Incorporation of trace elements on iron-rich concretions around plant roots of tagus estuary salt marsh (portugal)

Objective

The concentration gradients of solid Al, Fe, Mn, Zn, Cd, Pb, Cr and Ni in freshly formed concretions and in older concretions preserved in the sediment were evaluated in a millimetre resolution scale. These results provide a better understanding of the complex biogeochemical processes in the root-sediment system and elucidate the potentialities of marshes in the restoration of contaminated aquatic systems.

Methods

Tens (soll das vielleicht tonnes/tons heißen) of freshly formed concretions and preserved concretions were collected in the Rosário salt marsh. Each tubular structure and involving sediments were scraped in concentric layers of 2-mm thickness to form two sets of composite samples. The elemental composition of concretion and sediment samples were determined by atomic absorption spectrometry.

Results and Discussion

The sediment involving the concretions contained ～0.5 mmol g^-1 Fe, but concentrations increased towards the root up to 3.4 mmol g^-1 in 3-mm interval. Manganese, Zn, Cd and Pb exhibited similar radial distribution in both preserved and freshly formed concretions indicating that these metals remain in those structures with the age. Chromium and Ni showed a different pattern being lower in concretions with respect to involving sediments, and consequently, concretions do not act as a barrier for these elements. The estimated amount of Zn (59 tons), Pb (5 tons) and Cd (0.4 tons) retained in the concretions all over the Rosario salt marsh highlights the importance of these structures in sequestering metals.

Conclusions and Outlook

The sequestering may be particularly relevant in salt marshes located near urban and industrial zones, being areas working as a natural remediation system and contributing for the detoxification of aquatic environments and the well-being of coastal resources.