1. Comparison of methods for measurement of porewater iron(II) and sulfide: in situ colorimetric DET and DGT vs. ex situ centrifugation followed by colorimetric analysis
Nadeeka Rathnayake Kankanamge, Peter Teasdale, David Welsh and William Bennett
Environmental Futures Research Institute, Griffith University, Gold Coast campus, Queensland, Australia
BACKGROUND
In productive coastal sediments, the oxic and anoxic layers may only be separated by a few mm and have highly heterogeneous distributions, consistent with Aller’s model of heterogeneity (see adjacent Figure). As a result, different biogeochemical zones (e.g., Fe(III)- and sulfate-reduction) and are likely to be present at the same depth.
Conventional porewater sampling and analysis methods are unlikely to resolve solute changes over such small distances (Robertson et al., 2008). The purpose of this study was to compare the results of in situ determination of Fe(II) and sulfide using DET and DGT with conventional ex situ separation and analysis of pore waters.
EXPERIMENTAL
Aller’s heterogeneity model
Nine 10 cm diameter sediment cores were collected (with the bottom capped) from a seagrass meadow and a mud bank. Water was collected from each site.
Each core was incubated (24 °C) in a large container filled with aerated water. After allowing the cores to settle, a profile DGT-DET probe was deployed in the centre of each sediment core for 24 h (see adjacent Figure).
Sediment was extruded, sectioned (1cm) and prepared for analysis under N2. Pore waters were extracted by centrifugation. Solute analysis was by standard colorimetric methods (ferrozine for Fe(II) and methylene blue for sulfide).
DET Fe(II) was prepared and determined according to Bennett et al. (2008). DGT sulfide was prepared and determined according to Robertson et al. (2008).
RESULTS
(a) Original diffusive gel, (b) 2-D distribution of the DGT-DET Fe(II), (c) 1-D distribution of the Fe(II) of the DGT-DET and (d) 1 –D distribution of the Fe(II) core analysis
(a) Original diffusive gel, (b) 2-D distribution of the DGT-DET sulphid, (C) 1-D distribution of the sulphide of the DGT-DET and (d) 1 –D distribution of the sulphide core analysis
(Left) Average 1D (n = 9) distribution of the (left) mm-resolution Fe(II) profile obtained by DET and the (right) cm-resolution Fe(II) profile obtained using conventional methods.
(Left) Average 1D (n = 9) distribution of the (left) mm-resolution sulfide profile obtained by DGT and the (right) cm-resolution sulfide profile obtained using conventional methods.
REFERENCES
CONCLUSIONS
The comparison of Fe(II) and sulfide porewater measurements revealed quite different profiles and dramatically lower concentrations with the conventional methods, suggesting analyte losses as predicted by Robertson et al. (2008).
This study suggests that the colorimetric Fe(II)-DET and sulphide-DGT measurements are both more accurate and more representative than the conventional analysis of pore waters, despite minor artefacts with these techniques.
Robertson, David, Teasdale*, Peter R., & Welsh, David T. (2008). A novel gel-based technique for the high resolution, twodimensional determination of iron (II) and sulfide in sediment. Limnol. Oceanogr, 6, 502–512.
Bennett, William W., Teasdale, Peter R., Welsh, David T., Panther, Jared G., Stewart, Ryan R., Price, Helen L., & Jolley, Dianne F. (2012). Inorganic arsenic and iron(II) distributions in sediment porewaters investigated by a combined DGT colourimetric DET technique. Environmental Chemistry, 9(1), 31.