Soluble metal fraction

An example of a method suitable for the determination of cadmium, cobalt, copper, iron, manganese, nickel, and zinc in water, using chelation and sample extraction, is as follows [113]. The sample is filtered through an acid-washed membrane filter as soon as possible after collection. It is then acidified with nitric acid for preservation until analysis. This will give the soluble metal fraction. If the total metal content is to be found, the sample is acidified and allowed to stand for 4 days with occasional shaking. Then it is filtered. 

Figures 2.3 and 2.4 show breakthrough curves for total and soluble metal fractions, respectively, eluted in the absence and presence of colloids. In the absence of colloids (controls) practically none of the metals exhibited any meaningful breakthrough, suggesting nearly complete sorption by the soil matrix (Figure 2.3). The presence of colloids enhanced considerably total metal elution and in most cases even soluble metal elution, thus providing strong evidence for colloid-mediated metal transport.

The partition index (IR, which will be discussed in details below) of Cd, Cu, Cr, Ni and Zn in both soils rapidly increased from time zero (calculated value) to one day and further to one year. This was especially true for Cr, Cu and to some extent Ni and Cd (Table 6.5). This result indicates that added trace metals are initially and rapidly transferred from the labile EXC fraction into the more stable fractions. Furthermore, IR of trace metals in native arid soils incubated under the saturated paste regime decreased at the end of year. This indicates mobilization of trace elements in these soils as saturation (Table 6.5). Also, it can be seen that IR decreased, for any given time, with an increase of the loading level (Table 6.5, Fig. 6.5). This means that higher additions of soluble metals result in higher metal content in the labile fractions and lower metal binding intensity in soils. 

Trace metals are introduced to the ocean by atmospheric feUout, river runoff, and hydrothermal activity. The latter two are sources of soluble metals, which are primarily reduced species. Upon introduction into seawater, these metals react with O2 and are converted to insoluble oxides. Some of these precipitates settle to the seafloor to become part of the sediments others adsorb onto surfaces of sinking and sedimentary particles to form crusts, nodules, and thin coatings. Since reaction rates are slow, the metals can be transported considerable distances before becoming part of the sediments. In the case of the metals carried into the ocean by river runoff, a significant fraction is deposited on the outer continental shelf and slope. Hydrothermal emissions constitute most of the somce of the metals in the hydrogenous precipitates that form in the open ocean. 

ESR examination of nonchemically isolated fulvic acids showed that Mn2+ was the primary paramagnetic species observable (60, 61). Most likely, the soluble-colloidal fraction we identified in the speciation studies consisted primarily of such complexes. Because the ESR spectral characteristics of Mn in fulvic acid complexes are quite similar to Mn(H20)62+, Alberts et al. (62) suggested that the metal-fulvate interaction was weak. Stronger interaction would be expected to lead to changes in peak shape. This view leaves unexplained the ability of the complexes to survive the isolation procedure s long ultrafiltration steps, because weak interactions are usually associated with reversible complexation. 

Sposito, G., Holtzclaw, K. M., and Baham, J. (1976). Analytical properties of the soluble, metal-complexing fractions in sludge-soil mixtures II. Comparative structural chemistry of fulvic acid. Soil Sci. Soc. Am. J. 40, 691-697. 

Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be taken up by an organism and can react with its metabolic system (Campbell, 1995). Metals can be plant-bioavailable, if they come in contact with plants (physical accessibility) and have a form which can be uptaken by plant roots (chemical accessibility). Soil metals become accessible for humans by ingestion, inhalation and dermal contact. Available forms of PTMs are not necessarily associated with one particular chemical species or a specific soil component. Main soil PTMs pools of different mobility, target organisms and routes of transfer are sketched in Fig. 9.2. The most labile fraction, corresponding to the soluble metal pool, occurs as either free ions or soluble complexed ions and is considered the... 

The metal phases which are presumed to be sequentially extracted are step 1, soluble, exchangeable and carbonate-bound metals step 2, metals occluded in easily reducible manganese and iron oxides step 3, organically bound metals and sulphides step 4 , metals in non-silicate minerals lattice structure. It is important to emphasise that these metal phases are nominal target only, operationally defined by the extraction used. Consequently, is highly desirable and recommended to refer to the sequentially extracted metal fractions as easily extractable , reducible , oxidi-sable and residual , respectively. 

Colloid concentrations in the eluent were determined with a Bio-Tek multichannel (optical densitometer with fiber-optics technology Bio-Tek Instruments, Winooski, VT) microplate reader, precalibrated with known concentrations of each colloid at 540 nm. Total metal concentration in the eluents was allocated to solution phase and colloidal phase (colloid-bound contaminant). The eluent samples were centrifuged for 30 min at 3500 rpm (x 2750 g) to separate the soluble contaminant fraction from the colloid-bound contaminant fraction. The absence of colloidal material in the supernatant solution was verified by filtration through a 0.2-pm membrane filter. The soluble metal (Cu, Zn, Pb) fractions were analyzed by atomic absorption (concentrations >0.5 mg/H) or inductively coupled plasma (ICP) spectrometry (concentrations <0.5 mg/H). The colloid fraction was extracted with 1 M HNO3-HCI [32] solution and analyzed with the same methodology used for the soluble fraction. The results for the duplicate soil monoliths and for the two soils were combined for practical purposes, because the reproducibility between soil monoliths was within 15%. 

Metal transport increases were also metal specific, following the sequence Zn > Pb > Cu for total metal elution and Zn > Cu > Pb for soluble metal elution. Overall, however, between 30 and 90% of Cu was transported in the soluble fraction, while >60% of Zn and Pb were transported in the colloid-sorbed fraction. This is generally consistent with the metal affinities of the different colloids in conjunction with OC content and colloid size differences. Average increases of total Cu transport in the presence of colloids were three-fold for kaolinitic, five-fold for illitic, and six-fold for montmorillonitic colloids compared to the controls. The respective average... 

The soluble, metal-complexing, organic fraction of sewage sludge is a heterogeneous assembly of molecules that is characterized by wide ranges of chemical composition, molecular... 

Biomolecules are present in soils, especially in the rhizosphere, because of root exudates and microbial metabolites (Lynch, 1990a,b Marschner, 1998). Furthermore, in view of the stability constants of these biomolecular ligands with metals (NIST, 2004), a substantial fraction of the soluble metal ions in soil solution may be complexed with a series of biomolecules commonly present in microbial metabolites and root exudates. The concentrations in the rhizosphere of such metals as Cu, Mn, Zn, and Co vary with the seasons and are related to the presence of complexing biomolecules (Nielson, 1976 Linehan et al., 1989). The pH of the rhizosphere soil is generally lower than that of the corresponding... 

The washcoat composition is quite different to that used for three-way catalysts. The washcoat oxides used are chosen so as to ensure a minimal adsorption both of the soluble organic fraction and of the sulfur oxides SO2 and SO3. The properties of three different oxides in these respects are shown in Figs. 103 and 104. The composition of the washcoat is, together with the choice of precious metal formulation. 

A solution contains 0.0100 mol/L each of Cd and Mg ". (a) To what pH should it be raised to precipitate the maximum amount of one metal (identify which it is) as the hydroxide without precipitating the other (A) What fraction of the precipitated metal will still remain in solution (c ) To avoid accidentally precipitating any of the more soluble metal, a judicious chemist stops the addition of base when the pH is 0.50 units less than that calculated in part (a). Now what fraction of the less soluble metal remains in... 

Anodic stripping voltammetry (ASV) is ideally suited to the determination of metal ions that are reducible to mercury-soluble metals (Heineman et al., 1984 Wang, 1985). The technique consists of two steps. First, a fraction of the sample metal ion is deposited as amalgamated metal by reduction at a mercury electrode such as a hanging mercury drop electrode (HMDE). This is accomplished by applying a potential that is sufficiently negative to reduce the sample metal ion for a carefully measured time. This step, often called the deposition step, serves to preconcentrate sample as metal in the small mercury drop by what is... 

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