Distribution, mobility, and toxicity of metals in natural waters are strongly related to their aqueous speciation. To understand the behaviour of an aqueous element and the transformation between chemical species, there is a need for reliable methods that enable measurements of specific fractions of metals. Ultrafiltration has frequently been used to study speciation of metals in natural waters. Several disadvantages are, however, associated with ultrafiltration. The procedure is complicated, time consuming and implies sampling and storage of water which may result in a change in metal speciation. A possible alternative or complement to ultrafiltration is the technique of diffusive gradients in thin films (DGT) which provides an in situ measurement of labile metal species. DGT accumulates metals in a time- integrated way and produces a mean concentration over the chosen deployment period. The relatively common occurrence of aquatic moss and their ability to accumulate metals have led to an extensive use of moss as bioindicators in monitoring of aquatic environments. For waters where no native species can be used as bioindicator, transplant techniques have been developed. The aim of this study was to investigate differences and similarities between the trace metal speciation methods; DGT, 1 kDa ultrafiltration, 0.22um membrane filtration and transplanted aquatic moss, and to examine their dependence on water geochemistry. Two studies have been conducted and included in this work. In 2003 and 2004, DGT and 1 kDa ultrafiltration were simultaneously applied at two sampling stations in the Baltic Sea with different salinity and total trace metal concentrations. A total of 16 samplings were performed at the two sites. In 2004 and 2005, DGT, 1 kDa ultrafiltration and aquatic moss were simultaneously applied in the small fresh water stream Gråbergsbäcken in northern Sweden together with a standard 0.22um membrane filtration. The sampling was conducted 10 times over a whole ice free period. In the Baltic Sea concentrations of Mn, Zn and Cd measured by DGT were similar to the concentrations measured in 1 kDa ultrafiltered samples, especially for Mn. For Cu and Ni, the ultrafiltered concentrations clearly exceeded the DGT-labile concentrations. This indicates the existence of low molecular weight Cu and Ni species, small enough to pass through the 1 kDa ultrafilter but not labile enough to be retained in the DGT units. In Gråbergsbäcken, it was shown that 0.22um membrane filtration, 1 kDa ultrafiltration and DGT generally measure different metal fractions were <1 kDa ultrafiltered concentrations were lower than DGT labile concentrations which in turn were lower than <0.22um concentrations. No significant correlations were found between moss and any of the other speciation methods used, except for the relationship found between Fe in moss and particulate Fe, which suggests a substantial retention of Fe-rich particles on the surface of the moss plants. A comparison between moss samples and water samples regarding toxicological threshold levels for metals used by Swedish authorities, showed that high metal concentrations in water samples not necessarily are reflected in moss samples. This is the first comparison of DGT and 1 kDa ultrafiltration regarding trace metals in both fresh and brackish water. DGT labile colloids, to large to pass the ultrafilter, found in Gråbergsbäcken seems to be absent in the Baltic Sea, were a non-labile fraction was found small enough to pass the ultrafilter. Strong correlations between the methods implies that DGT can be a simple alternative to an ultrafiltration procedure. The concentration differences shown in fresh water can likely be reduced by using a restricted diffusion gel. This must, however, be tested and evaluated.
Luleå: Luleå tekniska universitet, 2005. , 12 p.