In situ trace metal speciation in lake surface waters using DGT, dialysis, and filtration

In situ measurements of Fe and Mn by dialysis and diffusive gradients in thin-films (DGT) in 5 lakes (pH 4.7-7.5, ionic strength 0.3-5 mmol l(-1)) and Cu and Zn in an acidic and circumneutral lake were compared to results from on site filtration. For the most acidic lake (pH 4.7) all measurements agreed, indicating an absence of colloids and negligible complexation by organic matter. There was little difference in the Mn concentrations measured by the three techniques for any lake, consistent with it being free from complexation. Zn measured by dialysis in circumneutral water was only slightly higher than DGT measurements, appropriate to only partial complexation. Substantial differences between dialysis and DGT for Cu were consistent with complexation by fulvic and humic substances, though not to the extent predicted by the speciation code WHAM. To achieve a good fit it was necessary to adjust the pK for Cu-fulvic binding from 0.8 to 1.3 and to assume that fulvic substances dominated. The presence of low molecular weight strong binding ligands would also be consistent with the data. Differences between the three measurement methods were greatest for Fe, attributable to the presence of large oxyhydroxide colloids, organic complexation and low molecular weight, reactive hydrolysis products. Fe and Mn concentrations measured by DGT on samples returned to the laboratory were much lower than in situ concentrations, illustrating the need for in situ measurements. While use of two in situ techniques provided useful information on the speciation of these natural waters, further refinements are required for unambiguous characterization of the solution. The use of DGT with a more restricted gel that excludes complexes with humic substances should provide complementary information to in situ dialysis.