Bioaccumulation and translocation of rare earth elements in two forage legumes grown in soils treated with coal fly ash

Abstract

Coal fly ash (CFA) is often used as a soil amendment to improve the physicochemical properties of the soil. However, CFA contains abundant rare earth elements (REEs), which may negatively affect plant growth and physiology and cause environmental hazards. We performed pot experiments to grow alfalfa (Medicago sativa L.) and erect milkvetch (Astragalus adsurgens Pall.) in soils treated with different rates of CFA, in order to investigate the impacts of CFA on the accumulation and translocation of REEs such as lanthanum (La), cerium (Ce), scandium (Sc) and yttrium (Y). The results showed that La, Ce, Sc and Y concentrations were considerably higher in the CFA than in soils, but adding CFA to soils did not always significantly increase their concentrations in plants. The element abundance in soils and CFA always followed the order of Ce > La > Y > Sc, but the element bioaccumulation factor was generally in the order of Sc > Y > La > Ce. In plants, La, Ce, Sc and Y concentrations generally followed the order of root > shoot and leaf > stem, with Ce concentrations being always higher than La and Y concentrations. Most of these elements absorbed were sequestered in roots without being further transported to the aerial parts. The root-to-shoot translocation factor of Sc was the highest, but the stem-to-leaf translocation factor of Sc was the lowest among the elements studied. The biogeochemical behaviours of Sc were quite different from those of La, Ce and Y. Adding CFA to the soils often considerably increased the amounts of La, Ce, Sc and Y accumulated in shoots due to significant biomass increase. Furthermore, REE speciations in the topsoil can be changed by both CFA addition and plant recycling. Therefore, land application of CFA for revegetation and soil reclamation proposes can affect the biogeochemical behaviours of La, Ce, Sc and Y.