Partitioning of metals in different binding phases of tropical estuarine sediments: importance of metal chemistry

Distribution of metals in different binding phases of estuarine sediments provides chemically significant description of metal–sediment interactions. This study describes the influences of ligand field stabilization energy (LFSE), Jahn–Teller effect, and water exchange rate (k ₋w) on metal distribution in different binding phases of estuarine sediments. It was found that Cu had highest affinity for organic binding phases in the studied sediments followed by Ni and Pb. However, Pb showed strong association with Fe/Mn oxide phases followed by Ni and Cu. Faster k ₋w of Cu (II) (1 × 10⁹ s⁻¹) increased the rate of complex formation of Cu²⁺ ion with ligand in the organic phases. The Cu–ligand (from organic phase) complexes gained extra stability by the Jahn–Teller effect. The combined effects of these two phenomena and high ionic potential increased the association of Cu with the organic phases of the sediments than Ni and Pb. The smaller ionic radii of Ni²⁺ (0.72 Å) than Pb²⁺ (1.20 Å) increase the stability of Ni–ligand complexes in the organic phase of the sediments. High LFSE of Ni(II) (compared with Pb²⁺ ions) also make Ni-organic complexes increasingly stable than Pb. High k ₋w (7 × 10⁹ s⁻¹) of Pb did not help it to associate with organic phases in the sediments. The high concentration of Pb in the Fe/Mn oxyhydroxide binding phase was probably due to co-precipitation of Pb²⁺ and Fe³⁺. High surface area or site availability for Pb²⁺ ion on Fe oxyhydroxide phase was probably responsible for the high concentration of Pb in Fe/Mn oxyhydroxide phase. Increasing concentrations of Cu in organic phases with the increasing Cu loading suggest that enough binding sites were available for Cu in the organic binding phases of the sediments. This study also describes the influence of nature of sedimentary organic carbon (terrestrial and marine derived OC) in controlling these metal distribution and speciation in marine sediment.