Measurements of trace metal species in situ in a softwater
river, a hardwater lake, and a hardwater stream were
compared to the equilibrium distribution of species calculated
using two models, WHAM 6, incorporating humic ion
binding model VI and visual MINTEQ incorporating NICA−Donnan. Diffusive gradients in thin films (DGT) and
voltammetry at a gel integrated microelectrode (GIME)
were used to estimate dynamic species that are both labile
and mobile. The Donnan membrane technique (DMT)
and hollow fiber permeation liquid membrane (HFPLM)
were used to measure free ion activities. Predictions of
dominant metal species using the two models agreed
reasonably well, even when colloidal oxide components
were considered. Concentrations derived using GIME were
generally lower than those from DGT, consistent with
calculations of the lability criteria that take into account
the smaller time window available for the flux to GIME. Model
predictions of free ion activities generally did not agree
with measurements, highlighting the need for further work
and difficulties in obtaining appropriate input data.
Several techniques for speciation analysis of Cu, Zn, Cd,
Pb, and Ni are used in freshwater systems and compared
with respect to their performance and to the metal
species detected. The analytical techniques comprise the
following: (i) diffusion gradients in thin-film gels (DGT);
(ii) gel integrated microelectrodes combined to voltammetric
in situ profiling system (GIME−VIP); (iii) stripping
chronopotentiometry (SCP); (iv) flow-through and hollow
fiber permeation liquid membranes (FTPLM and HFPLM); (v)
Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE−SV). All methods
could be used both under hardwater and under softwater
conditions, although in some cases problems with
detection limits were encountered at the low total
concentrations. The detected Cu, Cd, and Pb concentrations
decreased in the order DGT ≥ GIME−VIP ≥ FTPLM ≥
HFPLM ≈ DMT (>CLE−SV for Cd), detected Zn decreased
as DGT ≥ GIME−VIP and Ni as DGT > DMT, in agreement
with the known dynamic features of these techniques.
Techniques involving in situ measurements (GIME−VIP)
or in situ exposure (DGT, DMT, and HFPLM) appear to be
appropriate in avoiding artifacts which may occur
during sampling and sample handling.
The contamination of aquatic ecosystems by natural and anthropogenic metals has lead to a need to better characterize their impact in the environment. To a large extent, the fate and the (eco)toxicity of these elements in aquatic systems are related to their chemical speciation, which may vary continuously in space and time. Detailed measurements of the fraction of specific metal species or groups of homologous metal species and their variation as a function of the bio-physicochemical conditions of the natural media are thus of prime importance. To determine these metal fractions as well as redox chemical species regulating their distribution (dissolved oxygen, sulfides, iron and manganese oxides), new analytical tools capable of performing in situ, real-time monitoring in both water columns and sediments with minimum perturbation of the media are required. This paper reviews the challenges associated with metal speciation studies, and the progress made with state of the art voltammetric techniques to measure the speciation of metals in situ. More specifically, it summarizes the specific conceptual, analytical, and technical criteria that must be considered and/or fulfilled to develop rugged, field deployable, non-perturbing sensors and probes. Strategies used to satisfy these criteria are presented by describing the up-to-date most advanced voltammetric sensors, mini-/micro-integrated analytical systems, and submersible equipments developed for in situ measurements of trace metals and main redox species in aquatic systems. The spatial and temporal resolutions achieved by these news tools represent a significant advantage over traditional laboratory techniques, while simultaneously remaining cost effective. The application of these tools to aquatic systems is illustrated by several examples of unattended and remote in situ monitoring and/or profiling in water columns and sediments.
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