Rhizosphere modifications of iron-rich minerals and forms of heavy metals encapsulated in sulfidic tailings hardpan

Hardpan caps formed after extensive weathering of the top layer of sulfidic tailings have been advocated to serve as physical barriers separating reactive tailings in depth and root zones above. However, in a hardpan-based root zone reconstructed with the soil cover, roots growing into contact with hardpan surfaces may induce the transformation of Fe-rich minerals and release potentially toxic elements for plant uptake. For evaluating this potential risk, two representative native species, Turpentine bush (Acacia chisholmii, AC) and Red Flinders grass (Iseilema vaginiflarum, RF), of which pre-cultured root mats were interfaced with thin discs of crushed hardpan minerals in the rhizosphere (RHIZO) test. After 35 days, the surface dissolution of hardpan minerals occurred and Fe-rich cement minerals were transformed from ferrihydrite-like minerals to goethite-like and Fe(III)-carboxylic complexes, as revealed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and synchrotron-based X-ray absorption fine structure spectroscopy (XAFS) analysis. This transformation may result from the functions of root exudates. The transformation of hardpan cement minerals caused the co-dissolution of Cu and Zn initially encapsulated in the cements and their uptake by plants. Nevertheless, only was the minority of the plant Cu and Zn transported into shoots.