On metal diagenesis in contaminated sediments of the Deûle river (northern France)

The objective of the present work was to assess depth-related variations in the (bio)geochemical processes involved in anoxic sediments from the Deûle river, and to examine particularly their impacts on the distribution of anthropogenically sourced metals. Anoxic sediment samples were sliced and analyzed to determine total concentrations vs. depth of elements and corresponding pore waters were analyzed to determine concentration profiles with depth of pH, Eh, alkalinity, O2, dissolved organic carbon (DOC), and main inorganic anions and cations present in the medium. It was shown that rapid depletions of O2, NO 3 - and SO 4 2 - , accompanied with HCO 3 - generation and a sharp decrease in the redox potential occurred within the first centimeters of the surface sediment as a consequence of early diagenesis. Bacterial reductive dissolution of Mn(III and IV) and Fe(III) oxides/hydroxides to Mn(II) and Fe(II) accompanied by microbial degradation of organic matter took place as well, and resulted in trace metal increases in the pore water at levels that raised the possibility of mineral generation. Thermodynamic calculations predicted removal of metals from interstitial waters through combinations with carbonates and/or sulfides. These took place either by direct precipitation to form pure crystals, or by coprecipitation/sorption with/into calcite and with pyritic compounds. Chemical sequential extraction data were useful in this work to support, at least partially, some thermodynamic predictions concerning the existence of interactions between trace metals and carbonate and sulfide ions to generate (co)precipitates. Electron paramagnetic resonance (EPR) studies on Deûle sediments revealed the presence of Mn(II) adsorbed onto sedimentary calcite surfaces. X-ray diffractograms of heavy minerals from Deûle sediments extracted with CHBr3 by density separation exhibited peaks ascribed to galena, sphalerite and pyrite. In addition to these compounds, using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), micro-analyses of sedimentary heavy minerals confirmed successfully the existence of other thermodynamically predicted associations: covellite; chalcopyrite, greenockite and/or amorphous CdS.