Distribution of As, Cr, Ni, Rb, Ti and Zr between peat and its humic fraction along an undisturbed ombrotrophic bog profile (NW Switzerland)

Ombrotrophic bogs are useful records of the impact of historical human activity on heavy metal contamination. Several studies concerning the trace element record (mainly Pb and Hg) in these particular environments have been carried out in recent years, although the role of humic substances has often not been considered. In particular, of the components of peat organic matter, fulvic acids and low molecular weight compounds are generally responsible for the mobility of trace elements through the profile, while humic acids (HAs) are involved in the formation of more stable organo-mineral complexes. In order to study the parallel distribution of As, Cr, Ni, Rb, Ti and Zr in bulk peat and the corresponding HAs, a peat core (10 × 10 × 81 cm) was collected from Etang de la Gruère (Switzerland) and cut into 27 slices of 3 cm. The samples were freeze-dried and milled very finely, and HAs extracted from each sample. Both peat and HAs were analyzed using an energy-dispersive miniprobe X-ray fluorescence multielement analyser (EMMA-XRF). Of the considered elements, Ni showed a great affinity for the humic acid component, while Cr was concentrated mainly into humic material from the deeper layers. On the other hand, Ti, Zr and Rb seemed to reflect the variation in mineral material both in peat and HA samples, while the As content of both materials reflected the environmental conditions characterizing the bog.