Zn Speciation and Stable Isotope Fractionation in a Contaminated Urban Wetland Soil-Typha latifolia System

Wetlands play a key role in the immobilization of metallic contaminants. In this context the mechanisms of Zn sequestration and Zn transfer and storage in Typha latifolia L. colonizing a frequently flooded contaminated soil were studied. A combination of EXAFS spectroscopy, micro X-ray fluorescence (μXRF) and Zn isotope measurements was applied to soil, plant organs and decaying biomass. Zn was present in the soil as Zn-layered double hydroxide, as tetrahedral and octahedral sorbed Zn species, and as ZnS. Octahedral and tetrahedral Zn (attributed to symplastic Zn-organic acid and apoplasmic Zn-cell wall complexes, respectively) and Zn-thiol species were observed in the roots, rhizomes and stems. Iron plaque was present on the rhizomes and roots. Enrichment in light isotopes for Zn sorbed on the plaque relative to the soil (Δ⁶⁶Zn_plaque-soil = -0.3 to -0.1‰) suggested the dissolution of ZnS (enriched in light isotopes) in the rhizosphere with subsequent Zn²⁺ sorption on the root plaque. Furthermore, enrichment in light isotopes of stems relative to leaves (Δ⁶⁶Zn_stem-leaves = -0.2‰) suggested the remobilization of Zn via the phloem, from leaves back to the stems. Overall these data highlight the role of thiols in controlling Zn speciation during its transfer and storage in T. latifolia.