Dynamics of rare earth elements in water–soil systems: The case study of the Pineta San Vitale (Ravenna, Italy)

Abstract

The European Union Site of Community Importance Pineta San Vitale (Ravenna, Italy) is located in a complex lagoon system nearby the Adriatic Sea. This area is affected by relevant environmental pressures related to subsidence, groundwater salinization and industrial contamination. The occurrence of rare earth elements (REE), and the characterization of processes that control their migration in the water–soil system were performed for implementing the knowledge of the present environmental status in the area.Nine soil profiles were opened in the Pineta San Vitale and each horizon was characterized. In the parent material, dominant quartz, followed by plagioclase, mica, chlorite and calcite were recognized by XRD. Topsoil samples were characterized by neutral or sub-acid pH, relatively high organic carbon content (LOI up to 43wt.%; TOC: 38÷61g/kg), and low carbonate (11÷20g/kg CaCO3). Subsoil horizons were sub-alkaline or moderately alkaline, rich in carbonate (up to 114g/kg CaCO3), and depleted in organic matter. The electrical conductivity (EC: 0.2÷4.2 mS/cm) values along the soil profiles did depend on the water table fluctuations and the salinity of groundwater. Sequential extraction tests on two soil profiles showed the highest Y and ΣREE occurrence in the residual fraction of soils, with values generally increasing at depth, and total amounts similar to values estimated in the upper continental crust (UCC). Poor fractionation of REE was observed when the bulk soil horizon was considered, but different REE patterns occurred in the organic, oxide, and carbonate soil fractions, suggesting REE fractionation during weathering and pedogenic processes.Physical–chemical parameters and concentrations of total and dissolved elements were determined in 44 water samples. Waters showed near-neutral or slightly alkaline pH (6.8÷8.0) and total dissolved solids mostly in the range of 2 to 24g/L, with values increasing with increasing seawater influence. Concentrations of ΣREE determined in waters filtered through 0.4μm pore size filters varied from 0.1 to 189μg/L (0.7·10−9÷1.3·10−6mol/L ΣREE), with the highest values observed in soil waters. REE values were not related to salinity, nor to major ions, while were positively correlated to Al concentrations. Further filtration through 0.015μm pore size filters removed about 90% of REE compared to the 0.4μm filtered samples, i.e. the studied waters showed a small quantity of truly dissolved REE species. Speciation modeling results by Visual MINTEQ indicated that truly dissolved REE species are dominated by bi-dentate organic complexes in the soil waters rich in dissolved organic carbon (up to 74mg/L DOC); aqueous complex with the carbonate and bicarbonate ligands increased in waters having low DOC. UCC-normalized patterns of REE in the colloidal fraction of soil waters showed slight enrichments in the middle REE (MREE), likely due to preferential adsorption of the MREE onto <0.4μm particles. UCC-normalized patterns of REE truly dissolved in soil waters showed slight enrichments in the heavy REE (HREE), probably due to dominant low molecular weight DOC in the truly dissolved fraction.For evaluating the potential REE contribution from the atmosphere, 12 vegetation samples (ivy leaves) were submitted to leaching tests using acidulated water as a proxy of rain water. The REE leached from leaves were in the range of 0.5 to 10μg/kg ΣREE, suggesting a significant atmospheric contribution to the water–soil system. UCC-normalized patterns of REE leached from leaves were nearly flat, possibly indicating a dominant geogenic source of atmospheric REE.