Phytoextraction and recovery of rare earth elements using willow (Salix spp.).

Soil and water contaminations are caused by rare earth elements (REEs) due to mining and industrial activities, that threaten the ecosystem and human health. Therefore, phytoremediation methods need to be developed to overcome this problem. To date, little research has been conducted concerning the phytoremediation potential of Salix for REEs. In this study, two Salix species (Salix myrsinifolia and Salix schwerinii) and two Salix cultivars (Klara and Karin) were hydroponically exposed to different concentrations of six-REE for 4 weeks. The treatments were: T1 (Control: tap water), T2 (La: 50 mg/L) and T3 (La 11.50 + Y 11 + Nd 10.50 + Dy 10 + Ce 12 and Tb 11.50 in mg L-1). The effects of the REE on Salix growth indicators (height, biomass, shoot diameter and root length), concentrations of REE in the produced biomass, and accumulation of REE in different parts of the Salix (stem, root, and leaf) tissues, were determined. In addition, the retention of REE in ashes following Salix combustion (800 and 1000 °C) was determined. The result indicates that with La and REE exposure, the height growth, dry biomass, shoot diameter and root length of all Salix remained equivalent to the control treatment excluding Klara, which displayed relatively higher growth in all parameters. Further, among the REE studied, the highest La concentration (8404 µg g-1 DW) and La accumulation (10,548 µg plant-1) were observed in Karin and Klara root respectively. Translocations and bioconcentration factors were discovered at <1 for all Salix, which indicates their phytostabilization potential. The total REE concentrations in bottom ashes varied between 7 and 8% with retention rates between 85 and 89%. This study demonstrates that Salix are suitable candidates for REE phytostabilization and the remediation of wastewater sites to limit metals percolating to the water layers in the ecosystem.

Thermal treatment of municipal solid waste incineration fly ash: Impact of gas atmosphere on the volatility of major, minor, and trace elements.

Development of thermal processes for selective recovery of Zn and other valuable elements from municipal solid waste incineration (MSWI) fly ash requires comprehensive knowledge of the impact of gas atmosphere on the volatile behaviour of the element constituents of the ash at different reaction temperatures. This study assesses the partitioning of 18 elements (Al, As, Bi, C, Ca, Cd, Cl, Cu, K, Mg, Na, P, Pb, S, Sb, Sn, Ti, and Zn) between condensed and gaseous phases during thermal treatment of MSWI fly ash in both oxidising gas and reducing gas atmospheres, at different temperatures spanning the range 200-1050 °C. The operating atmosphere had major impacts on the partitioning of the following elements: As, Bi, C, Cd, Cu, Na, Pb, S, Sb, Sn, and Zn. The partitioning of these elements cannot be accurately predicted over the full range of investigated operating conditions with global thermodynamic equilibrium calculations alone, i.e. without also considering chemical kinetics and mass transfer. In oxidising conditions, the following elements were predominately retained in condensed phases, even at high temperatures: As, Bi, Sb, Sn, and Zn. All these elements, except As, were largely released to the gas phase (>70%) at high temperatures in reducing conditions. The impact of gas atmosphere on the volatility of Cd and Pb was greatest at low reaction temperatures (below ~750 °C). Results for volatile matrix elements, specifically C, Cl, K, Na, and S, are interpreted in terms of the mechanisms governing the release of these elements to the gas phase.

Volatilisation of major, minor, and trace elements during thermal processing of fly ashes from waste- and wood-fired power plants in oxidising and reducing gas atmospheres.

This study assesses the volatility of 15 elements (As, Bi, C, Cd, Cl, Cu, K, Mn, Na, P, Pb, S, Sb, Sn, and Zn) during thermal processing of fly ashes obtained from four waste-to-energy plants and one wood-combustion plant. Differences in volatility in oxidising and reducing atmospheres (air and 10% H2/90% N2) were assessed at two temperatures, 700 and 1000 °C. P and Mn were predominately retained in all ashes regardless of the operating atmosphere and temperature. Other elements showed significant variation in volatility depending on the type of fly ash, atmosphere, and temperature. Heat-treatment of the wood-combustion fly ash in the air atmosphere resulted in low release of K, Na, and all investigated heavy metals and metalloids. Several valuable elements, including Zn, Sb, Sn, and Bi, were significantly more volatile in the reducing atmosphere than in the oxidising atmosphere, particularly at 1000 °C. Other elements were either less volatile, equally volatile, or only marginally more volatile when the ashes were heated at 1000 °C in the reducing atmosphere. These elements include C, Cl, Cu, and, in the case of fly ashes derived from municipal solid waste, Cd and Pb. A two-step process, in which municipal solid waste incineration fly ash is first heated in an oxidising atmosphere and then in a reducing atmosphere, is proposed for production of a chloride-free zinc concentrate. Evaluation of the two-step process at 880 °C shows good potential for selective volatilisation of Zn with other valuable elements, including Sn, Sb, and Bi.