Trace Metal Speciation in Fresh and Brackish Waters using Ultrafiltration, DGT and Transplanted Aquatic Moss

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 specifi c fractions of metals. Ultrafi ltration has frequently been used to study speciation of metals in natural waters. Several disadvantages are, however, associated with ultrafi ltration. 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 ultrafi ltration is the technique of diffusive gradients in thin fi lms (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 mosses 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 ultrafi ltration, 0.22 μm membrane fi ltration 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 ultrafi ltration 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 ultrafi ltration and aquatic moss were simultaneously applied in the small freshwater stream Gråbergsbäcken in northern Sweden together with a standard 0.22 μm membrane fi ltration. The sampling was conducted 10 times over a whole icefree period. In the Baltic Sea concentrations of Mn, Zn and Cd measured by DGT were similar to the concentrations measured in 1 kDa ultrafi ltered samples, especially for Mn. For Cu and Ni, the ultrafi ltered 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 ultrafi lter but not labile enough to be retained in the DGT units. In Gråbergsbäcken, it was shown that 0.22 μm membrane fi ltration, 1 kDa ultrafi ltration and DGT generally measure different metal fractions, where <1 kDa ultrafi ltered concentrations were lower than DGT-labile concentrations, which in turn were lower than <0.22 μm concentrations. No signifi cant 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 are not necessarily refl ected in moss samples. This is the fi rst comparison of DGT and 1 kDa ultrafi ltration regarding trace metals in both fresh and brackish water. DGT labile colloids, to large to pass the ultrafi lter, found in Gråbergsbäcken seems to be absent in the Baltic Sea. Instead, in the brackish water, a non-labile fraction was found which was small enough to pass the ultrafi lter. Strong correlations between the methods implies that DGT can be a simple alternative to an ultrafi ltration 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.