Determination and Prediction of Zinc Speciation in Estuaries.

Lowering of the estuarine Environmental Quality Standard for zinc in the UK to 121 nM reflects rising concern regarding zinc in ecosystems and is driving the need to better understand its fate and behavior and to develop and parametrize speciation models to predict the metal species present. For the first time, an extensive data set has been gathered for the speciation of zinc within an estuarine system with supporting physicochemical characterization, in particular dissolved organic carbon. WHAM/Model VII and Visual MINTEQ speciation models were used to simulate zinc speciation, using a combination of measured complexation variables and available defaults. Data for the five estuarine transects from freshwater to seawater endmembers showed very variable patterns of zinc speciation depending on river flows, seasons, and potential variations in metal and ligand inputs from in situ and ex situ sources. There were no clear relationships between free zinc ion concentration [Zn2+] and measured variables such as DOC concentration, humic and biological indices. Simulations of [Zn2+] carried out with both models at high salinities or by inputting site specific complexation capacities were successful, but overestimated [Zn2+] in low salinity waters, probably owing to an underestimation of the complexation strength of the ligands present. Uncertainties in predicted [Zn2+] are consistently smaller than standard deviations of the measured values, suggesting that the accuracy of the measurements is more critical than model uncertainty in evaluating the predictions.

Predicting Copper Speciation in Estuarine Waters-Is Dissolved Organic Carbon a Good Proxy for the Presence of Organic Ligands?

A new generation of speciation-based aquatic environmental quality standards (EQS) for metals have been developed using models to predict the free metal ion concentration, the most ecologically relevant form, to set site-specific values. Some countries such as the U.K. have moved toward this approach by setting a new estuarine and marine water EQS for copper, based on an empirical relationship between copper toxicity to mussels (Mytilus sp.) and ambient dissolved organic carbon (DOC) concentrations. This assumes an inverse relationship between DOC and free copper ion concentration owing to complexation by predominantly organic ligands. At low DOC concentrations, the new EQS is more stringent, but above 162 µM DOC it is higher than the previous value. However, the relationship between DOC and copper speciation is poorly defined in estuarine waters. This research discusses the influence of DOC from different sources on copper speciation in estuaries and concludes that DOC is not necessarily an accurate predictor of copper speciation. Nevertheless, the determination of ligand strength and concentrations by Competitive Ligand Exchange Adsorptive Cathodic Stripping Voltammetry enabled the prediction of the free metal ion concentration within an order of magnitude for estuarine waters by using a readily available metal speciation model (Visual MINTEQ).

Physico-chemical factors controlling the speciation of phosphorus in English and Welsh rivers.

Phosphorus is a finite resource essential for global food production. However, excessive loss to river systems from diffuse sources (typically agricultural) and point sources (e.g. waste water treatment works and industrial effluent) can lead to negative environmental impacts, including changes to diatom and invertebrate community structure. Current environmental quality standards for phosphorus in the UK have been based on reactive phosphorus, which is poorly defined and comprises an unknown proportion of soluble reactive phosphorus and chemically extractable particulate phosphorus. This research assesses the influencing factors that may control soluble reactive phosphorus concentrations in rivers, including dissolved iron, as well as partitioning processes associated with the presence of total suspended solids, and questions the reliability of the assumptions used when setting environmental quality standards. The extensive phosphorus speciation monitoring carried out across a wide geographic area of England and Wales shows that not all phosphorus as measured by the molybdenum blue method is either soluble or necessarily bioavailable, particularly at concentrations in the range in which the Environmental Quality Standard for 'Good' status (typically less than 100 µg P L-1) has been set. Phosphorus speciation can change due to physico-chemical processes which vary spatially and/or temporally, including precipitation with iron and partitioning with suspended solids.

Mixtures of tritiated water, zinc and dissolved organic carbon: Assessing interactive bioaccumulation and genotoxic effects in marine mussels, Mytilus galloprovincialis.

Release of tritium (3H) in the marine environment is of concern with respect to its potential bioaccumulation and detrimental impact on the biota. Previous studies have investigated the uptake and toxicity of this radionuclide in marine mussels, and the interaction of 3H with dissolved organic ligands and elevated temperature. However, despite the well-established view that toxicity is partly governed by chemical speciation, and that toxic effects of mixture of contaminants are not always additive, there have been no studies linking the prevailing chemistry of exposure waters with observed biological effects and tissue specific accumulation of 3H in combination with other constituents commonly found in natural waters. This study exposed the marine mussel Mytilus galloprovincialis for 14 days to mixtures of 3H (as tritiated water, HTO) and zinc (Zn) at 5 Mbq L-1, and 383, 1913 and 3825 nM Zn, respectively, to investigate (a) 3H and Zn partitioning in soft tissues of mussels, and (b) DNA damage in haemocytes, determined using the single cell gel electrophoresis or the comet assay. Additionally, the extent of association of 3H with dissolved organic carbon (DOC, added as humic acid) over the exposure period was investigated in order to aid the interpretation of biological uptake and effects. Results concluded a clear antagonistic effect of Zn on 3H-induced DNA damage at all Zn concentrations used, likely explained by the importance of Zn in DNA repair enzymes. The interaction of DOC with 3H was variable, with strong 3H-DOC associations observed in the first 3 d of the experiment. The secretion of 3H-binding ligands by the mussels is suggested as a possible mechanism for early biological control of 3H toxicity. The results suggest risk assessments for radionuclides in the environment require consideration of potential mixture effects.

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) for the determination of [Zn(2+)] in estuarine waters.

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.