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Different mobilization mechanisms control the metal distribution in surface sediments of the Belgium coastal zone (BCZ) and the anoxic Gotland basin (GB). This mobilization was studied using DGT (diffusive gradients in thin films): vertical one-dimensional (1D) profiles of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn were measured at 5 mm intervals, while two-dimensional (2D) high-resolution (100 μm) images of smaller zones of the sediment profile were obtained on separate DGT probes. Removal of dissolved Cd, Cu, and Pb in BCZ sediments caused steep vertical gradients at the sediment-water interface that were well replicated in 1D profiles and 2D images. While 1D profiles showed apparent coincident maxima of Co, Mn, and Fe, 2D images revealed mutually exclusive Co and Fe mobilization. Correlation analysis supported this observation and showed a consistent linkage between Co and Mn. Sharp maxima of some metals in the vertical 1D profiles of GB sediment were attributed to localized mobilization in microniches. Examination of an ∼1 mm diameter Cu and Ni maximum in 2D, defined by ∼300 data points, showed that the metals were supplied from localized decomposition of reactive organic material, rather than from reductively dissolving Fe or Mn oxides, and that they were removed as their sulfides.

Belgian coastal sediment serves as an important sink for trace elements, yet a systematic study covering a wide range of elements including redox-sensitive metals (Fe, Mn, and Co), cationic trace metals (Cd, Pb, Ni, Cu, and Zn), oxyanions (P, V, As, and Mo), and sulfide has not been performed and the mechanisms controlling their mobilization were not investigated. Here, a passive sampling technique, Diffusive Gradients in Thin-films (DGT), was used in situ to obtain high resolution concentration profiles of these elements in the sediment porewater. Our results revealed two mobilization mechanisms of cationic trace metals and oxyanions in Belgian coastal sediments, both strongly linked to the cycling of Fe. Mobilization of Co, Pb, Ni, and Cu is controlled by electrogenic sulfur oxidation, acidification of the porewater and dissolution of FeS, while that of oxyanions (P, V, and As) is controlled by reductive dissolution of Fe oxyhydroxides. Constant cationic trace metal to Fe molar ratios were established in FeS, while the oxyanion to Fe ratios in Fe oxyhydroxides differ significantly between sampling stations, which is primarily caused by competing effects. We found no evidence that cationic trace metal mobilization was related to Fe oxyhydroxides, or oxyanion mobilization to FeS. This suggests that particulate organic matter forms the major pathway for cationic trace metal input in coastal sediments and that oxyanions will not be incorporated in FeS but form their own oxyanion-sulfide compound. These findings will contribute to a better understanding of the mobilization mechanisms of cationic trace metals and oxyanions in coastal sediments, and of their biogeochemical cycling in coastal ecosystems., (Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.)