Preliminary assessment (by optical and electron microscopy) of soil processes affecting metal distribution and dynamics in agricultural and industrial polluted soils of south Italy

In the framework of both characterization plan and sustainable land reclamation, required by the Italian law (D.L.152/2006) for contaminated sites, it is fundamental a detailed understanding of soil processes involved in contaminant dynamics. Indeed, particularly in subsurface horizons, localized metal accumulations can be found in discrete soil features such as coatings, infilling, concentrations, etc. Therefore, they could be considered valuable indicators for migration of PTE (potentially toxic elements), highlighting the interception of PTE by reactive soil phases or their co-precipitation. Here we studied two areas located in south Italy and formerly interested by anthropogenic metal contamination: i) one is a farmland potentially contaminated by Cr, Zn and heavy hydrocarbons, due to illegal burial of tannery sludges and wastes, ii) the other is an industrial area inside an automobile-battery recycling facility interested by disposal of Pb-battery wastes. In the first area, two main different waste materials buried in the soil were identified: a silty-grey colored mud and an organic compost-like. Both materials are found from the surface until a depth variable from 1 to 1.5 meters, but they are easily distinguishable only in the subsurface horizons not affected by tillage. The optical microscopy observation of organic compost-like material (95-105 cm) showed the presence of a chaotic mix of a dark brown material with the yellowish brown soil matrix incorporating angular calcium carbonate fragments, pumices, roots and mineral fragments, along with acicular light grey crystals as secondary minerals. Moreover, soil pores of the yellowish brown matrix commonly hold red fine coatings. This brown chaotic mix was found in presence of a massive greyish sandy soil matrix, crossed by rare angular pores. In the second area, a large accumulation of heterogeneous clasts chaotically mixed with soil in the first 40-45 cm of depth was found. At this depth, a darkened layer of sandy materials was occasionally found. All the identified microsites were analyzed by electron microscopy (SEM) associated with microanalysis (EDX). In soil from the first area, high concentration of Fe (52-56%), P (16-18%) and Ca (12-15%) in correspondence of the dark brown materials, associated with high Cr (1.0 to 1.8%) and Zn (0.9 to 1.5%) content, was observed. Cr was also found in very high concentration (62%) as discrete particles associated with Ca (19%). Low contents of Cr (0.3%) and Zn (0.5) were found associated to the pore coatings, which were generally enriched by Fe (48%) and P (25%). Acicular crystals were frequent and showed basically formed by Ca (61%) and S (38%). Results obtained are very promising giving access to PTE associations induced by soil processes. Further investigation using equipment with lower element detection limits, such as micro XRF and WDS, will move forwards the study of the colloidal transfer of metal pollutants along the soil profile.