Redox Speciation of Vanadium in Estuarine Waters Using Improved Methodology Based on Anion Exchange Chromatography Coupled to HR ICP-MS System

Abstract

An improved methodology was developed for V redox speciation in estuarine waters using a hyphenated technique consisting of ion chromatograph (IC) with an anion exchange column and a high-resolution inductively coupled plasma mass spectrometer (HR ICP-MS). This approach enables the direct determination of V(V), whereas reduced species (mainly V(IV)) are calculated by subtracting V(V) concentrations from the measured total V concentration. Based on the "on-column" V(V) chelation mechanism by EDTA, with the eluent composed of 40 mmol L^-1 ammonium bicarbonate, 40 mmol L^-1 ammonium sulphate, 8 mmol L^-1 ethylenediaminetetraacetic acid and 3% acetonitrile, the method was successfully used for analyses of V redox speciation in samples taken in the vertical salinity gradient of the highly stratified Krka River estuary. Due to the matrix effects causing different sensitivities, a standard addition method was used for V(V) quantification purposes. The limit of detection (LOD) was also found to be matrix related: 101.68 ng L^-1 in the seawater and 30.56 µg L^-1 in the freshwater. Performed stability tests showed that V redox speciation is preserved at least 7 days in un-treated samples, possibly due to the stabilization of V-reduced species with natural organic matter (NOM). The dominant V form in the analysed samples was V(V) with the reduced V(IV) accounting for up to 26% of the total dissolved pool. The concentration of V(IV) was found to correlate negatively with the oxygen concentration. Significant removal of dissolved V was detected in oxygen depleted zones possibly related to the particle scavenging.

Keywords: Krka River estuary; high-salinity matrix; ion chromatography; on-column complexation; vanadium(V) redox speciation.

Figure 1

(a) Chromatograms of solutions containing 40 nmol L⁻¹ V(V) (orange line) and 40 nmol L^–1 V(IV) (blue line) in MQ water with the EDTA added in solution (3 mmol L⁻¹, pH = 7) compared to blank (MQ water, grey line) using IC–ICP–MS. (b) Comparison of chromatograms obtained by IC-HR ICP-MS at different salinities; total V and V(V) concentration measured in samples collected at “Vrnaža port” (VP) sampling station (orange line: environmental sample of salinity = 5, V_tot = 24.5 ± 1.4 nmol L⁻¹, V(V) = 21.8 ± 0.1 nmol L⁻¹; blue line: environmental sample of salinity = 38, V_tot = 37.0 ± 0.8 nmol L⁻¹, V(V) = 31.1 ± 3.34 nmol L⁻¹). All chromatograms (a,b) were measured using the following eluent composition: 40 mmol L⁻¹ HCO₃^–, 40 mmol L^–1 SO₄^2–, 8 mmol L^–1 EDTA and 3% acetonitrile.

Figure 2

Temporal evolution of IC-UV/VIS chromatograms of V(V) (1 µmol L^–1) and EDTA (3 mmol L⁻¹) in two solutions of different pH values: (a) pH of 7 and (b) 2 (blue curve: solution measured right after preparation; orange curve: solution measured after 24 h; grey curve: solution measured after 7 days). (c) Comparison of different approaches to sample storage measured using IC–ICP–MS (orange curve: chromatogram of a sample filtered on-site and stored at +4 °C; blue curve: chromatogram of a same sample containing ligand (3 mmol L^–1 EDTA) added on-site, after filtration. (d) Stability of V(V) (40 nmol L⁻¹) spiked in natural sample of the Krka River estuary during the time period of 144 h.

Figure 3

(a) Comparison of chromatograms for measured solutions using eluent composed from 40 mmol L^–1 HCO₃^–, 40 mmol L^–1 SO₄^2–, 8 mmol L^–1 EDTA and 3% acetonitrile on IC-UV/Vis containing: V(V) (0.1 mmol L⁻¹) in MQ–blue line; V(V) (0.1 mmol L⁻¹) complexed withEDTA (1 mmol L⁻¹) in MQ– orange line; (b) comparison of IC–ICP–MS chromatograms in seawater sample (salinity= 38). Eluent containing 3 mmol L^–1 EDTA, 40 mmol L^–1 HCO₃^–, 40 mmol L^–1 SO₄^2– and 3% acetonitrile (blue line) and 8 mmol L^–1 EDTA, 40 mmol L^–1 HCO₃^–, 40 mmol L^–1 SO₄^2– and 3% acetonitrile (orange line) are used. (c) Eluents containing 80 mmol L^–1 HCO₃⁻, 8 mmol L^–1 EDTA and 3% acetonitrile (blue line, eluent 1) or 40 mmol L^–1 HCO₃^–, 40 mmol L^–1 SO₄^2–, 8 mmol L^–1 EDTA and 3% acetonitrile (orange line, eluent 2) are used.

Figure 4

The relationship between the total dissolved vanadium concentration and salinity at the MA (green circles), VP (blue circles) and SB (full orange circles—January 2020; empty orange circles—November 2019) sites. The theoretical dilution line between the two end members, open sea (red cross) and the Krka River (light brown cross), is represented by the black line.

Figure 5

Vertical profiles of salinity and dissolved oxygen, and total dissolved V (full circles) and dissolved V(V) (empty circles) for SB (a and d), MA (b and e) and VP (c and f) sites. Dashed lines on panel (a) as well as grey symbols on panel (d) represent November 2019 sampling at the SB station, while the rest of the data represent January 2020.

Figure 6

Percentage of reduced species in the water column of MA (orange markers), VP (green markers) and SB (purple markers—January 2020; blue markers—November 2019) sampling stations in relation to the dissolved oxygen concentration. Empty markers represent bottom seawater layer for each sampling station.

Figure 7

Map of the Krka River estuary with indicated sampling sites.