Potential bioaccumulation of trace metals in halophytes from salt marshes of a northern Adriatic coastal lagoon

Purpose

The Marano and Grado Lagoon (Italy) has been affected by trace metal(oid) contamination in the last century, especially mercury, from both industrial and long-term mining activities. The uptake and distribution of trace metal(oid)s in halophytes were determined in two selected salt marshes. To evaluate the potential activity of plants as phytoremediation, the bioconcentration and translocation factors (BCF and TF, respectively) were calculated.

Materials and methods

In both salt marshes, individuals of Sarcocornia fruticosa L. and Limonium vulgare L., two of the most abundant halophytes in this environment, were sampled. The aboveground biomass (stems and leaves) was collected and sealed in plastic bags. Once the stems were removed, the belowground biomass and the attached rhizo-sediment were sampled using a single gouge auger sampler. The sediment cores obtained were sectioned on field to a maximum depth of 15 cm. The roots were carefully separated from the rhizo-sediment in the laboratory. The sediment, roots, leaves, and stems were freeze-dried, finely ground, and homogenized. Samples were totally decomposed, using a mixture of mineral acids in a closed microwave system, and analyzed for trace metal(oid) content by ICP-AES. The total Hg content in the solid phase was determined by DMA-80.

Results and discussion

Metal(oid) concentrations in roots were usually up to one or two orders of magnitude higher than in stems and leaves. The exceptions are Cd and Ni, which levels were not detectable, and Cr in stems of both halophytes where the concentration reached up to four times more than in roots. Commonly, trace metal(oid) contents were higher in stems than in leaves, except for Zn. Considering all BCF data, a sequence of metal(oid)s preferentially transferred from sediment to belowground biomass of the two plants is Cd > Mn > As > Pb. This sequence does not coincide for the two salt marshes, except for Cd, probably due to the different source of metal(oid)s in sediments and/or some site-specific lithogenic properties. Metal(oid)s accumulated from rhizo-sediment were largely retained in roots as shown by TF values <1.

Conclusions

The general trend arising from BCF and TF reveals that root tissues accumulate significantly greater amounts of metal(oid)s than the aerial part, thus indicating high plant bioavailability of the substrate metal(oid)s as well as their limited translocation to the aboveground biomass. Our results suggest that both salt marshes investigated act as a sink, and only sporadically as a possible source, for several trace metal(oid)s which are not promptly available for the environment.