Applications of the DGT technique for measurements of anions and cations in natural waters
Sammanfattning: Since the toxicity and mobility of trace metals are related to the metals' speciation, robust methods for trace metal speciation analysis are of great interest. During the last 15 years, hundreds of scientific articles have been published on the development and applications of the diffusive gradients in thin films (DGT) passive sampling technique. In this work the commercially available DGT containing ferrihydrite adsorbent, used for determination of phosphate and inorganic arsenic, was characterised with respect to anionic molybdate, antimonate, vanadate and tungstate determination. Tests were performed in the laboratory as well as in the field. Diffusion coefficients were determined for the anions using two different methods with good agreement. Simultaneous measurements of arsenate were conducted as quality control to facilitate comparison of the performance with previous work. The ferrihydrite-backed DGT was concluded useful for application over the pH-range 4 to 10 for vanadate and tungstate, and 4 to<8 for molybdate and antimonate. At pH values ≥8, deteriorating adsorption was observed. The combination of a restricted pore (RP) version of DGT and the normal open pore (OP) DGT was used for speciation of copper and nickel at three brackish water stations with different salinities in the Baltic Sea. Time series and depth profiles were taken, and complementary membrane- (<0.22 μm) and ultrafiltration (<1 kDa) was conducted. Comparing DGT and ultrafiltration measurements indicated that copper and nickel were complexed. Due to small differences in results between the OP and RP DGTs it was suggested that the complexes were smaller than the pore size of the RP gel (~1 nm) resulting in that both DGTs accumulating essentially the same fraction. Further, there seemed to be a trend in copper speciation indicating higher degree of strong complexation with increasing salinity. The low salinity stations are more affected by fluvial inputs which will likely affect the nature and composition of the organic ligands present. Assuming that copper forms more stable complexes with ligands of marine rather than terrestrial origin would be sufficient to explain the observed trend.
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