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The role of organic ligand source and type on zinc and copper
speciation in the Tamar Estuary, UK
HOLLY B.C. PEARSON1, SEAN COMBER1, CHARLOTTE BRAUNGARDT1, PAUL WORSFOLD1
1 School of Geography, Earth and Environmental Sciences, Plymouth University, Devon, UK
holly.pearson@plymouth.ac.uk
In recent years, the Environment Agency (EA) has revised the EQSs for Cu
and Zn in UK fresh and salt waters (Table 1). For the first time, the EQSs
took into account bioavailability and background concentrations (through a
DOC correction factor for Cu, and ambient background concentrations for
Zn) for assessing compliance. Although there are reports available on the
complexation of the metals Cu and Zn in saline waters, data is limited,
particularly for Zn. As a result, there is currently no saline Cu or Zn BLM
available for use within the regulatory framework. In order to work towards a
more robust, metal speciation derived EQS for estuarine waters, a number
of areas of research have to be advanced, namely a better understanding of
the relationship between metal ions and natural ligands present in the water
column. Such interactions need to be considered in relation to different
ligand sources and types.
Introduction
Acknowledgements
With thanks to the International Zinc Association, European Copper Association, and Plymouth University for funding this research.
Table 1 Zn and Cu environmental quality standards (EQSs) under the Water Framework Directive.
“Dissolved” refers to the metal concentration present in a sample passed through a 0.45 µm filter. DOC:
Dissolved organic carbon, ABC: Ambient background concentration
Transect surveys of the Tamar
Estuary, across the full salinity
range (0 – 35), sought to provide
seasonal information regarding the
dominance and complexation
characteristics of ligands from
different sources on Cu and Zn by
use of competitive ligand exchange
adsorptive cathodic stripping
voltammetry and complexation
capacity titrations, and collection
from strategically planned locations
potentially influenced by varying
ligand sources (Fig. 1). The
complexation capacity [Lx] of
samples were determined using
competitive ligands of varying
strengths (10 & 2 μM SA for Cu, 4
& 40 μM APDC for Zn).
Characterisation of DOC in each
sample was carried out using 3D
fluorimetry to provide an indication
of the likely origin of the ligands
present.
Method Results (cont.)
Metal New EQS Old EQS
Cu 59 nM dissolved where DOC ≤ 83 µM
79 nM dissolved 59 nM + (42 x ((DOC/2)-0.5)) µM dissolved
where DOC > 83 µM
Zn 104 nM dissolved plus ABC (17 nM
dissolved) 612 nM dissolved
Fig. 1 Map of the Tamar Estuary, UK with sampling
station locations. Potential DOC sources are
marked with coloured symbols.
Conclusions
Fig 5 Ligand concentrations ([Lx]), Ligand excess ([Lx] – [total dissolved metal]), and labile, (organically)
complexed, and free metal as a percentage of total dissolved metal for each sampling occasion. The x-
axes represent salinity in all cases. Note that free Cu constitutes <1% of the total dissolved Cu
concentration.
Results
Total dissolved (TD) Cu and Zn (fig. 2) varied throughout the estuary with
each transect, but showed clear inputs of these metals in the upper and mid-
estuary, as a result of mining and sediment resuspension, respectively. DOC
concentrations (fig. 3) were relatively consistent during July and April, but
stark contrasts are apparent in the February transects.
Fig 4 The HIX and BIX indices determined for the Tamar transect samples
• Seasonal controls on zinc and copper speciation, and DOC source and
character could not be identified.
• Ligands of a stronger humic character appear more important in controlling
Cu and Zn complexation than those of a biogenic origin.
• Cu and Zn complexation appears directly related to [Lx], but the latter is
uncoupled from DOC, suggesting the use of DOC as an indicator of
potential bioavailability, and thus for setting the Cu EQS may need further
research.
Fig 2 Total dissolved and labile Cu and Zn plotted vs. salinity for samples from each Tamar
transect. Error bars represent the range about the mean (n = 2).
Fig 3 DOC plotted vs. salinity for samples from each Tamar transect. Error bars represent 95%
confidence intervals (n = 3).
The HIX and BIX indices for each sample (Fig. 4) revealed varying DOC
characters throughout the estuary and between surveys, with the winter
surveys showing contrasting DOC types.
Samples of the strongest humic character (high HIX index) were identified in
the upper and mid estuary during July (2014) and April, and at the seawater
end during February 2014. Biogenic-type ligands (indicated by a higher BIX
index) are generally more prevalent in the lower estuary.
Upper and mid-estuarine ligand inputs (fig. 5) mimic the dissolved metal
inputs, with a generally increasing complexation of Cu and Zn with increasing
[Lx]. In general, greater metal complexation coincides with the presence of
humic-type ligands, with the labile fraction increasing as the HIX signal
decreases and BIX increases.
No consistent correlation between DOC and complexation capacity, or free
metal was observed for the transects.
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