Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) for the determination of [Zn2+] in estuarine waters

Abstract

Zinc (Zn) has been classified as a "Specific Pollutant" under Annex VIII of the EU Water Framework Directive by two thirds of the EU member states. As a result, the UK Environmental Quality Standard (EQS) for Transitional and Coastal (TrAC) Waters has been reduced from 612 nM to 121 nM total dissolved Zn. It is widely accepted that the free metal ion ([Zn(2+)]) is the most bioavailable fraction, but there are few techniques available to determine its concentration in these waters. In this work, Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) has been applied, for the first time, to determine [Zn(2+)] in estuarine waters. The AGNES method had a mean RSD of ±18%, a (deposition time dependent) limit of detection of 0.73 nM and a [Zn(2+)] recovery of 112 ± 19% from a certified reference material (BCR-505; Estuarine Water). AGNES results for 13 estuarine samples (salinity 0.1-31.9) compared well (P = 0.02) with Competitive Ligand Exchange Cathodic Stripping Voltammetry (CLE-AdCSV) except for one sample. AGNES requires minimal sample manipulation, is unaffected by adsorption of interfering species at the electrode surface and allows direct determination of free zinc ion concentrations. Therefore AGNES results can be used in conjunction with ecotoxicological studies and speciation modelling to set and test compliance with water quality standards.

A review on electrochemical methods for trace metal speciation in environmental media

“…Once equilibrium between the natural ligands (Lx) and Ladd is achieved 71 (typically few hours), quantification of MLadd adsorbed on the sensor (Figure 1) by CSV 72 in a minimum of 12 aliquots at increasing concentrations of M leads to a titration curve 73 (Ip(MLadd) vs cT,M) that can be fitted with the presence of up to typically 2 classes of 74 ligands of specific concentrations [Lx] and conditional stability constant Kx'. Recent 75 CLE-CSV studies published in the last 2 years were applied almost exclusively to 76 marine systems, looking at the speciation of biologically important metals, mostly 77Cu [20][21][22][23][24][25][26][27][28][29] and Fe [30][31][32][33][34][35][36], but also Zn[37;38] or Co [39]. The analytical method is 78 powerful and can give insights into potential biological limitation (e.g.…”

“…Cu [20][21][22][23][24][25][26][27][28][29] and Fe [30][31][32][33][34][35][36], but also Zn[37;38] or Co [39]. The analytical method is 78 powerful and can give insights into potential biological limitation (e.g.…”

“…The capability of AGNES to determine free metal ion concentration has been validated 226 with other techniques, such as ISE [103], DMT [8] [112], SSCP [113], CLE-CSV [38] 227 as well as with speciation codes [101;114]. 228 A list of applications is given in Table 1.…”

“…Very low ionic strength systems, like river 229 waters, can be dealt accurately with AGNES due to the low product of residual current 230 times resistance when equilibrium is reached [90;102;115]. Higher ionic strength 231 systems, such as estuarine [38] and seawater samples [83;116], have also been analysed 232 with AGNES. The extent of complexation of metals to Dissolved Organic Matter 233 (DOM) [117][118][119][120], clays [85] or nanomaterials -from latex [96;113] to TiO2 234 nanoparticles [121;122] -has been assessed.…”

The effect of wastewater effluent derived ligands on copper and zinc complexation

“…This also suggests that this ligand category is present at much higher concentrations in B1 and B2 in comparison with other effluents. Further detailed analysis of the DOC characteristics of the wastewater at this WwTW would be required to fully elucidate this data, for example assessing the impacts that molecular size distribution has on the Zn speciation (Chaminda et al 2008) as well as determining the zinc forms such as the free metal ion concentration (Pearson et al 2016). …”