Screening dilute sources of rare earth elements for their circular recovery

Metals, including rare earth elements (REE), are the cornerstone of our current and future low-carbon urban infrastructure. This study looks at different waste resources and contaminated materials present in the urban setting as REE sources. Wastes and other dilute sources such as incineration ashes, sediments, and mine tailings are not only essential sources of REE in achieving a circular, carbon-neutral economy but may be the most realistic one. E-waste, being the most REE concentrated waste, faces serious reservations regarding handling in large-scale facilities, and this waste is generally landfilled. In this study, we analyzed REE total concentrations and pH desorption curves in ten dilute sources of REE: Ferrochrome slag from a mine in South Africa, sediments retrieved form stormwater ponds in Denmark, coal fly ashes, municipal solid waste (MSW) fly ashes, wood fly and bottom ashes and sewage sludge fly ashes (ashes from different sources). After analyzing different residues, we found that coal fly ashes and stormwater retention pond sediments present the most promising ones. While coal fly ashes have the highest critical REE contents from the studied wastes, the sediments collected from a stormwater retention pond showed the highest REE leachability. We can find Nd, Dy, and Er – all critical REE – in sediments/soils near highways, coal ashes, and bauxite residue. Overall, coal fly ashes contain the highest critical REE contents found in the studied wastes but sediments collected from stormwater water ponds present the highest leachable REE. In fact, up to 100% of total REE found in these sediments are leachable at room temperature and low pH. Future REE resource extraction efforts should account for REE speciation in wastes and not only total contents.