Biogeochemistry of arsenic in marina sediments
Résumé
The management of polluted sediments is problematic because to date no economically suitable process allows the complete and durable stabilization of pollutants. Some marine sediment contains arsenic at concentrations far higher than the limit that imposes a specific treatment to reduce their toxicity. The aim of ASEDMAR project (ANR-2008-CESA-003) was to quantify the phenomena of retention/mobilization of As by the marine compartments in marine sediments, in order to describe the link between their biogeochemical state, direct toxicity and ability to transfer As to the water column. Sediments were sampled in two French Mediterranean marinas. The project highlighted the importance of biogeochemistry on the behavior of arsenic and its mobility between the following different compartments: solid sediment, pore water and overlying water column. Initial characterization of the sediments revealed the occurrence of thio-arsenic species, probably related to the activity of sulfate-reducing bacteria. Diversity of bacteria directly involved in arsenic cycle was examined through the use of specific molecular tools based on the functional genes aoxB (AsIII oxidation) and arrA (AsV respiration). As we developed the DGGE fingerprinting technique on aoxB genes, the aoxB-carrying community was studied in more details. Altogether, our results showed a very active bacterial community against As in the marine sediments, where AsIII oxidizers and AsV reducers co-exist at both locations. Sediment microcosms were incubated in conditions that stimulated some bacterial processes potentially involved in arsenic mobility. Column experiments were performed in order to ob-serve at the laboratory level the effects of a punctual suspension event, for example linked to a dredging operation. Column experiments allowed studying arsenic distribution in interstitial water, at the interface and in the water column. The following events were observed: settling and deposit (0-7 days), settling, deposit and beginning of consolidation (7-20 days), consolidation (20-60 days), and stabilized sediment after 60 days. The total arsenic concentration was very low just after the re-suspension event that introduced oxygen into the sediment. Arsenic may have been oxidized and ad-sorbed on solid phases such as freshly formed iron oxides. Then, arsenic concentration increased in in-terstitial water and just above the interface, reaching several hundred µg/L. This phase was followed by a decrease of As concentration that however remained relatively high (100 µg/L) up to the end of the monitoring (150 days). All results suggest that along the settling, deposit and consolidation of the sediment, the progressive return to anaerobic conditions favored a succession of biogeochemical reactions linked to cycles of carbon, iron, manganese, nitrogen and sulfur characterized that induced flushes of iron, manganese and arsenic from the solid to the liquid phase. After the stabilization of sediment, a permanent system established in the columns accompanied by a continuous flux of arsenic toward the water column, that has continued for many months after the suspension event. A model was built of distinct blocks which calculate arsenic concentration and its evolution in the "sediment" following changes in physico-chemical and biological parameters. The overall results confirm the major influence of microbiological activities on the fate of pollutants. Those phenomena have to be considered when evaluating the impact of sediments management, both ex-situ and in-situ.