engleski

Vanadium speciation in the samples of "Krka River" estuary using ion chromatography inductively coupled plasma mass spectrometry

Vanadium speciation in environmental samples of high salinity still present a challenging task in environmental and analytical chromatographic chemistry. Despite relatively high concentrations in seawater (around 35 nmol dm-3), vanadium speciation analysis has been mainly neglected due to its complex chemistry and a vast number of oxidation states that vanadium takes form of. Redox states of vanadium are shown to be highly dependent upon pH and Eh values of the system, as well as biological activity and concentration [1]. In addition, seawater presents complex matrix for anion exchange chromatographic separation, where chloride anion is the source of most prominent interferences [2]. In oxic seawater (positive Eh and pH of 7-8), vanadium can be present as vanadium(V) in the form of hydrolysed HVO4 2- and H2VO4- ions forming polynuclear species at higher concentration. In more reducing acidic environment it is expected that a part of vanadium is present as VO2+ which can be easily oxidized to pentavalent state at higher pH values. Under the Eh, as well as pH, values characteristic for modern anoxic environment (Eh of approximately -300 to -100 mV and pH of 7-8) the predominating V redox state is vanadium(IV), while the predominating form is HV2O5-. Under these conditions vanadyl species can be reduced to vanadium(III) solid oxides V(OH)3 or V2O3. Reduced vanadium species can be then easily accumulated into the sediment [1]. Estuaries play an important role in geochemical cycling of trace metals, where various physical and chemical gradients affect solubility and mobilization of species [2]. In the freshwater-seawater mixing zone of estuaries, metal-organic interaction can easily affect speciation and established equilibria between chemical species [3]. Vanadium in „Krka River“ estuary was studied before, but only in reference to its bulk concentrations. Due to the dependence of the toxicity and redox state that vanadium takes form of in given environment, speciation method allows better insight on vanadium chemistry and bioavailability of vanadium species. A vanadium speciation method is conducted as on- line measurement via coupled IC with ICP MS as a detector with high detection limit. Presented analytical approach is suitable for quantification of vanadium species in „Krka River“ estuary water column samples. Separation of vanadium species on anion exchange column is based on the formation of negatively charged V-EDTA complexes, where present vanadium species within investigated samples are complexed on column with EDTA. Formed V(IV)-EDTA and V(V)-EDTA complexes have different charge enabling their separation using an eluent consisting of different anion (bicarbonate/sulphate/EDTA) and acetonitrile mixture as the mobile phase. Due to the high affinity of chloride anion from the seawater samples towards anion exchange column, optimization of conditions from sample storage to choice of eluent is needed in order to reduce interferences and obtain accurate results. Samples were measured on anion exchange column Metrosepp A Supp 5-50/4.0 , on the flow rate of 0.3 mL min-1 with eluent containing 40 mmol dm-3 ammonium carbonate, 40 mmol dm-3 ammonium sulphate, 8 mmol dm-3 EDTA and 3% acetonitrile. Since vanadium speciation is highly pH and Eh dependant, samples before analysis were simply filtrated and stored on +4°C in order to avoid speciation changes during the time frame from sampling to analysis. Samples were collected on several differrent locations along „Krka River“ estuary on three different layers (surface fresh layer, freshwater-seawtaer interface and sewater layer) of vertical variable salinity gradient. Based on developed chromatographic method for vanadium speciation on environmental samples of „Krka River“ estuary, it is found that vanadium is present mainly in the form of V(V) with concentrations varying with different salinity of collected layers. Determined concentrations of V(IV) could be attributed to stable complexes of V(IV) with organic matter from fluvial input of „Krka River“. Stated findings are in agreement with literature and suggest that various parameters such as pH, Eh and biology of the aquatic system are an important factor in establishing equilibrium between present vanadium species.

Vanadium speciation ; Krka River estuary ; ion chromatography

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