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Raewyn M Town

Publications and source records attributed to Raewyn M Town.

11 recordsLinked to original sources
PubMed2006

Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters.

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 > or = GIME-VIP > or = FTPLM > or = HFPLM approximately = DMT (>CLE-SV for Cd), detected Zn decreased as DGT > or = 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.

Artifacts↗
PubMed2006

Model predictions of metal speciation in freshwaters compared to measurements by in situ techniques.

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 forthe fluxto 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.

Benzopyrans↗
PubMed2006

Lability criteria for successive metal complexes in steady-state planar diffusion.

The lability of sequential metal complexes, ML, ML2, ML3, ... , up to a general 1:n metal/ligand stoichiometric ratio is considered for the case of metal ions (M) being accumulated at a surface (analytical sensor or organism). The analytical solution for the steady-state diffusion of M within a sequential complexation scheme allows quantification of the contribution from the dissociation of all of the complex species to the metal flux through the so-called lability degree, xi. A lability degree for each sequential complexation step is also defined which, due to the sequential character of the complexation scheme, depends not only on the proper kinetic constants of the given complexation step but also on the kinetics of the previous ones. When all contributions from the complexes are diffusion limited, the system is fully labile and xi=1. To provide simple lability criteria, the reaction layer approximation is extended to specifically deal with this sequential complexation scheme, so that a reaction layer thickness is defined when the existence of one particular rate-limiting step is assumed. Expressions for the classical lability parameter, L, are formulated using the reaction layer approximation. The change of the lability of the system as the diffusion layer thickness is modified is analyzed in detail. The contribution of the complex flux reflects the evolution of the system from labile to inert as the thickness of the sensor is appropriately decreased.

Journal Article↗
PubMed2005

Dynamic speciation analysis and bioavailability of metals in aquatic systems.

Dynamic metal speciation analysis in aquatic ecosystems is emerging as a powerful basis for development of predictions of bioavailability and reliable risk assessment strategies. A given speciation sensor is characterized by an effective time scale or kinetic window that defines the measurable metal species via their labilities. Here we review the current state of the art for the theory and application of dynamic speciation sensors. We show that a common dynamic interpretation framework, based on rigorous flux expressions incorporating the relevant diffusion and reaction steps, is applicable for a suite of sensors that span a range of time scales. Interpolation from a kinetic spectrum of speciation data is proposed as a practical strategy for addressing questions of bioavailability. Case studies illustrate the practical significance of knowledge on the dynamic features of metal complex species in relation to biouptake, and highlight the limitations of equilibrium-based models.

Bacteria↗
PubMed2005

Kinetic limitations in measuring stabilities of metal complexes by competitive ligand exchange-adsorptive stripping voltammetry (CLE-AdSV).

The kinetic limitations of Competitive Ligand Exchange-Adsorptive Stripping Voltammetry, CLE-AdSV, for the determination of very stable metal complexes are explained in detail. For a given type of metal, from a certain lower limit of the complex stability constant, K, the usual simple equilibrium interpretation of CLE-AdSV measurements is not generally valid. By critical assessment of data for natural waters we show that in many cases the reported stability constants appearto derive from nonequilibrium conditions in the bulk sample and hence overestimate the real values. Fe(II) is a special case due to the particular kinetic features of hydroxide as a ligand. Our results call for validation of such data by analysis on the basis of the kinetics involved and/or by independent kinetic-free experimental approaches. Earlier speculations from CLE-AdSV results on very strong ligands and derived features such as the potential bioavailability of trace metals in natural waters require reconsideration.

Adsorption↗
PubMed2004

Dinegative tetrahedral oxoanion complexation; structural and solution phase observations.

A structural and solution phase study of complexation of dinegative oxoanionic guests by protonated azacryptand hosts is presented. The interactions involve a complex balance of competing factors: anion free energy of hydration, host basicity and solvation, and complementarity of steric matching between host and guest. The formation constants of dinegatively charged anion cryptates are much larger than the analogous values for the mononegative analogues perchlorate or perrhenate.

Journal Article↗
PubMed2003

Steric complementarity in carboxylate anion recognition.

A 2,6 pyridino-linked aminocryptand acts as a good host for carboxylate anions, oxalate, malonate, acetate and lactate at low protonation levels as shown by aqueous complexation constants. With oxalate, at the hexaprotonated level of the host, the crystal structure shows formation of a dimeric unit linked by an oxalate/semioxalate short (2.474 A) H-bond. The protonated cryptand uses a cleft-binding mode in preference to the cavity-binding mode used by other aminocryptand hosts.

Journal Article↗
PubMed2003

Electrochemical metal speciation analysis of chemically heterogeneous samples: the outstanding features of stripping chronopotentiometry at scanned deposition potential.

The application of depletive stripping chronopotentiometry at scanned deposition potential (SSCP) to metal ion speciation analysis of chemically heterogeneous complex systems is described. In this electroanalytical stripping technique, metal which is accumulated in the electrode during the deposition step is quantitatively reoxidized during the detection step. SSCP is able to provide an unambiguous measure of heterogeneity that is not attainable by other nondepletive electroanalytical stripping techniques that measure only a proportion of the accumulated metal. SSCP data are not affected by the secondary interferences that plague conventional electrochemical techniques, that is, induced metal adsorption and insufficient ligand excess during stripping. Furthermore, the effect of heterogeneity, as manifested in flattening of the SSCP wave, can be distinguished from that of electrochemical irreversibility. Data are presented for complexation of Cu(II), Pb(II), and Cd(II) by several humic fractions: distinct heterogeneity differences are observed.

Adsorption↗
PubMed2003

Caged oxoanions.

The association between azacryptand hosts and oxoanion guests is reviewed. Positively charged hosts are the most effective; we focus on protonated azacryptands. Assessment of quantitative data suggests an anion cryptate effect and provides clear evidence for charge-based selectivity. Crystal structures show both cavity and cleft binding sites for anions within the series of cryptands studied. These two binding modes exhibit different pH dependence offering the possibility for design of monitoring/clean-up strategies based on a variation of appropriate host(s) and pH conditions.

Journal Article↗
PubMed2002

Implications of natural organic matter binding heterogeneity on understanding lead(II) complexation in aquatic systems.

A critical analysis of data published for complexation (conditional stability constants and complexation capacities) of lead(II) by natural organic matter (NOM) is presented. The same patterns are observed for the different types of NOM, i.e. isolated humic-type substances, isolated algae- and bacteria derived ligands and cell walls, and filtered whole water systems: apparent stronger binding sites are utilised at lower metal ion loadings, progressively weaker sites contribute to metal complexation at higher loadings. Continuous binding functions should thus be used to adequately describe lead-NOM complexation in natural waters. Two important observations arising from our analysis are that: (i) the binding curves (binding affinity as a function of metal ion loading) for whole natural water samples lie between those for compounds representative of the two main NOM types, viz., isolated humic compounds and biota, and (ii) binding constants commonly used in speciation codes probably underestimate the actual extent of lead(II) binding by NOM at natural concentration levels.

Bacteria↗