XRD-Thermal Combined Analyses: An Approach to Evaluate the Potential of Phytoremediation, Phytomining, and Biochar Production

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.

Mineralogy and Zn Chemical Speciation in a Soil-Plant System from a Metal-Extreme Environment: A Study on Helichrysum microphyllum subsp. tyrrhenicum (Campo Pisano Mine, SW Sardinia, Italy)

“…In this study, SEM and EDS analysis showed the presence of a mineral rim made up of Al-silicates coating the epidermis of H. tyrrhenicum roots (Figure 2), as previously observed in other autochthonous plant species growing in similar environments, like E. pithyusa subsp. cupanii, P. lentiscus, P. australis, and J. acutus [23,[76][77][78]109]. This structure was also detected by STXM ( Figure 6), also showing that Zn is mainly stored in the root epidermis.…”

Ex situ phytoremediation trial of Sardinian mine waste using a pioneer plant species

“…Biominerals were detected also in Sardinian autochthonous plant species growing in extreme metal environments like E. pithyusa subsp. cupanii (Medas et al 2015 ), Juncus acutus L. (Fancello et al 2019 ; Medas et al 2019 ), Pistacia lentiscus L. (De Giudici et al 2015 ), and Phragmites australis (Cav.) Trin.…”

“…Along the last 15 years, several studies were carried out on the Sardinian mine context through field sampling, in situ and ex situ phytoremediation experiments. In details, these studies highlighted that different autochthonous species are able to grow in these environments and to tolerate extremely high heavy metal concentrations (e.g., Cao et al 2004 , 2009 ; Jiménez et al 2005 , 2011 , 2014 ; Bacchetta et al 2012 , 2015 , 2017 , 2018 ; Concas et al 2015a ; De Giudici et al 2015 , 2017 ; Medas et al 2015 , 2019 ; Fancello et al 2019 ; Boi et al 2020a ; De Agostini et al 2020 ).…”

Plant-minerals-water interactions: An investigation on Juncus acutus exposed to different Zn sources

“…It exploits plant cover to prevent pollutants from spreading by erosion, water infiltration, leaching and from toxic dust dispersal by wind (Frérot et al 2006, and references therein). Juncus acutus L. has evolved adaptive mechanisms, in particular metal tolerance (Fancello et al, 2019;Freitas et al, 2009;Landsberger et al, 2010;Santos et al, 2014;Stefani et al, 1991;Syranidou et al, 2017a), allowing it to thrive on mine soils (Syranidou et al, 2017a). As with other macrophytes (Marchand et al, 2010), J. acutus plays a significant role in the riverbed processes, reducing metal mobility by limiting erosive processes, promoting the sedimentation of suspended particles, providing organic matter for bacterial metabolism, and potential sites for metal sorption.…”