Le 12 juin 2018 (amphi ISTE), Paulo Baisch, professeur à l’université Rio Grande à Porto Alegre (Brésil), nous propose un séminaire intitulé "ARSENIC ENRICHMENT IN BRAZILIAN TROPICAL BEACHES AND MARINE COASTAL AREAS".
résumé :
Arsenic is a metalloid which has been widely recognized for its ability to cause pollution in water and sediments by natural supergene enrichment processes. Hundreds of kilometers of Brazilian tropical beaches are enriched with arsenic (As). High As levels on beaches and nearshore sediments have exceeded legislative environmental thresholds. This environmental framework has been recently recognized and has caused serious limitations to dredging work, industrial activity and civil and port construction in these regions.
As enrichment has been observed not only in sediments but also in surface groundwater in some areas located in the tropical coastal region, such as the Paraiba river delta, Rio de Janeiro, where concentrations of 50 μg L−1 have been measured. As enrichment in waters has been attributed to arsenic fixed by authigenic sulfides. A study of palynomorphs confirmed that aquifer sediments were formed in inter-dune lakes/swamps after covered by eolian sands. Organic sediments in contemporaneous inter-dune lakes/swamps contain very high As concentration: up to 180 mg.kg−1 and 163 μg. L−1 in dry gyttja material and interstitial water, respectively. As in recent lake/swamp sediments is retained by iron hydroxides on upper layers and kept by sulfides on low ones. High As concentrations on sandy beaches correlate with detritus material of geniculate calcareous algae involved in diagenetic processes in the coastal zone. Presumably, carbonate bioclast gains high metalloid content in the surface oxic horizon of sediment during early diagenetic Fe and As redistribution. Cliffs and terraces of the Barreiras group sandstones in the nearshore zone in Bahia and Espirito Santo states are important local As sources. Due to the porous structure of these algae, precipitation of iron hydroxides and subsequent absorption of arsenic occur not only on the surface but also in the inner parts of algal particles. Subsequent diagenetic filling of cell lumen with calcium and magnesium carbonate preserves As inside the particles. This preservation contributes to the conservative behavior of As in changes in geochemical conditions in sediments. Besides, it greatly reduces the efficiency of surf abrasion in the purification of beach sand from high As concentrations. As enrichment process on beach sand may exist in other regions of the world, in addition to Brazil. Seven As species – arsenobetaine (AsB), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), monomethyl-arsonic acid (MMA), arsenite [As(III)], arsenate [As(V)] and p-Arsanilic acid (p-ASA) – were identified in muscle and liver of three species of marine fish (Bagre bagre, Isopisthus parvipinnis, Larimus breviceps), which were collected in the northern coast of Rio de Janeiro (Açu region).
Total As in muscle tissue of fish was between 2.3 μg.g−1 and 80.7 μg.g−1, above the guidelines issued by the Brazilian Health Surveillance Agency (1.0 μg.g−1). Organic As species (AsB, MMA, DMA, p-ASA) were prevalent in all liver and muscle samples, representing about 95% of total As. Arsenobetaine (AsB) was the dominant compound in all samples. Hazards to human health due to fish consumption seem to be low, since the As organic fraction is little toxic and AsB is considered a non-toxic species.