0 2 4 Mn DGT conc. (|mol L-1)

Fig. 16.6. Co, Cd and Mn measured in situ in sediment of the Black Sea by DGT (adapted from Ref. [65]).

another. In their comprehensive study of river sediment using DGT, Leermakers et al. [10] found that Pb and Zn were well supplied and that Co, Cu, Ni and Zn were only partly supplied. There was also very little supply of Fe and Mn, in contrast to previous work [2]. Strong geo-chemical linkages were apparent between As and Fe, and Co and Mn. Pronounced surface maxima of Cd, Cu, Ni, Pb and Zn were similar to previous observations [2] and attributed to release from fresh supplies of organic matter. Pronounced DGT maxima of Cd in the top 3 cm of sediment of a contaminated marine lagoon were also attributed to mineralization of organic matter, but generally this sediment acted as a strong sink for Cd, due to the formation of CdS [69].

The two-dimensional distribution of trace metals was measured in contaminated, marine, harbour sediment, by slicing the resin-gel into 3 x 3mm squares [11]. All features in the sediment were horizontally uniform, suggesting that localized features were absent in this sediment, which had been incubated for more than a year in a flume. Maxima of Cd, Cu and Ni observed within the surface centimetres of sediment were above the maxima in Co and Fe, suggesting that the primary source of these trace metals was release from freshly decomposing organic material.

Mercury was measured in river and marine sediments using DGT with an agarose diffusive gel and either Chelex 100 or Spheron-Thiol as the binding agent [70]. Measurements made using the thiol binding agent agreed well with Hg measured in pore-waters, indicating that total dissolved Hg was measured and that there was a good supply from the solid-phase. The lower concentrations measured using Chelex 100 suggested that only inorganic ions and weak complexes were measured.

16.5.3 Direct measurements of analytes in the binding layer

Metals have also been measured by DGT at much higher spatial resolution using a beam technique to analyse the binding layer, which is first dried onto a filter support using a gel drier. In the first application, proton-induced X-ray emissions (PIXE) was used for the analysis [15]. A resin with a very small bead size (0.2 mm) was used to ensure an effectively homogeneous, but immobilised binding layer. With the use of ceramic rather than plastic supports, it was possible to keep the total thickness of the DGT sediment probe to less than 1 mm. Images of concentrations, obtained in two dimensions by rastering the 1 mm proton beam, showed similar pronounced vertical gradients of Mn and Zn in a fresh-water sediment and overlying microbial mat. There was evidence of coincident release of As and Fe from an approximately spherical source. However, caution should be exercised when interpreting these very small scale features, as numerical simulations have shown that lateral diffusion within the diffusion gel will cause broadening of sharp peaks [71].

The same resin type and gel drying procedures have also been used to obtain 2D images of metal concentrations with analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) being used as the beam technique [72,73] (Fig. 16.7). While LA-ICP-MS has higher sensitivity than PIXE, data acquisition, which is based on a series of individual analyses, is tedious. The problems associated with internal standardization and energy stability at low power settings, to avoid penetration through the filter and the possibility of contamination, have been systematically investigated [73]. Once these issues are overcome, very precise quantitative measurements can be made. Early work without such good calibration procedures was still able to provide 2D images of metal concentrations in fresh-water [72] and marine [16] sediments.

A technique for measuring total dissolved sulphide by DGT was first developed for sediments by Teasdale et al. [74]. It uses a binding layer of AgI, which reacts with sulphide to form Ag2S. Quantification has been achieved in three ways: (1) the sulphide has been liberated with acid and measured colorimetrically [74], (2) it has been measured directly on the dried gel using LA-ICP-MS [75] and (3) by using the colour

Horizontal Distance (cm) 0.0 0.2 0.4 0.6 0.8 1.0 Co

0 0

Post a comment