Bioaccessible arsenic in soil of thermal areas of Viterbo, Central Italy: implications for human health risk.

Thermal waters near the city of Viterbo (Central Italy) are known to show high As contents (up to 600 µg/l). Travertine is precipitated by these waters, forming extended plateau. In this study, we determine the As content, speciation and bioaccessibility in soil and travertine samples collected near a recreational area highly frequented by local inhabitants and tourists to investigate the risk of As exposure through accidental ingestion of soil particles. (Pseudo)total contents in the studied soils range from 17 to 528 mg/kg, being higher in soil developed on a travertine substrate (197 ± 127 mg/kg) than on volcanic rocks (37 ± 13 mg/kg). In travertines, most As is bound to the carbonatic fraction, whereas in soil the semimetal is mostly associated with the oxide and residual fractions. Accordingly, bioaccessibility (defined here by the simplified bioaccessibility extraction test, SBET; Oomen et al., 2002.) is maximum (up to 139 mg/kg) for soil developed on a travertine substrate, indicating a control of calcite dissolution on As bioaccessibility. On the other hand, risk analysis suggests a moderate carcinogenic risk associated with accidental soil ingestion, while dermal contact is negligible. By contrast, ingestion of thermal water implies a higher carcinogenic and systemic health risk.

Microscopic biomineralization processes and Zn bioavailability: a synchrotron-based investigation of Pistacia lentiscus L. roots.

Plants growing on polluted soils need to control the bioavailability of pollutants to reduce their toxicity. This study aims to reveal processes occurring at the soil-root interface of Pistacia lentiscus L. growing on the highly Zn-contaminated tailings of Campo Pisano mine (SW Sardinia, Italy), in order to shed light on possible mechanisms allowing for plant adaptation. The study combines conventional X-ray diffraction (XRD) and scanning electron microscopy (SEM) with advanced synchrotron-based techniques, micro-X-ray fluorescence mapping (µ-XRF) and X-ray absorption spectroscopy (XAS). Data analysis elucidates a mechanism used by P. lentiscus L. as response to high Zn concentration in soil. In particular, P. lentiscus roots take up Al, Si and Zn from the rhizosphere minerals in order to build biomineralizations that are part of survival strategy of the species, leading to formation of a Si-Al biomineralization coating the root epidermis. XAS analysis rules out Zn binding to organic molecules and indicates that Zn coordinates Si atoms stored in root epidermis leading to the precipitation of an amorphous Zn-silicate. These findings represent a step forward in understanding biological mechanisms and the resulting behaviour of minor and trace elements during plant-soil interaction and will have significant implications for development of phytoremediation techniques.

The amorphous Zn biomineralization at Naracauli stream, Sardinia: electron microscopy and X-ray absorption spectroscopy.

An amorphous Zn biomineralization ("white mud"), occurring at Naracauli stream, Sardinia, in association with cyanobacteria Leptolyngbya frigida and diatoms, was investigated by electron microscopy and X-ray absorption spectroscopy. Preliminary diffraction analysis shows that the precipitate sampled on Naracauli stream bed is mainly amorphous, with some peaks ascribable to quartz and phyllosilicates, plus few minor unattributed peaks. Scanning electron microscopy analysis shows that the white mud, precipitated in association with a seasonal biofilm, is made of sheaths rich in Zn, Si, and O, plus filaments likely made of organic matter. Transmission electron microscopy analysis shows that the sheaths are made of smaller units having a size in the range between 100 and 200 nm. X-ray absorption near-edge structure and extended X-ray absorption fine structure data collected at the Zn K-edge indicate that the biomineral has a local structure similar to hemimorphite, a zinc sorosilicate. The differences of this biomineral with respect to the hydrozincite biomineralization documented about 3 km upstream in the same Naracauli stream may be related to either variations in the physicochemical parameters and/or different metabolic behavior of the involved biota.

Zn biomineralization processes and microbial biofilm in a metal-rich stream (Naracauli, Sardinia).

Several decades after the closure of the Ingurtosu mine (SW Sardinia), a variety of seasonal Zn biomineralizations occurs. In this work, waters, microbial consortia, and seasonal precipitates from the Naracauli stream were sampled to investigate chemical composition of stream waters and biominerals, and microbial strain identity. Molecular and morphological analysis revealed that activity of dominant cyanobacterium Leptolyngbya frigida results in precipitation of Zn silicate. The activity of the cyanobacterium was associated to other bacteria and many kind of diatoms, such as Halamphora subsalina and Encyonopsis microcephala, which are trapped in the process of biomineral growth. In this work, the precipitation process is shown to be the result of many different parameters such as hydrologic regime, microbial community adaptation, and biological mediation. It results in a decrease of dissolved Zn in the stream water, and is a potential tool for Zn pollution abatement.