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.