Geochemistry of rare earth elements in water and solid materials at abandoned mines in SW Sardinia (Italy)

This study investigates the behavior of rare earth elements (REE) in near-neutral waters that drain abandoned mines in SW Sardinia (Italy). The REE, together with other major and minor components, were determined in waters (146 samples) and solid materials (13 samples). Research was performed to evaluate the main factors that control the geochemical behavior of REE in non-acidic mining environments. Zinc, cadmium and lead are the main contaminants in the study area, either in the processing residues or in the waters that drain the mine site. Contamination of stream waters extends several km downstream of the mine, till the Mediterranean Sea. The mine tailing (MT) materials are the main REE source, with peak concentrations of 300mg/kg ΣREE. PAAS-normalized patterns of MT are characterized by light REE (LREE) enrichment with respect to heavy REE (HREE). The waters flowing out of the MT heaps are near-neutral (6.2 to 7.0 pH), with Zn-sulfate dominant composition, and a mean concentration of 53μg/l ΣREE. Drainages from MT and an adit flow in the Naracauli stream headwaters (mean 1.4μg/l ΣREE). Concentrations of REE in the Naracauli stream decrease dramatically about 400m downstream of source. The REE decrease matches Fe and Zn decrease in waters, probably because the REE are sorbed on freshly precipitated solid phases. Sorption processes, and/or co-precipitation with secondary phases, appear to control the REE geochemistry in the studied waters under near-neutral conditions. Also, despite the non-acidic environment, it is worth to observe that small changes in pH seem to affect the mobility of REE at Naracauli. The PAAS-normalized REE patterns in the waters generally reflect the PAAS-normalized REE patterns in the solid materials with which the water interacts, either MT or secondary phases such as Fe-hydroxide and bio-hydrozincite. As it is expected in waters under oxidizing conditions, patterns with negative Ce anomalies are developed due to the poor solubility of Ce+4 species, and its consequent scavenging in solid phases. However, there is an exception: Ce shows a poor affinity for the bio-hydrozincite mineral, probably reflecting the role of bacteria in the precipitation of the Naracauli hydrozincite. •Sorption processes, and co-precipitation with secondary phases control dissolved REE.•Secondary Fe-hydroxides host REE contents higher than mean values in mine tailings.•Cerium poor affinity to the Naracauli bio-hydrozincite•pH dependence of aqueous REE eve