Fe, Mn and 238U Accumulations in Phragmites australis Naturally Growing at the Mill Tailings Pond; Iron Plaque Formation Possibly Related to Root-Endophytic Bacteria Producing Siderophores

Mine drainage is a vital water problem in the mining industry worldwide because of the heavy metal elements and low pH. Rhizofiltration using wetland plants is an appropriate method to remove heavy metals from the water via accumulation in the rhizosphere. Phragmites australis is one of the candidate plants for this method because of metal accumulation, forming iron plaque around the roots. At the study site, which was the mill tailings pond in the Ningyo-toge uranium mine, P. australis has been naturally growing since 1998. The results showed that P. australis accumulated Fe, Mn, and 238U in the nodal roots without/with iron plaque compared with other plant tissues. Among the 837 bacterial colonies isolated from nodal roots, 88.6% showed siderophore production activities. Considering iron plaque formation around P. australis roots, we hypothesized that microbial siderophores might influence iron plaque formation because bacterial siderophores have catechol-like functional groups. The complex of catechol or other phenolics with Fe was precipitated due to the networks between Fe and phenolic derivatives. The experiment using bacterial products of root endophytes, such as Pseudomonas spp. and Rhizobium spp., showed precipitation with Fe ions, and we confirmed that several Pseudomonas spp. and Rhizobium spp. produced unidentified phenolic compounds. In conclusion, root-endophytic bacteria such as Pseudomonas spp. and Rhizobium spp., isolated from metal-accumulating roots of P. australis, might influence iron plaque formation as the metal accumulation site. Iron plaque formation is related to tolerance in P. australis, and Pseudomonas spp. and Rhizobium spp. might indirectly contribute to tolerance. Although there are many issues to be resolved in this research, we hope that the fundamental analysis of plant-microbe interactions would be helpful for phytoremediation at mine sites.

Analysis of the current state of technologies in the field of recycling technogenic gold-containing raw materials

In this article, we review existing approaches to recycling technogenic raw materials (ore dumps, metallurgical production slag, mill tailings of ore-dressing plants, etc.), containing non-ferrous and noble metals, which are accumulated in almost non-ferrous metallurgy industries. An analysis of existing technologies for processing technogenic raw materials (pyrite cinders and flotation tailings of concentration plants), which include enrichment, pyro- and hydrometallurgical and combined ways of extracting valuable components, was conducted on the basis of a review of published sources. It was shown that enrichment (screening, desliming in a hydrocyclone, enrichment using a concentration table, magneticliquid separation, flotation), pyrometallurgical and combined ways for extracting noble metals from this type of raw materials are unprofitable. The most satisfactory results were obtained using hydrometallurgical methods to extract valuable components from technogenic raw materials. Various solvents, such as sodium cyanide, thiocarbamide, sodium thiosulphate and sodium sulphite were tested as leaching agents. Cyanation proved to be the most effective way to extract noble metals from technogenic raw materials; however, this process is characterised by a high consumption of sodium cyanide. Therefore, it is of importance to discover an approach to extracting valuable components from such problematic products in order to make their processing more cost-effective by reducing cyanide consumption while maintaining gold extraction. According to the obtained results, gold-containing raw materials are promising in terms of extraction of nonferrous and noble metals using hydrometallurgical technologies. Future research should identify rational methods for processing technogenic gold-containing raw materials in order to make the technology more profitable for extracting valuable components.