Abstract
A method for evaluating the potential of reuse of biomasses for economic purposes is here presented starting from a case study. Juncus acutus plants and rhizospheres were harvested from abandoned Zn–Pb mine areas of southwest Sardinia (Italy). Thermogravimetry and Differential Thermal analyses were performed to evaluate the temperatures at which significant reactions occur. X-ray Diffraction (XRD) analysis was carried out on raw samples and on samples heated ex-situ (by a conventional diffractometer) or in-situ (by synchrotron-based diffraction). Raw samples mainly consist of quartz, phyllosilicates, and feldspars with minor amounts of sulfides, sulfates, and Fe, Pb, and Zn carbonates, concentrated in the rhizosphere. After heating, Zn and Fe oxides and willemite are observed in internal roots and stems, revealing the presence of these metals in the plant tissues. In-situ heating was less effective than ex-situ in revealing minor phases in organic samples, probably because the scarcity of oxygen within the sample holder did not allow the degradation of organic compounds and the oxidation of sulfides, resulting in a low quality XRD signal even if obtained with the high resolution ensured by a synchrotron light source. This method can be applied to plants from polluted sites for metal exploitation, and/or to biomasses from unpolluted sites for biochar production, since both applications take advantage of the knowledge of the minerals formed after heating.
Highlights
The chemical and biological interaction between roots and soil in contaminated environments is a complex research field that involves biology, geology, chemistry, and environmental engineering.Its significance comes from the multiple potential applications, from the determination of toxicity threshold for plants [1], to the transfer of contaminants in the food chain [2], to the use of plants in phytoremediation
We presented a specific case study of plant/soil interaction in an abandoned mine area contaminated by metals, and a methodology that could be applied elsewhere
The ex-situ heating turned out to be a useful tool in determining the occurrence of metal-bearing phases in plants, reducing or even removing the high background noise level produced by amorphous or low crystalline organic compounds and allowing to: (i) recognize minor minerals already present in the samples but previously hidden or barely recognizable and (ii) crystallize new phases reflecting the chemical composition of the samples
Summary
Its significance comes from the multiple potential applications, from the determination of toxicity threshold for plants [1], to the transfer of contaminants in the food chain [2], to the use of plants in phytoremediation. The latter field has acquired increasing interest during the last thirty years and it is widely recognized as one of the most promising techniques for limiting the diffusion of pollutants [3–7]. Phytoremediation is especially suitable for trace metals contamination because, unlike organic compounds, metals cannot be decomposed and they should be removed or at least immobilized. Public Health 2019, 16, 1976; doi:10.3390/ijerph16111976 www.mdpi.com/journal/ijerph
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