ABSTRACT
Elevated Arsenic water concentrations have been reported in alkaline lakes of Pantanal da Nhecolândia, with extreme values reaching 3 mg L−1. Silcrete occurrence, resulting from a currently forming process, is frequently observed in the beaches of black alkaline lakes. Two different silcrete types have been distinguished, namely light grey (GS) and black (BS) silcretes, with 3 different layers in each sample. The silcrete is formed and degraded on the beach of the lake according to the seasonal water regime. The silicification process alters various aspects of hydro-bio-geochemical functioning of the lakes and may also affect dissolved arsenic concentration. Arsenic contents increase from bottom to top layer, a trend which was less pronounced in BS samples due to high organic matter contents that can impact the arsenic trapping mechanism. About 71 to 76% of arsenic in silcrete samples were associated with silica, suggesting water arsenic immobilization during the silcrete formation. The nanoparticulate material, preserving arsenic within siliceous structures in surface waters, can come from the degradation of silcrete. Thus, the dissolved arsenic would be trapped during the formation of silcrete and released within small siliceous structures during its degradation.
