Effect of salt-marsh plants on the mobility of Cr in sediments

Abstract

Depth variation (1-cm resolution) of total Fe, Mn, and Cr concentrations was determined in below-ground biomass of Halimione portulacoides and in root sediments from two Tagus Estuary salt marshes (Portugal). The metal fractions extracted by hydroxylamine (HYD) and HCl were also quantified in sediment samples. Edaphic conditions of the sediment (O₂, pH, E_H, acidvolatile sulphides [AVS], and root biomass) were characterized in both marshes. Higher dissolved oxygen concentrations were found at depths with increased root biomass, while AVS were nearly absent in the root zone increasing with depth. Sediments colonized by H. portulacoides showed higher variability of total metal concentrations with depth than non-vegetated sediments, possibly due to intense exchanges between the below-ground plants and surrounding sediments. Concentrations of Fe_HYD (mainly Fe oxyhydroxides) were elevated in the root sediments since Fe(II) is oxidized by the O₂ released from roots. The normalization of Cr_HYD levels to Fe_HYD showed a sharp increase when oxygen is depleted, suggesting that Cr is sequestered in the Fe oxyhydroxide fraction. Roots of H. portulacoides from both marshes showed little ability to take up and accumulate Cr. A possible explanation is the reduction of soluble Cr(VI) to the stable Cr(III) and subsequent retention in the Fe oxyhydroxide fraction. The inability or limited defence mechanisms of H. portulacoides to accumulate Cr indicates that this halophyte is not adequate for successfully phytostabilizing Cr-polluted marshes.