Speciation of Co, Ni and Cu in the coastal and estuarine sediments: Some fundamental characteristics

Kinetic and sequential extraction methods were used to investigate chemical speciation and distribution of Co, Ni, and Cu in the estuarine and coastal sediments collected from the central east coast of India. The kinetic (single) extraction study provided a chemically significant description of the kinetics of metals–sediment interactions and correctly predicted the influences of ligand field stabilization energy (LFSE) and water exchange rate (k-w) on the dissociation rate constants of metals–sediments complexes in the natural systems. The results consistently demonstrate that the fast (kd1) and medium fast (kd2) dissociation rate constants of the 3d transition metals complexes in the estuarine and coastal sediments follow the trend as predicted from the LFSEs (weak field) (kd 1,2,Co(II) d7>kd 1,2, Ni(II) d8>kd 1,2, Cu(II) d9). However, a higher rate coefficient of water exchange (k-w) of Cu (II) helps them to form more strong inert complexes than predicted from its LFSE alone. Sequential extraction method was used to fractionate these metals depending upon their association with different phases in the studied sediments. The comparison and statistical analysis of the fractionation data obtained by two independent methods indicate that the transition metals formed strong complexes with the organic matter present in the sediments. However, Cu undergoes strong complexation reaction with organic matter within the sediments followed by Ni and Co. This study suggests that the non-residual fractions (obtained from sequential extraction study) may overestimate the bioavailable fraction of metals in a system. ► LFSE may control the stability of 3d transition metal-sediment complexes. ► The order of stability of metal complexes in sediments is Cu(II) > Ni(II) > Co(II). ► Cu (II) forms both inert and dynamic complexes with sediments. ► Multi-method approach provides a better physicochemical picture of metal speciation.