Pb and Zn imaging and in situ speciation at the geogenic/biogenic interface in sentinel earthworms using electron microprobe and synchrotron micro-focus X-ray spectroscopy

Using synchrotron- and electron microscope-based X-ray microanalyses, the distribution and speciation of Zn and Pb were examined in situ in two earthworm species (Dendrodrilus rubidus and Lumbricus rubellus) living in heavily-polluted soils. Main findings: (i) Zn spectra in ingested soil and in tissues more closely resembled Zn3(PO4)2 than ZnS; (ii) Zn speciation in tissues gave a best fit for Zn to the inner shell of 4 oxygens at 1.94 Å (or nitrogens at 1.96 Å); (iii) the best fit for Pb in tissue was with a shell of oxygens at 2.18 Å and a shell of sulphurs at 2.67 Å; (iv) a component of the Zn and much of the Pb detectable in gut contents was co-distributed with S; (v) Zn and Pb display ‘soft’ acid affinities in soil, but ‘hard’ acid affinities in tissue. This is the first metal characterisation study conducted on an invertebrate quench-frozen in the field.

► First in situ metal imaging & speciation in environment model organism.
► Innovative combination of synchrotron & electron probe μ-focus X-ray analyses.
► Tissues quench-frozen under field conditions preserving compositional integrity.
► Direct in situ imaging promotes understanding of metal fates & metabolism.
► Approach is compatible with future improvements in synchrotron technology.