Accumulation and partitioning of heavy metals in mangrove rhizosphere sediments

A field study was conducted to clarify the effect of rhizosphere processes on the accumulation and partitioning of heavy metals (Pb, Zn, Cu, Cr, Cd and Ni) in mangrove sediments. Metals were fractionated by a sequential extraction procedure into three chemically distinct fractions: water soluble, exchangeable and carbonate bound (B1), Fe–Mn oxide bound (B2), and organic and sulfide bound (B3). Results indicate that rhizosphere processes tend to increase the metal concentrations in the rhizosphere sediments. However, plant uptake may result in the decrease of the metal concentrations in the rhizosphere sediments when the metal concentrations are relatively low in the bulk sediments. Compared with the bulk sediments, the rhizosphere sediments have low concentrations of heavy metals in the B1 and B2 fractions and high concentrations in the B3 fraction. Either an increase or decrease in the residual fraction of heavy metals in the rhizosphere sediments may appear, depending on whether the formation of the refractory metal-organic compounds or the activation of the residual fractions dominates. Results also indicate that mangrove plants absorb and store non-essential metals in the perennial tissues, thus reducing the export of non-essential metals via leaf litter transport. Mangrove plants are excellent candidates for phytostabilization of heavy metals in intertidal substrates.