Spatial analysis of rare earth elements and lead in urban soils of Greater London Authority area and development of a new method for assessment of labile rare earth elements in soils

Abstract

Rare earth elements (REEs) are a group of seventeen elements, comprised of the fifteen lanthanides, yttrium (Y) and scandium (Sc), which have been known as ‘industrial vitamins’ due to their important roles in technical progress and the development of industries. Despite the increasing interest, there is current controversy about the health benefits and toxicity of these materials during the processing and utilization of REEs. Due to long residence time in soils and their potential toxicity effects, the contamination of urban soils has received wide attention in recent years. However, little information is available on REEs accumulation in urban soils, and the spatially varying relationships between geochemical elements in urban soils have also seldom been investigated. In addition to the complex land use and heterogeneity of urban soils, the sources of heavy metals and REEs in soils can be both natural and anthropogenic, and the spatial variation of their enrichments vary over space. On the other hand, for risk assessment of metal-contaminated soils, it is important to predict the bioavailability of metals. In this research, geographical information system (GIS) and spatial statistical techniques were applied to identify the contamination hotpots and to reveal the spatial variation of geochemical elements in urban soils of London. Meanwhile, a new method for simultaneous measurements of REEs using the diffusive gradients in thin films (DGT) was conduct in laboratory and deployment in soils. The research objectives were: (1) to investigate the concentration and distribution of lead and seven REEs in urban soils; (2) to identify contamination hotspots of REEs on sources and their influencing factors; (3) to reveal spatially varying relationship between Pb and Al in London soils and to explore the influencing factors of these elements in the study area; (4) to develop DGT for the simultaneous measurements of fifteen REE ions. The results showed that the index of local Moran’s I was a useful tool to identify contamination hotpots of Ce, La, Nd, Sc, Sm, Yb and Y in urban soils, and to classify them into spatial clusters and spatial outliers. Soil parent material was a natural factor leading to high-value spatial clusters of REEs in north and south area, while individual spatial outliers were associated with anthropogenic sources including agricultural practices, vehicular emission and urbanization. The relationships between Pb and Al were spatially varying in urban soils of London, with different relationships in different areas. The GWR models showed clear positive spatial relationships influenced by the natural geochemical factors found in large parklands and greenspaces in part of central, as well as southeast and southwest areas of London. Anthropogenic factors had a great impact on the concentration of Pb, leading to the weakened correlation in central London or even the changed relationship direction from positive to negative correlation between Pb and Al in the suburban area of northern London. This study highlights the value of using GWR to reveal spatially varying relationships in environmental variables. Furthermore, a new technique was developed to measure simultaneously fifteen REE ions in this study using the diffusive gradients in thin films (DGT) with Chelex® 100 binding gel. The results showed that DGT uptakes of all REE ions were independent of pH (3-9) and ionic strength (3 mM -100 mM). The fifteen REE ions were successfully extracted by elution using 2.0 M HCl, with elution rate ranging from 86.5% to 93.8%. The capacities of Chelex® 100 DGT for measurement of the mixed elements were determined at a range of 5.39-6.75 mg cm2, reflecting rapid binding dynamic to REEs in a mixed solution of the fifteen REE ions. This study demonstrates a significant advantage of Chelex® 100 DGT in simultaneous measurements of the fifteen REE ions, which could be an effective tool for the simultaneous measurements of REEs in the environment.