Evaluation of diffusive gradients in thin films technique (DGT) for measuring Al, Cd, Co, Cu, Mn, Ni, and Zn in Amazonian rivers.
ABSTRACT Studies concerning the lability and bioavailability of trace metals have played a prominent role in the search for contamination of water resources. This work describes the first application yet of the diffusive gradients in thin films technique (DGT) to the determination of the fraction of free plus labile metals in waters from the Amazon Basin. Due to the complexity of the use of DGT for samples with low ionic strength and high organic matter content (characteristic of Amazonian rivers), a new analytical procedure was developed. The method is based on the determinations of apparent diffusion coefficients (Dap) in the laboratory, by performing deployments in samples collected in the corresponding sites of study. The Dap thereby determined is then used for in situ measurements. The suitability of the proposed approach for determination of labile Al, Cd, Co, Cu, Mn, Ni, and Zn in the Amazon River and Rio Negro (English: Black River) was evaluated. Except for Co, Mn (in a deployment at Rio Negro), Ni and Zn (in a deployment at Amazon River), labile in situ measurements were lower or similar to dissolved concentrations, indicating suitability of the proposed approach.
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ABSTRACT: The diffusive gradients in thin films (DGT) technique, using a diffusive gel or a restrictive gel, was evaluated for the determination of labile inorganic and organic species of Al and Cu in model synthetic solutions and river water samples. Experiments were performed both in situ and in the laboratory. In the solutions containing Al ions, the major labile fraction consisted of inorganic species. The organic complex fractions were mainly kinetically inert. For the model Cu solutions, the most labile fraction consisted of inorganic species; however, significant amounts of labile organic complexes of Cu were also present. A comparison was made between the results obtained using restrictive gel DGT and tangential flow ultrafiltration (TF-UF). The Cu fraction determined by restrictive gel DGT (corresponding to the "free" ions plus the labile fraction of small molecular size complexes) was larger than that determined by TF-UF (corresponding to all small molecular size ions), suggesting that the techniques exhibited different porosities for discrimination of inorganic species. For the river water samples analyzed in the laboratory, less than 45% of the analytes were present in labile forms, with most being organic species. For the in situ measurements, the labile inorganic and organic fractions were larger than those obtained in the laboratory analyses. These differences could have been due to errors incurred during sample collection and storage. All results were consistent with those found using two different methods, namely, solid-phase extraction and the DGT technique employing the apparent diffusion coefficient.Analytical and Bioanalytical Chemistry 03/2011; 399(7):2563-70. · 3.66 Impact Factor
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ABSTRACT: The technique of diffusive gradients in thin-films (DGT) has been trialed in two river systems for in situ trace metal speciation measurements. This paper presents results for cadmium, copper and manganese concentrations in fresh and estuarine waters and demonstrates for the first time the effectiveness of using DGT to measure labile metal concentrations in such waters. This work has shown that even with very simple deployment systems the technique is sensitive and reproducible. The precision of in situ DGT measurements was typically 11% or better when the precision of subsequent analysis was good. The in-built metal pre-concentration procedure of DGT allowed Cd to be measured at concentrations below the detection limit of a direct determination by GF-AAS. The theoretically predicted linear increase in mass with time was obtained in river deployments of up to 72 h, confirming steady state river conditions and indicating no adverse effects due to biofouling over this period. Concentrations of DGT-labile Cu and Cd were equal to dissolved (0.45 micron) metal concentrations for the Ring and Stitt Rivers indicating the absence of tightly bound organic complexes or colloidal species. DGT-labile Cu and Cd were approximately 50% of the dissolved metal in the Que River and DGT-labile Cu was approximately 30% of dissolved Cu in the Savage River. A concentration-depth profile of labile manganese was obtained in a stratified estuary by deployment of a string of DGT devices at 0.3-m intervals across the redoxcline. Results revealed a large spike (maximum concentration = 1.4 mg/l) of DGT-labile Mn at the base of the redoxcline and demonstrate the utility of DGT to determine vertical changes in metal speciation across redox boundaries in stratified estuarine systems. It is in such dynamic systems as this that the in situ capabilities of DGT are likely to be most useful. DGT records multiple metal species as they exist in situ, overcoming the considerable problems of contamination often associated with sample collection and handling.Science of The Total Environment 11/1999; 239(1-3):71-80. · 3.26 Impact Factor
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ABSTRACT: The emerging technique of DGT (diffusive gradients in thin films) is shown to be capable of performing new speciation measurements in situ in natural waters. In DGT, metals are bound to a resin layer after passing through a well-defined diffusion layer. Cd was measured in solutions containing glycine, EDTA, and fulvic (FA) and humic acids (HA) by atomic absorption spectroscopy (AAS), anodic stripping voltammetry (ASV), and DGT. DGT measured similar labile fractions to ASV, with detailed differences being consistent with a thicker diffusion layer allowing more dissociation of labile complexes and a slower diffusion of FA and HA complexes through the gel. When single measurements are made in complex solutions with DGT, precise quantification is impossible due to uncertainties concerning the distribution of species with different diffusion coefficients. A new procedure was proposed based on the advantage of DGT of being able to control the pore size of the diffusive gel layer. Small (inorganic) species diffuse freely through all gels but larger FA and HA (organic) complexes diffuse less freely in more constrained gels. When measurements were made on known solutions of Cu and FA or HA, it was possible to quantify the inorganic and organic species separately. They agreed well with predictions made using the WHAM speciation code. Multiple DGT units were also deployed in situ in a stream with high dissolved organic carbon (14.6 mg/L). The systematic differences between the devices with different gel compositions enabled determination, for the first time, of the in situ concentrations of both labile inorganic and organic species in natural water. A single DGT device with a constrained gel can be used to quantify inorganic species directly, providing absolute accuracy is not required. This ability of DGT to measure well-defined fractions of metals in situ using a simple device gives it considerable potential as a regulatory tool. A direct speciation measurement may be preferable to modeling approaches which require diverse input data that are difficult to determine.Analytical Chemistry 10/2000; 72(18):4447-57. · 5.70 Impact Factor