Nickel can occur in a number of oxidation states, but only Ni (II) is stable in the pH and redox conditions found in the soil environment (McGrath 1995). Several studies have shown the strong geochemical affinity of Ni for Fe-Mn concretions and nodules (Palumbo et al. 2001; Liu et al. 2002). Gasparatos (2007) found that the contents of Ni in Fe-Mn concretions from Greece ranged from 186 to 592 ppm with an overall mean 312 ppm. These values compare favorably with those from Indian, Sicilian and French soils (Sidhu et al. 1977; Palumbo et al. 2001 ; Latrille et al. 2001). Literature data demonstrated that as for Co, high Ni content in concretions and nodules is mostly observed in the Mn rich phases while low Ni content is observed in the concretions where Mn content is low or in other words where Fe predominated over Mn (McKenzie 1975; Sanz et al. 1996). In that case the enrichment of Ni may be though to be due to its adsorption as Ni (II) on the negatively charged surface of Mn oxides (specific adsorption). Recent studies using synchrotron based spectro-scopic techniques such as extended X-ray absorption fine structure (EXAFS) spectroscopy have shown the preferential uptake of Ni by birnessite and not ferrihydrite supporting the strong affinity of Ni for Mn oxides which is interpreted by the formation of a pH-dependent inner sphere sorption complex (Manceau et al. 2007). In addition, Manceau et al. (2002) show that in soil ferromanganese nodules, Ni substitutes for Mn3+ in the manganese layer of the MnO2-Al(OH)3 mixed - layer oxide lithiophorite.

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