A comparison of two techniques for quantifying [Zn2+] in estuarine waters

H.B.C Pearson 1 , J. Galceran2, E. Companys2, J. Puy2, S.C. Comber1, C.B. Braungardt1, Paul Worsfold1

1Plymouth University, Devon, UK 2University of Lleida, Spain

holly.pearson@plymouth.ac.uk

Zinc (Zn) is an essential element for the healthy development of all organisms, but uptake in excess can be harmful. The free metal ion is known to be the most readily biologically available[1], therefore being of highest concern with respect to toxicity.

Zinc is now classified as a specific pollutant by the UK Environment Agency, and changes to Environmental Quality Standards (EQS) in 2013 under the Water Framework Directive (WFD) has resulted in a revision of the EQS for saline waters. Dissolved zinc is now restricted to 121 nM (previously 612 nM) including an ambient background concentration of 17 nM[2]. This change has driven further research into methods which may be used by scientists to investigate links between metal speciation and potential biological effects.

Water samples from the Tamar Estuary, UK have been collected over a full range of salinities (~0.1 – 35 psu) each season throughout a full calendar year. Samples were taken from locations suspected of being influenced by different ligand sources. Samples were filtered to 0.4 µm and 0.2 µm, and were analysed for DOC, nutrients, and chlorophyll-a concentrations. The total dissolved and (operationally defined) labile Zn fraction, complexation capacity (CC), natural ligand strengths (log Ks) and [Zn2+] have been measured.

The Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) technique for directly determining [Zn2+] is well established for marine[3] and low ionic strength waters[4], but this study is the first to apply AGNES to estuarine waters. Data obtained from traditional complexation capacity titrations with adsorptive cathodic stripping voltammetry is compared to that directly obtained via AGNES. The evolution of the different fractions of Zn (free, labile, non-labile) along the range of different salinities appears dependant on the season, which influences sources of ligands. Subsequently, seasonal variations in metal speciation should be considered when defining EQS.

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2. WFD-UKTAG 2013. Updated Recommendations on Environmental Standards. River basin Management (2015-21) Final Interim Report (SR3 - 2013).

3. Galceran, J., Huidobro, C., Companys, E. & Alberti, G. 2007. AGNES: a technique for determining the concentration of free metal ions. The case of Zn (II) in coastal Mediterranean seawater. Talanta, 71, 1795-1803.

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