Electrochemical characterization of Cobalt(II)-Complexes involved in marine biogeochemical processes. I. Co(II)-4-nitrocatechol and Co(II)-Humate
Graphical abstract
Introduction
Global climate changes involve complex processes in marine ecosystems, especially in the surface layers. Using a multidisciplinary, holistic approach, it is requisite to study the atmospheric influence on biogeochemical responses in the marine environment. A gradual approach and specific objectives for the assessment of concentrations, sources and fate of atmospheric components, the impact on marine biogeochemistry of macro- and micronutrients and inevitable effects in the biological organization of marine environment, are of great importance [1].
The sea surface micro-layer represents one of the largest natural phase boundaries on Earth, as an interfacial area it is crucial in gases, particles and energy exchange processes between sea and atmosphere [[2], [3], [4], [5], [6]]. The atmosphere impact is a significant external source of macro- and micronutrients.
Trace elements in oceans are present in very low concentrations <10−9 mol dm−3 [7]. Their bioavailability in surface waters regulates phytoplankton growth. As phytoplankton consumes atmospheric carbon dioxide and enters it into the seawater column, the key components that regulate the carbon cycle are bioavailable trace metals linked to the organic matter. In addition to iron, other bioactive micronutrients such as cobalt, copper, nickel and zinc play an important role in phytoplankton adaptation and growth regulation of, vital for the carbon biological pump [8,9].
Cobalt is an important micronutrient involved in vitamin B12 (cobalamin) biosynthesis and can replace zinc as a metal co-factor [[10], [11], [12], [13]]. Cobalt speciation in oceans is a dynamic process as it is a redox-active metal. Part of the dissolved cobalt under the euphotic zone remains unbound or weakly bound to the labile complexes [14]. The cobalt distribution in seawater often shows a maximum within the upper boundary of the thermocline as a result of atmosphere intake. In deep ocean layers it is exhausted due to the removal from seawater column by sedimentation processes [15]. Improvements in cobalt biogeochemistry understanding should give an insight into other current hypotheses, such as the Sunda Scenario [16], C–Co–Zn colimitation [12], as well as speciation influence on phytoplankton abundance.
In order to make cobalt speciation measurement at seawater, it is necessary to consider the redox state of cobalt. Co existing in +2 and + 3 oxidation state under typical environmental EH and pH conditions and is a redox active element that tends to be strongly bound to organic complexes in upper water column [17].
Cobalt (II) complexes formation with naturally occurring ligands at the hydrophobic mercury drop surface as a model for hydrophobic cell membranes in NaCl solution is studied, mimicking seawater conditions. Electrochemical characterization and speciation of cobalt (II) complexes with catecholate and humic acid type ligands was examined, as essential in the marine environment. Catecholates, ortho-dihydroxybenzene and 4-nitro-dihydroxybenzene, as simple organic ligands from aerosols are present as a structural part of complex marine ligands (humic acid, siderophores, etc.). Humic substances as heterogeneous biomolecules are a characteristic component of the dissolved organic carbon (DOC) pool in marine systems and important for phytoplankton communities [[18], [19], [20], [21]].
Section snippets
Electrochemical measurements
Electrochemical experiments were performed using a μ-AUTOLAB multi mode potentiostat (ECO Chemie, Utrecht, The Netherlands) equipped with a Metrohm 663 VA stand (Metrohm, Herisau, Switzerland). The instrument was computer-controlled using GPES 4.9 control software. The working electrode was a static mercury drop electrode (SMDE, size 2, i.e. 0.40 mm2), the counter electrode was a glassy carbon rode and the reference electrode was Ag/AgCl (sat. NaCl) (+0.197 V vs. SHE). Measurements were
Voltammetric characterization of Co(II)–4NC and Co(II)-HA complexes
Measurements were performed by square-wave (SW) and cyclic voltammetry (CV). Standard potential (E°) for the two-electron reduction of Co(II) (aq) ion in aqueous solutions is −0.28 V.
Catechol (1,2-dihydroxybenzene) and nitrocatechol are weak diprotic acids, present in a wide scale of environmental compounds as chelating ligands [25]. Nitro substituent leads to an increase hydroxyl groups acidity in phenol derivatives [26]. Several estimates of acidic dissociation constant in aqueous medium and
Conclusion
Cobalt speciation understanding is critical for its bioavailability evaluation in dynamic marine environment, involving living cells, aquatic and sediment compartments. Co(II) complexes have an important role in numerous biological processes, and therefore, a clear description of their chemistry is of substantial relevance.
Cobalt (II)–4NC and cobalt (II)-HA complexes characterization, using square-wave and cyclic voltammetry, in combination with UV–Vis spectrophotometry, were preformed.
CRediT authorship contribution statement
Anđela Bačinić: Methodology, Investigation, Validation, Data curation, Writing - original draft. Lidija-Marija Tumir: Investigation, Data curation, Writing - original draft. Marina Mlakar: Conceptualization, Supervision, Writing - review & editing, Project administration, Funding acquisition.
Acknowledgement
This work was realized within the scope of the project “New methodological approach to biogeochemical studies of trace metal speciation in coastal aquatic ecosystems” supported by the Croatian Science Foundation under the project number IP–2014–09–7530.
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