Spatial variation in pyritization of trace metals in salt-marsh soils

In the present study, the pyritization of trace metals was studied in 12 soils from 3 salt marshes in the Ria of Ortigueira (NW Spain). The concentrations of trace metals in the pyrite fraction were related to physicochemical conditions, physiographical position in the salt marsh, and the presence or absence and type of vegetation. Redox conditions in soils from the low-salt marsh and from the creek bottom were strongly reducing throughout the profile, and there were higher concentrations of Fe and some trace metals (Cu and Mn) in the pyrite fraction (soluble in HNO3) than in the reactive fraction (soluble in 1N HCl). In contrast, the trace-metal content in pyrite fraction in the surface layers of the high-salt marsh was low. In some of the soils, there was a significant increase in the pyrite content below 25 cm, and levels of Fe, Mn and Cu incorporated into this fraction were similar to those in the reactive fraction. The degree of pyritization varied greatly among metals in the order: Cu similar to Fe-1N HCl>dithionite- Fe>Ni similar to Mn>>Zn>Cr, although when we considered only the amorphous forms (ascorbate-Fe) as reactive-Fe, the order was: ascorbate-Fe>>Cu>Ni similar to Mn>>Zn>Cr. These differences appeared to be a consequence of the different geochemical behaviour of each metal (mainly in terms of the thermodynamic stability of sulfides and reaction kinetics), except for Zn. The low concentrations of Zn obtained may have been due to the solubility of ZnS in 1N HCl, which meant that it was extracted with the reactive fraction. Finally, we observed a direct relationship between DOP and DTMP, which was independent of the geochemical behaviour of each metal and of its concentration in the soil. Thus, the strong correlation between pyrite-Fe and the metals associated with this fraction appears to indicate that these metals coprecipitate with pyrite rather than form metal sulfides.