Evaluation of biogeochemical reactivity of fresh and weathered contaminated dredged sediments

Purpose Metal mobility in contaminated marine sediments is largely controlled by low-soluble sulfides. In dredged sediments exposed to air and water, geochemical and microbial-mediated processes may enhance the weathering and leaching of hazardous contaminants, especially trace metals. The objective of the present study was to thoroughly characterize and assess the biogeochemical reactivity of marine-dredged sediments. Materials and methods Two samples of dredged sediments, one stored (to preserve anoxic conditions) and another exposed to the air, were assessed in the perspective of their long-term management. Biogeochemical reactivity was evaluated, especially by a qualitative assessment of aerobic iron- and sulfur-oxidizing bacteria activity. Results and discussion Despite the high content of total sulfides (2 to 3.5 wt.% S), the acid-volatile fraction was low (4.3 × 10 −3 vs. 9.4 × 10 −5 g S kg −1 in raw and weathered sediments, respectively), as an indication of the high degree of crystallinity of sulfide minerals present in samples. The raw sediment was reactive, particularly to aerobic bacteria that decreased the pH from 7.0 to 4.2, for neutrophilic sulfur-oxidizing bacteria, and from 4.5 to 2.5, for acidophilic iron-oxidizing, within only 18 days. Even though only neutrophilic sulfur-oxidizing bacteria seem reactive in the aged sediment, they countered the major buffering effect due to the high amount of carbonates. Important differences in the temporal evolution of pH and Eh of the raw and aged sediments support these results. Conclusions Neutrophilic sulfur-oxidizing bacteria showed to be the most reactive in both sediments. Finally, despite the sharp pH decrease over time, acidophilic bacteria activity does not seem to be particularly enhanced.