Copper is a common contaminant in the environment. This element is essential, yet can be
toxic to plants at elevated concentrations. While researchers recognise the importance of
assessing metal speciation in relation to metal phytoavailability/toxicity, an accurate Cu risk
assessment procedure is hampered by the lack of an analytical technique that can be applied
to both aquatic and soil systems to provide a reliable assessment of Cu speciation, and the
lack of a thorough understanding of the relationship between speciation and
phytoavailability. In this study, selected analytical procedures for Cu speciation assessments
were evaluated. The relationships between the different forms of Cu measured and the
responses of a fresh water alga or a higher plant were examined.
The potential of electrospray ionisation mass spectrometry (ESI-MS) to provide a thorough
and accurate assessment of Cu speciation in solution, and to elucidate the interaction between
Cu and organic ligands in acidic waters was first evaluated. This technique was, however,
found unsuitable for Cu complexation and solution speciation assessments in
environmentally related systems. Even though the approach displayed some qualitative
potential in its ability to simultaneously detect a number of Cu and ligand species, and to
assess the trends of change in solution speciation, it failed to preserve the original solution
forms and distributions of Cu and ligand species during the ionisation process. This was
especially evident in solutions containing naturally occurring organic ligands such as oxalic,
citric and phthalic acids.
Following the assessment of ESI-MS, the ability of the Orion 94-29 solid state half-cell
cupric ion-selective electrode (Cu2+-ISE) to measure Cu2+, the ecotoxicologically important
form of Cu, was examined. A Cu-ethylenediamine (Cu-en) metal ion buffer calibration
procedure used by researchers to extend the Nernstian detection range of the electrode was
also evaluated. The Cu2+-ISE provided accurate and precise assessment of Cu2+ down to a
concentration range of 10-7 to 10-8 M in the presence of synthetic and naturally occurring
organic ligands. However, the current study found the Cu-en buffer calibration method
unsuitable for ISE measurement of Cu in contaminated acidic fresh waters, as it
overestimated the concentration of the Cu2+ in test solutions approximating acidic fresh
waters. The ISE is better calibrated using Cu standards prepared in the same medium as the
samples to avoid potential matrix effects.Employing a fresh water alga Chlorella prothecoides as a biological indicator, in conjunction
with chemical measurement techniques such as ICP-AES/MS, Cu2+-ISE and speciation
modelling, the relationship between Cu speciation and phytoavailability, the effect of
naturally occuring organic ligands on this relationship, and most importantly, the capacity of
the ISE to predict Cu phytoavailability/toxicity was investigated. Within experimental error,
Cu phytoavailability/toxicity in acidic fresh water systems was found to be governed by Cu
speciation, particularly the cupric ion activity, (Cu2+). Naturally occurring organic ligands
such as oxalic, tartaric, citric, malonic and a fulvic acid extracted from fresh water streams,
could reduce Cu phytoavailability/toxicity through complexation of the metal thus reducing
(Cu2+), an effect in keeping with the predictions of the ‘free ion activitiy model’ (FIAM).
However, the actual influence a particular organic ligand has on Cu speciation and the
consequent toxicity was also dependent on the ligand’s concentration and stability in the
aquatic environment, and possibly its physiological interactions with the organism.
Importantly, the Cu2+-ISE, calibrated in the modified MBL medium matching that of the
sample matrix, provided accurate measurements of (Cu2+), and a valid means of assessing Cu
phytoavailabilty/toxicity in acidic fresh waters.
The ability of a Cu2+- ISE to assess Cu phytoavailability/toxicity was further examined in a
complex soil-solution-plant system. This study showed that measurement of (Cu2+) in soil
solution with an Orion 96-29 combination Cu2+-ISE, calibrated in dilute CaCl2 solutions
comparable with that of the sample matrix, provided an accurate assessment of Cu
phytoavailability/toxicity to Rhodes grass (Chloris gayana, Kunth) grown in Cu
contaminated acidic soils.
Interestingly, comparable relationships of plant response as a function of log(Cu2+) were
observed across the unicellular alga – acidic nutrient medium (with or without organic
ligands) and the Rhodes grass - acidic soil systems. The comparability between such
different systems, confirmed that plant Cu uptake and toxicity was dependent on the (Cu2+)
as predicted by the FIAM. Most importantly, the Cu2+-ISE, calibrated in matrices that
resemble those of the samples, provided a valid means of assessing Cu phytoavailability and
toxicity in Cu contaminated acidic water and soil systems.