Inorganic arsenic and iron(II) distributions in sediment porewaters investigated by a combined DGT-colourimetric DET technique

A new approach for investigating the biogeochemistry of inorganic arsenic and iron(II) in freshwater, estuarine and marine sediments is reported. The recently developed Metsorb diffusive gradients in thin films (DGT) technique for the measurement of total inorganic arsenic and the colourimetric diffusive equilibration in thin films (DET) technique for the measurement of iron(II), were utilised in combination to determine co-located depth profiles of both solutes in sediment porewaters. DGT-measured porewater arsenic concentrations were typically less than 40 nM, whereas iron(II) concentrations reached up to 704 mu M. Statistically significant (P<0.0002) correlations between porewater arsenic and iron(II) profiles were observed (R>0.92) in mesocosms of each sediment type. This approach to investigating arsenic and iron geochemistry in sediments allows the in-situ determination of arsenic and iron species at exactly the same location in the sediment at 3-mm resolution for arsenic and 1-mm resolution for iron(II). The technique was capable of detecting very low concentrations of arsenic, with a detection limit of 0.27nM (0.02 mu g L-1) for a 48-h deployment time. Porewater iron(II), which is often present over a wide range of concentrations, was detectable up to 2000 mu M. This study shows the application of these recently developed DGT and DET techniques for the in-situ investigation of inorganic arsenic and iron biogeochemistry in sediments. This approach has the potential to enable simple, yet highly representative assessment of the biogeochemical status of arsenic and iron in a variety of natural sediments, including groundwater sediments where mobilised arsenic is responsible for significant human health risks.