutrals, humic and fulvic acids) from natural and drinking waters in optimal conditions was systematically investigated by solid-phase extraction method with porous polymer sorbents followed by isolation from general concentrate of antropogenic and/or toxic semivolatile compounds produced in chlorination and ozonation processes. 

Soil extracts are usually very complex. In water samples, humic and fulvic acids make analysis difficult, especially when polar substances are to be determined. Multidimensional chromatography can also make a significant contribution here to this type of analysis. 

Another example is the determination of bentazone in aqueous samples. Bentazone is a common medium-polar pesticide, and is an acidic compound which co-elutes with humic and/or fulvic acids. In this application, two additional boundary conditions are important. Eirst, the pH of the M-1 mobile phase should be as low as possible for processing large sample volumes, with a pH of 2.3 being about the best that one can achieve when working with alkyl-modified silicas. Secondly, modifier gradients should be avoided in order to prevent interferences caused by the continuous release of humic and/or fulvic acids from the column during the gradient (46). 

In multiresidue analysis, where more analytes with a wide polarity range need to be determined, large transfer volumes are required, and consequently, the selectivity is lower. However, since the major interferences in water analysis are the polar humic and fulvic acids, removing this early eluting interference in coupled-column RPLC will also be feasible in multiresidue methodology. 

The first attempts employing two Cjg columns showed that the selectivity was not high enough, although this improved when the first column was substituted by a 5 p.m GFF n internal surface rcversed-phase material. This is known as a restricted-access-material (RAM) column which, since it restricts some compounds because of their size and includes rcversed-phase interaction and ionic exchange, is very useful for analysing herbicides in samples with high contents of humic and fulvic acids (54). 

One problem with environmental samples that has already been mentioned concerns humic and fulvic acids which may be retained in the precolumn and co-elute with the more polar compounds. Of course, this depends on the selectivity of the sorbent in the precolumn. A simple solution is to add sodium sulphite to the solution prior to preconcentration. This approach has led to good results (37, 71). 

The breakdown of organic contaminants present in the MU water to produce ammonia (from nitrogenous contaminants), phenols and carboxylic acids (from humic and fulvic acids), and tri-halomethanes (from the by-products of chlorination)... 

Humic substances in sediments and soils have commonly been, defined as heteropolycondensates of decomposing plant and animal detritus 46. For lack of a better structural definition, these macromolecular substances have been divided into three categories fulvic acids and humic acid and humin. Fulvic acids and humic acids are soluble in dilute alkaline solutions, whereas humin is insoluble. 

Various destructive and non-destructive methods of analysis have been tested and H-l and C-13 NMR have, among other techniques provided valuable structural information on soluble humic acids and fulvic acids 48, Humin, on the other hand has withstood detailed non-destructive analysis. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

About half of the dissolved organic carbon may appear in humic or fulvic acids. These are high-molecular weight organic compounds of a composition which is somewhat uncertain. They contain aromatic hydroxyl and carboxyl groups which have the ability to bind to metal ions. Rivers and estuaries typically contain 10 mg/liter of acid with an exchange capacity of 5-10 mmol/g, mainly due to carboxylic... 

Humic acid precipitates at salt concentrations >0.05 M and pH <4, while the fulvic acids are still soluble. Thus, humic acid is hardly ever found in salt water ( O.S M). Titration experiments with alkali indicate that there are consecutive steps of deprotoni-zation. A typical average pka-value would be 7 (26), and for a selected humic acid values of 4 and 9 are given for pk and pk, (27, 28). 31 aZ... 

Little is known about actinide complexation by humic or fulvic acids although logg -values for Ara3+, Thlt+ and U022+ with humic acid at pH 4.0-4.5 as 6.8, 11.0 and 5.8, respectively, are reported (43). 

Acidolysis is a similar weathering reaction to hydrolysis in that is used to weather minerals, but in this case the source of is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H to weather minerals. All of these acids occur naturally in soils in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the... 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

---