Speciation of Co, Ni and Cu in the coastal and estuarine sediments: Some fundamental characteristics
Highlights
► 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.
Introduction
Interactions of trace metals with natural ligands (which are ubiquitous in freshwater and marine sediments) play an important role in their (trace metals) transport, fate, and bioavailability in natural systems (Buffle, 1988, Stumm and Morgan, 1988). The chemistry of trace metals and their interaction with natural ligands (such as, dissolved organic matter) are well studied in aquatic system as reflected by the large numbers of publications available in the literature (Achterberg and Braungardt, 1999, Bayen et al., 2006, Chakraborty, 2010, Chakraborty and Chakrabarti, 2006, Chakraborty and Chakrabarti, 2008, Chakraborty et al., 2006, Chakraborty et al., 2007, Chakraborty et al., 2009, Gopalapillai et al., 2008, Groschner and Appriou, 1994, Guthrie et al., 2005, Nicolau et al., 2008). However, the knowledge of speciation of trace metals and their interactions with heterogeneous binding sites present in sediments are not well understood.
Various experimental methods/techniques are being used to determine speciation of trace metals in soils and sediments. The approaches include batch techniques (Bermond et al., 1998, Bermond and Varrault, 2004, Carski and Sparks, 1987, Varrault et al., 2001, Ghestem and Bermond, 1999, Langford and Gutzman, 1992, Yu and Klarup, 1994), flow techniques (Beauchemin et al., 2002, Davison et al., 2000, Jimoh et al., 2004), kinetic extraction methods (Chakraborty et al., 2006, Chakraborty et al., 2011, Chakraborty et al., 2012a, Chakraborty et al., 2012b), as well as in situ techniques such as Diffusive Gradients in Thin Films (DGT) (Bermond et al., 1998, Fangueiro et al., 2002, Town et al., 2009, Van der Veeken et al., 2010). However, the kinetic extraction method is found to be cheaper, simpler and sensitive compared to the other methods. This method is capable of providing information on kinetically distinguishable metal complexes (in soil/sediment) and their corresponding dissociation rate constants in natural systems. This method helps us understand the distribution of trace metals in soil/sediment environment by providing information about the lability of various trace metal species in the systems (Varrault et al., 2001, Bermond and Varrault, 2004, Yu and Klarup, 1994).
In the kinetic extraction method, trace metals are quantified in an extract (containing a suitable chelating agent) as a function of time until equilibrium is reached. This time dependent metal extraction study from sediments can be useful in understanding the transport, fate, and bioavailability of trace metals (Sparks, 1989, Yu and Klarup, 1994) in sediments. The kinetic (single) extraction method has been successfully applied to predict uptake rate of metals by plants or other living organism in natural systems (Feng et al., 2005, Pinheiro and Van Leeuwen, 2001, van Leeuwen et al., 2005). This method has been widely used to ascertain the potential availability and mobility of the pollutants and their migration in a soil/sediment profile in polluted or naturally contaminated soils/sediments (Fangueiro et al., 2002, Margaret et al., 2003, Zhimang et al., 2001).
However, the association of metals in different phases of sediments cannot be understood by kinetic extraction method. Sequential extraction method is able to fractionate metals depending upon their association with different phases in sediments, such as: water soluble, exchangeable, bound to carbonates, bound to Fe/Mn-oxides, associated with organic matter, bound to sulfides, and residuals.
Sequential extraction method has been widely used for the fractionation study of metals in sediments from different sources, such as rivers, estuaries or marine sediments (Belzunce-Segarra et al., 1997, Singh et al., 1998, Tessier et al., 1979, Tsai et al., 1998). It is well known that binding sites present in sediment are more complex and heterogeneous compared to the heterogeneity and complexity of dissolved organic matter present in aquatic system.
To provide a better understanding of metal–sediment interactions, metals distribution and the influence of transition metal chemistry of three biologically important metals (Co, Ni and Cu) on their speciation in sediments, kinetic (single) extraction and sequential extraction methods were applied in this study.
The objective of this study was to test the hypotheses that the distribution and speciation of the three important transition metals in sediments are influenced by (i) metal-to-Total organic carbon (TOC) ratio, (ii) ligand field stabilization energy (LFSE). This study also aimed to show that the use of two independent methods (with different analytical windows) can provide a better physicochemical picture of metal speciation in sediments than either one of the method can do alone.
Section snippets
Study area and sampling
The sediment samples were collected from the three different environmentally significant sites of the central east coast of India.1) Kalingapatnam (KGP): The Kalingapatnam site is located in the north of Andhra Pradesh. This city is not industrially developed but an important minor port is located close to the sampling station. The approximate population of this area is 10,000.
Estuarine sediment samples were collected from (2) the Goutami Godavari estuary (GGE) (A 100 km2 area around Gautami
The kinetic model
The kinetic model proposed by Olson and Shuman (1985) was adapted (Lu et al., 1994, Mandal et al., 1999) to investigate the kinetic fractionation of Co, Ni and Cu in coastal and estuarine sediments. Consider sediment samples of n different components, in which each component, M–Sedimenti, exists in equilibrium with its dissociation products: the free metal ion or extractable metal complexes, M, and a naturally occurring, heterogeneous complexant, sediment i, such as humic acid adsorbed on
Total metals content in the sediments
The average concentrations of trace metals (Sc, V, Cr, Co, Ni, Cu, Zn, Pb, Cd, and Sn) in the coastal and estuarine sediments collected at the three environmentally significant sites in the central east coast of India are presented in Table 1. Evidently, the general levels of trace metals concentrations were relatively high in both the estuarine sediments compared to the coastal sediments. The correlation matrix and regression analysis suggest that a strong positive correlation exist between
Acknowledgments
Author is thankful to the Director, NIO (CSIR), Goa and the Scientist in charge, NIO RC, Visakhapatnam for their encouragement and support. Thanks to Drs G Parthiban and J.N. Pattan (from NIO, Goa) for carrying out ICP-MS analysis. Author gratefully acknowledges their unconditional help. This work is a part of the Council of Scientific and Industrial Research (CSIR) supported Supra Institutional Project (SIP 1308). This article bears NIO contribution number 5118.
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