Mineralogical, Chemical-physical and Mechanical Implications of Soil Contamination

This study evaluates alterations of soil properties as a result of soil contamination. The deviation from normal soil composition, with special reference to the mineralogical properties, was assessed for a Vertic Xerofluvent of the Lower Volturno River basin (Campania, Italy) polluted as a consequence of illegal disposal of foundry slags. The main crystalline phases present in the polluting slags were magnetite, zincite, hematite, calcite and quartz. Slags had an alkaline reaction (pH 10.1), variable grain size (50 mm > ∅ > 16 mm: 2.4%; 16 mm > ∅ > 2 mm: 46.3%; ∅ < 2 mm: 52.3%) and a high content of Cd and Pb (total Cd: 1082 mg kg-1, total Pb: 46941 mg kg-1, Cd-DPTA: 38.9 mg kg-1, Pb-DPTA: 207.8 mg kg-1). Contaminated soils showed relevant alterations in comparison with uncontaminated soils: an unnatural enrichment of the sandy fraction (from 438 g kg-1 to 777 g kg-1), a decrease of the organic C amount (from 11.03 g kg-1 to 3.87 g kg-1) and of the Cation Exchange Capacity (from 10.4 cmol[+] kg-1 to 3.6 cmol[+] kg-1), an increase in both Electrical Conductivity (from 0.201 dS m-1 to 0.578 dS m-1) and pH (from 7.6 to 9.1), all due to the intimate mixing of the polluting material with the soil matrix, favored by the soil vertic properties. Both Cd and Pb showed concentration values (total Cd 197 mg kg-1, total Pb 12174 mg kg-1, Cd-DPTA 17.0 mg kg-1, Pb-DPTA 53.1 mg kg-1) considerably higher than those observed in the reference soils (total Cd 3.8 mg kg-1, total Pb 63.5 mg kg-1, Cd-DPTA 0.1 mg kg-1, Pb-DPTA 2.1 mg kg-1). XRD and SEM analyses characterized the mineralogical properties of the slags and checked for the presence of toxic residuals in the soil. It was evident that the presence of magnetite and zincite in contaminated soils was connected both to the degree of contamination and to the intensity of the textural, chemical, chemical-physical alterations caused by the presence of the pollutant. Specifically, magnetite and zincite primarily collected in the coarser fraction (2-0.2 mm) of the surface layer (0-5 cm) of the Ap horizon, but they were still evident at 20 cm depth. SEM micrographs showed that slag particles appeared as spherules with a rugged surface. The EDS patterns confirmed the presence of Fe, Mn, and Zn compounds. In SEM micrographs of polluting slags found in contaminated soils samples, the original spheroidal morphology of the slag particles was still evident, exhibiting a smoother and cleaner surface. The espective EDS patterns substantiated the presence of Fe, Mn and Zn compounds, but also revealed the remarkable occurrence of peaks ascribed to carbonates and silicates, thus suggesting a close surface interaction of slag particles with soil matrix. Reflectance spectrometry measurements were very effective in discriminating among slags and non-contaminated or contaminated soils, providing evidence for different spectral bands features to be used for iron oxides or zincite. The concept of contamination should be extended to evaluate the general deviation from normal soil composition and thus give way to an in-depth and more articulated approach for the definition and checking of soil contamination