Easily reducible fractions

Cobalt is strongly adsorbed by Mn oxides. There are close relationships between Co and the easily reducible fraction of Mn (Mn oxides) in soils (Jarvis, 1984) and will be in detail discussed in next chapter. Cobalt is frequently accumulated in Mn nodules in soils (Mckenzie, 1975). It was suggested that the Co2+ ion was first sorbed, then slowly oxidized to Co3+ and became incorporated into the surface layers of the crystal lattice, releasing the Mn2+ ion into the solution (Bums, 1976 Mckenzie, 1975). X-ray photoelectron spectroscopy showed that Co3+ was present on the surface of bimessite after the sorption of the Co2+ ions took place (Murray and Dillard, 1979). Traina and Donor (1985) suggested that the Mn release during Co2+ sorption resulted not only from the oxidation of Co2+ to Co3+, but also... 

Zinc can be found in all investigated sediments, 25% in the carbonate fraction and 40% in the easily-reducible fraction (Fig. 8-12). Solely the content of the organic and resi-... 

The reducible fraction can be recovered in three steps easily reducible fraction (manganese oxides), moderately reducible fraction (amorphous iron oxides), and poorly reducible fraction (crystalline iron oxides). A few SEPs address all three fractions (Miller et al., 1986 Zeien and Briimmer, 1989 Krishnamurti et al., 1995). A number of SEPs are only able to differentiate between easily reducible and moderately reducible fractions (called readily and poorly available forms ofTE, respectively) (Shuman, 1983 Salomons and Forstner, 1984 Kersten and Forstner, 1986). Due to the lack of selectivity of reagents, with consequent overlapping of separated fractions, all reducible forms are typically separated together (Ure et al., 1993 Rauret et al., 1999). Despite the fact that the SEPs may be simplified, in this way, simultaneous recovery of readily and poorly available forms of TE seems rather disadvantageous for risk assessment purposes. 

I -- i Hydroxylamlne hydrochloride/EASILY REDUCIBLE FRACTION hUffllj-H Oxalate buffer/MODERATELY REDUCIBLE FRACTION Hydrogen peroxide/ORGANIC SULFIDIC PHASES RESIDUAL FRACTION... 

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. 

Cadmium in native arid soils predominates in the carbonate fraction. In 45 uncontaminated Israeli soils, Cd is mainly present in the carbonate bound fraction (43.5% 22.3%), followed by the easily reducible oxide bound fraction (22% 19.5%) (Table 5.4) (Banin et al., 1997a). The... 

Manganese in uncontaminated Israeli arid soils is predominantly in the easily reducible oxide fraction (35-40% of the total-HN03 Mn), followed by the carbonate fraction (18-25%), and the residual fraction (14-25%) (Han and Banin, 1996) (Table 5.4). Manganese in the organic fraction amounts to 9-12% and in the reducible oxide fraction to 5-11%. The exchangeable fraction of Mn in the soils is very low. In the sludge-amended calcareous soils of Southeast Spain, the residual and the carbonate bound Mn fractions are the major solid-phase (Moral et al., 2005). In comparison, the Mn in fine-textured soils from the southeastern United States is... 

The data for the six Israeli arid soils show that Co contents in the easily reducible oxide, the organic matter, the reducible oxide and the residual fractions and the total Co are linearly related to the Mn contents in each respective fraction. Correlation coefficients between the respective Co and Mn fractions are significant at the 5% probability level (Fig. 5.6). The Co/Mn molar ratios are 0.0186, 0.0177, 0.0075, 0.0205, 0.0364 in the total, the easily reducible oxide, the organic matter, the reducible oxide and the residual fractions, respectively. Therefore, Co is more preferentially incorporated in the reducible oxide (Fe oxide) fraction than Mn. Much of the Co in arid soils is included in the easily reducible oxide (Mn oxide) fraction,... 

Co2+ to Co3+, but also from a direct exchange of Co2+ for Mn2+ produced during the redox reaction. The cobalt in arid soils, as indicated by Han et al. (2002b), mainly occurs in the residual and the Mn oxide (easily reducible oxide) fractions. Furthermore, after water saturation, the Co is transferred mainly from the Mn oxide fraction into the carbonate and exchangeable fraction. This will be discussed in detail in the next chapter. 

For the deposition step, the working electrode is maintained at a potential cathodic (by at least 0.4 V) of the standard potential of the least easily reduced ion to be determined. Forced convection (via rotation, stirring, or flow) is usually used to facilitate the deposition step. Quiescent solutions may be employed in connection with ultramicroelectrodes. The deposition time required is dependent on the sample concentration, with 1- to 10-min periods usually being sufficient for measurements in the range of 10-7 M to 1 x 10 9 M. Because only a small fraction of the metal ions is deposited, it is essential that all experimental parameters be as reproducible as possible during a series of measurements. 

Lead can be found almost exclusively in the easily-reducible and in the residual fraction (Fig. 8-11). Its main binding forms are obviously the adsorption on iron and manganese hydroxides and mineral binding. The large increase in the lead concentration from Elsterberg to Greiz is not characterized by an increase of an individual binding form. 

Fe and lower rates of bioturbation. This work supports earlier studies which suggested that the availability of easily reducible Fe (Berner, 1970 Pyzik and Sommer, 1981), bioturbation, and sedimentation rate (Chanton et al., 1987a) are key in controlling sulfide retention in sediments. The relatively rapid cycling of S accounts for a significant fraction of C mineralization as well as dissolved O2 consumption in estuarine bottom waters. 

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. 

---