Revisiting the forms of trace elements in biogeochemical cycling: Analytical needs and challenges

Trace elements, existing as various forms at different dimensional levels, play an important role in the functioning of life on earth. These element forms, from small to large in size, include free ions, ionic compounds, nanoparticles, macromolecular complexes and bulk materials (i.e., minerals). They exhibit different chemical activities and hence profoundly determine their different transport, transformation, biological function and fate in environmental and biological systems. Recently, it is realized that nanoparticles and macromolecular complexes are the central of element forms, playing the dual roles of sources and sinks for ionic metals and bulk metals in various systems. In this paper, we revisited the notion of element forms from a more integrated perspective instead of any specific forms, reviewed the recent development of speciation methods and discussed the potential trends in technique improvement for analysis of various element forms at different dimensional levels in the future. (C) 2020 Elsevier B.V. All rights reserved.