Chromium and copper in micromorphological features and clay fractions of volcanic soils with andic properties

In volcanic soils of the Solofrana valley (south-western Italy), contamination by Cr and Cu occurs as a consequence of agricultural practices and surface deposition of polluted river sediments by periodic floods. In this study, the metal distribution in micromorphological features and clay fractions (< 2.0 µm and < 0.1 µm) of two selected soil profiles were investigated by microscopical (OM) and microanalytical (WDS) techniques in combination with clay mineralogical characterization (XRD, IR, DTA) and selective chemical extractions. In thin section, the sediments carried and deposited by river water occur on soil surface as massive structured flooded layers, and once incorporated in the soil matrix as fragments and round microaggregates. Features of illuvial origin as fine-particle infillings and coatings are also observed. From WDS analysis, the surface alluvial materials are found to contain high Cr amounts (1638 ± 61 mg kg−1). Both Cr and Cu are present in the subsurface infillings with concentrations of 273 and 243 mg kg−1. In deeper horizons, the fragments of stratified fine textured alluvial deposits, incorporated in the soil matrix, contain low concentrations of both metals (13-63 mg kg−1 for Cr and 18-24 mg kg−1 for Cu), suggesting that Cr and Cu migration, in colloidal or ionic form, is restricted to a limited distance. The contribution of the clay fraction (< 2.0 µm) to the soil total metal content is considerable and ranges from 26 to 82% for Cr and from 50 to 98% for Cu. Allophane and ferrihydrite, are the minerals occurring in clays and most likely involved in metal retention along with organic matter and organo-mineral complexes. Hydrated halloysite, the almost unique phyllosilicate in the < 0.1 µm clay fraction, may also play a role in contaminants adsorption on organo-mineral complexes.